JP7218869B2 - photon generator - Google Patents

Description

Application of Article 30, Paragraph 2 of the Patent Act (1) Date November 26-27, 2018 (Release date: November 26, 2018) Meeting name, venue 39th Quantum Information Technology Study Group Sponsor: General The Institute of Electronics, Information and Communication Engineers, Special Committee on Quantum Information Technology Co-sponsored by: The Japan Society of Applied Physics, Quantum Electronics Research Group, The University of Tokyo, Komaba Research Campus, Research Center for Advanced Science and Technology (2) Date: March 14, 2019 - 17th (Released date: March 16, 2019) Meeting name, Venue The 74th Annual Meeting of the Physical Society of Japan (2019) Kyushu University Ito Campus

The present invention relates to quantum technology, and more particularly to technology for generating photons.

In recent years, it has been shown that the use of quantum mechanics in calculations enables high-speed calculations that have never been possible before. In the quantum information processing, photons are important components as quantum bits that can be used for communication. It is important to generate the photons with high probability. The following photon generation methods have been known so far (see, for example, Non-Patent Document 1).

Non-Patent Document 1 discloses an improvement in the photon generation rate including the external coupling rate and a generation method in various coupling regions.

Hayato Goto, Shota Mizukami, Yuuki Tokunaga, and Takao Aoki, "Figure of merit for single-photon generation based on cavity quantum electrodynamics", Phys. Rev. A 99, 053843 (2019)

In reality, the photon pulse length is shortened to some extent to control the quantum gate time so that it does not become too long. However, Non-Patent Document 1 does not show the generation rate according to the finite pulse length, and shows how the pulse length can be appropriately shortened without significantly impairing the photon generation rate. It has not been.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a photon generator capable of generating photons more efficiently than before when the photon pulse length is finite.

A photon generation device according to a first aspect of the present invention comprises a control light generator that generates control light, and a control light that excites an initial state to an excited state upon receiving the control light, and generates photons when transitioning from the excited state to the ground state. A cavity QED system including atoms to be generated and a resonator that confines photons and emits them to the outside at a predetermined external coupling rate κ _ex , where κ _in is the internal loss rate of the cavity QED system, and the atoms and Let g be the photon coupling rate, γ be the attenuation rate of the induced polarization of the atom, C _in =g ² /2κ _in γ, and Ψ _i be the rate of change of the photon time waveform, and the external coupling rate κ _ex is , κ _ex =κ _in (1+2C _in +Ψ _i ) ^(1/2) .

A photon generating device according to a second aspect of the present invention comprises a control light generator that generates control light, and a control light that excites an initial state to an excited state upon receiving the control light, and generates photons when transitioning from the excited state to the ground state. A cavity QED system including atoms to be generated and a resonator that confines photons and emits them to the outside at a predetermined external coupling rate κ _ex , where κ _in is the internal loss rate of the cavity QED system, and the atoms and Let g be the photon coupling rate, γ be the decay rate of the induced polarization of the atom, and C _in =g ² /2κ _in γ, then the external coupling rate κ _ex is κ _ex =κ _in ((1+2C _in )( 1+γ/κ _in )) ^(1/2) .

According to the present invention, photons can be generated more efficiently than before when the pulse length of photons is finite.

FIG. 1 is a diagram showing an example of the functional configuration of a photon generator. FIG. 2 is a diagram for explaining the state transition of the cavity QED system. FIG. 3 is a diagram for explaining the state transition of the cavity QED system.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail. In the drawings, constituent parts having the same function are denoted by the same numbers, and redundant explanations are omitted.

The photon generator includes a control light generator 1 , a cavity QED system 2 and an external coupling rate controller 3 . A photon generator is a device that generates a single photon using a so-called CQED system (Cavity Quantum ElectroDynamics). See, for example, Non-Patent Literature 1 and Reference Literature 1 for details of a photon generator using a CQED system.
<Reference 1> Kazuhiro Hayasaka, "Deterministic Single Photon Generation by Cavity Quantum Electrodynamics", Laser Research, September 2003

A control light generator 1 generates control light for controlling a cavity QED system 2 . The generated control light is incident on the atoms of the cavity QED system section 2 .

The cavity QED system unit 2 receives the control light, excites the atoms from the initial state |u> to the excited state |e>, and generates photons when transitioning from the excited state |e> to the ground state |g>; and a resonator 21 for confining photons and emitting them to the outside with a predetermined external coupling rate κ _ex . Here, the atoms may be natural atoms or artificial atoms such as quantum dots and superconducting qubits. Photons generated from atoms are strongly confined within the resonator 21 and interact with the atoms. After that, the photon is emitted outside according to the external coupling rate κ _ex .

