JP4867621B2 - Quantum entangled photon pair generator - Google Patents

Description

  The present invention relates to a entangled photon pair generator, and more particularly to a entangled photon pair generator useful for a quantum cryptography communication system or the like in which a single quantum dot is embedded in a photonic crystal microresonator.

2. Description of the Related Art In recent years, the progress of information communication technology using the Internet or the like has been remarkable, and accordingly, research and development has been actively conducted on encryption technology for information transmission. As the encryption technology, quantum cryptography has recently attracted attention.
Quantum cryptography is an absolutely secure cryptographic communication technique in which the security of an encryption key for decrypting encrypted information is guaranteed by the principle of quantum mechanics.
An important element in quantum cryptography is quantum teleportation. Quantum teleportation is a technology that instantaneously moves only photon quantum information to another location. This quantum teleportation is realized by exchanging information between photons using entanglement of quantum states (quantum entanglement). This photon pair in the entangled state is also called a entangled photon pair or an entangled photon pair.
A entangled photon pair has the property that once the quantum state of one photon is determined, the quantum state of the other photon is also determined. In quantum cryptography, a quantum key distribution protocol or a configuration of a quantum repeater that enables long-distance transmission has been devised based on this physical property. Therefore, the means for generating entangled photon pairs is very important in quantum cryptography communication.
As a method for generating a entangled photon pair in a polarization state, parametric down conversion in a crystal having second-order optical nonlinearity is well known (Non-Patent Document 1). Hereinafter, quantum entanglement in the polarization state is simply expressed as quantum entanglement.
In addition, two photons are irradiated onto a semiconductor material such as cuprous chloride to generate exciton molecules with zero angular momentum by two-photon resonance excitation, and the generated exciton molecules are converted into two photons. A technique (hyperparametric scattering method) is also disclosed in which a pair of quantum entangled photons is generated (Patent Document 1).
Furthermore, in a single semiconductor quantum dot, a method of generating a entangled photon pair by a stepwise emission process (cascade process) of exciton molecules and excitons has been proposed (Non-patent Documents 2-4).
PGKwiat, E. Waks, AGWhite, I. Appelbaum, and PHEberhard, “Physical Review A” vol. 60, no. 2, R773-R776 Japanese Patent No. 3649408 O. Benson, C. Santori, M. Pelton and Y. Yamamoto, “Phys. Rev. Lett.” Vol. 84, no. 11, pp.2513-2516 C. Santori, D. Fattal, M. Pelton, G. Solomon and Y. Yamamoto, “Phys. Rev. Lett.” Vol. 66, p045308-1-4 RM Stevenson, RJ Young, P. Atkinson, K. Cooper, DA Ritchie and AJ Shields, “Nature” vol.439, no. 12, pp.179-182

However, when generating an entangled photon pair from one or two parent photons in the parametric down-conversion or hyperparametric scattering method, the generation efficiency is very low because a nonlinear optical process is used. Accordingly, when the intensity of the excitation light pulse including the parent photon is constant, the number of photon pairs generated per pulse follows a so-called Poisson distribution.
In order to guarantee absolute security in the quantum cryptography, it is required that only one pair of quantum entangled photons is generated per pulse. That is, the generation probability of two or more pairs of entangled photons per pulse must be reduced as much as possible. However, since the number of photon pairs generated by the parametric down-conversion or hyperparametric scattering method follows the Poisson distribution, the probability that two or more pairs of entangled photons are simultaneously generated cannot be zero in principle.
As a practical method for avoiding this problem, an idea of adjusting the average photon pair per pulse to about 0.1 has been considered. However, in this case, 9 or more of the 10 pulses are in the “photon number 0” state, which causes the transmission speed of the encryption key to be reduced to 1/10 or less.
On the other hand, when using the exciton molecule of a single semiconductor quantum dot, and the stepwise emission process of excitons, the exciton molecule emission and excitation are due to Pauli's exclusion rule that different electrons cannot occupy the same quantum state. Since photons are always generated one by one from the photon emission, only one pair of quantum entangled photons is generated in principle (Non-patent Document 2).

