Device and method for reading out the state of a qubit

Abstract

To read out the state of a qubit, a readout input waveform (1602) is injected into a system that includes an information storage element (101) for storing information about the state of the qubit and a readout resonator (102) that is electromagnetically coupled to the information storage element (101). A readout output waveform is extracted from the system and detected (1102). The injection (1602) of the readout input waveform is performed through an excitation port (103), which is also used to inject an excitation waveform into the information storage element (101) to affect the state of the qubit. During the injection of the readout input waveform into the system, the phase of the readout input waveform is controllably shifted (1604).

Description

Technical Field

[0001] The present invention generally relates to quantum computing technology. In particular, the present invention relates to techniques for reading out the state of qubits (quantum bits) in a fast and reliable manner. Background Art

[0002] In quantum computing, it has become common to use the term "qubit" to designate not only the basic information unit but also the information storage element for storing one qubit of information. As an example, a superconducting memory circuit having one or more qubits (i.e., information storage elements of qubit size) can be considered. In such an example, the qubit is a non-harmonic oscillator, such as a transmon, and it can be coupled to a nearby readout resonator to facilitate reading out the state of the qubit stored therein.

[0003] Figure 1 is a schematic diagram of an example of a memory circuit including four qubits (i.e., four information storage elements of qubit size). The qubit 101 in the upper left and its associated microwave resonator 102 are seen within the dashed rectangle. The transmission line represented by the thick solid line and the internal structure of the qubit are made of a material that becomes superconducting at low temperatures. During operation, the qubit 101 can be excited with an excitation waveform of a certain frequency, which is brought to the excitation port 103. The long horizontal transmission line is the readout line. To read out the stored value of the qubit, a readout waveform is coupled to the readout input port 104, and the phase of the resulting readout signal is detected at the readout output port 105. In this exemplary circuit, each of the four resonators 102, 106, 107, and 108 has a different resonance frequency, such that the frequency of the readout waveform determines which qubit is actually read.

[0004] Figure 2 is an equivalent circuit diagram showing the qubit 101 and its associated resonator 102. The qubit 101 consists of a Josephson junction 201 and a shunt capacitor 202. The resonator is shown as an LC circuit, which consists of a capacitor 203 and an inductor 204. The excitation port 103 corresponds to Figure 1 the excitation port of Figure 2 and it is coupled to the qubit through a coupling capacitor 205. Figure 1 The readout port 206 of

[0005] When photons of the readout waveform enter the resonator 102, they interact with the state of the qubit 101. Accordingly, the phase of the readout waveform that can be detected at the readout output port starts to change. It must be considered that the point in the I-Q space defined by the phase and amplitude of the readout waveform must be considered to belong to the probability distribution. Figure 3 The trajectory of the average point of the probability distribution as a function of time in the two-dimensional I-Q space is shown. Here, it is assumed that the injection of the readout waveform starts at time t = 0, and the average point of the probability distribution is observed at intervals of 40 nanoseconds. If the qubit is found to be in the excited state, each circle represents the position of the average point of the probability distribution at successive intervals of 40 nanoseconds. Correspondingly, if the qubit is found to be in the ground state, each cross represents the position of the average point of the probability distribution at successive intervals of 40 nanoseconds. The continuous curve represents the trajectory of the average point between 40-nanosecond intervals.

[0006] The units of the coordinate system are arbitrary and not important because what matters is the shape of the trajectory. Figure 3 It is shown that initially, the difference depending on the qubit state (i.e., the shortest distance between the two trajectories) increases relatively slowly; initially, the two trajectories point to the left from the origin, and they only gradually deviate from each other. It is possible to find the state of the qubit from the perspective of time or reliability. The more time one can wait before making the detection, the more reliable the result.

[0007] A long time delay for reading out the qubit state with reasonable reliability is disadvantageous because it sets a limit on the speed of performing quantum computation where the state has to be read. Ideally, there is a faster method for reading out the qubit state, in other words, it is desired that the reliability of reading out the qubit state can be enhanced only after a short time delay. SUMMARY OF THE INVENTION

[0008] An object of the present invention is to provide an apparatus and method for reading out the state of a qubit, which have a higher speed and / or better reliability than previously known techniques. Another object of the present invention is to be able to reset the readout of the qubit as quickly as possible after reading out the state of the qubit.

