JP3984240B2 - Optical quantum gate and operation method thereof - Google Patents

Description

  The present invention relates to a photon gate capable of greatly controlling the quantum state of weak controlled light with weak control light and an operation method thereof.

  2. Description of the Related Art In recent years, quantum information processing that performs information processing and communication by giving information to a quantum mechanical superposition state has been actively studied, and devices for practical use have been developed. As physical entities that carry quantum information, electromagnetic systems such as photons and matter systems such as nuclear spins are used. In particular, photons are convenient for carrying quantum information over long distances while suppressing deterioration of the quantum state called decoherence, and are considered to be optimal for information media in quantum communication. A quantum computer using photons has also been proposed.

  In the quantum information processing using these photons, a photon gate that changes the quantum state of the second light by the first light is necessary. For example, a quantum phase gate that changes the phase of the second light according to the intensity of the first light. However, in quantum communication and quantum computation using photons, it is necessary to control the quantum state of weak light (ideally, single photons) with weak light, which requires huge nonlinearity. Therefore, it has been extremely difficult to realize a photon gate capable of inducing a necessary and sufficient quantum state change in the second light.

  Under these circumstances, several methods have been proposed for inducing huge nonlinearities while avoiding losses due to absorption, using a physical phenomenon called electromagnetically induced transparency (EIT). In these methods, in the resonance region, the linear susceptibility that contributes to absorption cancels with quantum interference due to EIT, while the nonlinear susceptibility increases or takes a large value due to resonance. Properties that are advantageous for efficient use are utilized. H. Schmidt et al. Have proposed a light-based quantum phase gate utilizing this property (Non-patent Document 1). Thereafter, various methods improved from the method of Schmidt et al. Have been proposed.

  However, these methods assume a very simple substance that only needs to consider four energy states related to light. Actually, a substance consists of a group of physical systems such as atoms, ions, and molecules that have slightly different energy values corresponding to each physical system, and the distribution of transition energy has a width called a non-uniform width. Exists. In addition, there are many other energy states in the vicinity of the energy state involved in EIT. Since such an extra state causes absorption loss without causing EIT, it is still considered difficult to realize a quantum gate without loss according to the proposal of H. Schmidt et al. In particular, a solid material advantageous for device formation generally has a large non-uniform width and a complicated energy state structure. Therefore, it has been considered that it is very difficult to realize a quantum gate without loss.

  On the other hand, optical quantum gates or quantum computers that use only linear optical elements without relying on the nonlinearity of materials have been proposed, and recently, the number of qubits (quantum bits are units representing information in quantum information processing) has been extended. Some methods have been proposed and basic experiments have been conducted. However, this method requires a high-efficiency photon detector, a fast feedback system, etc. to introduce “non-linearity by human hand” instead of using the nonlinearity of the material, and the operation is stochastic. It has technical difficulties such as.

As described above, the conventional method for realizing the optical quantum gate has a problem that it is difficult to use a large non-linearity without a loss due to absorption in an actual system, and solves these problems. No realistic method has been known in the past.
H. Schmidt and A. Imamoglu, Opt. Lett. 21, 1936 (1996).

  The purpose of the present invention is to use a group of physical systems (atoms, ions, molecules, etc.) that interact with light, where the transition energy between energy states differs between individuals due to non-uniform widths. An object of the present invention is to provide an optical quantum gate capable of suppressing loss due to light absorption and greatly controlling the quantum state of weak controlled light with weak control light, and an operation method thereof.

The photon gate according to one embodiment of the present invention has at least four energy states (here, the four energy states are set as | 1>, | 2>, | 3>, | 4> in order from the lowest energy). A physical system group including a physical system and _a light A having an angular frequency ω _a that resonates with a transition between | 1> and | 3> and | 2> and | 3> A first light source system that generates light C having an angular frequency ω _c that resonates with a transition between and a control light B having an angular frequency ω _b that does not resonate with any of the two energy states. And the controlled light S having the same angular frequency ω _a as the light C and the light A or having an angular frequency band included in the angular frequency band of the light A. And before irradiating the control light B and causing the control light B to act on the controlled light S and performing the gate operation, the physical system group is irradiated with the light A and the light C in this order, And a control unit including an intensity control circuit for controlling the intensity of the light A and the light C.