Here, the stepwise emission process that relaxes from the exciton molecule to the ground state via the exciton that is an intermediate state will be described in detail.
FIG. 9A shows an energy diagram for electrons and holes in a single semiconductor quantum dot. For example, assuming an InAs quantum dot embedded in GaAs, GaAs is a barrier region and InAs is a dot region. A quantum dot is usually about several tens of nanometers in size, and its energy level is discretized by its quantum mechanical confinement effect. In FIG. 9A, black circles indicate electrons, white circles indicate holes, and arrows indicate the directions of spins.
Electron and hole pairs with opposite spins form a stable “exciton” state by Coulomb interaction. FIG. 9A shows a state in which there are two such excitons, and an “exciton molecule” state is formed by Coulomb interaction between excitons.

In FIG. 9B, an exciton molecule in an ideal single quantum dot that generates a entangled photon pair relaxes while emitting two photons to the ground state through the exciton state as an intermediate state. It shows the process of doing.
There are two pathways for the relaxation process. In the left-hand path, clockwise circularly polarized light (σ +) is emitted in the relaxation process from the exciton molecule to the exciton state, and then left-handed circularly polarized light (σ−) in the relaxation process from the exciton state to the ground state. ) Photons. On the other hand, in the right path, σ− photons are emitted in the relaxation process from the exciton molecule to the exciton state, and then σ + photons are emitted in the relaxation process from the exciton state to the ground state.
If it is impossible to distinguish these two paths quantum mechanically, there is a quantum entangled state with respect to the polarization state, and the function | ψ> representing this quantum mechanical state is the following (1) It is expressed as an expression.
| Ψ> = (| R> ₁ | L> ₂ + | L> ₁ | R> ₂ ) / √2 (1)
Here, R (L) represents right (left) circularly polarized light, and the subscript 1 (2) represents photons of exciton molecule (exciton) emission. In this state, only one of 1 and 2 is R and the other is L before the observation, but for example, at the moment when it is observed that photon 1 is in the R (L) state, 2 is determined to be in the state of L (R). This is quantum teleportation.
The relation between circularly polarized light and horizontal (H) and vertical (V) linearly polarized light is | R> = (| H> + i | V>) / √2 (2)
| L> = (| H> −i | V>) / √2 (3)
(Where i is an imaginary unit), the formula (1) is obtained by substituting the formulas (2) and (3) into the formula (1) as shown in the following formula (4): It is also possible to write in
| Ψ> = (| H> ₁ | H> ₂ + | V> ₁ | V> ₂ ) / √2 (4)
Therefore, the quantum mechanical correlation observed with circularly polarized light is also observed as linearly polarized light. Note that the horizontal axis is the same for any direction.

However, most of the actual quantum dots have an asymmetric structure, and an energy difference occurs between two exciton states that are intermediate states due to the exchange energy action of electrons and holes. The relaxation process of the exciton molecule in this case is shown in FIG.
At this time, two photons of horizontal polarization (H) are generated from the left path and two photons of vertical polarization (V) are generated from the right path, but the intermediate exciton state can be identified. A photon pair with no very entanglement or a very low fidelity that represents the degree of entanglement occurs (Non-patent Document 3).
In addition, by strictly controlling the quantum dot fabrication conditions and reducing the asymmetric structure as much as possible, even when quantum entangled photon pairs are generated, they are generated only from quantum dots of a certain size (ie, In addition, a strong magnetic field of 2 Tesla is required in some cases (Non-patent Document 4).
Therefore, the conventional method of generating entangled photon pairs using a cascade process is not directly applied to encryption technology in a general optical communication wavelength band of 1.3 to 1.5 μm. In addition, the fidelity of quantum entanglement measured in Non-Patent Document 4 is not as high as the above-described parametric down-conversion.
The present invention has been made in view of the above problems, and its purpose is to provide a high-fidelity quantum entangled photon pair generator using a single quantum dot embedded in a two-dimensional photonic crystal. It is to provide.

