Quantum bit control device and quantum computer

Abstract

To provide a quantum bit control device capable of satisfactorily generating an analog signal from a frequency-multiplexed pulse signal.SOLUTION: The quantum bit control device 16 includes a generation circuit group 21 including a plurality of waveform generation circuits 14a, 14b, and 14c that generate pulse signals at frequencies assigned to the respective quantum bits 24a, 24b, and 24c, a multiplexing circuit 22 that is commonly connected to the waveform generation circuits 24a, 24b, and 24c and frequency-multiplexes the pulse signals while adjusting the phases of the pulse signals on the basis of an algorithm for reducing the crest factor, and a digital-to-analog converter 23 that generates analog signals from the frequency-multiplexed pulse signals.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a qubit control device and a quantum computer for controlling the excitation of qubits. [Background technology]

[0002] In quantum computers, the state transitions of qubits are controlled by pulse signals. For example, an arbitrary waveform generator (AWG) is used to generate these pulse signals. An AWG comprises a waveform memory that stores shaped waveforms and outputs waveforms according to a frequency set by a clock signal source, and a digital-to-analog converter (DAC) that generates an analog signal from the output waveform. Since the amplitude of the waveform is generally set to a specific size, the DAC can generate an analog signal that corresponds well to the waveform. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2023-67604 [Non-patent literature]

[0004] [Non-Patent Document 1] JPGvan Dijk et al., “Impact of Classical Control Electronics on Qubit Fidelity”, Phys.Rev.Applied 12,044054(2019), Published October 24, 2019 [Non-Patent Document 2] Stephen Boyd, "Multitone Signals with Low Crest Factor," IEEE Transactions on Circuits and Systems, Vol. CAS-33, No. 10, published October 10, 1986. [Overview of the project] [Problems that the invention aims to solve]

[0005] Analog signals are generated for each individual qubit. For controlling a qubit, a transmission path for the analog signal is established for each individual qubit. This establishment of transmission paths is thought to impose physical constraints on the integration of qubits. For example, when exciting a superconducting qubit, each qubit is supplied with microwaves at its resonant frequency. These microwaves are guided via coaxial cables to the qubits in a dilution refrigerator. The same number of coaxial cables as qubits are installed in the dilution refrigerator. However, due to spatial constraints within the dilution refrigerator and heat generation issues in the attenuator, the integration of qubits is hindered as long as a coaxial cable is connected to each qubit. Non-patent document 1 proposes frequency multiplexing of pulse signals, but a method for generating analog signals from frequency multiplexed pulse signals has not yet been established.

[0006] The present invention aims to provide a qubit control device that can generate analog signals well from frequency-multiplexed pulse signals. [Means for solving the problem]

[0007] A qubit control device according to an embodiment of the present invention includes a group of generation circuits including a plurality of waveform generation circuits that generate pulse signals at frequencies assigned to each qubit; a multiplexing circuit that is commonly connected to the waveform generation circuits and frequency multiplexes the pulse signals while adjusting the phase of the pulse signals based on an algorithm that reduces the crest factor; and a digital-to-analog converter that generates an analog signal from the frequency multiplexed pulse signals.

[0008] A quantum computer according to an embodiment of the present invention comprises: a group of qubits including a plurality of qubits excited at individually assigned frequencies; a group of generation circuits including a plurality of waveform generation circuits that generate pulse signals at frequencies assigned to each of the qubits; a multiplexing circuit connected in common to the waveform generation circuits, which frequency multiplexes the pulse signals while adjusting the phase of the pulse signals based on an algorithm for reducing the crest factor; and a digital-to-analog converter that generates an analog signal from the frequency multiplexed pulse signals. [Effects of the Invention]

[0009] As described above, according to the embodiment of the present invention, a qubit control device is provided that can generate an analog signal well from a frequency-multiplexed pulse signal. [Brief explanation of the drawing]

[0010] [Figure 1] This is a conceptual diagram showing the configuration of a quantum computer system according to the first embodiment of the present invention. [Figure 2] This graph shows the relationship between the phase and amplitude of a frequency-multiplexed pulse signal. [Figure 3] This is a conceptual diagram showing the configuration of a quantum computer system according to the second embodiment of the present invention. [Modes for carrying out the invention]

[0011] One embodiment of the present invention will be described below with reference to the attached drawings.