[0009] The object of the present invention is achieved by injecting a readout waveform into a system including a qubit and its readout resonator through an excitation port and by performing an appropriate type of phase and amplitude matching of the waveform, the excitation port also being used for injecting an excitation waveform into the qubit.

[0010] According to a first aspect, there is provided an apparatus for reading out the state of a qubit. The apparatus includes an information storage element for storing information about the state of the qubit and a readout resonator electromagnetically coupled to the information storage element. The apparatus includes an excitation port for injecting an excitation waveform into the information storage element to affect the state of the qubit and one or more readout ports for injecting a readout input waveform into a system including the information storage element and the readout resonator and for extracting a readout output waveform from the system. The apparatus includes a readout waveform source for generating the readout input waveform and a readout waveform detector for detecting the readout output waveform. The readout waveform source is arranged to inject the readout input waveform into the system at least through the excitation port, and the readout waveform source is configured to controllably shift the phase of the readout input waveform during the injection of the readout input waveform into the system.

[0011] According to one embodiment, the excitation port is coupled to the information storage element, and one or more of the readout ports are coupled to the resonator and are different from the excitation port. This has the advantage that various possibilities are available for injecting the readout waveform into the system.

[0012] According to one embodiment, the information storage element and the readout resonator are made of a superconducting material. This has the advantage that a working model of the system can be constructed and its performance can be verified using practical measurements with known techniques.

[0013] According to one embodiment, the information storage element is a transmmon. This has the advantage that the operating theory of this information storage element is well-known and applicable to the theoretical analysis of the system operation.

[0014] According to one embodiment, the readout waveform source is configured to inject the readout waveform into the system simultaneously through the excitation port and a first readout port of the one or more readout ports, the first readout port being different from the excitation port. This has the advantage that the trajectory of the probability distribution of the resonator in the I-Q space can be controlled in various ways.

[0015] According to one embodiment, the readout waveform source is configured to control the phase and amplitude of the readout waveform injected into the system through the excitation port and the readout waveform injected into the system through the first readout port. This has the advantage that the trajectory of the probability distribution of the resonator in the I-Q space can be controlled in various ways.

[0016] According to one embodiment, the readout waveform source is configured to inject a first pair of simultaneous readout waveforms into the system through the excitation port and the first readout port, respectively, the phases and amplitudes of the first pair of readout waveforms being matched to maintain the average point of the first probability distribution at the origin of the I-Q space while moving the average point of the second probability distribution away from the origin of the I-Q space, the first probability distribution being associated with a first possible state of a qubit stored in the information storage element and the second probability distribution being associated with a second possible state of the qubit stored in the information storage element. The readout waveform source can be configured to subsequently inject a second pair of simultaneous readout waveforms into the system through the excitation port and the first readout port, respectively, wherein the phases and amplitudes of the second pair of readout waveforms are matched to move the average point of the second probability distribution back to the origin of the I-Q space. The advantage involved is that the resonator can be quickly reset after readout has been performed.

[0017] According to one embodiment, the readout waveform detector is configured to perform detection of the readout output waveform extracted from the system before the second pair of simultaneous readout waveforms is subsequently injected into the system. The advantage involved is that well-synchronized readout and reset operations are achieved.

[0018] According to a second aspect, there is provided a method for reading out the state of a qubit. The method includes injecting a readout input waveform into the system and detecting the readout output waveform extracted from the system, the system including an information storage element for storing the qubit state and a readout resonator electromagnetically coupled to the information storage element. The injection of the readout input waveform is performed through an excitation port that also serves as an injection port for injecting an excitation waveform into the information storage element to affect the state of the qubit. The phase of the readout input waveform is controllably shifted during its injection into the system.

