CN110945422B - Quantum computing device and implementation method of photon quantum logic gate - Google Patents
Quantum computing device and implementation method of photon quantum logic gate Download PDFInfo
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Abstract
A method of implementing a quantum computing device (10) and a photonic quantum logic gate, wherein the method comprises: a quantum computing device (10) receives input laser light to excite four-level quantum dots (12) to a bi-exciton state (402); in the case that the four-energy-level quantum dot (12) transits from a double exciton state to a single exciton state, the quantum computing device (10) receives a first single-photon pulse, a second single-photon pulse and a third single-photon pulse (404) which are input in sequence; input of the quantum computing device (10) with laser and single photon pulses can be realized (SWAP)1/2And a door.
Description
Technical Field
The invention relates to the field of calculation, in particular to a quantum calculation device and a realization method of a photon quantum logic gate.
Background
Quantum computing is a novel interdiscipline generated by combining quantum physics with computer science and information science. A Qubit (Qubit) can be in the quantum state |0>And quantum state |1>Is in a linear superposition state | ψ consisting of arbitrary complex coefficients>＝α|0>+β|1>The above. Quantum computing operates a Qubit through a logic gate, called a quantum logic gate or simply a quantum gate. Wherein, a quantum gate obtained by logic calculation of only one Qubit is called a single Qubit quantum gate, such as an AND gate, a NOT gate and the like; quantum gates obtained by finite combinations of multiple qubits through a single Qubit quantum gate are called multi-Qubit quantum gates (also called "universal quantum gates")"), for example, Control Non (CNOT) gates and swap squares (square root of swap,
) The gates all belong to a multi-Qubit quantum gate.
Photon calculation and quantum information processing use single photon as quantum bit carrier. However, since the interaction between single photons is usually very weak, it is difficult to directly implement the multi-Qubit quantum gate through the optical nonlinearity of a certain material, and therefore how to construct the multi-Qubit quantum gate for photons is a problem that is being researched by those skilled in the art.
Disclosure of Invention
The embodiment of the invention discloses a quantum computing device and an implementation method of a photon quantum logic gate, and provides a quantum computing device with four-energy-level quantum dots and optical resonance microcavity coupling, wherein laser and single photon pulses are input into the quantum computing device to realize the quantum computing device
And a door.
In a first aspect, an embodiment of the present invention discloses a quantum computing device, including four-energy-level quantum dots and an optical micro-resonant cavity, wherein: the four-energy-level quantum dot comprises a double exciton state, a first single exciton state, a second single exciton state and a vacuum state, and the energy of the first single exciton state and the energy of the second single exciton state are both equal to omegaXThe energy of the dual exciton state is equal to 2 omegaX- χ, where χ is the energy of the binding of the bisexciton and the energy of the vacuum state is equal to 0; the optical micro-resonant cavity comprises a first mode and a second mode, the first mode and the second mode are mutually orthogonal, and the resonant frequencies of the first mode and the second mode are both equal to omegaX- χ; the transition from the double exciton state to the first single exciton state and the transition from the first single exciton state to the vacuum state are coupled with the first mode and have a coupling strength, the transition from the double exciton state to the second single exciton state and the transition from the second single exciton state to the vacuum state are coupled with the second mode and have b coupling strength, a and b are both larger than 0 and a is equal to b. Four energy levels of singlet exciton states degenerated by two ground statesThe quantum dots are respectively coupled with two optical resonance micro-cavities with orthogonal resonance modes and equal resonance frequency to form a quantum computing device, and the quantum computing device can be input by using laser and single photon pulses
And a door.
In a second aspect, an embodiment of the present invention discloses a method for implementing a photonic quantum logic gate, including: the quantum computing device receives input laser light to excite the four-energy-level quantum dots to a double exciton state; under the condition that the four-energy-level quantum dot transits from a double-exciton state to a single-exciton state, the quantum computing device receives a first single-photon pulse, a second single-photon pulse and a third single-photon pulse which are input in sequence. And the first single-photon pulse, the second single-photon pulse and the third single-photon pulse are quantum bits encoded in a first mode and a second mode. The input first single-photon pulse and the second single-photon pulse are required to be executed
Operating qubits carrying specific information, the third single-photon pulse being input without carrying
Qubits of information of operation. Because the frequency of the first single-photon pulse and the third single-photon pulse is equal to the frequency required by the transition from the double-exciton state to the single-exciton state, and the frequency of the second single-photon pulse and the frequency of the third single-photon pulse are different by a frequency threshold value gamma, the quantum computing device can realize the quantum computing device after the three single-photon pulses output the quantum computing device
A gate for transferring the quantum state of the first single-photon pulse to a third single-photon pulse, wherein the output second single-photon pulse and the output third single-photon pulse are respectively subjected to a second single-photon pulse and a first single-photon pulse
The result after the door operation.
