Crosstalk compensation in quantum processing devices

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First Claim
1. A method of operating a quantum processing device comprising at least two quantum circuits coupled to a tunable coupler,wherein the quantum circuits are subject to crosstalk, the method comprising:
 applying a primary signal to the quantum circuits so as to drive one or more energy transitions between states spanned by the quantum circuits; and
applying a compensation signal to the tunable coupler, the compensation signal designed so as to shift at least one state spanned by the quantum circuits, in energy, to compensate for crosstalk between the quantum circuits, andwherein the crosstalk between the quantum circuits gives rise to hybridized states of the quantum circuits and the compensation signal is designed such that one of the hybridized states has an energy equal to an energy that one of the bare superposition state spanned by the quantum circuits would have in absence of crosstalk between the quantum circuits.
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Abstract
A method for operating a quantum processing device is provided including at least two quantum circuits coupled to a tunable coupler, wherein the quantum circuits are subject to crosstalk, the method including: applying a primary signal to the quantum circuits so as to drive one or more energy transitions between states spanned by the quantum circuits; and applying a compensation signal to the tunable coupler, the compensation signal designed so as to shift at least one state spanned by the quantum circuits, in energy, to compensate for crosstalk between the quantum circuits. Related quantum processing devices and chips are also provided.
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18 Claims
 1. A method of operating a quantum processing device comprising at least two quantum circuits coupled to a tunable coupler,
wherein the quantum circuits are subject to crosstalk, the method comprising: applying a primary signal to the quantum circuits so as to drive one or more energy transitions between states spanned by the quantum circuits; and applying a compensation signal to the tunable coupler, the compensation signal designed so as to shift at least one state spanned by the quantum circuits, in energy, to compensate for crosstalk between the quantum circuits, and wherein the crosstalk between the quantum circuits gives rise to hybridized states of the quantum circuits and the compensation signal is designed such that one of the hybridized states has an energy equal to an energy that one of the bare superposition state spanned by the quantum circuits would have in absence of crosstalk between the quantum circuits.  View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11)
 12. A quantum processing device, comprising
at least two quantum circuits; a tunable coupler, to which the at least two quantum circuits are coupled, the circuits subject to crosstalk; and a control circuit, configured in the quantum processing device to apply; a primary signal to the quantum circuits so as to drive one or more energy transitions between states spanned by the quantum circuits; and a compensation signal designed so as to shift at least one state spanned by the quantum circuits, in energy, to compensate for crosstalk between the quantum circuits; wherein the tunable coupler is designed such that a frequency thereof can be modulated so as to drive energy transitions in the quantum processing device and the control circuit is further configured in the quantum processing device to apply a synthesis signal to the tunable coupler, wherein the synthesis signal applied includes; said primary signal, the latter modulated so as to modulate the frequency of the tunable coupler and thereby drive said one or more energy transitions via the tunable coupler, in operation; and said compensation signal, so as to compensate for crosstalk between the quantum circuits.  View Dependent Claims (13, 14, 15, 16, 17)
 18. A quantum processing chip, including
a plurality of cells, each comprising: at least two quantum circuits; and a tunable coupler, to which the at least two quantum circuits are coupled, the quantum circuits being subject to crosstalk, wherein the quantum processing chip further comprises one or more further control circuits, configured in the chip to apply, for each of the cells; a primary signal to respective quantum circuits so as to drive one or more energy transitions between states spanned by the respective quantum circuits via a respective tunable coupler, in operation; and a compensation signal designed so as to shift at least one state spanned by the respective quantum circuits, in energy, to compensate for crosstalk between two of said respective quantum circuits, in operation of the chip, wherein the tunable coupler is designed such that a frequency thereof can be modulated so as to drive energy transitions in the quantum processing chip and the control circuit is further configured in the quantum processing chip to apply a synthesis signal to the tunable coupler, wherein the synthesis signal applied includes; said primary signal, the latter modulated so as to modulate the frequency of the tunable coupler and thereby drive said one or more energy transitions via the tunable coupler, in operation; and said compensation signal, so as to compensate for crosstalk between the quantum circuits.
1 Specification
The invention relates in general to the field of quantum processing devices and operation thereof. In particular, it is directed to methods of operating quantum processing devices with quantum circuits (e.g., superconducting qubits), coupled to a frequencytunable coupler, as well as related devices. Such methods aim at compensating for crosstalk between the quantum circuits.
