Control system and methods

Abstract

Methods and systems for interfacing with quantum devices are disclosed. A multiband input signal is divided into input signal components. Each input signal component is modified to apply an frequency and / or phase shift and is then provided to a quantum device.

Description

[0001] Control system and methods

[0002] The present disclosure relates to methods and systems for controlling and / or reading out quantum devices such as, for example, qubits.

[0003] In a quantum computer, the basic operating block is a qubit. One way to read out the state of the qubit is to couple a resonator to the qubit. The state of the qubit shifts the resonant frequency of the resonator. This shift can be detected by monitoring the phase and amplitude of a reference signal used to probe the resonator.

[0004] Qubits are generally operated at low temperatures (often in the mK range), with the circuitry for performing readout at room temperature. This is because the readout circuity does not work in low temperatures, and space in the low temperature environment is limited. The link between the low temperature environment and the room temperature environment is constricted and often represents a limitation in the design of a quantum computer. In order to optimise the use of available space, several resonators may be coupled to a single input feedline and a single output feedline. However, the input signals need to be separated out to address individual qubits, and the output signals need to be combined onto a single line.

[0005] Some multiplexing solutions are available to interface with multiple qubits at the same time. However, these are often inefficient, and lack precision since the mixers are fixed, and not adjustable. Frequency division multiplexing also faces challenges when interfacing with qubits operating at different frequencies.

[0006] It would be desirable to interface simultaneously with multiple qubits without compromising on control precision or flexibility.

[0007] According to a first aspect of the invention, there is provided a method of interfacing with a plurality of quantum devices, the method comprising: dividing a multiband input signal into a plurality of input signal components; modifying the frequency and / or phase of each input signal component to apply a frequency and / or phase shift; and after modifying the frequency and / or phase of each input signal component, providing each input signal component to a separate one of the plurality of quantum devices for input to the quantum devices. The invention disclosed herein provides scalable interfacing methods and systems for control and readout of quantum devices such as qubits. Existing solutions cannot scale efficiently for the large numbers of qubits required to achieve quantum advantage, whereas the present invention utilises multiband signals and frequency / phase modification to interface with multiple quantum devices in a scalable fashion.

[0008] The input signal components may be single band components.

[0009] The frequency and / or phase of each input signal component may be modified by a mixer, arranged to mix the input signal component with a further signal to cause the shift in frequency and / or phase.

[0010] The further signal may be shared by multiple mixers, or may be a separate signal for each mixer.

[0011] The further signal may be an internal clock signal of the mixer.

[0012] The mixer may be software controllable to allow variation in the frequency and / or phase shift applied to the input signal component. Software-controlled mixers are particularly well-suited to scalable quantum control and readout because they can be dynamically configured and calibrated for specific applications and quantum device characteristics.

[0013] The multiband input signal may be divided into the plurality of components by a demultiplexer.

[0014] Each input signal component may be configured to perform a control and / or read operation on the quantum device to which it is sent.

[0015] The steps of dividing the multiband input signal and modifying the frequency and / or phase of each input signal component may be performed on a single circuit or circuit board. The quantum devices may be coupled to the same circuit or circuit board.

[0016] The steps of dividing the multiband input signal and modifying the frequency and / or phase of each input signal component may be performed in a temperature-controlled environment. The environment may be held at cryogenic temperatures, and the quantum devices may be held in the same temperature-controlled environment.

[0017] According to a second aspect of the invention, there is provided a method of processing signals read from a plurality of quantum devices, the method comprising: receiving a plurality of output signal components, each output signal component from a one of the plurality of quantum devices; modifying the frequency and / or phase of each output signal component to apply a frequency and / or phase shift; and after modifying the frequency and / or phase of each output signal component, combining the signal components into a single multiband output signal.

[0018] The output signal components may be single band / tone components or may be broadband / multitone .

[0019] The frequency and / or phase of each output signal component may be modified by a mixer, arranged to mix the input signal component with a further signal to cause the shift.

[0020] The further signal may be shared by multiple mixers, or may be a separate signal for each mixer.

[0021] The further signal may be an internal clock signal of the mixer.

[0022] The mixer may be software controllable to allow variation in the frequency and / or phase shift applied to the output signal component.

[0023] The output signal components may be combined into the single multiband output signal by a multiplexer.

[0024] The steps of modifying the frequency and / or phase of each output signal component and combining the output signal components may be performed on a single circuit or circuit board. The quantum devices may be coupled to the same circuit or circuit board.