Optionally, the quantum computing device sequentially receives the input first single-photon pulse, the input second single-photon pulse, and the input third single-photon pulse, and includes: the quantum computing device sequentially receives a first single-photon pulse, a second single-photon pulse and a third single-photon pulse which are input at a preset time point. Since the difference between the preset time point and the start time of laser input and the difference of 5 times of the energy level lifetime of the dual exciton state is less than the preset threshold, the four-energy-level quantum dot can be considered to be transited from the dual exciton state to the single exciton state at the preset time point.
In a third aspect, an embodiment of the present invention provides an apparatus for implementing a photonic quantum logic gate, including: the first input unit is used for receiving input laser so as to excite the four-energy-level quantum dots to a double exciton state; and the second input unit is used for sequentially receiving the input first single-photon pulse, second single-photon pulse and third single-photon pulse under the condition that the four-energy-level quantum dot is transited from a double-exciton state to a single-exciton state. And the first single-photon pulse, the second single-photon pulse and the third single-photon pulse are quantum bits encoded in a first mode and a second mode. The input first single-photon pulse and the second single-photon pulse are required to be executed
Operating qubits carrying specific information, the third single-photon pulse being input without carrying
Information of the operation. Because the frequency of the first single-photon pulse and the third single-photon pulse is equal to the frequency required by the transition from the double-exciton state to the single-exciton state, and the frequency of the second single-photon pulse and the frequency of the third single-photon pulse are different by a frequency threshold value gamma, the quantum computing device can realize the quantum computing device after the three single-photon pulses output the quantum computing device
A gate for converting the quantum state of the first single photon pulseThe second single-photon pulse and the third single-photon pulse which are output are respectively subjected to the second single-photon pulse and the first single-photon pulse
The result after the door operation.
Optionally, the second input unit is specifically configured to sequentially receive the input first single-photon pulse, the input second single-photon pulse, and the input third single-photon pulse at a preset time point. Since the difference between the preset time point and the start time of laser input and the difference of 5 times of the energy level lifetime of the dual exciton state is less than the preset threshold, the four-energy-level quantum dot can be considered to be transited from the dual exciton state to the single exciton state at the preset time point.
In a fourth aspect, an embodiment of the present invention provides a storage medium for storing instructions, which when executed on a processor, enable the method described in the second aspect to be implemented.
In a fifth aspect, embodiments of the present invention provide a computer program product comprising instructions which, when run on a computer, cause the computer to perform the method described in the second aspect above.
In combination with the first aspect, or the second aspect, or the third aspect, or the fourth aspect, or the fifth aspect, in an alternative embodiment, the four-energy-level quantum dot includes a bi-exciton state, a mono-exciton state, and a vacuum state, the mono-exciton state includes a first mono-exciton state and a second mono-exciton state, and the first mono-exciton state and the second mono-exciton state both have an energy equal to ωXThe energy of the dual exciton state is equal to omegaB＝2ωX- χ, where χ is the energy of the binding of the bisexciton and the energy of the vacuum state is equal to 0; the optical micro-resonant cavity comprises a first mode and a second mode, the first mode and the second mode are mutually orthogonal, and the resonant frequencies of the first mode and the second mode are both equal to omegaX- χ; the transition from the double exciton state to the first single exciton state and the transition from the first single exciton state to the vacuum state are coupled with the first mode and the coupling strength is a, the transition from the double exciton state to the second single exciton state and the transition from the second single exciton state to the vacuum stateThe transitions in between are all coupled to the second mode with a coupling strength of b, a and b are both greater than 0 and a equals b.
With reference to the first aspect, or the second aspect, or the third aspect, or the fourth aspect, or the fifth aspect, in an alternative embodiment, the photons absorbed or emitted by the transition between the double exciton state and the first single exciton state are horizontally polarized, and the photons absorbed or emitted by the transition between the double exciton state and the second single exciton state are vertically polarized; the first mode is horizontal polarization and the second mode is vertical polarization.
With reference to the first aspect, the second aspect, the third aspect, the fourth aspect or the fifth aspect, in an alternative embodiment, the four-energy-level quantum dot is a self-organized semiconductor quantum dot, or a single-layer transition metal sulfide two-dimensional material quantum dot.
With reference to the first aspect, or the second aspect, or the third aspect, or the fourth aspect, or the fifth aspect, in an alternative embodiment, the optical microresonator includes one of a photonic crystal microcavity, a micro-cylindrical optical resonant cavity, a micro-disk optical resonant cavity, and a micro-ring optical resonant cavity.
