Electronic signal sensing apparatus and method for quantum computing system

Abstract

An electronic signal sensing apparatus for a quantum computing system, the apparatus being configured to receive a current signal from an electronic measurement device of the quantum computing system. The apparatus comprises a superconductor material sample configured such that the received current signal passes through the superconductor material sample; a current source configured to pass a base current through the superconductor material sample, wherein the base current is less than a critical current of the superconductor material sample; and a sensor configured to detect a phase transition of the superconductor material sample from a first state, which is a superconducting state, to a second state that occurs when the total current through the superconductor material sample increases from below the critical current to above the critical current.

Description

[0001] ELECTRONIC SIGNAL SENSING APPARATUS AND METHOD FOR QUANTUM COMPUTING SYSTEM

[0002] FIELD OF THE INVENTION

[0003] The invention relates to apparatus and methods for sensing signals produced by electronic devices in quantum computing systems, in particular those produced by spin measurement devices such as single-electron transistors and singleelectron boxes.

[0004] BACKGROUND

[0005] In quantum computing systems, electronic devices are used to perform measurements of the physical properties of the system, its constituent parts and its surroundings. For example, the spins of qudits in a quantum processor may be read using devices such as single electron transistors (SETs) and single electron boxes (SEBs), which perform spin to charge conversion to convert the spin of a carrier into a measurable current, the value of which depends on the spin of the carrier.

[0006] One manner of measuring the current signal produced by spin measurement devices such as SETs and SEBs which has been attempted previously involves measuring the properties of a resonant circuit. The SET or SEB is electrically connected to an LC resonator circuit, which comprises a superconducting element or elements (typically in this approach, the inductor and / or capacitor in the LC resonator are made of superconductor material). The state of the SET or SEB depends on the spin state that it is measuring, and the state of the SET or SEB affects its admittance. The admittance of the SET or SEB affects the behaviour of the resonant circuit such that a change in state of the SET or SEB typically results in a change in the resonant frequency and / or reflection coefficient of the LC resonator. For example, the resistance of an SET or quantum point contact changes in dependence on its state, thereby causing the proportion of a test signal that is reflected by the LC resonator (i.e. its reflection coefficient) to vary. In the case of an SEB, its different states exhibit difference capacitance values, which affect the resonant frequency of the LC resonator.

[0007] An example of this is shown in Fig. 1 , which is a plot of the real part of the reflection coefficient, F, of an LC resonator of the kind just described as a function of frequency, fc, while the connected spin measurement device is in a first state (dashed line 101 , “Off”) and in a second state (solid line 102, “On”). The reflection coefficient, F, is the ratio of the reflected voltage to the supplied voltage and is a complex number. The real part represents the ratio of the amplitudes of the reflected and supplied voltages while the imaginary part represents the phase difference between the reflected and supplied voltages. The resonant frequency is the frequency value at which the magnitude of the reflection coefficient F is lowest, which in both of the cases shown in Fig. 1 is about 329 MHz, though the resonant frequency when the spin measurement device is in the “On” state (i.e. that of line 102) is slight lower than when the current is absent (line 101). The reflection coefficient gradually increases to 1 (representing complete reflection of the electrical signal) either side of the resonant frequency. To detect the current signal, the reflection coefficient is monitored at a constant frequency 103 near the resonant frequency using an RF probe which transmits an AC test signal of the frequency 103 to the LC circuit and measures the proportion of the signal that is returned. When the state of the spin measurement device changes, a change in the reflection coefficient at the frequency 103 is observed (due to the change in admittance of the spin measurement device), whereby the current signal is detected. This approach to detecting the states of spin measurement devices is considered to be advantageous because measurements of the reflection coefficient enable very fast detection of the change in state. However, a drawback of this approach is that the change in reflection coefficient produced by the kinds of current signals generated by typical devices in a quantum computing system is small. Consequently, the sensitivity of this method is limited and it is susceptible to erroneous measurements. This ultimately contributes to the error rate suffered by computations performed by the quantum computing system. There is hence a need for an improved technique for sensing electrical signals in quantum computing systems. SUMMARY OF THE INVENTION

[0008] A first aspect of the invention provides an electronic signal sensing apparatus for a quantum computing system, the apparatus being configured to receive a current signal from an electronic measurement device of the quantum computing system, wherein the apparatus comprises: a superconductor material sample configured such that the received current signal passes through the superconductor material sample; a current source configured to pass a base current through the superconductor material sample, wherein the base current is less than a critical current of the superconductor material sample; and a sensor configured to detect a phase transition of the superconductor material sample from a first state, which is a superconducting state, to a second state that occurs when the total current through the superconductor material sample increases from below the critical current to above the critical current.