[0025] The steps of modifying the frequency and / or phase of each output signal component and combining the output signal components may be performed in a temperature-controlled environment. The environment may be held at cryogenic temperatures. The quantum devices may be held in the same temperature-controlled environment.

[0026] According to a third aspect of the invention, there is provided a method of controlling and / or reading a plurality of quantum devices comprising: providing input signals to the plurality of quantum devices according to the method of the first aspect; and processing output signals from the plurality of quantum devices according to the second aspect.

[0027] The multiband input signal may be generated by an external control system, and the multiband output signal may be readout by the external control system.

[0028] When the quantum devices are provided on a circuit / circuit board and / or are provided in a temperature-controlled environment, and when at least some of the steps of dividing the multiband input signal, modifying the frequency and / or phase of each input signal component, modifying the frequency and / or phase of each output signal component, and combining the output signal components are carried out on the same circuit or circuit board and / or in same the temperature-controlled environment the external control system may be separate, and may be outside the temperature-controlled environment.

[0029] Reading out the multiband output signal may comprise: receiving the multiband output signal comprising a plurality of frequency components, each corresponding to one of the plurality of quantum devices; determining operational filter parameters based on the plurality of frequency components; configuring a set of filters to filter the multiband output signal based on the determined operational filter parameters; and using the set of filters to divide the multiband output signal into a plurality of separate components corresponding to each of the quantum devices.

[0030] The set of filters, configured based on the operational filter parameters, may divide the multiband output signal into a plurality of channels, wherein each channel comprises, at most, one frequency component corresponding to one of the plurality of quantum devices. The operational filter parameters may define the total bandwidth of the output signal that is passed through the set of filters, and the bandwidth of each channel. The operational filter parameters may define the boundaries between the channels. The set of filters may split the output signal into a plurality of channels, each channel extending over a range of frequencies between a lower boundary and an upper boundary.

[0031] The operational filter parameters may comprise a number of channels that the output signal is split into and / or a maximum cut-off frequency of the set of filters.

[0032] The maximum cut-off frequency of the set of filters may be determined based on a sampling rate of an analogue-to-digital converter arranged to provide the output signal to the set of filters. The operational filter parameters may comprise the sampling rate of an analogue-to-digital converter and configuring the set of filters may comprise setting the sampling frequency of the analogue-to-digital converter.

[0033] The number of channels may be set at the minimum value at which there is at most a single one of the plurality of separate component corresponding to each of the quantum devices in each channel with lower and upper boundaries above 0Hz.

[0034] Alternatively, the number of channels may be set higher than the minimum value at which there is at most a single one of the plurality of separate component corresponding to each of the quantum devices in each channel with lower and upper boundaries above 0Hz.

[0035] The set of filters may be a digital polyphase filter bank arranged to allow the lower and upper boundaries of each filter in the set of filters to be modified by a controller.

[0036] The quantum devices may comprise qubits coupled to resonators for readout of states of the qubits.

[0037] According to a fourth aspect of the invention, there is provided a system for interfacing with a plurality of quantum devices, the system comprising: a divider arranged to split an multiband input signal into a plurality of input signal components; and for each input signal component, an input frequency shifter arranged to modify the frequency and / or phase of the input signal component, wherein each component is provided to a separate one of the plurality of quantum devices for input to the quantum devices, after modifying the frequency and / or phase of each input signal component. The input signal components may be single band components.

[0038] Each input frequency shifter may comprise a mixer, arranged to mix the input signal component with a further signal to cause the shift in frequency and / or phase.

[0039] The further signal may be shared by multiple mixers, or may be a separate signal for each mixer.

[0040] The further signal may be an internal clock signal of the mixer.

[0041] The input frequency shifter may be software controllable to allow variation in the frequency and / or phase shift applied by the input frequency shifter.

[0042] The divider may comprise a demultiplexer.

[0043] Each input signal component may be configured to perform a control and / or read operation on the quantum device to which it is sent.

[0044] The divider, the input frequency shifters and the plurality of quantum devices may be provided on a single circuit or circuit board.

[0045] The divider, the input frequency shifters and the plurality of quantum devices may be provided in a temperature-controlled environment. The temperature-controlled environment may be held at cryogenic temperatures.

[0046] According to a fifth aspect of the invention, there is provided a system for processing signals read from a plurality of quantum devices, the system comprising: a plurality of output frequency shifters, each output frequency shifter arranged to apply a frequency and / or phase shift to a one of a plurality of output signal components received from a plurality of quantum devices; and a combiner arranged to combine the output signal components into a multiband output signal.