Recent advances in quantum computing are making such a technology ever more relevant to industrial applications. Quantum computing makes direct use of quantummechanical phenomena, such as superposition and entanglement to perform operations on entangled quantum bits, i.e., states stored in quantum bits. Superconducting circuits are relatively easy to manufacture with current technologies and are thus promising candidates to further scale quantum information technologies.
Possible applications on quantum machines include the solving of hard optimization problems that are beyond the reach of classical algorithms. For example, quantum optimizations based on the variational principle are particularly appealing.
Quantum computing ideally needs a rapid and highfidelity generation of entangled qubit states. For example, twoqubit gates are known, which are implemented with transmon (fixedfrequency) qubits, where the qubits are coupled via a frequency tunable coupler element. In contrast to other approaches, this implementation takes advantage of the high coherence of fixedfrequency qubits to generate entangled two qubit states with fidelities of more than 97%.
In particular, an architecture has been proposed, wherein two qubits are connected to a single tunable coupler, by David C. McKay and coworkers (Phys. Rev. Appl. 6, 064007 (2016)). This architecture allows transitions between two states of the two qubits to be parametrically driven, by modulating the tunable coupler energy at a frequency that corresponds to the difference of the energy between the two states.
More generally, quantum computing relies upon the ability to accurately control the states of the quantum circuits (e.g., qubits) making up the system. This involves being able to individually address each quantum bit in a welldefined manner as well as to create at least two qubit operations. When crosstalk between qubits is present, it is no longer possible to address one qubit without selectively and accurately knowing the state of the others. The dynamics of one qubit depends on the state of the others, which amounts to an unwanted twoqubit gate (controlled unitary operation). Thus, the presence of crosstalk impedes the operation of quantum computing devices.
According to a first aspect, the present invention is embodied as a method of operating a quantum processing device. The device comprises at least two quantum circuits, which are coupled to a tunable coupler. This coupler is frequencytunable: its frequency can be modulated, e.g., so as to drive energy transitions in the quantum processing device. The quantum circuits are assumed to be subject to crosstalk in the device. Basically, the present method revolves around the application of two signals, which include a primary (modulated) signal and a compensation signal. On the one hand, the primary signal is applied to the quantum circuits (possibly via the tunable coupler) so as to drive one or more energy transitions between states spanned by the quantum circuits. On the other hand, the compensation signal applied is designed so as to shift (in energy) at least one state spanned by the quantum circuits, in order to compensate for crosstalk between two of the at least two quantum circuits.
According to another aspect, the invention is embodied as a quantum processing device. Consistently with the above method, the device comprises at least two quantum circuits and a tunable coupler, to which the circuits are coupled. The quantum circuits are again assumed to be subject to crosstalk. The device further comprises a control unit, which is configured to apply two signals, this including both a primary signal and a compensation signal for crosstalk, as described above.
According to a third and final aspect, the invention is embodied as a quantum processing chip. The chip includes a plurality of cells, each comprising at least two quantum circuits (again subject to crosstalk) and a tunable coupler, to which the quantum circuits are coupled. The quantum processing chip further comprises one or more control units, making up a control system. Consistently with the present method, the control units are configured in the chip to apply, for each of the cells, signals so as to drive energy transitions, while compensating for crosstalk between the quantum circuits in the cell, in operation of the chip.
Quantum processing devices, chips, and methods embodying the present invention will now be described, by way of nonlimiting examples, and in reference to the accompanying drawings.
The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views, and which together with the detailed description below are incorporated in and form part of the present specification, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present disclosure, in which:
The accompanying drawings show simplified representations of devices or parts thereof, as involved in embodiments. Technical features depicted in
In many quantum chip architectures, computational qubits are not directly coupled but linked together with coupling elements such as microwave resonators. This reduces crosstalk to some degree because the qubits can be spaced further apart. In addition to the indirect coupling via the resonators, some configurations may also give rise to unwanted qubitqubit couplings, e.g., via a direct capacitive coupling of the qubits, resulting in unwanted crosstalk. Moreover, crosstalk sets a lower bound on the distance between the qubits and, therefore, impedes qubit arrangements with small footprints.