The operation method of the photon gate according to another aspect of the present invention includes at least four energy states (where the four energy states are | 1>, | 2>, | 3>, | 4> in order from the lowest energy). For some physical systems, light A, | 2>, and | 3 with angular frequency ω _a that resonates with the transition between | 1> and | 3> Are irradiated in this order with light C having an angular frequency ω _c that resonates with a transition between> and the physical system group has the same angular frequency ω _a as that of light C and light A, or the angle of light A Irradiating the controlled light S having an angular frequency band included in the frequency band and the control light B having the angular frequency ω _b that does not resonate with any of the two energy states, the controlled light B A gate operation is performed by causing control light B to act on S.

In the embodiment of the present invention, “the controlled light S having the same angular frequency ω _a as the light A or having an angular frequency band included in the angular frequency band of the light A” refers to the central angular frequency ω _{S (c )} And the angular frequency width ΔS is ω _{a (c)} −ΔA ≦ ω _{S (c)} −ΔS and ω _{S (c)} + ΔS ≦ ω _{a (c)} + ΔA (where ω _{a (c)} and ΔA are Each of the controlled lights S satisfies the relationship between the central angular frequency and the angular frequency width of the light A).

  According to the present invention, there is no loss due to light absorption using a real physical system group that has many energy states that resonate with incident light and cause loss due to light absorption. An optical quantum gate capable of greatly controlling the quantum state of control light and a method thereof can be provided.

  Before describing the optical quantum gate of the present invention, first, a quantum gate that does not use EIT and an optical quantum gate that uses EIT by Schmidt et al. Will be described.

FIG. 1 shows a conventional photon gate scheme based on the optical Kerr effect without using EIT. This scheme uses a physical system with at least three energy states (with these energy states in order from the lowest energy | g>, | i>, | u>), and the Kerr effect (cross-phase modulation). using controls the controlled light of the phase of the angular frequency omega _a by the control light of angular frequency omega _b. In order to increase the value of the nonlinearity χ ⁽³⁾ related to the Kerr effect, it is more advantageous to decrease the detuning Δω _a and Δω _b from the state | i> or the state | u>, but the absorption increases. Therefore, the detuning cannot be reduced to a certain extent. Further, in the configuration as it is, self-phase modulation of the controlled light occurs, which also hinders the photon gate operation.

FIG. 2 shows a scheme proposed by Schmidt et al. For avoiding the above problems by using EIT. This scheme uses a physical system (EIT medium) that has at least four energy states (in order from the lowest energy | 1>, | 2>, | 3>, | 4>), EIT is induced by light (coupling light) having an angular frequency ω _c connecting the states | 2> and | 3> and controlled light having an angular frequency ω _a connecting the states | 1> and | 2>. Further, control light having an angular frequency ω _b having a transition between the states | 2> and | 4> and a detuning of Δω _b is incident. Γ ₃ and Γ _{4 in} FIG. 2 are relaxation rates from states | 3> and | 4>, respectively. In this scheme, the absorption of the controlled light disappears due to EIT, so there is no loss of the one-photon process by the medium, and a large phase shift due to the long interaction length in the medium is possible. Further, the high-order nonlinearity of the transition between the states | 1> and | 2> disappears, and self-phase modulation can be avoided. According to the calculation by Schmidt et al., The phase shift φ of the controlled light obtained by this scheme is expressed as in equation (1).

Where L is the length of the medium, N is the density of atoms (atoms, ions, molecules, etc. responsible for absorption in the medium), E _b is the electric field of the control light, μ ₁₃ and μ ₂₄ are the states | 1>, The transition dipole moment between | 3>, | 2>, and | 4>, Ω _c is the Rabbi angular frequency representing the magnitude of the interaction between the coupling light and the atom. Ω _c is expressed as in equation (2).

Where E _c is the electric field of the coupling light, and h is the Planck constant.

In the EIT-inducible medium, the relaxation rate Γ ₂ from state | 2> is very small, so Ω _c is made very small while satisfying the relationship of Ω _c ² > Γ ₂ Γ ₃ which is the condition for EIT induction. be able to. As a result, φ can be made very large.

The source of loss in this scheme is two-photon absorption. The phase shift with respect to the absorbance αL due to the two-photon process becomes the figure of merit of this photon gate, and its value is given by Equation (3).

