JP4824941B2 - Method for generating and detecting quantum entanglement state of electron spin pair - Google Patents

Description

  The present invention relates to calculation and communication technologies using quantum states such as quantum computation, quantum computers, and quantum cryptography communication, and in particular, quantum entanglement state electrons related to electron spin using quantum dots coupled by a tunnel barrier. The present invention relates to a method for generating a spin pair and a method for detecting a quantum entanglement state of the generated electron spin pair.

  As described in Non-Patent Document 1, the quantum entanglement element shown in FIG. 15 is used as a method for generating an electron pair in a quantum entanglement state related to a spin in a quantum dot coupled by a tunnel barrier. There is something. Here, a description will be given assuming that a micro conductive region having discrete energy levels is referred to as a quantum dot. In the quantum entanglement element of FIG. 1, the quantum dot 51 and the quantum dot 52 are coupled by a tunnel barrier 58, the quantum dot 51 and the quantum dot 53 are coupled by a tunnel barrier 60, and the electrode 54 and the quantum dot 51 are coupled by a tunnel barrier 57. In this configuration, the electrode 55 and the quantum dot 52 are coupled by a tunnel barrier 59, and the electrode 56 and the quantum dot 53 are coupled by a tunnel barrier 61.

The method of generating a quantum entanglement state in Non-Patent Document 1 uses such an element, and a) injecting two electrons into the quantum dot 51, so that one of the quantum dots 51 is generated by the two electrons. A spin singlet state is formed in one orbit, b) one of the two electrons is tunneled to the quantum dot 52, and c) the remaining one electron in the quantum dot 51 is transferred to the quantum dot 53. By tunneling, a quantum entanglement state of a spin singlet state is generated between the electrons of the quantum dots 52 and the electrons of the quantum dots 53. As a technique in this field, one described in Non-Patent Document 2 is known.
P. Recher, DS Saraga and D. Loss, “Creation and detection of mobile and non-local spin-entangled electrons,” http://arxiv.org/cond-mat/0408526, 25 Aug. 2004. S. Dickmann and P. Hawrylak, “Spin relaxation in a two-electron quantum dot,” http://arxiv.org/cond-mat/0208223, 27 Nov. 2002.

  However, the above generation method has two problems. One is that only spin singlet quantum entanglement states can be generated. The other is that a quantum entanglement state is not necessarily generated, so that a detector for detecting the generation of the quantum entanglement state is required. For this reason, there is a problem that the use efficiency of the quantum entanglement state is low with respect to the former point, and there is a problem that an additional device is further required with respect to the latter point.

  The present invention has been made in view of the above, and an object of the present invention is to enable generation of a quantum entanglement state with high utilization efficiency.

  Another object of the present invention is to make it possible to easily detect the generated quantum entanglement state.

In the quantum entanglement state generation method according to the first aspect of the present invention, a control voltage to be applied to the first quantum dot and the second quantum dot is applied to four types of spins in which one electron occupies each quantum dot. After the step of setting the first voltage, which is more stable in energy than the state where two electrons occupy one quantum dot in the state, and the step of setting the first voltage, the first quantum dot and the second After the step of injecting electrons one by one into each quantum dot and the step of injecting electrons, the control voltage is set so that two quantum electrons are occupied by one quantum dot. and setting the second voltage to energetically stable than a state of one by one occupied, after the step of setting the second voltage, caused by the control voltage from the first voltage to a second voltage After a predetermined time defined on the basis of the difference, respectively occurs is elapsed after the second voltage four spin relaxation time of energy relaxation, and re-setting the control voltage to the first voltage It is characterized by having.

  In the present invention, the control voltage applied to the first and second quantum dots is set to the first voltage, and then one electron is injected into each quantum dot, so that one of the four spin states is selected. Then, the control voltage is set to the second voltage. Then, paying attention to the difference in energy relaxation time in each of the four types of spin states, after passing a predetermined time defined based on this point, by resetting the control voltage to the first voltage, Enables reliable generation of the desired quantum entanglement state.

  In the quantum entanglement state generation method, the predetermined time is equal to or less than an energy relaxation time for a spin triplet state of zero in the quantum entanglement state among the four spin states. Is shorter than the energy relaxation time for the other three types of spin states, and the resetting step includes detecting the occurrence of energy relaxation after the predetermined time has elapsed, and A step of setting the control voltage to a first voltage when no energy relaxation occurs.