[0009] A superconductor material sample that is in the superconducting state can tolerate carrying only a limited electrical current before its superconductivity breaks down. The maximum current that the sample can sustain before undergoing this phase transition is referred to as its critical current, lc. The critical current is a property of the individual superconductor material sample (such that different samples of the same superconductor material with different dimensions will generally have different critical current values) and depends on the temperature of the superconductor material sample and the external magnetic field to which the sample is subjected. (Since the dependence of the critical current of the superconductor material on the applied magnetic field can be known, in some implementations, an applied magnetic field may be used to adjust the critical current to a desired value, e.g. to reduce the critical current, in turn reducing the magnitude of the required base current). When the current through a superconductor material which is initially in the superconducting state exceeds the critical current, the superconductor material undergoes a phase transition into a second state. In the case of type-l superconductors, the second state is typically a purely non-superconducting state, in which the material behaves as a conventional conductor with a non-zero resistivity. In the case of type-l I superconductor materials, the second state is typically an intermediate phase in which some parts (or “islands”) of the sample continue to have zero resistance while other parts acquire a non-zero resistivity.

[0010] In the electronic signal sensing apparatus defined above, a base current is supplied to the superconductor material sample. The value of the base current is selected such that, when the current signal which the apparatus is configured to detect passes through the superconductor material sample while the base current is also present, the critical current of the superconductor material sample is exceeded. This causes the superconductor material sample to undergo the phase transition from the first, superconducting, state to a second state. Such a phase change is associated with dramatic changes in the electrical properties of the superconductor material sample, in particular its resistance (which is effectively zero when in the first, superconducting state but non-zero in at least some parts of the sample when in the second state). This significant change in the electrical properties of the sample can be detected more easily than the comparatively small change in resonant frequency that occurs in the known technique described above, so the apparatus defined above enables measurement of electronic signals in a quantum computing system with a reduced error rate. For example, the electronic measurement device could be a spin measurement device such as an SET with two states (corresponding to different values of the measured spin) which each result in a different respective current being output by the SET, typically such that the higher current is associated with a state of interest such as an excited state. When the SET detects a change in the measured spin, the output current increases, resulting in the total current through the superconductor material sample (the base current plus the current output by the SET) exceeding the critical current of the superconductor material sample, thus causing the change in state of the superconductor material sample.

[0011] In preferred embodiments, the superconductor material sample is configured as an inductor and is electrically connected to a capacitor such that the superconductor material sample and the capacitor together form an LC resonator, whereby the LC resonator has a first reflection coefficient when the superconductor material sample is in the first state and a second reflection coefficient when the superconductor material sample is in the second state, and wherein the sensor is configured to detect the phase transition based on the change in the reflection coefficient that occurs when the superconductor material sample undergoes the phase transition. The different reflection coefficients exhibited in the first state and the second state (i.e. the first reflection coefficient and the second reflection coefficient) are a result of the impedance of the LC resonator being different in the two respective states. The capacitor may be any source of capacitance in the apparatus which in combination with the superconductor material behaves as an LC resonator - for example, in some implementations, this capacitance could be provided by parasitic capacitances associated with other elements of the apparatus rather than a dedicated capacitor component.

[0012] This configuration uses a change in the behaviour of the LC resonator formed by the superconductor material sample and the capacitor to enable detection of the phase transition, which is advantageous because typically changes in the reflection coefficient of the LC resonator in which the superconductor material sample is incorporated can be measured more quickly than direct measurements of the electrical properties of the spin measurement device can be performed. The slowness of direct measurements of the spin measurement device is a consequence of the fact that these devices have high impedances (typically greater than 25 kQ), which makes direct measurements of their properties very inefficient. Typically, the sensor will detect the phase transition by delivering a test signal, which is typically an AC test signal, to the LC resonator and measuring the proportion of the test signal that is reflected by the LC resonator. As explained above, the reflection coefficient represents the proportion of an electrical signal which is reflected by the LC resonator when received by the LC resonator (and any phase difference between the reflected and supplied signals). The reflection coefficient is generally dependent on the frequency of the test signal, with the reflection coefficient having a minimum value at one frequency, which is the resonant frequency of the LC resonator, and increasing gradually to a value of 1 (which indicates complete reflection of the signal) at frequencies either side of the resonant frequency. Typically, the first reflection coefficient and the second reflection coefficient which are measured by the sensor are both at the same frequency, e.g. the frequency of an AC test signal delivered to the LC resonator by the sensor.

[0013] Other approaches to detecting the phase transition are possible. For example, the voltage across the superconductor material sample could be monitored. In such embodiments, the voltage is preferably monitored using a four point measurement configuration, in which a current is passed through the superconductor material sample between two current terminals and the voltage dropped across the superconductor material sample is sensed using two separate voltage terminals, which are connected to the ends of the superconductor material sample between the current terminals (such that any voltage drop across the points of contact formed by the current terminals are not measured). The voltage will be about zero as long as the superconductor material is in the first, superconducting state but will increase significantly when the phase transition occurs. However, the approach based on measuring the reflection coefficient of an LC resonator which incorporates the superconductor material sample is preferred because of the improved measurement speeds that it achieves.

