JP2016012877A - Quantum entanglement generation method, quantum entanglement generation device, quantum communication apparatus, quantum communication system, quantum communication method and projection measuring method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To match timings at which two independent photons pass through a beam splitter by use of a beam splitter and a photon detector without using a spontaneously emitted photon to which timing control cannot be applied so that quantum entanglements of the two independent photons are securely generated.SOLUTION: In a quantum entanglement generation method, quantum entanglements of a first coupled quantum dot 1 and a second coupled quantum dot 2 are generated by the following steps of: injecting pump light into the first coupled quantum dot 1 coupling two quantum dots and the second coupled quantum dot 2 coupling two quantum dots; and detecting a stokes photon or an anti-stokes photon generated by Raman scattering when the pump light is injected through a beam splitter 3 by photon detectors 4 and 5.

Description

  The present invention relates to a quantum entanglement generation method, a quantum entanglement generation device, a quantum communication device, a quantum communication system, a quantum communication method, and a projection measurement method.

Quantum communication such as quantum cryptography communication or quantum key distribution (QKD) has attracted attention as a method having information theoretical security.
In order to realize quantum communication, for example, a quantum memory using a coupled quantum dot in which two quantum dots are coupled has been proposed.
When using such coupled quantum dots, it is common to use photons spontaneously emitted from the quantum dots.

  For example, when three electrons generated in a coupled quantum dot, one electron in one hole, and one hole are recombined and spontaneously emitted, the polarization of the spontaneously emitted photon and There is quantum entanglement between the spins of the remaining two electrons. Thus, it has been proposed to realize quantum memory devices and further quantum relay using this quantum entanglement.

JP 2007-335503 A JP 2013-97377 A

By the way, when generating quantum entanglement of two independent photons using a beam splitter and a photon detector, it is important to match the timings at which the two independent photons pass through the beam splitter.
However, as described above, when using photons spontaneously emitted from the quantum dots, the timing cannot be controlled because spontaneous emission is a stochastic phenomenon. For this reason, when generating quantum entanglement using photons spontaneously emitted from the quantum dots, it is difficult to match the timing at which the two independent photons pass through the beam splitter.

Therefore, without using the spontaneously emitted photons whose timing cannot be controlled, the beam splitter and the photon detector are used to match the timings of the two independent photons passing through the beam splitter. I want to make sure that entanglements can be generated.
In addition, it is desirable to use a beam splitter and a photon detector to make the projection measurement reliable by matching the timing.

  In this quantum entanglement generation method, pump light is incident on a first coupled quantum dot in which two quantum dots are coupled and a second coupled quantum dot in which two quantum dots are coupled, and by Raman scattering when the pump light is incident. Stokes photons or anti-Stokes photons are detected by a photon detector via a beam splitter to generate quantum entanglement of the first coupled quantum dots and the second coupled quantum dots.

  The present quantum entanglement generating apparatus pumps light to a first coupled quantum dot obtained by coupling two quantum dots, a second coupled quantum dot obtained by coupling two quantum dots, and the first coupled quantum dot and the second coupled quantum dot. A photon detector for detecting a Stokes photon or anti-Stokes photon due to Raman scattering when incident on the beam through a beam splitter, and a first coupled quantum dot and a second coupled quantum dot based on detection information from the photon detector And a controller for generating the quantum entanglement.

The quantum communication device includes the above-described quantum entanglement generation device.
This quantum communication system includes a plurality of quantum communication devices including the above-described quantum entanglement generation device, and a plurality of quantum communication devices are connected to each other.
This quantum communication method uses a plurality of quantum communication devices each including a first coupled quantum dot in which two quantum dots are coupled and a second coupled quantum dot in which two quantum dots are coupled, and is provided in a transmission-side quantum communication device. Quantum communication that performs quantum communication using quantum entanglement between transmission and reception of one of the first coupled quantum dot and the second coupled quantum dot and one of the first coupled quantum dot and the second coupled quantum dot provided in the quantum communication device on the receiving side In each of the quantum communication devices, the first coupled quantum dot and the second coupled quantum dot are set to the lower energy state of the exciton separation state in the first coupled quantum dot and the second coupled quantum dot, In the coupled quantum dot and the second coupled quantum dot, the energy state of the lower exciton separation state in the first coupled quantum dot and the second coupled quantum dot And the first pump light having an energy smaller than the energy difference between the higher energy states of the exciton localized state and the Stokes photon by Raman scattering when the first pump light is incident on the first beam splitter. When one of the two photon detectors serving as the first photon detector is detected, an in-device quantum entanglement is generated. In two adjacent quantum communication devices, two adjacent quantum One of the first coupled quantum dot and the second coupled quantum dot provided in one of the communication devices, and one of the first coupled quantum dot and the second coupled quantum dot provided in the other of the two adjacent quantum communication devices, The higher energy state of the exciton separation state and the higher energy of the exciton localized state in the first coupled quantum dot and the second coupled quantum dot The second pump light having an energy smaller than the energy difference of the state is incident, and anti-Stokes photons due to Raman scattering when the second pump light is incident are passed through the second beam splitter as 2nd photon detectors. When detected by any one of the two photon detectors, it is assumed that quantum entanglement between adjacent devices is generated, in-device quantum entanglement is generated in all quantum communication devices, and all adjacent two entanglements are generated. When quantum entanglement between adjacent devices is generated in the quantum communication device, quantum communication is performed using the quantum entanglement between transmission and reception, assuming that quantum entanglement between transmission and reception is generated.

  In this projection measurement method, pump light is incident on a standard coupled quantum dot in which two quantum dots are combined and the phase of the quantum state is specified, and a measurement target coupled quantum dot in which two quantum dots are coupled, and the pump light is incident. Then, the Stokes photon or anti-Stokes photon due to Raman scattering is detected by the photon detector via the beam splitter, and the phase of the quantum state of the measurement target coupled quantum dot is projected and measured.

  Therefore, according to the present quantum entanglement generation method, quantum entanglement generation device, quantum communication device, quantum communication system, and quantum communication method, a beam splitter and a photon detector can be used without using spontaneously emitted photons that cannot be controlled in timing. Is advantageous in that the quantum entanglement of two independent photons can be reliably generated by matching the timing when the two independent photons pass through the beam splitter. In addition, according to the present projection measurement method, there is an advantage that the projection measurement can be reliably performed by using the beam splitter and the photon detector to match the timing.

It is a schematic diagram for demonstrating the quantum entanglement production | generation apparatus and quantum entanglement production | generation method concerning this embodiment. (A), (B) is a schematic diagram showing an example of a coupled quantum dot used in the quantum entanglement generating apparatus and quantum entanglement generating method according to the present embodiment, (A) is a cross-sectional view thereof, (B) is a plan view thereof. It is a figure which shows the energy state of an example of the combined quantum dot used for the quantum entanglement production | generation apparatus concerning this embodiment, and a quantum entanglement production | generation method. It is a figure for demonstrating the initialization in the quantum entanglement production | generation apparatus concerning this embodiment, and a quantum entanglement production | generation method. It is a figure for demonstrating generation | occurrence | production of the Stokes photon by the Raman transition in the quantum entanglement production | generation apparatus and quantum entanglement production | generation method concerning this embodiment. It is a figure for demonstrating generation | occurrence | production of the anti-Stokes photon by the reverse Raman transition in the quantum entanglement production | generation apparatus and quantum entanglement production | generation method concerning this embodiment. (A), (B) is a figure for demonstrating the structure for implement | achieving the lifetime improvement by the spatial separation of the exciton in the quantum entanglement production | generation apparatus and quantum entanglement production method concerning this embodiment, (A) is a sectional view thereof, and (B) is a plan view thereof. (A), (B) is a figure for demonstrating the lifetime extension by the space separation of the exciton in the quantum entanglement production | generation apparatus and quantum entanglement production method concerning this embodiment. (A), (B) is a figure for demonstrating the lifetime extension by spin inversion in the quantum entanglement production | generation apparatus and quantum entanglement production method concerning this embodiment, (A) is for implement | achieving it. (B) is an energy diagram for explaining the change of the spin state. It is a schematic plan view which shows the structure of the modification of the coupling quantum dot group provided in the optical waveguide in the quantum entanglement production | generation apparatus and quantum entanglement production | generation method concerning this embodiment. It is a schematic diagram which shows an example of the optical waveguide used in the quantum entanglement production | generation apparatus and quantum entanglement production | generation method concerning this embodiment. It is a schematic diagram which shows the structural example of the resonator used in the quantum entanglement production | generation apparatus and quantum entanglement production | generation method concerning this embodiment. It is a schematic diagram which shows the structural example of the resonator used in the quantum entanglement production | generation apparatus and quantum entanglement production | generation method concerning this embodiment. It is a schematic diagram for demonstrating the quantum communication system and quantum communication method to which the quantum entanglement production | generation apparatus and quantum entanglement production | generation method concerning this embodiment are applied. It is a schematic diagram for demonstrating the structure of the quantum communication apparatus to which the quantum entanglement production | generation apparatus concerning this embodiment and the quantum entanglement production | generation method are applied, and the production | generation of the in-device quantum entanglement. It is a schematic diagram for demonstrating the structure of the quantum communication apparatus to which the quantum entanglement generation apparatus and quantum entanglement generation method concerning this embodiment are applied, and the production | generation of the quantum entanglement between adjacent apparatuses. It is a figure for demonstrating adjustment of the phase of the superimposition of the quantum entanglement in the quantum entanglement production | generation apparatus concerning this embodiment, and the quantum entanglement production | generation method. It is a schematic diagram for demonstrating the projection measuring method concerning this embodiment. (A), (B) is a figure for demonstrating the production method of the coupling | bonding quantum dot group used as the standard in the projection measuring method concerning this embodiment. It is a figure for demonstrating the production method of the coupling | bonding quantum dot group used as the standard in the projection measuring method concerning this embodiment. It is a schematic diagram for demonstrating the projection measurement in the transmission side in the quantum communication method concerning this embodiment, and the receiving side.

Hereinafter, with reference to the drawings, a quantum entanglement generation method, a quantum entanglement generation device, a quantum communication device, a quantum communication system, a quantum communication method, and a projection measurement method according to an embodiment of the present invention will be described with reference to FIGS. While explaining.
In the quantum entanglement generation method according to the present embodiment, as shown in FIG. 1, pump light is applied to the first coupled quantum dot 1 that couples two quantum dots and the second coupled quantum dot 2 that couples two quantum dots. Incident. Then, Stokes photons or anti-Stokes photons due to Raman scattering when the pump light is incident are detected by the photon detectors 4 and 5 through the beam splitter 3, and the first coupled quantum dots 1 and the second coupled quantum dots 2 are detected. Generate quantum entanglement of. Note that quantum entanglement is also referred to as quantum entanglement, quantum entanglement, quantum entanglement, or quantum correlation.

  For this reason, the quantum entanglement generation device according to the present embodiment includes a first coupled quantum dot 1 in which two quantum dots are coupled, a second coupled quantum dot 2 in which two quantum dots are coupled, and a first coupled quantum dot. From the photon detectors 4 and 5 for detecting Stokes photons or anti-Stokes photons by Raman scattering when the pump light is incident on the first and second coupled quantum dots 2 via the beam splitter 3, and the photon detectors 4 and 5 And a controller 6 that generates quantum entanglement of the first coupled quantum dot 1 and the second coupled quantum dot 2 based on the detected information.

In the present embodiment, the quantum entanglement generating device includes two coupled quantum dot groups 1X and 2X, a pump light source 7, two filters 8 and 9, a beam splitter 3, and two photon detectors 4 and 5. And a controller 6.
Here, the coupled quantum dot groups 1X and 2X include a plurality of coupled quantum dots 1 and 2 (cells) obtained by coupling two quantum dots. The coupled quantum dots 1 and 2 are two quantum dots provided close to each other so that electrons or holes can tunnel between the quantum dots. The coupled quantum dots 1 included in one coupled quantum dot group 1X are referred to as first coupled quantum dots, and the coupled quantum dots 2 included in the other coupled quantum dot group 2X are referred to as second coupled quantum dots. The coupled quantum dot groups 1X and 2X are also referred to as coupled quantum dot groups.

