CN111144573A - Method and device for reading superconducting qubits based on cascade relaxation - Google Patents
Method and device for reading superconducting qubits based on cascade relaxation Download PDFInfo
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Abstract
A method and a device for reading superconducting qubits based on cascade relaxation are disclosed, wherein the reading method comprises the following steps: before reading, the time of the superconducting qubit to be tested for relaxing to the ground state is prolonged in advance; and reading the state of the superconducting qubit to determine the corresponding logic state of the superconducting qubit to be tested. In one embodiment, prior to reading, the excited state component of the superconducting qubit in the unknown state to be measured is excited to a higher energy state to prolong the time for the superconducting qubit to be measured to relax to the ground state. By suppressing the rate of the qubit relaxing to the ground state, particularly the rate of the qubit relaxing to the ground state in the initial stage of measurement, the probability of the qubit relaxing to the ground state in the reading time is greatly reduced, the quantum bit is ensured to be kept in the excited state or the ground state with a larger probability in the measurement process, the measurement error caused by the fact that the qubit has already relaxed to the ground state in the reading process due to the fact that the rate of the qubit relaxing to the ground state is higher is avoided, and the reading time is prolonged to improve the reading fidelity.
Description
Technical Field
The disclosure belongs to the technical field of quantum computation, and relates to a method and a device for reading superconducting quantum bits based on cascade relaxation.
Background
The quantum computer has potential application value in solving the complex calculation aspect. The quantum computer is essentially a quantum mechanical system, encodes information by using the state of the quantum mechanical system, namely the so-called quantum state, executes an operation task according to the evolution law of quantum dynamics, and extracts a calculation result according to the measurement principle of quantum mechanics.
In the application of superconducting qubits, it is crucial for the accurate reading of the qubit state function. Currently, the reading method includes a dispersion measurement method, wherein the dispersion measurement is to couple the qubit to be measured with a linear resonant cavity. The state of the bit changes the frequency of the cavity due to the alternating Stark line magnetic splitting (AC Stark) effect, and thus can be measured indirectly by measuring the frequency of the cavity. However, the above-mentioned dispersion measurement method has a problem of large measurement error.
Disclosure of Invention
The disclosure provides a method and a device for reading superconducting qubits based on cascade relaxation, which at least partially solve the following technical problems: the existing quantum state reading/measurement has the problem of large measurement error due to relaxation.
In order to solve the above technical problem, according to an aspect of the present disclosure, there is provided a method for reading a superconducting qubit based on cascade relaxation, the method comprising: before reading, the time of the superconducting qubit to be tested for relaxing to the ground state is prolonged in advance; and reading the superconducting qubit state.
In an embodiment of the disclosure, the pre-prolonging the time for the superconducting qubit to be tested to relax to the ground state before reading includes: and pre-processing the superconducting qubit, and exciting the superconducting qubit to be detected to a higher energy state from an excited state A, wherein the excited state A refers to an excited state where the superconducting qubit to be detected is located before being read.
In an embodiment of the disclosure, the exciting the superconducting qubit to be tested from the excited state a to a higher energy state includes: exciting the superconducting qubit to be detected for one time from an excited state A to a higher energy state; or, the superconducting qubit to be tested is excited at least twice from the excited state A, the superconducting qubit to be tested is firstly excited from the excited state A to the excited state B with a higher energy level than the excited state A, then is excited from the excited state B to the excited state C with a higher energy level than the excited state B, and so on, and is excited for multiple times to a higher energy state.
In an embodiment of the present disclosure, the mode of exciting the superconducting qubit to be tested at least twice from the excited state a is cascade excitation, and the cascade excitation includes level-by-level successive excitation and interval level successive excitation.
In an embodiment of the disclosure, the reading of the superconducting qubit state includes: and reading the state of the preprocessed superconducting qubit, and determining the read logic state of the superconducting qubit to be tested by judging whether the type of the state of the superconducting qubit is a ground state or an excited state.
In an embodiment of the disclosure, the preprocessing the superconducting qubit, and exciting the superconducting qubit to be tested from the excited state a to a higher energy state adopts the following method: the operation of exciting the superconducting qubit from the excited state A to a higher energy state is realized by coupling the superconducting qubit through a first microwave transmission line and applying microwave pulses with specific frequency and waveform on the first microwave transmission line.