[0019] According to one embodiment, the injecting of the readout input waveform into the system includes injecting the readout waveform into the system simultaneously through the excitation port and a first readout port of the system, the first readout port being different from the excitation port. The advantage involved is that various possibilities are available for injecting the readout waveform into the system.

[0020] According to one embodiment, the method includes controlling the phases and amplitudes of the readout waveform injected into the system through the excitation port and the readout waveform injected into the system through the first readout port. The advantage involved is that the trajectory of the probability distribution of the resonator in the I-Q space can be controlled in various ways.

[0021] According to one embodiment, the method includes injecting a first pair of simultaneous readout waveforms into the system through the excitation port and the first readout port, respectively, the phases and amplitudes of the first pair of readout waveforms being matched so as to maintain the average point of the first probability distribution at the origin of the I-Q space while moving the average point of the second probability distribution away from the origin of the I-Q space, the first probability distribution being associated with a first possible state of a qubit stored in the information storage element and the second probability distribution being associated with a second possible state of the qubit stored in the information storage element. The method can include subsequently injecting a second pair of simultaneous readout waveforms into the system through the excitation port and the first readout port, respectively, the phases and amplitudes of the second pair of readout waveforms being matched so as to move the average point of the second probability distribution back to the origin of the I-Q space. The advantage involved is that the resonator can be quickly reset after readout has been performed.

[0022] According to one embodiment, the method can include detecting a readout output waveform extracted from the system before subsequently injecting the second pair of simultaneous readout waveforms into the system. The advantage involved is well-synchronized readout and reset operations. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The accompanying drawings, which are included to provide a further understanding of the invention and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings:

[0024] Figure 1 A superconducting quantum memory circuit is shown;

[0025] Figure 2 An equivalent circuit diagram is shown;

[0026] Figure 3 The separation of state-dependent probability distributions in a first case is shown;

[0027] Figure 4 A system having a qubit and a readout resonator is shown,

[0028] Figure 5 Many mathematical formulas explaining embodiments of the invention are shown,

[0029] Figure 6 Many mathematical formulas explaining embodiments of the invention are shown,

[0030] Figure 7 Many mathematical formulas explaining embodiments of the invention are shown,

[0031] Figure 8 Many mathematical formulas explaining embodiments of the invention are shown,

[0032] Figure 9 Many mathematical formulas are shown to explain embodiments of the present invention,

[0033] Figure 10 Many mathematical formulas are shown to explain embodiments of the present invention,

[0034] Figure 11 A device for reading out the state of a qubit is shown,

[0035] Figure 12 The separation of the state-dependent probability distribution in the second case is shown,

[0036] Figure 13 The separation of the state-dependent probability distribution in the third case is shown,

[0037] Figure 14 The separation of the state-dependent probability distribution in the fourth case is shown,

[0038] Figure 15 A comparison of the separation of the state-dependent probability distribution in many cases is shown,

[0039] Figure 16 Steps of a method for reading out the state of a qubit are shown,

[0040] Figure 17 Steps of a method for reading out the state of a qubit are shown, and

[0041] Figure 18 Steps of a method for reading out the state of a qubit are shown. Detailed Description

[0042] Figure 4 It is a schematic diagram of the principle of a device for reading out the state of a qubit. The device includes an information storage element 101 for storing information on the state of the qubit. Two horizontal lines in the figure mark two possible ground states that the qubit may have. In the technical field of hardware devices for quantum computing, it is customary to use the term qubit not only for the basic information unit of the concept but also for the hardware that constitutes the information storage element 101.

[0043] The device further includes a readout resonator 102, which is electromagnetically coupled to the information storage element or qubit 101. The readout resonator 102 is a harmonic oscillator and it has a certain resonance frequency. The strength of the electromagnetic coupling between the resonator 102 and the information storage element (or qubit) 101 can be described by a coupling coefficient g. For ease of reference, the qubit 101 and its readout resonator 102 are usually referred to as a "system".

