Control system

Abstract

A control system (1) for qubits in a quantum processor, comprises: an output line (7) to condition a driver signal and provide the conditioned driver signal to a set of qubits (5); the output line (7)

Description

The present disclosure relates to a control system arranged to control and readout an ensemble of qubits, a method of operating an ensemble of qubits in a quantum processor, and methods of calibrating output lines and readout lines of a control system. A quantum processor includes an ensemble of qubits. Each qubit, or a set of multiple qubits is driven by an output line (also referred to as a drive line, output strip or drive strip) and read by a readout line (also referred to as a readout strip, input strip or input line). Each output and readout strip includes a variable amplifier and an attenuator to condition a signal provided to or received from the qubit (or set of qubits). The output line also includes a digital to analogue converter (DAC) while the readout line includes a corresponding analogue to digital converter (ADC) so that the generation and processing of the signal can be handled on a suitable digital controller, such as a field programmable gate array (FPGA) or application specific integrated circuit (ASIC). The qubits, amplifiers, attenuators, low and high pass filters, mixers, transmission lines, DAC, and ADC are all variable due to manufacturing variability, operating temperature, frequency and other factors. When operations are performed on a qubit, a calibration stage is performed that allows this variability to be taken into account. In qubit control, the power provided to a qubit is changed by changing one or more of a DAC input code, gain of the amplifier and attenuation of the attenuator on the output line. In qubit readout the operation of the ADC is changed by changing the gain of the amplifier and attenuation of the attenuator on the readout line. A configuration is settled on by sweeping one or more of the different parameters to obtain a desired relative power to provide to the qubit on the output line and to optimise operation of the ADC on the readout line. For small qubit ensembles, the time taken to perform the sweep over the parameter space for each output line and readout line is not prohibitively time consuming on the overall duration of the operation. However, for large numbers of qubits, the calibration time is impractical. There is, therefore, a need to accommodate manufacturing variability for large qubit ensembles. According to a first aspect, there is provided a control system arranged to control qubits in a quantum processor, the control system having: one or more output lines, each arranged to condition a driver signal and provide the conditioned driver signal to a set of one or more qubits, each output line having one or more driver signal conditioning components; a memory having lookup tables comprising settings of the driver signal conditioning components for each output line; and one or more processors arranged to: for at least some of the one or more output lines, receive an input indicating a desired absolute signal power to be provided to the set of qubits; retrieve the settings of the driver signal conditioning components; and apply the retrieved settings to the driver signal conditioning components. The driver signal conditioning components may include one or more of: a variable gain amplifier; and a programmable attenuator, and wherein the settings comprise a gain of the amplifier and / or an attenuation of the programmable attenuator. The driver signal conditioning components may further include one or more of: a mixer; a tuneable filter; a passive filter; and an RF power detector. The lookup table may store the settings of the driver signal conditioning components in association with different frequencies of the driver signal and the temperature. The lookup table may store the settings of the driver signal conditioning components in association with different frequencies and powers of the driver signal and the temperature. If the desired absolute signal power to be provided to the set of qubits does not match an entry in the lookup table, the settings of the driver signal conditioning components may be interpolated from the lookup table. Each output line may include a digital to analogue converter, DAC, coupled to the input of the amplifier and attenuator. The DAC may be operated at full deflection. At least one of the output lines may include a signal power detector on the output to the set of qubits. The one or more processors may be arranged to modify the settings of the driver signal conditioning components based on the measured signal power in a closed control feedback loop. The closed control feedback loop may be used at regular intervals. The one or more processors may be arranged to: update the lookup tables based on the modification to the settings in the closed control feedback loop; and use the updated lookup tables in open loop control between instances of use of the closed control feedback loop. The one or more output lines may be held in a temperature-controlled environment, colder than room temperature. The qubits may be held at colder temperatures than the output lines. The control system may further have one or more readout lines, each arranged to receive a read signal from a set of one or more qubits, condition the read signal and provide the conditioned read signal for processing, each readout line having one or more readout signal conditioning components. The lookup table may comprise settings of the readout signal conditioning components for each readout line. The one or more processors may be further arranged to: for at least some of the readout lines, receive an input indicating an expected absolute signal power received from the set of qubits; retrieve the settings of the readout signal conditioning components corresponding to the received input; and apply the retrieved settings to the readout signal conditioning components. The set of qubits from which the read signal is received may be the same as the set of qubits to which the conditioned driver signal is provided. Each readout line may include an analogue to digital converter, ADC, coupled to the output of the amplifier and attenuator. The lookup table may store the setting of the readout signal conditioning components and measured ADC codes in association with different frequencies and powers of the signal received from the sets of qubits and the temperature. The settings may be selected such that the ADC operates at full deflection. If the absolute signal power measured from the set of qubits does not match an entry- in the lookup table, the settings of the readout signal conditioning components may be interpolated from the lookup table. At least one of the one or more readout lines may include a signal power detector on the input from the set of qubits. The one or more processors may be arranged to modify the settings of the readout signal conditioning components based on the measured signal power in a closed control feedback loop. The readout lines may be held in a temperature-controlled environment, colder than room temperature. The qubits may be held at colder temperatures than the readout lines. Each set of qubits may comprise a single qubit. According to a second aspect, there is provided a method of operating qubits in a quantum processor, the method comprising: generating a driver signal for one or more sets of qubits, each set having one or more qubits; performing one or more operations with the one or more sets of qubits, caused by the driver signal(s); and reading out a signal from each set of qubits, wherein, each generated driver signal is conditioned by driver signal conditioning components before transmission to the one or more sets of qubits, the method further comprising: for at least some of the one or more sets of qubits, receiving an input indicating a desired absolute signal power to be provided to the set of qubits; retrieving settings of the driver signal conditioning components corresponding to the received input from lookup tables; and applying the retrieved settings to the driver signal conditioning components. The driver signal conditioning components may include one or more of: a variable gain amplifier; and a programmable attenuator, and wherein the settings comprise a gain of the amplifier and / or an attenuation of the programmable attenuator. At least one of the output lines may include a signal power detector on the output to the qubit. The method may further comprise: modifying the settings of the driver conditioning components based on the measured signal power in a closed control feedback loop. The closed control feedback loop may be used at regular intervals. The method may comprise: updating the lookup tables based on the modification to the settings in the closed control feedback loop; and using the updated lookup tables in open loop control between instances of use of the