By implementing embodiments of the present invention, quantum computing devices are formed by coupling four-level quantum dots having two energetically degenerate singlet exciton states with an optically resonant microcavity having two mutually orthogonal, frequency-equal resonance modes. The input laser with the frequency equal to half of the frequency required by the transition of the vacuum state of the four-energy-level quantum dot to the double-exciton state is received, the quantum computing device can be excited to enable the quantum state of the four-energy-level quantum dot to be the double-exciton state, and when the four-energy-level quantum dot is transitioned from the double-exciton state to the single-exciton state, the three input single photon pulses are sequentially received. The first single-photon pulse, the second single-photon pulse and the third single-photon pulse are quantum bits encoded in a first mode and a second mode. The input first single-photon pulse and the second single-photon pulse are required to be executed
Quantum ratio of operation carrying specific informationParticularly, the third single-photon pulse of the input is carried out without carrying
Information of the operation. Because the frequency of the first single-photon pulse and the third single-photon pulse is equal to the frequency required by the transition from the double-exciton state to the single-exciton state, and the frequency of the second single-photon pulse and the frequency of the third single-photon pulse are different by a frequency threshold value gamma, the quantum computing device can realize the quantum computing device after the three single-photon pulses output the quantum computing device
A gate for transferring the quantum state of the first single-photon pulse to a third single-photon pulse, wherein the output second single-photon pulse and the output third single-photon pulse are respectively subjected to a second single-photon pulse and a first single-photon pulse
The result after the door operation.
Drawings
The drawings that accompany the detailed description can be briefly described as follows.
FIG. 1 is a schematic structural diagram of a quantum computing device according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of an energy level structure of a four-energy-level quantum dot according to an embodiment of the present invention;
FIG. 3 is a schematic diagram of a scenario in which a coupling interaction between a four-energy-level quantum dot and an optical micro-resonator enhances and suppresses a transition, according to an embodiment of the present invention;
FIG. 4 is a schematic flow chart of a method for implementing a photonic quantum logic gate according to an embodiment of the present invention;
fig. 5 is a schematic view of a scene of an implementation method of a photon quantum logic gate according to an embodiment of the present invention;
fig. 6 is a schematic structural diagram of an implementation apparatus of a photonic quantum logic gate according to an embodiment of the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention will be described below with reference to the drawings in the embodiments of the present invention.
Referring to fig. 1, fig. 1 is a schematic structural diagram of a quantum computing device 10 according to an embodiment of the present invention, where the quantum computing device 10 includes four-level quantum dots 12 and an optical resonant microcavity 14 coupled to the four-level quantum dots 12.
Optionally, the quantum computing device 10 may further include a memory for storing instructions and data, and a processor that may call the instructions in the memory to perform operations, such as the processor controlling the operations of performing the input operation of receiving laser light, sequentially receiving the input first single-photon pulse, second single-photon pulse, and third single-photon pulse, and so on, as shown in fig. 3.
Fig. 2 is a schematic diagram of an energy level structure of a four-energy-level quantum dot, and in fig. 2, the four-energy-level quantum dot 12 includes a dual exciton state, a first single exciton state, a second single exciton state, and a vacuum state. Wherein the energy of the vacuum state is equal to 0, the first and second singlet exciton states are energetically degenerate singlet exciton states, that is to say the energy of the first and second singlet exciton states is equal. For convenience of subsequent description, the energies of both the first and second singlet exciton states may be represented as ωXThe energy of the dual exciton state is denoted by ωBAnd the four-energy-level quantum dot 12 has double exciton binding energy chi, namely omegaBLess than twice the energy of a singlet exciton state, omegaBAnd omegaXThe relational expression of (1): omegaB＝2ωX-χ。
It is to be understood that the energy of a photon in embodiments of the present invention is determined by the frequency of the photon. That is, the following relationship exists between the photon energy E and the photon angular frequency ω:
wherein the content of the first and second substances,
is reduced Planck constant. Similarly, only energy and frequency of energy level and transition energy and transition frequency exist
Multiple of (d). The frequency mentioned in the invention can refer to angular frequency omega, and can also refer to line frequency v, and the following relation exists between the angular frequency omega and the line frequency v: ω ═ 2 π ν. According to convention, will generally be
Set to 1, energy is referred to in (angular) frequency.
In an embodiment of the present invention, the optical resonant microcavity 14 includes two resonant modes: a first mode and a second mode orthogonal to each other, and a resonance frequency of the first mode and the second mode is ωX- χ. The four-energy-level quantum dot 12 is located in the optical field in the optical resonant microcavity 14, the transition process of the four-energy-level quantum dot can absorb photons in the optical field in the optical resonant microcavity 14 or release photons matched with the first mode or the second mode, and the transition from the double exciton state to the first singlet exciton state and the transition from the first singlet exciton state to the vacuum state are both coupled with the first mode and have a coupling strength, the transition from the double exciton state to the second singlet exciton state and the transition from the second singlet exciton state to the vacuum state are both coupled with the second mode and have b coupling strength, wherein a and b are both greater than 0 and a is equal to b. When the four-level quantum dot 12 is located in the optical field within the optical microresonator 14, a coupling effect occurs. It should be noted that the coupling effect mentioned in the present embodiment is weak coupling, i.e. a < kappa/2, b < kappa/2. Where κ is the decay rate of the first and second modes of the optically resonant microcavity 14 dissipating to the environment outside the cavity.
