JP2010114336A - Quantum computing element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a one-photon polarization based on a π phase modulation between a one-photon polarization state having a high calculation success probability and a “exciton system coupled to a resonator mode” in consideration of integration and miniaturization A quantum operation element for a two-level system of states and quantum boxes is provided.
SOLUTION: There are provided a three-dimensional resonator in which a point defect mode of a two-dimensional photonic crystal is resonantly coupled with a Gaussian mode of a Fabry-Perot resonator, and means for resonantly coupling the two-level system of the three-dimensional resonator and a quantum box. A Q value of the three-dimensional resonator is configured to change a reflectance of the Fabry-Perot resonator and / or a circular hole closest to a point defect of the two-dimensional photonic crystal. The diameter can be controlled from a predetermined size and / or by changing the position of the center of the circular hole from the position of a predetermined lattice point.
[Selection] Figure 1

Description

  The present invention relates to a quantum operation element combining a quantum mechanical two-level system capable of optical transition and a three-dimensional optical resonator structure, and more particularly, a phase relaxation rate of a two-level system of a quantum box, three-dimensional. The present invention relates to a quantum arithmetic element in which the relaxation rate of an optical resonator and the magnitude relationship between the three components of a two-level system of a quantum box and a coupling coefficient of a three-dimensional resonator are controlled.

  A qubit is an innovative concept of a basic unit of information, and can express not only 1 and 0 but also a quantum mechanical superposition state of 1 and 0 unlike a classical bit. This superposition state has a characteristic that the bit value is not essentially determined until the bit value is measured. Encryption communication (quantum encryption communication) using the absolute uncertainty is one of the excellent applications of the concept of qubits.

  In the implementation of quantum cryptography, qubit information is encoded into the polarization or phase state of one photon. And a photon is transmitted using an optical fiber. However, since there is optical loss in the optical fiber, it is considered difficult to extend the distance exceeding 100 km for quantum key distribution. Three approaches have been considered to address this issue.

  (1) Increase the quantum efficiency of the photodetector. In the quantum key distribution experiment in the communication wavelength band, an avalanche photodiode photodetector is mainly used because the quantum efficiency is actually as low as 10%.

  (2) Develop a messenger single photon source with high luminous efficiency. This is because in the quantum key distribution experiment in the communication wavelength band, weak light at a single photon level is generated by applying laser light to an attenuator and used as a photon source, so that the influence of light loss in the optical fiber is significant.

  (3) Implement quantum relay by entanglement swapping. Although there is a limit to the long distance according to the above (1) and (2), by introducing this method, in principle, any long distance can be achieved.

  In the field of quantum information technology, theoretical and experimental researches are actively conducted with these three approaches to long-range quantum cryptography key distribution as the main axis (see Non-Patent Document 1 below).

The quantum relay of (3) described above requires a function (π phase modulation function) that modulates the phase of these combined systems by π according to the polarization state of the photon and the state of the two-level atomic system. To do. In the quantum information technology field, this π phase modulation is called quantum phase shift calculation (the following Non-Patent Document 2). This quantum phase shift operation is an operation that modulates the phase of the quantum state of the target bit by π when the bit values of the control qubit and the target qubit are 1. That is, in terms of an expression, | 1> _control | 1> _target → Exp [iπ] | 1> _control | 1> operation for conversion to _target .

  Which physical degree of freedom of which physical system is used as a qubit depends on the purpose of the application. For example, one-photon polarization degree of freedom (longitudinal polarization and transverse polarization) is used as a qubit for transmitting information, and an excited state and a ground state of an atomic system are used for storing information.

  The combined system of the resonator mode of the optical resonator (Fabry-Perot resonator composed of two spherical mirrors) and the quantum box can be used as the above-described quantum phase shift computing element. The quantum box only needs to include a two-level system capable of optical transition. As the quantum box, for example, a diamond quantum-vacancy center (NV center) [see the following Non-Patent Document 5] or a semiconductor quantum box is used. Below, although the case of a semiconductor quantum box is explained, it is not limited to a semiconductor.

  A semiconductor quantum box is composed of a discrete energy level structure in the conduction band and an energy level structure in the valence band. An optical transition occurs between the conduction band and the valence band, and this transition generates excitons. An exciton is a pair of an electron and a hole. Electrons are generated in the conduction band and vacancies are generated in the valence band. A state before exciton is generated is called a crystal ground state. In particular, a system composed of a crystal ground state (| G>) and an exciton state (| E>) is called an exciton system.

  Then, the optical resonator resonantly couples the resonator mode and the exciton system of the semiconductor quantum box to change the resonance frequency of the resonator if the exciton system is in the crystal ground state (| G>). In the field of quantum optics, this change is referred to as vacuum labis printing (see Non-Patent Document 3 below). As a result of this change, the one-photon absorption of the resonator is suppressed, and the incident one-photon is reflected by the spherical mirror constituting the resonator. At that time, the phase of the incident one photon changes by π (Non-patent Document 2). It is assumed that the center frequency of one incident photon is adjusted to match the frequency of the resonator mode when the exciton system is not resonantly coupled to the resonator mode.