[0012] Figure 1 schematically shows the configuration of a quantum computer system 11 according to the first embodiment of the present invention. The quantum computer system 11 comprises a quantum computer 12 that performs computational processing according to the theory of quantum mechanics, and a front-end server 13 connected to the quantum computer 12 that supplies a software program describing the computational processing required by the quantum computer 12. The front-end server 13 can be accessed by a user, for example, via the internet. The front-end server 13 generates the necessary software program according to the user's "question". The quantum computer 12 performs computational processing according to the generated software program. The front-end server 13 provides the user with the "solution" obtained from the computational processing of the quantum computer 12. The front-end server 13 may also hand over the software program supplied by the user directly to the quantum computer 12.

[0013] The quantum computer 12 comprises a qubit group 14 containing multiple qubits 14a, 14b, and 14c, a dilution refrigerator 15 that houses the qubits 14a, 14b, and 14c in an extremely cold space (under an extremely cold environment), and a qubit control device 16 connected to the dilution refrigerator 15. Each qubit 14a, 14b, and 14c is configured as a superconducting qubit. Each qubit 14a, 14b, and 14c is individually assigned a resonance frequency. When microwaves of the resonance frequency act on the qubits 14a, 14b, and 14c, the qubits 14a, 14b, and 14c can be excited. The resonance frequencies can be set apart, for example, with equal bandwidths. Here, the first frequency is assigned to qubit 14a. The second frequency is assigned to qubit 14b. The third frequency is assigned to qubit 14c. The number of qubits 14a, 14b, and 14c is not limited to three.

[0014] One distributor 17 is commonly connected to a plurality of qubits 14a, 14b, and 14c. The distributor 17 is arranged in a dilution refrigerator 15 together with the qubits 14a, 14b, and 14c. The distributor 17 is accommodated in a cryogenic space. The distributor 17 distributes a signal received commonly for the qubits 14a, 14b, and 14c to the individual qubits 14a, 14b, and 14c. A qubit controller 16 is connected to the distributor 17. The qubit controller 16 outputs an analog signal [microwave] that selectively causes excitation toward the plurality of qubits 14a, 14b, and 14c. One transmission path for the analog signal is established for each distributor 17. One transmission path can be constituted by one coaxial cable extending from the qubit controller 16 to the distributor 17. In the dilution refrigerator 15, the transmission path reaching the distributor 17 can be constituted by one coaxial cable.

[0015] The qubit controller 16 includes: a generation circuit group 21 that generates a pulse signal for each of the individual qubits 14a, 14b, and 14c; a multiplexing circuit 22 connected to the generation circuit group 21 that frequency-multiplexes the pulse signals; and a digital-to-analog converter (DAC) 23 that generates an analog signal from the frequency-multiplexed pulse signals. Here, the multiplexing circuit 22 can multiplex pulse signals, for example, as represented by the following formula. Each individual pulse signal is specified by a sine wave modulated with a Gaussian pulse.

Math

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[0016] The generation circuit group 21 includes a first waveform generation circuit 24a that generates a pulse signal based on the resonance frequency set for qubit 14a, a second waveform generation circuit 24b that generates a pulse signal based on the resonance frequency set for qubit 14b, and a third waveform generation circuit 24c that generates a pulse signal based on the resonance frequency set for qubit 14c. In the first waveform generation circuit 24a, the waveform is set based on the resonance frequency of qubit 14a. In the second waveform generation circuit 24b, the waveform is set based on the resonance frequency of qubit 14b. In the third waveform generation circuit 24c, the waveform is set based on the resonance frequency of qubit 14c.