closed control feedback loop. According to a third aspect, there is provided a method of calibrating an output line of a qubit control system, the output line configured to provide a driver signal to a set of one or more qubits, the method comprising: setting and / or measuring a temperature of the output line as a first temperature; controlling a digital to analogue converter, DAC, to provide a driver signal at a first frequency; setting an attenuator configured to attenuate the driver signal to have a first attenuation and / or setting an amplifier configured to attenuate the driver signal to have a first gain; measuring the signal power of the driver signal after it is amplified and / or attenuated, and recording the measured power in association with the first temperature, first frequency, first gain and / or first attenuation; and independently varying one or more of the temperature, frequency, attenuation level and amplification level and recording the signal power measured at each combination of temperature, frequency, attenuation level and / or amplification level. According to a fourth aspect, there is provided a method of calibrating a readout line of a qubit control system, the readout line configured to read a measurement signal from a set of one or more qubits, the method comprising: setting and / or measuring a temperature of the readout line; controlling a signal generator to provide an input signal having a first power and first frequency; setting an attenuator configured to attenuate the input signal to have a first attenuation and / or setting an amplifier configured to attenuate the input signal to have a first gain; measuring the input signal using an analogue to digital converter, ADC, after it is amplified and / or attenuated, and recording the codes output by the ADC in association with the first temperature, first frequency, first power, first attenuation and / or first gain. According to a fifth aspect, there is provided a control system arranged to control qubits in a quantum processor, the control system having: one or more output lines, each arranged to condition a driver signal and provide the conditioned driver signal to a set of one or more qubits, each output line having one or more driver signal conditioning components; on at least some of the outline lines, a signal power detector on the output to the set of qubits; and one or more processors arranged to, for the at least some of the one or more output lines: receive an input indicating a desired absolute signal power to be provided to the set of qubits; set the driver signal conditioning components to first settings based on the desired absolute signal power; receive an input indicating a measured power of the output line from the signal power detector; and modify the first settings of the driver signal conditioning components based on a difference between the desired absolute power and the measured power in a closed control feedback loop. The control system may further have one or more readout lines, each arranged to receive a read signal from a set of qubits, condition the read signal and provide the conditioned read signal for processing, each readout line having one or more readout signal conditioning components. On at least some of the readout lines, a signal power detector may be provided on the input from the set of qubits. The one or more processors may be further arranged to, for the at least some of the one or more readout lines: set the readout signal conditioning components to first settings based on an expected received power; receive an input indicative of a measured power of the signal received from the set of qubits by the signal power detector; and modify the first settings of the readout signal conditioning components based on a difference between the expected received power and the measured power in a closed control feedback loop. According to a sixth aspect, there is provided a control system arranged to control qubits in a quantum processor, the control system having: one or more output lines, each arranged to condition a driver signal and provide the conditioned driver signal to a set of one or more qubits, each output line having one or more driver signal conditioning component; a memory; and one or more processors arranged to calibrate at least some of the output lines by: setting and / or measuring a temperature of the output line as a first temperature; controlling a digital to analogue converter, DAC, to provide a driver signal at a first frequency; setting an attenuator configured to attenuate the driver signal to have a first attenuation and / or setting an amplifier configured to attenuate the driver signal to have a first gain; measuring the signal power of the driver signal after it is amplified and / or attenuated, and recording the measured power in association with the first temperature, first frequency, first gain and / or first attenuation; and independently varying one or more of the temperature, frequency, attenuation level and amplification level and recording the signal power measured at each combination of temperature, frequency, attenuation level and / or amplification level. According to a seventh aspect, there is provided a control system arranged to read a measurement from a set of one or more qubits in a quantum processor, the control system having: one or more readout lines, each arranged to receive a read signal from a set of one or more qubits, condition the read signal and provide the conditioned read signal for processing, each readout line having one or more readout signal conditioning component; a memory; and one or more processors arranged to calibrate at least some of the readout lines by: setting and / or measuring a temperature of the readout line; controlling a signal generator to provide an input signal having a first power and first frequency; setting an attenuator configured to attenuate the input signal to have a first attenuation and / or setting an amplifier configured to attenuate the input signal to have a first gain; measuring the input signal using an analogue to digital converter, ADC, after it is amplified and / or attenuated, and recording the codes output by the ADC in association with the first temperature, first frequency, first power, first attenuation and / or first gain. According to an eighth aspect, there is provided a method of operating qubits in a quantum processor, the method comprising: receiving an input indicating a desired absolute signal power of a driver signal to be provided to a set of one or more qubits; setting driver signal conditioning components arranged to generate the driver signal to first settings based on the desired absolute signal power; receiving an input indicating a measured power of the generated driver signal from a signal power detector; and modifying the first settings of the driver signal conditioning components based on a difference between the desired absolute power and the measured power in a closed control feedback loop. The method may further comprise: receiving a readout signal from the set of one or more qubits; setting readout signal conditioning components arranged to conditioning the readout signal to first settings based on an expected received power; receiving an input indicative of a measured power of the readout signal received from the set of qubits by a signal power detector; and modifying the first settings of the readout signal conditioning components based on a difference between the expected received power and the measured power in a closed control feedback loop. According to a ninth aspect, there is provided a control system arranged to read a measurement from a set of one or more qubits in a quantum processor, the control system having: one or more readout lines, each arranged to receive a read signal from a set of one or more qubits, condition the read signal and provide the conditioned read signal for processing, each readout line having one or more readout signal conditioning component; a memory having lookup tables comprising settings of the readout signal conditioning components for each output line; and one or more processors arranged to: for at least some of the readout lines, receive an input indicating an expected absolute signal power received from the set of qubits; retrieve the settings of the readout signal conditioning components corresponding to the received input; and apply the retrieved settings to the readout signal conditioning components. Each readout line may include an analogue to digital converter, ADC, coupled to the output of the amplifier and attenuator. The lookup table may store the setting of the readout signal conditioning components and measured ADC codes in association with different frequencies and powers of the signal received from the sets of qubits and the temperature. The settings may be selected such that the ADC operates at full deflection. If the absolute signal power measured from the set of qubits does not match an entry in the