  In the present invention, the predetermined time is equal to or shorter than the energy relaxation time for the state where the spin z component in the spin triplet state is 0, and is shorter than the energy relaxation time for the other three types of spin states. If a long time is set, and after the elapse of the predetermined time, the presence or absence of energy relaxation is detected, and when there is no energy relaxation, a spin triplet state spin z component of zero is generated. If it is detected that energy relaxation has occurred, it is detected that the other three types of spin states have been generated. Then, by setting the control voltage to the first voltage when no energy relaxation occurs, a state in which the spin z component of the spin triplet state having the quantum entanglement state is zero is reliably generated.

  In the quantum entanglement state generation method, the predetermined time is longer than any of energy relaxation times for the four types of spin states. In the present invention, the predetermined time is set to a time longer than any of the energy relaxation times for the four types of spin states, so that the predetermined time elapses after the control voltage is set to the second voltage. By making sure that a singlet ground state occupied by two electrons in one quantum dot is generated and then setting the control voltage to the first voltage, a spin singlet having a quantum entanglement state Make sure the state is generated.

  In the quantum entanglement state generation method, in place of the resetting step, a step of detecting whether energy relaxation has occurred, and when the energy relaxation has occurred, the control voltage is set to the first voltage. And a step of setting.

  In the present invention, instead of the step of resetting the control voltage to the first voltage after the lapse of the predetermined time, when the occurrence of energy relaxation occurs by detecting the presence or absence of energy relaxation, Since the singlet ground state is generated, the spin singlet state having the quantum entanglement state is reliably generated by setting the control voltage to the first voltage after the occurrence is detected.

In the quantum entanglement state generation method according to the second aspect of the present invention, the control voltage applied to the first quantum dot and the second quantum dot is in the state where two electrons occupy one quantum dot. After setting the second voltage that is more stable in terms of energy than the state in which one electron occupies each quantum dot and the step setting the second voltage, two electrons are injected into one quantum dot. After the step and the step of injecting electrons, the control voltage is energetically adjusted in a state where one electron occupies each quantum dot than in a state where two electrons occupy one quantum dot. And setting to a stable first voltage.

  In the present invention, by setting the control voltage to the second voltage and injecting two electrons into one quantum dot, the singlet ground state is reliably generated, and then the control is performed. By setting the voltage to the first voltage, a spin singlet state having a quantum entanglement state is reliably generated.

In the quantum entanglement state detection method according to the third aspect of the present invention, a control voltage to be applied to the first quantum dot and the second quantum dot is applied to four types of spins in which each electron occupies one quantum dot. After the step of setting the first voltage, which is more stable in energy than the state where two electrons occupy one quantum dot in the state, and the step of setting the first voltage, the first quantum dot and the second After the step of injecting electrons one by one into each quantum dot and the step of injecting electrons, the control voltage is set so that two quantum electrons are occupied by one quantum dot. and setting the second voltage to energetically stable than a state of one by one occupied, after the step of setting the second voltage, caused by the control voltage from the first voltage to a second voltage Measuring the relaxation time of the energy relaxation, after the step of measuring the relaxation time, it is a quantum entangled state by comparing the known energy relaxation time for the spin state of the measured relaxation times and four Detecting.

  In the present invention, by setting the control voltage applied to the first and second quantum dots to the first voltage and injecting one electron into each quantum dot, one of the four types of spin states can be obtained. After the generation, the control voltage is set to the second voltage. Then, the relaxation time of energy relaxation that occurs at this time is measured, and the measured relaxation time is compared with a known relaxation time to detect a quantum entanglement state. As described above, the present invention realizes the detection of the quantum entanglement state by a simple method of measuring the energy relaxation time by paying attention to the fact that the energy relaxation time varies depending on the four types of spin states.

  In the quantum entanglement state detection method, the measuring step includes detecting a charge of at least one quantum dot with a charge meter, setting a second voltage, and changing a detection result of the charge meter. It is characterized by measuring.

  In the present invention, paying attention to the fact that energy relaxation involves charge movement, the charge of at least one quantum dot is detected by a charge meter, and the charge meter detection result changes as the energy relaxation time. By measuring the time, the energy relaxation time is measured reliably and accurately.

  According to the quantum entanglement state generation method of the present invention, the quantum entanglement state can be reliably generated, and the utilization efficiency can be improved.

  Moreover, according to the quantum entanglement state detection method of the present invention, the generated quantum entanglement state can be easily detected.