When the quantum computing device meets the weak coupling condition, the transition frequency of the double exciton state to the first single exciton state is equal to the resonance frequency of the first mode, the transition frequency of the double exciton state to the second single exciton state is equal to the resonance frequency of the second mode, so that the four-energy-level quantum dot 12 and the optical micro-resonant cavity 14 can generate resonance, the transition of the double exciton state to the first single exciton state and the second single exciton state is enhanced, the spontaneous radiance of the double exciton state is improved, and the energy level service life is reduced; and because the transition frequency of the first single exciton state to the vacuum state is different from the resonance frequency of the first mode by x, and the transition frequency of the second single exciton state to the vacuum state is different from the resonance frequency of the second mode by x, the four-level quantum dot 12 and the optical micro-resonant cavity 14 can generate detuning, so that the transition of the first single exciton state to the vacuum state is inhibited, the transition of the second single exciton state to the vacuum state is inhibited, the spontaneous radiance of the double exciton is reduced, and the energy level service life is prolonged.
For example, as shown in fig. 3, fig. 3 is a schematic diagram of a scenario in which coupling interaction of four-energy-level quantum dots and an optical microresonator enhances and suppresses transitions. Assuming that the spontaneous radiance of a double exciton state of the four-energy-level quantum dot and the spontaneous radiance of a first single exciton state and a second single exciton state before the coupling action of the four-energy-level quantum dot and the optical micro-resonant cavity is not generated, is equal to 2 pi multiplied by 0.1GHz, the energy level life of the double exciton state is equal to 10ns, the coupling strength g is equal to 2 pi multiplied by 10GHz, the cavity mode attenuation rate kappa is equal to 2 pi multiplied by 40GHz, and the double exciton binding energy chi is equal to 2 pi multiplied by 720GHz (the energy is equal to 3 meV). Under the influence of the coupling interaction of the four-energy-level quantum dots and the resonant mode of the optical resonant cavity, the spontaneous emissivity of the double exciton state is about 2 pi x 2.7GHz, the energy lifetime is about 0.37ns, the spontaneous emissivity of the first single exciton state and the second single exciton state is about 2 pi x 0.0077GHz, and the energy lifetime is about 130 ns.
The embodiment of the present invention does not limit the specific directions of the first mode and the second mode. As a possible embodiment, the photons absorbed or emitted by the transition between the dual exciton state and the first singlet exciton state are horizontally polarized, and the photons absorbed or emitted by the transition between the dual exciton state and the second singlet exciton state are vertically polarized; the first mode is horizontally polarized and the second mode is vertically polarized.
That is, the photons absorbed or emitted by the transition of the dual exciton state to the first single exciton state have the same polarization direction as the photons of the first mode, the photons absorbed or emitted by the transition of the dual exciton state to the second single exciton state have the same polarization direction as the photons of the second mode, the transition frequency of the dual exciton state to the first single exciton state is equal to the resonance frequency of the first mode, the transition frequency of the dual exciton state to the second single exciton state is equal to the resonance frequency of the second mode, the transition of the first single exciton state is coupled to the first mode, and the transition of the second single exciton state is coupled to the second mode, that is, the four-level quantum dot 12 is coupled to the optical resonance microcavity 14.
As an alternative embodiment, the four-energy-level quantum dot 12 is a semiconductor quantum dot using self-organized growth, or a single-layer transition metal sulfide two-dimensional material quantum dot.
The semiconductor quantum dots adopting the self-organizing growth can be quantum dots formed by growing an indium gallium arsenide material on an indium GaAs material, quantum dots formed by growing an indium arsenide material on an indium GaAs material and quantum dots formed by growing an aluminum gallium arsenide material on an indium GaAs material, and the single-layer transition metal sulfide two-dimensional material quantum dots can comprise tungsten diselenide WSe2 and molybdenum disulfide MoS2And the material of the four-energy-level quantum dot and the single-layer transition metal sulfide two-dimensional material quantum dot is not limited in the embodiment of the invention.
As an alternative embodiment, the optical microresonator 14 can be a photonic crystal microcavity, a micro-cylindrical optical resonant cavity, a micro-disk optical resonant cavity, a micro-ring optical resonant cavity, or the like.
The optical resonant micro-cavity 14 is made of materials including GaAs, Si, SiN, and the like, and the embodiments of the present invention are not limited to the specific optical micro-resonant cavity and the materials used for the optical micro-resonant cavity.
For example, four-level quantum dots are formed from indium gallium arsenide InGaAs material grown on gallium arsenide GaAs material, and an optical microresonator forms a quantum computing device by etching the GaAs material around the four-level quantum dots.
As another example, the four-energy-level quantum dots are made of WSe2 material, the optical micro-resonant cavity is made of Si material, and the four-energy-level quantum dots are transferred and positioned on the optical micro-resonant cavity to be coupled to form the quantum computing device.
The embodiment shown in fig. 4 is used as an example to describe how to implement a photonic quantum logic gate based on the quantum computing device.
