Quantum bit control device and quantum computer

Abstract

Provided is a quantum bit control device and a quantum computer which are capable of contributing to the integration of quantum bits. A quantum bit control device 16 generates a pulse signal having a waveform that suppresses excitation at a second frequency and a third frequency which are different from a first frequency that drives excitation, the pulse signal being directed toward a plurality of quantum bits 14a, 14b, 14c excited at the first frequency, the second frequency, and the third frequency, which are allocated individually.

Description

Quantum bit controller and quantum computer

[0001] The present invention relates to a quantum bit control device that controls the excitation of a quantum bit, and a quantum computer.

[0002] In a quantum computer, a transmission path for the control signal is established for each individual quantum bit to control the excitation of that quantum bit. The establishment of such a transmission path is thought to impose physical constraints on the integration of quantum bits. For example, to excite a superconducting quantum bit, microwaves at the resonant frequency are supplied to each quantum bit. The microwaves are guided to the quantum bit inside the dilution refrigerator via a coaxial cable. The same number of coaxial cables as quantum bits are installed inside the dilution refrigerator. However, due to spatial constraints within the dilution refrigerator and the problem of heat generation in the attenuators, the integration of quantum bits is hindered as long as a coaxial cable is connected to each quantum bit.

[0003] JP 2023-67604 A

[0004] F. Lecocq et al., “Control and Readout of a Superconducting Qubit Using a Photonic Link”, Nature 591, 575 (2021), published March 24, 2021, R. Huang et al., “Cryogenic Multiplexing Control Chip for a Superconducting Quantum Processor”, Phys. Rev. Applied 18,064046 (2022), published December 15, 2022F. Motzoi et al., "Simple Pulses for Elimination of Leakage in Weakly Nonlinear Qubits", Phys. Rev. Lett. 103, 110501 (2009), published September 8, 2009 L. S. Theis et al., "Simultaneous gates in frequency-crowded multilevel systems using fast, robust, analytic control shapes," Phys. Rev. A93, 012324, published January 14, 2014

[0005] Non-Patent Document 1 proposes the use of laser light modulated to multiple quantum bits to control quantum bits. The laser light is guided through an optical fiber and supplied from a control device to the quantum bits. While this is expected to reduce the transmission path within the dilution refrigerator, concerns remain about the dark current and thermal load of the photodiode that generates microwaves from the laser light. Non-Patent Document 2 proposes frequency multiplexing of microwaves to control quantum bits. While multiplexing is expected to reduce the transmission path, the implementation of a demultiplexing circuit that is placed in the cryogenic space of the dilution refrigerator together with the quantum bits has not yet been established. Non-Patent Document 3 discloses a DRAG (Derivative Removal by Adiabatic Gate) pulse used in a single-qubit gate. The DRAG pulse can prevent higher-order excitations in a single quantum bit. Multiple quantum bits are not considered. Non-Patent Document 4 discloses the simultaneous excitation of two quantum bits that are close to each other. The two quantum bits are always excited simultaneously. Nothing other than simultaneous excitation is considered.

[0006] An object of the present invention is to provide a quantum bit control device and a quantum computer that can contribute to the integration of quantum bits.

[0007] A quantum bit control device according to one aspect of the present invention generates, for a plurality of quantum bits that are excited at individually assigned frequencies, pulse signals having a waveform that suppresses excitation at frequencies other than the frequencies that drive the excitation.

[0008] A quantum computer according to another aspect of the present invention comprises a plurality of quantum bits that are excited at individually assigned frequencies, a distributor connected in common to the quantum bits and that distributes a signal received in common to the quantum bits to each of the quantum bits, and a control device connected to the distributor and that generates, in common to the quantum bits, a pulse signal having a waveform that suppresses excitation at frequencies other than the frequency that drives the excitation.

[0009] As described above, according to this embodiment, it is possible to provide a quantum bit control device and a quantum computer that can contribute to the integration of quantum bits.