In order to achieve the above object, according to the present invention, an opening is provided at a lattice point excluding the lattice point at the center of the microresonator of the base material, and the inside of the opening is a medium having a refractive index different from that of the base material. It has a structure in which a single quantum dot is embedded in a microcavity made of a filled two-dimensional photonic crystal,
Means for generating a lowest-order exciton molecular state in the single quantum dot,
In the process of relaxing the exciton molecular state, a pair of a first photon corresponding to exciton light emission and a second photon corresponding to exciton light emission is generated, and the first photon and the first photon A entangled photon pair generator in which the state of a pair with two photons is entangled quantum mechanically,
Due to the Purcell effect caused by the coupling of the first resonance mode of the microresonator and the second photon, the exciton emission lifetime τ is shorter than the lifetime without the microresonator. Thus , there is provided a quantum entangled photon pair generator characterized in that the opening of the lattice point close to the center of the microresonator is changed to the original state.

Preferably, means for enhancing light emission to one side of the two-dimensional photonic crystal, such as disposing a distributed Bragg reflector parallel to the two-dimensional photonic crystal, is provided. Thereby, the extraction efficiency of the entangled photon pair can be improved.
Preferably, the microresonator of the two-dimensional photonic crystal is any one of the size, shape, or position of an opening to be disposed corresponding to the closest lattice point from the center of the microresonator, or its The combination is changed from that of the original grid point. By designing the shape of the microresonator, the fidelity of quantum entanglement can be improved. At this time, it is desirable that the size, shape, or position of the nearest opening is adjusted so that the wavelength of the resonator mode is independent of polarization in a two-dimensional plane.

Further, in the microresonator, the Q value of the microresonator mode is adjusted depending on the size, shape, or position of the nearest opening.
More specifically, the medium filling the openings is air, the shapes of the openings arranged periodically are circular holes, and the diameter of the circular holes closest to the resonator center is changed from a predetermined size. Or / and the center position is changed from the position of the predetermined grid point.
Regarding the means for generating the exciton molecular state, the Nth (N is an integer of 2 or more) excited state of the single quantum dot and the second resonator mode of the microresonator resonate, It is desirable to irradiate light having a wavelength that resonates both in the Nth excited state of the quantum dot and in the second resonator mode of the microresonator.
In order to obtain a entangled photon pair with high fidelity, the exciton emission lifetime τ is preferably in a relational expression of 1 / τ> Δ with respect to the split energy width Δ of the exciton state.

  The entangled photon pair generator according to the present invention has a single quantum dot structure embedded in a photonic crystal microresonator, and the lifetime of exciton and exciton molecular emission is shortened by the Purcell effect. Thus, the two exciton states, which are intermediate states, become indistinguishable, and the influence of phase relaxation of the exciton spin can be minimized by approaching the Fourier limit pulse. A highly entangled photon pair generator can be provided.

The present invention provides two stepwise light emission processes in which a single semiconductor quantum dot embedded in a photonic crystal microresonator relaxes from an exciton molecule to a ground state via an exciton that is an intermediate state. It is a entangled photon pair generator that uses photons as entangled photon pairs.
FIG. 1 shows a first embodiment of a entangled photon pair generator according to the present invention. The photonic crystal 11 has a structure in which circular holes 17 are arranged in a triangular lattice pattern in a semiconductor substrate 16 having a film thickness d, and a portion where only one circular hole is missing becomes a microresonator 15. A single quantum dot 14 made of InAs is embedded in this resonator portion. The closest hole (closest to the center of the microresonator 15) from the chipped hole is smaller in diameter than that of the other circular holes, and the center is from the center of the microresonator 15. It has been moved away. It is desirable that the single quantum dot is at or near the portion where the electric field strength of the resonator mode is strongest. The entangled photon pair generated from the quantum dot 14 is collected by the lens 18. The material of the semiconductor substrate 16 is not particularly limited, but is preferably a compound semiconductor, and GaAs is used in the present embodiment. The photonic crystal 11 is usually placed in the air, but may be in another medium including a vacuum.