lookup table, the settings of the readout signal conditioning components may be interpolated from the lookup table. At least one of the one or more readout lines may include a signal power detector on the input from the set of qubits. The one or more processors may be arranged to modify the settings of the readout signal conditioning components based on the measured signal power in a closed control feedback loop. The readout lines may be held in a temperature-controlled environment, colder than room temperature. The qubits may be held at colder temperatures than the readout lines. According to a tenth aspect, there is provided a method of reading measurements from a set of one or more qubits in a quantum processor, the method comprising: generating a driver signal for one or more sets of qubits, each set having one or more qubits; performing one or more operations with the one or more sets of qubits, caused by the driver signal(s); and reading out a signal from each set of qubits, wherein, each read signal is conditioned by readout signal conditioning components after receipt from the one or more sets of qubits, the method further comprising: receiving an input indicating an expected absolute signal power received from the set of qubits; retrieving settings of the readout signal conditioning components corresponding to the received input; and applying the retrieved settings to the readout signal conditioning components. The retrieved setting may include measured ADC codes in association with different frequencies and powers of the signal received from the sets of qubits and the temperature. The settings may be selected such that the ADC operates at full deflection. The method may further include, measuring an absolute power of the received signal; and modifying the settings of the readout signal conditioning components based on the measured signal power in a closed control feedback loop. According to an eleventh aspect, there is provided a control system arranged to read a measurement from a set of one or more qubits in a quantum processor, the control system having: one or more readout lines, each arranged to receive a read signal from a set of one or more qubits, condition the read signal and provide the conditioned read signal for processing, each readout line having one or more readout signal conditioning component; on at least some of the readout lines, a signal power detector on the input from the set of qubits; and one or more processors arranged to, for the at least some of the one or more readout lines: set the readout signal conditioning components to first settings based on an expected received power; receive an input indicative of a measured power of the signal received from the set of qubits by the signal power detector; and modify the first settings of the readout signal conditioning components based on a difference between the expected received power and the measured power in a closed control feedback loop. According to a twelfth aspect, there is provided a method of reading a measurement from a set of one or more qubits in a quantum processor, the method comprising: receiving an input indicating an expected absolute signal power of a read signal to be received from a set of one or more qubits; setting readout signal conditioning components arranged to condition the read signal to first settings based on the expected absolute signal power; receiving an input indicative of a measured power of the signal received from the set of qubits by the signal power detector; and modifying the first settings of the readout signal conditioning components based on a difference between the expected received power and the measured power in a closed control feedback loop. According to further aspects of the invention, there is provided non-transitory computer readable medium comprising instructions which, when executed by a processor, cause the processor to perform the steps of any one or more of the second, third, fourth, eighth, tenth and twelfth aspects. By using the look-up tables (which can be populated at the time of control system manufacture), a known absolute power (i.e. the power expressed in an SI unit such as Watts or Joules / second) can be set and provided to the qubits of the quantum processor. Furthermore, the qubit calibration of any operation only needs to include calibration of elements downstream of the controller. Therefore, the parameter space to be swept is significantly reduced, reducing the calibration time. Furthermore, setting an absolute power and / or measuring the power received from the qubit allows the ADC in the readout line to be operated using the full width of the output bits, maximising the signal to noise ratio. The calibration accounts for variation in manufacturing of the output and / or readout lines, and operating conditions such as temperature, frequency, input and output parameters and the like. The use of a closed control loop allows for any drift in the input or output power to be accounted for. The closed control loop is more time and resource intensive whilst performing a complete quantum circuit, and so this is used less frequently than the open control loop. The qubits may be held in a low temperature environment, such as a milliKelvin environment. The output line and readout line may be provided at a temperature staged between the qubits and room temperature. For example, the output line and readout line may be held at cryogenic temperatures, for example at 4.2K. This allows some multiplexing from the control electronics held at room temperature, which simplifies connections from room temperature into the temperature-controlled system (e.g., a cryostat). It will be appreciated that features disclosed in relation to a particular aspect or embodiment may be applied mutatis mutandis to any other aspect or embodiment. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 schematically illustrates a control system for controlling qubit operation; Figure 2A illustrates the output line of the system of Figure 1, configured for open loop operation; Figure 2B illustrates the readout line of the system of Figure 1, configured for open loop operation; Figure 3A illustrates a method of controlling the output line of Figure 2A; Figure 3B illustrates a method of controlling the readout line of Figure 2B; Figure 4 illustrates a method of controlling a qubit operation using the system of Figure 1; Figure 5A illustrates a method of calibrating the output line of Figure 2A Figure 5B illustrates a method of calibrating the readout line of Figure 2B; Figure 6A illustrates the output line of the system of a control system, configured for closed loop operation; and Figure 6B illustrates the readout line of the system of a control system, configured for closed loop operation. Figure 1 schematically illustrates a control system 1 having a first portion implemented in a temperature-controlled environment 3, such as a cryostat, dilution refrigerator or the like, and a second portion implemented at room temperature, used to control and readout the first portion. The temperature-controlled environment 3 is generally held at cryogenic temperatures and has a first region 3a held at a first temperature and second region 3b held at a second temperature colder than the first region 3a. For example, the first region 3a may be held at 4.2K, whilst the second region 3b may be held at milliKelvin temperatures. In the lower temperature region 3b of the temperature-controlled environment 3, a quantum device 5 is provided. For the sake of illustration, the quantum device 5 is taken to be a single qubit, including a resonator and the structure necessary to control and readout the qubit 5. It will be appreciated that this is only by way of example and multiple qubits 5 and other structures may be provided. Within the warmer region 3a of the temperature-controlled environment 3 an output line 7 is provided to send a driver (or drive / control / probe) signal to the qubit 5 and a readout line 9 is provided to receive the readout signal from the qubit 5 after the qubit operation is performed. The driver signal is generated by an external control unit 11 outside the temperature-controlled environment 3, for example at room temperature. The readout signal is processed by the same unit 11. In one example, the external control unit 11 may be implemented by a field programmable gate array (FPGA). The control unit 11 controls operation of the output line 7, readout line 9 and thus the quantum device 5. Figure 2A illustrates an example of an output line 7 in more detail. In this example, the output line 7 includes a digital to analogue converter (DAC) 13 arranged to generate the driver signal based on an input code provided by the external control unit 11. The signal generated by the DAC 13 is attenuated by a programmable attenuator 15, and then amplified by a variable gain amplifier 17. The DAC 13, attenuator 15 and amplifier 17 are arranged in series. The attenuation applied by the attenuator 