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 quantum bit control device that controls the excitation of a quantum bit, and a quantum computer. [Background technology]

[0002] In a quantum computer, the state transition of a quantum bit is controlled by the action of a pulse signal. An arbitrary waveform generator (AWG) is used to generate the pulse signal, for example. The AWG has a waveform memory that stores a shaped waveform and outputs the waveform 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 set to a roughly specific size, the DAC can generate an analog signal that corresponds to the waveform well. [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, October 10, 1986 Summary of the Invention [Problem to be solved by the invention]

[0005] Analog signals are generated for each individual quantum bit. To control the quantum bits, a transmission path for the analog signal is established for each quantum bit. Establishing 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 bits inside the dilution refrigerator via coaxial cables. The same number of coaxial cables as the quantum bits are installed inside the dilution refrigerator. However, due to spatial constraints within the dilution refrigerator and issues with heat generation in the attenuators, the integration of quantum bits is hindered as long as a coaxial cable is connected to each quantum bit. Non-Patent Document 1 proposes frequency multiplexing of pulse signals, but a method for generating an analog signal from a frequency-multiplexed pulse signal has not yet been established.

[0006] An object of the present invention is to provide a quantum bit control device that can generate an analog signal well from a frequency-multiplexed pulse signal. [Means for solving the problem]

[0007] A quantum bit control device according to one embodiment of the present invention comprises a generation circuit group including a plurality of waveform generation circuits that generate pulse signals at frequencies assigned to each quantum bit, a multiplexing circuit commonly connected to the waveform generation circuits and that frequency-multiplexes the pulse signals while adjusting the phase of the pulse signals based on an algorithm that reduces a 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 includes a quantum bit group including a plurality of quantum bits that are excited at individually assigned frequencies, a generation circuit group including a plurality of waveform generation circuits that generate pulse signals at frequencies assigned to each of the quantum bits, a multiplexing circuit commonly connected to the waveform generation circuits and that frequency-multiplexes the pulse signals while adjusting the phase of the pulse signals based on an algorithm that reduces a 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 aspects of the present invention, a quantum bit control device is provided that can generate an analog signal from a frequency-multiplexed pulse signal in a satisfactory manner. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a conceptual diagram showing the configuration of a quantum computer system according to a first embodiment of the present invention. [Figure 2] 10 is a graph showing the relationship between the phase and amplitude of a frequency-multiplexed pulse signal. [Figure 3] FIG. 10 is a conceptual diagram showing the configuration of a quantum computer system according to a second embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[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 a first embodiment of the present invention. The quantum computer system 11 includes a quantum computer 12 that performs computational processing 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 computational processing required for 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 in response to a user's "question." The quantum computer 12 performs computational processing in accordance with the generated software program. The front-end server 13 provides the user with a "solution" obtained through computational processing 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 qubit group 14 including multiple qubits 14a, 14b, and 14c, a dilution refrigerator 15 that accommodates qubits 14a, 14b, and 14c in a cryogenic space (cryogenic environment), and a qubit control device 16 connected to dilution refrigerator 15. Each qubit 14a, 14b, and 14c is configured as a superconducting qubit. A resonant frequency is individually set for each qubit 14a, 14b, and 14c. When microwaves of the resonant frequency act on qubits 14a, 14b, and 14c, qubits 14a, 14b, and 14c, qubits 14a, 14b, and 14c can be excited. The resonant frequencies can be set, for example, spaced apart by an equal bandwidth. Here, a first frequency is assigned to qubit 14a. A second frequency is assigned to qubit 14b. A third frequency is assigned to qubit 14c. The number of qubits 14a, 14b, and 14c is not limited to three.

[0014] A single distributor 17 is connected in common to multiple quantum bits 14a, 14b, and 14c. Distributor 17 is placed inside 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 in common by quantum bits 14a, 14b, and 14c to each of the quantum bits 14a, 14b, and 14c. A quantum bit controller 16 is connected to distributor 17. The quantum bit controller 16 outputs an analog signal [microwave] that selectively excites multiple quantum bits 14a, 14b, and 14c. One transmission path for the analog signal is established for each distributor 17. One transmission path can be composed of a single coaxial cable extending from quantum bit controller 16 to distributor 17. Within dilution refrigerator 15, the transmission path to distributor 17 can be composed of a single coaxial cable.