[0044] The apparatus includes an excitation port 103 for injecting an excitation waveform 401 into the information storage element 101. The excitation waveform affects the state of the qubit in a known manner. In the general parlance of the art, one typically talks about "exciting" the qubit, which is essentially synonymous with injecting the excitation waveform through the excitation port 103.

[0045] The apparatus includes one or more readout ports 104 for injecting a readout input waveform 402 into the system. The one or more readout ports 104 are also used to extract a readout output waveform 403 from the system. Injecting the readout input waveform 402 into the system is commonly referred to as driving the resonator 102. The coupling coefficient κ (the lowercase of the Greek letter kappa) describes the characteristic decay time from the resonator 102 to the readout port 104. The relative magnitudes of the constants g and κ are significant for the way the readout mechanism operates, which will be described in more detail later in this document.

[0046] The readout input waveform 402 originates from a readout waveform source, which is not shown in Figure 4 The detection of the readout output waveform extracted from the system is performed in a readout waveform detector, which is also not shown in Figure 4 this figure.

[0047] Contrary to conventional techniques in the art, the readout waveform input source can be arranged to inject at least some of the readout input waveform into the system through at least the excitation port 103. Thus, to some extent, the excitation port 103 simultaneously becomes a readout port of the system. This has a significant impact on the speed of performing the readout of the qubit state.

[0048] Conceptually, the situation can be explained as follows. In a traditional readout scheme, where the readout input waveform is injected only through the readout port 104, the resonator 102 starts empty. Before starting to interact with the state stored in the qubit 101, the readout input waveform or the readout photons (as they are also called) must first fill the resonator 102. The useful information obtained from the output waveform is proportional to the product of the amplitude and the phase. Therefore, it becomes reasonable to detect their phase only after the oscillation amplitude in the resonator 102 reaches a meaningful size and there is sufficient time to interact with the state in the qubit 101 through the coupling g.

[0049] When the readout input waveform is injected into the system through the excitation port 103, it immediately "meets" the state stored in the qubit 101 and thus can start interacting with it even before it finally enters the resonator 102. In other words, the resonator 102 starts filling with readout photons whose phase already reflects the state of the qubit to be read out. As a result, it becomes possible to detect the appropriate readout output waveform earlier than with traditional methods.

[0050] A more formal treatment of this situation is as follows. Let the eigenfrequency of the uncoupled qubit 101 be ω k = kω r + Δ k , where ω r is the resonance frequency of the resonator 102, and Δ k represents the detuning between the k-th energy level of the qubit and the resonator. For the ground state, Δ0 = 0, for the first excited state, Δ1 = Δ, for the second excited state, Δ2 = 2Δ + α, where α is the anharmonicity, and so on. In the dispersive regime, the detuning is larger than the qubit-resonator coupling g, meaning |Δ| >> g. The Hamiltonian describing the system can be written as shown in line (1) of Figure 5 . The free, interaction, qubit drive, and resonator drive Hamiltonians are given by lines (2), (3), (4), and (5) of Figure 5 respectively.

[0051] In the mathematical notation used, denotes the annihilation operator of the resonator mode, and |k> refers to the k-th eigenstate of the qubit. All subscripts "r" refer to the resonator, subscript "q" refers to the qubit, and subscript "d" refers to the readout (i.e., drive) waveform.

[0052] For transmon qubits, it is usually assumed that the coupling constants of different transmon layers are of the form The actual drive (i.e., readout) waveform at the drive frequency ω d is composed of the real and imaginary parts (i.e., I and Q quadratures) of the complex amplitude shown in line (6) of by Figure 5 respectively.

[0053] The Hamiltonian can be transformed to a frame rotating at the angular frequency ω d . Applying the single operator Figure 5 given by line (7) of and employing the rotating wave approximation justified by g << |2ω r | and |ω r - ω d | << |ω r + ω dr | gives the transformed Hamiltonian, which is given by lines (8), (9), (10), and (11) of Figure 6 . The acronym H.c. is used to denote the Hermitian conjugate, and denotes the shifted detuning.

[0054] Temporarily ignored Total transformed Hamiltonian Given by line 12 in Figure 7 .