  On the other hand, if the exciton system is in the exciton state (| E>), the resonance frequency of the resonator mode does not change, and the phase of the incident one photon does not change. From the above, it is noted that the optical transition of the exciton system is polarization-dependent, and the combined system of the optical resonator and the semiconductor quantum box changes the photon phase according to the state of the exciton system and the polarization state of the photon. It functions as a quantum phase shift computing element that changes by π.

Here, considering that the time of the quantum mechanical unitary interaction between the semiconductor quantum box and the photon is limited by the phase relaxation time T ₂ of several hundred pico order peculiar to the nano size quantum box, the unitary The optical interface, that is, the optical resonator, must be designed so that the interaction takes place in the order of several tens to several pico orders.

Normally, in order to maximize the function as a quantum phase shift operation element of the synthesis system of an optical resonator and a semiconductor quantum box, a resonator having a high Q value (see Non-Patent Document 4 below) is designed and phase relaxation is performed. It tends to seem that the resonator relaxation time 1 / κ longer than the time T ₂ must be secured, but this makes the phase relaxation of the exciton system faster than the resonator relaxation, and the quantum phase shift The operation fails.

For this reason, the electric field density is increased abnormally only at the position where the semiconductor quantum box is placed, thereby increasing the coupling strength g between the resonator mode and the exciton system, and the resonator relaxation from the phase relaxation time T _2. The optical resonator must be designed so that a sufficient interaction for quantum phase shift calculation can be performed even when the time (1 / κ) is short. That is, it is necessary to design an optical resonator that satisfies the inequality, g ≧ κ >> (1 / T ₂ ). This relational expression is a necessary condition for performing the quantum phase shift calculation. At this time, κ must be at least 40 times greater than 1 / T ₂ . Furthermore, in order to bring the success probability of the quantum phase shift operation closer to 1, the optical resonator must be designed so that the coupling efficiency between the incident photon and the resonator approaches 1. Here, the Q value is an index representing the performance index of the resonator, and this value increases as the optical loss of the resonator decreases. Further, Q = (1 / κ) × 2π × light velocity (C) × (1 / λ) is established between the Q value and the resonator relaxation time (1 / κ). λ is the optical transition wavelength of the exciton system.

In addition, a resonator satisfying g >> 1 / T ₂ is a quantum phase shift between a single-photon source with high emission efficiency useful for long-range quantum information communication and a photon-exciton system. It is conceivable to apply to a quantum memory necessary for synchronizing between a photon and an exciton system when performing an operation.

A quantum memory is a device for storing quantum information. In particular, the optical memory is a quantum memory specialized for storing optical information, and can be used for storing classical information as well as quantum information.
JP 2008-135591 A P. van Look et. Al., Physical Review Letters, Vol. 96, 240501 (2006). L. -M. Duan et. Al. , Physical Review Letters, Vol. 92, 127902 (2004). R. J. Thompson et. Al. , Physical Review A, Vol. 57, pp. 3084-3104 (1998) M. Shirane. et. al. , Journal of Applied Physics, Vol. 101, 073107 (2007). Xing-Fei He et. Al. , Journal of Applied Physics, Vol. 72, 211 (1992).

The conventional problem is that the quantum phase shift operation cannot be completed for the photon polarization state and the exciton state in a shorter time than the phase relaxation time T ₂ of the two-level system of the semiconductor quantum box. . This is because, in an existing resonator, the resonator mode and the exciton system are strongly coupled (coupling in which the relaxation rate κ of the resonator is smaller than the coupling strength g between the resonator mode and the exciton system). This is because the relaxation time (1 / κ) of the vessel is made longer than the phase relaxation time T ₂ . This is also because the coupling efficiency between the resonator and the incident photon is low.

  In view of the above situation, the present invention considers integration and miniaturization, and has a high probability of success in calculation. A π phase modulation between a one-photon polarization state and a “exciton system coupled to a resonator mode” state. It is an object to provide a quantum operation element (quantum phase shift operation element) for a one-photon polarization state and a two-level system of a quantum box based on the above.

  Another object of the present invention is to provide a quantum operation element having the functions of a single photon source and an optical memory useful for extending the distance of quantum information communication as an application of the quantum phase shift operation element according to the present invention.

In order to achieve the above object, the present invention provides
[1] A three-dimensional resonator in which a point defect mode of a two-dimensional photonic crystal is resonantly coupled with a Gaussian mode of a Fabry-Perot resonator, and means for resonantly coupling the two-level system of the three-dimensional resonator and a quantum box. The Q value of the three-dimensional resonator changes the reflectance of the Fabry-Perot resonator and / or the diameter of a circular hole closest to the point defect of the two-dimensional photonic crystal Can be controlled by changing from a predetermined dimension and / or changing the position of the center of the circular hole from the position of a predetermined lattice point. The quantum box may be a crystal including a two-level system capable of optical transition, and is not limited to a semiconductor. For example, it is possible to apply the NV center of diamond.