[0010]

[0023] Figure 2A is a conceptual diagram showing the configuration of a quantum computer system according to an embodiment of the present invention. Figure 2B is a conceptual diagram showing the excitation of a quantum bit, Figure 2C is a diagram showing the frequency profile of a pulse signal, and Figure 2C is a diagram showing the waveform of the pulse signal. Figure 3A is a conceptual diagram showing the excitation of a quantum bit, Figure 3B is a diagram showing the frequency profile of a pulse signal, and Figure 3C is a diagram showing the waveform of a pulse signal. Figure 4A is a conceptual diagram showing the excitation of a quantum bit, Figure 4B is a diagram showing the frequency profile of a pulse signal, and Figure 4C is a diagram showing the waveform of a pulse signal multiplexed for observing two quantum bits [Q0, Q1]. Figure 6A is a diagram showing the Rabi oscillations of quantum bit [Q0] using shading versus coefficient [α / Δ] and time, and Figure 6B is a graph showing the change in Rabi oscillations when coefficient [α / Δ] = 40 and coefficient [α / Δ] = 0. Regarding the quantum bit [Q1], FIG. 7A shows the Rabi oscillations in terms of shading versus the coefficient [α / Δ] and time, and FIG. 7B is a graph showing the change in the Rabi oscillations when the coefficient [α / Δ]=40 and the coefficient [α / Δ]=0.

[0011] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings.

[0012] FIG. 1 shows a schematic diagram of a quantum computer system 11 according to an embodiment of the present invention. The quantum computer system 11 includes a quantum computer 12 that performs computations according to the theory of quantum mechanics, and a front-end server 13 that is connected to the quantum computer 12 and supplies the quantum computer 12 with a software program describing the computations required. 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 in response to a user's "question." The quantum computer 12 then performs computations according to the generated software program. The front-end server 13 provides the user with a "solution" obtained through computations performed by the quantum computer 12. The front-end server 13 may also directly pass the software program provided by the user to the quantum computer 12.

[0013] Quantum computer 12 includes a plurality of quantum bits 14a, 14b, and 14c, a dilution refrigerator 15 that accommodates quantum bits 14a, 14b, and 14c in a cryogenic space (cryogenic environment), and a quantum bit control device 16 connected to dilution refrigerator 15. Each quantum bit 14a, 14b, and 14c is configured as a superconducting quantum bit. Each quantum bit 14a, 14b, and 14c is excited at an individually assigned frequency. Microwaves of the assigned frequency act on quantum bits 14a, 14b, and 14c during excitation. The frequencies can be set, for example, spaced apart by an equal bandwidth. Here, a first frequency is assigned to quantum bit 14a. A second frequency is assigned to quantum bit 14b. A third frequency is assigned to quantum bit 14c. The number of quantum bits 14a, 14b, and 14c is not limited to three.

[0014] One distributor 17 is commonly connected to the multiple quantum bits 14a, 14b, and 14c. Distributor 17 is disposed in dilution refrigerator 15 together with quantum bits 14a, 14b, and 14c. Distributor 17 is housed in an extremely low temperature space. Distributor 17 distributes a signal received commonly by quantum bits 14a, 14b, and 14c to each of quantum bits 14a, 14b, and 14c.

[0015] The quantum bit control device 16 includes a first control wave generating circuit 18a that generates a pulse signal based on a first carrier. The frequency of the first carrier is the resonant frequency of the quantum bit 14a, i.e., the first frequency. The pulse signal has a waveform that suppresses excitation at frequencies other than the first frequency. The waveform of the pulse signal selectively causes excitation in the quantum bit 14a specified by the frequency of the first carrier. To set the waveform, an Ωx signal and an Ωy signal are input to the first control wave generating circuit 18a. The Ωx signal and the Ωy signal are each converted to an analog signal and input to an orthogonal converter 19a. The orthogonal converter 19a outputs a pulse signal according to the frequency of the first carrier.

[0016] The quantum bit control device 16 includes a second control wave generating circuit 18b that generates a pulse signal based on the second carrier. The frequency of the second carrier is the resonant frequency of the quantum bit 14b, i.e., the second frequency. The pulse signal has a waveform that suppresses excitation at frequencies other than the second frequency. The waveform of the pulse signal selectively causes excitation in the quantum bit 14b specified by the frequency of the second carrier. To set the waveform, an Ωx signal and an Ωy signal are input to the second control wave generating circuit 18b. The Ωx signal and the Ωy signal are each converted to an analog signal and input to the orthogonal converter 19b. The orthogonal converter 19b outputs a pulse signal according to the frequency of the second carrier.

[0017] The quantum bit control device 16 includes a third control wave generating circuit 18c that generates a pulse signal based on a third carrier. The frequency of the third carrier is the resonant frequency of the quantum bit 14c, i.e., the third frequency. The pulse signal has a waveform that suppresses excitation at frequencies other than the third frequency. The waveform of the pulse signal selectively causes excitation in the quantum bit 14c specified by the frequency of the third carrier. To set the waveform, an Ωx signal and an Ωy signal are input to the third control wave generating circuit 18c. The Ωx signal and the Ωy signal are each converted to an analog signal and input to an orthogonal converter 19c. The orthogonal converter 19c outputs a pulse signal according to the frequency of the third carrier.

