JP6630302B2 - Quantum memory device - Google Patents

Description

  The present invention relates to a quantum memory used for quantum information processing. More specifically, the present invention relates to a technique for improving the security of information stored in a quantum memory.

  Information processing technology has been making remarkable progress, but its performance is said to be approaching its theoretical limit. Under such circumstances, a new information processing method based on the principle of quantum mechanics is expected to realize ultra-large capacity communication and ultra-high performance computer exceeding the performance limit based on the conventional classical information processing technology. . Conventionally, information processing based on quantum mechanics uses a stochastic state called “superposition” that is both “0” and “1” instead of the definitive state of “0” or “1”. It is said that it is possible to realize an extremely high-speed operation and an encryption communication that is secure in principle, which is orders of magnitude higher than that of a computer of a type. In realizing the new information processing technology described above, the key is to develop a quantum device that utilizes the properties unique to quantum mechanics.

  One of the most basic and important quantum devices is a quantum memory. The quantum memory is required to stably hold the above-described stochastic quantum state for a long time, and intensive research is being conducted along with a quantum gate for operating the quantum state. Various types of quantum memories have been developed so far, and for example, there is a type that stores a photon (Photon) state (Non-Patent Documents 1 to 3). In addition, any of atoms, superconducting quantum circuits, semiconductor quantum dots, and the like are generally considered as qubits, can store a quantum state, and can be used as a quantum memory.

  More specifically, research has been conducted on a quantum memory that stores (records) and reproduces a quantum state by coupling two stable quantum states with a resonating electromagnetic wave (Non-Patent Document 4, Non-Patent Document 5). As described above, the biggest feature of the quantum memory that is different from the classical memory is that the quantum memory stores the quantum superposition state. Although research on such a quantum memory is still in its infancy, improvement of confidential security of stored information is one of the important research subjects as in the case of conventional information processing technology.

Nicolas Gisin, et al .: Quantum storage of photonic entanglement in a crystal. Nature 469, 508-511 (2011) M. D. Lukin, et al .: Room-Temperature Quantum Bit Memory Exceeding One Second. Science 336, 1283-1286 (2012) Wolfgang Tittel, et al .: Broadband waveguide quantum memory for entangled photons.Nature 469, 512-515 (2011) Alexander I. Lvovsky, Barry C. Sanders, and Wolfgang Tittel ,: Optical quantum memory ", Nature Photonics, DOI: 10.1038 / NPHOTON.2009.231 (2009) Simon Bernon, Helge Hattermann, et al .: Manipulation and coherence of ultra-cold atoms on a superconducting atom chip ", Nature communications 4, DOI: 10.1038 / ncomms3380 (2013) Mukai, Imai: "Bose-Einstein condensation with permanent current atom chip", 2014, NTT Technical Journal 2014.6, p29-32

  Various methods can be considered for the configuration of the quantum memory. As a promising one, the inventors focused on a quantum memory based on a Ramsey interferometer as a basic operating principle. The fundamental principle of the Ramsey interferometer is to observe stable and accurate vibrations using two internal states such as atoms and molecules and an electromagnetic field that resonates therewith. Ramsey interferometers have already been used in applications such as high-precision clocks, high-precision measurement of physical quantities such as gravity and acceleration by atomic interferometers, and application to quantum information such as observation of quantum states.

  For example, as disclosed in Non-Patent Document 5, in a Ramsey interferometer, irradiation from a first π / 2 pulse (π / 2 pulse for writing) to a second π / 2 pulse (π / pulse for reading) are performed. Information is stored using the fact that the occupancy of the two quantum states that resonate changes according to the quantum phase that evolves until the irradiation of (two pulses). It is considered that a quantum memory can be operated by holding a state of “superposition” of two quantum states. More specifically, in a quantum memory using a Ramsey interferometer, a two-level atom is first placed in a ground state. Thereafter, the quantum phase of the atoms is initialized by irradiation of the first π / 2 pulse, and the quantum memory is set to the superimposed quantum state. By irradiating the second π / 2 pulse at a predetermined timing from the first π / 2 pulse, the stored quantum state can be read, and the quantum state can be stored and the quantum gate operation can be performed. .

  Here, the π / 2 pulse is an electromagnetic wave packet having a specific intensity (electric field amplitude) and a duration that resonates with an energy difference between the ground state and the excited state, and has a probability of 1/2 with any probability. It means a substance having a size (pulse area = pulse duration × pulse electric field amplitude) that can change a quantum state into a state. That is, by applying a π / 2 pulse, the quantum state can be changed from the ground state to the excited state or from the excited state to the ground state with a probability of 1/2.

  The quantum phase stored in the above-described Ramsey interferometer type quantum memory evolves with the quantum phase initialized at the time of irradiation of the writing π / 2 pulse. That is, the quantum phase set by the writing π / 2 pulse is set as the initial phase, and the quantum phase changes with time starting from the initial phase. As will be described later, when the reading π / 2 pulse is irradiated at a predetermined timing, the quantum state stored in the quantum memory can be reproduced.

  However, in the conventional quantum memory, there is a risk that the phase information of the pulse from the leaked light of the writing π / 2 pulse may be read by an eavesdropper. Once this phase information was known, the eavesdropper could reproduce this phase information with a commercially available laser and use it to steal data in the quantum memory. Therefore, in the prior art Ramsey interferometer type quantum memory, the security was not sufficiently high. Such vulnerability of the quantum memory has been a security problem of the quantum information system.

  The present invention has been made in view of such a problem, and an object of the present invention is to provide a quantum memory with improved confidentiality, a quantum memory device, and a method of storing or reading information in a quantum memory device. Where you do it.

  In order to achieve such an object, the present invention provides a storage medium holding two quantum states including a first quantum state and a second quantum state, Means for writing or reading information by irradiating the storage medium with a first π / 2 pulse of one or more types of first electromagnetic waves that resonate with a quantum state and couple the two quantum states; Irradiating the storage medium with a second π / 2 pulse having a phase different from that of the first electromagnetic wave due to at least one type of second electromagnetic wave different from the first electromagnetic wave of the means for writing or reading. Means for encrypting or decrypting the information written by the means for writing or reading the information.