[0014] In the first aspect of the invention, the base current is preferably at least 90% of the critical current of the superconductor material sample, preferably at least 95%.

[0015] A second aspect of the invention an electronic signal sensing apparatus for a quantum computing system , the apparatus being configured to receive a current signal from an electronic measurement device of the quantum computing system, wherein the apparatus comprises: a superconductor material sample configured such that the received current signal passes through the superconductor material sample; a current source configured to pass a base current through the superconductor material sample, wherein the base current is less than a critical current of the superconductor material sample; and a sensor configured to detect a change in the kinetic inductance of the superconductor material sample that occurs when the current through the superconductor material sample changes while the superconductor material is in a superconducting state. In this aspect of the invention, the sensor is configured to detect a change in the kinetic inductance of the superconductor material sample (rather than a phase transition, as in the first aspect). The “kinetic inductance” of the superconductor material sample arises due to the inertia of the charge carriers, which causes them to tend to resist changes in the flow of current through the sample. In most ordinary conductors, the effect of kinetic inductance is negligible in relation to other forms of impedance. In superconductors, however, the absence of any resistance makes the effect of kinetic inductance significant.

[0016] The kinetic inductance, Lk, of a superconductor material sample depends on its critical current, lc, and the instantaneous current, I, through the sample as follows:

[0017] Lk(l) = Lk(0)[1+(l / lc)2+...], where Lk(0) is the kinetic inductance of the sample in the absence of any current and lc is the critical current of the sample. Thanks to this dependence on current, when the current signal passes through the superconductor material sample, the kinetic inductance of the sample changes. Detecting this change of kinetic inductance (e.g. as a change in the resonant frequency of the circuit in which the superconductor material sample is incorporated, as will be described below) indicates that the current signal has passed through the superconductor material sample.

[0018] It will be noted that the dependence of Lkon the instantaneous current I goes as (l / lc)2. Therefore, the change in kinetic inductance produced by the current signal will be greatest when I is near lc. The provision of a base current thus increases the sensitivity of the kinetic inductance to the current signal relative to what would be observed if no base current were provided.

[0019] In preferred embodiments of the second aspect, the superconductor material sample is configured as an inductor and is electrically connected to a capacitor such that the superconductor material sample and the capacitor together form an LC resonator, whereby, when the superconductor material sample is in the superconducting state, the reflection coefficient of the LC resonator varies in dependence on the total current through the superconductor material sample, and wherein the sensor is configured to detect the change in kinetic inductance of the superconductor material sample based on the change in the reflection coefficient that occurs when the current signal passes through the superconductor material sample. Since the resonant frequency of the LC resonator depends on its inductance, the resonant frequency will change when the current signal passes through the superconductor material sample (causing the kinetic inductance of the sample to change). The current signal may thus be detected by sensing a change in the resonant frequency, e.g. by measuring a change in the reflection coefficient of the LC resonator at a fixed frequency.

[0020] It will be appreciated that alternative ways of detecting the change in kinetic inductance are possible. For example, an LCR meter or bridge circuit could be used. As a further alternative, the superconductor material sample could be placed in series with a known reference resistor. The voltage across the resistor or the superconductor sample would then be monitored. The monitored voltage would vary as a function of the frequency of the applied signal, which will characteristically depend on the ratio between the impedances of the superconductor material sample and the reference resistor. In this approach, the point at which the voltage across the reference resistor is half the input voltage may be used as a reference point, for example.

[0021] In the second aspect of the invention, the base current is preferably at least 85% of the critical current of the superconductor material sample, preferably at least 90%.

[0022] Preferred features that may be employed in implementations of each of the first aspect and the second aspect will now be discussed.

[0023] Preferably, the superconductor material sample comprises a type-ll superconductor material. Whereas type-l superconductor materials typically undergo a phase transition directly from the superconducting state to the resistive state, type-ll superconductor exhibit a “mixed” phase between the purely superconducting and purely resistive states, in which “islands” of the sample remain superconductive while other parts become resistive. Type-ll superconductors are preferred because they are capable of remaining superconducting in the presence of external magnetic fields, which is advantageous since in many implementations a magnetic field will need to be applied to a spin (e.g. qubit) being measured by the electrical device (e.g. a spin measurement device such as an SEB or SET). Furthermore, type-ll superconductors are capable of exhibiting low critical currents, which is particularly beneficial in the first aspect of the invention because the required phase transition can be induced by a low current, which minimises the amount of resistive heating experienced by the sample once after the phase transition has occurred. This in turn minimises thermal noise in the system and ensures that the temperature of the system is as uniform and steady as possible. NbTiN, NbTi, TiN and granular aluminium are examples of type-ll superconductor materials.