[0047] The output signal components may be single band / tone components or may be broadband / multitone . Each output frequency shifter may comprise a mixer, arranged to mix the output signal component with a further signal to cause the shift in frequency and / or phase.

[0048] The further signal may be shared by multiple mixers, or may be a separate signal for each mixer.

[0049] The further signal may be an internal clock signal of the mixer.

[0050] The output frequency shifter may be software controllable to allow variation in the frequency and / or phase shift applied by the output frequency shifter.

[0051] The combiner may be a multiplexer.

[0052] The output frequency shifters, the combiner and the quantum devices may be provided on a single circuit or circuit board.

[0053] The output frequency shifters, the combiner and the quantum devices may be provided in a temperature-controlled environment. The temperature-controlled environment may be held at cryogenic temperatures.

[0054] According not a sixth aspect of the invention, there is provided a system comprising: a plurality of quantum devices; a system according to the fourth aspect for providing input signals to the plurality of quantum devices; according to the fifth aspect for readout of the plurality of quantum devices.

[0055] The multiband input signal may be generated by an external control system, and the multiband output signal may be readout by the external control system.

[0056] Where at least some of the divider, the input frequency shifters, the quantum devices, the output frequency shifters, and the combiner are provided on a single circuit / circuit board and / or are provided in a temperature-controlled environment, the external control system may be separate, and may be outside the temperature-controlled environment.

[0057] The external control system may comprise: an analogue to digital converter arranged to receive a multiband output signal comprising information on the states of the plurality of quantum devices and convert the multiband output signal from an analogue signal to a digital signal; a set of filters arranged to divide the converted multiband output signal into a plurality of separate components corresponding to each of the quantum devices; and a processor arranged to determine operational filter parameters based on the resonant frequencies of the plurality of quantum devices; and configure the set of filters to filter the multiband output signal based on the determined operational filter parameters.

[0058] The set of filters may be configured to split the multiband output signal into a plurality of channels, each channel extending over a range of frequencies between a lower boundary and an upper boundary.

[0059] The operational filter parameters may comprise a number of channels that the multiband output signal is split into and / or a maximum cut-off frequency of the set of filters and / or a sampling frequency of the analogue to digital converter.

[0060] The set of filters may be a digital polyphase filter bank arranged to allow the filter frequencies to be modified by the processor.

[0061] The analogue to digital converter and the set of filters may be provided on an FPGA.

[0062] The quantum devices may comprise qubits coupled to resonators for readout of states of the qubits.

[0063] It will be appreciated that features discussed in relation to any embodiment or aspect may be applied, mutatis mutandis, to any other embodiment or aspect.

[0064] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0065] Figure 1 schematically illustrates an interface system for interfacing with qubits according to an embodiment;

[0066] Figure 2 illustrates a flow chart of a method of interfacing with qubits according to an embodiment; and

[0067] Figure 3 illustrates an external control system for controlling the interface system. Figure 1 schematically illustrates an interface system 1 used to control and readout a plurality of quantum devices 3i-3n. The interface system is implemented in a temperature-controlled environment 5, such as a cryostat, dilution refrigerator or the like.

[0068] In the temperature-controlled environment 5, a chip 7 is provided having n qubits 9i.n, which are operated and read by the interface system 1. Each qubit 9i.nis coupled to a resonator l li-n. The state of the qubit 7 influences the state of the resonator 9, which can be detected by a corresponding probe signal 13 i.n, to measure the qubit state.

[0069] The resonator l li-nis a structure that allows a signal (for example electrical) to be transmitted through it. Each resonator 1 li-nhas an individual transmission characteristic showing the amount of transmission as a function of frequency, with a peak at the natural resonant frequency fres,i (the resonant frequency without any external influence). By coupling an external system, such as a qubit 9i.nto the resonator 11 i-nthe state of the external system alters the transmission characteristic. This can be measured by a change in the transmission of a probe signal 13i-npassed through the resonator l li-n. Correlation of the qubit state to the change in transmission at a fixed frequency allows for measurement of the qubit state. It will be appreciated that this type of measurement can be employed with many types of qubits (e.g. charge, spin, electrical, magnetic) and can also be employed with reflective resonators instead of the transmissive.

[0070] In order to measure the state of the qubit 9i, the probe signal 13i for each resonator 1 l i.nis fixed at a measurement frequency fi.ncorresponding to a slope in the resonator response (preferably a high gradient slope). In this way, a small modification in the resonator state will result in a large shift of the probe signal 13i, and the state of the qubit 9 can be measured.