15 and the gain applied by the amplifier 17 are both variable under control of the external control unit 11. The amplitude of the DAC 13 and input codes for the DAC 13 are also provided by the external control unit 11. Figure 2B illustrates an example of a readout line 9 in more detail. In this example, the readout line includes a programable attenuator 19 to attenuate the signal received from the qubit 5. The attenuated signal is amplified by a variable gain amplifier 21 and converted to a digital signal by an analogue to digital converter (ADC) 23. As with the output line 7, the attenuator 19, amplifier 21 and ADC 23 are arranged in series and the attenuation applied by the attenuator 19 and the gain applied by the amplifier 21 are both variable under control of the external control unit 11. The amplitude of the ADC 23 is also provided by the external control unit 11. In order to perform an operation on the qubit 5, a driver signal of fixed power, frequency and duration is provided on the output line 7, and the signal from the qubit 5 is read by the readout line 9. Methods 100, 200 for operating the output line 7 and readout line 9 are shown in Figures 3A and 3B respectively. The gain of both amplifiers 17, 21 and the attenuation of both attenuators 15, 19 is set by the external control unit 11. The code input to the DAC 13 is also provided by the external control unit 11, as are the operating voltages of both the DAC 13 and ADC 23. The code output from the ADC 23 is provided to the external control unit 11 to give a reading of the measured signal from the qubit 5. The external control unit 11 includes at least a memory 25, a processor 27, such as a digital signal processor (DSP) and an interface or input / output unit 29 which allows for user input or connection to the control unit 11, as shown in Figure 1. The interface unit 29 may include input and output devices such as a keyboard, a monitor, a touchscreen and the like to allow for direct input to and monitoring of the control unit 11. Instead of or as well as the input and output devices, the interface unit 29 may provide a wired and / or wireless connection to allow for remote access, for example over Wi-Fi, 3G, 4G, 5G, the internet, Bluetooth, Local Area Networks, Wide area Networks and the like. The memory 25, DSP 27 and input / output unit 29 are all in communication with each other over a suitable communication bus 31. The memory 25 has a programme storage portion 33 and a data storage portion 35. The programme storage portion 33 includes various software modules 37a-f, 39, 41 which contain instructions that, when performed on the DSP 27 cause the external control unit 11 to perform various functions. The modules in the programme storage portion 33 include drivers 37a-f for controlling operation of the amplifiers 17, 21 and the attenuators 15, 19, the DAC 13 and ADC 23 of the output line 7 and readout line 9. The programme storage portion 33 also includes an output module 39 for controlling operation of the output line 7 and a readout module 41 for controlling operation of the readout line 9. Further modules (not shown) may be provided to allow for control of qubit operations. The different modules 37a-f, 39, 41 may call other modules where necessary and may retrieve data stored in the data storage portion 35 of the memory 25, as will be discussed below. For example, the output module 39 and readout module 41 may call the drivers 37a-f to apply the settings to configure and control the different components of the output line 7 and readout line 9. As shown in Figure 3A, the method 100 of controlling the output line 7 includes a first step 102, in which an input specifying the desired frequency and RF power of the signal to be provided to the qubit 5 is received. In one example, this may be received as a direct input from a user through the interface unit 29. Alternatively, the desired frequency and absolute power may be received from another software module on the external control unit 11 or from a remote system over the interface unit 29. In one example, the software of the external control unit 11 may provide users or programmers with a function, for example “set_drive_power( )”, with variables corresponding to the frequency and power that can be used in a programme to call the functions required to control the output line 7. The data storage portion 35 includes a first lookup table 43 for the output line 7. The first lookup table 43 includes a series of data entries corresponding to the gain, attenuation and DAC settings (DAC voltage, input codes) required to achieve different absolute powers and frequencies of driver signals at the operating temperature of the output line 7. In one example, it is assumed that the DAC settings are chosen such that the DAC 13 operates at full scale deflection, but this is by way of example only. The temperature of the output line 7 is determined either by direct measurement of the first region 3a of the temperature-controlled environment 3 or is provided from a source (for example another software module). In a second step 104 of the method 100 of controlling the output line 7. the first lookup table 43 is used to determine the gain, attenuation and DAC settings required to provide a signal with the desired RF power and frequency, at the determined temperature. Where the first lookup table 43 includes an entry corresponding exactly to the desired power and frequency at the determined temperature, the gain, attenuation and DAC settings from that entry are directly used. Where the lookup table does not include an entry corresponding exactly to the desired power and frequency at the determined temperature, a number of different methods may be used to determine the gain, attenuation and DAC settings. In one example, the first lookup table 43 may be searched to determine the closest matching entry, and this entry may be used. When looking for the closest matching entry, this may be considered as the entry with the smallest deviation in desired power, frequency and determined temperature. In some cases, power, frequency and temperature may be differently weighted, such that a variation in temperature, for example, is more acceptable than a variation in power. In other examples, the data in the first lookup table 43 may be used to interpolate the gain, attenuation and DAC settings to achieve the desired absolute power and frequency at the determined temperature. In a final step 106 of the method 100 of controlling the output line 7 the gain, attenuation and DAC settings determined form the first lookup table 43 are applied to the DAC 13, attenuator 15 and amplifier 17. The method 200 of controlling the readout line 9 includes a first step 202, in which the frequency and RF power of the signal received from the qubit 5 are determined. The frequency and RF power of the signal received from the qubit 5 may be known to the external control unit 11 based on the combination of the absolute power and frequency of the signal input to the qubit 5, the known characteristics of the qubit 5 (for example from a qubit calibration operation) and the calibration of the readout line 9. This information may be provided or retrieved from another module within the control unit 11. In one example, the software of the external control unit 11 may provide users with a function, for example “set^inputjower( )” with variables corresponding to the frequency and power. The function can be used in a programme to call the other functions required to control the readout line 9. The data storage portion 35 includes a second lookup table 45 for the readout line 9. The second lookup table 45 includes a series of data entries corresponding to the gain, attenuation and ADC settings (ADC voltage) required to operate the ADC 23 at full scale deflection for the given absolute input power at the operating temperature of the readout line 9. The readout line 9 is operating in the same region 3a of the temperature-controlled environment 3 as the output line 7. Therefore, in one example the same temperature is assumed for the readout line 9 and output line 7. In other examples, the temperature of the readout line 9 may be determined separately. This may be in similar ways to how the temperature of the output line 7 is determined. Separate sensors may be used to determine the temperature of the output line 7 and readout line 9 to account for temperature variations in the region 3a where they are located. In a similar fashion to the method 100 of controlling the input line 7, the method 200 of controlling the readout line 9, includes a second step 204, in which the second lookup table 45 is used to determine the gain, attenuation and ADC settings required to allow the ADC 23 to operate at full scale deflection, at the determined temperature. Where the second lookup table 45 includes an entry corresponding exactly to the desired power and frequency at the determined temperature, the gain, attenuation and ADC settings from that entry are