[0018] The first control wave generating circuit 18a, the second control wave generating circuit 18b, and the third control wave generating circuit 18c set the waveforms of the pulse signals according to the following equations: The pulse signals have a set frequency ω ｊ The frequency ω is set according to ｊ The pulse signal establishes a waveform that suppresses the output value below a threshold that drives excitation at a set frequency ω ｊ indicates the minimum value of the output value. where ω ｄ is the frequency of the carrier that carries the pulse signal, ω ｊ is the resonance frequency of the qubit that suppresses excitation, a ｊ and b ｊ is the order, δ is the detuning, and σ is the half-width of the Gaussian function.

[0019] Specifically, as shown in FIG. 2A, the carrier frequency ω that resonates with the quantum bit 14a [Q0] is ｄ When the above equation is set, as shown in FIG. 2B, a frequency profile that induces excitation in quantum bit 14a [Q0] and suppresses excitation in quantum bit 14b [Q1] and quantum bit 14c [Q2] according to [Equation 1] is derived. An output value equal to or greater than the threshold for driving excitation at the resonant frequency of quantum bit 14a [Q0] is ensured, and the output value is suppressed to below the threshold for driving excitation at the resonant frequencies of quantum bit 14b [Q1] and quantum bit 14c [Q2]. In this frequency profile, the carrier frequency (first frequency) ω ｄ The output value exhibits a maximum value at the first frequency of the quantum bit 14b [Q1] and a minimum value at the second frequency of the quantum bit 14b [Q1] and the third frequency of the quantum bit 14c [Q2]. At the minimum values, the output is nullified. As shown in FIG. 2C , the waveform can be determined based on the inverse Fourier transform. The pulse signal formed has a frequency profile spanning the resonant frequencies of the three quantum bits 14a, 14b, and 14c. When a pulse signal with this waveform is supplied to the quantum bits 14a, 14b, and 14c, excitation occurs in the quantum bit 14a selected by the pulse signal. Excitation is suppressed in the quantum bits 14b and 14c that are not selected. In this way, excitation is selectively induced among multiple quantum bits.

[0020] As shown in FIG. 3A, the carrier frequency ω ｄ When the above formula is set, as shown in FIG. 3B, a frequency profile that induces excitation in quantum bit 14b [Q1] and suppresses excitation in quantum bit 14a [Q0] and quantum bit 14c [Q2] according to [Formula 1] is derived. An output value equal to or greater than the threshold for driving excitation at the resonant frequency of quantum bit 14b [Q1] is ensured, and the output value is suppressed to below the threshold for driving excitation at the resonant frequencies of quantum bit 14a [Q0] and quantum bit 14c [Q2]. In this frequency profile, the carrier frequency (second frequency) ω ｄThe output value exhibits a maximum value at the first frequency of quantum bit 14a [Q0] and a minimum value at the third frequency of quantum bit 14c [Q2]. The output is nullified at the minimum value. As shown in FIG. 3C , the waveform can be determined based on the inverse Fourier transform. The pulse signal formed has a frequency profile spanning the resonant frequencies of the three quantum bits 14a, 14b, and 14c. When a pulse signal with this waveform is supplied to quantum bits 14a, 14b, and 14c, excitation occurs in quantum bit 14b selected by the pulse signal. Excitation is suppressed in quantum bits 14a and 14c that are not selected. In this way, excitation is selectively induced among multiple quantum bits.

[0021] As shown in FIG. 4A, the carrier frequency ω ｄ When the above equation is set, as shown in FIG. 4B, a frequency profile that induces excitation in quantum bit 14c [Q2] and suppresses excitation in quantum bit 14a [Q0] and quantum bit 14b [Q1] according to [Equation 1] is derived. An output value equal to or greater than the threshold for driving excitation at the resonant frequency of quantum bit 14c [Q1] is ensured, and the output value is suppressed to below the threshold for driving excitation at the resonant frequencies of quantum bit 14a [Q0] and quantum bit 14b [Q1]. In this frequency profile, the carrier frequency (third wave number) ω ｄ The output value exhibits a maximum value at the first frequency of quantum bit 14a [Q0] and a minimum value at the second frequency of quantum bit 14b [Q1]. At the minimum values, the output is nullified. As shown in FIG. 4C , the waveform can be determined based on the inverse Fourier transform. The pulse signal formed has a frequency profile spanning the resonant frequencies of the three quantum bits 14a, 14b, and 14c. When a pulse signal with this waveform is supplied to quantum bits 14a, 14b, and 14c, excitation occurs in quantum bit 14c selected by the pulse signal. Excitation is suppressed in quantum bits 14a and 14b that are not selected. In this way, excitation is selectively induced among multiple quantum bits.