  The invention according to claim 2 is the quantum memory device according to claim 1, wherein the first π / 2 pulse includes a π / 2 pulse for writing and a π / 2 pulse for reading, and the second π / 2 pulse. Contains an encrypted π / 2 pulse and a decrypted π / 2 pulse, and detunes the difference between the frequency-converted energy difference between the two quantum states and the sum of the energy of the second electromagnetic wave to δ (2π × frequency), the irradiation start timing of the decryption π / 2 pulse is (2n + 1) π / δ after the end of the encryption π / 2 pulse irradiation (n is an integer of 0 or more). It is characterized by.

  According to a third aspect of the present invention, in the quantum memory device according to the first or second aspect, the writing π / 2 pulse and the reading π / 2 pulse have the same phase relationship in the phase of an electromagnetic wave, and the encryption π / 2 pulse and the decrypted π / 2 pulse also have the same phase relationship in the phase of the electromagnetic wave, and the phase of the write π / 2 pulse and the read π / 2 pulse and the encrypted π / 2 pulse There is no correlation between the pulse and the phase of the decoded π / 2 pulse.

  According to a fourth aspect of the present invention, in the quantum memory device according to any one of the first to third aspects, the storage medium is an atomic group captured at a cryogenic temperature, and further includes an atom chip that captures the atomic group in the vicinity. , The first electromagnetic wave and the second electromagnetic wave are electromagnetic waves that resonate with a ground state and an excited state of atoms of the atomic group and couple the ground state and the excited state, respectively, and the surface of the atom chip In the vicinity, a spatial distribution of the magnetic flux density at which the magnetic potential has a minimum value is formed, and the difference in energy change between the ground state and the excited state is set to substantially zero.

According to a fifth aspect of the present invention, in the quantum memory device of the fourth aspect, the element of the atomic ensemble is an isotope ⁸⁷ Rb of a rubidium atom, and the first electromagnetic wave is the ground state and the excited state by induced Raman transition. a 2-wave state capable of binding to D ₁ line, said second electromagnetic wave is a combination of microwave (fm = 6.8 GHz) and radiofrequency (fr = 1.3 MHz), the vicinity of the atom chip characterized by using a generally B _min = 0.32mT as the value of the magnetic flux density in magnetic potential minimum point of the magnetic field traps in.

  The invention according to claim 6, wherein in the method of storing or reading information in a quantum memory device including a storage medium that holds two quantum states including a first quantum state and a second quantum state, the two quantum states Writing the information by irradiating the storage medium with a first π / 2 pulse of one or more first electromagnetic waves that resonates with the two quantum states and combines the two quantum states; Irradiating the storage medium with a second π / 2 pulse having a phase different from that of the first electromagnetic wave by one or more types of second electromagnetic waves different from the first electromagnetic wave when performing Encrypting the information written by writing the information; and irradiating the storage medium with the second π / 2 pulse to perform the encryption. Decrypting the encrypted information, and irradiating the storage medium with the first π / 2 pulse to read the information. is there.

  The invention according to claim 7 is the method according to claim 6, wherein the first π / 2 pulse includes a π / 2 pulse for writing and a π / 2 pulse for reading, and the second π / 2 pulse includes: A decimation δ (2π × frequency) including an encryption π / 2 pulse and a decryption π / 2 pulse, wherein a difference between a frequency-converted energy difference between the two quantum states and a sum of energies of the second electromagnetic waves. ), The timing of starting irradiation of the decrypted π / 2 pulse is (2n + 1) π / δ after the end of irradiation of the encrypted π / 2 pulse (n is an integer of 0 or more). And

  According to an eighth aspect of the present invention, in the method of the sixth or seventh aspect, the writing π / 2 pulse and the reading π / 2 pulse have the same phase relationship in the phase of the electromagnetic wave, and the encrypted π / 2 The pulse and the decrypted π / 2 pulse also have the same phase relationship in the phase of the electromagnetic wave. The phase of the π / 2 pulse for writing and the π / 2 pulse for reading, the phase of the encrypted π / 2 pulse, There is no correlation between the phase of the decoded π / 2 pulse and the phase.

  In the above method, preferably, the storage medium is an atomic group captured at a cryogenic temperature, further comprising an atom chip that captures the atomic group in the vicinity, wherein the first electromagnetic wave and the second electromagnetic wave are: A magnetic field that resonates with the ground state and the excited state of the atoms of the atomic group and couples the ground state and the excited state, respectively, and in the vicinity of the surface of the atom tip, the space of the magnetic flux density at which the magnetic potential has a minimum value. A distribution is formed, and a difference between energy changes in the ground state and the excited state can be set to substantially zero.

More preferably, the elements of the atomic population is isotope ⁸⁷ Rb rubidium atoms, said first electromagnetic wave by stimulated Raman transition two waves of the ground state and the excited state capable of binding D ₁ line The second electromagnetic wave is a combination of a microwave (fm = 6.8 GHz) and a radio wave (fr = 1.3 MHz), and has a magnetic flux density at a magnetic potential minimum point of a magnetic field trap near the atom tip. generally B _min = 0.32mT as values can be used.

  In the quantum memory device encrypted according to the present invention, confidential security against an attack that attempts to read the quantum memory is greatly improved.