directly used. Where the second lookup table 45 does not include an entry corresponding exactly to the desired power and frequency at the determined temperature, a number of different methods may be used to determine the gain, attenuation and ADC settings. In one example, the second lookup table 45 may be searched to determine the closest matching entry, and this entry may be used. When looking for the closest matching entry, this may be considered as the entry with the smallest deviation in desired power, frequency and determined temperature. In some cases, power, frequency and temperature may be differently weighted, such that a variation in temperature, for example, is more acceptable than a variation in power. In other examples, the data in the second lookup table 45 may be used to interpolate the gain, attenuation and DAC settings to achieve the desired power and frequency at the determined temperature. In a final step 206 of the method 200 of controlling the readout line 9 the gain, attenuation and ADC 23 settings determined form the second lookup table 45 are applied to the ADC 23, attenuator 19 and amplifier 21. By way of illustration only, Figure 4 illustrates a method of 300 of performing a qubit operation in the system 1 shown in Figure 1. In a first step 302, the gain, attenuation, DAC and ADC settings for the output line 7 and readout line 9 are determined and set based on the desired characteristics of the qubit operation. These are determined using the methods 100, 200 discussed above. In a second step 304, the driver signal is generated and transmitted by the output line 7. In a third step 306, the qubit operation is performed. In a fourth step 308, the signal is read out on the readout line 9. It will be appreciated that the read signal can be then further processed as necessary. In order to generate the first and second lookup tables 43, 45 the output line 7 and readout line 9 require calibration. Figures 5A and 5B show methods 150, 250 of calibrating the output line 7 and readout line 9 respectively. The calibration 150, 250 generally occurs at the time of manufacture, although it can be repeated if necessary. To calibrate the output line 7 an RF power meter is used. The RF power meter is connected on the output 47 of the output line 7, in place of the qubit 5. For example, the RF power meter is connected at the point the output line 7 connects into the lower temperature region 3b of the temperature-controlled environment 3. In one example, the RF power meter may have a calibration traceable to a national standards laboratory, such as the UK National Physical Laboratory (NPL), the German Physikalisch-Technische Bundesanstalt (PTB) or the US National Institute of Standards and Technology (NIST). It will be appreciated that in other examples, any other suitably calibrated or traceable power meter may be used. The method 150 of calibrating the output line 7 includes a first step 152 of setting and / or measuring a first temperature of the output line 7. At a second step 154, the DAC 13 is set to operate at a first frequency, operating at full scale deflection. At subsequent steps 156, 158, a first gain and a first attention are set at the attenuator 15 and amplifier 17. At a next step 160, the RF power of the signal generated with the first frequency, first gain and first attenuation, at the first temperature, is measured by the RF power meter as a first RF power. At a next step, 162, the first frequency, first gain, first attenuation, and first temperature are recorded in association with the first power, as an entry in the first lookup table 43. The method 150 is repeated 164 as the frequency, gain, attenuation, and temperature are independently varied. Each parameter is swept over a range between a minimum and a maximum, creating a sweep over a four-dimensional parameter space. Separate entries are created in the lookup table for each individual combination of the four parameters. Optionally, where the output line 7 includes filters (not shown) the cut off of the filter may also be varies as a fifth parameter. To calibrate the readout line 9, an external signal generator is used. The signal generator is placed at the input 49 of the readout line 9, in place of the qubit 5. In one example, the signal generator may have a calibration traceable to a national standards laboratory, such as the NPL, PTB or NIST. It will be appreciated that in other examples, any other suitably calibrated or traceable signal generator may be used. The method 250 of calibrating the readout line 9 includes a first step 252 of setting and / or measuring a first temperature of the readout line 9. At a second step 254 and third step 256, a first gain and a first attenuation are set at the attenuator 19 and amplifier 21. At a fourth step 258, the signal generator is controlled to generate a signal at a first frequency and first power. At a next step 260, the codes output from the ADC 23 with the first frequency, first power, first gain and first attenuation, at the first temperature, are measured as first ADC codes. At a subsequent step, 262, the first frequency, first power, first gain, first attenuation, and first temperature are recorded and associated with the first ADC codes as an entry in the second lookup table 45. The method 250 is repeated 264 as the frequency, power, gain, amplification, and temperature are independently varied. Each parameter is swept over a range between a minimum and a maximum, creating a sweep over a five-dimensional parameter space. Separate entries are created in the lookup table for each individual combination of the five parameters. In the embodiments discussed above, the output line 7 and readout line 9 are operated in an open loop mode. In this mode, the output line 7 and readout line 9 are controlled based only on the lookup tables 43, 45. In an alternative embodiment, a feedback loop may be used to provide closed loop control. Figures 6A and 6B show the output line 7’ and readout line 9’ respectively, for implementing closed loop control. The output line 7’ and readout line 9’ shown in Figures 6A and 6B are the same as the output line 7 and readout line 9 shown in Figures 2A and 2B, unless stated otherwise. In the output line 7’ used for closed loop control, an RF detector 51 is provided in a branch, at the output 47 of the output line 7’. The RF detector measures the power of the driver signal after amplification and attenuation. A proportional-integral-derivative (PID) controller 53 is provided to fine tune the gain and attenuation in the output line 7’, and the DAC settings to ensure the RF power of the signal from the output line 7’ matches the desired power. Similarly, in the readout line 9' used for closed loop control, an RF detector 55 is provided in a branch at the input 49 of the readout line 9’. The RF detector measures the power of the signal received from the qubit 5. A PID controller 57 is provided in the readout line 9’, to fine tune the gain and attenuation in the readout line 9', and the ADC settings to ensure the power reaching the ADC enables the ADC to operate at full scale deflection for the received signal. The use of the closed loop control ensures any drift from the status when the output line 7’ and readout line 9' were calibrated can be accounted for. It will be appreciated that use of the closed loop control is still faster than a full qubit calibration. This is because even with some drift, the parameters from the lookup tables 43, 45 will approximate the required gains, attenuations, and ADC and DAC settings. Therefore, the parameter space searched by the PID controllers 53, 57 is smaller than the space searched in a qubit calibration. It will be appreciated that while the output line 7 and readout line 9 shown in Figures 2A and 2B can only be operated in the open loop control, the output line 7’ and readout line 9’ shown in Figures 6A and 6B can be operated in closed loop control (where the detectors 53, 57 are used to fine tune the gains, attenuations, and ADC and DAC settings) and open loop control, where the gains, attenuations, and ADC and DAC settings are set only based on the lookup tables 43, 45. In at least some embodiments, the modifications made by the PID controllers 53, 57 in closed loop control may be recorded, such that they can be applied in open loop control. For example, the closed loop control may be repeated on a regular basis, with open loop control used between instances of closed loop control. For example, the open loop control may be used at the start of a day, or a set of operations or at any other regularity. In all the embodiments described above, the quantum processing device is a single qubit 5. It will be appreciated that this has limited usefulness. However, it will also be appreciated that the output lines 7, 7’ and readout lines 9, 9’ discussed above are readily scalable to any number of qubits 5 or other quantum devices. For example, the quantum processing device may include twenty qubits or