  [2] The quantum arithmetic device according to [1], further including an optical waveguide, wherein a spatial mode profile of light emission from the quantum box exciton can be controlled by the three-dimensional resonator, and the three-dimensional resonance The coupling efficiency of the emitted light to the optical waveguide can be controlled by optimizing the shape of the optical device and / or the optical waveguide. The Fabry-Perot resonator can be composed of a semiconductor multilayer film or a dielectric multilayer film.

  [3] In the quantum operation element according to [1] or [2], the Fabry-Perot resonator is configured by two spherical mirrors, and one of the two spherical mirrors is on the same substrate as the two-dimensional photonic crystal. It is characterized by being a semiconductor multilayer film fabricated in the above.

[4] In the quantum arithmetic device according to [1], [2], or [3], the coupling strength g between the two-level system of the quantum box and the point defect mode of the two-dimensional photonic crystal is 3 The relaxation rate κ of the ground mode of the three-dimensional resonator is greater than or similar to the relaxation rate κ of the three-dimensional resonator, and the phase relaxation rate 1 / T of the two-level system of the quantum box If it is greater than ₂ , that is, if the inequality g ≧ κ >> 1 / T ₂ is satisfied, the quantum operation element has a function of a quantum phase shift operation. This condition is made possible by the resonant coupling of the point defect fundamental mode of the two-dimensional photonic crystal resonator and the fundamental mode of the Fabry-Perot resonator.

[5] In the quantum arithmetic device according to [1], [2], or [3], the coupling strength g between the two-level system of the quantum box and the point defect mode of the two-dimensional photonic crystal is 3 When the relaxation rate κ of the three-dimensional resonator is smaller than the relaxation rate κ of the fundamental mode of the three-dimensional resonator, and larger than the phase relaxation factor 1 / T ₂ of the two-level system of the quantum box, that is, In the case where inequality κ> g >> 1 / T ₂ is satisfied, when g is smaller than κ / 2, the quantum arithmetic element emits light from an excited state in the direction of the resonator axis of the Fabry-Perot resonator. It has the function of a single photon source with directivity. This condition is made possible by the resonant coupling of the point defect fundamental mode of the two-dimensional photonic crystal resonator and the fundamental mode of the Fabry-Perot resonator.

[6] In the quantum arithmetic device according to [1], [2], or [3], the coupling strength g between the two-level system of the quantum box and the point defect mode of the two-dimensional photonic crystal is 3 When the relaxation rate κ of the three-dimensional resonator is smaller than the relaxation rate κ of the fundamental mode of the three-dimensional resonator, and larger than the phase relaxation factor 1 / T ₂ of the two-level system of the quantum box, that is, In the case where the inequality κ> g >> 1 / T ₂ is satisfied, the quantum arithmetic element has a point in a state where the Gaussian mode of the Fabry-Perot resonator and the point defect mode of the two-dimensional photonic crystal are resonantly coupled. By absorbing the light in the defect region and then changing the physical resonator length of the Fabry-Perot resonator, detuning the Gaussian mode of the Fabry-Perot resonator from the point defect mode of the two-dimensional photonic crystal, Light quantum It has a function as an optical memory that holds a state in an atomic quantum state.

  [7] The quantum operation element according to any one of [1] to [6], wherein the quantum box is a semiconductor quantum dot exciton system or a diamond NV center.

  According to the present invention, the following effects can be achieved.

  (1) Since a single quantum operation is completed with high calculation accuracy in a time shorter than the phase relaxation time of the semiconductor quantum box of about 1 nanosecond, quantum operations with high repetition efficiency can be performed.

  (2) Since the quantum arithmetic element of the present invention can be made into a solid element by using a semiconductor quantum box, a Bragg reflection type dielectric multilayer mirror and a photonic crystal slab, it can be miniaturized and integrated.

  The quantum arithmetic element of the present invention resonates a two-dimensional system of a three-dimensional resonator in which a point defect mode of a two-dimensional photonic crystal is resonantly coupled with a Gaussian mode of a Fabry-Perot resonator, and the three-dimensional resonator and the quantum box. A quantum operation element having means for coupling, wherein the Q value of the three-dimensional resonator changes a reflectance of the Fabry-Perot resonator and / or is closest to a point defect of the two-dimensional photonic crystal The diameter of the circular hole is changed from a predetermined size and / or the center position of the circular hole can be controlled by changing from the position of the predetermined lattice point.

  Hereinafter, embodiments of the present invention will be described in detail.

