WO2021155504A1 - 基于级联弛豫的超导量子比特的读取方法及装置 - Google Patents

基于级联弛豫的超导量子比特的读取方法及装置 Download PDF

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WO2021155504A1
WO2021155504A1 PCT/CN2020/074321 CN2020074321W WO2021155504A1 WO 2021155504 A1 WO2021155504 A1 WO 2021155504A1 CN 2020074321 W CN2020074321 W CN 2020074321W WO 2021155504 A1 WO2021155504 A1 WO 2021155504A1
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state
reading
superconducting qubit
qubit
excited
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PCT/CN2020/074321
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French (fr)
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陈明城
王粲
刘丰铭
应翀
王建文
张宇宸
尚仲夏
陆朝阳
潘建伟
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中国科学技术大学
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    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06N—COMPUTING ARRANGEMENTS BASED ON SPECIFIC COMPUTATIONAL MODELS
    • G06N10/00—Quantum computing, i.e. information processing based on quantum-mechanical phenomena

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  • the present disclosure belongs to the technical field of quantum computing, and relates to a method and device for reading superconducting qubits based on cascade relaxation.
  • Quantum computers have potential application value in solving complex calculations.
  • a quantum computer is essentially a quantum mechanical system. It uses the state of the quantum mechanical system, the so-called quantum state, to encode information, executes calculation tasks in accordance with the evolutionary law of quantum dynamics, and extracts the calculation results according to the measurement principle of quantum mechanics.
  • the present disclosure provides a method and device for reading superconducting qubits based on cascade relaxation, so as to at least partially solve the following technical problems: the existing quantum state reading/measurement has the problem of large measurement errors due to relaxation .
  • a method for reading superconducting qubits based on cascade relaxation includes: before reading, pre-extending the superconducting quantum The time for the bit to relax to the ground state; and to read the state of the superconducting qubit.
  • pre-extending the time for the superconducting qubit to be tested to relax to the ground state includes: pre-processing the superconducting qubit to remove the superconducting qubit from the excited state A is excited to a higher energy state, and the excited state A refers to the excited state that the superconducting qubit under test is in before it is read.
  • the exciting the superconducting qubit to be measured from the excited state A to a higher energy state includes: exciting the superconducting qubit to be measured from the excited state A once to excite to a higher energy state; Alternatively, the superconducting qubit to be tested is excited from the excited state A at least twice, and the superconducting qubit to be tested is excited from the excited state A to the excited state B with a higher energy level than the excited state A, and then from the excited state B Excited to the excited state C, which is a higher energy level than the excited state B, and so on, is excited many times to a higher energy state.
  • the mode of exciting the superconducting qubit to be tested from the excited state A at least twice is cascade excitation, and the cascade excitation includes repeated excitation by energy level and repeated excitation by interval energy level.
  • the reading of the state of the superconducting qubit includes: reading the state of the pre-processed superconducting qubit, and judging whether the state of the superconducting qubit is a ground state or The excited state is used to determine the read logic state of the superconducting qubit under test.
  • the pre-processed superconducting qubit is used to excite the superconducting qubit to be tested from the excited state A to a higher energy state using the following method: coupling with the superconducting qubit through the first microwave transmission line , And apply microwave pulses of specific frequency and waveform on the first microwave transmission line to realize the operation of exciting the superconducting qubit from the excited state A to a higher energy state.
  • the state of the superconducting qubit is read by the method of dispersion measurement, and the reading circuit includes: a second microwave transmission line and a microwave resonant cavity coupled with the superconducting qubit; wherein the microwave The resonant cavity is coupled to the superconducting qubit as a reading cavity, the second microwave transmission line is connected to the external circuit of the quantum chip where the superconducting qubit is located, and the transmission spectrum of the second microwave transmission line is measured Or reflect the spectrum to read the quantum state.
  • the superconducting qubit includes: Transmon qubit.
  • a reading device based on cascade relaxation of superconducting qubits.
  • the reading device includes: a relaxation time extension processing module for reading Before taking, the time for the superconducting qubit to be tested to relax to the ground state is extended in advance; and the quantum state reading module is used to read the state of the superconducting qubit.
  • the relaxation time extension processing module includes: an energy excitation sub-module for pre-processing the superconducting qubits to excite the superconducting qubits to be tested from the excited state A to a higher energy state;
  • the quantum state reading module includes: a reading sub-module for reading the state of the pre-processed superconducting qubit; and a judging sub-module for judging the state of the superconducting qubit by The category is ground state or excited state to determine the read logic state of the superconducting qubit to be tested.
  • the energy excitation sub-module includes: a first microwave transmission line, the first microwave transmission line is used to couple with the superconducting qubit, and the first microwave transmission line is loaded with Microwave pulses of specific frequencies and waveforms to achieve the operation of exciting the superconducting qubit from the excited state A to a higher energy state; and/or, the reading sub-module includes a reading circuit, and the reading circuit includes: a second microwave A transmission line and a microwave resonant cavity coupled with a superconducting qubit; wherein the microwave resonant cavity is used as a reading cavity to be coupled to the superconducting qubit, and the second microwave transmission line is connected to the superconducting qubit.
  • the external circuit of the quantum chip is connected, and the quantum state is read by measuring the transmission spectrum or the reflection spectrum of the second microwave transmission line.
  • the superconducting qubit includes: Transmon qubit.
  • the excited state A is the first excited state, the relaxation rate from the higher energy state to the ground state and the relaxation rate from the first excited state to the ground state
  • the high-energy state higher than the first excited state cannot directly transition back to the ground state, and it needs to go through an intermediate energy state; therefore, by pre-processing the superconducting qubit, the superconducting qubit under test is excited from excited state A to A higher energy state can extend the time for the superconducting qubit to be tested to relax to the ground state, suppress the rate at which the qubit relaxes to the ground state, and achieve the effect of reducing measurement errors. At the same time, it also helps to extend the reading time to improve reading security.
  • this scheme can be applied to Transmon qubits.
  • FIG. 1 is a schematic diagram of a method for reading superconducting qubits based on cascade relaxation according to a first embodiment of the present disclosure.
  • FIG. 2 is a flowchart of a method for reading superconducting qubits based on cascade relaxation according to the first embodiment of the present disclosure.
  • FIG. 3 is a schematic diagram showing the relationship between the probability of the bits in the initial state in different high-energy excited states relaxing to the ground state with time according to an embodiment of the present disclosure.
  • Fig. 4 is a schematic diagram of a reading circuit according to an embodiment of the present disclosure.
  • FIG. 5 is a schematic diagram of resolving quantum states from an IQ diagram generated based on dispersion measurement according to an embodiment of the present disclosure.
  • Fig. 6 is a schematic block diagram of reading a superconducting quantum chip by a reading device based on cascade relaxation of a superconducting qubit according to the second embodiment of the present disclosure.
  • the present disclosure provides a method and device for reading superconducting qubits based on cascade relaxation.
  • the rate of the ground state greatly reduces the probability that the qubit will relax to the ground state during the reading time, ensuring that the qubit is more likely to be in the original excited state or ground state during the measurement process, and avoiding reading time due to a faster relaxation rate
  • the measurement error caused by the relaxation to the ground state can extend the reading time to improve the reading fidelity.
  • the method and device for reading superconducting qubits based on cascade relaxation provided by the present disclosure extend the time for the superconducting qubit to be tested to relax to the ground state in advance before reading. Then read the state of the superconducting qubit.