[0071] In order to determine the resonator responses, and thus the measurement frequency, the resonators 11 i-nare calibrated. To calibrate the resonators 1 li-n, a frequency sweep measurement of the resonator is performed on a regular basis, with known (or no) input from the qubit 9i.n. During this operation, the probe signal 13i-nis swept across a frequency range Af centred on the natural resonant frequency fres,i and the response of the resonator Hi to the probe signal 131 is measured. In other words, for a given resonator I E, the frequency is swept between fres,i - Af / 2 to fres,i + Af / 2.

[0072] To generate the probe signals 13 i-n, for a state measurement a control system is provided outside the temperature-controlled environment 5. The control system generates a reference signal 15 that is provided on a single input line 15’ to the interface system 1. The reference signal 15 has n different frequency components 19i_nat different frequencies, forming the n probe signals 13i.n. Therefore, the reference signal 15 may be considered a multiband input signal. The control system will be discussed in more detail below, with reference to Figure 3.

[0073] The interface system 1 includes a demultiplexer 17 to split the reference signal 15 into the separate input signal components 19i-n. A separate input signal component 19i is provided for each qubit / re senator pair 7,9 in the system 1.

[0074] A mixer 21 i-nis provided for each separate input signal component 19i. The mixer 211-ncombines the received input signal component 19i with a signal from an internal clock 23i-n of the mixer 21 i-nto cause a shift in the frequency and / or phase of the input signal component 19i (and possibly also modify the amplitude). This creates a modified input signal component 13i-n, which is provided as the probe signal to the quantum devices 3i-n.

[0075] After interaction with the quantum devices 3i-nthe probe signals 13 i-nform output signal components 25i-n. A second mixer 27i-nis provided for each output signal component 25i-n. Each output signal component 25i_nis passed through the corresponding second mixer 27i-nto apply a frequency and / or phase shift, to form modified output signal components 3 l i.n. In the current example, the second mixer 27i-nreverses (or inverts) the shift from the first mixer 21 i_n, such that any change in the signal is solely due to the effect of the quantum devices 3i-n.

[0076] As with the mixers 21i-nthat apply a shift to the input signal components 19i-n, the mixers 27i-napply the frequency and / or phase shift to the output signal component 251- n (and optionally an amplitude modification) by mixing with signal from an internal clock 29i-n of the mixer 27i-n. The modified output signal components 3 li-nare provided to a multiplexer 33, which combines the components 31 i-ninto a single multiband output signal 35 which is provided on a single output feed line 37, to the control system for processing.

[0077] The mixers 21 i.nused for applying a phase and / or frequency shift to the input signal components 19i_nand the mixers 27i.nused for applying a phase and / or frequency shift to the output signal components 25i-nare software controlled.

[0078] By using software controlled mixers 21, 27 in the interface system 1 described above, the shifts (phase / amplitude / frequency) applied to the probe signals 13i-ncan be controlled in real-time, during an operation (rather than being set before the operation and remining constant throughout the operation). This allows mitigation of environmental noise and improves qubit coherence, maintaining system precision and stability. It also ensures probe signals 13i-nare at the correct frequencies and ensures the correct phase and amplitude modulation for each qubit 9i.n.

[0079] The software control for the mixers 21, 27 is provided by a separate connection 37 to the control system.

[0080] In the embodiment shown in Figure 1, the chip 7, demultiplexer 17, multiplexer 33 and mixers 21, 27 are provided on a single printed circuit board (PCB) 39, in the temperature-controlled environment 5. It will be appreciated that this is by way of example only. The chip 7, demultiplexer 17, multiplexer 33 and mixers 21, 27 may be mounted on the same or one or more PCBs or other types of substrate.

[0081] Figure 2 illustrates a flow chart showing the method 100 of interfacing with the set of quantum device 3.

[0082] At a first step 102, the multiband input signal 15 is received in the temperature- controlled environment 5.

[0083] At a second step 104, the multiband input signal 15 is divided into the separate input signal components 19i.n. At a third step 106, each of the input signal components 19i-nare modified by applying frequency and / or phase shifts (and / or amplitude control). This produces modified input signal components 13i-n.

[0084] In the next step 108, the modified input signal components 13 i-nare provided as probe signals to the quantum devices 3i_n, where a quantum operation is performed.

[0085] After the operation is performed, the output signal components 25 i_nare received from the quantum devices 3i.nat the next step 110.

[0086] At step 112, the output signal components 25i-nare modified by applying frequency and / or phase shifts (and / or amplitude control). This produces modified output signal components 3 li.n.