[0024] As noted above, preferably, the current source is a direct current, DC, current source. This is advantageous as a DC current does not produce significant magnetic noise and also enables the current through the superconductor material sample to be kept at a single, known value at all times in the absence of the current signal from the electronic measurement device. However, in principle any form of current which keeps the current through the superconductor material sample close enough to the critical current that the phase change would be induced (in the first aspect) by the current signal output by the electronic measurement device (e.g. an AC current with a DC offset which keeps it near the critical current of the superconductor material sample at all times).

[0025] Preferably the apparatus further comprises one or more inductors electrically connected to the superconductor material sample and configured such that the base current passes through the or each inductor, wherein preferably the one or more inductors comprise a first inductor electrically connected to a first end of the superconductor material sample and a second inductor electrically connected to a second end of the superconductor material sample. This configuration facilitates the delivery of the base current to the superconductor material sample and further promotes the stability and uniformity of the current through the superconductor material sample and helps to exclude electrical noise from outside sources. This reduces the risk of the phase transition and / or unwanted changes in the kinetic inductance of the sample erroneously being induced by current noise, thereby reducing the frequency of erroneous detections of the current signal.

[0026] Advantageously, the sensor may comprise a probe configured to deliver a test signal to the superconductor material sample and sense the proportion of the test signal that is reflected. This implementation is particularly beneficial when the superconductor material sample is configured as an inductor in an LC resonator, as discussed above, since this probe can be used to sense a change in the reflection coefficient of the LC resonator and hence detect the phase transition (in the first aspect) or the change in kinetic inductance (in the second aspect).

[0027] The invention also provides a spin measurement apparatus for a quantum computing system, the spin measurement device comprising: the electronic signal sensing apparatus of the first aspect or the second aspect; and an electronic spin measurement device configured to perform a spin to charge conversion on a qudit of the quantum computing system, thereby generating a current dependent on the spin of the qudit, and pass the generated current through the superconductor material sample.

[0028] This spin measurement apparatus thus uses the electronic signal sensing apparatus of the first aspect or the second aspect defined above for detecting a spin measurement in the quantum computing system. For example, the electronic spin measurement device may be a single-electron transistor (SET), a singleelectron box (SEB) or a quantum point contact, which are devices suitable for performing spin to charge conversions. The term “qudit” here means any quantum system capable of carrying information in a quantum computing system, e.g. a qubit.

[0029] The invention also provides a quantum computing chip comprising the spin measurement apparatus defined above, wherein the electronic signal sensing apparatus and the electronic measurement device are provided on the quantum computing chip. The components of the spin measurement apparatus are thus constructed on one quantum computing chip. On-chip constructions are favourable for several reasons such as simplifying the design of the quantum computing system, minimising the size of the quantum computing system, and reducing the distances travelled by electronic signals in the quantum computing system, which reduces noise.

[0030] The invention also provides a method of detecting a current signal from an electronic measurement device in a quantum computing system, the method comprising: passing a base current through a superconductor material sample, wherein the base current is less than the critical current of the superconductor material sample; passing the current signal from the electronic measurement device through the superconductor material sample while the base current is being passed through the superconductor material sample, wherein the output current signal and the base current in combination exceed a critical current of the superconductor material sample, thereby causing the total current through the superconductor material sample to increase from below the critical current of the superconductor material sample to above the critical current of the superconductor material sample, whereby the superconductor material sample undergoes a phase transition from a first state, which is a superconducting state, to a second state; and detecting the phase transition.

[0031] This method may be performed using the electronic signal sensing apparatus of the first aspect or the spin measurement apparatus or quantum computing chip incorporating that apparatus. All the optional and preferred features of the apparatus defined above are applicable to implementations of the method.

[0032] Detecting the phase transition may comprise delivering an electrical test signal to the superconductor material sample and detecting, based on the proportion of the test signal that is reflected, determining that the superconductor material sample is in a non-superconducting state. This may be performed using the probe defined above. When detecting the phase transition based on the proportion of the test signal that is reflected, whether the phase transition has occurred may be judged based on a change of a predetermined size in the proportion that is reflected or based on the proportion reflected passing some absolute threshold value (e.g. being above 90%).

[0033] The method may further comprise, after detecting the phase transition, reducing the base current such that the superconductor material sample returns to the superconducting state. The detection of the current and returning the superconductor material sample to its superconducting state may be performed cyclically, for example in a readout operation in which the spins of many carriers are measured. It is preferred that the base current is sufficiently large that the superconductor material sample does not return to the first, superconducting state after the current signal from the electronic measurement device has gone away: this enables the superconductor material sample to “latch” onto the second state and remain in this state indefinitely, which prolongs the period of time in which the phase transition (and hence the current signal) can be detected.