Referring to fig. 4, fig. 4 is a schematic flowchart of a method for implementing a photonic quantum logic gate according to an embodiment of the present invention, where the method shown in fig. 4 includes, but is not limited to, the following steps:
step 402: the quantum computing device receives input laser light to excite the four-level quantum dots to a bi-exciton state.
In the embodiment of the invention, the frequency of the laser is equal to half of the frequency required by the vacuum state transition of the four-energy-level quantum dot to the double-exciton state, namely the frequency of the laser is equal to
Lasers are classical optical pulses commonly used in the art, and four-level quantum dots in such quantum computing devices are resonantly and deterministically excited to a bi-exciton state. Referring to fig. 5, fig. 5 is a schematic view of a scenario of an implementation method for implementing a photon quantum logic gate. As shown in fig. 5, when the laser is input at time 0, the quantum state of the four-level quantum dot is a double exciton state.
Step 404: under the condition that the four-energy-level quantum dot transits from a double-exciton state to a single-exciton state, the quantum computing device receives a first single-photon pulse, a second single-photon pulse and a third single-photon pulse which are input in sequence.
And the SWAP gate is used for swapping the two quantum states. For example, assume that the quantum state of the first qubit A is equal to αA|0>+βA|1>The quantum state of the second qubit B is equal to alphaB|0>+βB|1>Then the quantum state of first qubit a equals alpha after the first qubit a and the second qubit B have undergone SWAP gatesB|0>+βB|1>The quantum state of the second qubit B is equal to alphaA|0>+βA|1>. Due to two
A SWAP gate may be obtained from a cascade of gates, and thus embodiments of the present invention will work with
The door is called a change-over square door.
Assuming a three-level atom has two energetically degenerate ground states |0>And the ground state |1>And an excited state |2>. Wherein the ground state |0>And the ground state |1>To excited state |2>The transition of the three-energy-level atoms needs to absorb photons with horizontal polarization and photons with vertical polarization respectively, and the transition frequencies of the two transition modes are equal to a frequency threshold value gamma due to the mutual coupling effect of the three-energy-level atoms and an external optical field. Inputting a first single-photon pulse and a second single-photon pulse to the three-energy-level atom in sequence, wherein the frequency of the first single-photon pulse and the ground state |0 of the three-energy-level atom>Or the ground state |1>Transition to excited state |2>Has the same transition frequency as the second single-photon pulse and has the same frequency as the ground state |0>Or the ground state |1>Transition to excited state |2>The transition frequency of (a) differs by a frequency threshold Γ. After the first single photon pulse is input into the three-energy-level atoms and reflected, the three-energy-level atoms are exchanged with the quantum states of the first photon pulse, namely, the first photon pulse undergoes SWAP gate operation. When the second single photon pulse is input into the three-energy-level atoms and reflected, the quantum states of the three-energy-level atoms and the second photon pulse are subjected to
The door is operated.
Based on the above theory, the frequency of the first single photon pulse and the third single photon pulse is equal to the frequency omega required for the transition from the dual exciton state to the single exciton stateX- χ, the frequency of the second single-photon pulses differing from the frequency of the third single-photon pulses by a frequency threshold Γ. And the first single-photon pulse, the second single-photon pulse and the third single-photon pulse are quantum bits encoded in a first mode and a second mode, and the input first single-photon pulse and the input second single-photon pulse are required to be executed
Operating qubits carrying specific information, the third single-photon pulse being input without carrying
Qubits of information of operation. As shown in fig. 5, at four levelsWhen the sub-point transits from the double-exciton state to the single-exciton state, the first single-photon pulse is input into the quantum computing device and then reflected, then the quantum computing device obtains the input quantum state of the first single-photon pulse, the second single-photon pulse is input into the quantum computing device and then reflected, and the input second single-photon pulse and the quantum computing device realize
A gate for reflecting the third single-photon pulse after inputting it into the quantum computing device, the third single-photon pulse acquiring the quantum state of the quantum computing device, i.e. the quantum state of the input first single-photon pulse, the output second single-photon pulse and the output third single-photon pulse being executed by the input first single-photon pulse and the input second single-photon pulse
The result after the door operation.
As an alternative embodiment, the quantum computing device receives the first single-photon pulse, the second single-photon pulse and the third single-photon pulse which are input in sequence from a preset time point, and the difference between the preset time point and the starting time of the laser input and the difference of 5 times of the energy level lifetime of the double exciton state are smaller than a preset threshold value.
The quantum computing device transitions the four-level quantum dots from a bi-exciton state to a mono-exciton state from the time of receiving the input laser to a preset time point. Wherein the preset time point is related to the energy level lifetime of the dual exciton state in the four-energy level quantum dot and the probability of the first and second singlet exciton states expected to be occupied at the preset time point, in fig. 5, the preset time point t1The calculation formula of (a) is as follows:
formula (1), τBIs the energy level lifetime of a bi-exciton state, η1Is the probability of occupation of a singlet exciton state at a predetermined time point. The probability of occupation of a singlet exciton state will affect the efficiency of the quantum computing device,the closer to 1, the better.