FIG. 1 is a diagram illustrating two quantum states of an isotope ⁸⁷ Rb of a rubidium atom that can be coupled using microwaves and radio waves and a transition between the two states. FIG. 2 is a diagram illustrating two quantum states of the isotope ⁸⁷ Rb of a rubidium atom that can be coupled by stimulated Raman transition using two wavelengths of the D ₁ line and transition between the two states. FIG. 3 is a conceptual diagram showing the configuration of the quantum memory device of the present invention. Figure 4 is a diagram for explaining a state transition between the 5 ² S _1/2 -5 ² P _3/2 of ⁸⁷ Rb atoms to measure the occupancy of states in the quantum memory of the present invention. FIG. 5 is a diagram illustrating a procedure for irradiating a read pulse without decryption after encryption in the quantum memory device of the present invention. FIG. 6 is a diagram showing the state occupancy when decoding is not performed in the quantum memory device of the present invention. FIG. 7 is a diagram for explaining a procedure of irradiating a read pulse with decryption after encryption in the quantum memory device of the present invention. FIG. 8 is a diagram showing the state occupancy when decoding is performed in the quantum memory device of the present invention.

  The quantum memory device according to the present invention emits an encrypted π / 2 pulse having a phase having a random relationship with the phase of the writing π / 2 pulse after the irradiation of the writing π / 2 pulse. Irradiation of the encrypted π / 2 pulse creates a state in which a random phase is added to the quantum phase stored in the quantum memory. By storing the randomized quantum phase, the security of security can be greatly improved as compared with the conventional quantum memory.

  When reading out information, a random phase added by the encrypted π / 2 pulse is neutralized by irradiating a decrypted π / 2 pulse having the same phase as the encrypted π / 2 pulse. By the irradiation of the decoded π / 2 pulse, the quantum phase initialized at the irradiation of the writing π / 2 pulse is recovered, and the information can be read.

  The quantum memory device encrypted according to the present invention irradiates the readout π / 2 pulse at a predetermined timing unless it is decoded by the π / 2 pulse having the same phase as the random phase of the encrypted π / 2 pulse. However, only information that evolves over time due to a completely random phase can be extracted. For this reason, confidential security against an attack that attempts to directly read the quantum memory can be significantly improved as compared with the related art. Further, the present invention can be implemented as a method of storing or reading information not only in a quantum memory device but also in a quantum memory device including a storage medium holding two quantum states.

  Hereinafter, as an example of a specific embodiment of the present invention, a case in which a cooled atom group is used as a storage medium of a quantum memory will be described. When a cooled atom group is used as a storage medium for information, a laser light source and a means for generating electromagnetic waves are required in addition to a medium and a mechanism for holding the medium, as described later. Therefore, the present invention can be implemented as a quantum memory device including a quantum memory as a storage medium. However, quantum memory can be realized by a method other than using a cooled atom group as a medium. In the case of a more integrated form of the medium and related components, a light source and a part of an electromagnetic wave generator described later are required. It should be noted that if not, there may be a case where other components different from the light source and the electromagnetic wave generator are included.

[Embodiment]
In the quantum memory device of the present invention, a cooled atomic aggregate (hereinafter, referred to as a cooled atomic group) that is less affected by noise from the surrounding environment than a solid-state element is used. As the atomic species of the cooled atomic group, the isotope ⁸⁷ Rb of rubidium atoms that can be stably captured spatially is considered in consideration of long-term memory retention.

FIG. 1 is a diagram illustrating two quantum states of the isotope ⁸⁷ Rb of a rubidium atom that can be coupled using microwaves and radio waves, and a transition between the two states. The ground state of the ⁸⁷ Rb atom splits into two hyperfine structures of F = 1 and F = 2 as shown in FIG. 1 because the spin state has a nuclear spin of 3/2 at 5 ² S _1/2 . I do.

Furthermore, considering the energy shift (Zeeman shift) due to the magnetic field, as shown in FIG. 1, the degeneracy is released by applying the magnetic field, and the energy level is split by the difference in the magnetic quantum number. That, F = 1 state is split into three spin states of different magnetic quantum number _{m F (m F = -1,0,1)} , F = 2 state five spins of different magnetic quantum numbers m _F Split into states (m _F = −2, −1, 0, 1, 2).

Among these spin states, atoms in two spin states | F = 1, m _F = −1> and | F = 2, m _F = 1> indicated by thick lines are used in the quantum memory device of the present invention. Used as a medium. Both of these spin states can be captured at the minimum point of the magnetic field, and when the value of the magnetic flux density is near B _min = 0.32 mT, the Zeeman shift exhibits the same magnetic field dependence up to the second order. (See Non-Patent Document 5).

Hereinafter, for simplicity, these two spin states will be represented by | 1> = | F = 1, m _F = −1> and | 2> = | F = 2, m _F = 1>. The | 1> state corresponds to the ground state, and the | 2> state corresponds to the excited state. In FIG. 1, the vertical axis represents the energy level of atoms. From the relationship of E = hν (h: Planck constant, ν: frequency), the vertical axis can also be represented by frequency.

The | 1> state and the | 2> state can be coupled by two-photon resonance because the difference in magnetic quantum number m _F between the two spin states is Δm _F = 2. Thus, as shown in FIG. 1, a microwave (MW: f _m = 6.8 GHz) and radio (wireless) wave: using (RF f _r = 1.3 MHz), by stimulated Raman transition two spin states Can be combined. In FIG. 1, the level of the dotted line near (upper) the | 1> state indicates energy separated from the ground state by δ described later. The level of the dotted line near (lower) the | 2> state indicates a virtual level assumed from the frequency of the radio wave. The | 1> state and the | 2> state can be combined by stimulated Raman transition also by laser light. Incidentally, slide into explaining the configuration and operation of the present invention using the example of a particular combination of microwave (f _m = 6.8GHz) and radio waves (f _r = 1.3MHz) is below, electromagnetic waves only this It is not limited to. That is, as can be understood from FIG. 1, since the difference between the magnetic quantum numbers m _F of the two quantum levels trapped at the minimum point of the magnetic field is 2, the conditions under which the | 1> state and | 2> state can be coupled by two-photon resonance Other electromagnetic waves can be used as long as the electromagnetic wave pulse satisfies the condition.