  Here, the plurality of coupled quantum dots 1 and 2 included in the coupled quantum dot groups 1X and 2X are arranged in series along the optical waveguides 10 and 11 in the optical waveguides 10 and 11 (here, the linear optical waveguide 10). , 11 in a line), and pump light from the outside is guided through the optical waveguides 10 and 11 so as to irradiate each of the plurality of coupled quantum dots 1 and 2. As a result, the coupling between the pump light and the coupled quantum dots 1 and 2 can be strengthened, the generation efficiency of Stokes photons or anti-Stokes photons can be improved, and the directivity of the Stokes light or anti-Stokes light can be improved.

The pump light source 7 emits pump light (here, pump pulse light) toward the two coupled quantum dot groups 1X and 2X.
Here, the pump light source 7 is a laser light source. Further, the pump light emitted from one pump light source 7 is branched by the beam splitter 12 so as to enter the two coupled quantum dot groups 1X and 2X.

Further, two coupled quantum dot groups are formed by the optical path length adjustment unit 13 so that the interference at the beam splitter 3 is maximized (here, the interference fringes detected by the photon detectors 4 and 5 are the strongest). The input time difference of the pump pulse light incident on 1X and 2X is adjusted.
In this way, the Stokes photons or anti-Stokes photons due to Raman scattering when the synchronized pump pulse light is incident on each of the two coupled quantum dot groups 1X and 2X are interfered by the beam splitter 3 to be detected by the photon detectors 4, 5 Can be detected.

In this case, when the pump light from the pump light source 7 is incident on each of the two coupled quantum dot groups 1X and 2X, Stokes photons or anti-Stokes photons due to Raman scattering are immediately generated.
For this reason, it becomes possible to match the timing at which Stokes photons or anti-Stokes photons generated independently by the two coupled quantum dot groups 1X and 2X pass through the beam splitter 3.

The filters 8 and 9 are filters that pass Stokes photons or anti-Stokes photons due to Raman scattering when the pump light is incident on the coupled quantum dot groups 1X and 2X and block the pump light.
The beam splitter 3 is, for example, a 50:50 beam splitter, and the Stokes photons or anti-Stokes photons generated by the two coupled quantum dot groups 1X and 2X and passing through the filters 8 and 9 have a probability of 50:50. It is designed to reflect or transmit.

  The photon detectors 4 and 5 detect Stokes photons or anti-Stokes photons reflected by the beam splitter 3 or transmitted through the beam splitter 3. Here, the photon detectors 4 and 5 are photon number detectors (PNDs) that can detect the number of Stokes photons or anti-Stokes photons reflected by the beam splitter 3 or transmitted through the beam splitter 3 (number of photons). .

The controller 6 generates quantum entanglements of the two coupled quantum dot groups 1X and 2X based on detection information from the photon detectors 4 and 5.
In the present embodiment, the controller 6 is based on detection information from one of the two photon detectors 4 and 5 that detect Stokes photons or anti-Stokes photons via the filters 8 and 9 and the beam splitter 3. Generate entanglement.

As described above, in this embodiment, the Stokes photon or the anti-Stokes photon is detected by one of the two photon detectors 4 and 5 via the filters 8 and 9 and the beam splitter 3, and two coupled quantum dots are detected. Generate group 1X, 2X quantum entanglements.
That is, in this embodiment, when either one of the two photon detectors 4 and 5 detects a Stokes photon or an anti-Stokes photon, the bell state is specified, and thereby, the two coupled quantum dot groups 1X and 2X are identified. Quantum entanglement is generated.

For this reason, the beam splitter 3, the photon detectors 4, 5 and the controller 6 are also referred to as a quantum entanglement generation unit or a bell state identification unit.
By the way, in the quantum dot formed using a semiconductor material, since the band gap of the semiconductor material which forms a quantum dot differs from the band gap of the semiconductor material surrounding it, an electron and a hole can be confined spatially.

In the coupled quantum dots 1 and 2, in addition to the exciton state (localized state; exciton localized state) in which excitons are localized in one of the two quantum dots, two quantum dots have electrons and An exciton state (separation state; exciton separation state) in which holes exist separately is possible.
Also, depending on the size (size difference), material, composition, distance between quantum dots, band alignment, etc. of two quantum dots, excitons are localized in one of the two quantum dots and excited in the other The energy state (energy level) can be different from the state where the child is localized.

In addition, an energy state (energy) between a separated state in which electrons are present in one of the two quantum dots and holes are present in the other, and a separated state in which holes are present in one and electrons are present in the other. The level can also be different.
For example, the lower energy state of the exciton separation state in the coupled quantum dots 1 and 2 is set, and the lower energy state of the exciton separation state and the higher energy of the exciton localized state in the coupled quantum dots 1 and 2 are determined. When pump light having an energy smaller than the energy difference between the states is incident, Stokes photons due to Raman scattering are generated.

  Also, for example, after generating Stokes photons due to Raman scattering and entering the higher energy state of the exciton separation state in the coupled quantum dots 1 and 2, the higher exciton separation state in the coupled quantum dots 1 and 2 When pump light having an energy smaller than the energy difference between the energy state and the higher energy state of the exciton localized state is incident, anti-Stokes photons due to Raman scattering are generated.

Therefore, in the present quantum entanglement generation method, first, the first coupled quantum dot 1 and the second coupled quantum dot 2 are replaced with the first coupled quantum dot 1 and the first coupled quantum dot 2 as an initialization state before the step of entering the pump light. The energy state of the lower exciton separation state in the two-bond quantum dot 2 is assumed.
Then, in the step of making the pump light incident, the first coupled quantum dot 1 and the second coupled quantum dot 2 have the lower energy state of the exciton separation state in the first coupled quantum dot 1 and the second coupled quantum dot 2 One pump light (non-resonant pump light) having an energy smaller than the energy difference between the higher energy states of the exciton localized state is incident (see FIG. 5).

As a result, Stokes photons are generated by Raman scattering (Raman transition). Further, in the step of generating quantum entanglement, Stokes photons due to Raman scattering when one pump light is incident are detected by the photon detectors 4 and 5 via the beam splitter 3, and one quantum entanglement is obtained. Generate.
In this case, the present quantum entanglement generation device uses the lower energy state of the exciton separation state and the higher exciton localization state of the first coupled quantum dot 1 and the second coupled quantum dot 2 as the pump light source 7. One pump light source that emits one pump light having an energy smaller than the energy difference between the energy states is provided.

  Then, after the controller 6 changes the first coupled quantum dot 1 and the second coupled quantum dot 2 to the lower energy state of the exciton separation state in the first coupled quantum dot 1 and the second coupled quantum dot 2, The one pump light source is controlled so that the one pump light is incident on the first coupled quantum dot 1 and the second coupled quantum dot 2, and Stokes photons due to Raman scattering when the one pump light is incident are obtained. One quantum entanglement is generated based on detection information from the photon detectors 4 and 5 detected via the beam splitter 3.

  Here, the initialization state, that is, the first coupled quantum dot 1 and the second coupled quantum dot 2 are set to the lower energy state of the exciton separation state in the first coupled quantum dot 1 and the second coupled quantum dot 2. For example, pump light having an excitation energy from a ground state in which no excitons exist in the first coupled quantum dot 1 and the second coupled quantum dot 2 to an energy state having a lower exciton separation state (an initialization pump) Light) may be applied to the first coupled quantum dot 1 and the second coupled quantum dot 2 (see FIG. 4).

  Further, for example, the exciton separation state is higher than the excitation energy from the ground state where no exciton exists in the first coupled quantum dot 1 and the second coupled quantum dot 2 to the lower energy state of the exciton separated state. The first coupled quantum dot 1 and the second coupled quantum dot 2 may be irradiated with pump light (initializing pump light) having an excitation energy lower than the excitation energy to one energy state (see FIG. 4). .

Multiple pumping can be prevented by using pump light having such excitation energy, that is, sufficiently weak initialization pump pulse light.
Further, in the present quantum entanglement generation method, after generating Stokes photons as described above to generate one quantum entanglement, anti-Stokes photons are generated as follows, and another quantum entanglement is generated. May be generated.

  That is, after one quantum entanglement is generated as described above, the exciton separation state in the first coupled quantum dot 1 and the second coupled quantum dot 2 is changed to the first coupled quantum dot 1 and the second coupled quantum dot 2. The other pump light (non-resonant pump light) having an energy smaller than the energy difference between the higher energy state of the exciton and the higher energy state of the exciton localized state is incident (see FIG. 6).

This generates anti-Stokes photons due to Raman scattering (reverse Raman transition). Further, in the step of generating quantum entanglement, anti-Stokes photons due to Raman scattering when other pump light is incident are detected by the photon detectors 4 and 5 via the beam splitter 3, and other quantum entanglement is detected. May be generated.
In this case, the present quantum entanglement generating apparatus uses the pump coupled light source 7 as the pump light source 7 and the exciton separation state in the first coupled quantum dot 1 and the second coupled quantum dot 2 in addition to the one pump light source that emits the one pump light. It is assumed that another pump light source that emits another pump light having an energy smaller than the energy difference between the higher energy state and the higher energy state of the exciton localized state is provided.

  Then, after the controller 6 generates one quantum entanglement for the first coupled quantum dot 1 and the second coupled quantum dot 2 as described above, the first coupled quantum dot 1 and the second coupled quantum dot 2 The photon detector 4 controls the other pump light source so that the other pump light is incident, and detects the anti-Stokes photon due to Raman scattering through the beam splitter 3 when the other pump light is incident. Based on the detection information from 5, another quantum entanglement is generated.

  In this quantum entanglement generation method, Stokes photons are generated as described above, and the first coupled quantum dot 1 and the second coupled quantum dot 2 are replaced with the first coupled quantum dot 1 and the second coupled quantum dot 2. After being initialized as the higher energy state of the exciton-separated state in, anti-Stokes photons due to Raman scattering when pump light is incident on the first coupled quantum dot 1 and the second coupled quantum dot 2 as described above May be detected by the photon detectors 4 and 5 via the beam splitter 3 to generate quantum entanglement of the first coupled quantum dot 1 and the second coupled quantum dot 2.

  In this case, the present quantum entanglement generation device detects, via the beam splitter 3, anti-Stokes photons due to Raman scattering when pump light is incident on the first coupled quantum dot 1 and the second coupled quantum dot 2. And the controller 6 that generates the quantum entanglement of the first coupled quantum dot 1 and the second coupled quantum dot 2 based on the detection information from the photon detectors 4 and 5. .

By the way, the coupled quantum dots 1 and 2 used in the quantum entanglement generating apparatus and the quantum entanglement generating method as described above can be formed as follows, for example.
That is, first, using, for example, MBE method, MOCVD method or the like, as shown in FIG. 2A, a GaAs buffer layer 21 (for example, a thickness of about 300 nm) and an n-GaAs layer 22 (for example, about 300 nm) are formed on the GaAs substrate 20. An AlGaAs layer 23 (for example, an i-Al _0.3 Ga _0.7 As layer having a thickness of about 100 nm) is grown.

Next, the lower InAs quantum dots 24 are formed on the AlGaAs layer 23 by SK mode growth. If necessary, further annealing is performed.
Next, an AlGaAs cap layer 25 (for example, Al _0.3 Ga _0.7 As layer) is grown to about 10 nm, for example, so that the lower InAs quantum dots 24 are embedded.
Next, upper InAs quantum dots 26 are formed on the AlGaAs cap layer 25 by SK mode growth. If necessary, further annealing is performed.