[0017] The excitation of individual qubits 14a, 14b, and 14c is controlled by pulse signals. Pulse signals are generated at frequencies assigned to each qubit 14a, 14b, and 14c. Analog signals are generated from the frequency-multiplexed pulse signals. These analog signals are supplied to qubits 14a, 14b, and 14c. In this way, the excitation of qubits 14a, 14b, and 14c is controlled. Since the pulse signals are frequency-multiplexed before generating the analog signals, the transmission path (wiring) for the analog signals can be reduced. When the phase is adjusted based on an algorithm that reduces the crest factor during frequency multiplexing, the amplitude of the analog signal output from DAC23 can be kept within a specified range. Analog signals can be generated well from the frequency-multiplexed pulse signals. Even when pulse signals are multiplexed, the waveforms of individual pulse signals can effectively act on the corresponding qubits 14a, 14b, and 14c. The excitation of qubits 14a, 14b, and 14c can be driven effectively.

[0018] Each of the qubits 14a, 14b, and 14c is supplied with a non-resonant excitation pulse signal. These non-resonant excitation pulse signals can be generated, for example, by the first waveform generation circuit 24a, the second waveform generation circuit 24b, and the third waveform generation circuit 24c, respectively. The non-resonant excitation pulse signals cause an AC Stark shift in the qubits 14a, 14b, and 14c. When non-resonant excitation pulse signals are supplied to the qubits 14a, 14b, and 14c, an interaction described by the following Hamiltonian occurs in the qubits 14a, 14b, and 14c.

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[0019] The qubit control device 16 includes a resonator that reads out the states of qubits 14a, 14b, and 14c. The front-end server 13 generates a "solution" defined by a software program according to the read states.

[0020] The resonance frequencies assigned to qubits 14a, 14b, and 14c are separated by equal bandwidth. As mentioned above, the phase of the pulse signals is adjusted based on an algorithm that reduces the crest factor, so interference between pulse signals can be effectively avoided.

[0021] In this embodiment, the first waveform generation circuit 24a, the second waveform generation circuit 24b, and the third waveform generation circuit 24c generate waveforms that specify the z-axis rotation of qubits 14a, 14b, and 14c that cancel out the z-axis rotation caused by the phase shift. When non-resonant excitation pulse signals are supplied to qubits 14a, 14b, and 14c based on the waveforms, z-axis rotation is induced in qubits 14a, 14b, and 14c. The z-axis rotation caused by the phase shift can be canceled out. In this way, qubits 14a, 14b, and 14c can be driven effectively.

[0022] In this embodiment, the distributor 17 is connected to the DAC 23 and distributes the analog signal to the individual qubits 14a, 14b, and 14c. A single-qubit gate operator acts on each of the individual qubits 14a, 14b, and 14c. The analog signal can be used in common by multiple qubits 14a, 14b, and 14c. The waveform of each pulse signal can act well on the corresponding qubits 14a, 14b, and 14c. The qubits 14a, 14b, and 14c can be driven effectively.

[0023] The waveform generation circuits 24a, 24b, and 24c generate pulse signals based on the frequencies that cause AC Stark shifts in the corresponding qubits 14a, 14b, and 14c. When pulse signals that cause AC Stark shifts are supplied to qubits 14a, 14b, and 14c, an interaction can be realized that produces a z-axis rotation described by the Hamiltonian. Thus, the z-axis rotation can be canceled out in response to the supply of non-resonant excitation pulse signals.

[0024] The inventors verified the effect of phase shift used in frequency multiplexing. In the verification, the inventors calculated [Equation 1]. At all frequencies, the amplitude is a i =1 was set. The base frequency was set to ω0 / 2π=5[GHz]. The frequency interval was set to Δω / 2π=0.1[GHz]. The standard deviation was set to σ=15. The sample size was set to N=30. When φ=0 was set for the phase at all frequencies, it was confirmed that pulses were concentrated at the maximum value Maxi and the minimum value Mini, as shown in Figure 2. Only a few pulses were observed outside of the maximum value Maxi and the minimum value Mini. When the phase was set according to Schroeder's algorithm, it was confirmed that the amplitude fell within a specific range Sch and was leveled along the time axis. When the phase was set randomly, it was confirmed that the amplitude fell within a range Rdm that was larger than Schroeder's range Sch. The superiority of the algorithm for reducing the crest factor was confirmed.