[First Embodiment]
FIG. 1 is a schematic diagram illustrating an example of a configuration of a quantum entanglement element used in a quantum entanglement state generation method / detection method according to the present embodiment. The element shown in FIG. 1 has quantum dots 1 (first quantum dots) and quantum dots 2 (second quantum dots) coupled by a tunnel barrier 6, and electrodes 3 and quantum dots 1 are coupled by a tunnel barrier 5. The quantum dots 2 are coupled by a tunnel barrier 7.

  When one electron is injected into these two quantum dots 1 and 2, a state in which the one electron occupies the quantum dot 1 and a state in which the quantum dot 2 is occupied can be considered. The quantum mechanical superposition of these two states can be used as a quantum information memory. This superposition can be controlled by applying a pulsed voltage to the gate electrode between the electrodes 3 and 4 (not shown). The positional relationship between the electrodes 3 and 4 and the gate electrode will be described later with reference to FIG.

  When one electron is injected into each of the quantum dots 1 and 2, there are four types of electron spin states. FIG. 2 is a schematic diagram showing the relationship between these four types of spin states. As shown in the figure, the spin state is a spin singlet state (hereinafter referred to as “| 11S>”. 11S means a singlet state in which one electron is entered in each of the left and right quantum dots) In the spin triplet, there are states where the spin z component is +1, 0, −1 (represented as | 11T +>, | 11T0>, | 11T->, respectively, where T is the meaning of Triplet). Among them, | 11S> is in a quantum entanglement state in which the direction of the electron spin in the quantum dot 1 and the direction of the electron spin in the quantum dot 2 have a negative correlation. | 11T0> is also in a quantum entanglement state with a negative correlation. However, | 11T0> is in a different phase from | 11S>. On the other hand, in | 11T +> and | 11T->, the direction of the electron spin in the quantum dot 1 is the same as the direction of the electron spin in the quantum dot 2, and is not in the state of quantum entanglement.

  Usually, when electrons are injected into two quantum dots one by one, it is not clear which of these four states will result, so only a mixed state in which the four states are mixed can be obtained. In such cases, there is no quantum entanglement. Therefore, a method for generating and detecting quantum entanglement by some method is required.

  In the generation method of the present embodiment, in addition to the above four spin states, the fifth state is a spin singlet state in which two electrons occupy one orbit in any one quantum dot (| 02S> or | 20S>, where 02S and 20S indicate the singlet state in which two electrons are contained in one quantum dot. Either | 02S> or | 20S> may be used, but here, | 02S> is described as the fifth state.

  In general, it is known that a quantum dot structure has properties of spin orbit interaction and electron lattice interaction. Since this spin-orbit interaction works between states having different orbits and the spin z component being different by one, the spin-orbit interaction works between the above five states | 11T +> and | 02S> And only between | 11T-> and | 02S>. In addition, the electron-lattice interaction causes energy relaxation when preserving spin, and thus occurs remarkably between | 11S> and | 02S>. Furthermore, when spin-orbit interaction and electron-lattice interaction coexist, energy relaxation occurs at a low frequency between | 11T +> and | 02S> and between | 11T-> and | 02S>. The existence of such a selection rule is also known theoretically (see Non-Patent Document 2). The generation method in the present embodiment uses this selection rule.

  By applying a voltage to the gate electrode between the electrodes 3 and 4 (not shown), the potential of each quantum dot 1 and 2 can be changed. FIG. 3 is a graph showing the relationship between energy and voltage for each of the five states. Reference numerals 8, 9, 10, 11, 12, and 13 in the drawing correspond to the states of | 11S>, | 11T +>, | 11T0>, | 11T->, | 20S>, and | 02S>, respectively. Here, since the relative positional relationship between | 20S> and | 02S> and the energy of other states is important, the representative control voltages are the first voltage a and the second voltages b and b '.

  The first voltage a is 2 for one quantum dot in each of the four types of spin states (| 11S>, | 11T +>, | 11T0>, | 11T->) where one electron occupies each quantum dot. This is a voltage that is more stable in terms of energy than the state occupied by one electron (| 20S>, | 02S>). In the figure, at the first voltage a, the energy of | 20S>, | 02S> is higher than the other | 11S>, | 11T> states.

  The second voltages b and b 'are voltages that are more stable in terms of energy in a state where two electrons occupy one quantum dot than in a state where one electron occupies each quantum dot. In the figure, at the second voltages b and b ', the states of | 20S> and | 02S> are ground states with the lowest energy.