more. In this case, the functions for controlling the output line 7, 7’ and readout line 9, 9’ may include a further variable identifying which lines are to be controlled by the function. In one case, each qubit 5 may have a dedicated output line 7 and readout line 9. Each output line 7 and readout line 9 is calibrated separately as discussed above, with a separate lookup table for each line. In other cases, each output line 7 and readout line 9 may control a set of qubits 5, arranged in series or parallel. It will be appreciated that where a set of qubits 5 is addressed by a single output line 7 (that drives the resonator of the qubits 5), the same qubits 5 may be addressed by a single readout line 9. Alternatively, where a set of qubits 5 is addressed by a single output line 7, the qubits in the set may be read by two or more readout lines 9, each reading one or more of the qubits 5. Similarly, where a set of qubits 5 is read by a single readout line 9, the qubits 5 in the set may be addressed by two or more output lines 7. In the examples discussed above, both the output line(s) 7, 7’ and readout line(s) 9, 9’ are controlled using the lookup tables 43, 45. However, this is by way of example only. In some cases, only the output lines 7, 7’ may be controlled in this way, and in other cases only the readout lines 9, 9’ may be controlled in this way. Furthermore, in some cases, some qubits 5 (or sets of qubits) may have only the output lines 7. 7’ controlled in this way, some qubits 5 (or sets of qubits) may have only the readout lines 9, 9’ controlled in this way, and some qubits 5 (or sets of qubits) may have both lines 7, 7’, 9, 9’ controlled in this way. In a single device, there may be any combination of different qubits 5 (or sets of qubits) have only the output line 7, 7’ controlled and / or only the readout line 9, 9' controlled and / or both lines 7, 7’, 9, 9' controlled. In the example discussed above, the input line 7, 7’ includes a DAC 13, an attenuator 15 and an amplifier 17, and the readout line 9, 9’ includes an attenuator 19, an amplifier 21 and an ADC 23. These are just examples of signal conditioning components that may be used to condition the signals on the output line 7, 7’ and readout line 9, 9’. In some cases, the qubit readout may be achieved by different techniques to the readout lines 9 discussed above. For example, optical camera based readout technologies or other suitable technologies may be used. The readout may be within any suitable region 3a, 3b of the temperature controlled environment, or outside it. Where different readout techniques are used, the output lines 7 only may be controlled by the lookup tables 43, 45 as discussed above. In some examples, the attenuator 15, 19 or amplifier 17, 21 may be omitted from one or both lines. 7, 7’, 9, 9’. Other signal conditioning components may also be included in one or both lines. For example, the line 7, 7’, 9, 9’ may include mixers, tuneable filters, passive filters, RF power detectors and the like. These may be formed of separate components and / or by combinations of inductors / capacitors / resistors, These additional components may be variable / programmable based on settings, or fixed. Where signal conditioning components on either line are variable / programmable based on settings, the settings may be incorporated into the lookup table and controlled based on the methods discussed above. It will be appreciated that where an output line 7, 7’ or readout line 9, 9’ includes multiple variable / programmable signal conditioning components, all the parameters may be varied during the calibration operation 150, 250. Alternatively, only a subset of the parameters may be varied. Any one or more parameters may be included in the subset that is varied. For example, in an output line 7. 7’ or readout line 9, 9’ including a variable amplifier 17, 21 and a variable attenuator 15, 19, the gain of the amplifier 17, 21 may be fixed and some or all of the other parameters varied. In the examples discussed above, the readout line 9, 9’ mirrors the output line 7. 7‘. This need not be the case. Furthermore, where a larger ensemble is controlled, the output lines 7, 7' and readout lines 9, 9’ may be different for different qubits 5 or sets of qubits. This may include both the signal conditioning components, and also the ability to use closed loop control. In the examples discussed above, the signal is attenuated and then amplified. This is by way of example only, the order of attenuation and amplification may be swapped, and additional signal conditioning components may be provided between attenuation and amplification, before attenuation and amplification or after attenuation and amplification. In the examples discussed above, the DAC 13 and ADC 23 are operated at full scale deflection. This is not necessarily always the case, and the DACs 13 and / or ADCs 23 may be operated away from full scale deflection. The external control unit 11 discussed above is given by way of example only. Any suitable type of processor may be used instead of a DSP. Furthermore, the implementation on a FPGA is only by way of example. Functions of the control unit 11, including but not limited to the memory 25 and DSP 27 may be distributed across a number of entities including but not limited to being distributed across the cloud. Any type of non-volatile memory may be used for the long-term storage of the lookup tables 43, 45 and other information. In the examples discussed above, software drivers 37a-f are used to apply the settings to the signal conditioning components. This is by way of example only, and any suitable driver may be used. The connectivity for the control of the signal conditioning components may be through the same connection as the output signal and readout signal into the temperature-controlled environment 3, or by a separate connection. On the example discussed above, the qubit(s) 5 are in the lowest temperature region of the temperature-controlled environment, and the output line 7, 7’ and readout line 9, 9’ are in the warmer region 3b. It will be appreciated that the temperature-controlled environment may have two or more different temperature regions. Furthermore, the output line 7, 7’ and readout line 9, 9’ may be distributed across two or more of the different regions 3a, 3b, or even within the room temperature environment outside the temperature-controlled environment 3. This may mean that different components / parts of the lines are at different temperatures. This will provide further variables to be includes in the lookup tables 43, 45. Depending on the implementation of the qubits 5 and other components, cryogenic temperatures may not be necessary. In some cases, all components may even be at room temperature. In one example, the PID controllers 53, 57 are provided as part of the output line 7, 7’ and readout line 9, 9’. This is by way of example only. The PID controllers 53, 57 may be part of the external control unit 11 or may be provided elsewhere. Furthermore, the use of PID controllers 53, 57 is only by way of example. Various different methods and controllers may be used to fine tune the settings of the signal conditioning components in the closed loop control. During the calibration of the output line 7 and readout line 9, the gain of the amplifiers and attenuation of the attenuators may be continuously variable or stepped, in which case the gain and / or attenuation are varied in discrete steps. When using stepped amplifiers and / or attenuators, the sweep in the calibration may use each individual step or may vary by multiple steps at once. Other parameters to be calibrated may be varied in any suitable way over any suitable range. The ranges over which the parameters of the output line 7, 7’ and readout line 9, 9’ are varied depends on the desired accuracy, the physical setup of the qubits 5 and the intended operation. For example, the frequency may be swept over a range defined by the frequency ranges of the qubit resonators. Other parameters may be calibrated over other suitable ranges. In the example discussed above, the signals are RF power signals. However, this is by way of example only. The qubits 5 may be operated by signals of any suitable frequency, including, but not limited to optical signals, RF signals, or electromagnetic signals at any other suitable wavelength. Where the signals are detected (for example in the closed loop control), suitable detectors may be provided, such as photodiodes or similar. In one example embodiment, the qubit(s) 5 are superconducting qubits. Typically, resonators for readout of superconducting qubits have resonant frequencies in the range of 4 GHz to 12 GHz. These frequencies are by way of example only, and superconducting qubits may have higher or lower resonant frequencies. Whilst a superconducting qubit is given by way of example, the control system 1 discussed above can be used for driving and readout of any type of qubit or resonator at any frequency range.