[0034] The invention also provides a method of detecting a current signal from an electronic measurement device in a quantum computing system, the method comprising: passing a base current through a superconductor material sample, wherein the base current is less than the critical current of the superconductor material sample; passing the current signal from the electronic measurement device through the superconductor material sample while the base current is being passed through the superconductor material sample and the superconductor material sample is in a superconducting state, wherein the output current signal and the base current in combination are less than a critical current of the superconductor material sample such that the superconductor material sample remains in the superconducting state while the current signal is present, thereby causing the total current through the superconductor material sample to change, whereby the kinetic inductance of the superconductor material sample changes; and detecting the change in kinetic inductance.

[0035] This method may be performed using the electronic signal sensing apparatus of the second aspect, or the spin measurement apparatus or quantum computing chip incorporating that apparatus. All the optional and preferred features of the apparatus defined above are applicable to implementations of the method. In the method above, the base current is chosen such that the sum of the base current and the current signal is less than the critical current of the superconductor material sample. This ensures that the superconductor material sample remains in the superconducting state.

[0036] Preferably, detecting the phase transition comprises delivering an electrical test signal to the superconductor material sample and detecting, based on the proportion of the test signal that is reflected, determining that the kinetic inductance of the superconductor material sample has changed. In implementations of the methods defined above, the electronic measurement device is preferably a spin measurement device configured to perform a spin measurement and generate the current signal based on the outcome of the spin measurement, preferably a single-electron transistor, a single-electron box or a quantum point contact.

[0037] BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Fig. 1 is a graph of reflection coefficient of an LC resonator including a superconducting component as a function of the frequency of the applied signal, which illustrates a known technique for detecting the electrical signal generated by a spin measurement device;

[0039] Fig. 2 is a graph of voltage across a sample of superconductor material as a function of applied current, which illustrates the phrase transitions of the superconductor material sample;

[0040] Fig. 3 shows an example of a spin measurement apparatus in accordance with an embodiment of the invention, which may be configured to implement each of the first aspect of the invention and the second aspect of the invention;

[0041] Fig. 4 shows measurements of the reflection coefficient of the LC resonator in the apparatus of Fig. 3, when configured to implement the first aspect of the invention, as a function of the frequency of the applied signal, which illustrates a method of detecting a current signal from an electronic measurement device in a quantum computing system in accordance with an embodiment of the invention; Fig. 5 shows measurements the reflection coefficient of the LC resonator in the apparatus of Fig. 3 as a function of the applied current and the frequency of the applied test signal;

[0042] Fig. 6 shows measurements of the magnitude of the current signal required to trigger detection in the apparatus of Fig. 3, when configured to implement the first aspect of the invention, as a function of the applied base current;

[0043] Fig. 7A illustrates the layout of components in an exemplary implementation of the apparatus of Fig. 3 and Fig. 7B shows a variant of the layout shown in Fig. 7A;

[0044] Fig. 8 illustrates a first method in accordance with an embodiment of the invention;

[0045] Fig. 9 shows another example of a spin measurement apparatus in accordance with an embodiment of the first aspect of the invention;

[0046] Fig. 10 illustrates a second method in accordance with an embodiment of the invention.

[0047] DETAILED DESCRIPTION

[0048] The physical principle which the first aspect of the present invention exploits will be explained with reference to Fig. 2. Then, embodiments of the invention will be described with reference to Fig. 3.

[0049] Fig. 2 shows how the voltage (on the vertical axis, with units of volts, V) measured across a sample of a superconductor material sample varies as a function of the current (on the horizontal axis, with units of microamps, pA) through the superconductor material sample. For the purposes of this example, it is assumed that the superconductor material sample remains below its critical temperature Tcof the superconductor material sample such that it is capable of exhibiting superconductivity.

[0050] Initially, the current is zero and there is correspondingly no voltage across the superconductor material sample. At this point, the superconductor material is in the superconducting state and as such has no resistance. In this scenario, as the current was increased while the sample was in the superconducting state, a gradual, linear increase in the voltage was measured (despite the zero resistance of the sample) - it should be noted that this linear current-voltage measured in the superconducting state was an artefact caused by the measurement setup that was used, which was not a four-point configuration and therefore sensed a small voltage due to the resistance of elements such as the contacts and wires in the system. When the current reaches a critical current value lc, indicated by the numeral 201 , the superconductor material undergoes a phase transition into a second state. This second state may be a purely non-superconducting state, in the case of type-l superconductor materials, but in the case of type-ll superconductor materials may be an “intermediate” state, in which parts of the sample continue to exhibit superconductivity. In either case, the resistance of the sample becomes non-zero once the critical current lchas been exceeded and consequently a rapid increase in the measured voltage occurs. The measured voltage remains large as the current is further increased above lcto about 200 pA. The current is then decreased, but as it returns below the critical current, the superconductor material does not immediately return to the superconducting state. The sample instead only returns to the superconducting state at a lower value (indicated by the numeral 202) than the initial critical current.

[0051] As Fig. 2 shows, the transition from the superconducting state to the second state is associated with a dramatic change in the electrical properties of the superconductor material sample. This change is exploited by the present invention to enable detection of a current signal, as will now be shown with reference to Fig. 3.

