Single photon generation device and single photon generation method
By employing a four-level atom resonator with controlled laser beams, the generation of mixed-state single photons is minimized, enhancing quantum coherence and device accuracy.
Patent Information
- Application Number
- JP2024023284
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2024-02-19
- Publication Date
- 2025-08-29
AI Technical Summary
When generating single photons using three-level atoms, the single photon state becomes a mixed state rather than a pure state, leading to degraded quantum coherence.
A resonator that traps a four-level atom is used, with two control laser beams irradiated to suppress the transition to a mixed state, thereby generating single photons with high coherence.
The rate at which single photons become mixed is reduced, improving the quantum coherence and accuracy of devices using these photons.
Smart Images
Figure 2025126852000001_ABST
Abstract
Description
[Technical Field]
[0001] The present invention relates to the generation of single photon states (hereafter referred to as single photon generation). [Background technology]
[0002] Single-photon generation is one of the essential elements for maximizing the quantum nature of light in optical quantum communication, quantum cryptography, and quantum computing. One method for single-photon generation is to use cavity quantum electrodynamics (QED). In this method, a single photon is emitted into the cavity when an atom trapped in the cavity transitions from an excited state to a ground state. By extracting the photon into a waveguide connected to the cavity, a coherent single-photon pulse can be generated on the waveguide.
[0003] Among the methods for generating single photons using a resonator, the method using three energy levels in an atom (called a three-level atom) is expected to be a promising single photon generation method because it is possible to shape the pulse shape of the single photon. This method is disclosed in, for example, Non-Patent Document 1. [Prior art documents] [Non-patent literature]
[0004] [Non-Patent Document 1] GS Vasilev, D. Ljunggren, and A. Kuhn, "Single photons made-to-measure", New J. Phys. 12, 063024 (2010). Summary of the Invention [Problem to be solved by the invention]
[0005] However, when generating single photons using three-level atoms, the single photon state becomes a mixed state rather than a pure state, which causes a problem of degraded quantum coherence.
[0006] The present invention has been made in view of the above points, and aims to provide a technique for reducing the rate at which single photons generated become mixed when single photons are generated using a resonator. [Means for solving the problem]
[0007] According to the disclosed technology, there is provided a resonator that traps a four-level atom having a first ground level, a first excited level, a second ground level, and a second excited level, A first laser beam coupled to a transition between the first ground level and the second excited level, and a second laser beam coupled to a transition between the first excited level and the second excited level are irradiated onto the four-level atom, thereby generating a single photon. A single photon generating device is provided. [Effects of the Invention]
[0008] According to the disclosed technology, when single photons are generated using a resonator, it is possible to reduce the rate at which the generated single photons are in a mixed state. [Brief explanation of the drawings]
[0009] [Figure 1] FIG. 1 illustrates single-photon generation using a three-level atom. [Figure 2] FIG. 1 illustrates single-photon generation using a four-level atom. [Figure 3] FIG. 3 is a configuration diagram of a quantum computing system 300. [Figure 4] FIG. 2 illustrates an example of the hardware configuration of a computer. DETAILED DESCRIPTION OF THE INVENTION
[0010] Hereinafter, an embodiment of the present invention (the present embodiment) will be described with reference to the drawings. The embodiment described below is merely an example, and the embodiment to which the present invention is applied is not limited to the following embodiment.
[0011] The configuration and operation of the device according to this embodiment will be described in detail below.
[0012] (About resonators) In this embodiment, a resonator 10 is used, and therefore, first, the resonator 10 will be described with reference to Fig. 1. As shown in Fig. 1, the resonator 10 in this embodiment has a configuration in which two high-reflectivity mirrors 11 and 12 face each other. A single atom is trapped as an atomic quantum bit inside the resonator 10 shown in Fig. 1.
[0013] Of the mirrors 11 and 12 that make up the resonator 10, mirror 11 has a slight transparency, allowing photons from a waveguide (or optical fiber) to be incident on the resonator 10 via mirror 11 and reflected (output) from the resonator 10.
[0014] In this embodiment, the resonator uses two mirrors as shown in Fig. 1, but this is just one example. For example, a resonator using a nano-optical fiber may also be used.
[0015] (Single photon generation) As mentioned above, among the methods for generating single photons using the resonator 10, the method using a three-level atom is expected to be a promising single photon generation method because it is possible to shape the pulse shape of the single photon.
[0016] The basic principle of this method will be explained with reference to Figure 1. As shown in Figure 1, an atom with three Λ-type levels |u>, |e>, and |f> is trapped in a resonator 10. As shown in Figure 1, the |u>-|e> transition resonates with the control laser, and the |e>-|f> transition resonates with the resonator mode. The resonator mode is connected to the waveguide mode.
[0017] First, the atom is in the initial state |u>, and is then irradiated with a control laser from the outside to transition to the excited state |e>. After that, the atom interacts with the cavity mode of the cavity 10, transitioning to |f>, and a single-photon state is generated in the cavity 10.
[0018] Finally, the single photon in the resonator 10 is generated as a single photon pulse in the waveguide. At this time, the shape of the single photon pulse can be shaped by changing the amplitude of the irradiated laser light over time.