  • the relaxation time to the ground state By extending the relaxation time to the ground state, the relaxation rate of the qubit to the ground state is suppressed, ensuring that the qubit is more likely to be in the original excited state or ground state during the measurement process, and avoiding reading time due to the faster relaxation rate
  • the measurement error caused by the relaxation to the ground state can extend the reading time to improve the reading fidelity.
  • the pre-extending the relaxation time of the superconducting qubit under test to the ground state before reading includes: pre-processing the superconducting qubit, and removing the superconducting qubit under test from the excited state A is excited to a higher energy state, and the excited state A refers to the excited state that the superconducting qubit under test is in before it is read.
  • the exciting the superconducting qubit to be measured from the excited state A to a higher energy state includes: exciting the superconducting qubit to be measured from the excited state A once to excite to a higher energy state; Alternatively, the superconducting qubit to be tested is excited from excited state A at least twice, first excited state A is excited to excited state B that is higher in energy level than excited state A, and then excited from excited state B to more than excited state B The high-energy excited state C, and so on, is excited many times to a higher energy state.
  • the reading of the state of the superconducting qubit includes: reading the state of the pre-processed superconducting qubit, and judging whether the state of the superconducting qubit is a ground state or The excited state is used to determine the read logic state of the superconducting qubit under test.
  • the pre-processed superconducting qubit is used to excite the superconducting qubit to be tested from the excited state A to a higher energy state using the following method: coupling with the superconducting qubit through the first microwave transmission line , And apply microwave pulses of specific frequency and waveform on the first microwave transmission line to realize the operation of exciting the superconducting qubit from the excited state A to a higher energy state.
  • the state of the superconducting qubit is read by the method of dispersion measurement, and the reading circuit includes: a second microwave transmission line and a microwave resonant cavity coupled with the superconducting qubit; wherein the microwave The resonant cavity is coupled to the superconducting qubit as a reading cavity, the second microwave transmission line is connected to the external circuit of the quantum chip where the superconducting qubit is located, and the transmission spectrum of the second microwave transmission line is measured Or reflect the spectrum to read the quantum state.
  • the superconducting qubit includes: Transmon qubit.
  • Transmon is a newly coined vocabulary to name a new structure of superconducting qubits, which was proposed by Yale University researchers in their paper in 2007.
  • Transmon is actually an abbreviation of "a transmission-line shunted plasma oscillation qubit", and its Chinese translation is "plasma oscillation bits connected in parallel with transmission lines”.
  • the first exemplary embodiment of the present disclosure provides a method for reading superconducting qubits based on cascade relaxation.
  • FIG. 1 is a schematic diagram of a method for reading superconducting qubits based on cascade relaxation according to a first embodiment of the present disclosure.
  • 2 is a flowchart of a method for reading superconducting qubits based on cascade relaxation according to the first embodiment of the present disclosure.
  • the right side shows the energy levels of the superconducting qubits corresponding to 1, 2, 3,...n
  • the left side shows the corresponding energies E1, E2, E3,..., En, corresponding to the ground state, the first excited state, the second excited state, ... the n-1th excited state
  • the dotted rectangle indicates the reading process
  • the downward arrow between the energy levels in the dotted rectangle indicates the high energy
  • the upward arrow indicates the process of excitation from low energy level to high energy level.
  • the ground state and excited state of the superconducting qubit are encoded to correspond to logic 0 and logic 1.
  • the ground state can be encoded as logic 0
  • the excited state can be encoded as logic 1, or vice versa.
  • the method for reading superconducting qubits based on cascade relaxation in this embodiment includes operation S101 and operation S102.
  • the superconducting qubit is preprocessed to excite the superconducting qubit to be tested from the excited state A to a higher energy state.
  • the excited state A refers to the excited state that the superconducting qubit to be tested is in before being read.
  • the rectangular frame 1 represents the reading process in the prior art
  • the rectangular frames 2-4 represent the reading process of the reading method of the present disclosure.
  • the ground state is energy level 1
  • the corresponding energy is E1
  • the excited state A is the first excited state, which corresponds to energy level 2
  • the corresponding energy is E2.
  • the qubit has a certain probability to relax from the first excited state (excited state A) to the ground state, as shown by the dotted down arrow in Figure 1, because the first excited state (excited state A) and the ground state are between The adjacent energy level corresponds to a higher transition rate.
  • the superconducting qubit under test has already transitioned from the excited state A to the ground state.
  • the measured result is that the superconducting qubit is in the ground state.
  • the reading logic of is 0, and the superconducting qubit under test is actually tested to be in an excited state to be the correct measurement result, which leads to a certain probability of error in the measurement, corresponding to a larger reading error.
  • the superconducting qubit to be tested is excited from the excited state A to a higher energy state by pre-processing the superconducting qubit.
  • the implementation manner can be referred to as shown in the rectangular boxes 2-4 in FIG. 1.
  • the relaxation rate from the higher energy state to the ground state is different from the relaxation rate from the excited state A to the ground state.
  • the high energy state higher than the excited state A cannot directly transition back to the ground state. Go through intermediate states.
  • the process of relaxing from a higher energy state to the ground state prolongs the time for the superconducting qubit to relax from the excited state to the ground state, correspondingly prolonging the reading time and reducing the measurement error caused by the relaxation.
  • the relaxation of E3 to E1 in Figure 1 requires a transition from E3 to E2 and then from E2 to E1. Compared with the transition from E2 to E1, the relaxation time to the ground state is prolonged.
  • the above method is only an example of extending the time to relax to the ground state to avoid measurement errors due to the qubits having relaxed to the ground state during the reading process. Others can extend the time to relax to the ground state to prolong the reading time. Therefore, the technical ideas for reducing or eliminating measurement errors are within the protection scope of the present disclosure.
  • the superconducting qubit can be excited only once to excite the superconducting qubit from the excited state A to the excited state B having a higher energy level than the excited state A.
  • the excited state B may be the second excited state.
  • the excited state B may also be the fourth excited state, as shown in the rectangular box 3 in FIG. 1.
  • multiple (at least two) excitations can also be performed.
  • the superconducting qubit can be excited from the excited state A to the excited state B that is higher in energy than the excited state A, and then the superconducting qubit can be excited. Excited from the excited state B to the excited state C with a higher energy level than the excited state B, as shown by the rectangular box 4 in Figure 1.
  • the way to excite the superconducting qubit to be tested from the excited state A at least twice is cascade excitation
  • the cascade excitation includes level-by-level repeated excitation and interval level repeated excitation, for example , Can be excited from the first excited state to the second excited state, from the second excited state to the third excited state, etc.
  • Level-by-level repeated excitation means that the excitation is repeated from the adjacent two energy levels to the higher energy level
  • the interval energy level repeated excitation means that the excitation process can occur from two spaced apart energy levels, for example, from the first excitation
  • the state can be excited to the third excited state, and then can be excited from the third excited state to the fourth excited state, from the fourth excited state to the sixth excited state, and so on.
  • the present disclosure does not limit the number of excitations, and can be performed multiple times before reaching the highest energy level without limitation.
  • the energy level between the higher energy level that is excited to and the energy level that is excited may be adjacent energy levels, as indicated by the rectangular frame 2 in FIG. 1.
  • the energy level that is excited to the higher energy level and the energy level that is excited can also be spaced apart energy levels, as shown by the rectangular box 3 in Fig. 1.