[0052] Fig. 3 shows schematically an example of a spin measurement apparatus in accordance with an embodiment of the invention. This spin measurement apparatus includes a single electron transistor (SET) 301 , which is configured to perform a spin to charge conversion on a qudit or other quantum element in a quantum computing system. The other components shown (i.e. those inside the dashed box) form an electronic signal sensing apparatus 302, which is configured to receive the current signal generated by the SET 301 as it performs a spin to charge conversion. Atypical SET 301 contains a quantum dot between its source and drain, the electrical potential of which can be controlled by application of a gate voltage VGto the gate of the SET 301 . The voltage between the source and drain of the SET 301 may be controlled by application of a voltage to a contact 313.

[0053] The SET 301 is configured to output the current I SET generated by the spin to charge conversion to a superconductor material sample 303, which is configured as an inductor with inductance Lres. The superconductor material sample has a first end 303i and a second end 303ii, and SET 301 is electrically connected to the first end 303i such that the current from the SET 301 passes through the superconductor material from the first end 303i to the second 303ii. In use, the superconductor material sample 303 is cooled to a sufficiently low temperature that it is in the superconducting state in the absence of any applied current (and is therefore capable of sustaining a non-zero critical current lc). In other implementations, the SET 301 could be replaced with a different kind of electronic measurement device configured to generate a produce a current signal, such as a single electron box (SEB) or quantum point contact.

[0054] A grounded capacitor 305 with capacitance Cmter is electrically connected to the second end 303ii of the superconductor material sample 303. In this configuration, the parasitic capacitances of the SET 301 and the superconductor material sample 303 and the coupling capacitor 309c form an LC resonator, which is confined by the grounded capacitor 305.

[0055] A first inductor 307a with inductance Lbias is connected to the first end 303i of the superconductor material sample 303. A second inductor 307b, also with inductance Lbias, is connected to the second end 303ii of the superconductor material sample 303. In this case the first inductor 307a and second inductor 307b have the same inductance although in other implementations, the two inductors could have different inductance values. A direct current (DC) current source 311 is configured to pass a constant DC base current lbias through the superconductor material sample 303. The first inductor 307a and second inductor 307b facilitate the provision of the base current I bias to the superconductor material sample 303 and serve to stabilise the base current through the superconductor material sample 303. They also contain high-frequency test signals that are used to probe the behaviour of the LC resonator, as will be described below.

[0056] The value of the base current lbias is less than the critical current of the superconductor material sample but, in the first aspect, is of such a value that the sum of the base current I bias snd the current ISET generated by the SET 301 exceeds the critical current lc. Therefore, while only the base current lbias is applied, the superconductor material sample remains in the superconducting state. Once the SET 301 performs a spin to charge conversion and generates the current ISET, the current through the superconductor material sample exceeds the critical current and the phase transition described above with reference to Fig. 2 occurs. In the second aspect, an implementation of which using the circuit of Fig. 3 will be described below, the value of the base current lbias is chosen such that the base current lbias and the current ISET in combination do not exceed the critical current lc. The current I SET snd the bese current lbias pass out of the LC resonator through a current collector 315.

[0057] In implementations of the first aspect, the phase transition is detected as follows. The apparatus includes a probe 309, which is configured to deliver an AC test signal of a frequency that is equal to or close to the resonant frequency of the LC resonator formed by the superconductor material sample 303 and the capacitor 305 when the superconductor material sample 303 is in the superconducting state. In this implementation, the probe 309 delivers the test signal via a coupling capacitor 309c. The probe 309 also senses the signal reflected by the LC resonator and thereby measures the reflection coefficient of the LC resonator. While the superconductor material sample 303 is in the superconducting state, a significant proportion of the test signal is absorbed by the LC resonator and consequently a relatively low reflection coefficient is measured. When the current ISET from the SET 301 passes through the superconductor material sample 303 and the critical current lcis exceeded, superconductor material sample 303 undergoes the phase transition from the superconducting state to the second state (e.g. an intermediate phase, in the case of a type-ll superconductor, or a completely non-superconducting phase in the case of the type-l superconductor). This causes the superconductor material sample 303 to acquire a non-zero ohmic resistance, which significantly increases the proportion of the test signal which is reflected and therefore raises the reflection coefficient of the LC resonator. This causes a large, easily detectable change in the measured reflection coefficient to occur as the current ISET passes through the superconductor material sample 303. Moreover, because the superconductor material sample 303 does not instantly return to the superconducting state once the current ISET is removed (as explained above with reference to Fig. 2), the change in reflection coefficients persists for a time after the current ISET passes through the superconductor material sample. The persistence of the change in state of the superconductor material sample 303 further enhances the ease of detecting the current ISET.