FIG. 2 is a diagram illustrating two quantum states of the isotope ⁸⁷ Rb of a rubidium atom that can be coupled by stimulated Raman transition using two wavelengths of the D ₁ line and transition between the two states. As shown in FIG. 2, two spin states of | 1> state and | 2> state can be coupled by stimulated Raman transition using two wavelengths (795 nm) of the D ₁ line as one set. In FIG. 2, an arrow 31 indicates an energy difference between a virtual level that causes a stimulated Raman transition and F ′ = 1, mF ′ = 0.

A quantum memory device of the present invention, in order to operate as a quantum memory Ramsey interferometric, as a write [pi / 2 pulse and read [pi / 2 pulse of quantum states, stimulated Raman to 2 wavelengths pair the D ₁ line laser A transition is used (corresponding to FIG. 2). Further, as an encryption [pi / 2 pulse and the decoding [pi / 2 pulses, microwave (MW: f _m = 6.8GHz) and radiofrequency: using a set of (RF f _r = 1.3MHz) (Fig. 1). As already mentioned, these π / 2 pulses are not limited to combinations of the particular types of electromagnetic waves mentioned above. In the example of two quantum states of ⁸⁷ Rb in FIG. 1 and FIG. 2, two magnetic quantum numbers m _F have transitioned, so that a two-photon transition is used and a pair of two electromagnetic waves is used. However, when the atoms used in the Ramsey interferometer are other than ⁸⁷ Rb and two quantum states can be combined by one-photon transition, one type of electromagnetic wave can be used. Also in the present embodiment, if the electromagnetic wave pulse satisfies the condition (two-photon transition) that can combine two quantum states, ie, | 1> state and | 2> state, and can resonate with an energy difference, other than the D ₁ line laser Other combinations of electromagnetic waves can also be used.

What should be noted here is the phase relationship of each π / 2 pulse in each electromagnetic wave described above. The phase of the electromagnetic wave of the D ₁ line laser is uniform between the two wavelengths. In addition, the phases of the electromagnetic waves of microwaves and radio waves are also aligned between the two electromagnetic waves. On the other hand, an electromagnetic wave of the phase of the D ₁ line laser, without any relationship between the microwave and radio waves of the electromagnetic waves of the phase should be noted that has a random phase relationship uncorrelated. In other words, the π / 2 pulse for writing and the π / 2 pulse for reading have the same phase relationship in the phase of the electromagnetic wave, and the encrypted π / 2 pulse and the decrypted π / 2 pulse also have the same phase in the phase of the electromagnetic wave. And there is no correlation between the phases of the π / 2 pulse for writing and the π / 2 pulse for reading and the phases of the encrypted π / 2 pulse and the decrypted π / 2 pulse. become.

More specifically, the phase of the electromagnetic wave mentioned here corresponds to φ when the electromagnetic wave is expressed as Ecos (2πft + φ). Here, f is the frequency of the electromagnetic field, E is the electric field amplitude, and t is the time. Considering _two frequencies f ₁ and f ₂ corresponding to the two wavelengths with respect to the two wavelengths of the D ₁ line, and assuming that the phases of the respective frequencies are φ ₁ and φ ₂ , each electric field is E ₁ cos (2πf ₁ t + φ ₁ ). , And E ₂ cos (2πf ₂ t + φ ₂ ). Here, when the relationship of φ ₁ −φ ₂ = constant 1 holds, it is said that the two electromagnetic waves have the same phase. Similarly, also for the above-mentioned microwave and radio waves, the phase difference between these two electromagnetic waves satisfies the condition of constant (constant 2), and the phases between the two electromagnetic waves are aligned. On the other hand, the relationship between the phase difference between two wavelengths of D ₁ line and (constant 1), the phase difference between the microwaves and radio waves (the constant 2) is completely random.

FIG. 3 is a conceptual diagram showing the configuration of the quantum memory device of the present invention. In FIG. 3, the components of the quantum memory device are conceptually represented, and some components are omitted or are represented by abstraction / symbolization in order to simplify the description. Please note. The quantum memory device 10 includes an atom chip 1 placed in a vacuum region 8 and a ⁸⁷ Rb atomic group 2 trapped in the vicinity immediately below a lower surface of the atom chip 1. Further, inside the vacuum region 8, a radio wave generating antenna 3 for irradiating an atomic group with radio waves (radio waves), and outside the vacuum region 8, a microwave generating antenna 4 for irradiating a microwave. Prepare. Although not shown in FIG. 3, a light source for supplying a set of two wavelengths of D ₁ ray laser light 5, absorption measurement laser light 6, and re-pump laser light 7 to the atomic group is provided. FIG. 3 shows the atom chip 1 symbolically, and the actual shape and operation will be described later. Further, for one coordinate axis, a magnetic field potential curve for capturing the atomic group 2 in the vicinity of the surface of the atom tip 1 is also conceptually drawn.

  An atom chip is known as an atomic optical device in which fine electrodes and the like are formed on a substrate like an electronic device, and an extremely low temperature atom is guided and manipulated using an electric field, a magnetic field, or light. The realization of laser cooling and Bose-Einstein condensation (BEC) enables the generation and manipulation of atoms at cryogenic temperatures of 1 μK or less, and realizes a specific quantum state in an atom chip. Various configurations can be applied to the specific configuration of the atom chip. Hereinafter, an example of a configuration using superconductivity will be described (see Non-Patent Document 6).

In the atom chip 1, a wiring pattern of a superconducting thin film (MgB ₂ ) is drawn on a quartz substrate. More specifically, the substrate has a square closed circuit pattern whose basic shape is approximately 9 mm on a side. This closed circuit pattern has a shape in which one corner of the substantially square closed circuit pattern is recessed inward by a few mm square. A Z-shaped electric wire pattern is locally formed by one concave shape, and an atomic group is captured in the vicinity of the Z-shaped electric wire pattern.