  The electrons in the quantum dots are weakly coupled due to the lattice vibration mode of the host crystal constituting the quantum dots, that is, the electron lattice interaction with the phonons. Further, due to the material or structure constituting the quantum dot, the electron orbit and the electron spin in the quantum dot are weakly coupled by the spin orbit interaction. These electron lattice interaction and spin orbit interaction exist naturally in the material constituting the quantum dot, or can be obtained by artificial material design or structural design.

  Note that the states | 11T +>, | 11T0>, and | 11T-> are degenerated in the zero magnetic field and are split by applying the magnetic field. In the figure, the triplet state is shown to be slightly split. In addition, since the state | 11S> and | 20S>, | 02S> cause a large anticrossing due to the tunnel coupling between the quantum dots, the anticrossing is also illustrated.

  When one electron is injected into each of the quantum dots 1 and 2 at the first voltage a, one of the states | 11S>, | 11T +>, | 11T0>, and | 11T-> is formed. . Here, when the control voltage is changed from the first voltage a to the second voltage b or b ′ in a non-adiabatic manner, the respective states are initially maintained in the second voltages b and b ′, but finally, Energy relaxes to the ground state of | 02S> or | 20S>. The relaxation time at this time depends on whether the state is | 11S>, | 11T +>, | 11T0>, or | 11T->.

  FIG. 4 is a diagram showing the relationship between the energy relaxation process and the relaxation time for each spin state. Energy relaxation from | 11S> to | 02S> (or | 20S>) occurs efficiently by electron-lattice interaction, so the relaxation time τ1 is the shortest. Since energy relaxation from | 11T +> and | 11T-> to | 02S> (or | 20S>) is caused by spin orbit interaction and electron lattice interaction, their relaxation times τ2 and τ4 are relatively long. Furthermore, energy relaxation from | 11T0> to | 02S> (or | 20S>) is not caused by low-order spin-orbit interaction and electron lattice interaction, so the relaxation time τ3 is the longest. In a typical quantum dot, it is considered that τ1 reaches about 10 nsec, τ2 and τ4 reach about 1 μsec to 100 μsec, and τ3 reaches 500 μsec or more.

  Next, a method for generating a quantum entanglement state in the present embodiment will be described with reference to the flowchart of FIG.

  In step 1 (shown as S1 in the figure, the same applies hereinafter), the control voltage applied to the quantum dots 1 and 2 is set to the first voltage a. This control voltage is applied to the gate electrode between the electrodes 3 and 4 (not shown).

  In step 2, one electron is injected into each of the quantum dots 1 and 2. As a result, a state of | 11S>, | 11T +>, | 11T0>, or | 11T-> is formed.

  In step 3, the control voltage is set from the first voltage a to the second voltage b or b '.

  In step 4, after the elapse of a predetermined time defined based on differences in the relaxation times of energy relaxation caused by changing the control voltage from the first voltage to the second voltage in the four types of spin states, the control voltage Is reset to the first voltage. By appropriately setting the predetermined time in step 4, a quantum entanglement state of | 11T0> or | 11S> is generated.

  First, processing for generating the state | 11T0> will be described. Here, the predetermined time is set to a time longer than the relaxation times τ1, τ2, and τ4 and shorter than τ3. Thereby, in step 4, when this predetermined time has elapsed, only | 11T0> having a long relaxation time remains in the excited state without being relaxed. The presence or absence of energy relaxation can be detected by using a charge meter. In the relaxation process, since electrons always move from one quantum dot to the other quantum dot, a charge meter is provided to detect the charge of at least one quantum dot, and the output result of the charge meter has not changed. It can be specified that the quantum entanglement state | 11T0>. If no energy relaxation occurs after the lapse of the predetermined time, the quantum entanglement state of | 11T0> is obtained by resetting the control voltage to the first voltage.

  If the output result of the charge meter changes, it indicates that energy relaxation has occurred. Therefore, specify that the spin state is one of | 11S>, | 11T +>, or | 11T->. it can. In this case, return to step 1, set the first voltage again, discharge the electrons of at least one quantum dot to the electrode, and then inject another electron into the corresponding quantum dot, and the quantum entanglement by | 11T0> Each of the above steps is repeated until a mental state is obtained. On average, a quantum entanglement state of | 11T0> can be obtained by repeating the operation about four times.