[0087] At the subsequent step 114, the modified output signal components 3 li-nare combined into a single multiband output signal 35. At a final step 116, the multiband output signal 35 is provided out of the temperature-controlled environment 5.

[0088] In the above examples, a demultiplexer 17 and multiplexer 33 are used to divide and combine the multiband signals. This is by way of example only. The input signal 15 may be divided into the input signal components 19 i-nby any suitable signal divider and the modified output signal components 31i.nmay be combined by any suitable signal combiner.

[0089] Likewise, any suitable shifter may be used to apply the phase / frequency shifts to the different components.

[0090] In the above examples, the mixers 21, 27 apply a phase shift by mixing a signal to be shifted with an internal clock signal of the mixer 21, 27. This is by way of example only. One or more reference signals may be provided by an external source. Furthermore, at least some mixers 21, 27 may share the same reference signal. The shared reference signal may be from an external source, or the internal clock of one of the mixers 21, 27. It will be appreciated that in the above example, the input side of the interface system 1 and the output side are symmetrical, in that they both use a demultiplexer 17 / multiplexer 33, and mixer 21, 27. It may be, however, that the two sides are asymmetrical. In one example, the functions of combining / dividing the signals and / or applying the shifts to the signal components may be achieved by different technologies in the input and output sides. In other examples, the input or output side may not include the functions of combining and / or shifting at all.

[0091] In the above example, the shift applied by the second mixers 27 reverses the shift from the first mixer. This may be by way of example only, and the two mixers may apply different shifts. Signal processing may allow the effect of the qubit 9 to be separated from the shifts.

[0092] It will be appreciated that various signal processing methods are known for forming the multiband input signal 15 and for processing the multiband output signal 35. Any of these methods may be used. Figure 3, discussed below, provides one example for the control system 201, using polyphase filtering.

[0093] The input signal components 19 may be single band / single tone, or may cover a range of frequencies.

[0094] In the embodiment of the control system 201 based on polyphase filtering, the multiband input signal 15 from which the probe signals 13 i-nfor a state measurement or calibration operation is generated by a Digital -to-Analogue Converter 203 (DAC).

[0095] For a state measurement, the frequency components correspond to the measurement frequencies fi-nof the different resonators 1 l i.n. For a calibration operation, the frequency component is set to fres,i - Af / 2 and then varied over time to fres,i + Af / 2. It will be appreciated that the sweep may follow a different pattern to this to measure the desired response curve.

[0096] Readout of the phase and amplitude shift of the probe signals 13i-nis achieved by heterodyne measurement, which upconverts the signal generated by the DAC 203 and downconverts the multiband output signal 35. To do this, the output signal 35 is mixed with a signal from a local oscillator 205 after the DAC 203 and before being provided to an Analogue-to-Digital converter (ADC) 207, used for analysing the signal. The local oscillator 205 is referenced to the same clock as the DAC 203.

[0097] After the ADC, the signal is provided to a digital polyphase filter bank 209 to separate out the signals from the different qubit resonator pairs 9i-n, 1 h-n.

[0098] The digital polyphase filter bank 209 comprises a set of filters, spread over the range - fs / 2 to +fs / 2, where fsis the sampling frequency of the ADC 207. The filters split the range into a plurality of equally spaced bands (also referred to as channels).

[0099] Each band may be thought of as a band-pass filter, which allows passage of signals having frequency between a lower frequency boundary and an upper frequency boundary. The set of filters are arranged such that the upper frequency boundary of one band forms the lower frequency boundary of the next filter.

[0100] Where the sampling rate of the ADC is fsand the signal is divided in N channels / bands, the filter boundaries, ft>,xare defined by:

[0101] Where x = 0, 1, 2...N.

[0102] As can be seen from equation (1), the minimum cut-off frequency of the set of filters (i.e. the lowest frequency allowed to pass) is -fs / 2 whilst the maximum cut-off frequency (i.e. the highest frequency allowed to pass) is +fs / 2. Apart from the minimum cut-off and maximum cut-off, each boundary for a filter can be considered the high pass for the channel below the boundary, and the low pass for the channel above the boundary.

[0103] The ADC 207 and digital polyphase filter bank 209 are software controlled so that both fsand N are selectable parameters to allow the upper and lower boundaries of the channels to be controlled, as will be discussed below.