In order to generate a magnetic field by passing a current through the above-described closed circuit, the property of superconductivity is used. As a characteristic of the superconductor, there is a property (the Meissner effect) that the magnetic flux that has penetrated the closed circuit at the time of transition to superconductivity is preserved. Utilizing this property, first, when the temperature is higher than the superconducting transition temperature, a magnetic field is applied from the outside in a direction perpendicular to the closed circuit, and the superconductor is cooled to a temperature lower than the superconducting transition temperature as it is. After that, a permanent current flows so that the magnetic flux penetrating the closed circuit is preserved even if the externally applied magnetic field is cut off. A magnetic field is generated around the electric current as described by Biot-Savart's law, and this magnetic field is used for trapping atoms. Actually, atoms cannot be trapped only by the magnetic field of the atom tip. Therefore, an auxiliary magnetic field is applied from outside the atom tip 1 and a minimum point of the magnetic field is created by “the magnetic field of the atom tip + auxiliary magnetic field”. At the minimum point of this magnetic field, ⁸⁷ Rb atoms are captured. ⁸⁷ Rb atoms are captured at a position approximately 10 to 100 μm away from the surface of the atom chip 1. The position at which ⁸⁷ Rb atoms are captured from the surface of the atom tip 1 can be controlled by the strength of the auxiliary magnetic field applied from the outside.

The ⁸⁷ Rb atoms to be captured are supplied from outside the atom chip 1. ^{The 87} Rb atom is a metal Rb atom, and floats in a vacuum in the vacuum region 8 in a monatomic gas state by heating. The Rb atom is located in the vacuum region 8 and at a position approximately 1 m away from the atom tip 1. At room temperature, individual atoms are flying at an initial velocity (about 1000 km / h) similar to that of a pistol bullet. When the speed of ⁸⁷ Rb atoms decelerates radiation pressure of the light (the force due to momentum change with photon) to about per second several centimeters, bring close to the minimum point of the above-described magnetic field, ⁸⁷ Rb by the magnetic field of the above Atoms are trapped. Note that FIG. 3 does not show a cooling device, a magnetic field generator, a cooling laser, and the like necessary for the atom trapping operation around the atom chip 1. It should be noted that there are various variations of the cooling device, magnetic field generator, cooling laser, etc. necessary for the atom trapping operation, including the necessity, depending on the specific configuration of the atom chip and the principle of atom trapping. I want to.

⁸⁷ Rb atoms to be irradiated with a laser pulse of D ₁ ray laser light, microwaves and radio waves are captured by the above-described atom chip 1 using a magnetic field potential for capturing atoms. That is, as conceptually shown by the potential curve in FIG. 3, ⁸⁷ Rb atoms were | 1> in a magnetic field trap in which the value of the magnetic flux density at the minimum point of the magnetic field was adjusted to B _min = 0.32 mT by the magnetic field generator. It is captured in a state (ground state) and is condensed by evaporative cooling in a Bose state. However, the atom density is set to a relatively low value (about 10 ¹³ cm ⁻³ ) in order to suppress the influence of interatomic collision.

  If atoms are not trapped in the vacuum region 8 by a magnetic field, the atoms will fall freely by gravity. In order to capture atoms, a magnetic field potential having a minimum point is used as described above. The magnetic field potential curve in FIG. 3 is schematically drawn, and the horizontal axis in the curve corresponds to space, and the vertical axis corresponds to the amount of energy shift due to the magnetic field. For example, if the horizontal axis is the x-axis, the magnetic field is minimal at a certain point on the x-axis, that is, the amount of energy shift is minimized. By setting similar magnetic minimum points for the y-axis and the z-axis, the atoms receive a force toward one point in the three-dimensional space, and the atoms are captured at the magnetic minimum points. Here, the fact that an atom is captured by receiving a force toward the magnetic field minimum point means that the amount of energy shift (Zeeman shift) of the atom between the center of the trap (that is, the magnetic field minimum point) and the periphery thereof. It means slightly different. Therefore, when the number of primitives to be captured is increased and the spatial spread of the atomic group is increased, the variation of the quantum phase over time due to the atoms increases.

  Furthermore, if the energy shift between the ground state and the excited state is different, the variation of the time change of the quantum phase between the ground state and the excited state becomes larger, and when the population average of N is taken, the quantum phases cancel each other. In addition, interference fringes become invisible, that is, coherence is lost.

In the space where the atomic group is captured, a minimum point of the magnetic flux density is set in order to capture the atoms, and a potential is necessarily formed with a non-uniform magnetic field. As described above, when the value of the magnetic flux density is in the vicinity of B _min = 0.32 mT, since the Zeeman shift has the same magnetic field dependence up to the second order, | 1> state and | 2> state Is almost zero. Therefore, the quantum phase of the group of atoms captured by the magnetic field trap evolves uniformly over time. That is, in an atomic group having a certain spatial extent, the quantum phase of the atoms at each point changes with time by almost the same magnitude. As a result, the time variation of the quantum phase is suppressed, so that the coherence time can be extended, and a long storage time can be realized as a quantum memory.

  Next, in the quantum memory device having the configuration shown in FIG. 3, the operation of the quantum memory including the encryption / decryption of the quantum state specific to the present invention will be described in terms of each state with and without decryption processing. This will be further described while making a comparison.

When storing the quantum state in the quantum memory, the ground state atomic group 2 prepared in the vicinity of the atom chip 1 is first irradiated with a writing π / 2 pulse 5 by a set of D ₁ line lasers, Is initialized. The two quantum states of ⁸⁷ Rb and the transition between the two states by the set of D ₁ -line lasers are as shown in FIG. By irradiating the π / 2 pulse for writing, N / 2 atoms of the N atoms in the trapped atomic group are excited to an excited state, and the remaining N / 2 atoms are in the ground state. Become. As is well known, at this stage, it is unknown which atom is excited, and if attention is paid to only one of the N atoms, the atom is not determined as a ground state or an excited state. It is a so-called superimposed state of two states. Such a non-classical state of superposition of two states is stored in the quantum memory of the present invention. That is, there is a state in which there are N atoms in the superposed state of the ground state and the excited state.