[0055] In a traditional readout scheme, where the readout waveform is injected into the system only through the readout port, Ω d = 0. Thus, the phase space distribution of the resonator will rotate around the origin at an angular frequency depending on the qubit state. Here we make a crucial observation that the frame is shifted by α VO = -Ω q λ k / g k = -Ω q / g. Thus, in the non-shifted frame, the phase space distribution of the resonator should rotate around the point α VO . The position of α VO is fully controllable by the readout waveform Ω qd and ω d .

[0056] To explain the decay of the resonator state, we use the Lindblad master equation given by line (13) in Figure 7 , where ρ is the reduced density operator of the resonator, κ represents the energy decay rate of the resonator, and

[0057] To make the observation more obvious, we perform the standard dispersive approximation. We first use the operator given by line (14) in Figure 7 for another transformation. Based on the assumption of in , we calculate in to the second order. For clarity, we can restrict the formula to only the first three levels of the transmons ({|g>, |e>, |f>} = {|0>, |1>, |2>}). The Hamiltonian is assumed to be in the form given by lines (15), (16), and (17) in . Here we have defined the dispersive constants Figure 7 and and Ω′ and Ω′ rd = (Ω rd / 2)exp(it(ω r - ω d )). Finally, introducing the displacement operator The total Hamiltonian is assumed to be in the form given by lines (18) to (21) in Figure 8 .

[0058] Line (18) describes the constant frequency shift caused by coupling and driving. Line (19) shows that the driving from the qubit side, i.e., injecting the readout waveform into the system through the excitation port, tilts the qubit Hamiltonian. Line (20) is important for the readout scheme considered here because it predicts that any coherent state will rotate around the point α VO Rotate. By choosing ω r -ω d =χ1 / 2, the angular frequencies of these rotations can be set to be equal to +χ≡χ1 / 2 - χ0 and -χ for α g and α e . Figure 8 Line (21) in r shows that the transformation has an impact on the amplitude of the resonator, which can be compensated by changing Ω VO . The traditional dispersive system is obtained by setting α

[0059] Using the equation of line (13) with an approximate Hamiltonian of Figure 7 , we obtain the analytical equations for the expectation values j∈{g, e}, as shown in line (22) of Figure 9 . Assuming that the resonator is initially empty (the so-called "vacuum state") and the readout pulse does not change, the solution is given by line (23) of Figure 9 .

[0060] The formal treatment given above is valid for the general case and is not restricted by any specific physical implementation of the qubit, for example. The following three special cases can be noted.

[0061] The first special case is the traditional readout scheme where no readout waveform is injected into the system through the excitation port, meaning Ω qd =0. In this case, the probability distributions associated with the two qubit states |g> and |e> will rotate around different points in the phase space. They will initially advance in the same direction, as described by the trajectories shown above in conjunction with Figure 3 . The state separation at t < χ -1 increases quadratically in time, as shown in line (24) of Figure 10 .

[0062] The second special case is the situation where the readout waveform is injected into the system only through the excitation port, which means Ω rd =0. This readout scheme can be called the backdoor readout scheme to illustrate its difference from the traditional alternative. The probability distributions associated with the two qubit states |g> and |e> will rotate around the point z, but at different frequencies. At t < χ -1The state separation at time increases linearly in time, as shown in Figure 10 line (25) of back . Further evolution depends on the magnitude of κ. For small κ, the probability distributions will rotate around z while slowly converging to their respective steady states. This can make single-shot readout more challenging because S

[0063] will oscillate. The same potential problem exists in traditional readout schemes, but can be avoided by having a larger κ. Figure 9 A third special case is injecting the readout waveform into the system through the excitation and readout ports such that the numerator in equation (23) of |g> equals zero. This will cause the probability distribution associated with the qubit state |g> to remain at the origin. The probability distribution associated with the qubit state |e> will rotate around the point α(1 - χ |e> ) / χ -1 . The state separation at t < χ back increases as in the previous case, i.e., as S

[0064] Figure 11 shows a device for reading out the state of qubit 101, where an equivalent circuit diagram represented by Figure 2 is used for illustrative comparison. The device includes a readout waveform source 1101 for generating a readout input waveform. The device also includes a readout waveform detector 1102 for detecting a readout output waveform extracted from the system, which includes a qubit (or information storage element) 101 and a readout resonator 102. The readout waveform source 1101 is arranged to inject the readout input waveform into the system at least through the excitation port 103. In the Figure 11 illustrated embodiment, the readout waveform source 1101 is additionally arranged to inject the readout input waveform into the system through the readout port 206.