[0015] The quantum bit control device 16 includes a generation circuit group 21 that generates a pulse signal for each quantum bit 14a, 14b, and 14c, a multiplexing circuit 22 connected to the generation circuit group 21 and 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 the pulse signals, for example, as shown in the following equation: Each pulse signal is specified by a sine wave modulated with a Gaussian pulse.

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

[0017] The excitation of each quantum bit 14a, 14b, and 14c is controlled by a pulse signal. The pulse signal is generated at a frequency assigned to each quantum bit 14a, 14b, and 14c. An analog signal is generated from the frequency-multiplexed pulse signal. The analog signal is supplied to quantum bits 14a, 14b, and 14c. In this way, the excitation of quantum bits 14a, 14b, and 14c is controlled. Because the pulse signal is frequency-multiplexed in advance to generate the analog signal, the transmission path (wiring) for the analog signal can be reduced. If the phase is adjusted based on an algorithm that reduces the crest factor during frequency multiplexing, the amplitude of the analog signal output from DAC 23 can be kept within a predetermined range. An analog signal can be generated satisfactorily from the frequency-multiplexed pulse signal. Even when the pulse signal is multiplexed, the waveform of each pulse signal can effectively affect the corresponding quantum bit 14a, 14b, and 14c. The excitation of quantum bits 14a, 14b, and 14c can be effectively driven.

[0018] Non-resonant excitation pulse signals are supplied to the individual quantum bits 14a, 14b, and 14c. The non-resonant excitation pulse signals can be generated by, for example, a first waveform generating circuit 24a, a second waveform generating circuit 24b, and a third waveform generating circuit 24c, respectively. The non-resonant excitation pulse signals induce AC Stark shifts in the quantum bits 14a, 14b, and 14c. When the non-resonant excitation pulse signals are supplied to the quantum bits 14a, 14b, and 14c, an interaction described by the following Hamiltonian occurs in the quantum bits 14a, 14b, and 14c:

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[0019] The quantum bit controller 16 includes a resonator that reads out the states of the quantum bits 14a, 14b, and 14c. The front-end server 13 generates a "solution" defined by a software program in response to the read-out states.

[0020] The resonant frequencies assigned to the qubits 14 a, 14 b, and 14 c are spaced apart by an equal bandwidth, and as described above, the phases of the pulse signals are adjusted based on an algorithm that reduces the crest factor, thereby effectively avoiding interference between the pulse signals.

[0021] In this embodiment, the first waveform generating circuit 24a, the second waveform generating circuit 24b, and the third waveform generating circuit 24c generate waveforms that specify z-axis rotations of the quantum bits 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 the quantum bits 14a, 14b, and 14c based on the waveforms, z-axis rotations are induced in the quantum bits 14a, 14b, and 14c. The z-axis rotation caused by the phase shift can be canceled out. In this way, the quantum bits 14a, 14b, and 14c can be driven successfully.

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

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

[0024] The inventors have verified the effect of phase shifting used in frequency multiplexing. In the verification, the inventors have calculated [Equation 1]. 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 number of samples was set to N = 30. When the phase was set to φ = 0 at all frequencies, pulses were confirmed to be concentrated at the maximum value Maxi and the minimum value Mini, as shown in Figure 2. Only a few pulses were confirmed outside 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, which was larger than the Schroeder range Sch. The superiority of the algorithm in reducing the crest factor was confirmed.

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

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[0026] 3 shows a schematic 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 qubit group 35 including a plurality of qubits 33a, 33b, 33c, and 33d, and a control qubit 34 coupled to qubits 33a, 33b, 33c, and 33d to form cross resonance. Qubit group 35 is housed in dilution refrigerator 15, as described above. A quantum bit controller 36 is connected to dilution refrigerator 15.