[0019] (About the assignment) In the single photon generation by the three-level atom described above, there is a process in which the atom is excited to the state |e> and then de-excited back to the initial state |u>. From this point, the atom is re-excited and a single photon is generated. However, if this process exists, the single photon state becomes a mixed state rather than a pure state, and quantum coherence deteriorates. This reduction in coherence leads to a decrease in the accuracy of devices in future applications. Therefore, a mechanism to suppress this re-excitation process is needed.
[0020] (Overview of the Technology Relating to the Embodiments) In this embodiment, a four-level atom is used and an additional control laser beam is irradiated in the technology for generating single photons using the resonator 10. This solves the above-mentioned problem and makes it possible to generate single photon pulses with high coherence.
[0021] (Example) An embodiment of the technology using the four-level atom will now be described. Figure 2 shows a resonator 10 in this embodiment. The configuration of the resonator 10 itself is the same as that shown in Figure 1.
[0022] A configuration including the resonator 10 may be called a single-photon generating device. In addition to the resonator 10, the single-photon generating device may also include a laser light irradiation device (a light source, an optical system, etc.) for irradiating atoms with a control laser light. Furthermore, the single-photon generating device does not necessarily have to include a laser light irradiation device as long as it is configured to be able to irradiate the resonator 10 with a control laser light.
[0023] In this embodiment, as shown in FIG. 2, an atom having four levels |u>, |e1>, |e2>, and |f> is trapped in a resonator 10. The four-level atom has an additional |e2> level compared to a three-level atom. Two control laser beams are irradiated onto the four-level atom. The two control laser beams are referred to as control laser beam 1 and control laser beam 2.
[0024] |u> may be called the first ground level, |e1> may be called the first excited level, |e2> may be called the second excited level, and |f> may be called the second ground level.
[0025] The |u>-|e2> transition is coupled to the control laser light 1 with a coupling constant Ω(t), the |e1>-|e2> transition is coupled to the control laser light 2 with a coupling constant Ω2, and the |e1>-|f> transition is coupled to the cavity mode with a coupling constant g. The cavity mode is coupled to the waveguide mode with a coupling constant κ ex are connected.
[0026] Also, the detuning between the |u>-|e2> transition and the control laser light 1 is Δ1, and the detuning between the |e1>-|f> transition and the resonator mode is Δ2.
[0027] The excited state |e2> is γ u The relaxation rate from |e2> to other levels is assumed to be negligibly small. The relaxation rate of the excited state |e1> to the level |u> is assumed to be negligibly small, and the relaxation rates to other levels are collectively γ o To suppress the generation of mixed states, γu It is necessary to suppress the relaxation from |e2> to |u>, which is expressed by the rate
[0028] <Single photon generation and manipulation> Here, the operation for generating single photons using control laser beam 1 and control laser beam 2 will be described.
[0029] The atoms are constantly irradiated with control laser light 2. Then, the atoms are irradiated with control laser light 1, the intensity of which is varied over time according to the desired photon pulse shape.
[0030] The above operation causes the atomic state to transition from |u> to |f>, generating a photon.
[0031] <Probability of generating a single photon in a mixed state> In the single photon generation using the above four-level atom, the probability of generating a mixed-state single photon, that is, the probability of an unwanted photon generation process occurring, is calculated as follows: The Hamiltonian of the four-level atom is expressed by the following equation (1).
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[0035] From here on, we assume that the intensity of the control laser light is weak and that the occupation probability of the excited states |e1> and |e2> is always small. Under these assumptions, it becomes possible to solve the quantum master equation in an approximate form using the effective operator formalism. The effective operator formalism is described in "F. Reiter and AS Sorensen. "Effective operator formalism for open quantum systems", Phys. Rev. A, 85, 032111 (2012)." The density matrix after a sufficient time has passed can be expanded as shown in equation (3) below.
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[0039] (Example of system configuration using a single photon generator) An example configuration of a quantum computing system 300 using a single-photon generating device is shown in Figure 3. A "quantum computing system" may also be called a "quantum computer."
[0040] 3, quantum computing system 300 includes a quantum computing device 100 and a control device 200. The quantum computing device 100 includes, for example, a single-photon generating device and a plurality of resonators (a group of resonators) in which atoms are trapped. A quantum gate between a photon quantum bit and an atomic quantum bit is realized by making a photon generated by the single-photon generating device enter the resonator and causing the photon and the atom to interact quantum mechanically.
[0041] In the example shown in FIG. 3, the single-photon generating device includes a resonator and a laser light irradiator.
[0042] The control device 200 performs quantum computation by transmitting control signals to the quantum computing device 100 to instruct photon generation, measurement, manipulation, etc., and obtaining measurement results from the quantum computing device 100. The control device 200 can be realized by a classical computer.
[0043] The quantum computing device 100 is not limited to a device that uses actual atoms. For example, the quantum computing device 100 may be a simulator that simulates single-photon generation using four-level atoms, a quantum gate between a photon qubit and an atomic qubit, or the like on a classical computer.