In more detail, crosstalk manifests itself by a shift of the qubit frequency conditional on the state of the other qubits on the processor (assuming a twoqubit configuration); this is referred to as an unwanted {circumflex over (z)}_{1}{circumflex over (z)}_{2 }interaction where {circumflex over (z)}_{i }is the zPauli operator on qubit i. More specifically, let ω_{ij }be the transition energy from the qubit ground state ij to the excited state with qubit 1 in state i and qubit 2 in state j, where i,jϵ{0,1}. In the absence of crosstalk the relation ω_{11}=ω_{01}+ω_{10 }holds, i.e., the energy of the doubleexcited state equals the sum of the individual qubit energies. However, when an unwanted {circumflex over (z)}_{1}{circumflex over (z)}_{2 }interaction is present this relation no longer holds, i.e., ω_{11}≠ω_{01}+ω_{10}. When this happens the fidelity of the single qubit and two qubit gates deteriorates since the gate parameters no longer match the qubit parameters. All the more, this implies that transitions/gates on one qubit depends on the state of the other qubit in an uncontrolled manner.
A quantum gate for superconducting qubits is an electrical signal whose spectral content depends both on the desired gate and on the qubit parameters. Even a slight shift in qubit parameters (such as frequency) creates a mismatch between the signal applied and the system that this signal is driving. This hinders the operation of the qubits, inasmuch as it makes it difficult to achieve and calibrate single qubit gates that do not depend on the state of other qubits. Thus, such mismatches quickly deteriorate the accuracy of the quantum evolution. Accordingly, crosstalk between the quantum circuits is detrimental to the operation of the quantum processing system.
When the frequency of the qubits is controllable the unwanted qubitqubit crosstalk can to some degree be mitigated using the control lines that tune the frequency of the qubits. Setting the qubits far away in frequency space reduces crosstalk. This, however, is not possible when the frequency of the qubits cannot be tuned. Moreover, since the coupling falls off only with the inverse of the difference frequency, a large frequency tuning range is required to sufficiently suppress the crosstalk. Tunable coupling elements have been used in quantum computing architectures to reduce crosstalk whilst relaxing the required spectral separation between the qubits. There, however, the tunable coupler must be biased at a welldefined flux to suppress crosstalk. This exact bias point is in practice hard to achieve and qubitqubit crosstalk, despite being suppressed, is still present.
Fixedfrequency qubits do not feature a SQUID loop and are thus more robust against flux noise. This increases their coherence times but comes at the expense of qubit frequency control. Some quantum gates such as the crossresonance gate have been developed specifically for fixedfrequency qubits coupled dispersively via a nontunable coplanar waveguide resonator. In such systems, gates have been created by using a microwave pulse to induce a 2g coupling between the qubits.
Fixedfrequency qubit architectures that feature tunable couplings openup additional possibilities. For instance, an architecture based on fixedfrequency qubits and a tunable coupler passively mitigate measurement crosstalk by biasing the coupling element at a specific point.
Contrary to the prior art approaches, the present approach involves quantum circuits (e.g., fixedfrequency qubits), coupled to a frequencytunable coupler, wherein signals are applied (e.g., as a synthesis signal to the tunable coupler), which incorporate a compensation signal (in addition to signals for driving transitions) for directly compensating for crosstalk between the quantum circuits, when driving the desired transitions among states of the quantum circuit system.
In reference to
Most embodiments discussed herein assume that the quantum processing device is a computing device 1, in which the quantum circuits 11, 12 are meant to be used as computational circuits (e.g., computational qubits). Still, the skilled person may appreciate that the present invention further extends to quantum sensing devices, where the quantum circuits are implemented as sensors. In principle, the less crosstalk or parasitic signals the better the sensitivity of the sensors. Note, however, that the active compensation scheme proposed herein may impact the sensitivity of the sensor. In all cases, the tunable coupler 20 is, in the present context, not meant to be used as a computational quantum circuit in the processing device 1, even though the coupler 20 can practically be embodied as a qubit, as discussed later in detail. Rather, the role of the coupler 20 is merely to couple the quantum circuits, e.g., to achieve desired energy transitions between states spanned (i.e., subtended) by such circuits, when operating the device.
The device 1 thus comprises two or more quantum circuits 11, 12, which are, each, coupled to the tunable coupler 20, as depicted in
The present method basically revolves around the application of two functionally different types of signals. On the one hand, a primary signal is applied to the quantum circuits 11, 12 (possibly via the tunable coupler 20), so as to drive energy transitions between states spanned by the quantum circuits, while, on the other hand, a compensation signal is applied to the tunable coupler, which signal is designed so as to shift one or more states spanned by the quantum circuits (in energy or, equivalently, in frequency), in order to compensate for crosstalk between the quantum circuits.