In an embodiment of the present disclosure, a state of the superconducting qubit is read using a dispersion measurement method, and the reading circuit includes: a second microwave transmission line and a microwave resonant cavity coupled to the superconducting qubit; the microwave resonant cavity is used as a reading cavity to be coupled with the superconducting qubit, the second microwave transmission line is connected with an external circuit of a quantum chip where the superconducting qubit is located, and the quantum state is read by measuring the transmission spectrum or the reflection spectrum of the second microwave transmission line.
In an embodiment of the present disclosure, the superconducting qubit includes: transmon qubits.
In order to solve the above technical problem, according to another aspect of the present disclosure, there is provided a superconducting qubit reading device based on cascade relaxation, the reading device including: the relaxation time prolonging processing module is used for prolonging the time of the superconducting qubit to be tested for relaxing to the ground state in advance before reading; and the quantum state reading module is used for reading the superconducting quantum bit state.
In an embodiment of the present disclosure, the relaxation time extension processing module includes: the energy excitation submodule is used for preprocessing the superconducting qubit and exciting the superconducting qubit to be detected from an excited state A to a higher energy state; and/or, the quantum state reading module comprises: the reading submodule is used for reading the state of the preprocessed superconducting qubit; and the judgment submodule is used for determining the read logic state of the superconducting qubit to be detected by judging whether the type of the state of the superconducting qubit is a ground state or an excited state.
In an embodiment of the present disclosure, the energy excitation sub-module includes: a first microwave transmission line for coupling with the superconducting qubit and loaded with microwave pulses of a specific frequency and waveform to effect excitation of the superconducting qubit from an excited state A to a higher energy state; and/or, the read submodule comprises a read circuit, the read circuit comprises: a second microwave transmission line and a microwave resonant cavity coupled to the superconducting qubit; the microwave resonant cavity is used as a reading cavity to be coupled with the superconducting qubit, the second microwave transmission line is connected with an external circuit of a quantum chip where the superconducting qubit is located, and the quantum state is read by measuring the transmission spectrum or the reflection spectrum of the second microwave transmission line.
In an embodiment of the present disclosure, the superconducting qubit includes: transmon qubits.
According to the technical scheme, the method and the device for reading the superconducting qubits based on the cascade relaxation have the following beneficial effects:
(1) by prolonging the time of the superconducting qubit to be tested for relaxing to the ground state and suppressing the rate of the qubit relaxing to the ground state, the probability of the qubit relaxing to the ground state in the reading time is greatly reduced, the higher probability of the qubit in the measurement process is ensured to be in the original excited state or the ground state, the measurement error caused by the fact that the qubit has already relaxed to the ground state in the reading process due to the higher relaxation rate is avoided, and the reading time can be prolonged to improve the reading fidelity;
(2) in one embodiment, in some qubit systems, the excited state a is a first excited state, and the relaxation rate for relaxation from a higher energy state to the ground state is different from the relaxation rate for transition from the first excited state to the ground state, and the high energy state higher than the first excited state cannot directly transition back to the ground state, and needs to pass through an intermediate energy state; therefore, the superconducting qubit to be measured is excited from the excited state A to a higher energy state by preprocessing the superconducting qubit, so that the time for the superconducting qubit to be measured to relax to the ground state is prolonged, the rate for the qubit to relax to the ground state is suppressed, the effect of reducing measurement errors is realized, meanwhile, the reading time is prolonged to improve the reading fidelity, and the scheme can be suitable for the Transmon qubit.
Drawings
Fig. 1 is a schematic diagram of a method for reading superconducting qubits based on cascade relaxation according to a first embodiment of the disclosure.
Fig. 2 is a flowchart of a method for reading superconducting qubits based on cascade relaxation according to a first embodiment of the disclosure.
Fig. 3 is a schematic diagram illustrating a relationship between probability of bit relaxation to a base state of an initial state in different high-energy excited states over time according to an embodiment of the disclosure.
FIG. 4 is a schematic diagram of a read circuit according to an embodiment of the disclosure.
FIG. 5 is a diagram illustrating the resolution of quantum states from IQ plots generated based on dispersion measurements according to one embodiment of the present disclosure.
Fig. 6 is a schematic block diagram of a superconducting quantum chip read by a superconducting qubit reading apparatus based on cascade relaxation according to a second embodiment of the disclosure.
[ notation ] to show
1-a quantum chip;
11-superconducting qubits;
2-a reading device;
21-relaxation time extension processing module;
211-energy excitation submodule;
2111-a first microwave transmission line;
2112-microwave pulses;
22-a quantum state read module;
221-read submodule;
2210-a read circuit;
22101 — a second microwave transmission line;
22102-microwave resonant cavity
222-a judgment sub-module.