Equation (3) means that the figure of merit does not depend on the light intensity. Therefore, the figure of merit and Ω _c can be set independently. That is, even with very weak light, a desired phase shift can be obtained while suppressing the loss ratio.
Furthermore, various improved schemes have been proposed based on this scheme.

  By the way, when trying to implement these EIT utilization schemes with actual EIT media, especially solid materials that are advantageous for device development, it is difficult to completely eliminate the loss due to absorption with EIT alone. This is because, in an actual physical system, in many cases, many extra energy states that do not cause EIT contribute to absorption, and absorption loss does not disappear.

In the following, superposition of solids can be realized and EIT is observed (K. Ichimura, K. Yamamoto, N. Gemma, Phys. Rev. A58, 4116 (1998).), Rare earth ions Pr ³⁺ Taking the dispersed oxide crystal Y ₂ SiO ₅ as an example, the state in which absorption loss is not completely eliminated by the conventional method and the mechanism by which the present invention can avoid the absorption will be described.

FIG. 3 shows energy states actually used for EIT observation in Y ₂ SiO ₅ in which Pr ³⁺ ions are dispersed. Consider the case where EIT is executed according to Schmidt's scheme in these states. That is, these states (a part of) correspond to the energy states | 1>, | 2>, and | 3> in FIG.

  As shown in FIG. 3, the upper three states and the lower three states connected by optical transition are each split into three by hyperfine interaction. The interval of the division is several MHz to several tens of MHz in frequency. In addition, although not shown in FIG. 3, the optical transition connecting the upper three states and the lower three states of this material has a variation in transition energy called non-uniform width, and the distribution width is about 10 GHz. .

In order to induce EIT, as shown in the figure, two wavelengths of light connecting the lower two states and the upper one, that is, coupling light and controlled light are irradiated. Since EIT occurs when two-wavelength irradiated light undergoes two-photon resonance, the frequency difference between the coupling light and the controlled light is 17.3 MHz, which corresponds to the energy difference between the two states selected from the three states below. Consider the case of irradiation with setting. Below, the six energy states of the Pr ³⁺ ion shown in FIG. 3 are in order from the lowest energy | 1>, | 2>, | 3>, | 4>, | 5>, | 6> And

  It should be noted that for such materials, the energy difference between states split by hyperfine interactions is less than the non-uniform width of the optical transition. Consider a case in which the frequency of the coupling light and the controlled light is irradiated in accordance with the frequency of the optical transition connecting the lower state and the upper state in FIG. FIG. 3 shows a case where the coupling light and the controlled light manipulate ions that resonate with the transition between the states | 2> and | 6> and the transition between the states | 1> and | 6>, respectively. In fact, as shown in Fig. 4, in addition to the ions that resonate with the transition between states | 2> and | 6> and the transition between states | 1> and | 6>, respectively, between | 2> and | 5> There are also ions that resonate between transitions, | 1>, | 5> transitions, and ions that resonate between | 2>, | 4> transitions, | 1>, | 4> transitions (ions 1, 2 in FIG. 4). , 3). These cause EIT to satisfy the two-photon resonance condition. However, the controlled light resonates with the transition between | 3>-| 4> and there is no transition where the coupling light resonates (ion 4 in FIG. 4), or the controlled light transitions between | 2> and | 4>. There are also ions that contribute to the absorption of controlled light without causing EIT, such as there is no transition in which the coupling light resonates (ion 5 in FIG. 4). Due to the presence of these ions, absorption does not go away.

From the above considerations, the non-uniform width between states | 1>, | 2>, | 3>, or the nonuniform width between states | 4>, | 5>, | 6>, the uniform width of light transitions ( The resonance width inherent to each ion) and the spectral width of the irradiated light are sufficiently small compared to the energy difference between these states, and the population of Pr ³⁺ ions before irradiation is distributed in the three states below. Is assumed. This assumption can be actually satisfied by a method such as irradiating light from a narrow band ring dye laser onto Pr ³⁺ : Y ₂ SiO ₅ cooled to liquid helium temperature (about 4K).