  • the excited state A and the excited state B may be adjacent energy levels, or may be separated energy levels.
  • the superconducting qubit may be coupled to the superconducting qubit through a microwave transmission line, and a microwave pulse of a specific frequency and waveform can be applied to the microwave transmission line to realize the operation of exciting the superconducting qubit from the excited state A to a higher energy state.
  • the state of the pre-processed superconducting qubit is read, and the read logic state of the superconducting qubit to be tested is determined by judging whether the state of the superconducting qubit is the ground state or the excited state.
  • the state of the superconducting qubit may be read by a dispersion measurement method, and it is judged whether the superconducting qubit is in the ground state or the excited state by measuring the transmission spectrum or reflection spectrum of the reading cavity. In an embodiment, if it is determined that the superconducting qubit is in the ground state, it is considered that the read logic state of the superconducting qubit to be tested is logic 0. If it is judged that the superconducting qubit is in an excited state, the read logic state of the superconducting qubit to be tested is logic 1. It should be noted that here it is only necessary to judge whether the bit is in the ground state or the excited state. The excited state here is only a concept corresponding to the ground state.
  • Both the excited state A and the higher energy state belong to the excited state, and there is no need to specifically judge whether the superconducting qubit is in the ground state or excited state.
  • the specific state in the excited state that is, there is no need to determine whether the superconducting qubit is in the excited state A or a higher energy state.
  • FIG. 3 is a schematic diagram showing the relationship between the probability of the bits in the initial state in different high-energy excited states relaxing to the ground state with time according to an embodiment of the present disclosure.
  • the main factor affecting the relaxation rate of superconducting qubits is the size of the charge transition matrix.
  • the interaction between the charge and the cavity mode read on the quantum chip, noise, etc. causes the superconducting qubit to relax from the excited state to the ground state.
  • the Transmon qubit In some superconducting qubit structures, such as Transmon qubits, only transition matrix elements exist between two energy states with adjacent energies. Among them, the Transmon qubit is a superconducting circuit, which represents the state of the qubit according to the different energy of the oscillating current.
  • the technical solution of the present disclosure is applicable to superconducting Transmon qubits that are currently widely used.
  • the Transmon qubit referred to here can be a three-dimensional Transmon qubit or a coplanar two-dimensional Transmon qubit.
  • Transmon qubits Let’s take Transmon qubits as an example to illustrate.
  • the transition matrix elements of Transmon qubits can be written as follows:
  • E J and E C are Josephson energy and charge energy, respectively, which are symbols commonly used in this field.
  • Figure 3 is a simulation diagram of the relationship between the probability of the qubit relaxing to the ground state and the time when the initial state of the Transmon qubit is in different excited states. It shows that the probability of relaxing to the ground state after the qubit is excited to a higher energy state decreases significantly.
  • the abscissa corresponding to the dashed line is 2 ⁇ s, which is the typical reading time of current superconducting qubits.
  • the reading error due to relaxation is about 12.7%.
  • this error is reduced to 1.1%; when the bit is excited to the third excited state, it is reduced to 0.098%; when the bit is excited to the fourth excited state, it is reduced to 0.001%.
  • superconducting qubits have other relaxation channels, but the error caused by relaxation is still expected to be reduced by about 100 times.
  • the energy levels are level-by-level excitation, that is, superconducting qubits only have charge transition matrix elements between two adjacent energy states, and will not directly relax from a higher energy state to The ground state can only relax to the ground state through the intermediate energy level in a level-by-level cascade attenuation. Therefore, the process of exciting the Transmon qubit in the excited state A to a higher energy state and relaxing to the ground state in a cascade attenuation greatly reduces the relaxation rate, thereby reducing the superconducting qubit due to the read process The error caused by relaxation also makes it possible to take a longer reading time.
  • Fig. 4 is a schematic diagram of a reading circuit according to an embodiment of the present disclosure.
  • XY represents a manipulation line
  • R represents a read line
  • the operation of operation S101 corresponds to the control line XY.
  • the box above the control line XY in FIG. 4 represents the operation process of pre-exciting the superconducting qubit to be tested from the excited state A to a higher energy state.
  • Example X12 gate represents the process of exciting the superconducting qubit under test from the first excited state to the second excited state
  • gate X23 represents the process of exciting the superconducting qubit under test from the second excited state to the third excited state
  • the X34 gate represents the operation process of exciting the superconducting qubit under test from the third excited state to the fourth excited state.
  • the dashed box in Figure 4 indicates that further excitation can be performed on the basis of the operation of the solid line box, that is, the operation of multiple excitations can be performed.
  • the content in the above dashed box is an optional operation, or it can be performed only once. Process. Since the situation of one-shot and multiple-shot has been described in detail above, it will not be repeated here.
  • the X12, X23, and X34 gates can be realized by coupling microwave transmission lines with superconducting qubits, and applying microwave pulses of specific frequencies and waveforms on the microwave transmission lines.
  • the typical generation method of the required microwave pulse is: produced by a microwave signal source, an arbitrary waveform generator and an image rejection mixer (IQ mixer).
  • the rectangular frame above the reading line R in FIG. 4 represents the measurement or reading process, which is illustrated as "measurement/reading".
  • the reading/measurement process on the reading line R occurs after the pre-processing time of the superconducting qubit.
  • the operation of operation S102 occurs on the read line R correspondingly.
  • the reading circuit includes a microwave transmission line and a microwave resonant cavity coupled with a superconducting qubit.
  • the microwave resonant cavity is used as a reading cavity to couple with the superconducting qubit.
  • the microwave transmission line is connected with the circuit outside the quantum chip, and the quantum state can be read by measuring the transmission spectrum or reflection spectrum of the microwave transmission line.
  • frequency division multiplexing can be used to read the quantum state.
  • a Josephson Parametric Amplifier JPA
  • JPA Josephson Parametric Amplifier
  • FIG. 5 is a schematic diagram of resolving quantum states from an IQ diagram generated based on dispersion measurement according to an embodiment of the present disclosure.
  • the abscissa is the real part of transmittance/reflectance, and the ordinate is the imaginary part of transmittance/reflectance.
  • the cascade relaxation-based superconducting qubit reading of the present disclosure is implemented
  • the energy state position distribution corresponding to the ground state and the excited state is correspondingly obtained.
  • the ground state and the excited state need to be distinguished in the read operation.
  • draw the dividing line between the excited state and the ground state so that logic 0 and logic 1 can be obtained.
  • the dividing line that distinguishes the ground state from the excited state may be a straight line, and of course the dividing line may also be other curves.
  • advanced algorithms such as machine learning can also be used to distinguish.
  • a reading device based on cascade relaxation of superconducting qubits is provided, and the reading device can be used to perform the above-mentioned reading method.
  • Fig. 6 is a schematic block diagram of reading a superconducting quantum chip by a reading device based on cascade relaxation of a superconducting qubit according to the second embodiment of the present disclosure.
  • the double arrow indicates the coupling relationship
  • the single arrow indicates the transmission direction or the loading direction; the line indicates the connection relationship.
  • the reading device 2 of this embodiment includes: a relaxation time extension processing module 21 and a quantum state reading module 22.
  • the relaxation time extension processing module 21 is used to pre-extend the time for the superconducting qubit to be tested to relax to the ground state before reading.
  • the quantum state reading module 22 is used to read the state of the superconducting qubit 11.
  • the relaxation time extension processing module 21 includes: an energy excitation sub-module 211.