In embodiments, the primary signals can be applied to the quantum circuits via drive lines 31, 32 (see
In the following, it is assumed that both the primary and compensation signals are applied as a synthesis signal to the tunable coupler 20, via the line 30, see step S60 in the flowchart of FIG. 9, as well as the diagram of
Thus, and as the present Inventors have realized, an additional signal component can be embedded in the applied signals, which component is devised so as to shift (in energy or frequency) at least one state spanned by the quantum circuits, in order to compensate for crosstalk between the quantum circuits 11, 12, as exemplified latter in reference to
Applying the additional compensation signal makes it possibly to cure the mismatch between the primary signals and the system that this primary signal is meant to drive. In particular, the present approach makes it possible to achieve and calibrate single qubit gates that do not depend on the state of other qubits, such that the accuracy of the quantum evolution realized can be maintained over a longer period of time.
Moreover, embodiments of the present invention leverage short operation times allowed by the signals applied S60, as well as the highcoherence of the fixedfrequency circuits. In that respect, we note that concomitant transitions may possibly be contemplated, e.g., by concomitantly modulating the frequency of the coupler 20 atm frequencies, e.g., to achieve high gate rates, while altogether embedding crosstalk correction signals within the applied signals. Using a suitable synthesis signal, for example, the crosstalk compensation does not impact the operation times of the quantum circuits. This way, the quantum circuits can possibly be entangled within a single, short operation, such that short state preparation can be obtained. This, in turn, preserves the possibility for a quantum processing device to be scaled to a larger number of quantum circuits. In addition, the present approach may be leveraged for a quantum computer to perform with less error.
As assumed in
Operations as described herein may further require to suitably set amplitudes and phases of the signals applied S60, e.g., so as to achieve an entangled state of the quantum circuits. This way, quantum processing devices (such as described later in reference to
For example, referring to
This, in turn, may for instance be exploited to implement gate operations S90. E.g., the frequency modulation of the coupler 20 can be used to drive transitions as necessary to implement S80 a quantum gate operating on the quantum circuits 11, 12 (but effectively not on the tunable coupler 20), while altogether compensating for crosstalk.
Note, in the present context, such quantum gates will typically operate on two or more of the quantum circuits. However, since the crosstalk compensation improves coherence, the present compensation scheme may also be leveraged to improve single qubit gates.
The frequency of the tunable coupler 20 may notably be modulated S70 so as to implement S80 a universal set of gates based on states of the at least two quantum circuits 11, 12, where such gates are agnostic to states of the tunable coupler, for the same reasons as mentioned above. Amplitudes and phases of signals applied to the tunable coupler 20 may typically need be set, prior to, or while modulating S51 the frequency of the primary signals, so as to achieve, e.g., an iSWAP gate, a cPhase gate, a Fredkin gate (also known as also CSWAP gate) or a square root of SWAP gate (commonly denoted by √{square root over (SWAP)}), or any combination of such gates.
The signal synthesis is now described in detail. While the signal synthetized may possibly be adequately shaped ab initio (e.g., thanks to an arbitrary waveform generator with sufficient sampling rate and bandwidth) so as to effectively include the two types of signal contributions needed, it will be much easier (and less costly) in practice to separately form the two signal components and then combine them, as assumed in
The compensation signal may for instance simply be obtained by first generating S52 a signal with a frequency generator 52, prior to gating S53 the generated signal to obtain a desired signal pulse, e.g., using a gate 54 controlled by a gate controller 53, as illustrated in
Note, preferred embodiments are directed to microwave activated qubits (e.g., transmontype, superconducting qubits). In such cases, the primary signal can be modulated S51 so as to generate a harmonic microwave signal, using techniques known per se. Meanwhile, the compensation signal can be obtained by first generating S52 a fixedfrequency, continuouswave microwave signal, and subsequently gating S53 (if needed) the microwave signal generated. This way, a simple and costeffective compensation scheme is achieved. The resulting signal components are combined at step S55 and subsequently applied S60 to the tunable coupler 20, as illustrated in
Referring now to
In practice, crosstalk may typically give rise to hybridized states of the quantum circuits (excluding the coupler), see
Variants of the present method as described above may be combined in multiple ways. For example, preferred embodiments of the present method are based on a system including fixedfrequency, superconducting qubits 11, 12 and tunable couplers 20, which allows turning on and off the qubit crosstalk. The tunable coupler 20 is then preferably implemented as a SQUID loop. Threading a magnetic flux through the SQUID loop allows its transition frequency to be controlled, see
The tunable coupler 20 may for instance be operated using a control unit as shown in
In the example of
Scanning the frequency of the compensation tone at a fixed power and measuring the induced frequency shift of the − state reveals that there is a point where the crosstalk induced 22 shift disappears, as identified by the dashed lines in
Applying a tone at the frequency where the 22 shift disappears compensates the qubit crosstalk. This can be seen by performing four Ramsey measurements on the second qubit Q2, in an arrangement such as depicted in
More generally, a compensation signal can be adjoined to each driving signal, to accordingly compensate for crosstalk. Yet, since a tunable coupler can be concomitantly driven at distinct frequencies, several compensation signals may be embedded in the applied signals, so as to simultaneously shift and thus compensate several states of the system. Thus, as one understands, the present invention can more generally be embodied so as to shift and thus compensate one or more states of the quantum circuit system (excluding states of the tunable coupler) for crosstalk between the quantum circuits.