FIG. 2 is a diagram for explaining the state transition of the cavity QED system section 2. FIG. An atom can have at least three states: an initial state |u>, an excited state |e> and a ground state |g>.

Atoms in the initial state |u> are excited to the excited state |e> upon receiving the control light. Ω is the transition frequency between the initial state |u> and the excited state |e> generated by applying the control light. Δe is the detuning corresponding to the excited state |e>, eg Δe=E _|e> −E _|g> −g. E _|e> is the energy level of the excited state |e>, and E _|g> is the energy level of the ground state |g>. Δu is the detuning corresponding to the initial state |u>, eg Δu=Ω−(E _|e> −E _|u> −Δe). E _|u> is the energy level of the initial state |u>.

An atom in the excited state |e> generates a photon when transitioning to the ground state |g>. An atom in the excited state |e> decays with a predetermined natural decay rate γ. γ can also be said to be the decay rate of the induced polarization. g is the atomic-photon coupling rate.

The generated photons are confined in the resonator 21 . The resonator 21 is, for example, two mirrors facing each other. In this case, the external coupling rate κ _ex is the transmittance of one of the two mirrors forming the resonator 21 . Resonator 21 may be a microring, microtoroid, microsphere or photonic crystal resonator instead of two mirrors.

κ _in is the internal loss factor of the cavity QED system 2 and can be said to be the attenuation factor of the optical electric field inside the cavity QED system 2 . Note that κ=κ _ex +κ _in .

The external coupling rate controller 3 controls the external coupling rate κ _ex . If the resonator 21 is two mirrors, the external coupling ratio controller 3 can change the transmittance of the mirrors by, for example, applying tension or heat to the mirrors, thereby changing the external coupling ratio κ _ex can be controlled.

Specifically, the external coupling rate control unit 3 performs control so that the external coupling rate κ _ex satisfies κ _ex =κ _in (1+2C _in +Ψ _i ) ^(1/2) . where C _in =g ² /2κ _in γ. Here, Ψ _i is a quantity relating to the rate of change of the photon time waveform, and is defined by Equation (1), for example.

where ψ ₀ (t) is the amplitude of the photon time waveform.

When the photon time waveform is a Gaussian function, Ψ _i is defined by Equation (2), for example.

where σ is the standard deviation of the Gaussian function. By controlling the waveform of the control light generated by the control light generator 1, a desired photon time waveform can be realized. See Reference 2, for example, for the method of controlling the time waveform of photons.
<Reference 2> V., Genko S., D. Ljunggren, and A. Kuhn, "Single photons made-to-measure," New Journal of Physics 12, 063024 (2010)

By setting the external coupling rate κ _ex =κ _in (1+2C _in +Ψ _i ) ^(1/2) , photons can be generated more efficiently than before when the photon pulse length is finite. (For example, see Non-Patent Document 1).

In the photon generation method described in Non-Patent Document 1, it is known that the maximum value P _S of the photon generation probability follows Equation (3).

From equations (1) and (3), when the photon time waveform is a Gaussian function, in order to maximize the photon generation efficiency P _s , the standard deviation σ, which is an index of the photon time width, is expressed by equation (4 ). This condition can be used to determine the duration of photons to be generated.

The state transition in the cavity QED system can also be described as shown in FIG. 3, for example. The state |φ ₀ > is a state in which |φ ₀ >=|u>|0>, atoms are in the initial state |u>, and no photons exist in the resonator 21 . The state |φ ₁ > is a state in which |φ ₁ >=|e>|0>, the atom is in the excited state |e>, and no photons exist in the resonator 21 . The state |φ ₂ > is a state in which |φ ₂ >=|g>|1>, the atom is in the ground state |g>, and photons are present in the resonator 21 . The state |φ ₃ > is |φ ₃ >=|g>|0>, the atoms are in the ground state |g>, and the photons have left the cavity 21, resulting in It is a state in which no photons are present in 21 . The states |φ ₀ > and |φ ₁ > are reversible. Similarly, the states |φ ₁ > and |φ ₂ > are reversible. On the other hand, the states |φ ₂ > and |φ ₃ > are irreversible. In other words, the state |φ ₂ > can be transitioned to the state |φ ₃ >, but the state |φ ₃ > cannot be transitioned to the state |φ ₂ >.