  A first embodiment of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a configuration diagram of a quantum arithmetic element according to a first embodiment of the present invention, FIG. 2 is a configuration diagram of a point defect type two-dimensional photonic crystal layer of the present invention, and FIG. FIG. 2B is a sectional view thereof. FIG. 3 is an explanatory diagram of a method of changing the Q value of the point defect type two-dimensional photonic crystal resonator of the quantum arithmetic element of the present invention, and FIG. 4 is a diagram illustrating the dielectric / It is explanatory drawing of the spherical mirror comprised by a semiconductor multilayer film.

  Referring to FIG. 1, the embodiment of the present invention comprises optical waveguides 1 and 5, two spherical mirrors 2 and 4 constituting a Fabry-Perot resonator, and a point defect type two-dimensional photonic crystal layer 3. Yes. The point defect type two-dimensional photonic crystal layer 3 is on a plane parallel to the XY plane, and the axis of the Fabry-Perot resonator is arranged perpendicular to the point defect type two dimensional photonic crystal layer 3, that is, in the Z direction. .

  As shown in FIG. 2A, the point defect type photonic crystal layer 3 has a structure in which circular holes 11 are arranged in a triangular lattice pattern with respect to the semiconductor thin film 10. This periodicity creates a forbidden band for light propagating in the XY plane, and light of a certain wavelength cannot propagate. If one of the circular holes 11 is removed in this state, it becomes a point defect 12 that disturbs the periodicity. Then, the forbidden band light cannot propagate in the plane and stays at the point defect 12. This is called a point defect type optical localized mode (point defect mode) here.

  The six circular holes 11 ′ closest to the point defect 12 have a diameter changed from a predetermined size and / or a center position of the circular holes 11 ′ has been changed from a predetermined lattice point position. . As a result, a point defect mode with very small light leakage to the outside of the point defect type two-dimensional photonic crystal layer 3 is formed in the forbidden band, and the spatial profile thereof is localized in the quantum box formation region 13 (the above-mentioned patent). Reference 1).

On the other hand, the fundamental mode of the Fabry-Perot resonator composed of the two spherical mirrors 2 and 4 is a Gaussian mode, and the aforementioned point defect type two-dimensional photonic crystal layer 3 is formed at a position where the beam waist is minimized. ing. If the physical resonator length of the Fabry-Perot resonator is L and the curvature radii of the spherical mirrors 2 and 4 are R ₁ and R ₂ , respectively,
0 ≦ (1-L / R ₁ ) × (1-L / R ₂ ) ≦ 1
Is satisfied. This relationship suppresses light loss in the direction perpendicular to the Z axis.

  Further, the physical resonator length L can be shortened to a little less than 1.5 times the wavelength λ. If the length is shorter than this length, the fundamental mode of the Fabry-Perot resonator cannot be approximated only by the Gaussian mode.

  As shown in FIG. 4, the spherical mirrors 2 and 4 have a structure in which two types of dielectric / semiconductor films having different refractive indexes are alternately laminated in a spherical shape. The dielectric / semiconductor film 14 has a higher refractive index than the dielectric / semiconductor film 15. The dielectric / semiconductor films 14 and 15 are laminated to a thickness such that the optical length in the film is λ / 4. However, the wavelength λ is the transition wavelength of the quantum box exciton system.

  Furthermore, the point defect mode of the point defect type two-dimensional photonic crystal layer 3 includes a fundamental mode (Gaussian mode) of a Fabry-Perot resonator formed in the Z-axis direction, and a point defect type two-dimensional photonic of this point defect mode. The crystal layer 3 is resonantly coupled through a vertical leak mode (electromagnetic field leaching component).

  The dielectric / semiconductor films 14 and 15 to be laminated are so arranged that the Q value of the Fabry-Perot resonator is much lower than the Q value of the point defect mode of the point-defect type two-dimensional photonic crystal without the Fabry-Perot resonator. By adjusting the number of. The Q value is an index representing the performance index of the resonator, and this value increases as the optical loss of the resonator decreases. This promotes light emission through the spherical mirrors 2 and 4 which are dielectric / semiconductor multilayer films due to the coupling between the leak mode and the Gaussian mode in the point defect mode. That is, the Q value of the synthesis system using the point defect mode and the Fabry-Perot resonator can be controlled by adjusting the number of dielectric / semiconductor films 14 and 15 to be stacked, and there is no Fabry-Perot resonator. The Q value of the point defect mode can be made much lower, and is comparable to the Q value of the Fabry-Perot resonator alone.

  The coupling strength between the point defect mode and the Gaussian mode of the Fabry-Perot resonator changes the diameter of the circular hole 11 ′ closest to the point defect 12 from a predetermined size and / or changes the position of the center of the circular hole 11 ′. By changing from the position of the predetermined lattice point, it is possible to control by changing the depth of the point defect mode out of the point defect type two-dimensional photonic crystal layer 3 in the leak mode. The relative change in the ratio of the leaking mode in the point defect mode can be determined by measuring the Q value when there is no Fabry-Perot resonator.