Detailed Description
Due to the influences of noise of measurement electronics, the limit of quantum decoherence rate and the like, long-time integration needs to be carried out on measurement signals to better distinguish quantum states. However, due to the relaxation phenomenon of qubits, the qubits have a certain probability of relaxing from the excited state to the ground state over a longer measurement time, which causes great measurement errors.
Therefore, based on the above analysis, the present disclosure provides a method and an apparatus for reading a superconducting qubit based on cascade relaxation, in which a time for a superconducting qubit to be measured to relax to a ground state is prolonged, and a rate for the qubit to relax to the ground state is suppressed, so that a probability that the qubit relaxes to the ground state within a reading time is greatly reduced, a higher probability that the qubit is in an original excited state or ground state during a measurement process is ensured, a measurement error caused by the fact that the qubit has already relaxed to the ground state during reading due to a faster relaxation rate is avoided, and the reading time can be prolonged to improve reading fidelity.
According to the method and the device for reading the superconducting qubits based on the cascade relaxation, before reading, the time for the superconducting qubits to be measured to relax to the ground state is prolonged in advance. Then a read of the superconducting qubit state is performed. By prolonging the time of relaxation to the ground state, the relaxation rate of the qubit from the relaxation to the ground state is suppressed, the qubit is ensured to be in the original excited state or the ground state with higher probability in the measurement process, the measurement error caused by relaxation to the ground state during reading due to the higher relaxation rate is avoided, and the reading time can be prolonged to improve the reading fidelity.
In an embodiment of the present disclosure, the pre-prolonging the time for the superconducting qubit to be tested to relax to the ground state before reading includes: and pre-processing the superconducting qubit, and exciting the superconducting qubit to be detected to a higher energy state from an excited state A, wherein the excited state A refers to an excited state where the superconducting qubit to be detected is located before being read. In an embodiment of the disclosure, the exciting the superconducting qubit to be tested from the excited state a to a higher energy state includes: exciting the superconducting qubit to be detected for one time from an excited state A to a higher energy state; or, the superconducting qubit to be tested is excited at least twice from the excited state A, the excited state A is firstly excited to the excited state B with a higher energy level than the excited state A, then the excited state B is excited to the excited state C with a higher energy level than the excited state B, and so on, and the superconducting qubit is excited to a higher energy state for many times.
In an embodiment of the disclosure, the reading of the superconducting qubit state includes: and reading the state of the preprocessed superconducting qubit, and determining the read logic state of the superconducting qubit to be tested by judging whether the type of the state of the superconducting qubit is a ground state or an excited state.
In an embodiment of the disclosure, the preprocessing the superconducting qubit, and exciting the superconducting qubit to be tested from the excited state a to a higher energy state adopts the following method: the operation of exciting the superconducting qubit from the excited state A to a higher energy state is realized by coupling the superconducting qubit through a first microwave transmission line and applying microwave pulses with specific frequency and waveform on the first microwave transmission line.
In an embodiment of the present disclosure, a state of the superconducting qubit is read using a dispersion measurement method, and the reading circuit includes: a second microwave transmission line and a microwave resonant cavity coupled to the superconducting qubit; the microwave resonant cavity is used as a reading cavity to be coupled with the superconducting qubit, the second microwave transmission line is connected with an external circuit of a quantum chip where the superconducting qubit is located, and the quantum state is read by measuring the transmission spectrum or the reflection spectrum of the second microwave transmission line.
In an embodiment of the present disclosure, the superconducting qubit includes: transmon qubits. "Transmon" is a newly created vocabulary for naming a superconducting qubit of new structure, proposed by yale university researchers in their paper in 2007. "Transmon" is actually a shorthand of "a transmission-line shared plasma oscillation bit", where the text translates to "plasma oscillation bit with parallel transmission lines".
For the purpose of promoting a better understanding of the objects, aspects and advantages of the present disclosure, reference is made to the following detailed description taken in conjunction with the accompanying drawings.
A 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 disclosure. Fig. 2 is a flowchart of a method for reading superconducting qubits based on cascade relaxation according to a first embodiment of the disclosure.