When the upper InAs quantum dots 26 are grown, the upper InAs quantum dots 26 can be grown immediately above the lower InAs quantum dots 24 by adjusting the growth conditions.
Here, the size is different between the lower InAs quantum dots 24 and the upper InAs quantum dots 26. That is, the size of the lower InAs quantum dot 24 is increased, and the size of the upper InAs quantum dot 26 is decreased. The size of the quantum dots can be designed in the range of about 10 to about 40 nm, and the variation can be about 10% or less.

Next, an AlGaAs cap layer 27 (for example, an Al _0.3 Ga _0.7 As layer) is grown to about 300 nm, for example, so that the upper InAs quantum dots 26 are embedded.
Finally, an n-GaAs layer 28 (for example, a thickness of about 200 nm) is formed on the AlGaAs cap layer 27.
In this manner, as shown in FIGS. 2A and 2B, coupled quantum dots 1 and 2 are formed by combining two quantum dots, the lower InAs quantum dot 24 and the upper InAs quantum dot 26. be able to.

Hereinafter, the energy state (energy level) in the coupled quantum dots 1 and 2 thus formed will be specifically described.
In the coupled quantum dots 1 and 2 formed as described above, in which quantum dot the electrons constituting the exciton exist, and in which quantum dot the holes constituting the exciton exist. Thus, four energy states (energy levels; exciton levels) can be obtained as shown in FIG.

Here, the energy state (energy level) of the exciton localized state in which the exciton is localized in one of the two quantum dots (here, the smaller size) is represented by | e>.
Also, the energy state (energy level) of the exciton localized state in which the exciton is localized in the other of the two quantum dots (here, the larger size) is represented by | vac>.
In addition, an energy state (energy level) of an exciton-separated state in which holes are present in one of the two quantum dots (here, the smaller size) and electrons are separated in the other (here, the larger size). ) Is defined as | g>.

In addition, an energy state (energy level) of an exciton-separated state in which electrons are separated into one of two quantum dots (here, smaller size) and holes are separated from the other (here larger size). ) Is defined as | s>.
These energy states | e>, | vac>, | g>, and | s> are expressed as follows.

Here, | e> has higher energy (binding energy) than | vac>.
Further, | s> has higher energy (binding energy) than | g>. Thus, there are two ways of distributing the separated electrons and holes to the two quantum dots, and there is an energy difference between the energy states of the two. In addition, in | g> and | s>, electrons and holes are oppositely arranged (symmetrical arrangement).

In addition, | e> has higher energy (binding energy) than | s>.
In addition, the energy of | e> and | vac> is shifted by the amount of the Coulomb energy (Coulomb binding energy) of electrons and holes. For example, an SK quantum dot having a diameter of about 20 nm causes an energy shift of about 10 to about 30 meV.
Here, the Coulomb binding energy, | vac>, | g> , | s>, | for each _e>, U L, 0,0, and _{U S,} 2 single electronic ground level energy of the quantum dots When the difference is ΔE _e and the hole ground level energy difference between the two quantum dots is ΔE _h , the energy difference E ₁ between | g> and | e> and the energy difference E ₂ between | s> and | e> are It becomes as follows.

  Thus, the system of coupled quantum dots 1 and 2 formed as described above is a rhombus 4-level system of | g>, | s>, | e>, and | vac>. Therefore, in addition to the Raman process by | g>, | s>, and | e>, the Raman process by | g>, | vac>, and | s> and the actual transition from | s> to | vac> also contribute. This complicates the physical process and makes it difficult to control normal operation.

However, in the system of coupled quantum dots 1 and 2 formed as described above, the energy of | e> and | vac> is shifted by the amount of Coulomb energy.
Therefore, the detuning energy related to the Raman transition is the sum of the coulomb energy U _L + U _S for the values related to | g>, | vac>, | s> than the values related to | g>, | e>, | s>. It will increase by the amount. If the detuning energy is large, the transition amplitude becomes inversely proportional to it, and the contribution of the Raman process and actual transition due to | g>, | vac>, | s> is greatly suppressed.

Therefore, in the coupled quantum dots 1 and 2 formed as described above, three energy levels | g>, | e>, and | s> among the four energy levels have three levels called a lambda system. Construct a unit system. That is, the system of coupled quantum dots 1 and 2 formed as described above constitutes a lambda system (three-level lambda system) with three levels among the four levels.
As a result, Stokes photons are emitted by Raman scattering by the pump light, and the initial state | g> is changed to the final state | s>. Further, anti-Stokes photons are emitted by Raman scattering by the pump light, and a transition is made from the initial state | s> to | g>.

Next, a specific description will be given of how quantum entanglement is generated using such three-level lambda-based coupled quantum dots 1 and 2.
Here, two coupled quantum dots, each having N coupled quantum dots 1 and 2 of the three-level lambda system as described above arranged in a row in the optical waveguides 10 and 11, are provided. Generate quantum entanglements of groups 1X, 2X (see FIG. 1).

First, as an initialization state, the coupled quantum dots 1 and 2 included in each of the two coupled quantum dot groups 1X and 2X are set to an energy state in a lower exciton separation state, that is, | g> (FIG. 3). FIG. 4).
Then, the coupled quantum dots 1 and 2 included in each of the two coupled quantum dot groups 1X and 2X have an energy difference between the lower energy state of the exciton separation state and the higher energy state of the exciton localized state. As non-resonant pump light having a small energy, non-resonant laser light (detuning Δ) having a polarization state intensity Ω and a pulse width τ that can interact with the coupled quantum dots 1 and 2 is incident (see FIG. 5).

The incident non-resonant laser light is subjected to Raman scattering, and a part thereof is converted into Stokes light having a longer wavelength. In this way, Stokes photons due to Raman scattering are generated. Here, since the N coupled quantum dots 1 and 2 are arranged in a line in the optical waveguides 10 and 11, Raman scattering occurs efficiently and the ratio of spontaneous emission photons is small.
Here, a case where the energy state of excitons of all the coupled quantum dots 1 and 2 included in the coupled quantum dot groups 1X and 2X is | g> is referred to as a ground state. Also, only the energy state of excitons of one coupled quantum dot 1 and 2 included in the coupled quantum dot groups 1X and 2X is | s>, and the energy state of excitons of the remaining coupled quantum dots 1 and 2 is | g. The case of> is referred to as a symmetric 1 excited state. Further, the change from | g> to | s> by Raman scattering is called excitation to a symmetric state by Raman scattering.

  Here, when the energy states of excitons of all the coupled quantum dots 1 and 2 included in the coupled quantum dot groups 1X and 2X are | g>, the energy states of the coupled quantum dot groups 1X and 2X are as follows. This quantum state is made to correspond to | 0> of the qubit.

  Also, only the energy state of excitons of one coupled quantum dot 1 and 2 included in the coupled quantum dot groups 1X and 2X is | s>, and the energy state of excitons of the remaining coupled quantum dots 1 and 2 is | g. In the case of>, the energy states of the coupled quantum dot groups 1X and 2X are expressed as follows, and the quantum state (exciton coherent state) is made to correspond to | 1> of the qubit.

In this case, in the initialized state, the energy states of the two coupled quantum dot groups 1X and 2X are both set to | G> described above. That is, in the initialized state, the two coupled quantum dot groups 1X and 2X are set to the states of | 0> and | 0> with qubits.
As a result of the above-described Raman scattering, the quantum states of the two coupled quantum dot groups 1X and 2X are coherently superimposed on the symmetric 1 excited state and the ground state, as shown below.

Is the n-Stokes photon state.
Note that the Stokes photons from the two coupled quantum dot groups 1X and 2X can be obtained with a fixed probability amplitude by adjusting the pump light intensity so that the amplitude f and the higher-order term of the symmetrical one excitation state are sufficiently smaller than 1. Can be generated.
When the photon detectors 4 and 5 detect that one Stokes photon is generated from the two coupled quantum dot groups 1X and 2X (see FIG. 1) via the beam splitter 3, the controller 6 As a result of Raman scattering, the energy state of one of the two coupled quantum dot groups 1X and 2X becomes the above-described | S ₁ >, and | 0>, | 1>, or | 1> , | 0>, and one of the four bell states is specified as shown below, so that quantum entanglement is generated.

  After generating quantum entanglement as described above, each coupled quantum dot included in each of the two coupled quantum dot groups has a higher energy state in the exciton separation state and a higher exciton localized state. When non-resonant pump light having an energy smaller than the energy difference between the two energy states is incident, it undergoes Raman scattering, and a part thereof is converted into anti-Stokes light having a shorter wavelength (see FIG. 6).

That is, anti-Stokes photons are generated by Raman scattering. One anti-Stokes photon due to Raman scattering is generated from two coupled quantum dot groups.
Then, when the photon detector detects that one anti-Stokes photon has been generated from the two coupled quantum dot groups, the controller, as described above, selects one of the four bell states. It is assumed that quantum entanglement has been generated assuming that one state has been identified.

Note that the above-described quantum entanglement generating device may include two electrodes 30 and 31 provided on both sides of each of the first coupled quantum dot 1 and the second coupled quantum dot 2 as shown in FIG. good.
Then, after the controller 6 initializes the first coupled quantum dot 1 and the second coupled quantum dot 2 to the lower energy state of the exciton separation state, the excitons are configured as shown in FIG. Before the operation of pumping light into the first coupled quantum dot 1 and the second coupled quantum dot 2, the voltage is applied between the two electrodes 30 and 31 so that electrons and holes are spatially separated. Control to turn off the voltage applied between the two electrodes 30 and 31 may be performed.

  That is, in the above-described quantum entanglement generation method, after the initialization of setting the first coupled quantum dot 1 and the second coupled quantum dot 2 to the lower energy state of the exciton separation state, the electrons constituting the exciton and A voltage is applied between the two electrodes 30 and 31 provided on both sides of the first coupled quantum dot 1 and the second coupled quantum dot 2 so that the holes are spatially separated, and the first coupled quantum dot 1 and The voltage applied between the two electrodes 30 and 31 may be turned off before the operation in which the pump light is incident on the second coupled quantum dot 2.

By doing so, electrons and holes constituting excitons are spatially separated by an electric field generated between the two electrodes 30 and 31 during standby from initialization to operation. The child state is stabilized, and the lifetime of the exciton state can be extended.
For example, as shown in FIG. 7, a gate electrode 30 is provided on one side in the vicinity of each coupled quantum dot 1 and 2, a ground electrode 31 is provided on the other side, and a gate voltage is applied during standby, An electric field is applied in a direction orthogonal to the stacking direction of the two quantum dots constituting the coupled quantum dots 1 and 2.

As a result, electrons and holes move in opposite directions within the quantum dot, and as shown in FIG. 8, spatial separation of electron / hole wave functions occurs. Such spatial separation (Stark effect) greatly reduces the exciton recombination probability. As a result, the lifetime of excitons during standby is greatly extended.
Further, as shown in FIGS. 9A and 9B, the above-described quantum entanglement generating device generates an external magnetic field and an alternating magnetic field in each of the first coupled quantum dot 1 and the second coupled quantum dot 2. It is good also as a thing provided with the magnetic field generation mechanism to do.

  Then, after the controller 6 initializes the first coupled quantum dot 1 and the second coupled quantum dot 2 to the lower energy state of the exciton separation state, one of the electrons and holes constituting the exciton The magnetic field generation mechanism is controlled so that the spin direction is changed to be in an optically inactive state, and excitons are formed before the pump light is incident on the first coupled quantum dot 1 and the second coupled quantum dot 2. The magnetic field generation mechanism may be controlled so that the spin direction of one of electrons and holes is changed to be in an optically active state.