Next, what kind of transition energy (transition frequency) causes EIT, what kind of ion does not cause EIT and causes absorption, or what state the ion that absorbed light moves to, etc. ^Therefore , we consider the distribution of Pr ³⁺ ions on a two-dimensional transition frequency plane with two transition frequencies of two transitions.

EIT is caused by light irradiation that resonates with the two transitions that connect the two most common states with the highest energy state. FIG. 5 shows the larger transition frequency of two transition frequencies in which EIT occurs by irradiating light that resonates with the transition, on the x-axis (horizontal axis), and the smaller transition frequency on the y-axis (vertical axis). FIG. 3 is a diagram schematically showing a range in which Pr ³⁺ ions are distributed in a Y ₂ SiO ₅ crystal. When considering states | 1> to | 6>, the transition pairs that cause EIT by light irradiation are (| 2>-| 4>, | 3>-| 4>), (| 2>-| 5 >, | 3>-| 5>), (| 2>-| 6>, | 3>-| 6>), (| 1>-| 4>, | 2>-| 4>), (| 1 >-| 5>, | 2>-| 5>), (| 1>-| 6>, | 2>-| 6>), (| 1>-| 4>, | 3>-| 4>) , (| 1>-| 5>, | 3>-| 5>), (| 1>-| 6>, | 3>-| 6>). In each parenthesis, a transition with a larger transition frequency is marked on the left. Nine overlapping ellipses in FIG. 5 schematically show the distribution of ions on the transition frequency plane corresponding to each of the above-described transition sets.

The width (distribution) of each ellipse in the x-axis direction corresponds to the non-uniform width of the optical transition on the left side of the corresponding transition set. The width of each ellipse in the y-axis direction corresponds to the non-uniform width of the optical transition on the right side of the corresponding transition set. In addition, 2 ^1/2 times the length of each ellipse in the minor axis direction corresponds to the non-uniform width between the lower two states that are not connected by light among the three states constituting the corresponding transition set. . Actually, the non-uniform width between the lower two states is several tens of kHz, whereas the non-uniform width of the optical transition is about 10 GHz, which is more than 100,000 times different. However, in FIG. 5, the short axis direction of the ellipse representing the distribution is enlarged and the long axis direction is reduced for easy understanding of the overlapping of the ellipses.

FIG. 6 schematically shows the relationship between the distribution of ions on the transition frequency plane and the irradiation light. FIG. 6 shows that the crystal is irradiated with coupling light having a frequency ν _c and controlled light having _a frequency ν _a (ν _a −ν _c = 17.3 MHz). Of the portions A to K indicated by the circles in FIG. 6, only the portion of E in which all the ions contained in the portion become transparent by EIT, and at least some of the ions existing in other regions are 1 It interacts in the same way as when irradiating light of a wavelength. In particular, at least some ions of each of D, F, I, J, and K will absorb the controlled light.

  In the present invention, first, the physical system group constituting the photon quantum gate is irradiated with light A having the same angular frequency as the controlled light or including the angular frequency band of the controlled light. It is desirable that the irradiation time of light A is longer than the lifetime of the upper state, and that the product of the pan plate (excitation probability due to light irradiation for 1 second) of the ion by light A and the irradiation time is greater than 1. desirable. By irradiation with light A, the ions distributed in D, E, F, I, J, and K go through the upper state to the state | 3> or | 2>, | 2> or | 3>, | 3> Or | 1>, | 2> or | 1>, | 1> or | 3>, | 1> or | 2>. After irradiation with this light A, if no other light is irradiated, the absorption of the controlled light disappears or becomes extremely small for all ions due to the movement of the population during the longitudinal relaxation time between the lower states. .

  In the present invention, the coupling light C is irradiated immediately after the light A irradiation. As a result, the ions distributed in A, B, C, G, E, and H are the states | 3> or | 2>, | 2> or | 3>, | 3> or | 1>, or | 2>, respectively. Move to | 1>, | 1> or | 3>, | 1> or | 2>. In the state after the irradiation with the coupling light C, ions that may absorb the coupling light or the controlled light are distributed only in the region E.

  In other words, in the method of the present invention, a physical system group having a large non-uniform width and an extra state is distributed as if only in the E part during the longitudinal relaxation time between the lower states, and almost by EIT. It behaves like a physical system with a narrow non-uniform width that does not have an extra state that is easy to completely eliminate absorption.