In fig. 1, the right side shows that the energy levels of the superconducting qubits are 1, 2, 3, and … … n, the left side shows the energies E1, E2, E3, … …, and En corresponding to the respective energy levels, and the energies E1, E2, E3, E … …, and En corresponding to the ground state, the first excited state, the second excited state, and the n-1 th excited state of … …, respectively, the reading process is shown by the dashed rectangle, the downward arrow between the energy levels in the dashed rectangle indicates the transition from the high energy level to the low energy level, and the upward arrow indicates the excitation from the low energy level to the high energy level.
In quantum computing, the encoding is performed with the ground state and the excited state of the superconducting qubit to correspond to a logical 0 and a logical 1, for example, the encoding may be performed with the ground state as a logical 0 and the encoding may be performed with the excited state as a logical 1, or vice versa.
Referring to fig. 2, the method for reading a superconducting qubit based on cascade relaxation according to the present embodiment includes operations S101 and S102.
In operation S101, the superconducting qubit is preprocessed to excite the superconducting qubit to be tested from an excited state a to a higher energy state, where the excited state a is the excited state of the superconducting qubit to be tested before being read.
Referring to fig. 1, a rectangular box ① represents a reading process in the prior art, and rectangular boxes ② - ④ represent a reading process of the reading method of the present disclosure, for example, referring to a rectangular box ①, in the related art, a ground state is an energy level 1, corresponding to energy E1, an excited state a is a first excited state, corresponding to an energy level 2, corresponding to an energy level E2, and during the reading process, since a qubit has a certain probability of relaxation from the first excited state (excited state a) to the ground state, as shown by a downward arrow with a broken line in fig. 1, since an adjacent energy level is between the first excited state (excited state a) and the ground state, a corresponding transition rate is large, and therefore, at the instant of reading, it is possible that the superconducting qubit to be measured has transited from the excited state a to the ground state, and the measured result is the ground state, corresponding to a read logic 0, and the qubit to be actually tested in the excited state is a correct measurement result, thereby causing an error in the measurement with a certain probability, corresponding to a large reading error.
In this operation S101, the superconducting qubit to be measured is excited from the excited state a to a higher energy state by preprocessing the superconducting qubit, an embodiment may be illustrated with reference to rectangular boxes ② - ④ in fig. 1. in some qubit systems, the relaxation rate from the higher energy state to the ground state differs from the relaxation rate from the excited state a to the ground state, the high energy state above the excited state a cannot transition directly back to the ground state, and an intermediate energy state needs to be passed.
In operation S101, only one excitation may be performed to excite the superconducting qubit from excited state a to excited state B at a higher energy level than excited state a, e.g., in one embodiment, excited state B may be a second excited state, as illustrated by rectangular box ② in fig. 1, and in another embodiment, excited state B may also be a fourth excited state, as illustrated by rectangular box ③ in fig. 1.
In operation S101, multiple (at least two) excitations may also be performed, for example, the superconducting qubit may be first excited from excited state a to excited state B having a higher energy level than excited state a, and then excited from excited state B to excited state C having a higher energy level than excited state B, as indicated by rectangular box ④ in fig. 1.
In an embodiment of the present disclosure, the mode of exciting the superconducting qubit to be tested at least twice from the excited state a is cascade excitation, which includes level-by-level successive excitation and interval level successive excitation, for example, the superconducting qubit may be excited from a first excited state to a second excited state, from the second excited state to a third excited state, and so on. The energy level-by-energy level progressive excitation means excitation from two adjacent energy levels to a high energy level, and the interval level progressive excitation means that an excitation process can occur from two spaced energy levels, for example, from a first excited state to a third excited state, then from the third excited state to a fourth excited state, from the fourth excited state to a sixth excited state, and the like.
The number of excitations is, of course, not limited by the present disclosure and may be performed a number of times before reaching the highest energy level, and is not limited in that the higher energy level to which the excitation is directed and the energy level to which the excitation is directed may be adjacent energy levels, as illustrated by the rectangular box ② in FIG. 1.
In one embodiment, the superconducting qubit may be excited from the excited state A to a higher energy state by coupling the superconducting qubit via a microwave transmission line and applying microwave pulses of a particular frequency and waveform to the microwave transmission line, for example.
In operation S102, the state of the preprocessed superconducting qubit is read, and a read logic state of the superconducting qubit to be tested is determined by determining whether the type of the state of the superconducting qubit is a ground state or an excited state.