[0058] The value of the base current I bias may be chosen to be below the current value at which the superconductor material sample returns from the second state to the superconducting state (which in the example shown in Fig. 2 is the current value labelled 202), such that the base current does not need to be removed in order for the superconductor material sample to return to the superconducting state ready for a further measurement to be performed. Alternatively, the base current may have a value greater than the current value at which the superconductor material sample returns from the second state to the superconducting state. In this scenario, the superconductor sample remains in the second state for as long as the base current continues to be applied, which enables the period of time in which the current signal may be detected to be extended indefinitely.

[0059] The components of the spin measurement apparatus of Fig. 3 may be provided on a quantum computing chip. Examples of two manners of forming some of the components on a quantum computing chip will be described below with reference to Figs. 7A and 7B.

[0060] The change in measured reflection coefficient that occurs when the superconductor material sample 303 undergoes the phase transition is illustrated in Fig. 4, which is a graph showing the attenuation of a test signal, in decibels, by the LC resonator of Fig. 3 as a function of frequency. The line 401 shows the attenuation of the signal when the superconductor material is in its superconducting state. A large proportion of each frequency in the vicinity of the resonant frequency, which is about 460 MHz, is absorbed. The line 402 shows the attenuation of the signal after the phase transition, when the superconductor material sample 303 is in the second state. The effect of the increased impedance of the superconductor material sample 303 in the second state is to make the absorption peak extremely broad and shallow such that, at any frequency in the vicinity of the resonant frequency that was exhibited while in the superconducting state, the proportion of the signal that is reflected is now far greater than in the superconducting state.

[0061] Fig. 5 shows more detailed measurements of the behaviour of the LC resonator of Fig. 3 across a range of applied currents. To obtain these measurements, the current through the superconductor material sample 303 was gradually increased from zero to a value well above the critical current lc. The values on the horizontal axis are the voltage applied across the SET (which produced the applied current) and therefore are representative of the current through the superconductor material sample. The SET in this case had a resistance of about 250 kQ, so the current varied between 0 and about 1 V / 250 kQ = 4 pA. At each current value, a test signal was applied and its frequency was swept from a value of about 485 MHz to about 515 MHz. The vertical axis represents the frequency of the applied test signals and the shade at each point inside the graph represents the degree of attenuation of the test signal for each combination of current and frequency (the values of which are shown in decibels by the scale on the right-hand side of the graph, such that darker values indicate more absorption). The dark band which extends from 0 to about 0.5 on the horizontal axis at about 496 MHz on the vertical axis indicates the resonant frequency of the LC resonator while in the superconductor material sample 303 is superconducting state. This dark band ends abruptly at about 0.5 on the horizontal axis, indicating that the critical current was reached at this point. At all current values above this point, the degree of absorption was relatively low, indicating a significant increase in the reflection coefficient at values close to the resonant frequency. Fig. 6 shows a set of measurements which illustrate how the base current lbias supplied by the current source 311 and the current signal ISET combine to create the critical current lcthat induces the phase transition in the superconductor material sample 303. At each of several values of the base current lbias (represented by the horizontal axis) between about 119.5 pA and 120.5 pA, the additional current required to induce the phase transition (plotted on the vertical axis), as determined by observing a large change in reflection coefficient, was measured. The results show a linear, inverse correlation between the base current and the additional current required to cause the phase transition, which confirms that the superconductor material sample exhibited a uniform critical current across the measurements. The measurements contain three outliers, visible in the graph as the three values which lie well below the other data points, which were discounted when fitting the line shown on the graph. These anomalous measurements were attributed to electrical and / or thermal noise causing the phase transition to occur at a lower applied current than expected.

[0062] Fig. 7A shows example of how the superconductor material sample 303, first inductors 307a and second inductor 307b of the schematic of Fig. 3 may be constructed on a two-dimensional chip surface. The superconductor material sample 703 is provided as a thin strip of material which extends along a rectilinear pathway, alternating in a stepped fashion so as to form a plurality of turns which result in the superconductor material sample 703 behaving as an inductor. At the first end of the superconductor material sample 703 there is a first electrical contact 703i and at the second end of the superconductor material sample 703 there is a second electrical contact 703ii. The first electrical contact 703i is electrically connected to a first inductor 707a, which is a two-dimensional coil of conductive material. The first electrical contact 703i is electrically connected to one end of the coil and, at the other end of the coil, there is another electrical contact 723. Similarly, a second inductor 707b, also formed by a two-dimensional coil, is connected to the second electrical contact 703ii at one end and, at its other end, is connected to an electrical contact 724. In use, the base current is passed between the electrical contacts 723, 724 of the inductors, which causes it to pass through the first inductor 707a, the superconductor material sample 703 and the second inductor 707b.