  • the energy excitation submodule 211 is used to pre-process the superconducting qubit 11 to excite the superconducting qubit 11 to be tested from the excited state A to a higher energy state.
  • the excited state A refers to the superconducting qubit to be tested before being read.
  • the quantum state reading module 22 includes: a reading sub-module 221 and a judgment sub-module 222.
  • the reading sub-module 221 is used to read the state of the superconducting qubit after pre-processing.
  • the judging submodule 222 is used to determine the read logic state of the superconducting qubit under test by judging whether the state of the superconducting qubit 11 is a ground state or an excited state.
  • the energy excitation sub-module 211 includes a first microwave transmission line 2111 on which a microwave pulse 2112 is loaded.
  • the first microwave transmission line 2111 is used for coupling with the superconducting qubit 11, and the first microwave transmission line 2111 is loaded with microwave pulses 2112 of a specific frequency and waveform to realize the excitation of the superconducting qubit 11 from the excited state A to more.
  • Operation in a high-energy state; and/or, in an embodiment, the reading sub-module 221 includes a reading circuit 2210.
  • the reading circuit 2210 includes: a second microwave transmission line 22101 and a microwave resonant cavity 22102 coupled with the superconducting qubit 11.
  • the microwave resonant cavity 22102 is used as a reading cavity to couple with the superconducting qubit 11, and the second microwave transmission line 22101 is connected to the external circuit of the quantum chip 1 where the superconducting qubit 11 is located, and the second microwave transmission line 22101 is measured.
  • the transmission spectrum or reflection spectrum is used to read the quantum state.
  • the superconducting qubit includes: Transmon qubit.
  • Transmon qubits used as an example, and other qualified qubit systems can also apply the reading method of the present disclosure, as long as the qubit system meets the following conditions: relaxation from a higher energy state to a ground state The rate is different from the relaxation rate of the transition from excited state A to the ground state. From the high energy state higher than the excited state A, it cannot directly transition back to the ground state. It needs to go through an intermediate energy state, so the process of relaxing from the higher energy state to the ground state is prolonged The time to relax to the ground state is correspondingly prolonged and the reading time is reduced and the measurement error caused by the relaxation is reduced.
  • the present disclosure provides a method and device for reading superconducting qubits based on cascade relaxation, which suppresses the superconducting quantum to be measured by extending the time for the unknown superconducting qubit to relax to the ground state.
  • the rate at which the bit relaxes to the ground state greatly reduces the probability that the qubit relaxes to the ground state during the reading time, and ensures that the greater probability of the qubit is at the original value during the measurement process.
  • the excited state or the ground state can avoid the measurement error caused by the faster relaxation rate that has been relaxed to the ground state during reading, and the reading time can be prolonged to improve the reading fidelity; in one embodiment, in some qubits