  Next, a method for manufacturing the photonic crystal structure of the present embodiment will be briefly described with reference to FIG. AlGaAs as the sacrificial layer 12 is epitaxially grown on the semiconductor substrate 13 made of GaAs [FIG. 2A]. Next, GaAs serving as the semiconductor substrate 16 is grown to a thickness of d / 2 [FIG. 2 (b)]. Then, quantum dots 14 made of InAs are arranged on the semiconductor substrate 16 having a half film thickness [FIG. 2 (c)]. Next, GaAs is further grown to form a semiconductor substrate 16 having a film thickness d on the sacrificial layer 12 (FIG. 2D). Subsequently, by combining the lithography technique and the dry etching process, the circular holes 17 arranged periodically are formed to form the photonic crystal 11 having the microresonators 15 [FIG. 2 (e)]. Next, the sacrificial layer 12 under the photonic crystal is wet-etched through the circular hole 17 to form a cavity under the photonic crystal [FIG. 2 (f)]. The wet etching is performed so that the size of the cavity in the plane is larger by about 5 to 10 μm than the region where the circular holes 17 constituting the photonic crystal 11 are formed. The remaining sacrificial layer 12 functions as a base for supporting the photonic crystal 11 which is a thin film.

FIG. 3 is a diagram for explaining the mechanism of the entangled photon pair generator according to the present invention. The discussion proceeds with an example of an InAs quantum dot embedded in GaAs having an exciton emission wavelength of approximately 1.3 μm.
As an initial state, it is necessary to generate an exciton molecular state, which is the wavelength of the absorption region of the medium surrounding the quantum dot, or an ultra-thin layer of dot material that can be created when creating a quantum dot called a wetting layer. The quantum dots can be generated by irradiating the quantum dots with an appropriate intensity with an excitation light having a resonance wavelength of 1 or a wavelength that resonates with a higher-order exciton level. Alternatively, by introducing a p-i-n structure, it can be generated by injecting electrons and holes into the quantum dot region by applying an external electric field of an appropriate magnitude.

The exciton molecular state typically has a lifetime of about 1 ns, and after releasing one photon to relax to the exciton state, it emits another photon with a lifetime of about 1 ns to the ground state. And relax. As described above, in almost all quantum dots, due to the shape asymmetry, as shown in FIG. 9C, the two exciton states that are intermediate states are horizontal (H) or vertical (V ) Is an eigenstate, and an energy difference Δ is generated.
Now, when the lifetime in the absence of the photonic crystal microresonator is τ ₀ = 1.0 ns, the full width at half maximum δ of the spread (natural width) on the spectrum is about 0.7 μeV due to the uncertainty principle. That is led.
It has been confirmed by various experiments that the value of Δ is about several to several tens of μeV. Therefore, Δ >> δ in the absence of the microresonator, and the two exciton states that are intermediate states can be sufficiently distinguished.
According to the uncertain principle, if the emission lifetime of the exciton is shortened, the spectral spread δ increases in inverse proportion to it. Therefore, by using this principle, if the aforementioned energy difference becomes smaller than the natural width, that is, if a situation of Δ <δ is created, it should be impossible to identify. The photonic crystal microresonator plays this role.
When light of an isolated level such as an exciton of an electromagnetic field is placed in a photonic crystal microresonator, its emission lifetime τ is compared with τ _{0 in the} case of no resonator as It is shortened by the expressed parcel factor F times.