  Next, processing for generating a state of | 11S> will be described. Here, the predetermined time is set to a time longer than the relaxation times τ1, τ2, τ3, and τ4. As a result, in step 4, energy relaxation can be performed so that the ground state is always | 20S> or | 02S>. After obtaining the ground state, the control voltage is changed non-adiabatically from the second voltage b or b 'to the first voltage a. As a result, the ground state at the second voltage b or b ′ changes to the excited state at the first voltage a. At the first voltage a, energy relaxation from the excited state to the ground state occurs most efficiently, so that the quantum entanglement state by | 11S> can be obtained almost certainly by passing the predetermined time.

  Next, another process for generating the state | 11S> will be described. Here, instead of the process of step 4, the presence or absence of occurrence of energy relaxation is detected using a charge meter. That is, it waits until the output result of the charge meter changes. Since this energy relaxation also involves a change in charge state, the relaxation process can be detected by a charge meter. When the output result changes, it means that energy relaxation has occurred, so that the ground state | 02S> or | 20S> is formed by the relaxation. In this case, by changing the control voltage from the second voltage to the first voltage in a non-adiabatic manner, the ground state at the second voltage changes to the excited state at the first voltage. This also makes it possible to obtain the quantum entanglement state by | 11S> almost certainly.

  Next, still another process for generating the state | 11S> will be described. Here, the following processing is performed instead of the processing of step 1 to step 4. First, the control voltage applied to the quantum dots 1 and 2 is set to the second voltage b or b ′. Then, two electrons are injected into one quantum dot. This reliably forms the ground state. Thereafter, by changing the control voltage from the second voltage to the first voltage in a non-adiabatic manner, the ground state at the second voltage changes to an excited state at the first voltage. This also makes it possible to obtain the quantum entanglement state by | 11S> almost certainly.

  Next, a method for detecting a quantum entanglement state in the present embodiment will be described. This detection method uses the fact that differences occur in the four types of spin states in the relaxation time when energy relaxation occurs.

  Although there are a plurality of methods for measuring the occurrence of energy relaxation, it is desirable to use a highly sensitive charge meter 14 in the vicinity of the quantum dots as shown in FIG. By capacitively coupling at least one of the quantum dots and the charge meter 14 by the capacitor capacitance 15, the charge meter 14 can measure the charge state of the quantum dot, indicating that charge transfer, that is, energy relaxation has occurred. It becomes detectable.

  On the other hand, in any case of the first voltage a and the second voltage b, b ′, the energy from any state of | 11S>, | 11T +>, | 11T0>, | 11T-> to any state Relaxation is not caused by spin orbit interactions and electron lattice interactions.

  Based on the above, this quantum entanglement state detection method will be described with reference to the flowchart of FIG. Steps 11 to 13 in the figure are the same as the processes in steps 1 to 3 in FIG.

  In step 14, the energy relaxation time caused by changing the control voltage from the first voltage a to the second voltage b or b 'is measured. Specifically, the relaxation time is measured by measuring the time T from when the control voltage is set to the second voltage until the output result of the charge meter 14 changes.

  If the measured relaxation time is about τ1 or shorter, the spin state is found to be the quantum entanglement state | 11S>, and if about τ3 or longer, the quantum entanglement state | 11T0>. I understand that. Also, if it is about τ2 or τ4, it can be seen that the spin state is | 11T +> or | 11T-> which is not in the quantum entanglement state.

  Therefore, in step 15, the quantum entanglement state is detected by comparing the measured relaxation time with the known energy relaxation times τ1 to τ4 for the four types of spin states.

  In reality, other interactions may occur, and the magnitude relation of the energy relaxation time for each spin state may be reversed. However, even in that case, since there is a difference in relaxation time between the respective spin states, it is possible to detect which state it was.

  In step 14, the relaxation time can be measured without using a charge meter, for example, by using an ammeter. At the second voltage b or b ′, the current generated by the tunneling of electrons from the quantum dot 1 to the electrode 3 or from the quantum dot 2 to the electrode 4 can be detected only when no energy relaxation occurs. . Therefore, in order to measure the time until energy relaxation occurs, the time from when the control voltage is set to the second voltage until no current is detected by the ammeter is measured.