  In FIG. 7, the light A is irradiated and then the coupling light C is irradiated or the coupling light is irradiated again within the longitudinal relaxation time between the lower states after the coupling light irradiation is stopped once. The relationship between the energy state of the ions in the state | 1> among the ions distributed in the region E and the light before the controlled light S is incident is shown. There is no ion in the state | 2> in the region E, and those in the state | 3> can ignore the interaction with the irradiation light. Therefore, when discussing the gate operation, only ions in the state | 1> need be considered.

The state of the physical system in which the coupling light C is irradiated and the ions are in the state | 1> is a special case of an EIT state (called a dark state or a population trapping state). The state of EIT is generally expressed as follows.

Here, Ω _c1 (t) and Ω _s (t) are Rabbi angular frequencies representing the strength of interaction between the coupling light and the controlled light and the ions, respectively, and are given by the following equation (5).

However, mu _2u, mu _1u each state | 2>, | 1> and the transition dipole moment between the upper state, E _c1, E _s, respectively coupling light represents the electric field of the controlled light.

The state of the physical system shown in FIG. 7 can be regarded as a special case in which the controlled light intensity is zero (Ω _s (t) = 0) among the EIT states expressed by Equation (4). it can. Therefore, by changing the Ω _s (t) from zero to adiabatic when the controlled light is incident, EIT from the beginning to the end (without paying attention to the intensity and phase of the coupling light) The gate operation can be performed while maintaining this state (JR Kuklinski, U. Gaubatz, FT Hioe, and K. Bergmann, Phys. Rev. A40 (11), 6741 (1989). NT Vitanov and S. Stenholm, Opt. Commun. 135, 394 (1997).). Therefore, the physical system is never excited to the state | 3>, and the gate operation that avoids the absorption loss until the EIT is induced becomes possible. In the case of ions in the general starting state, the controlled light rising from zero is irradiated, and the system is maintained in the EIT state, and the desired coupling light intensity and controlled light intensity EIT state are transferred to the adiabatic state. It is practically impossible from the viewpoint of intensity and phase control, and a loss due to absorption occurs until the state shifts to an EIT state of desired coupling light intensity and controlled light intensity. On the other hand, in the method of the present invention, the related ions are prepared in the state | 1> by the coupling light before the controlled light irradiation. As a result, the state before the input when the controlled light intensity is zero can be automatically set to the EIT state (without paying attention to the light intensity and phase in particular), so the gate operation is started from that state. The adiabatic passage allows gate operation without absorption loss throughout.

  When the incident non-control light is a pulse, if the coupling light intensity is gradually changed after the non-control light is incident on the physical system group, the non-control light is decelerated, and the same control light is caused by a long interaction time. Even with intensity, a larger phase change can be induced in the uncontrolled light.

Embodiments of the present invention will be described below with reference to the drawings.
Example 1
FIG. 8 is a block diagram of the photon gate according to the present embodiment. In this embodiment, substituted Pr ³⁺ to O.O5% of Y ³⁺ ions in Pr ^3+: Y ₂ a SiO ₅ crystal 1 is used as a solid material constituting the quantum gates. Some Pr ³⁺ ions in the crystal are used as a physical system, and nonlinearity for quantum gate operation is developed using at least four energy states. Since a large number of Pr ³⁺ ions are contained in the crystal, a physical system group is formed. This crystal 1 is placed in a cryostat 2 with an optical window and is kept at the liquid helium temperature.

  A ring dye laser 12 excited by an argon ion laser 11 is prepared as a light source (first light source) for initializing the photon gate, and the line width of the output laser light is stabilized to suppress jitter at the center frequency. It is narrowed within 100kHz by the integrated feedback system 13. This laser beam is separated into two by a 50/50 beam splitter 14, one laser beam goes straight and passes through the first optical path, and the other laser beam is reflected by the mirror 15 and reflected in the second optical path. Pass through.

The intensity and the center frequency (wavelength) of the laser light in the first optical path are adjusted by the intensity control and frequency shift acoustooptic elements 16a, and the Pr ³⁺ : Y ₂ SiO ₅ crystal 1 is irradiated as laser light A . The center frequency (wavelength) of the laser light A is set in the vicinity of 605.98 nm which coincides with the center frequency (wavelength) of the controlled light described later within the range of the laser line width. This frequency ν _a resonates with an optical transition between | 1> and | 3> for some Pr ³⁺ ions in the crystal.