  That is, in the above-described quantum entanglement generation method, the electrons constituting the exciton and the first coupled quantum dot 1 and the second coupled quantum dot 2 are initialized after being set to the lower energy state of the exciton separation state, and An external magnetic field and an alternating magnetic field are generated in each of the first coupled quantum dot 1 and the second coupled quantum dot 2 so that the spin direction of one of the holes is changed to be in an optically inactive state, and the first coupled quantum dot is generated. Before the operation in which the pump light is incident on the first and second coupled quantum dots 2, the first coupled quantum is such that the spin direction of one of the electrons and holes constituting the exciton is changed to an optically active state. An external magnetic field and an alternating magnetic field may be generated in each of the dot 1 and the second coupled quantum dot 2.

By doing this, since the spin direction of one of the electrons and holes constituting the exciton changes and becomes an optically inactive state during standby from initialization to operation, the exciton state is It is possible to stabilize and extend the lifetime of the exciton state.
For example, as shown in FIGS. 9 (A) and 9 (B), by using an electron spin resonance (ESR) scheme, a static magnetic field as an external magnetic field and a micro magnetic field as an alternating magnetic field are applied to the coupled quantum dots 1 and 2. By simultaneously applying the wave pulse, one of the spins of electrons and holes constituting the excitons generated in the coupled quantum dots 1 and 2 is inverted. In this case, as a magnetic field generation mechanism, a device that generates a uniform static magnetic field and a device that generates an electromagnetic field corresponding to an alternating magnetic field (for example, a microwave pulse generator) are provided.

Moreover, the area (pulse width) of the microwave pulse as an alternating magnetic field is set to such a size that the spin of one of electrons and holes is just reversed, so that the microwave pulse as an alternating magnetic field is converted into electrons and holes. It can act as a π pulse that just reverses one of the spins.
When the spin of one of the electrons and holes constituting the exciton is reversed in this way, the spin needs to be reversed again for spontaneous emission, but when there is no such final state, Since it is a forbidden transition, spontaneous emission is suppressed.

Thus, by changing the spin state of the exciton, it is possible to stabilize the exciton state during standby and to prolong the lifetime of the exciton state. In addition, since the microwave pulse as an alternating magnetic field is generally far from the mode frequency of the optical waveguide, it is not always necessary to irradiate from the same direction as the pump light.
Here, in order to recombine electrons and holes constituting the exciton to spontaneously emit, that is, to obtain an optically active state, the values of j and m indicating the spin state of the exciton Must be ± 1, that is, the difference between the values of j and m indicating the spin state of the electrons constituting the exciton and the values of j and m indicating the spin state of the hole must be ± 1. .

For example, the values of j and m indicating the spin state of the holes constituting the exciton are 3/2 and -3/2, respectively, and the values of j and m indicating the spin state of the electrons constituting the exciton are 1 respectively. When it becomes / 2, 1/2, the values of j and m indicating the spin state of the exciton are 1, -1, and the optically active state is obtained.
On the other hand, in order to prevent recombination of electrons and holes constituting the exciton and spontaneous emission, that is, to make an optically inactive state, j, m indicating the spin state of the exciton Is different from ± 1, that is, the difference between the values of j and m indicating the spin states of electrons constituting the exciton and the values of j and m indicating the spin states of holes is other than ± 1. good.

For example, the values of j and m indicating the spin state of the holes constituting the exciton are 3/2 and -3/2, respectively, and the values of j and m indicating the spin state of the electrons constituting the exciton are 1 respectively. When / 2 or -1/2, the values of j and m indicating the spin state of the exciton become 2 and -2, and the optically inactive state is obtained.
Thus, by changing the value of m indicating the spin state of the electrons constituting the exciton by exactly 1, excitation from the optically active state to the optically inactive state, or from the optically inactive state to the optically active state. The child's state can be changed.

Therefore, as described above, after initialization, a microwave pulse as an alternating magnetic field is added as a π pulse, so that the optical inactive state becomes an inactive state during standby, and the optical response becomes impossible. Again, by applying a microwave pulse as an AC magnetic field as a π pulse, the optically active state is set to an optically active state during operation.
Further, in the above quantum entanglement generating device, the plurality of coupled quantum dots 1 and 2 constituting each coupled quantum dot group 1X and 2X are arranged in the linear optical waveguides 10 and 11, and the linear optical waveguide 10 However, the present invention is not limited to this.

For example, a plurality of coupled quantum dots 1 and 2 constituting each coupled quantum dot group 1X and 2X may be provided in a line along a curved optical waveguide in a curved optical waveguide. Also in this case, the plurality of coupled quantum dots 1 and 2 constituting each coupled quantum dot group 1X and 2X are provided in series along the optical waveguide in the optical waveguide.
Further, for example, as shown in FIG. 10, the number density and the surface density of the plurality of coupled quantum dots 1 and 2 constituting each coupled quantum dot group 1X and 2X are set to linear or curved optical waveguides 10 and 11, respectively. You may make it change along. That is, the distribution of the plurality of coupled quantum dots 1 and 2 constituting each coupled quantum dot group 1X and 2X may be modulated along the linear or curved optical waveguides 10 and 11.

Further, the optical waveguide provided with the plurality of coupled quantum dots 1 and 2 constituting each coupled quantum dot group 1X and 2X is a mesa-type optical waveguide or light by a photonic crystal in which, for example, microstructures 40 are periodically arranged. A waveguide (for example, see FIG. 11) is preferable.
By doing so, it is possible to strengthen the coupling between the pump light and the coupled quantum dots 1 and 2, improve the generation efficiency of Stokes photons or anti-Stokes photons, and improve the directivity of the Stokes light or anti-Stokes light. it can.

  For example, in order to increase the generation efficiency and directivity of Stokes photons or anti-Stokes photons, it is preferable to superimpose contributions from the coupled quantum dots 1 and 2 coherently. For this purpose, it is effective to perform the arrangement of the coupled quantum dots 1 and 2 and the surface density modulation. In addition, optimization of the shape and size of the optical waveguides 10 and 11 is also effective. It is also effective to improve the efficiency by resonating with resonators 50 and 51 such as ring resonators (see FIGS. 12 and 13). These designs involve data such as the wavelength of pump light, Stokes light, or anti-Stokes light, and physical property parameters of the coupled quantum dots 1 and 2.

Further, as shown in FIGS. 12 and 13, the quantum entanglement generation device described above includes coupled quantum dots 1 and 2 on the path, and is based on Raman scattering when pump light is incident on the coupled quantum dots 1 and 2. A resonator 50 having a resonance frequency that matches at least one of the frequencies of the Stokes photon and the anti-Stokes photon may be provided.
For example, the above-described quantum entanglement generating device includes the first coupled quantum dot 1 on the path, and at least the frequencies of Stokes photons and anti-Stokes photons due to Raman scattering when pump light is incident on the first coupled quantum dots 1. Stokes photons and anti-Stokes photons due to Raman scattering when the first resonator 50 having a resonance frequency matching one of the first resonator 50 and the second coupled quantum dot 2 on the path is incident on the second coupled quantum dot 2. And a second resonator 50 having a resonance frequency that matches at least one of these frequencies.

  That is, in the above quantum entanglement generation method, the coupled quantum dots 1 and 2 are provided on the path, and at least the frequencies of the Stokes photons and anti-Stokes photons due to Raman scattering when the pump light is incident on the coupled quantum dots 1 and 2 A Stokes photon and anti-Stokes photon caused by Raman scattering when pump light is incident on the coupled quantum dots 1 and 2 may be made to resonate by the resonator 50 having a resonance frequency that coincides with one of them.

  For example, in the quantum entanglement generation method described above, the first coupled quantum dot 1 is provided on the path, and at least the frequencies of Stokes photons and anti-Stokes photons due to Raman scattering when pump light is incident on the first coupled quantum dots 1 are used. The first resonator 50 having a resonance frequency that coincides with one of them causes the Stokes photon and anti-Stokes photon caused by Raman scattering when the pump light is incident on the first coupled quantum dot 1 to resonate, and the second coupled quantum dot on the path 2 and a second resonator 50 having a resonance frequency that matches at least one of the Stokes photon and anti-Stokes photon frequencies due to Raman scattering when pump light is incident on the second coupled quantum dot 2. Stokes photon due to Raman scattering when pump light is incident on dot 2 Fine anti-Stokes photons may be caused to resonate.

Here, as the resonator 50, for example, a Fabry-Perot resonator 53 as shown in FIG. 12, a ring resonator 58 as shown in FIG.
For example, as shown in FIG. 12, a Fabry-Perot resonator 53 may be configured by an optical element or an optical waveguide. That is, for example, the Fabry-Perot resonator 53 may be configured by including two half mirrors 51 and 52.

Further, for example, as shown in FIG. 13, a ring resonator 58 may be configured by an optical element or an optical waveguide. That is, for example, the ring resonator 58 may be configured as including two half mirrors 54 and 55 and two mirrors 56 and 57.
In addition, when a mechanism for controlling the resonance frequency of the resonator 50 [for example, the phase modulator 59 (see FIG. 12), etc.] is provided, Control may be performed so as to match at least one of the frequencies of Stokes photons and anti-Stokes photons due to Raman scattering.

By doing in this way, the directivity of Stokes light or anti-Stokes light can be improved. It is also possible to reinforce the coupling between the pump light and the coupled quantum dots 1 and 2. It is also possible to improve conversion efficiency and speed.
In addition, the life of the exciton state is extended by designing the resonance frequency of the resonator 50 to deviate from the exciton, that is, the transition frequency of recombination of electrons and holes constituting the exciton during standby. Is possible.

  In addition, the quantum entanglement generation method and the quantum entanglement generation device as described above include, for example, quantum cryptography communication or quantum key distribution such as a quantum transmitter, a quantum repeater, a quantum receiver, and a quantum memory (quantum storage device). The present invention can be applied to a quantum communication device used for quantum communication. A quantum repeater is also referred to as a quantum repeater. The quantum memory can be used not only for quantum communication but also for quantum computation.

For example, in a quantum communication system in which a plurality of quantum communication devices such as a quantum transmitter, a quantum repeater, and a quantum receiver are connected to each other, each of the quantum transmitter, the quantum repeater, and the quantum receiver is as described above. A quantum entanglement generation method and a quantum entanglement generation apparatus can be provided.
Here, quantum communication such as quantum cryptography communication or quantum key distribution is attracting attention as a method having information theoretical security. However, transmission without relay of quantum key distribution is limited to about 50 to about 100 km. It is. For this reason, it is expected to realize quantum relay.

  And it is possible to implement | achieve quantum relay by applying the above quantum entanglement production | generation methods and quantum entanglement production | generation apparatuses to a quantum transmitter, a quantum repeater, and a quantum receiver. In particular, by using Stokes photons or anti-Stokes photons that are immediately emitted by pump light irradiation, rather than a method that relies on spontaneous emission that cannot be controlled in timing, two coupled quantum dot groups 1X, 2X (first and second) The two independent photons from the two-coupled quantum dots 1 and 2) coincide with the timing at which the two independent photons pass through the beam splitter 3 to reliably generate quantum entanglement of the two independent photons (see FIG. 1), and the quantum relay Can be realized. Further, the Raman transition and the reverse Raman transition are effectively resonant excitation, and phase relaxation does not occur. Further, since the spin degree of freedom of carriers in the quantum dots is not used, the quantum entanglement is also resistant to deterioration due to spin relaxation.

  When quantum entanglement is generated using spontaneously emitted photons, the timings at which two independent photons from the two coupled quantum dot groups (first and second coupled quantum dots) pass through the beam splitter coincide with each other. It is difficult to do. In other words, since spontaneous emission is a stochastic phenomenon, using spontaneously emitted photons, the beam splitter and photon detector are used to match the timing of two independent photons passing through the beam splitter, It is in principle impossible to generate photon quantum entanglement. Of course, if the Purcell effect can be enhanced to the limit, the time constant of spontaneous emission may be asymptotic to zero and the timing may be matched, but the Purcell effect has a feasible upper limit. It is becoming clear that it cannot be at a level where two independent photons can interfere. In addition, when excited by resonance pump pulse light, it is difficult to excite only a single exciton. Further, in ordinary non-resonant excitation that is not Raman scattering, phase relaxation is likely to occur when the excitonic ground state falls after excitation. Further, for example, as described in Japanese Patent Application Laid-Open No. 2007-335503, when three electrons are present in one quantum dot to generate quantum entanglement, spin relaxation due to interaction between carriers is reduced. Occurs and the generation of quantum entanglement is incomplete.