[0022] A single multiplexing circuit 21 is commonly connected to the first control wave generating circuit 18a, the second control wave generating circuit 18b, and the third control wave generating circuit 18c. The multiplexing circuit 21 is composed of, for example, a synthesizer. The multiplexing circuit 21 frequency-multiplexes the pulse signal of the first frequency, the pulse signal of the second frequency, and the pulse signal of the third frequency. The output terminal of the multiplexing circuit 21 is connected to the distributor 17. One transmission path for analog signals is established for each distributor 17. One transmission path can be composed of a single coaxial cable extending from the quantum bit control device 16 to the dilution refrigerator 15. Within the dilution refrigerator 15, the transmission path to the distributor 17 can be composed of a single coaxial cable.

[0023] The quantum bit control device 16 includes a readout circuit 22 that reads out the states of the quantum bits 14 a, 14 b, and 14 c. When reading out the states, the readout signal multiplexes the first, second, and third frequencies assigned to the individual quantum bits 14 a, 14 b, and 14 c. The front-end server 13 generates a "solution" defined by a software program in accordance with the readout states.

[0024] In the quantum computer 12 according to this embodiment, a pulse signal is supplied as a multiplexed analog signal from the quantum bit control device 16 to the distributor 17. The pulse signal is distributed from the distributor 17 to the individual quantum bits 14a, 14b, and 14c. When the pulse signal is supplied to the multiple quantum bits 14a, 14b, and 14c, excitation occurs in the quantum bit 14a, 14b, or 14c selected by the pulse signal. The excitation of the quantum bit 14a, 14b, or 14c is selectively suppressed based on the waveform of the pulse signal. In this way, excitation is selectively generated among the multiple quantum bits 14a, 14b, and 14c. Because the pulse signal suppresses excitation in the quantum bits 14a, 14b, or 14c other than the selected quantum bit 14a, 14b, or 14c, the pulse signal need only be simply distributed to the multiple quantum bits. Such simple distribution can greatly contribute to the integration of quantum bits.

[0025] The generated pulse signal is shaped according to a set frequency to establish a waveform that suppresses the output value below a threshold that drives excitation at the set frequency. The frequency assigned to each quantum bit is specified when designing the waveform of the pulse signal. The output value can be suppressed below a threshold that drives excitation at the specified frequency. The output value can be reliably controlled at the frequency assigned to quantum bits other than the quantum bit that excites.

[0026] The pulse signal exhibits a minimum output value at a set frequency. If the first frequency, second frequency, and third frequency assigned to each quantum bit 14 a, 14 b, and 14 c are specified, each frequency can be used as a parameter when designing the pulse signal. The pulse signal exhibits a minimum output value at that frequency. The frequency can be clearly specified when designing the pulse signal.

[0027] The pulse signal has a waveform that selectively excites the quantum bit 14 a, 14 b, or 14 c specified by the carrier frequency. The carrier can be set for each quantum bit 14 a, 14 b, or 14 c. Since a pulse signal is formed for each quantum bit 14 a, 14 b, or 14 c, reliable control can be achieved for each quantum bit 14 a, 14 b, or 14 c.

[0028] The multiplexing circuit 21 frequency-multiplexes the pulse signal. The multiplexed signal can be transmitted over a single transmission path (wiring). The number of signal transmission paths can be reduced. Since excitation of quantum bits 14a, 14b, or 14c other than those specified by the carrier frequency is suppressed, the multiple quantum bits 14a, 14b, and 14c can be well controlled.

[0029] The waveform of the pulse signal is formed according to Equation 1. Because a generalized equation has been devised in this way, the quantum bits 14a, 14b, and 14c can be reliably and selectively excited according to the parameter settings.