Claims

1. A control system arranged to control qubits in a quantum processor, the control system having:one or more output lines, each arranged to condition a driver signal and provide the conditioned driver signal to a set of one or more qubits, each output line having one or more driver signal conditioning components;a memory having lookup tables comprising settings of the driver signal conditioning components for each output line; andone or more processors arranged to:for at least some of the one or more output lines, receive an input indicating a desired absolute signal power to be provided to the set of qubits;retrieve the settings of the driver signal conditioning components; andapply the retrieved settings to the driver signal conditioning components.

2. The control system of claim 1, wherein the driver signal conditioning components include one or more of: a variable gain amplifier; and a programmable attenuator, and wherein the settings comprise a gain of the amplifier and / or an attenuation of the programmable attenuator.

3. The control system of claim 2, wherein the driver signal conditioning components further include one or more of: a mixer; a tuneable filter; a passive filter; and an RF power detector.

4. The control system of any preceding claim, wherein the lookup table stores the settings of the driver signal conditioning components in association with different frequencies of the driver signal and the temperature.

5. The control system of claim 4, wherein the lookup table stores the settings of the driver signal conditioning components in association with different frequencies and powers of the driver signal and the temperature.

6. The control system of any preceding claim, wherein if the desired absolute signal power to be provided to the set of qubits does not match an entry in the lookup table, the settings of the driver signal conditioning components are interpolated from the lookup table.

7. The control system of any preceding claim, wherein each output line includes adigital to analogue converter, DAC, coupled to the input of the amplifier and attenuator, wherein the DAC is operated at full deflection.

8. The control system of any preceding claim, whereinat least one of the output lines includes a signal power detector on the output to the set of qubits;wherein the one or more processors are arranged to modify the settings of the driver signal conditioning components based on the measured signal power in a closed control feedback loop.