Hereinafter, a quantum communication method using a quantum communication system including a quantum transmitter, a quantum repeater, and a quantum receiver to which the quantum entanglement generation method and the quantum entanglement generation device as described above are applied will be described with reference to FIGS. explain.
As shown in FIG. 14, the present quantum communication method uses a plurality of quantum communication devices 63 including a first coupled quantum dot 1 that couples two quantum dots and a second coupled quantum dot 2 that couples two quantum dots, One of the first coupled quantum dot 1 and the second coupled quantum dot 2 provided in the transmission-side quantum communication device 63 (quantum transmitter 60) and the first quantum communication device 63 (quantum receiver 61) provided in the reception-side quantum communication device 63. This is a quantum communication method in which quantum communication is performed using quantum entanglement between transmission and reception of one of the coupled quantum dots 1 and the second coupled quantum dots 2.

  Here, as shown in FIG. 15, in each quantum communication device (quantum transmitter 60, multiple quantum repeaters 62, quantum receiver 61) 63, the first coupled quantum dot 1 and the second coupled quantum dot 2 are In the first coupled quantum dot 1 and the second coupled quantum dot 2, the energy state of the lower exciton separation state is set, and the first coupled quantum dot 1 and the second coupled quantum dot 2 are connected to the first coupled quantum dot 1 and the second coupled quantum dot 2. A first pump light having an energy smaller than the energy difference between the lower energy state of the exciton separation state and the higher energy state of the exciton localized state in the coupled quantum dot 2 is incident, and the first pump light is incident. When the Stokes photon due to Raman scattering is detected by one of the two photon detectors 4 and 5 as the first photon detector via the first beam splitter 3 Shall apparatus in the quantum entanglement is generated.

  Further, as shown in FIG. 16, in two adjacent quantum communication devices 63, one of the first coupled quantum dot 1 and the second coupled quantum dot 2 provided in one of the two adjacent quantum communication devices 63, and One of the first coupled quantum dot 1 and the second coupled quantum dot 2 provided in the other of the two adjacent quantum communication devices 63 has a high exciton separation state in the first coupled quantum dot 1 and the second coupled quantum dot 2. The second pump light having an energy smaller than the energy difference between the upper energy state and the higher energy state of the exciton localized state is incident, and the anti-Stokes photon due to Raman scattering when the second pump light is incident, When one of the two photon detectors 4 and 5 serving as the second photon detector is detected via the second beam splitter 3, the quantum interanging between adjacent devices is detected. It is assumed that the instrument has been generated.

Then, in-device quantum entanglement is generated in all the quantum communication devices 63, and when the inter-adjacent device quantum entanglement is generated in all the two adjacent quantum communication devices 63, transmission / reception quantum entanglement is generated. As a result, quantum communication is performed using quantum entanglement between transmission and reception.
Such a quantum communication method using a quantum communication system (quantum relay method using a quantum relay system) to which the above-described quantum entanglement generation method or quantum entanglement generation device is applied, includes excitons generated in coupled quantum dots 1 and 2. The quantum relay is realized by utilizing the Raman interaction of the pump light.

Hereinafter, the operation of the quantum communication system (each quantum communication device), that is, the quantum communication method will be described more specifically with reference to FIGS.
First, the quantum communication system, that is, each quantum communication device 63 (quantum transmitter 60, multiple quantum repeaters 62, quantum receiver 61; node) is initialized. In other words, excitons are generated in the coupled quantum dots 1 and 2 included in the two coupled quantum dot groups 1X and 2X provided in each quantum communication device 63, and are set in an initialized state. The quantum repeater 62 is also referred to as a communication point or a relay point.

Here, in order to prevent multiple excitation, a sufficiently weak initialization pump pulse light is incident on the two coupled quantum dot groups 1X and 2X via the optical waveguides 10 and 11 using an initialization laser light source. (See FIG. 4).
Here, the initialization pump pulse light resonates with the excitation energy from the ground state (the state where no exciton is present) to the lower energy state | g> of the exciton-separated state or is excited from the ground state. It is assumed that the energies are higher than the excitation energy to the lower energy state | g> of the exciton-separated state and lower than the excitation energy to the higher energy state | s> of the exciton-separated state. The energy state is lowered to the lower energy state | g> of the exciton-separated state by energy relaxation by scattering or the like, or Raman transition accompanied by Stokes light. Thus, if the condition of weak excitation is satisfied, the probability of multicarrier excitation is extremely low.

  In this way, excitons are generated in a certain proportion of the coupled quantum dots 1 and 2 of the plurality of coupled quantum dots 1 and 2 included in the two coupled quantum dot groups 1X and 2X. The energy state | g> of the lower separation state can be obtained. That is, | g> state excitons are generated in this way. The coupled quantum dots 1 and 2 in which excitons are not generated are not used until the next routine.

Here, the two coupled quantum dot groups 1X and 2X provided in the i-th quantum communication device 63 are α _i and β _i , respectively, and the quantum states of these coupled quantum dot groups α _i and β _i are as follows: It represents as follows.

In this way, after excitons are generated in the coupled quantum dots 1 and 2 included in the two coupled quantum dot groups 1X and 2X to be in an initialized state, the initialized exciton state is stabilized here. The standby state can be maintained.
Here, by applying a gate voltage to the gate electrode 30 (see FIG. 7) provided in the vicinity of each coupled quantum dot group 1X, 2X, all coupled quantum dots included in each coupled quantum dot group 1X, 2X. A gate voltage (gate electric field) is applied adiabatically to 1 and 2, and the electrons and holes constituting the | g> -state excitons are separated laterally by the Stark effect. When the gate voltage is applied (ε ≠ 0), it is expressed by the following wave equation of the electron of a quantum dot having a large size when the electric field ε is applied, and the following lower equation. In addition, since the overlap with the wave function of the holes of the quantum dots having a small size when the electric field ε is applied is reduced (see FIG. 8), the recombination lifetime is dramatically increased. For example, the lifetime of excitons having an order of several ns to 10 ns is dramatically increased.

In this manner, the lifetime of the exciton is prolonged, and the initialized exciton state (quantum state of each coupled quantum dot group) is set to a standby state that can be stably maintained.
Next, as shown in FIG. 15, quantum entanglement is generated in each quantum communication device 63. That is, in-device quantum entanglement is generated.
Here, the following operations are performed on the two coupled quantum dot groups 1X, 2X (α _i , β _i ) provided in each quantum communication device 63.

First, the gate voltage is adiabatically returned to zero.
Next, using a laser light source 7 for pump (for Raman process; for Stokes process), detuning is set as Δ, and non-resonant pump pulse light (energy E ₁ −Δ) between | g> − | e> is introduced. Then, this is irradiated to two coupled quantum dot groups 1X, 2X (α _i , β _i ), and weak Stokes light (energy E ₂ −Δ) having an average photon number of 1 or less is extracted by a Raman process (Stokes process). .

At this time, the time difference of the input to the coupled quantum dot groups 1X, 2X (α _i , β _i ) of the pump pulse light from the pumping laser light source 7 is finely adjusted to provide two coupled quantum dot groups 1X, 2X. The scattered output from (α _i , β _i ) is set so as to pass through a 50:50 beam splitter (BS) 3 synchronously, and two beams are detected by two photon detectors (photon number detectors) 4 and 5. Capture. That is, the Stokes photons are detected by the two photon detectors 4 and 5 by causing the beam splitter 3 to interfere with the Stokes scattered light from the coupled quantum dot groups 1X and 2X (α _i , β _i ) by the synchronized pump pulse light. To do.

When only one of the two photon detectors 4 and 5 detects one Stokes photon, a bell state as shown below is specified and each quantum communication device 63 is provided with 2 Quantum entanglement (in-device quantum entanglement) is generated between the coupled quantum dot groups 1X and 2X (α _i , β _i ). This is called post-selection (post-selection) by detecting one Stokes photon.

If such an operation is successful, the gate voltage is applied again to return to the standby state.
Although there is a high probability that no Stokes photons are detected, in this case, the above operation is repeated again. In addition, although the probability is small, when two or more Stokes photons are detected, the gate voltage is left at zero, waits for all excitons to recombine, and starts again from initialization.

  Next, as shown in FIG. 16, quantum entanglement is generated in two adjacent quantum communication devices 63. That is, as described above, the quantum entanglement between adjacent devices is generated starting from the state in which the in-device quantum entanglement has already been formed in all the quantum communication devices 63 on the transmission / reception path. This is also called quantum entanglement swapping.

Here, one of the two adjacent quantum communication devices 63 (for example, the quantum transmitter 60 and the first quantum repeater 62, the two adjacent quantum repeaters 62, the last quantum repeater 62 and the quantum receiver 61) is provided. The following operation is performed on one coupled quantum dot group 2X (β _i ) and one coupled quantum dot group 1X (α _{i + 1} ) (here, a pair of β _i and α _{i + 1} ) provided on the other side. .

First, the gate voltage is adiabatically returned to zero.
Next, using a laser light source 7 for pump (for reverse Raman process; for anti-Stokes process), detuning is Δ, and non-resonant pump pulse light (energy E ₂ −Δ) between | s> − | e> Is irradiated to two coupled quantum dot groups, and weak anti-Stokes light (energy E ₁ −Δ) having an average photon number of 1 or less is extracted by an inverse Raman process (anti-Stokes process). This non-resonant pump pulse light is also referred to as retrieval pulse light.

At this time, the time difference of the input to the coupled quantum dot groups 1X, 2X (α _i , β _{i + 1} ) of the pump pulse light from the pumping laser light source 7 is finely adjusted to provide two coupled quantum dot groups 1X, 2X. The scattered output from (α _i , β _{i + 1} ) is set so as to pass through a 50:50 beam splitter (BS) 3 synchronously, and two beams are detected by two photon detectors (photon number detectors) 4 and 5. Capture. That is, anti-Stokes photons from the coupled quantum dot groups 1X and 2X (α _i , β _{i + 1} ) by the synchronized pump pulse light are caused to interfere with each other by the beam splitter 3 and the two photon detectors 4 and 5 provide anti-Stokes photons. Is detected.

Note that anti-Stokes photons generated in one coupled quantum dot group 2X (β _{i + 1} ) provided in one of the two adjacent quantum communication devices 63 are transmitted to the other via, for example, the optical fiber 64. For this reason, it is desirable that the energy E ₁ -Δ of the anti-Stokes photons be in the low-loss transmission wavelength band of the optical fiber 64. Further, the anti-Stokes photons may be transmitted to any of the two adjacent quantum communication devices 63.

When only one of the two photon detectors 4 and 5 detects one anti-Stokes photon, the bell state as shown below is specified, and two adjacent quantum communication devices 63 are detected. It is assumed that a quantum entanglement (quantum entanglement between adjacent devices) is generated between the two coupled quantum dot groups 1X and 2X (α _i , β _{i + 1} ) included in FIG. This is called post-selection (post-selection) by detecting one anti-Stokes photon.

If such an operation is successful, the gate voltage is applied again to return to the standby state.
If no anti-Stokes photons are detected, the above operation is repeated again. When two or more anti-Stokes photons are detected, the two adjacent quantum communication devices 63 are left with the gate voltage set to zero and wait for all excitons to recombine. Redo from the operations to initialize and generate in-device quantum entanglement.

  As described above, in-device quantum entanglement is generated in each of all the quantum communication devices 63, and the inter-adjacent device quantum entanglement in each of the two adjacent quantum communication devices 63 in which the in-device quantum entanglement is generated. As a result, the bell state as shown below is specified, and the inter-transmission / reception quantum entanglement is generated at both ends of the quantum relay communication path (transmission line) (see FIG. 14). That is, quantum entanglement is generated between one coupled quantum dot group 1X provided in the quantum transmitter 60 and one coupled quantum dot group 2X provided in the quantum receiver 61 (see FIG. 14).