[0025] Here, the phase φ i Rotational operation R achieved by microwave pulses φi (θ) is defined by the following equation.

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[0026] FIG. 3 schematically shows the configuration of a quantum computer system 31 according to a second embodiment of the present invention. In this embodiment, a quantum computer 32 includes a quantum bit group 35 including a plurality of quantum bits 33a, 33b, 33c, 33d, and a control quantum bit 34 coupled to the quantum bits 33a, 33b, 33c, 33d to form cross resonance. The quantum bit group 35 is housed in the dilution refrigerator 15 in the same manner as described above. A quantum bit controller 36 is connected to the dilution refrigerator 15.

[0027] Each individual qubit 33a, 33b, 33c, 33d and the control qubit 34 are composed of superconducting qubits. Each individual qubit 33a, 33b, 33c, and 33d is assigned a unique resonance frequency. When microwaves of the resonance frequency act on qubits 33a, 33b, 33c, and 33d, they can be excited. The resonance frequencies can be set apart, for example, with equal bandwidths. Here, the first frequency is assigned to qubit 33a. The second frequency is assigned to qubit 33b. The third frequency is assigned to qubit 33c. The fourth frequency is assigned to qubit 33d. The number of qubits 33a, 33b, 33c, and 33d is not limited to four. One control qubit 34 may be assigned for every multiple qubits 33a, 33b, 33c, and 33d.

[0028] A two-qubit gate operator acts on the control qubit 34. An analog signal [microwave] output from the qubit control device 36 is supplied to the control qubit 34. The analog signal selectively excites individual qubits 33a, 33b, 33c, and 33d based on cross-resonance. One transmission path of the analog signal is established for each control qubit 34. One transmission path can consist of one coaxial cable extending from the qubit control device 36 to the control qubit 34. Within the dilution refrigerator 15, the transmission path to the control qubit 34 can consist of one coaxial cable.

[0029] The qubit control device 36 comprises a group of generation circuits 37 that generate pulse signals for each individual qubit 33a, 33b, 33c, and 33d, a multiplexing circuit 38 connected to the group of generation circuits 37 that frequency multiplexes the pulse signals, and a digital-to-analog converter (DAC) 39 that generates an analog signal from the frequency multiplexed pulse signals. The multiplexing circuit 38 can multiplex microwaves as shown in [Equation 1]. When frequency multiplexing, the multiplexing circuit 38 adjusts the phase of the pulse signals based on an algorithm that reduces the crest factor. The amplitude of the multiplexed pulse signals can be leveled according to the phase adjustment. The amplitude of the analog signal output from the DAC 39 can be kept within a predetermined range.

[0030] The generation circuit group 37 includes a first waveform generation circuit 43a that generates a pulse signal based on the resonance frequency set for qubit 33a, a second waveform generation circuit 43b that generates a pulse signal based on the resonance frequency set for qubit 33b, a third waveform generation circuit 43c that generates a pulse signal based on the resonance frequency set for qubit 33c, and a fourth waveform generation circuit 43d that generates a pulse signal based on the resonance frequency set for qubit 33d. In the first waveform generation circuit 43a, the waveform is set based on the resonance frequency of qubit 33a. The waveform is supplied to the control qubit 34 and causes cross-resonance in qubit 33a. In the second waveform generation circuit 43b, the waveform is set based on the resonance frequency of qubit 33b. The waveform is supplied to the control qubit 34 and causes cross-resonance in qubit 33b. In the third waveform generation circuit 43c, the waveform is set based on the resonance frequency of qubit 33c. The waveform is supplied to the control qubit 34 and causes cross-resonance in qubit 33c. In the fourth waveform generation circuit 43d, the waveform is set based on the resonance frequency of qubit 33d. The waveform is supplied to control qubit 34, which causes cross-resonance in qubit 33d.