In one embodiment, in operation S102, the state of the superconducting qubit may be read by using a dispersion measurement method, and whether the superconducting qubit is in the ground state or the excited state may be determined by measuring a transmission spectrum or a reflection spectrum of the read cavity. In one embodiment, if the superconducting qubit is determined to be in the ground state, the read logic state of the superconducting qubit to be tested is considered to be a logic 0. And if the superconducting qubit is judged to be in the excited state, the read logic state of the superconducting qubit to be detected is logic 1. It should be noted that, here, it is only necessary to determine whether the bit is in the ground state or the excited state, where the excited state is only a concept corresponding to the ground state, and both the excited state a and the higher energy state belong to the excited state, and it is not necessary to specifically determine whether the superconducting qubit is in the specific state in the excited state, that is, it is not necessary to determine whether the superconducting qubit is in the excited state a or the higher energy state.
The working principle and technical effect of the reading method of the present disclosure are described below with reference to fig. 3.
Fig. 3 is a schematic diagram illustrating a relationship between probability of bit relaxation to a base state of an initial state in different high-energy excited states over time according to an embodiment of the disclosure.
The main factor affecting the relaxation rate of a superconducting qubit is the size of its charge transition matrix element. In superconducting qubits, the interaction of charge with the read cavity modes, noise, etc. on the quantum chip causes the superconducting qubit to relax from the excited state to the ground state. The larger the charge transition matrix element, the greater the relaxation rate.
In some superconducting qubit structures, such as the Transmon qubit, only the transition matrix elements exist between two energy states that are energy-adjacent. The Transmon qubit is a superconducting circuit, and states of the qubits are represented according to different energies of the oscillation current. The technical scheme disclosed by the invention is suitable for the superconducting Transmon qubit which is widely used at present. The Transmon qubit referred to here can be a three-dimensional Transmon qubit or a two-dimensional Transmon qubit that is coplanar.
Taking the Transmon qubit as an example, the transition matrix elements of the Transmon qubit can be written in the form:
wherein E isJAnd ECRespectively josephson energy and charge energy, are symbols commonly used in the art,
and (3) representing the transition matrix element from the jth energy state to the j +1 th energy state, and characterizing the relaxation rate of the qubit from the j +1 th energy state to the jth energy state. The larger this value, the higher the relaxation rate. In the same way as above, the first and second,
characterizing qubits from the j + k th energy state to the j + k th energy state for transition matrix elements from the j 'th energy state to the j + k' th energy stateRelaxation rates of j energy states.
Fig. 3 is a simulation diagram of probability of relaxation of qubits to the base state of the Transmon qubit in different excited states as a function of time, which shows that the probability of relaxation to the base state after the qubits are excited to a higher energy state is significantly reduced. In the simulation diagram of the relationship shown in FIG. 3, the time for the superconducting qubit to relax from the first excited state to the ground state is T 115 μ s. And supposing that under ideal conditions, only the influence of charge transition matrix elements exists, and the relaxation time between other energy levels is Tn，n-1＝T1N, n denotes the number of the energy level states, Tn，n-1Representing the relaxation time of the nth energy level to the (n-1) th energy level. The abscissa at the dashed line corresponds to 2 mus, which is the typical read time for a current superconducting qubit. At this parameter, the read error due to relaxation is about 12.7% when the superconducting qubit is in the first excited state. This error drops to 1.1% when the bit is excited to the second excited state at the beginning of the read; when the excitation state is excited to a third excitation state, the excitation state is reduced to 0.098%; when excited to the fourth excited state, it drops to 0.001%. In practical cases, superconducting qubits have other relaxation channels, but errors due to relaxation are expected to fall by a factor of about 100.
For a Transmon qubit, level-by-level excitation is performed between energy levels, that is, a superconducting qubit only has a charge transition matrix element between two energy states adjacent in energy, and does not relax directly from a higher energy state to a ground state, but only relaxes to the ground state through an intermediate energy level in a level-by-level cascade decay manner. Therefore, the process of exciting the Transmon qubit in excited state a to a higher energy state and relaxing to the ground state in a cascade decay manner greatly reduces the relaxation rate, thereby reducing the error of the superconducting qubit caused by relaxation during reading and also providing the possibility of adopting a longer reading time.
FIG. 4 is a schematic diagram of a read circuit according to an embodiment of the disclosure.