  This will be described using a specific example. Assuming that the wavelength of light is 1.55 μm of the communication band, the semiconductor thin film is made of GaAs (refractive index 3.4) with a thickness of 320 nm. Air holes having a radius r = 130 nm are arranged in a two-dimensional triangular lattice pattern. The center interval between adjacent circular holes is a = 400 nm. A plan view of the point defect type two-dimensional photonic crystal layer is shown in FIG.

  In this point defect mode, in order to change the Q value, the six circular holes 11 ′ closest to the Fabry-Perot resonator are shifted outwardly from the original positions derived from the periodicity indicated by the broken lines by s, and these circles The radius r ′ of the hole 11 ′ is smaller than the radius r of the other circular holes 11. FIG. 3B shows the result of simulating the Q value change when the shift amount to the outside of the center of the circular hole is changed under the condition of s = r−r ′. A maximum Q value of 33,000 is obtained at a shift amount s = 0.09a = 36 nm.

  Although there is actually a manufacturing error, it is difficult to obtain an ideal calculated Q value, but Q = 17,000 is realized in a microresonator structure manufactured in a situation close to the design value of the above parameters. (Non-Patent Document 3). Accordingly, the Q value can be controlled within a range of about two digits from about 300 for the lower side to about 17,000 for the higher side. That is, the resonant coupling strength of the Gaussian mode and the point defect mode of the Fabry-Perot resonator can be controlled within a range of about two digits.

As a result of the simulation by the time domain difference method, the radius of curvature of the spherical mirrors 2 and 4 of the Fabry-Perot resonator that is resonantly coupled to the point defect mode is 36 μm with respect to the above point defect type two-dimensional photonic crystal layer 3. The physical resonator length L is 1.87 μm at the minimum. However, it is a result when Ta ₂ O ₅ (refractive index 2.0411) and SiO ₂ (refractive index 1.455) are alternately laminated as a dielectric film constituting the multilayer mirror. In order to make the Q value of the Fabry-Perot resonator lower than the Q value of the point defect mode, when a total of five layers are stacked, a three-dimensional resonator (point-defect type two-dimensional photonic crystal + Fabry-Perot type resonance) The relaxation rate κ of the vessel was approximately 200 GHz. Note that Q = relaxation time (1 / κ) × 2π × light velocity (C) × (1 / λ) holds between the resonator relaxation time (1 / κ) and the Q value. The Q value when 200 GHz is approximately 6000. This value is approximately one third of the Q value of the point defect mode in the absence of the Fabry-Perot resonator. This result means that the Q value of the point defect mode is lowered due to the resonance coupling between the Gaussian mode and the point defect mode of the Fabry-Perot resonator.

  At this time, since the cross-sectional mode profile of the light emitted from the Fabry-Perot resonator is a Gaussian mode, the emitted light and the optical waveguides 1 and 5 (such as optical fibers) are changed by changing the waveguide shape. The coupling efficiency can be controlled.

On the other hand, when the Q value of the Fabry-Perot resonator is higher than the Q value of the point-defect mode when there is no Fabry-Perot resonator, the center frequency of the point-defect mode is caused by resonance coupling with the Fabry-Perot resonator. It often happens that it splits into two frequencies. However, this frequency splitting effect is not necessary for the purpose of efficiently interacting the external light and the exciton system via the point defect mode in a time shorter than the phase relaxation time T _{2 of the} exciton system. In addition, the interaction time required for the computation may be longer than the phase relaxation time. Therefore, it is desirable that the Q value of the Fabry-Perot resonator is lower than the Q value of the point defect mode.

  Now, as shown in FIG. 2, the semiconductor quantum box 13 is disposed substantially at the center of the point defect 12 in the XY and Z directions. This is because the exciton of the semiconductor quantum box 13 and the point defect mode interact most strongly where the electromagnetic field strength of the ground mode of the point defect 12 is the largest.

  The exciton system of a semiconductor quantum box has an energy level structure that is qualitatively similar to an atom. The point defect mode is resonantly coupled with the exciton system. When the spatial extent of the point defect ground mode in the XY direction is about 1 μm, the magnitude of the resonant coupling strength g is 20 D (Debye) and the refractive index of the semiconductor quantum box is 3.4. When it is, it becomes about 200 GHz. The resonance coupling strength g≈200 GHz is a resonance coupling strength sufficient for the excitons to interact unitarily with the light because the phase relaxation rate inherent to the semiconductor quantum box exciton system is about 5 GHz. The resonance coupling strength g was calculated using the formula shown in the following formula (1).