[0044] In this case, the quantum computing device 100 may be a classical computer such as a PC, or may be a virtual machine on the cloud. When the quantum computing device 100 is a simulator, the quantum computing device 100 may be provided inside the control device 200. That is, in this case, the quantum computing system 300 may be configured with a classical computer.
[0045] (Classical computer configuration example) As described above, when a classical computer is used as the quantum computing device 100, the control device 200, or the quantum computing system 300, the device (quantum computing device 100, the control device 200, or the quantum computing system 300) can be realized by having the classical computer execute a program. This classical computer may be a physical computer or a virtual machine on the cloud. Hereinafter, the classical computer will be referred to as a "computer."
[0046] That is, the device can be realized by executing a program corresponding to the processing performed by the device using hardware resources such as a CPU and memory built into a computer. The program can be recorded on a computer-readable recording medium (such as a portable memory) and stored or distributed. The program can also be provided via a network such as the Internet or email.
[0047] Fig. 4 is a diagram showing an example of the hardware configuration of the computer. The computer in Fig. 4 includes a drive device 1000, an auxiliary storage device 1002, a memory device 1003, a CPU 1004, an interface device 1005, a display device 1006, an input device 1007, an output device 1008, and the like, all of which are interconnected via a bus BS. The computer may further include a GPU.
[0048] A program for realizing processing on the computer is provided by a recording medium 1001 such as a CD-ROM or a memory card. When the recording medium 1001 storing the program is set in the drive device 1000, the program is installed from the recording medium 1001 to the auxiliary storage device 1002 via the drive device 1000. However, the program does not necessarily have to be installed from the recording medium 1001, but may be downloaded from another computer via a network. The auxiliary storage device 1002 stores the installed program as well as necessary files, data, etc.
[0049] The memory device 1003 reads and stores a program from the auxiliary storage device 1002 when an instruction to start the program is received. The CPU 1004 realizes the functions related to the device in accordance with the program stored in the memory device 1003. The interface device 1005 is used as an interface for connecting to a network, etc. The display device 1006 displays a GUI (Graphical User Interface) or the like according to the program. The input device 1007 is composed of a keyboard, mouse, buttons, a touch panel, etc., and is used to input various operation instructions. The output device 1008 outputs the results of calculations.
[0050] (Summary of implementation form, effects, etc.) As described above, the technique described in this embodiment can reduce the rate at which the generated single photons are mixed when single photons are generated using a resonator.
[0051] The following additional notes are provided regarding the above-described embodiments.
[0052] <Additional Notes> (Additional note 1) a resonator that traps a four-level atom having a first ground level, a first excited level, a second ground level, and a second excited level; A first laser beam coupled to a transition between the first ground level and the second excited level, and a second laser beam coupled to a transition between the first excited level and the second excited level are irradiated onto the four-level atom, thereby generating a single photon. Single photon generator. (Additional note 2) The single photon is generated by the transition of the state of the four-level atom from the first ground level to the second ground level. Item 1. A single-photon generating device according to claim 1. (Additional note 3) The coupling constant between the transition between the first excitation level and the second excitation level and the second laser light is increased to reduce the rate at which mixed-state single photons are generated. 3. The single-photon generating device according to claim 1 or 2. (Additional note 4) A single-photon generating method in a single-photon generating device including a resonator trapping a four-level atom having a first ground level, a first excited level, a second ground level, and a second excited level, comprising: A first laser beam coupled to the transition between the first ground level and the second excited level and a second laser beam coupled to the transition between the first excited level and the second excited level are irradiated onto the four-level atom, thereby generating a single photon. Single photon generation method.
[0053] Although the present embodiment has been described above, the present invention is not limited to such a specific embodiment, and various modifications and changes are possible within the scope of the gist of the present invention described in the claims. [Explanation of symbols]
[0054] 10 resonator 11, 12 Mirror 100 Quantum computing device 200 control device 300 Quantum Computing System 1000 Drive Device 1001 Recording media 1002 Auxiliary storage device 1003 Memory device 1004 CPU 1005 Interface device 1006 Display device 1007 Input Device 1008 Output Device
Claims
1. a resonator trapping a four-level atom having a first ground level, a first excited level, a second ground level, and a second excited level; A single photon is generated by irradiating the four-level atom with a first laser beam coupled to a transition between the first ground level and the second excited level and a second laser beam coupled to a transition between the first excited level and the second excited level. Single photon generator.
2. The single photon is generated by the state of the four-level atom transitioning from the first ground level to the second ground level. The single-photon generating device according to claim 1 .
3. The coupling constant between the transition between the first excitation level and the second excitation level and the second laser light is increased to reduce the rate at which mixed-state single photons are generated. The single-photon generating device according to claim 1 or 2.
4. A single-photon generating method in a single-photon generating device including a resonator trapping a four-level atom having a first ground level, a first excited level, a second ground level, and a second excited level, the method comprising: A first laser beam coupled to the transition between the first ground level and the second excited level and a second laser beam coupled to the transition between the first excited level and the second excited level are irradiated onto the four-level atom, thereby generating a single photon. Single photon generation method.