[0027] Each of the quantum bits 33a, 33b, 33c, and 33d and the control quantum bit 34 are composed of superconducting quantum bits. A resonant frequency is set for each of the quantum bits 33a, 33b, 33c, and 33d. When microwaves of the resonant frequency act on the quantum bits 33a, 33b, 33c, and 33d, the quantum bits 33a, 33b, 33c, and 33d, the quantum bits 33a, 33b, 33c, and 33d can be excited. The resonant frequencies can be set, for example, so that they are spaced apart by an equal bandwidth. Here, a first frequency is assigned to the quantum bit 33a. A second frequency is assigned to the quantum bit 33b. A third frequency is assigned to the quantum bit 33c. A fourth frequency is assigned to the quantum bit 33d. The number of quantum bits 33a, 33b, 33c, and 33d is not limited to four. One control quantum bit 34 may be assigned for each of the multiple quantum bits 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 controller 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 for the analog signal is established for each control qubit 34. One transmission path can be composed of a single coaxial cable extending from the qubit controller 36 to the control qubit 34. Within the dilution refrigerator 15, the transmission path to the control qubit 34 can be composed of a single coaxial cable.

[0029] The quantum bit control device 36 includes a generator circuit group 37 that generates pulse signals for each quantum bit 33a, 33b, 33c, and 33d, a multiplexer circuit 38 connected to the generator circuit group 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 multiplexer circuit 38 can multiplex microwaves as shown in Equation 1. During frequency multiplexing, the multiplexer circuit 38 adjusts the phase of the pulse signal based on an algorithm that reduces the crest factor. The amplitude of the multiplexed pulse signal can be smoothed in response to the phase adjustment. The amplitude of the analog signal output from the DAC 39 can be kept well 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 quantum bit 33a, a second waveform generation circuit 43b that generates a pulse signal based on the resonance frequency set for quantum bit 33b, a third waveform generation circuit 43c that generates a pulse signal based on the resonance frequency set for quantum bit 33c, and a fourth waveform generation circuit 43d that generates a pulse signal based on the resonance frequency set for quantum bit 33d. The first waveform generation circuit 43a sets a waveform based on the resonance frequency of quantum bit 33a. The waveform is supplied to the control quantum bit 34 to cause cross resonance in quantum bit 33a. The second waveform generation circuit 43b sets a waveform based on the resonance frequency of quantum bit 33b. The waveform is supplied to the control quantum bit 34 to cause cross resonance in quantum bit 33b. The third waveform generation circuit 43c sets a waveform based on the resonance frequency of quantum bit 33c. The waveform is supplied to the control quantum bit 34 to cause cross resonance in quantum bit 33c. The waveform is set in the fourth waveform generating circuit 43d based on the resonant frequency of the quantum bit 33d, and is supplied to the control quantum bit 34 to cause cross-resonance in the quantum bit 33d.

[0031] Non-resonant excitation pulse signals are supplied to the individual quantum bits 33a, 33b, 33c, and 33d. The non-resonant excitation pulse signals can be generated by, for example, a first waveform generating circuit 43a, a second waveform generating circuit 43b, a third waveform generating circuit 43c, and a fourth waveform generating circuit 43d, respectively. The non-resonant excitation pulse signals induce AC Stark shifts in the quantum bits 33a, 33b, 33c, and 33d. When the non-resonant excitation pulse signals are supplied to the quantum bits 33a, 33b, 33c, and 33d, an interaction described by the Hamiltonian in [Equation 4] occurs in the quantum bits 33a, 33b, 33c, and 33d. The Hamiltonian corresponds to an interaction that realizes a rotation operation around the z-axis.

[0032] The quantum bit controller 36 includes a resonator that reads out the states of the quantum bits 33a, 33b, 33c, and 33d. The front-end server 13 generates a "solution" defined by a software program in response to the read-out states.

[0033] The excitation of each quantum bit 33a, 33b, 33c, and 33d is controlled by a pulse signal. The pulse signal is generated at a frequency assigned to each quantum bit 33a, 33b, 33c, and 33d. An analog signal is generated from the frequency-multiplexed pulse signal. An analog signal common to multiple quantum bits 33a, 33b, 33c, and 33d can be supplied to the control quantum bit 34. A quantum bit gate can act on each quantum bit 33a, 33b, 33c, and 33d in response to cross-resonance. Because the pulse signal is frequency-multiplexed in advance to generate the analog signal, the transmission path (wiring) for the analog signal can be reduced. Simultaneous execution of multiple quantum bit gates can be achieved in response to 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 DAC 39 can be kept within a predetermined range. An analog signal can be generated efficiently from the frequency-multiplexed pulse signal. Even when the pulse signal is multiplexed, the amplitude of the analog signal can be maximized according to the phase shift. The response speed of the quantum bit gate can be improved. The excitation of quantum bits 33 a, 33 b, 33 c, and 33 d can be driven efficiently.