この（５）式において、λｘは励起子発光波長、ｎは媒質の屈折率、Ｑは共振器モードのＱ値（Quality Factor）、Ｖは共振器のモード体積、Ｅは量子ドットの位置での電場強度、μは励起子の双極子モーメント、λｃは共振器の共鳴波長、Δλｃは共振器幅で、λｃ／Ｑに等しい。
一方、自然幅は
δ＝１／τ＝Ｆ／τ_０ （６）
と表されるため、大きなＦを得ることが出来ると、それに比例して自然幅を大きくすることが可能となる。例えばＦ＝５０が得られたとすれば、自然幅δ＝０．７×５０＝３５μｅＶとなり、典型的な値Δ＝１０μｅＶの分裂幅が十分に隠れ、２つの経路の区別がつかなくなる状況（δ＞Δ）が作り出されて、忠実度の高い量子もつれ光子対の発生が可能となる。
励起子発光寿命τが小さくなることは、別の観点からも望ましい。有限温度の固体中ではフォノンの影響によって、励起子状態のスピン緩和が起こり、量子もつれの度合いを低減する別の要因となる。τが小さくなることで、スピン緩和の影響を低減できるからである。
次に、発生する量子もつれ光子対の忠実度について定量的に議論する。スピン緩和を無視できる状況での理想的な忠実度は自然幅δと励起子分裂幅Δを用いて、次の（７）式で表される。

In this equation (5), λx is the exciton emission wavelength, n is the refractive index of the medium, Q is the Q value (Quality Factor) of the resonator mode, V is the mode volume of the resonator, and E is the position of the quantum dot. The electric field strength, μ is the exciton dipole moment, λc is the resonance wavelength of the resonator, Δλc is the resonator width, and is equal to λc / Q.
On the other hand, the natural width is δ = 1 / τ = F / τ ₀ (6)
Therefore, if a large F can be obtained, the natural width can be increased in proportion thereto. For example, if F = 50 is obtained, the natural width δ = 0.7 × 50 = 35 μeV, and the split width of a typical value Δ = 10 μeV is sufficiently hidden, so that the two paths cannot be distinguished (δ > Δ) is created, allowing the generation of highly entangled quantum entangled photon pairs.
It is desirable from another viewpoint that the exciton emission lifetime τ is reduced. In a solid at a finite temperature, the spin relaxation of the exciton state occurs due to the effect of phonons, which is another factor for reducing the degree of quantum entanglement. This is because the effect of spin relaxation can be reduced by reducing τ.
Next, we discuss quantitatively the fidelity of the generated entangled photon pairs. The ideal fidelity in a situation where spin relaxation can be ignored is expressed by the following equation (7) using the natural width δ and the exciton splitting width Δ.

（５）式〜（７）式により、理想的な忠実度は共振器のＱ値によって制御可能であることが分かる。ここで、フォトニック結晶や量子ドットでの典型的な数値をあてはめることで、Ｑ値の変化に対する忠実度の変化をシミュレートする。

From equations (5) to (7), it can be seen that the ideal fidelity can be controlled by the Q value of the resonator. Here, a typical numerical value in a photonic crystal or a quantum dot is applied to simulate a change in fidelity with respect to a change in Q value.

The exciton emission wavelength λx is 1300 nm and substantially coincides with the resonator wavelength λc, and the exciton molecular emission wavelength λxx is 1.6 nm longer than λx, and the inner product E · μ of the electric field and the dipole moment 4 represents the relationship between the Q value and the fidelity when the absolute value of the product is ½ of the maximum value and the mode volume is V = 0.4 (λx / n) ³ . + Shown in black circles. The greater the Q value, the closer the fidelity is to the value 1 of the fully entangled state.
However, increasing the Q value is not only good. As the Q value increases, the exciton molecule emission wavelength λxx deviates from the resonance spectrum width Δλc, and the emission efficiency to the outside of the resonator decreases, that is, the generation efficiency of the entangled photon pair decreases. The simulation result is shown by the dotted line in FIG. The generation efficiency of 100% assumes a state where both excitons and exciton molecules resonate with the resonator.
FIG. 4 shows that the fidelity of the entangled photon pair and its generation efficiency are in a trade-off relationship. Therefore, in reality, there is an optimum Q value for making both the fidelity of the entanglement pair and the generation efficiency be a certain value or more. In this calculation example, when Q is set to about 1200, values of fidelity 0.95 and generation efficiency 11% (for an ideal situation) can be obtained.
By the way, the above-mentioned optimum Q value is a value that assumes τ ₀ = 1.0 ns and Δ = 10 μeV, and is a value that varies depending on the characteristics of the quantum dots. Therefore, it is desirable that the resonator Q value can be designed freely according to the characteristics of the quantum dots.