9. The control system of claim 8, wherein the closed control feedback loop is used at regular intervals, and wherein the one or more processors are arranged to: update the lookup tables based on the modification to the settings in the closed control feedback loop; and use the updated lookup tables in open loop control between instances of use of the closed control feedback loop.

10. The control system of any preceding claim, wherein the one or more output lines are held in a temperature-controlled environment, colder than room temperature, optionally wherein the qubits are held at colder temperatures than the output lines.

11. The control system of any preceding claim, further having:one or more readout lines, each arranged to receive a read signal from a set of one or more qubits, condition the read signal and provide the conditioned read signal for processing, each readout line having one or more readout signal conditioning components,wherein the lookup table comprises settings of the readout signal conditioning components for each readout line,wherein the one or more processors are further arranged to:for at least some of the readout lines, receive an input indicating an expected absolute signal power received from the set of qubits; retrieve the settings of the readout signal conditioning components corresponding to the received input; andapply the retrieved settings to the readout signal conditioning components.

12. The control system of claim 11, wherein the set of qubits from which the read signal is received is the same as the set of qubits to which the conditioned driver signal is provided.

13. The control system of claim 11 or claim 12, wherein each readout line includes an analogue to digital converter, ADC, coupled to the output of the amplifier and attenuator and the lookup table stores the setting of the readout signal conditioning components and measured ADC codes in association with different frequencies and powers of the signal received from the sets of qubits and the temperature, preferably wherein the settings are selected such that the ADC operates at full deflection.

14. The control system of any of claims 11 to 13, wherein if the absolute signal power measured from the set of qubits does not match an entry in the lookup table, the settings of the readout signal conditioning components are interpolated from the lookup table.

15. The control system of any of claims 11 to 14, whereinat least one of the one or more readout lines includes a signal power detector on the input from the set of qubits,wherein the one or more processors are arranged to modify the settings of the readout signal conditioning components based on the measured signal power in a closed control feedback loop.

16. The control system of any of claims 11 to 15, wherein the readout lines are held in a temperature-controlled environment, colder than room temperature,preferably wherein the qubits are held at colder temperatures than the readout lines.

17. The control system of any preceding claim, wherein each set of qubits comprises a single qubit.

18. A method of operating qubits in a quantum processor, the method comprising: generating a driver signal for one or more sets of qubits, each set having one or more qubits;performing one or more operations with the one or more sets of qubits, caused by the driver signal(s); andreading out a signal from each set of qubits,wherein, each generated driver signal is conditioned by driver signal conditioning components before transmission to the one or more sets of qubits, the method further comprising:for at least some of the one or more sets of qubits, receiving an input indicating a desired absolute signal power to be provided to the set of qubits;retrieving settings of the driver signal conditioning components corresponding to the received input from lookup tables; andapplying the retrieved settings to the driver signal conditioning components.

19. The method of claim 18, wherein the driver signal conditioning components include one or more of: a variable gain amplifier; and a programmable attenuator, and wherein the settings comprise a gain of the amplifier and / or an attenuation of the programmable attenuator.

20. The method of claim 18 or claim 19, wherein at least one of the output lines includes a signal power detector on the output to the qubit, the method further comprising:modifying the settings of the driver conditioning components based on the measured signal power in a closed control feedback loop.

21. The method of claim 20, wherein the closed control feedback loop is used at regular intervals, the method comprising:updating the lookup tables based on the modification to the settings in the closed control feedback loop; andusing the updated lookup tables in open loop control between instances of use of the closed control feedback loop.

22. A method of calibrating an output line of a qubit control system, the output line configured to provide a driver signal to a set of one or more qubits, the method comprising:setting and / or measuring a temperature of the output line as a first temperature;controlling a digital to analogue converter, DAC, to provide a driver signal at a first frequency;setting an attenuator configured to attenuate the driver signal to have a first attenuation and / or setting an amplifier configured to attenuate the driver signal to have a first gain;measuring the signal power of the driver signal after it is amplified and / or attenuated, and recording the measured power in association with the first temperature, first frequency, first gain and / or first attenuation; andindependently varying one or more of the temperature, frequency, attenuation level and amplification level and recording the signal power measured at each combination of temperature, frequency, attenuation level and / or amplification level.

23. A method of calibrating a readout line of a qubit control system, the readout line configured to read a measurement signal from a set of one or more qubits, the method comprising:setting and / or measuring a temperature of the readout line;controlling a signal generator to provide an input signal having a first power and first frequency;setting an attenuator configured to attenuate the input signal to have a first attenuation and / or setting an amplifier configured to attenuate the input signal to have a first gain;measuring the input signal using an analogue to digital converter, ADC, after it is amplified and / or attenuated, and recording the codes output by the ADC in association with the first temperature, first frequency, first power, first attenuation and / or first gain.

24. A control system arranged to control qubits in a quantum processor, the control system having:one or more output lines, each arranged to condition a driver signal and provide the conditioned driver signal to a set of one or more qubits, each output line having one or more driver signal conditioning components;on at least some of the outline lines, a signal power detector on the output to the set of qubits; andone or more processors arranged to, for the at least some of the one or more output lines:receive an input indicating a desired absolute signal power to be provided to the set of qubits;set the driver signal conditioning components to first settings based on the desired absolute signal power;receive an input indicating a measured power of the output line from the signal power detector; andmodify the first settings of the driver signal conditioning components based on a difference between the desired absolute power and the measured power in a closed control feedback loop.

25. The control system of claim 24, further having:one or more readout lines, each arranged to receive a read signal from a set of qubits, condition the read signal and provide the conditioned read signal for processing, each readout line having one or more readout signal conditioning components; andon at least some of the readout lines, a signal power detector on the input from the set of qubits,wherein the one or more processors are further arranged to, for the at least some of the one or more readout lines:set the readout signal conditioning components to first settings based on an expected received power;receive an input indicative of a measured power of the signal received from the set of qubits by the signal power detector; and modify the first settings of the readout signal conditioning components based on a difference between the expected received5 power and the measured power in a closed control feedback loop.

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