  • the relaxation rate from the higher energy state to the ground state is different from the relaxation rate from the excited state A to the ground state, and the high energy state (non-excited state) higher than the excited state A cannot directly transition back to the ground state. It needs to pass through the intermediate energy state; therefore, by pre-processing the superconducting qubit, the superconducting qubit under test is excited from the excited state A to a higher energy state, so as to extend the time for the superconducting qubit under test to relax to the ground state. Suppresses the relaxation rate of the qubit to the ground state to achieve the effect of reducing measurement errors. At the same time, it can also extend the reading time to improve the fidelity of the reading. This technical solution can be applied to Transmon qubits.
  • modules in the embodiments can be combined into one module, and in addition, they can be divided into multiple sub-modules. Except that at least some of such features and/or processes or units are mutually exclusive, any combination can be used to compare all the features disclosed in this specification (including the accompanying claims, abstract and drawings) and any method or methods disclosed in this manner or All processes or units of the device are combined. Unless expressly stated otherwise, each feature disclosed in this specification (including the accompanying claims, abstract and drawings) may be replaced by an alternative feature providing the same, equivalent or similar purpose. Moreover, in the unit claims that list several devices, several of these devices may be embodied by the same hardware item.
  • the first excited state refers to the first excited state energy level adjacent to the ground state
  • the second excited state refers to the second excited state energy level adjacent to the first excited state
  • the third excited state refers to the adjacent first excited state energy level. The energy level of the third excited state of the second excited state, and so on.

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Abstract

一种基于级联弛豫的超导量子比特的读取方法及装置，该读取方法包括：在读取之前，预先延长待测超导量子比特弛豫到基态的时间；以及进行超导量子比特状态的读取，以确定待测超导量子比特对应的逻辑状态。在一实施例中，在读取之前，将待测未知态超导量子比特的激发态分量激发至更高能态以延长待测超导量子比特弛豫到基态的时间。通过压制量子比特弛豫到基态的速率，特别是在测量初始阶段弛豫到基态的速率，使得读取时间内量子比特弛豫到基态的概率大大降低，保证测量过程中量子比特较大概率保持在激发态或基态，避免由于弛豫到基态的速率较快导致的读取时已经弛豫至基态引发的测量误差，也有助于延长读取时间以提升读取保真度。

Description

基于级联弛豫的超导量子比特的读取方法及装置 技术领域
本公开属于量子计算技术领域，涉及一种基于级联弛豫的超导量子比特的读取方法及装置。
背景技术
量子计算机在解决复杂计算方面具有潜在的应用价值。量子计算机本质上为一个量子力学系统，利用量子力学系统的状态，即所谓的量子态来编码信息，按照量子动力学的演化规律来执行运算任务，并根据量子力学的测量原理来提取计算结果。
在超导量子比特的应用中，对于量子比特状态函数的精确读取至关重要。目前读取方式包括色散测量方式，其中色散测量是指将待测量子比特与一线性谐振腔相耦合。由于交流斯塔克谱线磁裂(AC Stark)效应，比特的状态会改变谐振腔的频率，由此可以通过测量谐振腔的频率来间接测量比特的状态。然而上述色散测量的方式存在测量误差大的问题。
发明内容
本公开提供了一种基于级联弛豫的超导量子比特的读取方法及装置，以至少部分解决以下技术问题：现有的量子态读取/测量由于弛豫存在测量误差较大的问题。
为了解决上述技术问题，根据本公开的一个方面，提供了一种基于级联弛豫的超导量子比特的读取方法，该读取方法包括：在读取之前，预先延长待测超导量子比特弛豫到基态的时间；以及进行超导量子比特状态的读取。
在本公开的一实施例中，所述在读取之前，预先延长待测超导量子比特弛豫到基态的时间，包括：预先处理超导量子比特，将待测超导量子比特从激发态A激发至更高能态，激发态A是指该待测超导量子比特读取之前所处的激发态。
在本公开的一实施例中，所述将待测超导量子比特从激发态A激发至更高能态包括：将待测超导量子比特从激发态A进行一次激发，激发至更高能态； 或者，将待测超导量子比特从激发态A进行至少两次激发，先将待测超导量子比特从激发态A激发至比激发态A更高能级的激发态B，再从激发态B激发至比激发态B更高能级的激发态C，以此类推，激发多次至更高能态。
在本公开的一实施例中，将待测超导量子比特从激发态A进行至少两次激发的方式为级联激发，该级联激发包括逐能级累次激发和间隔能级累次激发。
在本公开的一实施例中，所述进行超导量子比特状态的读取，包括：读取预先处理后的超导量子比特的状态，通过判断超导量子比特所处状态的类别为基态还是激发态来确定待测超导量子比特的读取逻辑状态。
在本公开的一实施例中，所述预先处理超导量子比特，将待测超导量子比特从激发态A激发至更高能态采用如下方法：通过第一微波传输线与超导量子比特进行耦合，并在该第一微波传输线上加以特定频率和波形的微波脉冲，实现超导量子比特从激发态A激发至更高能态的操作。
在本公开的一实施例中，采用色散测量的方法读取超导量子比特的状态，读取电路包括：第二微波传输线以及与超导量子比特相耦合的微波谐振腔；其中，所述微波谐振腔作为读取腔与所述超导量子比特相耦合，所述第二微波传输线与所述超导量子比特所在的量子芯片的外电路相连接，通过测量所述第二微波传输线的透射谱或反射谱来读取量子态。
在本公开的一实施例中，所述超导量子比特包括：Transmon量子比特。
为了解决上述技术问题，根据本公开的另一个方面，提供了一种基于级联弛豫的超导量子比特的读取装置，该读取装置包括：弛豫时间延长处理模块，用于在读取之前，预先延长待测超导量子比特弛豫到基态的时间；以及量子态读取模块，用于进行超导量子比特状态的读取。
在本公开的一实施例中，所述弛豫时间延长处理模块包括：能量激发子模块，用于预先处理超导量子比特，将待测超导量子比特从激发态A激发至更高能态；和/或，所述量子态读取模块包括：读取子模块，用于读取预先处理后的超导量子比特的状态；以及判断子模块，用于通过判断超导量子比特所处状态的类别为基态还是激发态来确定待测超导量子比特的读取逻辑状态。