[0104] The channels separated by the filter bank 209 are individually provided to a digital signal processor (DSP) 211 for comparison to the output from the DAC 203 and hence I / Q demodulation and readout. This may provide measurement of the qubit states or characterisation of the resonators 11 i-n. The DSP 211 includes a local control unit 213i-nfor each individual qubit 9i.nand a multi qubit control unit 215 for control of the ensemble. In addition to providing readout, the local control units 213 i-nand a multi qubit control unit 215 provide inputs for operation and control of the qubits 9i.nand resonators 1 li-n. The outputs from the local control units 213i-nare provided to a software defined channel combiner 217, which combines the outputs into a single signal, which is provided to the DAC 203. This includes the components necessary for generating the multiband input signal 15.

[0105] A memory 219 may also be provided. The memory 219 may include a programme storage portion 221 and data storage portion 223. The program storage portion 221 may store computer program code which ,when executed on the DSP 211 or other suitable processor, causes the operation of the qubits 9i.nand control system 201. The stored programme code may also operate the software defined mixers 21, 27 in the interface system 1.

[0106] The data storage portion 223 may contain data on the qubits 9i-n, resonators 11 i-n, resonator transmission characteristic and the like

[0107] In use, the multiband input signal 15 is generated by the DSP 211 and DAC 203. As discussed above, the multiband input signal 15 comprises frequency components for probing each resonator l li-n.

[0108] The natural resonant frequency fres,i of each resonator l li-nis fixed by the structure of the resonator l li-n. However, different operations (different qubit operations or calibration operations) may address a different subset of qubits 1 li-non the chip 7 and where the probe signal 13i_nis swept across a range, different sweeps may be used for different subsets of the qubits l li-nand in different situations. Therefore, the components of the reference signal 15 may vary from operation to operation.

[0109] The operational filter parameters (fsand N) of the ADC 207 and filter bank 209 are determined based on the frequency components of the multiband input signal 15. This information may be based directly on the multiband input signal 15 and / or may be based on known information regarding the multiband input signal 15 and / or the qubit resonator pairs 9i-n, l li-n to be addressed. fsis selected so that the resonant frequencies fall within the first Nyquist zone of the ADC 207, after up and down conversion. fsand N are also chosen such that the each of the different frequency components of the received signal is in a dedicated channel, with no other frequency component. As discussed above, where a sweep is performed in a calibration operation, the full range of the sweep is within the channel.

[0110] As can be seen from equation 1, channels with negative frequency are defined by the filters. This is an inherent property of the polyphase filter. The channels having any frequency component < 0 Hz (i.e. channels in which any part of the frequency range covered by the channel is negative) are ignored as these do not form real components of the signal. Therefore, for M qubits, the minimum number of channels Nmin = 2M.

[0111] The ADC 207 and filter 209 are configured based on the determined parameters.

[0112] After the qubit operation has been performed, the output signal 35 is processed to divide the signal into the separate channels by the polyphase filter 209. The readout operation is then performed. The readout operation may be part of qubit statement measurement or resonator characterisation.

[0113] Any suitable search method could be used to find the value of fsand N. For example, linear search algorithms and / or minimisation algorithms may be used to identify the values of fsand N to be used.

[0114] The lower limit of possible values for fsis set by the requirement of the frequencies being measured to fall within the first Nyquist zone whilst the lower limit of N (Nmin) is set by the number of qubits to be measured.

[0115] The upper limit of fsis set by the limitations of the ADC 29.

[0116] The upper limit of N is typically set by available memory and / or the minimum size of channel allowed by Af. As N increases, further memory is required and so keeping N as low as possible (as in example 1 above) reduces memory requirements.

[0117] In at least some embodiments, it is preferable to set N = Nmin to reduce memory usage. However, in some cases, this may not be possible. For example, the upper limit of fs may be such that N = Nmin cannot separate each component of the signal into a dedicated channel and so N has to be increased. Furthermore, in other situations for example, where memory is less constricted it may not be necessary to keep N = Nmin.

[0118] The variation of the operational filter parameters fsand N used to divide the output signal 35 into channels also allows for dynamic variation of the filtering, which accommodates the variability in frequencies that may be needed for different operations and applications.

[0119] In the example shown in Figure 3, the DAC 203, ADC 207, filter bank 209, DSP 211 and combiner 217 are implemented on an FPGA chip 225. However, any suitable type of control circuit may be used.

[0120] A digital polyphase filter bank is an example of a software controllable linear filter bank (i.e. the filter bank divides the incoming signal into equal size channels). Other types of filter bank may also be used. For example, a digital filter bank with non-equal channels may be used. In this case, the operational filter parameters may include upper and lower boundaries of each individual filter, in addition to N and fs. This will allow for greater control over filters. It will, however, be appreciated, that this will significantly increase memory requirements.