Referring to fig. 4, XY denotes a manipulation line and R denotes a read line. In an embodiment, the operation of operation S101 occurs corresponding to a steering line XY, and a box above the steering line XY in fig. 4 represents an operation process of pre-exciting the superconducting qubit to be tested from the excited state a to a higher energy state, and exemplarily, an operation process of exciting the superconducting qubit to be tested from the first excited state to the second excited state is represented by an X12 gate; the X23 gate represents the operation process of exciting the superconducting qubit to be tested from the second excited state to the third excited state; the X34 gate represents the operation of exciting the superconducting qubit under test from the third excited state to the fourth excited state. The dashed box in fig. 4 indicates that the excitation may be performed further on the basis of the operation of the solid box, that is, the excitation may be performed multiple times, and of course, the contents in the dashed box are optional operations, and may also be performed only once. Since the above description has been given in detail for the case of one excitation and multiple excitations, no further description is given here.
The X12, X23 and X34 gates can be realized by coupling a microwave transmission line with the superconducting qubit and applying microwave pulses of specific frequency and waveform on the microwave transmission line.
The typical generation method of the required microwave pulse is as follows: the device is produced by a microwave signal source, an arbitrary waveform generator and an image rejection mixer (IQ mixer).
The rectangular box above the read line R in fig. 4 represents the measurement or read process, illustrated as "measurement/read". Where the read/measurement process on read line R occurs after a time of pre-processing of the superconducting qubit. In one embodiment, the operation of operation S102 corresponds to occurring on the read line R. In one example, a read circuit includes: a microwave transmission line and a microwave resonant cavity coupled to the superconducting qubit. The microwave resonant cavity is coupled with the superconducting quantum bit as a reading cavity. The microwave transmission line is connected with an external circuit of the quantum chip, and the quantum state can be read by measuring the transmission spectrum or the reflection spectrum of the microwave transmission line. The quantum states may be read, for example, in a frequency division multiplexed manner. Optionally, in an embodiment, in order to further improve the reading efficiency and fidelity, a Josephson Parametric Amplifier (JPA) may be optionally installed at an outlet of the microwave transmission line in the reading circuit.
FIG. 5 is a diagram illustrating the resolution of quantum states from IQ plots generated based on dispersion measurements according to one embodiment of the present disclosure. The abscissa is the real part of the transmittance/reflectance and the ordinate is the imaginary part of the transmittance/reflectance.
According to the dispersion measurement method, it is necessary to form an IQ diagram by measuring transmittance or reflectance and to resolve quantum states on the IQ diagram, as shown in fig. 5, the positions of the superconducting qubits corresponding to IQ diagrams are different when they are in different energy states, as shown in fig. 5 by a legend "○" corresponding to ground state measurement data, and legends "+", "△" corresponding to different excited states, such as "+" corresponding to first excited state measurement data, "-" corresponding to second excited state measurement data, and "△" corresponding to third excited state measurement data, the superconducting qubits to be measured are prepared in a ground state (corresponding to a logical 0) and a first excited state (corresponding to a logical 1), respectively, and during reading, the cascade relaxation-based reading method of the superconducting qubits is implemented, and by performing operation S101 and operation S102, the energy state position distributions corresponding to the ground state and excited state are obtained, and as can be understood from the schematic diagram in fig. 5, the boundary between the ground state and excited state is drawn according to the measured IQ diagram, so that logical 1 can be obtained.
In the example of fig. 5, the boundary line that distinguishes the ground state from the excited state may be a straight line, but the boundary line may be another curve. Besides the mathematical fitting, advanced algorithms such as machine learning can be adopted for resolution.
Second embodiment
In a second exemplary embodiment of the present disclosure, a superconducting qubit reading device based on cascade relaxation is provided, which may be used to perform the above-described reading method.
Fig. 6 is a schematic block diagram of a superconducting quantum chip read by a superconducting qubit reading apparatus based on cascade relaxation according to a second embodiment of the disclosure. The double arrows indicate the coupling relation, and the single arrows indicate the transmission direction or the loading direction; the connecting lines indicate the connection relationship.
Referring to fig. 6, the reading apparatus 2 of the present embodiment includes: a relaxation time extension processing module 21 and a quantum state reading module 22.
The relaxation time extension processing module 21 is configured to extend in advance 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 for reading the state of the superconducting qubit 11.
In an embodiment of the present disclosure, referring to fig. 6, the relaxation time extension processing module 21 includes: the energy excites the sub-module 211. The energy excitation submodule 211 is configured to pre-process the superconducting qubit 11, and excite the superconducting qubit 11 to be detected from an excited state a to a higher energy state, where the excited state a is an excited state where the superconducting qubit to be detected is located before being read; and/or, in one embodiment, the quantum state read block 22 comprises: a read sub-module 221 and a decision sub-module 222. The read submodule 221 is configured to read a state of the preprocessed superconducting qubit. The judgment submodule 222 is configured to determine a read logic state of the superconducting qubit to be tested by judging whether the type of the state of the superconducting qubit 11 is a ground state or an excited state.
