JP2020144163A - Photon generator - Google Patents

Abstract

To provide technology with which it is possible to generate photons more efficiently than before.SOLUTION: A photon generator includes a control light generation unit 1 for generating control light, and a cavity QED system unit 2 including an atom excited from an initial state into an excited state upon receiving control light and generating a photon when transitioning from the excited state to a ground state, and a resonator 21 for confining the photons and releasing to the outside at a prescribed external binding rate κex. The external binding rate κex is set so as to satisfy κex=ακin(1+2Cin)(1/2), where κin represents the internal loss rate of the cavity QED system unit 2, g represents the binding rate of atoms and photons, γ represents the decay rate of induced polarization of atoms, Cin=g2/2κinγ, and α represents a prescribed correction coefficient.SELECTED DRAWING: Figure 1

Description

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 qubits that can be used for communication. It is important to generate the photon with high probability. The following photon generation methods are known so far (see, for example, Non-Patent Document 1 and Non-Patent Document 2).

Non-Patent Document 1 discloses a method of generating a photon by using an atom captured in a resonator as a single photon source. Weakly coupled regions are used here.

Further, Non-Patent Document 2 describes a method of generating photons in a strong coupling region by using an atom captured in a resonator as a single photon source.

C.K.Law, H.J.Kimble, "Deterministic generation of a bit-stream of single-photon pulses", Journal of Modern Optics, Volume 44, NO.11-12, p.0267-2074, 1997 A.Kuhn, M.Hennrich, T.Bondo, and G.Rempe, "Controlled generation of single photons from a strongly coupled atom-cavity system", Applied Physics B, Volume 69, Issue 5-6, p.373-377 , 1999

However, in Non-Patent Document 1 and Non-Patent Document 2, the photon generation rate is not improved in consideration of the external coupling rate of the resonator.

An object of the present invention is to provide a photon generator capable of generating photons more efficiently than before.

The photon generator according to one aspect of the present invention has a control light generator that generates control light, excites the excited state from the initial state by receiving the control light, and generates photons when transitioning from the excited state to the base state. A cavity QED system including an atom and a resonator that confine a photon and emit it to the outside at a predetermined external coupling rate κ _ex , and the internal loss rate of the cavity QED system is κ _in , and the atom and photon Let g be the bond rate, γ be the decay rate of the induced polarization of the atom, C _in = g ² / 2κ _in γ, and α be the predetermined correction factor, and the outer bond rate κ _ex is κ _ex = ακ _in (1). + 2C _in ) ^(1/2) is set to be satisfied.

By considering the outer coupling rate, photons can be generated more efficiently than before.

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

Hereinafter, embodiments of the present invention will be described in detail. In the drawings, the components having the same function are given the same number, and duplicate description is omitted.

The photon generator includes a control light generation unit 1, a cavity QED system unit 2, and an external coupling rate control unit 3. The photon generator is a device that generates a single photon using a so-called CQED system (Cavity Quantum ElectroDynamics). For details of the photon generator using the CQED system, refer to, for example, Reference 1 and Reference 2.

<Reference 1> Hayato Goto, Shota Mizukami, Yuuki Tokunaga, Takao Aoki, "Fundamental Limit on the Efficiency of Single-Photon Generation Based on Cavity Quantum Electrodynamics", arXiv: 1808.10077, August 30, 2018
<Reference 2> Kazuhiro Hayasaka, "Deterministic Single Photon Generation by Resonator Quantum Electrodynamics", Laser Research, September 2003 The control light generator 1 is the control light for controlling the cavity QED system part 2. To generate. The generated control light is incident on the atoms of the cavity QED system unit 2.

Cavity QED system part 2 receives control light, excites 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>. It includes a resonator 21 that traps photons and emits them to the outside at a predetermined external coupling rate κ _ex . Here, the atom may be a natural atom or an artificial atom such as a quantum dot or a superconducting qubit. Photons generated from an atom are strongly confined in the resonator 21 and interact with the atom. The photons are then emitted to the outside according to the outer binding rate κ _ex .

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

Atoms in the initial state | u> are excited to the excited state | e> by receiving control light. Ω is the transition frequency between the initial state | u> and the excited state | e>, which is generated by applying the control light. Δe is the detuning corresponding to the excited state | e>, for example 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>, for example, Δu = Ω-(E _{| e>} -E _{| u>} -Δe). E _{| u>} is the energy level of the excited state | u>.

Atoms in the excited state | e> generate photons when transitioning to the ground state | g>. Atoms in the excited state | e> are attenuated at a predetermined natural attenuation factor γ. γ can also be said to be the decay rate of induced polarization. g is the bond rate between the atom and the photon.

The generated photons are confined in the resonator 21. The resonator 21 is, for example, two mirrors facing each other. In this case, the outer coupling ratio κ _ex is the transmittance of one of the two mirrors constituting the resonator 21. The resonator 21 may be a micro ring, a microphone toroid, a microsphere, or a photonic crystal resonator instead of the two mirrors.

κ _in is the internal loss rate of the cavity QED system part 2, and can be said to be the attenuation rate of the photoelectric field in the cavity QED system part 2. In addition, κ = κ _ex + κ _in .

The outer bond rate control unit 3 controls the outer bond rate κ _ex . When the resonator 21 is two mirrors, the external coupling rate control unit 3 can change the transmittance of the mirror by, for example, applying tension or heat to the mirror, whereby the external coupling rate κ. You can control _ex .