在本公开的一实施例中，所述能量激发子模块包括：第一微波传输线，所述第一微波传输线用于与所述超导量子比特进行耦合，并且所述第一微波传输线上加载有特定频率和波形的微波脉冲，以实现超导量子比特从激发态A激发至更高能态的操作；和/或，所述读取子模块包括读取电路，该读取电路包括： 第二微波传输线以及与超导量子比特相耦合的微波谐振腔；其中，所述微波谐振腔作为读取腔与所述超导量子比特相耦合，所述第二微波传输线与所述超导量子比特所在的量子芯片的外电路相连接，通过测量所述第二微波传输线的透射谱或反射谱来读取量子态。
在本公开的一实施例中，所述超导量子比特包括：Transmon量子比特。
从上述技术方案可以看出，本公开提供的基于级联弛豫的超导量子比特的读取方法及装置，至少具有以下有益效果：
(1)通过延长待测超导量子比特弛豫到基态的时间，压制量子比特弛豫到基态的速率，从而使得读取时间内量子比特弛豫到基态的概率大大降低，保证测量过程中量子比特较大概率处于原先的激发态或基态，避免由于弛豫速率较快导致的读取时已经弛豫至基态引发的测量误差，可以延长读取时间以提升读取保真度；
(2)在一实施例中，在一些量子比特体系中，激发态A为第一激发态，从较高能态弛豫至基态的弛豫速率与从第一激发态跃迁至基态的弛豫速率存在差异，从高于第一激发态的高能态无法直接跃迁回基态，需要经过中间能态；因此，通过对超导量子比特进行预先处理，将待测超导量子比特从激发态A激发至更高能态，以延长待测超导量子比特弛豫到基态的时间，压制量子比特弛豫到基态的速率，实现减少测量误差的效果，同时还有助于延长读取时间以提升读取保真度，该方案可以适用于Transmon量子比特。
附图说明
图1为根据本公开第一实施例所示的基于级联弛豫的超导量子比特的读取方法的示意图。
图2为根据本公开第一实施例所示的基于级联弛豫的超导量子比特的读取方法的流程图。
图3为根据本公开一实施例所示的初态在不同高能激发态的比特弛豫到基态的概率随时间的关系示意图。
图4为根据本公开一实施例所示的读取线路的示意图。
图5为根据本公开一实施例所示的从基于色散测量产生的IQ图中分辨量子态示意图。
图6为根据本公开第二实施例所示的基于级联弛豫的超导量子比特的读取 装置对超导量子芯片进行读取的示意框图。
【符号说明】
1-量子芯片；
11-超导量子比特；
2-读取装置；
21-弛豫时间延长处理模块；
211-能量激发子模块；
2111-第一微波传输线；
2112-微波脉冲；
22-量子态读取模块；
221-读取子模块；
2210-读取电路；
22101-第二微波传输线；
22102-微波谐振腔
222-判断子模块。
具体实施方式
由于测量电子学设备噪声、量子退相干速率的限制等方面的影响，要较好地区分量子态需要对测量信号做较长时间的积分。然而，由于量子比特存在弛豫现象，在较长的测量时间内量子比特有一定概率从激发态弛豫到基态，这造成了极大的测量误差。
因此，基于上述分析，本公开提供了一种基于级联弛豫的超导量子比特的读取方法及装置，通过延长待测超导量子比特弛豫到基态的时间，压制量子比特弛豫到基态的速率，从而使得读取时间内量子比特弛豫到基态的概率大大降低，保证测量过程中量子比特较大概率处于原先的激发态或基态，避免由于弛豫速率较快导致的读取时已经弛豫至基态引发的测量误差，可以延长读取时间以提升读取保真度。
本公开提供的基于级联弛豫的超导量子比特的读取方法及装置，通过在读取之前，预先延长待测超导量子比特弛豫到基态的时间。然后进行超导量子比特状态的读取。通过延长弛豫到基态的时间，压制量子比特弛豫到基态的弛豫速率，保证测量过程中量子比特较大概率处于原先的激发态或基态，避免由于 弛豫速率较快导致的读取时已经弛豫至基态引发的测量误差，可以延长读取时间以提升读取保真度。
本公开的一实施例中，所述在读取之前，预先延长待测超导量子比特的弛豫到基态的时间，包括：预先处理超导量子比特，将待测超导量子比特从激发态A激发至更高能态，激发态A是指该待测超导量子比特读取之前所处的激发态。在本公开的一实施例中，所述将待测超导量子比特从激发态A激发至更高能态包括：将待测超导量子比特从激发态A进行一次激发，激发至更高能态；或者，将待测超导量子比特从激发态A进行至少两次激发，先将激发态A激发至比激发态A更高能级的激发态B，再由激发态B激发至比激发态B更高能级的激发态C，以此类推，激发多次至更高能态。
在本公开的一实施例中，所述进行超导量子比特状态的读取，包括：读取预先处理后的超导量子比特的状态，通过判断超导量子比特所处状态的类别为基态还是激发态来确定待测超导量子比特的读取逻辑状态。
在本公开的一实施例中，所述预先处理超导量子比特，将待测超导量子比特从激发态A激发至更高能态采用如下方法：通过第一微波传输线与超导量子比特进行耦合，并在该第一微波传输线上加以特定频率和波形的微波脉冲，实现超导量子比特从激发态A激发至更高能态的操作。
在本公开的一实施例中，采用色散测量的方法读取超导量子比特的状态，读取电路包括：第二微波传输线以及与超导量子比特相耦合的微波谐振腔；其中，所述微波谐振腔作为读取腔与所述超导量子比特相耦合，所述第二微波传输线与所述超导量子比特所在的量子芯片的外电路相连接，通过测量所述第二微波传输线的透射谱或反射谱来读取量子态。
在本公开的一实施例中，所述超导量子比特包括：Transmon量子比特。“Transmon”是为了命名一种新结构的超导量子比特而新造的词汇，由耶鲁大学研究人员在2007年在其论文中提出。“Transmon”实际为“a transmission-line shunted plasma oscillation qubit”的简写，其中文翻译为“传输线并联的电浆振荡比特”。
为使本公开的目的、技术方案和优点更加清楚明白，以下结合具体实施例，并参照附图，对本公开进一步详细说明。
本公开的第一个示例性实施例提供了一种基于级联弛豫的超导量子比特的读取方法。
图1为根据本公开第一实施例所示的基于级联弛豫的超导量子比特的读取方法的示意图。图2为根据本公开第一实施例所示的基于级联弛豫的超导量子比特的读取方法的流程图。
图1中，右侧示意了超导量子比特所处的能级分别对应为1、2、3、……n，左侧示意了各能级下对应的能量E1、E2、E3、……、En，分别对应基态、第一激发态、第二激发态、……第n-1激发态，虚线矩形框示意的为读取过程，虚线矩形框内的能级间的向下箭头示意由高能级向低能级跃迁的过程，向上箭头示意由低能级激发至高能级的过程。
进行量子计算时，以超导量子比特的基态和激发态进行编码，以对应逻辑0和逻辑1，例如，可以利用基态编码为逻辑0，利用激发态编码为逻辑1，或者反之也可以。
参照图2所示，本实施例的基于级联弛豫的超导量子比特的读取方法，包括操作S101和操作S102。
在操作S101，预先处理超导量子比特，将待测超导量子比特从激发态A激发至更高能态，激发态A是指该待测超导量子比特读取之前所处的激发态。
参照图1所示，矩形框①表示现有技术中的读取过程，矩形框②-④表示本公开读取方法的读取过程。示例性的，参照矩形框①所示，相关技术中，基态为能级1，对应能量为E1，激发态A为第一激发态，对应为能级2，对应能量为E2，在读取过程中，由于量子比特有一定概率从第一激发态(激发态A)弛豫到基态，如图1中的虚线向下箭头所示，由于第一激发态(激发态A)与基态之间为邻近能级，对应的跃迁速率较大，因此，在读取的瞬间有可能待测超导量子比特已经由激发态A跃迁至基态，此时测得的结果为超导量子比特处于基态，对应的读取逻辑为0，而该待测超导量子比特实际上被测试出处于激发态才是正确的测量结果，由此导致了测量在一定概率上的错误，对应会产生较大的读取误差。
该操作S101中，通过对超导量子比特进行预先处理，将待测超导量子比特从激发态A激发至更高能态，实施方式可以参照图1中矩形框②-④所示。在一些量子比特体系中，从较高能态弛豫至基态的弛豫速率与从激发态A跃迁至基态的弛豫速率存在差异，从高于激发态A的高能态无法直接跃迁回基态，需要经过中间能态。从较高能态弛豫至基态的过程延长了超导量子比特从激发态弛豫到基态的时间，对应延长了读取时间以及减少了由于弛豫带来的测量误差。 例如图1中的E3弛豫至E1需要先从E3跃迁至E2，然后从E2跃迁至E1的过程，相较于从E2跃迁至E1的跃迁过程延长了弛豫到基态的时间。当然，上述方式仅作为延长弛豫到基态的时间的一种示例，避免在读取过程中量子比特已经弛豫至基态导致测量错误，其他能够延长弛豫到基态的时间，以延长读取时间从而降低或消除测量误差的技术构思均在本公开的保护范围之内。
在操作S101中，可以仅进行一次激发，将超导量子比特从激发态A激发至比激发态A更高能级的激发态B，例如，在一实施例中，激发态B可以是第二激发态，如图1中矩形框②示意的情况，在另一实施例中，激发态B也可以是第四激发态，如图1中矩形框③中示意的情况。