With further reference to fig. 6, in one embodiment, the energy excitation sub-module 211 includes a first microwave transmission line 2111, the first microwave transmission line 2111 being loaded with microwave pulses 2112. Wherein the first microwave transmission line 2111 is configured to couple with the superconducting qubit 11, and the first microwave transmission line 2111 is loaded with microwave pulses 2112 of specific frequency and waveform to achieve the operation of exciting the superconducting qubit 11 from the excited state a to a higher energy state; and/or, in an embodiment, the read submodule 221 includes read circuitry 2210. The read circuit 2210 includes: a second microwave transmission line 22101 and a microwave resonant cavity 22102 coupled to the superconducting qubit 11. The microwave resonant cavity 22102 is coupled to the superconducting qubit 11 as a reading cavity, the second microwave transmission line 22101 is connected to an external circuit of the quantum chip 1 where the superconducting qubit 11 is located, and the quantum state is read by measuring a transmission spectrum or a reflection spectrum of the second microwave transmission line 22101.
In an embodiment of the present disclosure, the superconducting qubit includes: transmon qubits. Of course, the present disclosure takes a Transmon qubit as an example, and other qualified qubit systems can also apply the reading method of the present disclosure as long as the qubit system satisfies the following conditions: 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 jump back to the ground state, and the intermediate energy state needs to pass, so that the relaxation time from the higher energy state to the ground state is prolonged, the reading time is correspondingly prolonged, and the measurement error caused by the relaxation is reduced.
In summary, the disclosure provides a method and an apparatus for reading a superconducting qubit based on cascade relaxation, which suppress a rate of relaxation of an unknown superconducting qubit to be measured to a ground state by prolonging a time of relaxation of the unknown superconducting qubit to be measured to the ground state, and particularly suppress the rate of relaxation to the ground state at an initial stage of measurement, so that a probability of relaxation of the qubit to the ground state within a reading time is greatly reduced, it is ensured that a large probability of the qubit is in an original excited state or the ground state during the measurement, a measurement error caused by relaxation to the ground state during reading due to a fast relaxation rate is avoided, and the reading time can be prolonged to improve reading fidelity; in one embodiment, in some qubit systems, the relaxation rate from the higher energy state to the ground state differs from the relaxation rate from the excited state a to the ground state, and a transition from a higher energy state (non-excited state) above the excited state a to the ground state cannot be made directly back, requiring the passage of intermediate energy states; therefore, the superconducting qubit to be measured is excited from the excited state A to a higher energy state by preprocessing the superconducting qubit, so that the time for the superconducting qubit to be measured to relax to the ground state is prolonged, the relaxation rate of the qubit to relax to the ground state is suppressed, the effect of reducing the measurement error is realized, meanwhile, the reading time can be prolonged to improve the reading fidelity, and the technical scheme can be suitable for the Transmon qubit.
Those skilled in the art will appreciate that the modules in the apparatus of an embodiment may be adaptively changed and disposed in one or more apparatuses other than the embodiment. The modules in the embodiments may be combined into one module and furthermore they may be divided into a plurality of sub-modules. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and all of the processes or elements of any method or apparatus so disclosed, may be combined in any combination, except combinations where at least some of such features and/or processes or elements are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Also in the unit claims enumerating several means, several of these means may be embodied by one and the same item of hardware.
As used herein and in the claims, the use of ordinal numbers such as "first", "second", "third", etc., to modify a corresponding element does not by itself connote any ordinal number of the element or represent the order of one element from another or the order of manufacture, and are used merely to distinguish one element having a certain name from another element having a same name, wherein the terms "first excited state", "second excited state", "third excited state", etc., are used throughout to describe terms of art and are used in a sequential sense to define an order of energy levels, a first excited state refers to an energy level of a first excited state adjacent to a ground state, a second excited state refers to an energy level of a second excited state adjacent to the first excited state, and a third excited state refers to an energy level of a third excited state adjacent to the second excited state, and so on.
Furthermore, the word "comprising" or "comprises" does not exclude the presence of elements or steps other than those listed in a claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.
Unless a technical obstacle or contradiction exists, the above-described various embodiments of the present invention may be freely combined to form further embodiments, which are within the scope of the present invention.