Specifically, the outer bond rate control unit 3 controls so that the outer bond rate κ _ex satisfies κ _ex = ακ _in (1 + 2C _in ) ^(1/2) . Where C _in = g ² / 2κ _in γ. Here, α is a correction coefficient that is a predetermined real number. The correction coefficient α is appropriately determined so as to obtain a desired result. α is, for example, 1/2 <α <2. Theoretically, α = 1, but in reality, it is necessary to adjust the value of κ _ex appropriately so that the desired result can be obtained. α is a correction coefficient for making this adjustment. By setting the outer binding rate κ _ex = ακ _in (1 + 2C _in ) ^(1/2) , photons can be generated more efficiently than before (see, for example, Reference 1).

It is known that the photon generation probability P _S follows the following equation.

Considering that κ = κ _ex + κ _in and C = g ² / (2κγ), as κ _ex increases, the value of the first term of the above equation increases and the value of the second term decreases. Therefore, it is assumed that there is an appropriate value of κ _ex that maximizes the upper limit of P _s . The inventors have found that the appropriate value of kappa _ex that maximizes the upper limit of P _s is _{κ ex = ακ in (1 +} 2C in) (1/2). See reference 1, for example, for more information on why a suitable κ _ex value is κ _ex = ακ _in (1 + 2C _in ) ^(1/2) .

The state transition in the cavity QED system can also be described as shown in FIG. 3, for example. The state | φ ₀ > is | φ ₀ > = | u> | 0>, the atom is in the initial state | u>, and there is no photon in the resonator 21. The state | φ ₁ > is | φ ₁ > = | e> | 0>, the atom is in the excited state | e>, and there are no photons in the resonator 21. The state | φ ₂ > is | φ ₂ > = | g> | 1>, the atom is in the ground state | g>, and the photon is present in the resonator 21. The state | φ ₃ > is | φ ₃ > = | g> | 0>, the atom is the ground state | g>, and the photon has gone out of the resonator 21. There are no photons in 21. The state | φ ₀ > and the state | φ ₁ > are reversible. Similarly, the state | φ ₁ > and the state | φ ₂ > are reversible. On the other hand, the state | φ ₂ > and the state | φ ₃ > are irreversible. In other words, it is possible to transition from state | φ ₂ > to state | φ ₃ >, but not from state | φ ₃ > to state | φ ₂ >.

The overall 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 weight is a complex amplitude corresponding to the state.

In this case, the rate of change of the weights α ₀ , α ₁ and α ₂ can be described as follows. Where i is an 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. This enables photon generation even in the intermediate region (see, for example, Reference 1). The definition of the intermediate region will be described later.

Therefore, the control light generation unit 1 may control the increase rate of Ω so as to satisfy the following equation (1) with T as 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 resonator 21.

In Non-Patent Document 1, photons are generated in the weakly bound region, and in Non-Patent Document 2, photons are generated in the strongly bound region, but photons are not generated in the intermediate region. Here, the weak binding region is a region where g << κ, and the strong binding region is a region where g> κ, γ. The intermediate region is the region where g to κ. Here, "~" are about the same value, in other words, they are not far apart. For example, if the values on the left and right sides of "~" are separated by one digit, these values can be said to be approximately the same, but if these values are separated by three digits, these values are It can be said that they are not almost the same. Also, "<<" means that the value to the left of "<<" is sufficiently smaller than the value to the right of "<<".

On the other hand, by controlling the increase rate of Ω so that the control light generation unit 1 satisfies the above equation (1), in other words, by making the increase rate of the control light sufficiently slow, photons are also generated in the intermediate region. It can be generated (see, for example, Reference 1).

The control light generation unit 1 may control the rate of increase of Ω so as to satisfy the following equation (2) with T as a predetermined time. This enables more stable photon generation in the intermediate region.

[Modification example]
Although the embodiments of the present invention have been described above, the specific configuration is not limited to these embodiments, and even if the design is appropriately changed without departing from the spirit of the present invention, 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 order described, but may also be executed in parallel or individually as required by the processing capacity of the device that executes the processes.

For example, the photon generator does not have to include the outer coupling rate control unit 3. In this case, it is assumed that the outer binding rate κ _ex is initialized to κ _ex = ακ _in (1 + 2C _in ) ^(1/2) .

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

Claims (3)

A control light generator that generates control light,
It excites from the initial state to the excited state by receiving the control light, traps the atom that generates a photon when transitioning from the excited state to the ground state, and the photon, and emits it to the outside with a predetermined external coupling rate κ _ex. Cavity QED system including resonator and
Including
Wherein the internal loss factor kappa _in cavity QED systems unit, the coupling ratio of the said atoms photons and g, the attenuation rate of the induced polarization of the atoms and gamma, and ^{_{C in = g 2 / 2κ in}} γ, predetermined The outer coupling rate κ _ex is set to satisfy κ _ex = ακ _in (1 + 2C _in ) ^(1/2) , where α is the correction coefficient of.
Photon generator. The photon generator according to claim 1.
External coupling ratio control unit that the external coupling factor kappa _ex is controlled to satisfy the _{κ ex = ακ in (1 +} 2C in) (1/2),
A photon generator that further comprises. The photon generator according to claim 1 or 2.
Let T be a predetermined time, let α ₂ (t) be the weight at time t corresponding to the state where the atom is in the ground state and the photon exists in the resonator 21, and κ = κ _ex + κ. _In, the attenuation factor of the photoelectric field in the cavity QED system is γ, the transition frequency between the initial state and the excited state generated by applying control light is Ω, and C = g ² / ( As 2κγ), the rate of increase in Ω is controlled so as to satisfy the following equation.

Photon generator.

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