  Therefore, according to the present embodiment, as a method of generating the quantum entanglement state, the control voltage applied to the quantum dots 1 and 2 is set to the first voltage a and one electron is injected into each quantum dot. Thus, after generating any of the four spin states, the control voltage is set to the second voltage b or b ′. Then, after a lapse of a predetermined time defined based on the difference in energy relaxation time between the four types of spin states, the control voltage is reset to the first voltage a, so that only the spin singlet state is obtained. In addition, it is possible to reliably generate a quantum entanglement state in the spin triplet state, thereby improving the utilization efficiency of the quantum entanglement state. Thereby, the more excellent application to a quantum computer etc. can be aimed at.

  According to the present embodiment, as the predetermined time, the energy relaxation time τ3 for the state | 11T0> in which the spin z component in the spin triplet state is 0 is shorter than the other three types of spin states | 11S> and | 11T + When energy relaxation time does not occur by setting the time longer than energy relaxation time τ1, τ2, τ4 for>, | 11T->, and detecting the occurrence of energy relaxation after the elapse of this predetermined time , It is detected that a state in which the spin z component of the spin triplet state is 0 is generated, and when energy relaxation occurs, it is detected that the other three types of spin states have been generated. . Then, by setting the control voltage to the first voltage a when no energy relaxation occurs, a state in which the spin z component of the spin triplet state having the quantum entanglement state is zero can be reliably generated.

  According to the present embodiment, the predetermined time is set to a time longer than any of the energy relaxation times for the four types of spin states, so that the predetermined voltage is set after the control voltage is set to the second voltage b or b ′. Make sure that a singlet ground state | 20S> or | 02S> occupied by two electrons in one quantum dot is generated after the passage of time, and then set the control voltage to the first voltage. Thus, a spin singlet state having a quantum entanglement state can be reliably generated.

  According to the present embodiment, as another method of generating the quantum entanglement state, instead of step 4 of resetting the control voltage to the first voltage a after the elapse of a predetermined time in FIG. By detecting this, the energy relaxation occurs when the singlet ground state | 20S> or | 02S> is generated, and the control voltage is changed to the first voltage after this occurrence is detected. By setting, a spin singlet state having an entanglement state can be reliably generated.

  According to the present embodiment, as yet another method of generating the quantum entanglement state, two electrons are applied to one quantum dot after setting the control voltage of the quantum dots 1 and 2 to the second voltage b or b ′. To ensure that the singlet ground state | 20S> or | 02S> is generated, and then the control voltage is set to the first voltage a, thereby providing a spin singlet having a quantum entanglement state. The term state can be generated reliably.

  According to the present embodiment, as a method for detecting the quantum entanglement state, by setting the control voltage applied to the quantum dots 1 and 2 to the first voltage a and injecting electrons one by one into each quantum dot, After generating any of the four types of spin states, the control voltage is set to the second voltage b or b ′. Then, the relaxation time of energy relaxation that occurs at this time is measured, and the measured relaxation time is compared with a known relaxation time to detect a quantum entanglement state. Thus, by focusing on the fact that the energy relaxation time varies depending on the four types of spin states, the quantum entanglement state can be detected by a simple method of measuring the energy relaxation time.

  According to the present embodiment, paying attention to the fact that energy relaxation involves charge movement, the charge meter 14 detects the charge of at least one quantum dot, and the charge meter detection result changes as the energy relaxation time. By measuring the time until the energy relaxation time, the energy relaxation time can be measured reliably and accurately.

  The quantum entanglement state generation / detection method described above can be realized with one quantum entanglement element.

[Second Embodiment]
In the present embodiment, a more specific configuration and procedure of a quantum entanglement element for realizing the quantum entanglement state generation method and detection method described in the first embodiment will be described.

  FIG. 8 is a perspective view showing the configuration of the quantum entanglement element. The device shown in the figure is formed by etching and forming a gate electrode of stacked structures 26 and 27 using gallium arsenide or the like. The device shown in FIG. 1 can be realized by applying an appropriate gate voltage to this device at an extremely low temperature of about 0.1K. In the quantum dots 1 and 2 of the device thus manufactured, the charging energy is about 1 meV, the quantum confinement energy is about 0.1 to 1 meV, the electrostatic coupling energy between dots is about 0.2 meV, and the tunnel coupling energy between quantum dots is about 1 μeV. About 0.1 meV can be obtained.