For example, if the predetermined time point is 5 times the energy level lifetime of the double exciton state, that is, 0.37 × 5 — 1.85ns, the sum of the occupancy probabilities of the first and second single exciton states is 99.7% from the time when the laser excites the quantum computing device to the predetermined time point, and it is considered that the four-level quantum dot transits from the double exciton state to the single exciton state.
Since transitions of the first and second singlet exciton states to the vacuum state are suppressed, the four-level quantum dot is approximately considered to be always in the first and second singlet exciton states from the preset time point to the first preset time point.
And at a first preset time point, the four-energy-level quantum dot starts to transition to a vacuum state from a single exciton state, and at a second preset time point, the four-energy-level quantum dot transitions to the vacuum state.
Wherein a difference between the first preset time point and the preset time point and a difference of 0.1 times of the energy level lifetime of the singlet exciton state are less than a preset threshold. The first predetermined time point is related to the energy level lifetime of the singlet exciton state in the four-energy level quantum dot, and the probability of occupation of the singlet exciton state at the first predetermined time point is expected. In fig. 5, a first preset time point t2The calculation formula of (a) is as follows:
in the formula (2), τXEnergy level lifetime of singlet exciton state, η1Is the probability of occupation of a singlet exciton state at a predetermined time point, η2Is the probability of occupation of a singlet exciton state at the first predetermined time point. Of course eta2Will affect the efficiency of the quantum computing device, the closer to 1 the better.
For example, if the difference t2 between the first predetermined time point and the predetermined time point is 0.1 times the lifetime of the singlet state energy level, i.e., 130 × 0.1 — 13ns, the occupancy probability of the first singlet state and the second singlet state is still over 90% when the first predetermined time point is reached, i.e., the quantum state of the four-level quantum dot is still the singlet state.
It should be noted that the difference between the first preset time point and the preset time point is greater than or equal to the input time of the three single-photon pulses, that is, the first single-photon pulse, the second single-photon pulse, and the third single-photon pulse are guaranteed to complete the logic operation in the time period.
For example, assuming that the time interval between the centers of the three single photon pulses is 3ns, the difference t2 between the first predetermined time point and the predetermined time point must be greater than or equal to 3 × 3ns — 9ns, so as to ensure that the logic operation is completed when the first predetermined time point is reached.
And the difference between the second preset time point and the first preset time point and the difference of 5 times of the energy level service life of the singlet exciton state are smaller than a preset threshold value. The second predetermined time point is related to the energy level lifetime of the singlet exciton state in the four-energy level quantum dot, and the probability of occupation of the singlet exciton state at the second predetermined time point is expected. In fig. 5, a second preset time point t3The calculation formula of (a) is as follows:
in the formula (3), τXEnergy level lifetime of singlet exciton state, η2Is the probability of occupation of a singlet exciton state at a first predetermined point in time, η3Is the probability of occupation of the singlet exciton state at the second predetermined time point. Of course eta3Will affect the efficiency of the quantum computing device, the closer to 0 the better.
For example, if the third predetermined time t3 is 5 times the energy level lifetime of the singlet exciton state, i.e., 5 × 130ns is 650ns, the occupancy probability of the singlet exciton state is about 0.5% when the third predetermined time arrives, and it can be considered that the quantum state of the four-level quantum dot is restored to the vacuum state.
It should be noted that, because the quantum gate does not work only once in a practical application scenario, but works in one clock cycle like a device in a computer, each time the laser is input to the quantum computing device, the laser goes through
The gate time is taken as one operation cycle.
In the method shown in fig. 4, receiving an input laser with a frequency equal to half of a frequency required for a vacuum state transition of the four-level quantum dot to a dual exciton state excites the quantum computing device such that the quantum state of the four-level quantum dot is a dual exciton state, and sequentially receiving three single photon pulses as the four-level quantum dot transitions from the dual exciton state to a single exciton state. And the first single-photon pulse, the second single-photon pulse and the third single-photon pulse are quantum bits encoded in a first mode and a second mode. The input first single-photon pulse and the second single-photon pulse are required to be executed
Operating qubits carrying specific information, the third single-photon pulse being input without carrying
Information of the operation. Because the frequency of the first single-photon pulse and the third single-photon pulse is equal to the frequency required by the transition from the double-exciton state to the single-exciton state, and the frequency of the second single-photon pulse and the frequency of the third single-photon pulse are different by a frequency threshold value gamma, the quantum computing device can realize the quantum computing device after the three single-photon pulses output the quantum computing device
A gate for transferring the quantum state of the first single-photon pulse to a third single-photon pulse, wherein the output second single-photon pulse and the output third single-photon pulse are respectively subjected to a second single-photon pulse and a first single-photon pulse
The result after the door operation.
The method of embodiments of the present invention is set forth above in detail and the apparatus of embodiments of the present invention is provided below.