  Then, by applying a phase factor, this quantum entanglement state can be made substantially into the following form.

  Here, after generating in-device quantum entanglement in each of all the quantum communication devices 63, in each of the two adjacent quantum communication devices 63 in which the in-device quantum entanglement is generated, between adjacent devices quantum entanglement However, the present invention is not limited to this. For example, the generation of in-device quantum entanglement and the generation of inter-adjacent quantum entanglement are performed on the transmission side or reception with respect to two adjacent quantum communication devices 63. You may make it perform in order from the side.

In this way, quantum entanglement can be generated even when the distance between the sender and the receiver of communication is long, and quantum relay can be realized. Quantum communication (quantum communication protocol) such as key extraction and quantum teleportation can be realized.
For example, the quantum entanglement generated as described above can be used as a resource of a QKD protocol (for example, the Ekert91 system). The Ekert91 system is described, for example, in Artur E. Ekert, “Quantum cryptography based on Bell's Theorem”, Physical Review Letters, vol. 67, No. 6, pp. 661-663, 5 August 1991.

  Therefore, according to the quantum entanglement generation method, the quantum entanglement generation device, the quantum communication device, the quantum communication system, and the quantum communication method according to the present embodiment, the beam splitter can be used without using spontaneously emitted photons that cannot be controlled in timing. And the photon detector to match the timing of the two independent photons passing through the beam splitter, so that the quantum entanglement of the two independent photons (ie, the first coupled quantum dot 1 and the second coupled quantum dot 2). (Quantum entanglement) can be reliably generated.

In addition, this invention is not limited to the structure described in embodiment mentioned above, A various deformation | transformation is possible in the range which does not deviate from the meaning of this invention.
For example, in the quantum entanglement generation method of the above-described embodiment, at least one of the first coupled quantum dots and the second coupled quantum dots is provided on the front surface side with respect to the potential of the back surface electrode provided on the back surface side. A voltage may be applied to the surface electrode to adjust the superposition phase of the quantum entanglement of the first coupled quantum dots and the second coupled quantum dots.

  In this case, the quantum entanglement generating device according to the above-described embodiment includes the back surface electrode provided on the back surface side of each of the first coupled quantum dot and the second coupled quantum dot, the first coupled quantum dot, and the second coupled quantum dot. And a surface electrode provided on each surface side. The controller performs control for applying a voltage to the front surface electrode with respect to the potential of the back surface electrode in at least one of the first coupled quantum dot and the second coupled quantum dot, so that the first coupled quantum dot and the second coupled quantum dot The phase of superposition of the quantum entanglements may be adjusted.

  For example, as shown in FIG. 17, the back surface side of each of the coupled quantum dot group 1X as the first coupled quantum dot 1 and the coupled quantum dot group 2X as the second coupled quantum dot 2, here, the substrate 70 The surface provided on the surface side with respect to each of the back surface electrode 71 provided on the back surface side and the coupled quantum dot group 1X as the first coupled quantum dot 1 and the coupled quantum dot group 2X as the second coupled quantum dot 2 An electrode 72 is provided. Here, it is assumed that the surface electrode 72 is provided on both sides of the coupled quantum dot group 1X as the first coupled quantum dot 1 and the coupled quantum dot group 2X as the second coupled quantum dot 2.

Then, in at least one of the coupled quantum dot group 1X and the coupled quantum dot group 2X, a voltage is applied to the front electrode 72 with respect to the potential of the back electrode 71, and the | s> state based on the energy of the | g> state The phase of superposition of the quantum entanglement of the coupled quantum dot group 1X and the coupled quantum dot group 2X is increased or decreased by increasing or decreasing the energy of | S ₁ > state based on the energy of | G> state. You may adjust it.

Here, the energy in the | g> state is the lower energy state from the ground state to the exciton separation state. The energy in the | s> state is the higher energy state in the exciton separation state. Also, the energy of the | G> state is the energy of the coupled quantum dot group 1X (2X) when the energy states of excitons of all the coupled quantum dots 1 and 2 included in the coupled quantum dot groups 1X and 2X are | g>. Energy state. Also, the energy of the | S ₁ > state is | s> only in the energy state of the exciton of one coupled quantum dot 1 or 2 included in the coupled quantum dot groups 1X and 2X, and the remaining coupled quantum dots 1 and 2 This is the energy state of the coupled quantum dot groups 1X and 2X when the energy state of the exciton of | g>.

For example, in one of the coupled quantum dot group 1X and the coupled quantum dot group 2X, the same voltage of the same polarity (same sign) is applied to the front surface electrode 72 on both sides with respect to the potential of the back surface electrode 71 for a predetermined time t. By increasing or decreasing | s> state energy E _{s based} on | g> state energy E _g , and increasing or decreasing | S ₁ > state energy based on | G> state energy, Quantum entanglement states shown below

The phase of the superposition of

And can be adjusted.
Here, δ = (E _s ′ −E _s ) + (E _g −E _g ′). E _s ′ and E _g ′ are the energy of the | s> state and the | g> state after application of the electric field.
By having the function of adjusting the phase of superposition of such quantum entanglement states, it is possible to share standard quantum entanglement between the transmitting side and the receiving side as described below. Become.

  Further, for example, as shown in FIG. 18, the phase of the quantum state of the standard coupled quantum dot group α (first coupled quantum dot 1; coupled quantum dot group 1X) is prepared in a predetermined phase state, which becomes the standard. Pump light from the laser light source 7 for the reverse process is incident on the coupled quantum dot group α and the coupled quantum dot group β (second coupled quantum dot 2; coupled quantum dot group 2X) to be measured. Then, anti-Stokes photons due to Raman scattering when the pump light is incident are detected by two photon detectors (photon number detectors) 4 and 5 via the beam splitter 3 and measured based on the detected information. Projection measurement of the phase of the quantum state of the target coupled quantum dot group β can be performed.

Here, preparing the phase of the quantum state of the standard coupled quantum dot group α in a predetermined phase state means a standard coupled quantum dot group in which the phase of the quantum state is specified, that is, a standard state used for projection measurement. Is to create a group of coupled quantum dots.
The coupled quantum dot group α in the standard state used for the projection measurement can be created as follows, for example.

That is, first, the coupled quantum dot group α is initialized to the | G> state by the laser light source for initialization. Next, as shown in FIGS. 19A and 19B, pump light Pp and Pr from both the forward process laser light source and the reverse process laser light source are simultaneously applied to the coupled quantum dot group α. Irradiate. And the intensity | strength of the pump light from both laser light sources is temporally changed with an attenuator, as shown in FIG. That is, the reverse process laser light source is first started, and then the forward process laser light source is started. Then, the intensity is shifted to zero so that the intensity is the same. As a result, the wave function in the final state is an even superposition of the | G> state and the | S ₁ > state, as shown in FIG. This physical process is called Stimulated Raman Adiabatic Passage (STIRAP) in atomic optics. Since this process hardly depends on the absolute intensity of the laser, it is easy to control. After such a state, the coupled quantum dot group α in the standard state used for the projection measurement is further adjusted by adjusting the phase to a desired value using the above-described method for adjusting the phase of the superposition of the quantum entanglement state. That is, it is possible to create a coupled quantum dot group α that is a standard in which the phase of the quantum state is specified. In this way, after the superposition state is created, by adjusting the superposition phase, it is possible to create a coupled quantum dot group α in a standard state used for projection measurement, in which the vector direction is also defined.

  Here, pump light from the laser light source 7 for the reverse process is incident, and anti-Stokes photons due to Raman scattering when the pump light is incident are detected by the photon detectors 4 and 5 via the beam splitter 3. Thus, the phase of the quantum state of the coupled quantum dot group β to be measured is projected and measured. However, the present invention is not limited to this, and the pump light from the forward process laser light source 7 is incident. The Stokes photons due to Raman scattering may be detected by the photon detectors 4 and 5 via the beam splitter 3 to projectively measure the phase of the quantum state of the coupled quantum dot group β to be measured.

  As described above, in the projection measurement method according to the present embodiment, pump light is applied to the standard coupled quantum dot in which two quantum dots are coupled and the phase of the quantum state is specified, and the measurement target coupled quantum dot in which the two quantum dots are coupled. The Stokes photon or anti-Stokes photon due to Raman scattering when the pump light is incident is detected by the photon detector via the beam splitter, and the phase of the quantum state of the measurement target coupled quantum dot is projected and measured. Accordingly, there is an advantage that the projection measurement can be surely performed by using the beam splitter and the photon detector to match the timing.

Furthermore, using such a projection measurement of the phase of the quantum state of the coupled quantum dot group, for example, as shown in FIG. 21, the transmission-side quantum communication device 63 (quantum transmitter 60) and the reception-side quantum communication device 63 In each of the (quantum receiver 61), the projection measurement of the phase of the quantum state of the two coupled quantum dot groups included therein can be performed, and the key extraction and the security check can be performed according to the Ekert91 protocol. That is, for each quantum entanglement state of the quantum transmitter 60 and the quantum receiver 61, the quantum state of each coupled quantum dot group is obtained by using the projection measurement of the phase of the coupled quantum dot group as described above. The phase can be projected and key extraction and security checks can be performed according to the Ekert91 protocol. In this case, for the coupled quantum dot group α ₁ provided in the quantum transmitter 60 and the coupled quantum dot group β _{N + 1} provided in the quantum receiver 61, as described above, as the standard coupled quantum dot group, its quantum state Are prepared in a predetermined phase state. In this way, standard quantum entanglement can be shared between the transmitting side and the receiving side.

  Here, when performing the projection measurement of the phase of the quantum state of each coupled quantum dot group using the projection measurement of the phase of the quantum state of the coupled quantum dot group as described above, an arbitrary direction on the equator of the qubit is set. Projection measurement with respect to the Bloch vector facing is possible. In other words, by using the projection measurement of the phase of the quantum state of the coupled quantum dot group as described above, after creating the superposition state, by adjusting the superposition phase, the vector orientation is also specified. A coupled quantum dot group in a standard state to be used is created, and projection measurement is performed using the group as described above, thereby enabling projection measurement regarding a Bloch vector pointing in an arbitrary direction on the equator of the qubit.

For this reason, for example, in quantum cryptography communication, it is possible to perform projection measurement with a plurality of bases. For example, when the transmission / reception quantum entanglement is generated on the transmission side and the reception side, each of the transmission-side quantum communication device and the reception-side quantum communication device includes two coupled quantum dot groups included in the plurality of coupled quantum dot groups. It is possible to perform projective measurement on the basis.
In other words, the projection measurement on the calculation basis, that is, the measurement for determining whether | G> or | S ₁ > is performed, the pump light is incident in the setting of the forward process or the reverse process, and the Stokes photons or anti-Stokes photons emitted This can be done with or without detection. On the other hand, it may be necessary to perform projection measurements on multiple bases, for example, when quantum cryptography communication, when transmission and reception quantum entanglement is generated on the transmission side and reception side, etc. In addition, by using the projection measurement of the phase of the quantum state of the coupled quantum dot group as described above, the projection measurement of the phase of the quantum state of each coupled quantum dot group can be performed to perform projection measurement on a plurality of bases. It becomes possible.

  Thus, in the quantum communication method of the above-described embodiment, in each of the quantum communication device on the transmission side and the quantum communication device on the reception side, the other of the first coupled quantum dots and the second coupled quantum dots is changed to the phase of the quantum state. The coupled quantum dot becomes the specified standard, and the phase measurement of the quantum state of the first coupled quantum dot and the second coupled quantum dot is measured, and quantum communication can be performed by using quantum entanglement between transmission and reception. That is, quantum communication can be realized by combining the quantum entanglement generation method of the above-described embodiment and the above-described projection measurement method.