[0031] Each of the qubits 33a, 33b, 33c, and 33d is supplied with a non-resonant excitation pulse signal. These non-resonant excitation pulse signals can be generated, for example, by the first waveform generator 43a, the second waveform generator 43b, the third waveform generator 43c, and the fourth waveform generator 43d, respectively. The non-resonant excitation pulse signals cause an AC Stark shift in the qubits 33a, 33b, 33c, and 33d. When the non-resonant excitation pulse signals are supplied to the qubits 33a, 33b, 33c, and 33d, an interaction described by the Hamiltonian of [Equation 4] occurs in the qubits 33a, 33b, 33c, and 33d. The Hamiltonian corresponds to the interaction that realizes a rotation operation around the z axis.

[0032] The qubit control device 36 is equipped with a resonator that reads the states of qubits 33a, 33b, 33c, and 33d. The front-end server 13 generates a "solution" defined by a software program according to the read states.

[0033] The excitation of individual qubits 33a, 33b, 33c, and 33d is controlled by pulse signals. Pulse signals are generated at frequencies assigned to each qubit 33a, 33b, 33c, and 33d. An analog signal is generated from the frequency-multiplexed pulse signals. The analog signal common to multiple qubits 33a, 33b, 33c, and 33d can be supplied to a control qubit 34. In accordance with cross-resonance, qubit gates can be applied to individual qubits 33a, 33b, 33c, and 33d. Since the pulse signals are frequency-multiplexed in advance for analog signal generation, the transmission path (wiring) of the analog signal can be reduced. Simultaneous execution of multiple qubit gates can be achieved in accordance with cross-resonance.

[0034] When the phase is adjusted based on an algorithm that reduces the crest factor during frequency multiplexing, the amplitude of the analog signal output from DAC39 can be kept within a specified range. A good analog signal can be generated from the frequency multiplexed pulse signal. Even when pulse signals are multiplexed, the amplitude of the analog signal can be maximized according to the phase shift. The response speed of the qubit gate can be significantly improved. The excitation of qubits 33a, 33b, 33c, and 33d can be driven effectively.

[0035] The resonance frequencies assigned to qubits 33a, 33b, 33c, and 33d are separated by equal bandwidth. As mentioned above, the phase of the pulse signals is adjusted based on an algorithm that reduces the crest factor, so interference between pulse signals can be effectively avoided.

[0036] In this embodiment, the waveform generation circuits 43a, 43b, 43c, and 43d generate waveforms that specify the z-axis rotation of the qubits 33a, 33b, 33c, and 33d that cancel out the z-axis rotation caused by the phase shift. When a pulse signal is supplied to the control qubit 34 based on the waveform, z-axis rotation is induced in the qubits 33a, 33b, 33c, and 33d in accordance with the action of cross-resonance. The z-axis rotation caused by the phase shift can be canceled out. In this way, the excitation of the qubits 33a, 33b, 33c, and 33d can be driven effectively.

[0037] The waveform generation circuits 43a, 43b, 43c, and 43d generate pulse signals based on the frequencies that cause AC Stark shifts in the corresponding qubits 33a, 33b, 33c, and 33d. When pulse signals that cause AC Stark shifts are supplied to qubits 33a, 33b, 33c, and 33d, an interaction can be realized that produces a z-axis rotation described by the Hamiltonian. Thus, the x-axis rotation can be canceled out in response to the supply of non-resonant excitation pulse signals.

[0038] The inventors examined the effects of phase shift associated with frequency multiplexing in order to realize cross-resonance. Cross-resonance can be realized according to the following equation.

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[0039] In the quantum computer 32 using cross-resonance, the waveform generation circuits 43a, 43b, 43c, and 43d may generate pulse signals that complement the pulse signals that cause cross-resonance in the qubits 33a, 33b, 33c, and 33d. The waveform generation circuits 43a, 43b, 43c, and 43d form waveforms that specify z-axis rotation based on a combination of rotation operations around the x-axis and rotation operations around the y-axis. When the waveforms from the waveform generation circuits 43a, 43b, 43c, and 43d are supplied to the qubits 33a, 33b, 33c, and 33d, z-axis rotation occurs in each of the qubits 33a, 33b, 33c, and 33d. In this way, the z-axis rotation can be canceled out in each of the qubits 33a, 33b, 33c, and 33d.