该操作S101中，也可以进行多次(至少为两次)激发，例如可以先将超导量子比特从激发态A激发至比激发态A更高能级的激发态B，然后将超导量子比特从激发态B激发至比激发态B更高能级的激发态C，如图1中矩形框④示意的情况。
在本公开的一实施例中，将待测超导量子比特从激发态A进行至少两次激发的方式为级联激发，该级联激发包括逐能级累次激发和间隔能级累次激发，例如，从第一激发态可以被激发至第二激发态，从第二激发态被激发至第三激发态等。逐能级累次激发表示从相邻的两个能级之间累次往高能级进行激发，间隔能级累次激发表示可以从间隔开的两个能级之间发生激发过程，比如，从第一激发态可以被激发至第三激发态，然后从第三激发态可以被激发至第四激发态，从第四激发态可以被激发态至第六激发态等。
当然，本公开不限制激发的次数，在达到最高能级之前可以进行多次，不进行限制。上述能级中，激发至的更高能级与被激发的能级之间可以是相邻的能级，如图1中矩形框②示意的情形。激发至的更高能级与被激发的能级之间也可以是间隔开的能级，如图1中矩形框③示意的情形。例如，上述实施例中，激发态A与激发态B可以是邻近的能级，也可以是间隔开的能级。
在一实施例中，例如可以通过微波传输线与超导量子比特进行耦合，并在该微波传输线上加以特定频率和波形的微波脉冲实现超导量子比特从激发态A激发至更高能态的操作。
在操作S102，读取预先处理后的超导量子比特的状态，通过判断超导量子比特所处状态的类别为基态还是激发态来确定待测超导量子比特的读取逻辑状态。
在一实施例中，该操作S102中，可以采用色散测量的方法读取超导量子比特的状态，通过测量读取腔的透射谱或反射谱等方法判断超导量子比特处于基态还是激发态。在一实施例中，如果判断超导量子比特处于基态，则认为待测超导量子比特的读取逻辑状态为逻辑0。如果判断超导量子比特处于激发态，则待测超导量子比特的读取逻辑状态为逻辑1。需要注意的是，这里只需要判断比特处于基态或是激发态，这里的激发态仅为与基态对应的概念，激发态A和更高能态均属于激发态，不需要具体判断超导量子比特处于激发态中的具体状态，即不需要判断超导量子比特处于激发态A还是更高能态。
下面结合同图3来介绍本公开的读取方法的工作原理和技术效果。
图3为根据本公开一实施例所示的初态在不同高能激发态的比特弛豫到基态的概率随时间的关系示意图。
影响超导量子比特弛豫速率的主要因素为其电荷跃迁矩阵元的大小。在超导量子比特中，电荷与量子芯片上读取腔腔模、噪声等相互作用使得超导量子比特从激发态弛豫到基态。电荷跃迁矩阵元越大，弛豫速率越大。
在某些超导量子比特结构，如Transmon量子比特中，只有能量相邻的两个能态之间存在跃迁矩阵元。其中，Transmon量子比特为一超导电路，根据振荡电流的不同能量来表示量子比特的状态。本公开的技术方案适用于目前被广泛使用的超导Transmon量子比特。这里所指的Transmon量子比特可以为三维Transmon量子比特，也可以为共平面的二维Transmon量子比特。
下面以Transmon量子比特为例来说明，Transmon量子比特的跃迁矩阵元可写为如下形式：
Figure PCTCN2020074321-appb-000001
其中，E J和E C分别为约瑟夫森能量和电荷能，均为本领域通用的符号，
Figure PCTCN2020074321-appb-000002
表示从第j个能态到第j+1个能态的跃迁矩阵元，表征量子比特从第j+1个能态到第j个能态的弛豫速率。这一数值越大，弛豫速率越高。同样地，
Figure PCTCN2020074321-appb-000003
为从第j个能态到第j+k个能态的跃迁矩阵元，表征量子比特从第j+k个能态到第j个能态的弛豫速率。
图3为Transmon量子比特初态在不同激发态时量子比特弛豫到基态的概率随时间的关系模拟图，它展示了将量子比特激发到更高能态后弛豫到基态的概 率显著下降。图3所示的关系模拟图中，超导量子比特从第一激发态弛豫到基态的时间为T 1＝15μs。并假设在理想情况下，只有电荷跃迁矩阵元的影响，其它能级间的弛豫时间为T n，n-1＝T 1/n，n表示能级状态的序号，T n，n-1表示第n个能级到第n-1个能级的弛豫时间。虚线处所对应的横坐标为2μs，这是目前超导量子比特的典型的读取时间。在这一参数下，当超导量子比特处于第一激发态时，因弛豫产生的读取误差约为12.7％。当比特在开始读取时被激发到第二激发态时，这一误差降为1.1％；激发到第三激发态时，降为0.098％；激发到第四激发态时，降为0.001％。在实际情况下，超导量子比特会有其它的弛豫通道，但因弛豫所造成的误差预计仍有约100倍的下降。
对于Transmon量子比特而言，能级之间为逐能级激发，即超导量子比特仅在能量相邻的两个能态之间存在电荷跃迁矩阵元，不会直接从更高能态弛豫到基态，而只能通过中间能级以逐能级级联衰减的方式弛豫到基态。因此，将处于激发态A的Transmon量子比特激发到更高能态，以级联衰减的方式弛豫到基态的过程极大地降低了弛豫速率，从而降低了超导量子比特由于在读取过程中弛豫所造成的误差，也为采取更长的读取时间提供了可能。
图4为根据本公开一实施例所示的读取线路的示意图。
参照图4所示，XY表示操控线，R表示读取线。在一实施例中，操作S101的操作对应在操控线XY发生，图4中操控线XY上面的方框表示预先将待测超导量子比特从激发态A激发至更高能态的操作过程，示例性，以X12门表示将待测超导量子比特从第一激发态激发至第二激发态的操作过程；X23门表示将待测超导量子比特从第二激发态激发至第三激发态的操作过程；X34门表示将待测超导量子比特从第三激发态激发至第四激发态的操作过程。图4中虚线方框表示可以在实线方框的操作基础上进一步进行激发，即可以进行多次激发的操作，当然，上述虚线框中的内容为可选操作，也可以是只进行一次激发过程。由于上文已经详细描述过一次激发和多次激发的情形，这里不再赘述。
X12、X23、X34门的实现方式可为微波传输线与超导量子比特进行耦合，并在微波传输线上加以特定频率和波形的微波脉冲实现。
其中，所需微波脉冲的典型产生方法为：由微波信号源、任意波形发生器及镜像抑制混频器(IQ混频器)制作产生。
图4中读取线R上面的矩形框表示测量或读取过程，以“测量/读取”进行示意。其中读取线R上读取/测量过程发生在超导量子比特预先处理的时间之后。 在一实施例中，操作S102的操作对应在读取线R发生。在一实例中，读取电路包括：微波传输线以及与超导量子比特相耦合的微波谐振腔。微波谐振腔作为读取腔与超导量子比特相耦合。微波传输线与量子芯片外电路相连接，可通过测量微波传输线的透射谱或反射谱来读取量子态。例如可以采取频分复用方式来对量子态进行读取。可选的，在一实施例中，为进一步提高读取效率及保真度，读取电路中微波传输线出口处可选装约瑟夫森参量放大器(JPA)。
图5为根据本公开一实施例所示的从基于色散测量产生的IQ图中分辨量子态示意图。横坐标为透射率/反射率的实部，纵坐标为透射率/反射率的虚部。
按照色散测量方法，需要通过测量透射率或反射率形成IQ图，并在IQ图上分辨量子态，如图5所示。超导量子比特处于不同能态时对应IQ图的位置不同，如图5中图例“○”对应基态测量数据，图例“+”“●”“△”分别对应不同的激发态，比如“+”对应第一激发态测量数据；“●”对应第二激发态测量数据；“△”对应第三激发态测量数据。待测超导量子比特分别被制备在基态(对应逻辑0)和第一激发态(对应逻辑1)上，读取过程中，实施本公开的基于级联弛豫的超导量子比特的读取方法，通过执行操作S101和操作S102，对应得到基态和激发态对应的能态位置分布，参照图5中的示意可知，在读取操作中需要将基态与激发态区分开来。根据测量得到的IQ图，画出激发态和基态之间的分界线，从而可以得到逻辑0和逻辑1。
在图5的示例中，区分基态与激发态的分界线可以是一条直线，当然分界线也可以是其它曲线。除了采用数学拟合的方式获得之外，还可以采取机器学习等高级算法进行分辨。
第二实施例
在本公开的第二个示例性实施例中，提供了一种基于级联弛豫的超导量子比特的读取装置，该读取装置可以用于执行上述读取方法。
图6为根据本公开第二实施例所示的基于级联弛豫的超导量子比特的读取装置对超导量子芯片进行读取的示意框图。双箭头示意耦合关系，单箭头示意传输方向或加载方向；连线示意连接关系。
参照图6所示，本实施例的读取装置2包括：弛豫时间延长处理模块21和量子态读取模块22。
其中，弛豫时间延长处理模块21用于在读取之前，预先延长待测超导量子比特弛豫到基态的时间。量子态读取模块22用于进行超导量子比特11状态的 读取。