Hereinafter, additional notes will be disclosed regarding the above-described embodiment.
(Appendix 1)
Pump light is incident on a first coupled quantum dot obtained by coupling two quantum dots and a second coupled quantum dot obtained by coupling two quantum dots,
A Stokes photon or anti-Stokes photon due to Raman scattering when the pump light is incident is detected by a photon detector via a beam splitter, and quantum entanglement between the first coupled quantum dot and the second coupled quantum dot is performed. A quantum entanglement generation method comprising: generating a quantum entanglement.

(Appendix 2)
Prior to the step of injecting the pump light, the first coupled quantum dot and the second coupled quantum dot are placed in the lower energy state of the exciton separation state in the first coupled quantum dot and the second coupled quantum dot. age,
In the step of injecting the pump light, the first coupled quantum dot and the second coupled quantum dot are excited in the lower energy state of the exciton separation state in the first coupled quantum dot and the second coupled quantum dot. 2. The quantum entanglement generating method according to appendix 1, wherein pump light having an energy smaller than an energy difference between energy states of higher child localized states is incident.

(Appendix 3)
The step of injecting the pump light includes the step of exciting the first coupled quantum dot and the second coupled quantum dot with the lower energy state of the exciton separation state in the first coupled quantum dot and the second coupled quantum dot. A step of entering a first pump light having an energy smaller than an energy difference between the higher energy states of the child localized states, and the first coupled quantum dots on the first coupled quantum dots and the second coupled quantum dots And injecting the second pump light having an energy smaller than the energy difference between the higher energy state of the exciton separation state and the higher energy state of the exciton localized state in the second coupled quantum dot. ,
The step of generating the quantum entanglement includes detecting the Stokes photon due to Raman scattering when the first pump light is incident by the photon detector via the beam splitter, and generating the first quantum entanglement as the first quantum entanglement. A step of generating quantum entanglement, and the anti-Stokes photons due to Raman scattering when the second pump light is incident are detected by the photon detector via the beam splitter, and second quantum entanglement is detected. Generating quantum entanglement,
Prior to the step of entering the first pump light, the first coupled quantum dot and the second coupled quantum dot are moved to a lower exciton separation state in the first coupled quantum dot and the second coupled quantum dot. Energy state,
Performing a step of generating the first quantum entanglement after the step of entering the first pump light;
After the step of generating the first quantum entanglement, performing a step of entering the second pump light,
The quantum entanglement generation method according to appendix 1, wherein a step of generating the second quantum entanglement is performed after the step of entering the second pump light.

(Appendix 4)
In the step of generating the quantum entanglement, the Stokes photon or the anti-Stokes photon caused by Raman scattering when the pump light is incident on the first coupled quantum dot is passed, and the pump light is blocked. A filter, passing through the Stokes photon or the anti-Stokes photon due to Raman scattering when the pump light is incident on the second coupled quantum dot, and passing through the second filter and the beam splitter that block the pump light Any one of the first photon detector and the second photon detector serving as the photon detector generates the quantum entanglement according to any one of appendices 1 to 3. The quantum entanglement generation method as described.

(Appendix 5)
The first coupled quantum dot and the second coupled quantum dot are initialized so as to have an energy state having a lower exciton separation state, so that the electrons and holes constituting the exciton are spatially separated. An operation in which a voltage is applied between two electrodes provided on both sides of each of the first coupled quantum dot and the second coupled quantum dot, and the pump light is incident on the first coupled quantum dot and the second coupled quantum dot. The method of generating quantum entanglement according to any one of appendices 1 to 4, wherein the voltage applied between the two electrodes is turned off before time.

(Appendix 6)
After the initialization of the first coupled quantum dot and the second coupled quantum dot to the lower energy state of the exciton separation state, the direction of one of the spins of electrons and holes constituting the exciton is changed. An external magnetic field and an alternating magnetic field are generated in each of the first coupled quantum dot and the second coupled quantum dot so as to be in an optically inactive state, and the pump is coupled to the first coupled quantum dot and the second coupled quantum dot. Before the operation of entering light, the first coupled quantum dot and the second coupled quantum dot are arranged such that the spin direction of one of electrons and holes constituting the exciton is changed to an optically active state. The quantum entanglement generation method according to any one of appendices 1 to 5, wherein an external magnetic field and an alternating magnetic field are generated in each of the above.

(Appendix 7)
The first coupled quantum dots and the second coupled quantum dots are provided in an optical waveguide, and include a plurality of first coupled quantum dots and a plurality of second coupled quantum dots provided in series along the optical waveguide. The quantum entanglement generation method according to any one of appendices 1 to 6, characterized in that:

(Appendix 8)
The first coupled quantum dot is provided on a path, and has a resonance frequency that matches at least one of the frequencies of the Stokes photon and the anti-Stokes photon due to Raman scattering when the pump light is incident on the first coupled quantum dot. The first resonator causes the Stokes photon and the anti-Stokes photon due to Raman scattering to resonate when the pump light is incident on the first coupled quantum dot,
The second coupled quantum dot is provided on a path, and has a resonance frequency that matches at least one of the frequencies of the Stokes photon and the anti-Stokes photon due to Raman scattering when the pump light is incident on the second coupled quantum dot. Any one of appendices 1 to 7, wherein the second resonator causes the Stokes photon and the anti-Stokes photon caused by Raman scattering to resonate when the pump light is incident on the second coupled quantum dot. The quantum entanglement generation method described in 1.

(Appendix 9)
In at least one of the first coupled quantum dots and the second coupled quantum dots, a voltage is applied to the front surface electrode provided on the front surface side with respect to the potential of the back surface electrode provided on the back surface side, and the first coupling The quantum entanglement generation method according to any one of appendices 1 to 8, wherein a phase of superposition of quantum entanglements of quantum dots and the second coupled quantum dots is adjusted.

(Appendix 10)
A first coupled quantum dot in which two quantum dots are coupled;
A second coupled quantum dot in which two quantum dots are coupled;
A photon detector that detects Stokes photons or anti-Stokes photons by Raman scattering when pump light is incident on the first coupled quantum dots and the second coupled quantum dots via a beam splitter;
A quantum entanglement generation device comprising: a controller that generates quantum entanglement of the first coupled quantum dots and the second coupled quantum dots based on detection information from the photon detector.

(Appendix 11)
Pump light having an energy smaller than the energy difference between the energy state of the lower exciton separation state and the energy state of the higher exciton localized state in the first coupled quantum dot and the second coupled quantum dot is emitted. With a pump light source,
The controller sets the first coupled quantum dot and the second coupled quantum dot to the first energy state of the lower exciton separation state in the first coupled quantum dot and the second coupled quantum dot. 11. The quantum entanglement generation device according to appendix 10, wherein the pump light source is controlled so that the pump light is incident on a coupled quantum dot and the second coupled quantum dot.

(Appendix 12)
A first pump light having an energy smaller than an energy difference between an energy state of a lower exciton separation state and an energy state of a higher exciton localized state in the first coupled quantum dot and the second coupled quantum dot; A first pump light source that emits;
Second pump light having an energy smaller than the energy difference between the higher energy state of the exciton separation state and the higher energy state of the exciton localized state in the first coupled quantum dot and the second coupled quantum dot. A second pump light source that emits,
The controller sets the first coupled quantum dot and the second coupled quantum dot to the first energy state of the lower exciton separation state in the first coupled quantum dot and the second coupled quantum dot. The first pump light source is controlled so that the first pump light is incident on the coupled quantum dots and the second coupled quantum dots, and the Stokes photons due to Raman scattering when the first pump light is incident are Based on detection information from the photon detector detected through a beam splitter, the first quantum entanglement is generated as the quantum entanglement, and then the first coupled quantum dot and the second coupled quantum dot are The second pump light source is controlled so that the second pump light is incident, and the Raman scattering when the second pump light is incident. The second quantum entanglement is generated as the quantum entanglement based on detection information from the photon detector that detects the anti-Stokes photon by the beam splitter via the beam splitter. Quantum entanglement generator.

(Appendix 13)
A first filter that passes the Stokes photon or the anti-Stokes photon due to Raman scattering when the pump light is incident on the first coupled quantum dot, and blocks the pump light;
A second filter that passes the Stokes photon or the anti-Stokes photon due to Raman scattering when the pump light is incident on the second coupled quantum dot, and blocks the pump light;
The controller detects the Stokes photon or the anti-Stokes photon via the first filter, the second filter, and the beam splitter, and the first photon detector and the second photon detector as the photon detector. The quantum entanglement generation device according to any one of appendices 10 to 12, wherein the quantum entanglement is generated based on detection information from any one of the above.

(Appendix 14)
Two electrodes provided on both sides of each of the first coupled quantum dots and the second coupled quantum dots,
The controller may spatially separate the electrons and holes constituting the excitons after the first coupled quantum dots and the second coupled quantum dots are initialized to have a lower energy state in the exciton separation state. And applying a voltage between the two electrodes before the pump light is incident on the first coupled quantum dot and the second coupled quantum dot. 14. The quantum entanglement generation device according to any one of appendices 10 to 13, wherein control for turning off a voltage is performed.

(Appendix 15)
A magnetic field generation mechanism for generating an external magnetic field and an alternating magnetic field in each of the first coupled quantum dots and the second coupled quantum dots;
The controller sets the first coupled quantum dot and the second coupled quantum dot to the lower energy state of the exciton-separated state, and then initializes one of the spins of electrons and holes constituting the exciton. The magnetic field generation mechanism is controlled so that the direction is changed to an optically inactive state, and the exciton is set before the operation of injecting the pump light into the first coupled quantum dot and the second coupled quantum dot. 15. The quantum entanglement according to any one of appendices 10 to 14, wherein the magnetic field generation mechanism is controlled so that one of the electrons and holes constituting the spin direction changes to be in an optically active state. Generator.

(Appendix 16)
The first coupled quantum dots and the second coupled quantum dots are provided in an optical waveguide, and include a plurality of first coupled quantum dots and a plurality of second coupled quantum dots provided in series along the optical waveguide. The quantum entanglement generation device according to any one of appendices 10 to 15, characterized in that:

(Appendix 17)
The first coupled quantum dot is provided on a path, and has a resonance frequency that matches at least one of the frequencies of the Stokes photon and the anti-Stokes photon due to Raman scattering when the pump light is incident on the first coupled quantum dot. A first resonator;
The second coupled quantum dot is provided on a path, and has a resonance frequency that matches at least one of the frequencies of the Stokes photon and the anti-Stokes photon due to Raman scattering when the pump light is incident on the second coupled quantum dot. The quantum entanglement generation device according to any one of appendices 10 to 16, further comprising a second resonator.

(Appendix 18)
A back electrode provided on each back side of the first coupled quantum dot and the second coupled quantum dot;
A surface electrode provided on each surface side of the first coupled quantum dots and the second coupled quantum dots,
The controller performs control for applying a voltage to the front surface electrode with respect to the potential of the back surface electrode in at least one of the first coupled quantum dot and the second coupled quantum dot, and the first coupled quantum dot and the 18. The quantum entanglement generation device according to any one of appendices 10 to 17, wherein a phase of superposition of quantum entanglements of the second coupled quantum dots is adjusted.

(Appendix 19)
A quantum communication device comprising the quantum entanglement generation device according to any one of appendices 10 to 18.
(Appendix 20)
A plurality of quantum communication devices including the quantum entanglement generation device according to any one of appendices 10 to 18,
The quantum communication system, wherein the plurality of quantum communication devices are connected to each other.