Referring now more specifically to
This device 1 includes n quantum circuits 11, 12 (n≥2, preferably fixedfrequency circuits), which normally have distinct frequencies (to simplify the operation of the system), in addition to a tunable coupler 20, to which the quantum circuits are coupled. More than two quantum circuits may possibly be involved (n≥3).
The coupler 20 is designed in such a manner that its frequency can be modulated upon application S60 of a signal, as described earlier. Again, the coupler 20 may possibly be concomitantly modulated at m frequencies, m≥2, and the coupling element 20 is not meant to be used as a computational qubit.
In addition, the device 1 includes a control unit 50, configured in the processing device 1 to apply two types of signals, including a compensation signal to compensate for crosstalk between the circuits, as described earlier. In particular, the unit 50 may be designed to modulate the frequency of the tunable coupler 20. In all cases, the unit 50 makes it possible to drive energy transitions between pairs of states spanned by the quantum circuits and achieve entangled states, as already discussed in reference to the present methods. The control unit 50 typically includes a control line 30 (i.e., a physical wire), to apply the signals. Preferably, the unit 50 is designed so as to allow a composite (e.g., parametric, multifrequency) signal to be applied S60 and, if necessary, to suitably set amplitudes and phases of the applied signals. In particular, and as explained earlier, the unit 50 may be adapted to apply a synthesis signal, which embeds both a primary signal (modulated) and a compensation signal to compensate for crosstalk between the circuits 11, 12, in operation.
The control unit 50 may be implemented as a circuit, such as a linear circuit for example, whereby the synthesis signal applied amounts to a linear transformation of the modulated signal and the compensation signal. Referring to
In embodiments, the second signal generator 52 comprises a frequency generator to generate a fixedfrequency, continuouswave microwave signal, as well as a gate 54 and a gate controller 53 to gate the continuouswave microwave signal generated and obtain the desired compensation pulses, in operation.
Contrary to the depictions of
In all cases, the tunable coupler 20 may be capacitively coupled to each of the quantum circuits 11, 12. In the example of
The coupling is performed by modulating the frequency of the tunable coupler 20 near or at the frequency corresponding to the desired transitions. The coupler can be regarded as formed by one or more electrodes, as well as the SQUID loop (the latter containing the two Josephson junctions). I.e., the tunable coupler 20 may for instance couple to the transmons 11, 12 via separate electrodes (the latter separated by the SQUID loop) or via a single electrode, with the SQUID loop coupling to the ground.
A final aspect of the invention is now described in reference to
A controller system (not shown in
While the present invention has been described with reference to a limited number of embodiments, variants and the accompanying drawings, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the present invention. In particular, a feature (devicelike or methodlike) recited in a given embodiment, variant or shown in a drawing may be combined with or replace another feature in another embodiment, variant or drawing, without departing from the scope of the present invention. Various combinations of the features described in respect of any of the above embodiments or variants may accordingly be contemplated, that remain within the scope of the appended claims. In addition, many minor modifications may be made to adapt a particular situation or material to the teachings of the present invention without departing from its scope. Therefore, it is intended that the present invention not be limited to the particular embodiments disclosed, but that the present invention will include all embodiments falling within the scope of the appended claims. In addition, many other variants than explicitly touched above can be contemplated.