  Next, a method for designing the Q value of the photonic crystal resonator will be described using a specific example. In the photonic crystal, air holes with a radius r = 0.10 μm are arranged in a two-dimensional triangular lattice pattern on a GaAs thin film (refractive index 3.4) having a thickness of 0.24 μm. The center distance a of each circular hole is 0.36 μm. A plan view of the photonic crystal is shown in FIG. Such a photonic crystal has a structure in which one central circular hole is filled with GaAs, that is, a point defect, and this portion becomes the center of the resonator. A resonator having such a shape is called an H1-type resonator.

In the H1 type resonator shown in FIG. 5 (a), six circular holes closest to the resonator are introduced from the periodicity shown by a broken line as shown in FIG. 5 (b) in order to change the Q value. They are shifted from the original position by s, and their radius r ′ is smaller by s than the other circular hole radii r. In FIG. 5B, only the right circular hole among the six is shown in an enlarged manner. It should be particularly noted here that the H1 resonator must maintain 6-fold symmetry even when the nearest circular hole is modulated. That is equivalent to the fact that the resonator mode should not have a polarization-dependent resonance wavelength, ie double degenerate. If this condition is not met, it will be very difficult to realize the entangled state of the polarization state.
FIG. 5C shows the result of simulating the Q value change when the radius of the circular hole is reduced from r → r ′ and the center displacement s to the outside is changed under the condition of s = r−r ′. Show. A maximum Q value of 33,000 is obtained at the center displacement s = 0.09a = 32 nm.

Although there is a manufacturing error when manufacturing the photonic crystal, it is difficult to obtain an ideal calculated Q value, but Q = 17,000 is realized in the microresonator structure manufactured with the design values of the above parameters. The measured wavelength spectrum of the resonator mode is shown in FIG.
In FIG. 6, black circles indicate the measurement results of the spectrum in which the quantum dots emit light through the resonator. The result of fitting this spectrum using the Lorentz function is also shown by a solid line. The arrow in the figure represents the width that is half of the maximum value in this Lorentz function, that is, the full width at half maximum (FWHM), and the value was estimated to be 0.077 nm. The Q value can be obtained by dividing the resonator wavelength, here 1327.6 nm, by FWHM, and the result is Q = 17,000. On the other hand, a measurement result of Q = 300 was obtained by performing the same experiment when the center displacement s = 0. Therefore, it has been found that the Q value can actually be controlled within a two-digit range of 300 to 17,000.
From the above, it has been shown that the resonator structure can be designed by selecting the optimum Q value according to the characteristics of the quantum dots.
Although a triangular lattice type photonic crystal is mentioned here as a specific example, the same effect can be obtained with other crystal structures such as a square lattice. Further, the shape of the opening is not limited to the circular hole, and the same effect can be obtained even if the opening is an elliptical hole, a semicircular hole, or a polygonal hole. In the above embodiment, the example in which the farthest point of the nearest circular hole is fixed is shown. However, it is not always necessary to do so. If the 6-fold symmetry is maintained (triangular lattice) Any change is possible. That is, it is possible to fix the center position by changing the radius, move the center position without changing the radius, or add another circular hole.