The intensity and center frequency (wavelength) of the laser light in the second optical path are adjusted by the acoustooptic device 16c for controlling the intensity and frequency, and the Pr ³⁺ : Y ₂ SiO ₅ crystal 1 is irradiated as the laser light C. The center frequency (wavelength) of the laser beam C is set to the 17.3 MHz lower frequency side than the center frequency (wavelength) of the laser beam A. This frequency ν _c resonates with an optical transition between | 2> and | 3> for some Pr ³⁺ ions in the crystal.

The acousto-optic elements 16a and 16c are configured so that the Pr ³⁺ : Y ₂ SiO ₅ crystal 1 is irradiated with the laser light A and the laser light C at a predetermined timing and intensity before and during the gate operation. Is connected to the intensity control circuit 18.

In the Pr ³⁺ : Y ₂ SiO ₅ crystal 1, the laser light from the second optical path is made thicker so that the laser light C spatially includes the laser light A, and the laser light C and the laser light A are 5 It is adjusted to meet at a small angle within a degree. The laser beam C is trapped by the photon trap 19 after passing through the crystal.

  It should be noted that the light incident surface or the light exit surface of the crystal 1 is provided with an antireflection coating for light emitted or incident, respectively. The first optical path is provided with a beam splitter 20 having a reflectivity of 99% and a transmittance of 1% in order to guide the controlled light from the outside to the region irradiated with the laser light A in the crystal 1. ing. The portion described so far substantially corresponds to the portion surrounded by a broken line in FIG.

A ring dye laser 22 excited by an argon ion laser 21 is prepared as a light source for controlled light, and the line width of the output laser light is narrowed within 100 kHz by a stabilizing feedback system 23 that suppresses jitter at the center frequency. Turn into. The intensity and central angular frequency (wavelength) of this laser light are adjusted by the acousto-optic element 24 for controlling the intensity and frequency, and the central angular frequency (wavelength) is the same within the range of the laser light A and the laser line width. Set to This laser light is separated into two by a 50/50 beam splitter 25, one is used as a reference light through a delay path 26, and the other is Pr ³⁺ : Y via a beam splitter 20 as controlled light S. ₂ Irradiation to SiO ₅ crystal 1.

A ring dye laser 32 excited by an argon ion laser 31 is prepared as a control light source (second light source), and the line width of the output laser light is stabilized and a stabilized feedback system 33 that suppresses jitter at the center frequency. It narrows within 100kHz. This laser light is adjusted in intensity and center frequency (wavelength) by the acoustooptic element 34 for intensity and frequency control, reflected by the mirrors 35 and 36, and irradiated to the Pr ³⁺ : Y ₂ SiO ₅ crystal 1 as the control light B Is done. The center frequency (wavelength) of the control light B is set 300 GHz higher than the controlled light S. This frequency ν _b does not resonate with any transition consisting of two of the four energy states of the Pr ³⁺ ion used for quantum gate operation.

In the Pr ³⁺ : Y ₂ SiO ₅ crystal 1, the control light B is made thicker than the control light S so that the control light B spatially includes the control light S, and the control light B and the control light S are It is adjusted to meet at a small angle within 5 degrees. The control light B is trapped by the photon trap 37 after passing through the crystal.

The controlled light S transmitted through the Pr ³⁺ : Y ₂ SiO ₅ crystal 1 is reflected by the mirror 41, superimposed on the reference light by the 50/50 beam splitter 42, and the intensity after being superimposed by the photodetector 43. By detecting, the phase change of the controlled light S can be detected.