Therefore, the quantum memory device according to the present invention includes a storage medium 2 that holds two quantum states including a first quantum state and a second quantum state, and the two quantum states resonating with the two quantum states. Means for writing or reading information by irradiating the storage medium with a first π / 2 pulse of one or more kinds of first electromagnetic waves to be coupled, and the first electromagnetic wave for means for writing or reading the information Means for writing or reading the information by irradiating the storage medium with a second π / 2 pulse having a phase different from that of the first electromagnetic wave by one or more types of second electromagnetic waves different from the first electromagnetic wave Means for encrypting or decrypting the written information. Here, the first π / 2 pulse includes a π / 2 pulse for writing and a π / 2 pulse for reading, and the second π / 2 pulse includes an encrypted π / 2 pulse and a decrypted π / 2 pulse. Includes two pulses. The storage medium is an atomic group captured at a cryogenic temperature, and further includes an atom chip that captures the atomic group in the vicinity, and the first electromagnetic wave and the second electromagnetic wave are each of the atomic group. An electromagnetic wave that resonates with a ground state and an excited state of an atom and couples the ground state and the excited state.In the vicinity of the surface of the atom tip, a spatial distribution of a magnetic flux density at which a magnetic potential has a minimum value is formed. , The difference in energy change between the ground state and the excited state is set to substantially zero. Further, the element of the atomic group is an isotope ⁸⁷ Rb of a rubidium atom, and the first electromagnetic wave is two waves of a D ₁ line capable of coupling the ground state and the excited state by induced Raman transition, The second electromagnetic wave is a combination of a microwave (fm = 6.8 GHz) and a radio wave (fr = 1.3 MHz), and the value of the magnetic flux density at the magnetic potential minimum point of the magnetic field trap near the atom tip. generally B _min = 0.32mT as can be used.

FIG. 5 is a diagram illustrating a procedure for irradiating a read pulse without decryption after encryption in the quantum memory device of the present invention. FIG. 7 is a diagram for explaining a procedure of irradiating a read pulse with decryption after encryption in the quantum memory device of the present invention. Referring to FIG. 5, a writing π / 2 pulse 21 having a predetermined electric field intensity and a predetermined pulse time _Td is irradiated. As described above, the phase difference (φ ₁ −φ ₂ ) between the two electromagnetic waves is a constant for a set of two wavelengths of the D ₁ line laser light, and a specific position between the two laser lights is obtained. Note that the two electromagnetic waves have a phase difference and are in phase.

Next, in the quantum memory device of the present invention, a microwave is generated from the microwave generating antenna 3 and a radio (radio) wave is generated from the radio wave generating antenna 4 at the same time. The atomic group 2 is irradiated with an encrypted π / 2 pulse 22 having a pulse time T _m and encryption is performed by adding a random phase to the quantum phase. The transition between the two quantum states of ⁸⁷ Rb by microwave and radio waves is as shown in FIG. Note that the phase difference between the two electromagnetic waves, microwave and radio, is a constant, there is a specific phase difference between the two electromagnetic waves, and the two electromagnetic waves are in phase. .

Further, it is important that the microwave and radio wave of the encrypted π / 2 pulse have a random phase relationship with the set of two wavelengths of laser light of the D ₁ line. Here, having a random phase relationship means that the phase difference (constant 1) between two electromagnetic waves of a set of two wavelengths of laser light of the D ₁ line and the phase difference (constant 2) of two electromagnetic waves of a microwave and a radio wave. ) Are phaseless and completely unrelated and random. In this way, the encrypted π / 2 pulse 22 is irradiated, and the phase difference of the electromagnetic wave in the encrypted π / 2 pulse 22 is set at random with respect to the phase difference between the electromagnetic waves in the written π / 2 pulse 21. Thus, the security of the quantum memory is greatly improved as compared with the related art.

To decrypt the quantum state encrypted by the above-described encrypted π / 2 pulse 22, the decrypted π / 2 pulse 24 having the same phase as the encrypted π / 2 pulse 22 is used as shown in FIG. Irradiate. The initial quantum phase is recovered by neutralizing the random phase added to the quantum phase by the decryption π / 2 pulse 24 using the same microwave and radio waves as in the encryption. The microwave and radio waves of the decrypted π / 2 pulse 24 must have the same phase difference between the two electromagnetic waves as the encrypted π / 2 pulse 22. At this time, the timing (T _{2 in} FIG. 7) for irradiating the decrypted π / 2 pulse 24 is T ₂ = (2n + 1) π / δ (where n is an integer, from the irradiation of the encrypted π / 2 pulse 22). δ needs to be the energy difference in FIG. 1). Here, referring again to FIG. 1, when the energy in the state on the vertical axis is expressed in units of frequency, the energy difference δ is expressed by the following equation (1).
δ = 2π × [(| 1> state and | 2> energy difference between states [frequency unit])
− (Microwave frequency + radio wave frequency)] Equation (1)

  This energy difference δ is also called detuning. Therefore, the timing condition for irradiating the decryption π / 2 pulse 24 is as follows: the energy difference in frequency conversion between the two quantum states and the energy of the (second) electromagnetic wave in the π / 2 pulse for encryption / decryption. When the difference from the sum is detuned δ (2π × frequency), the irradiation start timing of the decryption π / 2 pulse is (2n + 1) π / δ (n is 0) from the end of the encryption π / 2 pulse irradiation. It can be said that later.

After decoding by irradiating the decoded π / 2 pulse 24, as shown in FIG. 7, by irradiating the reading π / 2 pulse 23 with a set of two wavelengths of laser light of the D ₁ line, , The occupancy of the | 1> state and | 2> state according to the time evolution of the quantum phase.

FIG. 4 is a diagram for explaining a state transition between 5 ² S _1/2 −5 ² P _3/2 of ⁸⁷ Rb atoms for measuring the state occupancy in the quantum memory of the present invention. To determine the state of the quantum memory, the occupancy of the states | 1> and | 2> can be measured as follows. As shown in FIG. 4, an atomic group is first irradiated with an absorption measurement laser that resonates between the | 2> state and the ² P _3/2 : F = 3 state, so that the amount of light absorbed by the laser is measured. | 2> Count the number of atoms in the state. That is, in the quantum memory device 10 of the present invention shown in FIG. 3, the atomic group 2 after the irradiation with the reading π / 2 pulse 23 is irradiated with the laser beam 6 for absorption measurement, and the atoms in the state | 2> Count the number.