  FIG. 7 is a sectional view showing a second embodiment of the entangled photon pair generator according to the present invention. In FIG. 7, parts that are the same as the parts in FIG. 1 showing the first embodiment are given the same reference symbols, and redundant descriptions are omitted. In this example, a distributed Bragg reflector (DBR) 22 supported by the semiconductor substrate 21 is arranged via a spacer 23 with respect to the first embodiment. The DBR has an effect of increasing the probability of being guided to the condensing lens 18 by reflecting a component radiated to the lower side of the drawing among the entangled photon pairs generated from the quantum dots. The DBR has a structure in which, for example, AlGaAs and GaAs are alternately stacked by 1/4 wavelength.

In the explanation so far, only an ideal state where only one quantum dot whose position is controlled exists inside the resonator has been considered. However, it is very difficult to accurately control the position with the current production technique, and in most cases, self-forming quantum dots are used. The self-formed quantum dots are formed at random positions, and there is rarely a situation where there is only one quantum dot in the resonator, and it is highly possible that several quantum dots are formed. Therefore, it is necessary to selectively excite only the quantum dot at the center of the resonator. Note that light emission from other quantum dots acts as excess noise during device operation and must be reduced as much as possible.
In the third embodiment of the entangled photon pair generator according to the present invention, the wavelength (energy) of the higher-order excited state of the quantum dot exciton and the wavelength of the higher-order mode of the resonator resonate, And a means for irradiating excitation light resonating therewith. FIG. 8 illustrates the operating principle.

Since quantum dots have a deep confinement potential, there can be multiple discretized exciton energy levels. Usually, exciton or exciton molecular light emission refers to light emission from the lowest order state among the plurality of energy levels. The second lowest energy (or long wavelength) state is called the second excited state, and is called the third and fourth excited states in this order. These wavelength intervals are typically about 50 nm.
For example, in the case of the H1 resonator shown in FIG. 5, the resonator has resonator modes at intervals of 30 to 100 nm. Accordingly, by appropriate resonator design, the situation of FIG. 8 is created in which the excitons in the ground state and the first resonator mode resonate and the excitons in the second excited state and the second resonator mode resonate. Is possible.
At this time, when the excitation light that resonates both the exciton in the second excitation state and the second resonator mode is irradiated, the second excitation state is generated in the quantum dot. Thereafter, the energy is relaxed to a low energy state in about 10 ps, and the lowest-order exciton molecular state is generated. The subsequent photon emission process is as described above.
Such an excitation method that resonates in a higher-order excited state can selectively excite only specific quantum dots, and is very effective when there are a plurality of quantum dots in a resonator, for example. This is because the conventional excitation method that is not resonance excitation simultaneously brings a plurality of quantum dots into an excited state, and emits photons from other than the quantum dots for which a entangled photon pair is to be generated, leading to an increase in noise.
It should be noted that the excitation light itself can be one of the causes of noise increase in a entangled photon pair light source that handles one photon level, and therefore it is desirable that the excitation light has a low power. In the present invention, the higher-order resonator mode resonating with the higher-order excited state of the exciton can generate the exciton molecular state with lower excitation light power due to the electric field enhancement effect.
Noise is reduced by the above action, and as a result, it contributes to improvement of the entanglement fidelity.

As a specific example, the case where the second excited state of the quantum dot excitons and the higher-order resonator mode are resonating has been described. However, the quantum dot excitons are in the third or higher-order excited state. It doesn't matter.
The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope shown in the claims. That is, embodiments obtained by combining technical means appropriately modified within the scope of the claims are also included in the technical scope of the present invention.

  According to the present invention, a photon pair having high-fidelity quantum entanglement with respect to polarization can be generated, so that its security is proved by the principle of quantum mechanics and applied to quantum key distribution and quantum relay systems. Can contribute to the development of absolutely safe communication technology.