With reference to FIG. 9, the time change of the light irradiation intensity | strength of the photon gate in a present Example is demonstrated. First, the Pr ³⁺ : Y ₂ SiO ₅ crystal 1 was irradiated with 50 W / cm ² laser light A with a beam diameter reduced to 100 μm for 1 ms, and then 50 W / cm ² laser light C with a beam diameter reduced to 1 mm. Was irradiated for 1 ms. 1 ms after the end of the irradiation of laser beam C, Pr ³⁺ : Y ₂ SiO ₅ crystal 1, laser beam C with set value C _g = 2 W / cm ² with the beam diameter reduced to 1 mm, and beam diameter of 100 μm The weakly controlled light S (intensity set value S _g ) whose intensity was reduced by the aperture acoustooptic device 24 was simultaneously irradiated. The rise time of the light intensity at the start of irradiation with the laser light C and the controlled light S was 100 ns. Delaying the work of a series of up to record the output of the photodetector 43 at time T _r of the irradiation after 500μs of the controlled light S, and records the output of the start photodetector 43 from the irradiation of the laser beam A in this state The delay path 26 was fixed at a position where the output from the photodetector 43 was maximized by repeatedly changing the setting of the path 26 little by little.

Next, with the delay path 26 fixed in this state, the series of operations starting from the irradiation of the laser beam A, excluding the setting change of the delay path 26, is performed by reducing the beam diameter to 1 mm and increasing the intensity with the acoustooptic device 34. The test was repeated while irradiating the adjusted weak control light B. During the repetition, when the intensity of the control light B was gradually increased from zero, the output from the photodetector 43 gradually decreased, and increased again when the intensity of the control light B was further increased. This is presumably because the phase of the controlled light S changes depending on the intensity of the control light B. Further, the transmittance of the controlled light S with respect to the Pr ³⁺ : Y ₂ SiO ₅ crystal 1 at the time of output recording of the photodetector 43 was 90% or more. This indicates that a phase gate in which absorption by ions that do not cause EIT is suppressed can be realized.

(Example 2)
An embodiment in which the method of light irradiation to the photon gate is changed will be described. With reference to FIG. 10, the intensity change of the light irradiation of the photon quantum gate in a present Example is demonstrated. First, the Pr ³⁺ : Y ₂ SiO ₅ crystal 1 was irradiated with 50 W / cm ² of laser light A with a beam diameter reduced to 100 μm for 1 ms, and then with 50 W / cm ² of laser light C with a beam diameter reduced to 1 mm. Irradiated for 1 ms. 1 ms after the end of irradiation with laser beam C, laser beam C with a set value C _g = 2 W / cm ² with a beam diameter reduced to 1 mm on Pr ³⁺ : Y ₂ SiO ₅ crystal 1 (the rise time of the light intensity is 100 ns), the beam diameter is reduced to 100 μm, and the weakly controlled light S whose intensity is reduced by the acoustooptic device 24 is started from zero intensity and applied for 500 μs, and the same intensity (set value) as in FIG. 9 (Example 1) Irradiation was performed by gradually increasing the intensity until S _g ). The output of the photodetector 43 at this time _Tr was recorded, and thereafter the same operation as in Example 1 was performed. As a result, as in Example 1, it was confirmed that the output from the photodetector 43 decreased and increased with respect to the intensity of the control light B. Further, the transmittance of the controlled light S with respect to the Pr ³⁺ : Y ₂ SiO ₅ crystal 1 at the time of output recording of the photodetector 43 was 90% or more.

Further, the integrated intensity of the controlled light S transmitted through the crystal by the output recording time _Tr of the photodetector 43 was measured and compared with the case of Example 1. As a result, it was observed that the transmittance of the integrated intensity in this example was higher than that in Example 1. This is considered to be due to the following reasons. That is, in Example 1, the loss due to absorption until the transition to the EIT state is made in the state where the ions are not in the state of causing EIT for the laser light C and the controlled light S immediately after the irradiation of the controlled light S. Occurs. On the other hand, in this example, Pr ³⁺ ions are kept in an EIT state from the time immediately before irradiation of the controlled light S until the start of measurement of 500 μs after irradiation, and the loss due to absorption until EIT occurs. Is reduced.

Incidentally, after the same time, or increases when the continuously increasing the intensity of the control light S from zero to a set value S _g, a lower set value C _g1 to continuously the intensity of the light C from the set value C _g You may make it reduce. When the non-control light S is a pulse, after the pulse is incident on the Pr ³⁺ : Y ₂ SiO ₅ crystal 1, the intensity of the light C is gradually decreased from the set value C _g to the lower set value C _g1 continuously. As a result, the uncontrolled light is decelerated and slowly advanced in the Pr ³⁺ : Y ₂ SiO ₅ crystal 1, thereby causing a larger phase shift in the controlled light S coming out of the crystal with the same control light intensity. I was able to.