ここで、ｇは半導体量子箱励起子系と定在波共振器モードの共鳴結合強度、ｖは半導体量子箱励起子系の遷移周波数、μは半導体量子箱励起子の双極子モーメント、ε₀ は真空での誘電率、ｈはプランク定数、Ｖ_m は共振器定在波モードの体積、ｎは半導体量子箱の屈折率、ｒは励起子の発生する位置座標、Ψ_m （ｒ）は共振器定在波モード関数であり、∫｜Ψ_m （ｒ）｜² ｄｒ＝Ｖ_m である。

Where g is the resonance coupling strength between the semiconductor quantum box exciton system and the standing wave resonator mode, v is the transition frequency of the semiconductor quantum box exciton system, μ is the dipole moment of the semiconductor quantum box exciton, and ε ₀ is Dielectric constant in vacuum, h is Planck's constant, V _m is volume of resonator standing wave mode, n is refractive index of semiconductor quantum box, r is position coordinate where exciton is generated, Ψ _m (r) is resonator It is a standing wave mode function and ∫ | Ψ _m (r) | ² dr = V _m .

The resonator standing wave mode function Ψ _m (r) is standardized so that the maximum value is 1, and the three-dimensional resonator according to the present invention (point defect two-dimensional photonic crystal layer + fabry-perot resonator) Then, it becomes maximum at the position of the semiconductor quantum box 13. If the spatial extent of the point defect mode in the XY direction is about 1 μm and the physical resonator length L is about 1 to 2 μm, the value of this function at the position of the semiconductor quantum box 13 is the point defect in the Fabry-Perot resonator. More than 10 times larger than that of the region that is not the two-dimensional photonic crystal layer 3. With this feature, even if the relaxation rate κ of the three-dimensional resonator of the present invention is about 200 GHz, a relatively large resonance coupling strength g = 200 GHz can be achieved. However, the quantum box may be a crystal including a two-level system capable of optical transition, and is not limited to the semiconductor described above. For example, an impurity level such as the NV center of diamond may be used instead of the semiconductor quantum box.

From the above, the resonance coupling strength g and the relaxation rate κ of the three-dimensional resonator are 1 / T _{2 to} 5 GHz at ₂₀₀ GHz. That is, this corresponds to a three-dimensional resonator that satisfies the necessary condition g ≧ κ >> 1 / T ₂ of the quantum phase shift calculation. Furthermore, since the mode profile of the three-dimensional resonator is a Gaussian mode, the coupling efficiency of the light emitted from the three-dimensional resonator is brought close to 100% by appropriately designing the shapes of the optical waveguides 1 and 5 such as optical fibers. It is possible to remarkably increase the quantum phase shift repetition calculation efficiency.

Further, functions other than the quantum phase shift calculation included in the quantum arithmetic element according to the present invention are a function as a single photon source and a function as an optical memory. The necessary condition for having these functions is κ> g >> 1 / T ₂ .

  Hereinafter, the features of the quantum arithmetic device according to the present invention as a single photon source and as an optical memory will be described.

  Note that the point defect mode has a very high optical confinement effect in the XY plane direction. Since the semiconductor quantum box excitons are resonantly coupled to the point defect mode, the exciton-specific radiation relaxation process occurs in the XY plane direction. It is suppressed. Therefore, when the coupling strength g between the semiconductor quantum box exciton and the point defect mode is less than or equal to half of the relaxation rate κ of the three-dimensional resonator, the quantum arithmetic element of the present invention has a point defect mode vacuum rabbi. Since splitting disappears, the quantum operation element has a function as a single photon source in which the light emitted from the exciton has a strong directivity in the direction of the resonator axis of the Fabry-Perot resonator.

  In addition, light is absorbed in the point defect region in a state where the Gaussian mode and the point defect mode of the Fabry-Perot resonator are resonantly coupled, and then the physical resonator length L is changed to change the Gaussian mode of the Fabry-Perot resonator. When the resonance frequency is detuned from the resonance frequency of the point defect mode, these modes do not resonate with each other. As a result, light can be confined in the point defect region, so that the quantum arithmetic element according to the present invention becomes a quantum arithmetic element having a function as an optical memory.

  Here, the manufacturing method of the quantum arithmetic element which shows 1st Example of this invention is demonstrated easily using FIG.

  (1) An appropriate buffer layer is laminated (not shown) on the semiconductor substrate 21 and then the semiconductor thin film 10 constituting the point defect type two-dimensional photonic crystal layer 3 is laminated. The semiconductor quantum box 13 is also included in this part. Further, a sacrificial layer 25 is laminated thereon (FIG. 5A).

  (2) A mesa structure 31 is formed on the sacrificial layer 25 by using a lithography technique and a dry etching process (FIG. 5B).

  (3) After the mesa structure 31 is formed, the same material as the sacrificial layer 25 is stacked. When about 1 μm is laminated, the original angular shape gradually becomes smooth and approaches the convex lens shape. This becomes the convex forming layer 32 [FIG. 5 (c)].