  FIG. 9 is a cross-sectional view showing a detailed configuration of the laminated structures 26 and 27. As shown in the figure, the laminated structure 26 has a 500 nm thick aluminum gallium arsenide layer, a 10 nm thick indium gallium arsenide layer, a 20 nm thick aluminum gallium arsenide layer on a gallium arsenide layer 27. A silicon-added aluminum gallium arsenide layer having a thickness of 80 nm is formed in this order. With such a semiconductor stacked structure, Rashba type or dresser house type spin-orbit interaction can be generated. Furthermore, a piezoelectric or deformation potential type electron lattice interaction caused by the semiconductor crystal can be generated. In addition, the gate voltages VL and VR can be applied to the plurality of gate electrodes 20, respectively, whereby the potentials of the two quantum dots 1 and 2 can be independently changed. Therefore, the first voltage shown in FIG. The voltage a and the second voltages b and b ′ can be changed at high speed. Here, it is assumed that the gate voltage VL is applied to the quantum dot 1 and the gate voltage VR is applied to the quantum dot 2. FIG. 10 shows a schematic equivalent circuit thereof.

  Since the quantum point contact structure functions as a charge meter by measuring the current flowing through the barrier, the charge meter is stronger than the quantum dot 1 by making it near the quantum dot 1, and the quantum dot 2 Since they are weakly coupled, it becomes possible to detect the charge state of each quantum dot.

  Next, a more specific procedure of the quantum entanglement state generation method described in the above embodiment will be described with reference to the waveform diagram of FIG. First, the gate voltages VL and VR are changed. The initial state is VL = VL0, VR = VR0, and the quantum dots 1 and 2 have no electrons. At time t1, by changing the gate voltage VL from VL0 to VL1, one electron enters the quantum dot 1 and the current value Idet of the charge meter changes from Id0 to Id1. Similarly, by changing the gate voltage VR from VR0 to VR1 at time t2, one electron enters the quantum dot 2 and the current value Idet of the charge meter changes from Id1 to Id2. Here, by setting the gate voltages VL1 and VR1 to be the first voltage a in FIG. 3, a state where electrons are injected into the quantum dots 1 and 2 one by one can be realized. At time t3, after changing the gate voltage VR to VR2, the process waits for a predetermined time tw. The gate voltage VR2 is set to be the second voltage b in FIG. The predetermined time tw is set to a time that is shorter than the relaxation time τ3 in the quantum entanglement state of the spin triplet and sufficiently longer than the other relaxation times τ1, τ2, and τ4. If energy relaxation does not occur during this waiting time, that is, if there is no change in the output result of the charge meter, it will be in a quantum entanglement state with a very high probability. By returning to VR1, a quantum entanglement state in the spin triplet can be generated. On the other hand, when energy relaxation occurs during this waiting time, that is, when a change occurs in the output result of the charge meter, electrons are emitted from the quantum dots 1 and 2 because they are not in the quantum entanglement state, Start over from the beginning.

  Next, a more specific procedure of another method for generating a quantum entanglement state described in the above embodiment will be described with reference to the waveform diagram of FIG. The procedure from time t1 to t3 is the same as that described with reference to FIG. After the gate voltage VR is changed to VR2 at time t3, energy relaxation is caused by waiting for a sufficiently long predetermined time tw '. The predetermined time tw ′ is longer than any of the relaxation times τ1, τ2, τ3, and τ4. Alternatively, as another generation method, energy relaxation may be caused by waiting until energy relaxation occurs and the current value of the charge meter changes to IdR. After causing energy relaxation as described above, the quantum entanglement state in the spin singlet can be generated by returning the gate voltage VR to VR1.

  Next, a more specific procedure of the quantum entanglement state detection method described in the above embodiment will be described with reference to the waveform diagram of FIG. The procedure from time t1 to t3 is the same as that described with reference to FIG. When the gate voltage VR is changed to VR2 at time t3, the charge movement from the quantum dot 1 to the quantum dot 2 finally occurs, and the current value of the charge meter changes from Id2 to IdR. Here, the relaxation time from the time t3 until the current value changes is measured. This relaxation time varies greatly depending on the spin state formed when the first voltage a is applied, and statistically has a distribution as shown in FIG. Therefore, the measured relaxation time is compared with the previously known relaxation time shown in FIG. 14 to determine in which region the measured relaxation time is, thereby detecting whether or not the quantum entanglement state exists. be able to.