在本公开的一实施例中，参照图6所示，弛豫时间延长处理模块21包括：能量激发子模块211。该能量激发子模块211用于预先处理超导量子比特11，将待测超导量子比特11从激发态A激发至更高能态，激发态A是指该待测超导量子比特读取之前所处的激发态；和/或，在一实施例中，量子态读取模块22包括：读取子模块221和判断子模块222。读取子模块221用于读取预先处理后的超导量子比特的状态。判断子模块222用于通过判断超导量子比特11所处状态的类别为基态还是激发态来确定待测超导量子比特的读取逻辑状态。
进一步参照图6所示，在一实施例中，能量激发子模块211包括第一微波传输线2111，该第一微波传输线2111上加载有微波脉冲2112。其中该第一微波传输线2111用于与超导量子比特11进行耦合，该第一微波传输线2111上加载有特定频率和波形的微波脉冲2112，以实现超导量子比特11从激发态A激发至更高能态的操作；和/或，在一实施例中，读取子模块221包括读取电路2210。该读取电路2210包括：第二微波传输线22101以及与超导量子比特11相耦合的微波谐振腔22102。其中，微波谐振腔22102作为读取腔与所述超导量子比特11相耦合，第二微波传输线22101与超导量子比特11所在的量子芯片1的外电路相连接，通过测量第二微波传输线22101的透射谱或反射谱来读取量子态。
在本公开的一实施例中，所述超导量子比特包括：Transmon量子比特。当然，本公开以Transmon量子比特作为示例，其他符合条件的量子比特系统也可以应用本公开的读取方法，只要该量子比特体系满足如下条件即可：从较高能态弛豫至基态的弛豫速率与从激发态A跃迁至基态的弛豫速率存在差异，从高于激发态A的高能态无法直接跃迁回基态，需要经过中间能态，由此从较高能态弛豫至基态的过程延长了弛豫到基态的时间，对应延长了读取时间以及减少了由于弛豫带来的测量误差。
综上所述，本公开提供了一种基于级联弛豫的超导量子比特的读取方法及装置，通过延长待测未知超导量子比特弛豫到基态的时间，压制待测超导量子比特弛豫到基态的速率，特别是在测量初始阶段弛豫到基态的速率，从而使得读取时间内量子比特弛豫到基态的概率大大降低，保证测量过程中量子比特较大概率处于原先的激发态或基态，避免由于弛豫速率较快导致的读取时已经弛豫至基态引发的测量误差，可以延长读取时间以提升读取保真度；在一实施例中，在一些量子比特体系中，从较高能态弛豫至基态的弛豫速率与从激发态A 跃迁至基态的弛豫速率存在差异，从高于激发态A的高能态(非激发态)无法直接跃迁回基态，需要经过中间能态；因此，通过对超导量子比特进行预先处理，将待测超导量子比特从激发态A激发至更高能态，以延长待测超导量子比特弛豫到基态的时间，压制量子比特弛豫到基态的弛豫速率，实现减少测量误差的效果，同时还可以延长读取时间以提升读取保真度，该技术方案可以适用于Transmon量子比特。
本领域那些技术人员可以理解，可以对实施例中的装置中的模块进行自适应性地改变并且把它们设置在与该实施例不同的一个或多个装置中。可以把实施例中的模块组合成一个模块，以及此外可以把它们分成多个子模块。除了这样的特征和/或过程或者单元中的至少一些是相互排斥之外，可以采用任何组合对本说明书(包括伴随的权利要求、摘要和附图)中公开的所有特征以及如此公开的任何方法或者装置的所有过程或单元进行组合。除非另外明确陈述，本说明书(包括伴随的权利要求、摘要和附图)中公开的每个特征可以由提供相同、等同或相似目的的替代特征来代替。并且，在列举了若干装置的单元权利要求中，这些装置中的若干个可以是通过同一个硬件项来具体体现。
说明书与权利要求中所使用的序数例如“第一”、“第二”、“第三”等的用词，以修饰相应的元件，其本身并不意味着该元件有任何的序数，也不代表某一元件与另一元件的顺序、或是制造方法上的顺序，该些序数的使用仅用来使具有某命名的一元件得以和另一具有相同命名的元件能做出清楚区分，需要说明的是，全文中的“第一激发态”、“第二激发态”、“第三激发态”等用语为本领域惯用技术术语，是按照能级顺序依次的称谓，具有确定的能级顺序，第一激发态指的是邻近基态的第一个激发态能级，第二激发态指的是邻近第一激发态的第二个激发态能级，第三激发态指的是邻近第二激发态的第三个激发态能级，以此类推。
再者，单词“包含”或“包括”不排除存在未列在权利要求中的元件或步骤。位于元件之前的单词“一”或“一个”不排除存在多个这样的元件。
除非存在技术障碍或矛盾，本发明的上述各种实施方式可以自由组合以形成另外的实施例，这些另外的实施例均在本发明的保护范围中。
以上所述的具体实施例，对本公开的目的、技术方案和有益效果进行了进一步详细说明，所应理解的是，以上所述仅为本公开的具体实施例而已，并不用于限制本公开，凡在本公开的精神和原则之内，所做的任何修改、等同替换、 改进等，均应包含在本公开的保护范围之内。

Claims (12)

  1. 一种基于级联弛豫的超导量子比特的读取方法，其特征在于，包括：
    在读取之前，预先延长待测超导量子比特弛豫到基态的时间；以及
    进行超导量子比特状态的读取。
  2. 根据权利要求1所述的读取方法，其特征在于，所述在读取之前，预先延长待测超导量子比特弛豫到基态的时间，包括：
    预先处理超导量子比特，将待测超导量子比特从激发态A激发至更高能态，激发态A是指该待测超导量子比特读取之前所处的激发态。
  3. 根据权利要求2所述的读取方法，其特征在于，所述将待测超导量子比特从激发态A激发至更高能态包括：
    将待测超导量子比特从激发态A进行一次激发，激发至更高能态；或者，
    将待测超导量子比特从激发态A进行至少两次激发，先将待测超导量子比特从激发态A激发至比激发态A更高能级的激发态B，再从激发态B激发至比激发态B更高能级的激发态C，以此类推，激发多次至更高能态。
  4. 根据权利要求3所述的读取方法，其特征在于，所述将待测超导量子比特从激发态A进行至少两次激发的方式为级联激发，该级联激发包括逐能级累次激发和间隔能级累次激发。
  5. 根据权利要求2所述的读取方法，其特征在于，所述进行超导量子比特状态的读取，包括：
    读取预先处理后的超导量子比特的状态，通过判断超导量子比特所处状态的类别为基态还是激发态来确定待测超导量子比特的读取逻辑状态。
  6. 根据权利要求2所述的读取方法，其特征在于，所述预先处理超导量子比特，将待测超导量子比特从激发态A激发至更高能态采用如下方法：
    通过第一微波传输线与超导量子比特进行耦合，并在该第一微波传输线上加以特定频率和波形的微波脉冲，实现超导量子比特从激发态A激发至更高能态的操作。
  7. 根据权利要求5所述的读取方法，其特征在于，采用色散测量的方法读取预先处理后的超导量子比特的状态，读取电路包括：第二微波传输线以及与超导量子比特相耦合的微波谐振腔；
    其中，所述微波谐振腔作为读取腔与所述超导量子比特相耦合，所述第二 微波传输线与所述超导量子比特所在的量子芯片的外电路相连接，通过测量所述第二微波传输线的透射谱或反射谱来读取量子态。
  8. 根据权利要求1-7中任一项所述的读取方法，其特征在于，所述超导量子比特包括：Transmon量子比特。
  9. 一种基于级联弛豫的超导量子比特的读取装置，其特征在于，包括：
    弛豫时间延长处理模块，用于在读取之前，预先延长待测超导量子比特弛豫到基态的时间；以及
    量子态读取模块，用于进行超导量子比特状态的读取。
  10. 根据权利要求9所述的读取装置，其特征在于，
    所述弛豫时间延长处理模块包括：能量激发子模块，用于预先处理超导量子比特，将待测超导量子比特从激发态A激发至更高能态，激发态A是指该待测超导量子比特读取之前所处的激发态；和/或，
    所述量子态读取模块包括：读取子模块，用于读取预先处理后的超导量子比特的状态；以及判断子模块，用于通过判断超导量子比特所处状态的类别为基态还是激发态来确定待测超导量子比特的读取逻辑状态。
  11. 根据权利要求10所述的读取装置，其特征在于，
    所述能量激发子模块包括：第一微波传输线，所述第一微波传输线用于与所述超导量子比特进行耦合，并且所述第一微波传输线上加载有特定频率和波形的微波脉冲，以实现超导量子比特从激发态A激发至更高能态的操作；和/或，
    所述读取子模块包括读取电路，该读取电路包括：第二微波传输线以及与超导量子比特相耦合的微波谐振腔；其中，所述微波谐振腔作为读取腔与所述超导量子比特相耦合，所述第二微波传输线与所述超导量子比特所在的量子芯片的外电路相连接，通过测量所述第二微波传输线的透射谱或反射谱来读取量子态。
  12. 根据权利要求9-11中任一项所述的读取装置，其特征在于，所述超导量子比特包括：Transmon量子比特。
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