  (4) In order to obtain a desired reflectance by alternately laminating two kinds of dielectrics (high refractive index dielectric 23 and low refractive index dielectric 22) having different refractive indexes on the convex forming layer 32. A multilayer film 33 is formed by laminating the required number of layers. The film thickness is λ / 4n (n is the refractive index of each layer), where λ is the exciton transition wavelength. Further, a buffer layer 34 having a sufficient thickness is stacked so as to withstand the following process [FIG. 5 (d)].

  (5) Next, the semiconductor substrate 21 is removed by a technique such as etching. In FIG. 5 (e) and below, FIG. 5 (d) is shown upside down.

  (6) Periodic holes are formed in the semiconductor thin film 10 using a lithography technique and a dry etching process. Further, the lower sacrificial layer 25 and the convex shape forming portion 32 of the same material are removed by a wet etching process to create a cavity 35 [FIG. 5 (f)].

  (7) In the processes up to (6) above, a structure in which the dielectric spherical mirror and the photonic crystal layer are integrated is formed, but if there is no photonic crystal layer portion, the structure is only the spherical mirror. Therefore, by combining these two structures, a structure in which a photonic crystal layer is embedded between spherical mirrors can be obtained [FIG. 5 (g)].

  From the above, in the quantum arithmetic element showing the first embodiment of the present invention, the quantum mechanical relationship between the external light necessary for the quantum phase shift calculation and the quantum box exciton system is shorter than the phase relaxation time of the exciton. It is configured to complete unitary interaction. Therefore, it is possible to perform quantum computation with high efficiency and high computation accuracy.

  In addition, when the coupling strength g between the semiconductor quantum box exciton and the point defect mode is less than half of the relaxation rate κ of the three-dimensional resonator, the quantum arithmetic element of the present invention has a vacuum defect in the point defect mode. Since splitting disappears, the quantum operation element has a function as a single photon source in which the light emitted from the exciton has a strong directivity in the direction of the resonator axis of the Fabry-Perot resonator.

  Further, in a situation where the Gaussian mode and the point defect mode of the Fabry-Perot resonator are resonantly coupled, after the light is absorbed in the point defect region, the physical resonator length L is changed, and the Gaussian mode of the Fabry-Perot resonator is changed. When the resonance frequency is detuned from the resonance frequency of the point defect mode, these modes do not resonate with each other, and light can be confined in the point defect mode. Therefore, the quantum operation element according to the present invention has a quantum operation functioning as an optical memory. It becomes an element.

  Next, a second embodiment according to the present invention will be described with reference to FIG.

  In the second embodiment, one of the spherical mirrors constituting the Fabry-Perot resonator in the first embodiment is formed on a multilayer film 24 formed on the same substrate as the point defect type two-dimensional photonic crystal layer 3. Has been replaced. Since the point defect type two-dimensional photonic crystal layer 3 and one of the mirrors are integrated, the mechanical stability is high.

  In this structure, since one mirror constituting the Fabry-Perot resonator is a plane mirror made of the multilayer film 24, the operating conditions are different from those of the Fabry-Perot resonator according to the first embodiment configured by the two spherical mirrors 2 and 4. Is different. However, by appropriately setting the reflectivity and position of the two mirrors, the beam waist of the resonator mode can be designed to be minimal in the vicinity of the position of the quantum box.

  A method for fabricating the quantum arithmetic element structure of the second embodiment will be briefly described with reference to FIG.

  Two kinds of dielectrics (low refractive index dielectrics) 22 and high refractive index dielectrics 23 having different refractive indexes are alternately stacked on the semiconductor substrate 21. The film thicknesses are λ / 4n (n is the refractive index of each layer), where λ is the exciton transition wavelength. After the number of layers necessary for obtaining a desired reflectance is stacked to form a multilayer film 24, a sacrificial layer 25 is stacked and the semiconductor thin film 10 constituting the point defect type photonic crystal layer 3 is stacked.

  Subsequently, by combining a lithography technique and a dry etching process, the circular holes 11 periodically arranged with respect to the semiconductor thin film 10 are formed to form a photonic crystal structure. Further, only the sacrificial layer 25 is selectively removed by a wet etching process. When the semiconductor thin film 10 is made of GaAs and the sacrificial layer 25 is made of AlGaAs, this process can be performed by using a hydrofluoric acid solution.

  In addition, this invention is not limited to the Example mentioned above, A various change is possible in the range shown to the claim. That is, embodiments obtained by combining technical means appropriately modified within the scope of the claims are also included in the technical scope of the present invention.

  The quantum arithmetic element according to the present invention can be applied to light-light switching between light having a weak single photon level, and can be miniaturized and integrated, so that it can be used as a space-saving / power-saving optical signal processing apparatus. Applicable to. Further, since the exciton system is three-dimensionally confined in the resonator, the direction of light emission can be controlled, and the present invention can be applied to a single photon source with high directivity. Furthermore, the three-dimensional confinement can be applied to an application such as an optical memory.