Referring to fig. 6, fig. 6 is a schematic structural diagram of an implementation apparatus 60 of a photonic quantum logic gate according to an embodiment of the present invention. As shown in fig. 6, the apparatus 60 includes a first input unit 62 and a second input unit 64, specifically: the first input unit 62 is used for receiving input laser to enable the quantum state of the four-energy-level quantum dot to be a double-exciton state, the quantum computing device comprises the four-energy-level quantum dot and an optical micro-resonant cavity coupled with the four-energy-level quantum dot, and the frequency of the laser is equal to half of the frequency required by transition of a vacuum state of the four-energy-level quantum dot to the double-exciton state; the second input unit 64 is configured to receive a first single-photon pulse, a second single-photon pulse, and a third single-photon pulse that are input in sequence when the four-energy-level quantum dot transitions from the dual-exciton state to the single-exciton state, where frequencies of the first single-photon pulse and the third single-photon pulse are equal to a frequency required for the dual-exciton state to transition to the single-exciton state, a frequency difference between frequencies of the second single-photon pulse and the third single-photon pulse is a frequency threshold value Γ, and the first single-photon pulse, the second single-photon pulse, and the third single-photon pulse are qubits encoded in the first mode and the second mode.
In an alternative embodiment, the second input unit 64 is specifically configured to sequentially receive the first single-photon pulse, the second single-photon pulse and the third single-photon pulse at a preset time point, where a difference between the preset time point and a start time of the laser input and a difference between 5 times of an energy level lifetime of the dual exciton state are smaller than a preset threshold.
In an alternative embodiment, the four-level quantum dot includes a bi-exciton state, a mono-exciton state, and a vacuum state, the mono-exciton state includes a first mono-exciton state and a second mono-exciton state, and the energy of the first mono-exciton state and the second mono-exciton state are both equal to ωXThe energy of the dual exciton state is equal to omegaB＝2ωX- χ, where χ is the energy of the binding of the bisexciton and the energy of the vacuum state is equal to 0; the optical micro-resonant cavity comprises a first mode and a second mode, the first mode and the second mode are mutually orthogonal, and the resonant frequencies of the first mode and the second mode are both equal to omegaX- χ; the transition from the double exciton state to the first single exciton state and the transition from the first single exciton state to the vacuum state are coupled with the first mode and the coupling strength is a, the transition from the double exciton state to the second single exciton state is performed to obtain the final productAnd the transition of the second singlet exciton state to the vacuum state is coupled to the second mode with a coupling strength of b, a and b are both greater than 0 and a is equal to b.
In an alternative embodiment, the photons absorbed or emitted by the transition between the bi-exciton state and the first mono-exciton state are horizontally polarized, and the photons absorbed or emitted by the transition between the bi-exciton state and the second mono-exciton state are vertically polarized; the first mode is horizontal polarization and the second mode is vertical polarization.
In an alternative embodiment, the four-energy-level quantum dot adopts a semiconductor quantum dot grown by self-organization, or a single-layer transition metal sulfide two-dimensional material quantum dot.
In an alternative embodiment, the optical microresonator comprises one of a photonic crystal microcavity, a micro-cylindrical optical resonant cavity, a micro-disk optical resonant cavity, and a micro-ring optical resonant cavity.
It is understood that the functions of the functional units of the apparatus according to the embodiment of the present invention can be implemented according to the method of the embodiment of the method shown in fig. 2, and are not described herein again.
In the device shown in fig. 6, receiving an input laser with a frequency equal to half of the frequency required for the vacuum state transition of the four-level quantum dot to the dual exciton state excites the quantum computing device such that the quantum state of the four-level quantum dot is the dual exciton state, and when the four-level quantum dot transitions from the dual exciton state to the single exciton state, three single photon pulses are sequentially received. And the first single-photon pulse, the second single-photon pulse and the third single-photon pulse are quantum bits encoded in a first mode and a second mode. The input first single-photon pulse and the second single-photon pulse are required to be executed
Operating qubits carrying specific information, the third single-photon pulse being input without carrying
Information of the operation. Because the frequency of the first single-photon pulse and the third single-photon pulse is equal to that of the dual excitonThe frequency required by the state transition to the singlet exciton state is different from the frequency of the third single-photon pulse by a frequency threshold gamma, and then the three single-photon pulses are output to a quantum computing device which can realize
A gate for transferring the quantum state of the first single-photon pulse to a third single-photon pulse, wherein the output second single-photon pulse and the output third single-photon pulse are respectively subjected to a second single-photon pulse and a first single-photon pulse
The result after the door operation.