It is a schematic diagram which shows the structure of a quantum entanglement element. It is a schematic diagram which shows the relationship of each spin state. It is a graph which shows the relationship between energy and voltage about each spin state. It is a figure which shows the relationship between an energy relaxation process and relaxation time about each spin state. It is a flowchart which shows the production | generation process of a quantum entanglement state. It is a schematic diagram which shows the state which provided the charge meter in the vicinity of the quantum dot. It is a flowchart which shows the detection process of a quantum entanglement state. It is a perspective view which shows the more concrete structure of a quantum entanglement element. It is sectional drawing which shows the semiconductor laminated structure of a quantum entanglement element. It is a typical equivalent circuit which shows the electrical constitution of a quantum entanglement element. It is a wave form diagram which shows the gate voltage and electric current in the production | generation process of a quantum entanglement state. It is a wave form diagram which shows the gate voltage and electric current in another production | generation process of a quantum entanglement state. It is a wave form diagram which shows the gate voltage and electric current in the detection process of a quantum entanglement state. It is a graph which shows the relationship between energy relaxation and its frequency. It is a schematic diagram which shows the structure of the conventional quantum entanglement element.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 2 ... Quantum dot 3, 4 ... Electrode 5, 6, 7 ... Tunnel barrier 14 ... Charge meter 15 ... Capacitor capacity 20 ... Gate electrode 26, 27 ... Laminated structure 51, 52, 53 ... Quantum dot 54, 55, 56 ... Electrodes 57-61 ... Tunnel barrier

Claims (7)

The control voltage applied to the first quantum dot and the second quantum dot is occupied by one electron in each quantum dot, and in the four types of spin states, two electrons are occupied in one quantum dot. Setting the first voltage to be more energetically stable than the state;
After the step of setting to the first voltage, injecting one electron into each of the first quantum dot and the second quantum dot;
After the step of injecting electrons, the control voltage is stabilized more energetically in a state where two electrons occupy one quantum dot than in a state where one electron occupies each quantum dot. Setting to two voltages;
After the step of setting to the second voltage, a predetermined value is defined based on differences in energy relaxation relaxation times caused by changing the control voltage from the first voltage to the second voltage in the four types of spin states. Resetting the control voltage to the first voltage after the time has elapsed from the second voltage ; and
A method for generating a quantum entanglement state, comprising: The predetermined time is equal to or shorter than the energy relaxation time of the spin triplet state of the spin entanglement state in the quantum entanglement state among the four types of spin states, and the other three types of spins. A time longer than the energy relaxation time for the state,
The resetting step includes:
Detecting the presence or absence of energy relaxation after the elapse of the predetermined time;
When no energy relaxation occurs, setting the control voltage to the first voltage;
The quantum entanglement state generation method according to claim 1, wherein:   The method for generating a quantum entanglement state according to claim 1, wherein the predetermined time is longer than any of energy relaxation times for four types of spin states. Instead of the resetting step,
Detecting the occurrence of energy relaxation;
When energy relaxation has occurred, setting the control voltage to a first voltage;
The quantum entanglement state generation method according to claim 1, wherein: The control voltage applied to the first quantum dot and the second quantum dot is more energetic in the state where two electrons occupy one quantum dot than in the state where one electron occupies each quantum dot. Setting the second voltage to be stable,
After the step of setting to the second voltage, injecting two electrons into one quantum dot;
After the step of injecting electrons, the control voltage is more stable in terms of energy in a state where one electron is occupied in each quantum dot than in a state where two electrons are occupied in one quantum dot. Setting to 1 voltage;
A method for generating a quantum entanglement state, comprising: The control voltage applied to the first quantum dot and the second quantum dot is occupied by one electron in each quantum dot, and in the four types of spin states, two electrons are occupied in one quantum dot. Setting the first voltage to be more energetically stable than the state;
After the step of setting to the first voltage, injecting one electron into each of the first quantum dot and the second quantum dot;
After the step of injecting electrons, the control voltage is stabilized more energetically in a state where two electrons occupy one quantum dot than in a state where one electron occupies each quantum dot. Setting to two voltages;
After setting the second voltage, measuring a relaxation time of energy relaxation caused by changing the control voltage from the first voltage to the second voltage;
After the step of measuring the relaxation time, detecting the quantum entanglement state by comparing the measured relaxation time with known energy relaxation times for the four spin states;
A method for detecting a quantum entanglement state, comprising:   7. The step of measuring, wherein the charge of at least one quantum dot is detected by a charge meter, and the time from when the second voltage is set until the detection result of the charge meter changes is measured. The method for detecting a quantum entanglement state as described.

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