[0065] The readout waveform source 1101 is arranged to controllably shift the phase of the readout input waveform during the process of injecting the readout input waveform into the system. This ability is schematically illustrated in Figure 11 which has controllable phase shifters 1103 and 1104 in the line that leads from the output of the readout waveform source 1101 to the readout port 206 and the excitation port 103, respectively. The readout waveform source 1101 can also be arranged to control the amplitude of the readout input waveform. This ability of the readout waveform source 1101 is schematically illustrated in Figure 11 which has controllable attenuators 1105 and 1106 in the line that leads from the output of the readout waveform source 1101 to the readout port 206 and the excitation port 103, respectively.

[0066] As shown in Figure 11As shown, the excitation port 103 is coupled to the qubit (or information storage element) 101, and the readout port 206 is coupled to the resonator 102. Specifically, in Figure 11 since the excitation port 103 also serves as a readout port (because the readout input waveform is injected through it), it can be said that another readout port 206 different from the excitation port 103 is coupled to the resonator 102.

[0067] The qubit (or information storage element) 101 and the resonator 102 can be made of superconducting material: by way of example, they can appear on a superconducting quantum memory circuit as Figure 1 shown. However, this is not a requirement, and other kinds of qubit technologies can be used. Superconducting material here means a material that can be made superconducting by cooling it to a low enough temperature. Examples of such materials are aluminum, but other superconducting materials such as molybdenum, niobium, tin, tantalum or lead can also be used. For operation, the superconducting quantum memory circuit is cooled to a very low temperature, which can be on the order of a few Kelvin, or below one Kelvin, or a few tens of millikelvin. The qubit 101 is preferably a non-harmonic oscillator, such as a transmon.

[0068] Figure 12 The example shown is how the average points of the two probability distributions associated with the two qubit states |g> and |e> move in the phase space in the case of "backdoor readout". This refers to the case where the readout waveform source 1101 injects the readout waveform into the system only through the excitation port 103. In Figure 12 the graphical symbols are the same as those in an earlier Figure 3 .

[0069] Figure 13 The example shown is how the average points of the two probability distributions associated with the two qubit states |g> and |e> move in the phase space in the case of "asymmetric backdoor readout". This refers to the case where the readout waveform source 1101 injects the readout waveform into the system through both the excitation port 103 and the readout port 206 simultaneously. Generally speaking, it can be said that the device can include one or more readout ports, and the port 206 is the first readout port of the one or more readout ports and is different from the excitation port 103.

[0070] Specifically, in Figure 13 the case, the readout waveform source 1101 can be configured to control the phase and amplitude of the readout waveform injected into the system through the excitation port 103 and the readout waveform injected into the system through the first readout port 206 such that in Figure 9 the numerator in equation (23) is equal to zero. This causes one of the probability distributions to remain at or very close to the origin of the I-Q space, while the other probability distribution moves away from it along a curved trajectory.

[0071] Figure 14 illustrates how to perform a fast reset of a readout resonator using the principles considered above. First, the readout waveform source 1101 injects a first pair of simultaneous readout waveforms into the system through the excitation port 103 and the readout port 206, respectively. The phase and amplitude of the first pair of readout waveforms match the following principle (explained above with reference to Figure 13 which explains this principle): the matching is performed so as to maintain the average point of the first probability distribution at the origin of the I-Q space while moving the average point of the second probability distribution away from the origin of the I-Q space (arrow 1401). Here, the first probability distribution is associated with the first possible state of the qubit stored in the information storage element 101, and the second probability distribution is associated with the second possible state of the qubit stored in the information storage element 101.