[0121] In the above examples, both N and fsare varied. It will be appreciated that one of N and fsmay be fixed, and only one of the parameters varied.

[0122] In the above, the local oscillator 205 is provided externally of the FPGA 227. However, it will be appreciated that the local oscillator 205 may be provided on the FPGA 227, and the heterodyning may even be done on the DSP 211.

[0123] In the above examples, it is assumed that the sampling frequency of the DAC 203 is set to be the same as the sampling frequency of the ADC 207. This need not be the case, and in some cases, the sampling frequencies may be different. In this case, negative frequencies may be generated by the mixing of the DAC signal and ADC signal during the readout operation. Reflections of these frequencies can then fall within the same channels as signals to be measured. However, correct selection of the operating parameters of the filters can allow these to be filtered out, as the frequencies of these reflections will be known.

[0124] In the examples discussed above, local control units are provided for qubit control. It will be appreciated that this is by way of example only. A single control unit may be provided or groups of qubits may be controlled by a single control unit. Any suitable controller for control and readout from the chip 3 may be used.

[0125] In the above examples, the interface system 1 is provided in the temperature-controlled environment 5, whilst the control system 201 is provided outside. The feed lines 15’, 35’ thereby provided the connectivity between the inside and outside of the temperature controlled environment 5. As can be seen, by using the demultiplexer 17, multiplexer 33, and mixers 21, 27 n qubits can be controlled by a single input feed line 15’ and a single output feed line 35’ rather than requiring n input feed lines and n output feed lines.

[0126] In one example embodiments, the qubits 9i.nare superconducting qubits. Typically, resonators for readout of superconducting qubits have resonant frequencies in the range of 4 GHz to 12 GHz, with spacing of between 10 MHz and 100 MHz between resonant frequencies. In these examples, Af is between 100 KHz and 5 MHz. These frequencies are by way of example only, and superconducting qubits may have higher or lower resonant frequencies, with higher or lower spacing and larger or smaller sweep.

[0127] Whilst a superconducting qubit is given by way of example, the control system 201 and interface system 1 discussed above can be used for multiplexed readout of any type of qubit or resonator at any frequency range may be used.

[0128] Typically, between three and five qubits may be coupled on a single feedline 15’, 35’ . However, this is by way of example. In some examples, more than five qubits may be coupled on a single feedline 15’, 35’. Where a large number of qubits are provided, multiple interface systems 1 and control systems 201 as described above may be provided, each coupled to a different subset of the qubits.

[0129] The interface system 1 (and control system 201) discussed above has been described in relation to readout of qubits 9i.nand resonators 1 li-n. It will, however, be appreciated that the control system has applicability to the readout of any type of quantum device (such as quantum memory, or other quantum devices) or other types of system requiring multiplexed readout from the low temperature environment. In addition, while the illustrated system shows components (including the qubits 9i-n) in a low temperature environment, it should be understood that the present invention can also be used with non-cryogenic qubits (i.e. without a low temperature environment / temperature- controlled zone).

Claims

Claims1. A method of interfacing with a plurality of quantum devices, the method comprising: dividing a multiband input signal into a plurality of input signal components; modifying the frequency and / or phase of each input signal component to apply a frequency and / or phase shift; and after modifying the frequency and / or phase of each input signal component, providing each input signal component to a separate one of the plurality of quantum devices for input to the quantum devices.

2. The method of claim 1, wherein the frequency and / or phase of each input signal component is modified by a mixer, arranged to mix the input signal component with a further signal to cause the shift in frequency and / or phase.

3. The method of claim 2, wherein the further signal is an internal clock signal of the mixer.

4. The method of any claim 2 or claim 3, wherein the mixer is software controllable to allow variation in the frequency and / or phase shift applied to the input signal component.

5. The method of any preceding claim wherein the multiband input signal is divided into the plurality of components by a demultiplexer.

6. The method of any preceding claim wherein each input signal component is configured to perform a control and / or read operation on the quantum device to which it is sent.

7. The method of any preceding claim, wherein the steps of dividing the multiband input signal and modifying the frequency and / or phase of each input signal component are performed on a single circuit or circuit board, and the quantum devices are coupled to the same circuit or circuit board.

8. The method of any preceding claim wherein the steps of dividing the multiband input signal and modifying the frequency and / or phase of each input signal component are performed in a temperature-controlled environment, preferably held at cryogenic temperatures, and the quantum devices are held in the same temperature-controlled environment.