After measuring the number of atoms in the | 2> state, the laser in the | 2> state is physically pushed out of the trap region (observation region) of the atom chip 1 and blown off using an absorption measurement laser with the radiation pressure of light as an external force. . | 2> after blowing off state of the atom, now | 1> and status, ² P _3/2: the Ripanpu laser resonating between F = 2 state is irradiated to the atomic population. That is, in the quantum memory device 10 of the present invention shown in FIG. 3, after the atoms in the | 2> state are blown off, the atomic group 2 is irradiated with the laser beam 7 for repump, and the number of atoms in the | 2> state is measured. By irradiating the same laser for absorption measurement as described above, the number of atoms in the | 1> state can be measured. The occupancy of | 1> state and | 2> state is calculated from the number of atoms of | 1> state and | 2> state measured as described above, and the measurement of the occupancy of quantum state is completed. .

FIG. 6 is a diagram showing a measurement result of the occupancy of the | 2> state when the quantum memory device according to the present invention is encrypted and read out without performing decryption. In the procedure of irradiating each π / 2 pulse shown in FIG. 5, reading was performed by irradiating the reading π / 2 pulse 23 after “encrypting π / 2 pulse was not irradiated” after the encryption π / 2 pulse. Corresponds to the quantum state. That is, the waiting time T ₂ from the end of the irradiation of the encrypted π / 2 pulse 22 to the start of the irradiation of the reading π / 2 pulse 23 is changed, and each state is captured for the atomic group captured under the same conditions. The measurement of the occupancy was repeated. Read from the quantum memory | 2> occupancy state against waiting time T _2, which change randomly, were confirmed to have been encrypted.

FIG. 8 is a diagram showing a measurement result of the occupancy of the | 2> state when the quantum memory device of the present invention is encrypted and then decrypted and read. In the procedure of irradiating each π / 2 pulse shown in FIG. 7, “irradiate a decryption π / 2 pulse” after an encryption π / 2 pulse, and then irradiate a reading π / 2 pulse 23 Corresponds to the read quantum state. In other words, each state by changing the waiting time T ₃ of the irradiation to the start of the read [pi / 2 pulse 23 from the end of the irradiation of the decoding [pi / 2 pulse 24, with respect to atomic population was trapped under the same conditions The measurement of the occupancy was repeated. By performing the decoding, the quantum phase initialized by the writing π / 2 pulse is restored, and the coherent oscillation of the state occupancy can be confirmed with respect to the waiting time T ₃ , similarly to the normal Ramsey interference. Was. The excitation rate of the Ramsey interferometer oscillates at a frequency of δ / (2π). For Figure 8, since the [delta] of the write pulse (D ₁ line) and [delta] = 2 [pi × 120Hz, occupancy vibrates at approximately 120Hz frequency. From the coherent oscillation of the state occupancy in FIG. 8, it was confirmed that quantum information was stored.

In the quantum memory device of the present invention, if a specific example of each π / 2 pulse is given, the pulse width of the writing π / 2 pulse is _Td = 0.3 ms, and the pulse width of the decoding π / 2 pulse is _Tm = 1.48 ms. When the electric field strength is described by the Rabi frequency when the electric field is applied, the R ₁ frequency of the D ₁ line laser is 830 Hz, and the Rabi frequency of the microwave + radio wave is 169 Hz. Therefore, according to the quantum memory device of the present invention, by irradiating the encrypted π / 2 pulse and the decrypted π / 2 pulse with a predetermined waiting time, the concealment performance is far improved compared to the related art. A quantum memory can be realized.

In the above-described embodiment, an example of the Ramsey interferometer type quantum memory device that sets a quantum state for an atomic group of ⁸⁷ Rb atoms has been described. A similar encryption / decryption of a quantum memory with improved concealment performance is possible. In addition, although ⁸⁷ Rb atoms have been described as examples of cooling atoms, laser cooling can be applied to Li, Na, K, Cs, Ca, Sr, Ba, Yb, Cr, Dy, etc. May be used if there is an appropriate energy level. In addition, the possibility that the encryption / decryption of the present invention can be applied to other than the cooled atom, such as a quantum two-level type superconducting flux quantum or semiconductor quantum dot, which can measure Ramsey interference. There is.
Further, in the quantum memory device of the present invention, information is stored and read through irradiation of two different pulses of writing π / 2 pulse, reading π / 2 pulse and encryption π / 2 pulse / decoding π / 2 pulse. I do. This can be further increased to three or more stages. As a result, it is possible to further improve the performance of concealment.

In the quantum memory device of the present invention, the phase difference of the electromagnetic waves between the two wavelengths of the D ₁ line laser is a constant and the phases are uniform, and the phase difference is constant between the two electromagnetic waves of the microwave and the radio wave. And the phases are aligned. On the other hand, the point that the phase of the electromagnetic wave between the two wavelengths of the D ₁ line laser and the phase of the electromagnetic wave of the microwave and the radio wave are uncorrelated and random phases with each other is that the quantum memory device of the present invention is implemented. It is important in doing. Accordingly, an eavesdropper attempting to read information in the quantum memory implemented according to the present invention needs to know each phase difference (two constants) between the two pairs of electromagnetic waves, and furthermore, encrypts π / The timing from the irradiation of the two pulses 22 to the irradiation of the decoding π / 2 pulse 22 is also limited to the timing that satisfies a predetermined relationship determined by the energy difference δ. Therefore, it is possible to realize a quantum memory with much higher concealment performance than the prior art.