(Appendix 21)
The first coupled quantum dot provided in the transmission-side quantum communication device using a plurality of quantum communication devices including a first coupled quantum dot coupled with two quantum dots and a second coupled quantum dot coupled with two quantum dots. And one of the second coupled quantum dots and one of the first coupled quantum dots and one of the second coupled quantum dots provided in the quantum communication device on the receiving side perform quantum communication using quantum entanglement between transmission and reception A method,
In each of the quantum communication devices,
The first coupled quantum dot and the second coupled quantum dot are in the lower energy state of the exciton separation state in the first coupled quantum dot and the second coupled quantum dot,
In the first coupled quantum dot and the second coupled quantum dot, the lower energy state of the exciton separation state and the higher energy of the exciton localized state in the first coupled quantum dot and the second coupled quantum dot The first pump light having an energy smaller than the energy difference between the states is incident,
In-device quantum entanglement when Stokes photons due to Raman scattering when the first pump light is incident are detected by one of the two photon detectors as the first photon detector via the first beam splitter. That the
In the two adjacent quantum communication devices,
One of the first coupled quantum dot and the second coupled quantum dot provided in one of the two adjacent quantum communication devices, and the first coupled quantum dot provided in the other of the two adjacent quantum communication devices And an energy difference between the higher energy state of the exciton-separated state and the higher energy state of the exciton localized state in the first coupled quantum dot and the second coupled quantum dot. A second pump light having an energy smaller than
When an anti-Stokes photon due to Raman scattering when the second pump light is incident is detected by one of the two photon detectors as the second photon detector via the second beam splitter, it is between adjacent devices. Suppose quantum entanglement has been generated,
The intra-device quantum entanglement is generated in all the quantum communication devices, and the inter-adjacent device quantum entanglement is generated in all the adjacent two quantum communication devices. As another example, a quantum communication method comprising performing quantum communication using the inter-transmission / reception quantum entanglement.

(Appendix 22)
In each of the quantum communication device on the transmission side and the quantum communication device on the reception side, the other of the first coupled quantum dot and the second coupled quantum dot is a coupled quantum dot serving as a standard in which the phase of the quantum state is specified. Supplementary note 21, characterized in that the phase measurement of the quantum state of the first coupled quantum dot and the second coupled quantum dot is measured and the quantum communication is performed using the inter-transmission / reception quantum entanglement. Quantum communication method.

(Appendix 23)
The pump light is incident on a standard coupled quantum dot in which two quantum dots are coupled and a phase of the quantum state is specified, and a measurement target coupled quantum dot in which two quantum dots are coupled,
A Stokes photon or anti-Stokes photon due to Raman scattering when the pump light is incident is detected by a photon detector via a beam splitter, and the phase of the quantum state of the measurement target coupled quantum dot is projected and measured. Projection measurement method.

1 coupled quantum dots (first coupled quantum dots)
2 coupled quantum dots (second coupled quantum dots)
1X, 2X coupled quantum dot group 3 beam splitter 4, 5 photon detector 6 controller 7 pump light source 8, 9 filter 10, 11 optical waveguide 12 beam splitter 13 optical path length adjustment unit 20 GaAs substrate 21 GaAs buffer layer 22 n-GaAs layer 23 AlGaAs layer 24 Lower InAs quantum dot 25 AlGaAs cap layer 26 Upper InAs quantum dot 27 AlGaAs cap layer 28 n-GaAs layer 30 Gate electrode (electrode)
31 Ground electrode (electrode)
40 Microstructure 50 Resonator 51, 52 Half mirror 53 Fabry-Perot resonator 54, 55 Half mirror 56, 57 Mirror 58 Ring resonator 59 Phase modulator 60 Quantum transmitter 61 Quantum receiver 62 Quantum repeater 63 Quantum communication equipment 64 Optical fiber 70 Substrate 71 Back electrode 72 Front electrode

Claims (14)

Pump light is incident on a first coupled quantum dot obtained by coupling two quantum dots and a second coupled quantum dot obtained by coupling two quantum dots,
A Stokes photon or anti-Stokes photon due to Raman scattering when the pump light is incident is detected by a photon detector via a beam splitter, and quantum entanglement between the first coupled quantum dot and the second coupled quantum dot is performed. A quantum entanglement generation method comprising: generating a quantum entanglement. Prior to the step of injecting the pump light, the first coupled quantum dot and the second coupled quantum dot are placed in the lower energy state of the exciton separation state in the first coupled quantum dot and the second coupled quantum dot. age,
In the step of injecting the pump light, the first coupled quantum dot and the second coupled quantum dot are excited in the lower energy state of the exciton separation state in the first coupled quantum dot and the second coupled quantum dot. 2. The quantum entanglement generation method according to claim 1, wherein pump light having an energy smaller than an energy difference between energy states of higher child localized states is incident. The step of injecting the pump light includes the step of exciting the first coupled quantum dot and the second coupled quantum dot with the lower energy state of the exciton separation state in the first coupled quantum dot and the second coupled quantum dot. A step of entering a first pump light having an energy smaller than an energy difference between the higher energy states of the child localized states, and the first coupled quantum dots on the first coupled quantum dots and the second coupled quantum dots And injecting the second pump light having an energy smaller than the energy difference between the higher energy state of the exciton separation state and the higher energy state of the exciton localized state in the second coupled quantum dot. ,
The step of generating the quantum entanglement includes detecting the Stokes photon due to Raman scattering when the first pump light is incident by the photon detector via the beam splitter, and generating the first quantum entanglement as the first quantum entanglement. A step of generating quantum entanglement, and the anti-Stokes photons due to Raman scattering when the second pump light is incident are detected by the photon detector via the beam splitter, and second quantum entanglement is detected. Generating quantum entanglement,
Prior to the step of entering the first pump light, the first coupled quantum dot and the second coupled quantum dot are moved to a lower exciton separation state in the first coupled quantum dot and the second coupled quantum dot. Energy state,
Performing a step of generating the first quantum entanglement after the step of entering the first pump light;
After the step of generating the first quantum entanglement, performing a step of entering the second pump light,
2. The quantum entanglement generation method according to claim 1, wherein a step of generating the second quantum entanglement is performed after the step of entering the second pump light.   In the step of generating the quantum entanglement, the Stokes photon or the anti-Stokes photon caused by Raman scattering when the pump light is incident on the first coupled quantum dot is passed, and the pump light is blocked. A filter, passing through the Stokes photon or the anti-Stokes photon due to Raman scattering when the pump light is incident on the second coupled quantum dot, and passing through the second filter and the beam splitter that block the pump light 4. The quantum entanglement is generated by detecting one of the first photon detector and the second photon detector as the photon detector. 5. The quantum entanglement generation method described in 1.   The first coupled quantum dot and the second coupled quantum dot are initialized so as to have an energy state having a lower exciton separation state, so that the electrons and holes constituting the exciton are spatially separated. An operation in which a voltage is applied between two electrodes provided on both sides of each of the first coupled quantum dot and the second coupled quantum dot, and the pump light is incident on the first coupled quantum dot and the second coupled quantum dot. 5. The quantum entanglement generation method according to claim 1, wherein a voltage applied between the two electrodes is turned off before time.   After the initialization of the first coupled quantum dot and the second coupled quantum dot to the lower energy state of the exciton separation state, the direction of one of the spins of electrons and holes constituting the exciton is changed. An external magnetic field and an alternating magnetic field are generated in each of the first coupled quantum dot and the second coupled quantum dot so as to be in an optically inactive state, and the pump is coupled to the first coupled quantum dot and the second coupled quantum dot. Before the operation of entering light, the first coupled quantum dot and the second coupled quantum dot are arranged such that the spin direction of one of electrons and holes constituting the exciton is changed to an optically active state. The quantum entanglement generation method according to any one of claims 1 to 5, wherein an external magnetic field and an alternating magnetic field are generated respectively. The first coupled quantum dot is provided on a path, and has a resonance frequency that matches at least one of the frequencies of the Stokes photon and the anti-Stokes photon due to Raman scattering when the pump light is incident on the first coupled quantum dot. The first resonator causes the Stokes photon and the anti-Stokes photon due to Raman scattering to resonate when the pump light is incident on the first coupled quantum dot,
The second coupled quantum dot is provided on a path, and has a resonance frequency that matches at least one of the frequencies of the Stokes photon and the anti-Stokes photon due to Raman scattering when the pump light is incident on the second coupled quantum dot. 7. The Stokes photon and the anti-Stokes photon caused by Raman scattering when the pump light is incident on the second coupled quantum dot are caused to resonate by the second resonator. The quantum entanglement generation method according to item.   In at least one of the first coupled quantum dot and the second coupled quantum dot, a voltage is applied to the front surface electrode provided on the front surface side with respect to the potential of the back surface electrode provided on the back surface side, and the first coupled quantum dot The quantum entanglement generation method according to claim 1, wherein a phase of superposition of quantum entanglements of quantum dots and the second coupled quantum dots is adjusted. A first coupled quantum dot in which two quantum dots are coupled;
A second coupled quantum dot in which two quantum dots are coupled;
A photon detector that detects Stokes photons or anti-Stokes photons by Raman scattering when pump light is incident on the first coupled quantum dots and the second coupled quantum dots via a beam splitter;
A quantum entanglement generation device comprising: a controller that generates quantum entanglement of the first coupled quantum dots and the second coupled quantum dots based on detection information from the photon detector.   A quantum communication device comprising the quantum entanglement generation device according to claim 9. A plurality of quantum communication devices including the quantum entanglement generation device according to claim 9,
The quantum communication system, wherein the plurality of quantum communication devices are connected to each other. The first coupled quantum dot provided in the transmission-side quantum communication device using a plurality of quantum communication devices including a first coupled quantum dot coupled with two quantum dots and a second coupled quantum dot coupled with two quantum dots. And one of the second coupled quantum dots and one of the first coupled quantum dots and one of the second coupled quantum dots provided in the quantum communication device on the receiving side perform quantum communication using quantum entanglement between transmission and reception A method,
In each of the quantum communication devices,
The first coupled quantum dot and the second coupled quantum dot are in the lower energy state of the exciton separation state in the first coupled quantum dot and the second coupled quantum dot,
In the first coupled quantum dot and the second coupled quantum dot, the lower energy state of the exciton separation state and the higher energy of the exciton localized state in the first coupled quantum dot and the second coupled quantum dot The first pump light having an energy smaller than the energy difference between the states is incident,
In-device quantum entanglement when Stokes photons due to Raman scattering when the first pump light is incident are detected by one of the two photon detectors as the first photon detector via the first beam splitter. That the
In the two adjacent quantum communication devices,
One of the first coupled quantum dot and the second coupled quantum dot provided in one of the two adjacent quantum communication devices, and the first coupled quantum dot provided in the other of the two adjacent quantum communication devices And an energy difference between the higher energy state of the exciton-separated state and the higher energy state of the exciton localized state in the first coupled quantum dot and the second coupled quantum dot. A second pump light having an energy smaller than
When an anti-Stokes photon due to Raman scattering when the second pump light is incident is detected by one of the two photon detectors as the second photon detector via the second beam splitter, it is between adjacent devices. Suppose quantum entanglement has been generated,
The intra-device quantum entanglement is generated in all the quantum communication devices, and the inter-adjacent device quantum entanglement is generated in all the adjacent two quantum communication devices. As another example, a quantum communication method comprising performing quantum communication using the inter-transmission / reception quantum entanglement.   In each of the quantum communication device on the transmission side and the quantum communication device on the reception side, the other of the first coupled quantum dot and the second coupled quantum dot is a coupled quantum dot serving as a standard in which the phase of the quantum state is specified. The phase measurement of the quantum state of the first coupled quantum dot and the second coupled quantum dot is measured, and the quantum communication is performed using the inter-transmission / reception quantum entanglement. The quantum communication method as described. The pump light is incident on a standard coupled quantum dot in which two quantum dots are coupled and a phase of the quantum state is specified, and a measurement target coupled quantum dot in which two quantum dots are coupled,
A Stokes photon or anti-Stokes photon due to Raman scattering when the pump light is incident is detected by a photon detector via a beam splitter, and the phase of the quantum state of the measurement target coupled quantum dot is projected and measured. Projection measurement method.