[0030] In this embodiment, quantum bits 14a, 14b, and 14c and distributor 17 are placed inside dilution refrigerator 15. The multiplexed signal from quantum bit controller 16 to distributor 17 can be transmitted over a single transmission line. Even if the number of quantum bits placed inside dilution refrigerator 15 increases, the number of wires between quantum bit controller 16 and dilution refrigerator 15 can be effectively reduced compared to the number of quantum bits. The number of coaxial cables inside dilution refrigerator 15 that lead to distributor 17 can be reduced. The integration of quantum bits can be effectively achieved.

[0031] The inventor verified the frequency selectivity of [Equation 1]. In the verification, the inventor observed the Rabi oscillation of two qubits. When multiplexing two qubits [Q0, Q1], b ｉ = 0, δ = 0 and ω ｄ =ω Ｑ０ It can be seen that the following equation holds in response to the inverse Fourier transform: however, where ω Ｑ０ denotes the resonant frequency that causes excitation in the qubit [Q0]. Ｑ１ indicates the resonant frequency at which excitation occurs in the quantum bit [Q1]. As shown in Figure 5, the obtained pulse signal was repeatedly irradiated to the quantum bits [Q0, Q1]. As a result, as shown in Figures 6A and 7A, Rabi oscillations were obtained in the quantum bits [Q0] and [Q1]. Here, the inventors varied the coefficient [α / Δ] of the imaginary component of [Equation 2]. When the coefficient [α / Δ] = 0 and 40, the change in amplitude was observed. As shown in Figure 6B, excitation occurred in the quantum bit [Q0] regardless of the change in the coefficient [α / Δ]. On the other hand, as shown in Figure 7B, excitation was suppressed in the quantum bit [Q1] when the coefficient [α / Δ] = 40. When a common pulse signal was irradiated, it was confirmed that excitation was selectively induced in the quantum bits [Q0, Q1] depending on the setting of the coefficient [α / Δ]. From FIGS. 6A and 7A, it can be seen that simultaneous excitation occurs in the two qubits [Q0, Q1] depending on the selection of the coefficient [α / Δ].

[0032] 12 quantum computer 14a quantum bit 14b quantum bit 14c quantum bit 15 dilution refrigerator 16 quantum bit control device 17 distributor 21 multiplexing circuit

Claims

1. A quantum bit control device that generates, for a plurality of quantum bits that are excited at individually assigned frequencies, a pulse signal having a waveform that suppresses excitation at frequencies other than the frequencies that drive the excitation.

2. The quantum bit control device according to claim 1, wherein the pulse signal has a frequency spectrum spanning the resonant frequencies of a plurality of the quantum bits.

3. The quantum bit control device of claim 1, wherein the pulse signal is shaped according to a set frequency to establish a waveform that suppresses output values ​​below a threshold that drives excitation at the set frequency.

4. The quantum bit control device according to claim 3, wherein the pulse signal exhibits a minimum value of the output value at the set frequency.

5. The quantum bit control device according to claim 4, wherein the pulse signal has a waveform that selectively causes excitation in the quantum bit specified by the carrier frequency.

6. The quantum bit control device according to claim 5, further comprising a multiplexing circuit that frequency-multiplexes the pulse signal.

7. The quantum bit control device according to claim 6, wherein the waveform is formed according to the following formula: However, ω ｄ is the frequency of the carrier that carries the pulse signal, ω ｊ is the resonance frequency of the quantum bit that suppresses the excitation, a ｊ and b ｊ is the order, δ is the detuning, and σ is the half-width of the Gaussian function.

8. A quantum computer comprising: a plurality of quantum bits that are excited at individually assigned frequencies; a distributor connected in common to the quantum bits and distributing a signal received in common to the quantum bits to each of the quantum bits; and a control device connected to the distributor and generating, in common to the quantum bits, a pulse signal having a waveform that suppresses excitation at frequencies other than the frequency that drives the excitation.

9. The quantum computer according to claim 8, wherein the pulse signal has a frequency profile spanning the resonant frequencies of a plurality of the quantum bits.

10. The quantum computer of claim 8, wherein the pulse signal is shaped according to a set frequency to establish a waveform that suppresses output values ​​below a threshold that drives excitation at the set frequency.

11. The quantum computer according to claim 10, wherein the pulse signal exhibits a minimum value of the output value at the set frequency.

12. The quantum computer according to claim 11, wherein the pulse signal has a waveform that selectively induces excitation in the quantum bit identified by the carrier frequency.

13. The quantum computer according to claim 12, wherein the control device includes a multiplexing circuit that frequency-multiplexes the pulse signal.

14. The quantum computer according to claim 13, wherein the qubits and the distributor are disposed within a dilution refrigerator.

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