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

[0036] In this embodiment, waveform generation circuits 43a, 43b, 43c, and 43d generate waveforms that specify z-axis rotations of quantum bits 33a, 33b, 33c, and 33d that cancel out the z-axis rotation caused by the phase shift. When a pulse signal based on the waveform is supplied to control quantum bit 34, z-axis rotations are induced in quantum bits 33a, 33b, 33c, and 33d in response to the action of cross resonance. The z-axis rotation caused by the phase shift can be canceled out. In this way, the excitations of quantum bits 33a, 33b, 33c, and 33d can be successfully driven.

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

[0038] The inventors have examined the influence of the phase shift caused by frequency multiplexing in realizing 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 a rotation operation around the x-axis and a rotation operation around the y-axis. When the waveforms of the waveform generation circuits 43a, 43b, 43c, and 43d are supplied to the qubits 33a, 33b, 33c, and 33d, z-axis rotation is induced 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 formula:

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[0041] 12. Quantum Computers 14 qubits 14a Qubit 14b qubit 14c qubit 16 qubit controller 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 Computer 33a qubit 33b qubit 33c qubit 33d qubit 34 control qubits 35 qubits 36 qubit controller 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 generation circuit group including a plurality of waveform generation circuits that generate pulse signals at frequencies assigned to each quantum bit; a multiplexing circuit connected in common to the waveform generating circuits, which frequency-multiplexes the pulse signals while adjusting the phases of the pulse signals based on an algorithm for reducing a crest factor; a digital-to-analog converter that generates an analog signal from the frequency-multiplexed pulse signal; 1. A quantum bit control device comprising:

2. The frequencies are spaced apart by equal bandwidths The quantum bit control device of claim 1 .

3. The generation circuit group further includes a waveform generation circuit that generates a waveform that specifies a z-axis rotation of the quantum bit that cancels out the z-axis rotation caused by the phase shift. The quantum bit control device of claim 1 .

4. The waveform generation circuit generates a pulse signal based on a frequency that causes an AC Stark shift in the quantum bit. The quantum bit control device of claim 3 .

5. The generation circuit group further includes a waveform generation circuit that is supplied to a control qubit coupled to the qubit to complement a pulse signal that causes cross resonance, and that is supplied to the qubit to generate a waveform that specifies a z-axis rotation based on a combination of a rotation operation about the x-axis and a rotation operation about the y-axis. The quantum bit control device of claim 3 .

6. a qubit group including a plurality of qubits excited at individually assigned frequencies; a generation circuit group including a plurality of waveform generation circuits that generate pulse signals at frequencies assigned to each of the quantum bits; a multiplexing circuit connected in common to the waveform generating circuits, which frequency-multiplexes the pulse signals while adjusting the phases of the pulse signals based on an algorithm for reducing a crest factor; a digital-to-analog converter that generates an analog signal from the frequency-multiplexed pulse signal; A quantum computer equipped with

7. The frequencies are spaced apart by equal bandwidths The quantum computer of claim 6.

8. The generation circuit group further includes a waveform generation circuit that generates a waveform that specifies a z-axis rotation of the quantum bit that cancels out the z-axis rotation caused by the phase shift. The quantum computer of claim 6.

9. a distributor connected to the digital-to-analog converter for distributing the analog signal to the quantum bit and causing the quantum bit to function as a one-qubit gate; The quantum computer of claim 8.

10. The waveform generation circuit generates a pulse signal based on a frequency that causes an AC Stark shift in the quantum bit. The quantum computer of claim 9.

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

12. The waveform generation circuit generates a pulse signal based on a frequency that causes an AC Stark shift in the quantum bit. The quantum computer of claim 11.

13. The generation circuitry group further includes a waveform generation circuit that complements the pulse signal supplied to the control qubit to cause cross-resonance in the target qubit and generates a waveform that specifies a z-axis rotation based on a combination of a rotation operation about the x-axis and a rotation operation about the y-axis in the target qubit. The quantum computer of claim 11.

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