[0040] Here, the operation of the rotating gate is expressed by the following equation.

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[0041] 12 Quantum Computers 14 qubit group 14a qubit 14b qubit 14c qubit 16-qubit control device 21 Generation circuit group 22 Multiplexing circuit 23. Digital-to-Analog Converter (DAC) 24a Waveform generation circuit (1st waveform generation circuit) 24b Waveform generation circuit (second waveform generation circuit) 24c Waveform generation circuit (3rd waveform generation circuit) 32 Quantum Computers 33a Qubit 33b qubit 33c qubit 33d qubit 34 control qubits 35 qubit group 36-qubit control device 37 Generation circuit group 38 Multiplexing circuit 39. Digital-to-Analog Converter (DAC) 43a Waveform generation circuit (1st waveform generation circuit) 43b Waveform generation circuit (second waveform generation circuit) 43c Waveform generation circuit (3rd waveform generation circuit) 43d Waveform generation circuit (4th waveform generation circuit)

Claims

1. A group of generation circuits including multiple waveform generation circuits that generate pulse signals at frequencies assigned to each qubit, A multiplexing circuit is connected in common to the waveform generation circuit and frequency multiplexes the pulse signal while adjusting the phase of the pulse signal based on an algorithm that reduces the crest factor, A digital-to-analog converter that generates an analog signal from the frequency-multiplexed pulse signal, A qubit control device equipped with the following features.

2. The aforementioned frequencies are separated by equal bandwidth. The qubit control device according to claim 1.

3. The aforementioned group of generation circuits further comprises a waveform generation circuit that generates a waveform that specifies the z-axis rotation of the qubit, which cancels out the z-axis rotation caused by the phase shift. The qubit control device according to claim 1.

4. The waveform generation circuit generates a pulse signal based on the frequency at which the qubit causes an AC Stark shift. The qubit control device according to claim 3.

5. The aforementioned generation circuit group further includes a waveform generation circuit that complements the pulse signal supplied to the control qubit coupled to the qubit to cause cross-resonance, and supplies the qubit to generate a waveform that specifies the z-axis rotation based on a combination of rotation operations around the x-axis and rotation operations around the y-axis. The qubit control device according to claim 3.

6. A group of qubits including multiple qubits excited at individually assigned frequencies, A group of generation circuits including multiple waveform generation circuits that generate pulse signals at frequencies assigned to each individual qubit, A multiplexing circuit is connected in common to the waveform generation circuit and frequency multiplexes the pulse signal while adjusting the phase of the pulse signal based on an algorithm that reduces the crest factor, A digital-to-analog converter that generates an analog signal from the frequency-multiplexed pulse signal, A quantum computer equipped with [the necessary components].

7. The aforementioned frequencies are separated by equal bandwidth. The quantum computer according to claim 6.

8. The aforementioned group of generation circuits further comprises a waveform generation circuit that generates a waveform that specifies the z-axis rotation of the qubit, which cancels out the z-axis rotation caused by the phase shift. The quantum computer according to claim 6.

9. The digital-to-analog converter is connected to a distributor that distributes the analog signal to the qubit, causing the qubit to function as a single-qubit gate. The quantum computer according to claim 8.

10. The waveform generation circuit generates a pulse signal based on the frequency at which the qubit causes an AC Stark shift. The quantum computer according to claim 9.

11. The group of qubits further comprises a control qubit that receives the analog signal and is coupled to the qubit to form a cross-resonance. The quantum computer according to claim 8.

12. The waveform generation circuit generates a pulse signal based on the frequency at which the qubit causes an AC Stark shift. The quantum computer according to claim 11.

13. The aforementioned generation circuit group further includes a waveform generation circuit that complements the pulse signal supplied to the control qubit to cause cross-resonance at the target qubit, and generates a waveform that identifies the z-axis rotation based on a combination of rotation operations around the x-axis and rotation operations around the y-axis at the target qubit. The quantum computer according to claim 11.

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