Also, by using a value near B _min = 0.32 mT as the value of the magnetic flux density at the minimum point of the magnetic field trap, the difference in energy change between the | 1> state and the | 2> state is made almost zero, and the magnetic trap is captured. By equalizing the time evolution of the quantum phase of the set of atomic groups, a long-term quantum memory can be realized. According to the present invention, a memory holding time of about several seconds can be realized. Considering the application to a quantum computer, an operation to change the quantum state about 1000 times within a few seconds is possible, and the application to a quantum computer is possible.

The quantum memory device of the present invention can be used for general purposes. For example, but not limited to, expected applications, write π / 2 pulses and read π / 2 pulses are used as a first key, and encrypted π / 2 pulses and decrypted π / 2 pulses are used as a second key. Considering this, by dividing the owner of each key into two parties, the present invention can be applied to technical fields such as secret sharing in which stored information is read out only when two parties cooperate.
As described in detail above, in the quantum memory device encrypted according to the present invention, confidential security against an attack that attempts to read the quantum memory is greatly improved.

  The present invention can be used for information processing technology. In particular, it can be used for high-speed, large-capacity computers and communications.

DESCRIPTION OF SYMBOLS 1 Atom chip 2 Rb atom 3 Radio wave generation antenna 4 Microwave generation antenna 5 D _1- ray laser 6 Laser beam for absorption measurement 7 Laser beam for repump 8 Vacuum region
Reference Signs List 10 Quantum memory device 21 π / 2 pulse for writing 22 π / 2 pulse for reading 23 Encrypted π / 2 pulse 24 Decoded π / 2 pulse 31 Virtual level causing induced Raman transition and F ′ = 1, mF ′ = Energy difference from zero level

Claims (8)

A storage medium for holding two quantum states including a first quantum state and a second quantum state;
Writing or reading information is performed by irradiating the storage medium with a first π / 2 pulse of one or more types of first electromagnetic waves that resonate with the two quantum states and couple the two quantum states. Means,
Irradiating the storage medium with a second π / 2 pulse having a phase different from that of the first electromagnetic wave due to at least one type of second electromagnetic wave different from the first electromagnetic wave of the means for writing or reading. And a means for encrypting or decrypting the information written by the means for writing or reading the information. The first π / 2 pulse includes a π / 2 pulse for writing and a π / 2 pulse for reading,
The second π / 2 pulse includes an encrypted π / 2 pulse and a decrypted π / 2 pulse;
When the difference between the frequency-converted energy difference between the two quantum states and the sum total of the energy of the second electromagnetic wave is detuned δ (2π × frequency), the irradiation start of the decoding π / 2 pulse is started. 2. The quantum memory device according to claim 1, wherein the timing is (2n + 1) π / δ (n is an integer of 0 or more) after the end of irradiation of the encrypted π / 2 pulse. 3. The writing π / 2 pulse and the reading π / 2 pulse have the same phase relationship in the phase of the electromagnetic wave,
The encrypted π / 2 pulse and the decrypted π / 2 pulse also have the same phase relationship in the phase of the electromagnetic wave,
There is no correlation between the phases of the writing π / 2 pulse and the reading π / 2 pulse and the phases of the encrypted π / 2 pulse and the decrypted π / 2 pulse. The quantum memory device according to claim 1 or 2, wherein: The storage medium is an atomic group captured at a cryogenic temperature, further comprising an atom chip that captures the atomic group in the vicinity,
The first electromagnetic wave and the second electromagnetic wave are electromagnetic waves that resonate with a ground state and an excited state of the atoms of the atomic group and combine the ground state and the excited state, respectively.
In the vicinity of the surface of the atom tip, a spatial distribution of the magnetic flux density at which the magnetic potential has a minimum value is formed, and the difference in energy change between the ground state and the excited state is set to substantially zero. The quantum memory device according to claim 1. The element of the atomic population is the isotope ⁸⁷ Rb of rubidium atoms,
It said first electromagnetic wave is a two-wave of the ground state and the excited state capable of binding D ₁ line by stimulated Raman transition,
The second electromagnetic wave is a combination of a microwave (fm = 6.8 GHz) and a radio wave (fr = 1.3 MHz),
Quantum memory device according to claim 4, characterized by using a generally B _min = 0.32mT as the value of the magnetic flux density in magnetic potential minimum point of the magnetic field traps in the vicinity of the atom chip. A method for storing or reading information in a quantum memory device including a storage medium that holds two quantum states including a first quantum state and a second quantum state,
Writing information by irradiating the storage medium with a first π / 2 pulse of one or more types of first electromagnetic waves that resonate with the two quantum states and couple the two quantum states; ,
The storage medium is irradiated with a second π / 2 pulse having a phase different from that of the first electromagnetic wave due to at least one type of second electromagnetic wave different from the first electromagnetic wave when performing the writing or reading. And encrypting the information written by the step of writing the information,
Irradiating the second π / 2 pulse to the storage medium to decrypt the encrypted information encrypted by the encrypting step; and transmitting the first π / 2 pulse to the storage medium. Irradiating the storage medium to read the information. The first π / 2 pulse includes a π / 2 pulse for writing and a π / 2 pulse for reading,
The second π / 2 pulse includes an encrypted π / 2 pulse and a decrypted π / 2 pulse;
When the difference between the frequency-converted energy difference between the two quantum states and the sum total of the energy of the second electromagnetic wave is detuned δ (2π × frequency), the irradiation start of the decoding π / 2 pulse is started. The method according to claim 6, wherein the timing is (2n + 1) π / δ (n is an integer of 0 or more) after the end of irradiation of the encrypted π / 2 pulse. The writing π / 2 pulse and the reading π / 2 pulse have the same phase relationship in the phase of the electromagnetic wave,
The encrypted π / 2 pulse and the decrypted π / 2 pulse also have the same phase relationship in the phase of the electromagnetic wave,
There is no correlation between the phase of the writing π / 2 pulse and the reading π / 2 pulse and the phase of the encrypted π / 2 pulse and the decrypted π / 2 pulse. The method according to claim 6 or 7, wherein