9. A method of processing signals read from a plurality of quantum devices, the method comprising: receiving a plurality of output signal components, each output signal component from a one of the plurality of quantum devices; modifying the frequency and / or phase of each output signal component to apply a frequency and / or phase shift; and after modifying the frequency and / or phase of each output signal component, combining the signal components into a single multiband output signal.

10. The method of claim 9, wherein the frequency and / or phase of each output signal component is modified by a mixer, arranged to mix the input signal component with a further signal to cause the shift.

11. The method of claim 10, wherein the further signal is an internal clock signal of the mixer.

12. The method of claim 10 or claim 11, wherein the mixer is software controllable to allow variation in the frequency and / or phase shift applied to the output signal component.

13. The method of any of claims 9 to 12 wherein the output signal components are combined into the single multiband output signal by a multiplexer.

14. The method of any of claims 9 to 13, wherein the steps of modifying the frequency and / or phase of each output signal component and combining the output signal components are performed on a single circuit or circuit board, and the quantum devices are coupled to the same circuit or circuit board.

15. The method of any of claims 9 to 14 wherein the steps of modifying the frequency and / or phase of each output signal component and combining the output signal components are performed in a temperature-controlled environment, preferably held at cryogenic temperatures, and the quantum devices are held in the same temperature-controlled environment.

16. A method of controlling and / or reading a plurality of quantum devices comprising: providing input signals to the plurality of quantum devices according to the method of any one of claims 1 to 8; and processing output signals from the plurality of quantum devices according to the method of any one of claims 9 to 15.

17. The method of any preceding claim wherein the quantum devices comprise qubits coupled to resonators for readout of states of the qubits.

18. A system for interfacing with a plurality of quantum devices, the system comprising: a divider arranged to split a multiband input signal into a plurality of input signal components; and for each input signal component, an input frequency shifter arranged to modify the frequency and / or phase of the input signal component, wherein each component is provided to a separate one of the plurality of quantum devices for input to the quantum devices, after modifying the frequency and / or phase of each input signal component.

19. The system of claim 18, wherein each input frequency shifter comprises a mixer, arranged to mix the input signal component with a further signal to cause the shift in frequency and / or phase.

20. The system of claim 19, wherein the further signal is an internal clock signal of the mixer.

21. The system of any of claims 18 to 20, wherein the input frequency shifter is software controllable to allow variation in the frequency and / or phase shift applied by the input frequency shifter.

22. The system of any of claims 18 to 21 wherein the divider comprises a demultiplexer.

23. The system of any of claims 18 to 22, wherein each input signal component is configured to perform a control and / or read operation on the quantum device to which it is sent.

24. The system of any of claims 18 to 23, wherein the divider, the input frequency shifters and the plurality of quantum devices are provided on a single circuit or circuit board.

25. The system of any of claims 18 to 24, wherein the divider, the input frequency shifters and the plurality of quantum devices are provided in a temperature- controlled environment, preferably held at cryogenic temperatures.

26. A system for processing signals read from a plurality of quantum devices, the system comprising: a plurality of output frequency shifters, each output frequency shifter arranged to apply a frequency and / or phase shift to a one of a plurality of output signal components received from a plurality of quantum devices; and a combiner arranged to combine the output signal components into a multiband output signal.

27. The system of claim 26, wherein each output frequency shifter comprises a mixer, arranged to mix the output signal component with a further signal to cause the shift in frequency and / or phase.

28. The system of claim 27, wherein the further signal is an internal clock signal of the mixer.

29. The system of any of claims 26 to 28, wherein the output frequency shifter is software controllable to allow variation in the frequency and / or phase shift applied by the output frequency shifter.

30. The system of any of claims 26 to 29, wherein the combiner is a multiplexer.

31. The system of any of claims 26 to 30, wherein the output frequency shifters, the combiner and the quantum devices are provided on a single circuit or circuit board.

32. The system of any of claims 26 to 31, wherein the output frequency shifters, the combiner and the quantum devices are provided in a temperature-controlled environment, preferably held at cryogenic temperatures.

33. A system comprising: a plurality of quantum devices; a system as claimed in any of claims 18 to 25 for providing input signals to the plurality of quantum devices; and a system as claimed in any of claims 26 to 32 for readout of the plurality of quantum devices.

34. The system as claimed in any of claims 18 to 33, wherein the quantum devices comprise qubits coupled to resonators for readout of states of the qubits.

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