In summary, quantum computing devices are formed by coupling a four-level quantum dot having two energetically degenerate singlet exciton states with an optically resonant microcavity having two mutually orthogonal resonant modes of equal resonant frequency. The input laser with the frequency equal to half of the frequency required by the transition of the vacuum state of the four-energy-level quantum dot to the double-exciton state is received, the quantum computing device can be excited to enable the quantum state of the four-energy-level quantum dot to be the double-exciton state, and when the four-energy-level quantum dot is transitioned from the double-exciton state to the single-exciton state, the three input single photon pulses are sequentially received. And the first single-photon pulse, the second single-photon pulse and the third single-photon pulse are quantum bits encoded in a first mode and a second mode. The input first single-photon pulse and the second single-photon pulse are required to be executed
Operating qubits carrying specific information, the third single-photon pulse being input without carrying
Information of the operation. Since the frequency of the first and third single-photon pulses is equal to the frequency required for the transition of the two-exciton state to the single-exciton state, and the frequency of the second single-photon pulse differs from the frequency of the third single-photon pulse by a frequency threshold Γ, the three single-photon pulses are defined asAfter the quantum computing device is output, the quantum computing device can be realized
A gate for transferring the quantum state of the first single-photon pulse to a third single-photon pulse, wherein the output second single-photon pulse and the output third single-photon pulse are respectively subjected to a second single-photon pulse and a first single-photon pulse
The result after the door operation.
It will be understood by those skilled in the art that all or part of the processes of the methods of the embodiments described above can be implemented by a computer program, which can be stored in a computer-readable storage medium, and can include the processes of the embodiments of the methods described above when the computer program is executed. And the aforementioned storage medium includes: various media that can store program codes, such as ROM, RAM, magnetic or optical disks.
Claims (7)
1. A method for implementing a photonic quantum logic gate, comprising:
a quantum computing device receives input laser light to enable a quantum state of a four-energy-level quantum dot to be a double exciton state, the quantum computing device comprises the four-energy-level quantum dot and an optical micro-resonant cavity coupled with the four-energy-level quantum dot, and the frequency of the laser light is equal to half of the frequency required for a vacuum state of the four-energy-level quantum dot to transition to the double exciton state; the four-energy-level quantum dots comprise the dual exciton state, the single exciton state and the vacuum state, the single exciton state comprises a first single exciton state and a second single exciton state, and the energy of the first single exciton state and the second single exciton state is both equal to omegaXThe energy of the said bisexciton state is equal to ωB＝2ωX- χ, wherein χ is the energy of the binding of the bisexciton, the energy of said vacuum state being equal to 0; the optical micro-resonant cavity comprises a first mode and a second mode, the first mode and the second mode are mutually orthogonal, and the resonant frequencies of the first mode and the second mode are bothIs equal to omegaX- χ; the transition between the bi-exciton state to the first mono-exciton state and the transition between the first mono-exciton state to the vacuum state are both coupled to the first mode with a coupling strength a, the transition between the bi-exciton state to the second mono-exciton state and the transition between the second mono-exciton state to the vacuum state are both coupled to the second mode with a coupling strength b, a and b are both greater than 0 and a is equal to b;
under the condition that the four-energy-level quantum dot is transited from the double-exciton state to the single-exciton state, the quantum computing device receives a first single-photon pulse, a second single-photon pulse and a third single-photon pulse which are input in sequence, the frequency of the first single-photon pulse and the frequency of the third single-photon pulse are equal to the frequency required by the transition from the double-exciton state to the single-exciton state, the frequency of the second single-photon pulse and the frequency of the third single-photon pulse are different by a frequency threshold value gamma, and the first single-photon pulse, the second single-photon pulse and the third single-photon pulse are quantum bits which are encoded in the first mode and the second mode.
2. The method of claim 1 wherein the quantum computing device receives input first, second and third single-photon pulses in sequence, comprising:
the quantum computing device receives the first single-photon pulse, the second single-photon pulse and the third single-photon pulse which are input in sequence from a preset time point, and the difference between the preset time point and the starting time of laser input and the difference of 5 times of the energy level life of the double exciton state are smaller than a preset threshold value.
3. The method of claim 1, wherein the photons absorbed or emitted by the transition between the bi-exciton state and the first mono-exciton state are horizontally polarized and the photons absorbed or emitted by the transition between the bi-exciton state and the second mono-exciton state are vertically polarized;
the first mode is horizontal polarization and the second mode is vertical polarization.
4. The method of claim 2, wherein the photons absorbed or emitted by the transition between the bi-exciton state and the first mono-exciton state are horizontally polarized and the photons absorbed or emitted by the transition between the bi-exciton state and the second mono-exciton state are vertically polarized;
the first mode is horizontal polarization and the second mode is vertical polarization.
5. The method according to any one of claims 1 to 4, wherein the four-energy-level quantum dots are self-organized semiconductor quantum dots or transition metal sulfide two-dimensional material quantum dots with a single layer.
6. The method of any of claims 1-4, wherein the optical microresonator comprises one of a photonic crystal microcavity, a micro-cylindrical optical resonant cavity, a micro-disk optical resonant cavity, and a micro-ring optical resonant cavity.
7. The method of claim 5, wherein the optical microresonator comprises one of a photonic crystal microcavity, a micro-cylindrical optical resonator, a micro-disk optical resonator, and a micro-ring optical resonator.
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