[0072] Subsequently, the readout waveform source 1101 injects a second pair of simultaneous readout waveforms into the system through the excitation port and the first readout port, respectively. The phase and amplitude of the second pair of readout waveforms are matched so as to move the average point of the second probability distribution back to the origin of the I-Q space ( Figure 14 arrow 1402 in). Instead of simply waiting for the photons of the readout waveform to naturally decay from the readout resonator, a further step is performed, which ultimately brings the two probability distributions back to the same point in the I-Q space again.

[0073] Not only can the reset of the resonator but also the detection of the readout waveform extracted from the system be performed faster than conventional methods. The readout waveform detector 1102 can be configured to perform the detection before the above-mentioned further step, i.e., before the readout waveform source 1101 injects the second pair of simultaneous readout waveforms into the system. Due to the linear increase in state separation, a shorter integration time in the detection gives sufficiently reliable results. If a slightly different view is adopted, the detection results can be made more reliable if the same integration time as in the prior art methods is used.

[0074] Figure 15 shows a graphical comparison of how state separation evolves over time in conventional (graph 1501), backdoor (graph 1502), and asymmetric backdoor (graph 1503) readout schemes.

[0075] Figure 16It is a basic schematic diagram of a method for reading out the state of a qubit. Step 1601 represents the assumption that something may have changed the qubit state since the last readout, because otherwise there would be no reason to read it out again. Step 1602 includes injecting a readout input waveform into a system that includes an information storage element for storing information about the state of the qubit and a readout resonator electromagnetically coupled to the information storage element. Step 1603 includes detecting a readout output waveform extracted from the system. According to the description of the device referenced above, the injection of the readout input waveform in step 1602 is performed through an excitation port that is also used to inject an excitation waveform into the information storage element to affect the state of the qubit. There is also step 1604, which shows how the phase of the readout input waveform can be controllably shifted during the injection of the readout input waveform into the system.

[0076] Figure 17 It shows how step 1602 of injecting the readout input waveform into the system includes injecting the readout waveform into the system simultaneously through the excitation port (sub-step 1701) and through a first readout port of the system (sub-step 1702). For clarity, it can be emphasized that the first readout port described here is different from the excitation port. Figure 17 It also shows how step 1604, which can controllably shift the phase of the readout input waveform, involves controlling the phase and amplitude of the readout waveform injected into the system through the excitation port and the readout waveform injected into the system through the first readout port.

[0077] Figure 18 Corresponding to the previous reference Figure 14 explanation. Figure 18 The method includes injecting a first pair of simultaneous readout waveforms into the system in step 1801 through the excitation port and the first readout port respectively. According to step 1802, the phase and amplitude of the first pair of readout waveforms are matched so as to maintain the average point of the first probability distribution at the origin of the I-Q space while moving the average point of the second probability distribution away from the origin of the I-Q space. Here, the first probability distribution is associated with a first possible state of the qubit stored in the information storage element, and the second probability distribution is associated with a second possible state of the qubit stored in the information storage element.

[0078] Figure 18The method includes subsequently injecting a second pair of simultaneous readout waveforms into the system through the excitation port and the first readout port, respectively, in step 1803. The phases and amplitudes of the second pair of readout waveforms can be matched according to step 1804 so as to shift the average point of the second probability distribution back to the origin of the I-Q space. In step 1603, the detection of the readout output waveform extracted from the system can be performed before subsequently injecting the second pair of simultaneous readout waveforms into the system in step 1803.

[0079] It will be apparent to those skilled in the art that, as technology advances, the basic idea of the present invention can be implemented in various ways. Therefore, the present invention and its embodiments are not limited to the above examples, but may vary within the scope of the claims.