Sectional drawing and top view which show 1st Embodiment of the quantum entangled photon pair generator by this invention. Sectional drawing of the process order which shows the manufacturing process of 1st Embodiment of the quantum entangled photon pair generator by this invention. FIG. 3 is a diagram illustrating the principle of entangled photon pair generation according to the present invention. The graph showing the Q value dependence of the entanglement fidelity of the entangled photon pair generated from the quantum dot embedded in the microresonator and the light collection efficiency. (A) Planar structure figure of H1 type | mold resonator, (b) The partial enlarged view, (c) The graph showing the change of Q value. The graph which shows the characteristic of the produced H1 type | mold resonator. Sectional drawing which shows 2nd Embodiment of the quantum entangled photon pair generator by this invention. The figure showing the relationship of the exciton energy (wavelength), resonator wavelength, and excitation light wavelength in 3rd Embodiment of the quantum entangled photon pair generator by this invention. (A) Energy diagram in a quantum dot, (b) Generation process of an entangled photon pair from an ideal quantum dot, (c) Generation process of a two-photon from an actual quantum dot.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Photonic crystal 12 Sacrificial layer 13, 21 Semiconductor substrate 14 Quantum dot 15 Microresonator 16 Semiconductor base material 17 Circular hole 18 Condensing lens 22 Distributed Bragg reflector 23 Spacer

Claims (9)

A microresonator comprising a two-dimensional photonic crystal in which an opening is provided at a lattice point excluding the lattice point at the center of the microresonator of the base material, and the inside of the opening is filled with a medium having a refractive index different from that of the base material It has a structure in which a single quantum dot is embedded,
Means for generating a lowest-order exciton molecular state in the single quantum dot,
In the process of relaxing the exciton molecular state, a pair of a first photon corresponding to exciton light emission and a second photon corresponding to exciton light emission is generated, and the first photon and the first photon A entangled photon pair generator in which the state of a pair with two photons is entangled quantum mechanically,
Due to the Purcell effect caused by the coupling of the first resonance mode of the microresonator and the second photon, the exciton emission lifetime τ is shorter than the lifetime without the microresonator. Thus , the quantum entangled photon pair generator is characterized in that the opening of the lattice point close to the center of the microresonator is changed to the original state. 2. The entangled photon pair generator according to claim 1, further comprising optical means for enhancing the emission of the first and second photons to one side of the two-dimensional photonic crystal. . The entangled photon pair generator according to claim 2 , wherein the optical means is a distributed Bragg reflector arranged in parallel to the two-dimensional photonic crystal. In the microresonator of the two-dimensional photonic crystal, any size, shape or position of an opening to be arranged corresponding to the closest lattice point from the center of the microresonator or a combination thereof is an original grating. characterized in that it is changed from that of the point, entangled photon pair generator according to any one of claims 1 to 3. The microresonator of the two-dimensional photonic crystal has a size, shape or position of an opening to be arranged corresponding to the closest lattice point from the center of the microresonator, or a combination thereof. wherein the wavelength of the mode is adjusted to be polarization independent in a two-dimensional plane, entangled photon pair generator according to any one of claims 1 to 4. In the microresonator, the Q value of the microresonator mode is either the size, shape, or distance from the center of the opening to be arranged corresponding to the closest lattice point from the center of the microresonator or characterized in that it is adjusted depending on the combination, entangled photon pair generator according to any one of claims 1 to 5. The entangled photon pair generator according to any one of claims 1 to 6 , wherein the medium having a refractive index different from that of the substrate is air. The Nth (N is an integer of 2 or more) excited state of the single quantum dot and the second resonator mode of the microresonator resonate, and the lowest-order exciton molecular state in the single quantum dot The means for generating a light is a means for irradiating light that resonates in both the Nth excited state of the single quantum dot and the second resonator mode of the microresonator. 8. The entangled photon pair generator according to any one of 7 above. The exciton emission lifetime tau is characterized in that a relation expression of 1 / tau> delta against splitting energy width delta exciton states, entangled photon pairs according to any of claims 1 to 8 Generator.

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