It is a block diagram of the quantum arithmetic element which shows 1st Example of this invention. It is a block diagram of the point defect two-dimensional photonic crystal layer of this invention. It is explanatory drawing of the method of changing Q value of the point defect type two-dimensional photonic crystal resonator of the quantum arithmetic element of this invention. It is explanatory drawing of the spherical mirror comprised by the dielectric material / semiconductor multilayer film of the quantum arithmetic element which shows 1st Example of this invention. It is sectional drawing of the manufacturing process of the quantum arithmetic element which shows 1st Example of this invention. It is sectional drawing of the quantum arithmetic element which shows 2nd Example of this invention. .

Explanation of symbols

DESCRIPTION OF SYMBOLS 1,5 Optical waveguide 2,4 Spherical mirror which comprises Fabry-Perot type resonator 3 Point defect type two-dimensional photonic crystal layer 10 Semiconductor thin film 11, 11 'Circular hole 12 Point defect 13 Semiconductor quantum box 14 High refractive index semiconductor / dielectric 15 Low refractive index semiconductor / dielectric 21 Semiconductor substrate 22 Low refractive index dielectric 23 High refractive index dielectric 24 Multilayer film (plane mirror)
25 Sacrificial layer 31 Mesa structure 32 Convex forming layer 33 Multilayer film (spherical mirror)
34 Buffer layer 35 Cavity

Claims (7)

Quantum operation comprising a three-dimensional resonator in which a point defect mode of a two-dimensional photonic crystal is resonantly coupled with a Gaussian mode of a Fabry-Perot resonator, and means for resonantly coupling the two-level system of the three-dimensional resonator and a quantum box The Q value of the three-dimensional resonator changes a reflectance of the Fabry-Perot resonator and / or sets a diameter of a circular hole closest to the point defect of the two-dimensional photonic crystal to a predetermined value. A quantum computing element configured to be controlled by changing the size and / or changing the position of the center of the circular hole from the position of a predetermined lattice point. 2. The quantum arithmetic device according to claim 1, further comprising an optical waveguide, wherein a spatial mode profile of light emission from the quantum box excitons can be controlled by the three-dimensional resonator, and the three-dimensional resonator and / or A quantum arithmetic element configured to control the coupling efficiency of the light emission to the optical waveguide by optimizing the shape of the optical waveguide. 3. The quantum arithmetic element according to claim 1, wherein the Fabry-Perot resonator is composed of two spherical mirrors, and one of the two spherical mirrors is formed on the same substrate as the two-dimensional photonic crystal. A quantum computing element characterized by being a film. 4. The quantum operation element according to claim 1, wherein the coupling strength g between the two-level system of the quantum box and the point defect mode of the two-dimensional photonic crystal is a relaxation of the fundamental mode of the three-dimensional resonator. If the relaxation rate κ of the fundamental mode of the three-dimensional resonator is larger than or similar to the rate κ and larger than the phase relaxation rate 1 / T ₂ of the two-level system of the quantum box, that is, When the inequality g ≧ κ >> 1 / T ₂ is satisfied, the quantum operation element has a function of a quantum phase shift operation. 4. The quantum operation element according to claim 1, wherein the coupling strength g between the two-level system of the quantum box and the point defect mode of the two-dimensional photonic crystal is a relaxation of the fundamental mode of the three-dimensional resonator. When the relaxation rate κ of the ground mode of the three-dimensional resonator is smaller than the rate κ and larger than the phase relaxation rate 1 / T ₂ of the two-level system of the quantum box, that is, the inequality κ> g >> 1 / In the case where T ₂ is satisfied, when g is smaller than κ / 2, the quantum arithmetic element is a single photon source whose light emission from the excited state has a strong directivity in the resonator axial direction of the Fabry-Perot resonator A quantum arithmetic element having the following function. 4. The quantum operation element according to claim 1, wherein the coupling strength g between the two-level system of the quantum box and the point defect mode of the two-dimensional photonic crystal is a relaxation of the fundamental mode of the three-dimensional resonator. When the relaxation rate κ of the ground mode of the three-dimensional resonator is smaller than the rate κ and larger than the phase relaxation rate 1 / T ₂ of the two-level system of the quantum box, that is, the inequality κ> g >> 1 / In the case where T ₂ is satisfied, the quantum arithmetic element absorbs light in the point defect region under a situation where the Gaussian mode of the Fabry-Perot resonator and the point defect mode of the two-dimensional photonic crystal are resonantly coupled. Then, by changing the physical resonator length of the Fabry-Perot resonator and detuning the Gaussian mode of the Fabry-Perot resonator from the point defect mode of the two-dimensional photonic crystal, the quantum state of light is reduced. A quantum computing element having a function as an optical memory for holding a quantum state of a child system. The quantum operation element according to claim 1, wherein the quantum box is a semiconductor quantum dot exciton system or an NV center of diamond.

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