Claims

1. A device for reading out the state of a qubit, comprising: An information storage element (101) for storing the qubit state; A readout resonator (102) that is electromagnetically coupled to the information storage element (101); An excitation port (103) for injecting an excitation waveform (401) into the information storage element (101) to affect the state of the qubit; One or more readout ports (104, 105, 206) for injecting a readout input waveform (402) into a system including the information storage element (101) and the readout resonator (102), and for extracting a readout output waveform (403) from the system; A readout waveform source (1101) for generating the readout input waveform, and A readout waveform detector (1102) for detecting the readout output waveform; Characterized in that The readout waveform source (1101) is configured to inject the readout input waveform into the system simultaneously through the excitation port (103) and a first readout port (206) of the one or more readout ports, the first readout port (206) being different from the excitation port (103), and The readout waveform source (1101) is configured to controllably shift the phase of the readout input waveform during the process of injecting the readout input waveform into the system.

2. The device according to claim 1, characterized in that The excitation port (103) is coupled to the information storage element (101), and One or more of the readout ports (104, 105, 206) are coupled to the resonator (102).

3. The device according to claim 2, characterized in that, The information storage element (101) and the readout resonator (102) are made of superconducting material.

4. The device according to claim 3, characterized in that, The information storage element (101) is a transmmon.

5. The device according to claim 1, characterized in that The readout waveform source (1101) is configured to control the phase (1103, 1104) and amplitude (1105, 1106) of the readout input waveform injected into the system through the excitation port (103) and the readout input waveform injected into the system through the first readout port (206).

6. The device according to claim 5, characterized in that The readout waveform source (1101) is configured to: Inject a first pair of simultaneous readout input waveforms into the system through the excitation port (103) and the first readout port (206) respectively, wherein the phase and amplitude of the first pair of readout input waveforms match, so as to maintain the average point of the first probability distribution at the origin of the I-Q space while moving the average point of the second probability distribution away from the origin of the IQ space, the first probability distribution being associated with a first possible state of the qubit stored in the information storage element (101), and the second probability distribution being associated with a second possible state of the qubit stored in the information storage element (101), and Subsequently, a second pair of simultaneous readout input waveforms are injected into the system through the excitation port (103) and the first readout port (206) respectively, and the phases and amplitudes of the second pair of readout input waveforms are matched so as to shift the average point of the second probability distribution back to the origin of the I-Q space.

7. The device according to claim 6, characterized in that, The readout waveform detector (1102) is configured to perform detection of the readout output waveform extracted from the system before injecting the second pair of simultaneous readout input waveforms into the system subsequently.

8. A method for reading out a qubit state, comprising: injecting (1602) a readout input waveform into a system, the system including an information storage element for storing information of the qubit state and a readout resonator electromagnetically coupled to the information storage element, and detecting (1603) the readout output waveform extracted from the system, wherein, the injection (1602) of the readout input waveform is performed through an excitation port and simultaneously through a first readout port, the excitation port is also used for injecting an excitation waveform into the information storage element to affect the state of the qubit, the first readout port is different from the excitation port, and the phase of the readout input waveform can be controllably shifted (1604) during the process of injecting the readout input waveform into the system.

9. The method according to claim 8, comprising controlling the phases and amplitudes (1604) of the readout input waveform injected into the system through the excitation port and the readout input waveform injected into the system through the first readout port.

10. The method according to claim 9, comprising: injecting (1801) a first pair of simultaneous readout input waveforms into the system through the excitation port and the first readout port respectively, wherein the phases and amplitudes of the first pair of readout input waveforms are matched (1802) so as to maintain the average point of the first probability distribution at the origin of the I-Q space when moving the average point of the second probability distribution away from the origin of the I-Q space, the first probability distribution is associated with a first possible state of the qubit stored in the information storage element, and the second probability distribution is associated with a second possible state of the qubit stored in the information storage element, and, subsequently injecting (1803) a second pair of simultaneous readout input waveforms into the system through the excitation port and the first readout port respectively, and the phases and amplitudes of the second pair of readout input waveforms are matched (1804) so as to shift the average point of the second probability distribution back to the origin of the I-Q space.

11. The method according to claim 10, comprising: detecting (1603) the readout output waveform extracted from the system before injecting the second pair of simultaneous readout waveforms into the system subsequently.

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