The above-mentioned embodiments are intended to illustrate the objects, aspects and advantages of the present disclosure in further detail, and it should be understood that the above-mentioned embodiments are only illustrative of the present disclosure and are not intended to limit the present disclosure, and any modifications, equivalents, improvements and the like made within the spirit and principle of the present disclosure should be included in the scope of the present disclosure.
Claims (10)
1. A method for reading superconducting qubits based on cascade relaxation is characterized by comprising the following steps:
before reading, the time of the superconducting qubit to be tested for relaxing to the ground state is prolonged in advance; and
reading of the superconducting qubit state is performed.
2. The method according to claim 1, wherein the pre-extending the time for the superconducting qubit under test to relax to the ground state before reading comprises:
preprocessing the superconducting qubit, and exciting the superconducting qubit to be detected to a higher energy state from an excited state A, wherein the excited state A refers to an excited state where the superconducting qubit to be detected is located before reading;
optionally, the exciting the superconducting qubit to be tested from the excited state a to a higher energy state includes: exciting the superconducting qubit to be detected for one time from an excited state A to a higher energy state; or, the superconducting qubit to be tested is excited at least twice from the excited state A, the superconducting qubit to be tested is firstly excited from the excited state A to the excited state B with a higher energy level than the excited state A, then is excited from the excited state B to the excited state C with a higher energy level than the excited state B, and so on, and is excited for multiple times to a higher energy state;
optionally, the mode of exciting the superconducting qubit to be detected at least twice from the excited state a is cascade excitation, and the cascade excitation includes energy level-by-energy level successive excitation and interval energy level successive excitation.
3. The method of claim 2, wherein the performing a read of the superconducting qubit state comprises:
and reading the state of the preprocessed superconducting qubit, and determining the read logic state of the superconducting qubit to be tested by judging whether the type of the state of the superconducting qubit is a ground state or an excited state.
4. The method of claim 2, wherein the pre-processing the superconducting qubit and the exciting the superconducting qubit to be tested from the excited state a to the higher energy state are performed by:
the operation of exciting the superconducting qubit from the excited state A to a higher energy state is realized by coupling the superconducting qubit through a first microwave transmission line and applying microwave pulses with specific frequency and waveform on the first microwave transmission line.
5. A method for reading according to claim 3, characterized in that the state of the preprocessed superconducting qubit is read by means of dispersion measurement, the reading circuit comprising: a second microwave transmission line and a microwave resonant cavity coupled to the superconducting qubit;
the microwave resonant cavity is used as a reading cavity to be coupled with the superconducting qubit, the second microwave transmission line is connected with an external circuit of a quantum chip where the superconducting qubit is located, and the quantum state is read by measuring the transmission spectrum or the reflection spectrum of the second microwave transmission line.
6. The method of reading according to any one of claims 1 to 5, wherein the superconducting qubit comprises: transmon qubits.
7. A superconducting qubit reading device based on cascade relaxation, comprising:
the relaxation time prolonging processing module is used for prolonging the time of the superconducting qubit to be tested for relaxing to the ground state in advance before reading; and
and the quantum state reading module is used for reading the superconducting quantum bit state.
8. The reading apparatus according to claim 7,
the relaxation time extension processing module includes: the energy excitation submodule is used for preprocessing the superconducting qubit and exciting the superconducting qubit to be detected from an excited state A to a higher energy state, wherein the excited state A refers to the excited state of the superconducting qubit to be detected before reading; and/or the presence of a gas in the gas,
the quantum state read module includes: the reading submodule is used for reading the state of the preprocessed superconducting qubit; and the judgment submodule is used for determining the read logic state of the superconducting qubit to be detected by judging whether the type of the state of the superconducting qubit is a ground state or an excited state.
9. The reading apparatus according to claim 8,
the energy excitation sub-module comprises: a first microwave transmission line for coupling with the superconducting qubit and loaded with microwave pulses of a specific frequency and waveform to effect excitation of the superconducting qubit from an excited state A to a higher energy state; and/or the presence of a gas in the gas,
the read submodule includes a read circuit including: a second microwave transmission line and a microwave resonant cavity coupled to the superconducting qubit; the microwave resonant cavity is used as a reading cavity to be coupled with the superconducting qubit, the second microwave transmission line is connected with an external circuit of a quantum chip where the superconducting qubit is located, and the quantum state is read by measuring the transmission spectrum or the reflection spectrum of the second microwave transmission line.
10. The reading apparatus according to any one of claims 7 to 9, wherein the superconducting qubit comprises: transmon qubits.
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