The entire state inside the resonator 21 is expressed as |ψ>=α ₀ |φ ₀ >+α ₁ |φ ₁ >+α ₂ |φ ₂ >. α ₀ is the weight corresponding to the state |φ ₀ >, α ₁ is the weight corresponding to the state |φ ₁ >, and α ₂ is the weight corresponding to the state |φ ₂ >. Here the weights are the complex amplitudes corresponding to the states.

In this case, the rate of change of weights α ₀ , α ₁ , α ₂ can be described as follows. where i is the imaginary unit.

Here, when the rate of increase of Ω is small, the transition from the initial state |u> to the ground state |g> is performed with little loss.

The control light generator 1 may control the rate of increase of Ω so as to satisfy the following equation (5) where T is a predetermined time. α ₂ (t) is the weight at time t corresponding to the state where the atom is in the ground state |e> and the photon is present in the cavity 21 .

Non-Patent Document 1 describes a photon generation method using a three-level atomic system, but the single-photon generation method includes photon generation using a two-level atomic system described in Reference There are also methods.
<Reference 3> G. Cui and MG Raymer, "Quantum efficiency of single-photon sources in the cavity-QED strong-coupling regime," OPTICS EXPRESS Vol. 13, No. 24 (2005)

It is known that when a two-level atomic system is used, the maximum value P _S of the photon generation probability follows equation (6).

To achieve this, the external coupling rate control unit 3 sets the external coupling rate κ _ex to satisfy κ _ex =κ _in ((1+2C _in )(1+γ/κ _in )) ^(1/2) . to control. In a two-level atomic system, the photon pulse length is determined approximately by κ + γ, and the photon time waveform cannot be controlled. be able to.

[Variation]
Although the embodiments of the present invention have been described above, the specific configuration is not limited to these embodiments. Needless to say, it is included in the present invention.

The various processes described in the embodiments are not only executed in chronological order according to the described order, but may also be executed in parallel or individually according to the processing capacity of the device that executes the processes or as necessary.

For example, the photon generator does not have to include the external coupling rate controller 3 . In this case, the external coupling rate κ _ex is κ _ex =κ _in (1+2C _in +Ψ _i ) ^(1/2) when using a three-level atomic system, and is initialized to κ _ex =κ _in ((1+2C _in )(1+γ/κ _in )) ^(1/2) .

1 Control light generator 2 Cavity QED system 21 Resonator 3 External coupling rate controller

Claims (5)

a control light generator that generates control light;
Atoms that receive the control light and are excited from an initial state to an excited state, generate photons when transitioning from the excited state to the ground state, confine the photons, and emit the photons to the outside at a predetermined external coupling rate κ _ex a cavity QED system including a resonator;
including
Let _κin be the internal loss rate of the cavity QED system, g be the coupling rate between the atoms and the photons, γ be the decay rate of the induced polarization of the atoms, Cin = g ² / _{2κin γ} , and The external coupling rate κ _ex is set so as to satisfy κ _ex =κ _in (1+2C _in +Ψ _i ) ^(1/2) , where Ψ _i is the amount related to the rate of change of the photon time waveform.
Photon generator. The photon generator of claim 1, comprising:
an external coupling rate control unit that controls the external coupling rate κ _ex to satisfy κ _ex =κ _in (1+2C _in +Ψ _i ) ^(1/2) ;
a photon generator, further comprising: The photon generator of claim 1 or 2,
The amount Ψ _i related to the rate of change is defined by the following equation, where the amplitude of the photon time waveform is ψ ₀ (t),

Photon generator. The photon generator of claim 1 or 2,
The photon time waveform is a Gaussian function,
The quantity Ψ _i related to the rate of change is defined as Ψ _i =1/(2σ ² κ _in ² ), where σ is the standard deviation of the Gaussian function.
Photon generator. a control light generator that generates control light;
Atoms that receive the control light and are excited from an initial state to an excited state, generate photons when transitioning from the excited state to the ground state, confine the photons, and emit the photons to the outside at a predetermined external coupling rate κ _ex a cavity QED system including a resonator;
including
Let _κin be the internal loss rate of the cavity QED system, g be the coupling rate between the atoms and the photons, and γ be the attenuation rate of the induced polarization of the atoms, and Cin = g ² / _{2κin γ} . The external coupling ratio κ _ex is set to satisfy κ _ex =κ _in ((1+2C _in )(1+γ/κ _in )) ^(1/2) ,
Photon generator.

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