The figure for demonstrating the scheme of the conventional optical quantum gate which does not utilize EIT. The figure for demonstrating the scheme of the conventional optical quantum gate using EIT. Pr ^3+: in Y ₂ SiO ₅ crystal, shows an energy state that is used to observe the EIT. The figure showing a mode that the transition which connects between the various states of the ion in a crystal | crystallization resonates with irradiation light due to non-uniform width. The figure which shows typically distribution of the ion on a transition frequency plane. The figure which shows typically the relationship between the distribution of ion on a transition frequency plane, and irradiation light. The figure for demonstrating the state of the energy of the ion involved in a photon gate in the time before performing gate operation | movement about the photon gate of this invention. 1 is a block diagram illustrating a photon gate according to a first embodiment of the present invention. The figure which shows the time change of the laser irradiation intensity | strength in Example 1 of this invention. The figure which shows the time change of the laser irradiation intensity | strength in Example 2 of this invention.

Explanation of symbols

1 ... Pr ³⁺ : Y ₂ SiO ₅ crystal, 2 ... cryostat, 11 ... argon ion laser, 12 ... ring dye laser, 13 ... stabilized feedback system, 14 ... beam splitter, 15 ... mirror, 16a, 16c ... acoustic optics Element, 17 ... Control unit, 18 ... Intensity control circuit, 19 ... Photon trap, 20 ... Beam splitter, 21 ... Argon ion laser, 22 ... Ring dye laser, 23 ... Stabilized feedback system, 24 ... Acoustic optical element, 25 ... Beam splitter, 26 ... Delay path, 31 ... Argon ion laser, 32 ... Ring dye laser, 33 ... Stabilized feedback system, 34 ... Acousto-optic element, 35, 36 ... Mirror, 37 ... Photon trap, 41 ... Mirror, 42 ... beam splitter, 43 ... photodetector.

Claims (5)

A physical system group including a physical system having at least four energy states (here, the four energy states are set as | 1>, | 2>, | 3>, | 4> in order from the lowest energy);
Light A of angular frequency ω _a that resonates with the transition between | 1> and | 3> and angular frequency ω that resonates with the transition between | 2> and | 3> with respect to a part of the physical system. a first light source system that generates light C of _c ;
A second light source system that generates the control light B of the angular frequency ω _b that does not resonate with any transition consisting of two of the four energy states;
The physical system group is irradiated with controlled light S and control light B having the same angular frequency ω _a as the light C or the light A or having an angular frequency band included in the angular frequency band of the light A, and controlled An intensity control circuit that controls the intensity of the light A and the light C by irradiating the physical system group with the light A and the light C in this order before performing the gate operation by causing the control light B to act on the light S. And a control unit including the optical quantum gate.   The photonic gate according to claim 1, wherein the physical system has an oxide crystal in which rare earth ions are dispersed. For a physical system group including a physical system having at least four energy states (where the four energy states are set as | 1>, | 2>, | 3>, | 4> in order from the lowest energy) Light A of angular frequency ω _a that resonates with the transition between | 1> and | 3> and light C with angular frequency ω _c that resonates with the transition between | 2> and | 3> for some physical systems In this order,
In the physical system group, the light C, the controlled light S having the same angular frequency ω _a as the light A or the angular frequency band included in the angular frequency band of the light A, and the four energy states Operation of a photon gate characterized by irradiating the control light B with an angular frequency ω _b that does not resonate with any of the two transitions and performing the gate operation by causing the control light B to act on the controlled light S Method. After irradiation with the light C and the light A in this order to the physical system, set the intensity of the light C to the set value C _g, continuously the intensity of the controlled light S from zero to a set value S _g The physical system that resonates with the irradiation light in the physical system group is maintained in an adiabatic state in which the energy states | 1> and | 2> that do not absorb the light C are superimposed. Item 4. A method of operating a photon gate according to Item 3. Wherein after simultaneously or increased when the continuously increasing intensities of the controlled light S from zero to the set value S _g, continuously the intensity of light C to a lower set value C _g1 from the set value C _g 5. The method of operating an optical quantum gate according to claim 4, wherein the number is reduced.

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