[0063] A further electrical contact 721 is electrically connected to the superconductor material sample 703 for the purpose of receiving the current signal from the SET or other electronic device whose current output is to be measured. Additionally, a further electrical contact 722, also electrically connected to the superconductor material sample 703, is provided for the purpose of receiving test signals from a probe used to measure the reflection coefficient of the LC resonator formed by the superconductor material sample 703 and the associated capacitor.

[0064] In this implementation, the electrical contacts 703i, 703ii, 721 , 722, 723, 724 and inductors 707a, 707b may be made of the same superconductor material as the superconductor material sample or could be made of a different material (superconductor or otherwise), for example gold.

[0065] Fig. 7B shows a second example of how the superconductor material sample 303, first inductors 307a and second inductor 307b of the schematic of Fig. 3 may be constructed on a two-dimensional chip surface. This example is a variant of that shown in Fig. 7A, with the following differences. Relative to Fig 7A, the portion of the superconductor material sample disposed between the contact 703i and the contacts 721 , 722 has been omitted. The area of the contact 703i has been reduced, which has correspondingly reduced the spatial separation between the first inductor 707a and the superconductor material sample 703. This example therefore provides a more physically compact implementation than that of Fig. 7A.

[0066] Fig. 8 illustrates the steps of a method in accordance with an embodiment of the invention, which may be performed using the spin measurement detection apparatus of Fig. 3 and will be described with reference to that apparatus. In a first step S801 , a constant DC base current lbias is supplied through a superconductor material sample 303 using the DC current source 311 . The base current is less than the critical current of the superconductor material sample 303. While the base current is being current passed through the superconductor material sample 303, the superconductor material sample receives in step S801 a current signal from an electronic measurement device - in the case of the Fig. 3 apparatus, the SET 301 - which passes through the superconductor material sample. The current signal ISET from the SET 301 and the base current lbias in combination exceed the critical current lcof the superconductor material sample 303, so the superconductor material sample 303 undergoes a phase transition when the current signal passes through it. The phase transition is detected in step S803 using the RF probe 309 in the manner described previously. A change in the reflection coefficient of the LC resonator formed by the superconductor material sample 303 and the capacitor 305 is observed, confirming the current signal from the SET 301 has passed through the superconductor material sample and thereby indicating the outcome of the spin measurement.

[0067] It will be appreciated that, throughout the method steps, the superconductor material sample S303 is cooled to a temperature sufficiently low that it is in the superconducting phase before the current signal from the electronic measurement device passes through it.

[0068] Fig. 9 shows a second example of a spin measurement apparatus in accordance with an embodiment of the invention. In this spin measurement apparatus, an SET 901 is configured to pass a current ISET, which is generated by a spin to charge conversion performed by the SET, through a superconductor material sample 903 that is part of an electronic signal sensing apparatus 902. The current ISET passes through the superconductor material sample from a first end 903i to a second end 903ii. In this embodiment, the superconductor material sample 903 is typically not in the form of an inductor (as was the case in the Fig. 3 embodiment) but rather has the form of a thin film or wire with a small cross- sectional area in the direction along which the current I SET passes, which ensures its critical current is small (which minimises the current that is required to cause the phase transition, thereby minimising the resistive heating that occurs after the phase transition has occurred).

[0069] Like in the Fig. 3 embodiment, a direct current (DC) current source 911 is configured to pass a constant DC base current lbias through the superconductor material sample 903. A first inductor 907a connected to the first end 903i and a second inductor 907b connected to the second end 903ii are provided to stabilise the current through the superconductor material sample 903. The value of the base current lbias is less than the critical current of the superconductor material sample but is of such a value that the sum of the base current lbias and the current ISET generated by the SET 901 exceeds the critical current lc. Therefore, while only the base current I bias is applied, the superconductor material sample remains in the superconducting state. When the SET 901 performs a spin to charge conversion and the resulting current signal ISET passes through the superconductor material sample 903, the critical current is exceeded and the phase transition described above with reference to Fig. 2 occurs.

[0070] The embodiment of Fig. 9 differs from that of Fig. 3 as to how the phase transition is detected. In this embodiment, the detection is based on the large change in the resistance of the superconductor material sample 903 that occurs as the superconductor material sample 903 undergoes the phase transition. To enable this change in resistance to be detected, a voltage sensor 909 is configured to measure the voltage across the superconductor material sample 903. While the superconductor material sample 903 is in the superconducting state, the voltage sensor measures essentially zero voltage. When the current ISET passes through the superconductor material sample 903, the phase transition occurs, causing the superconductor material sample 903 to develop a non-zero resistance, as a result of which the measured voltage increases significantly in magnitude. The current ISET is thus detected by observing a large, rapid change in the voltage across the superconductor material sample 903.

[0071] The method illustrated in Fig. 8 may be performed using the spin measurement apparatus of Fig. 9. In step S801 , the base current lbias is passed through the superconductor material sample 903 by the DC current source 911. While the base current lbias is applied, the SET 901 performs a spin to charge conversion, thereby generating a current signal I SET which passes through the superconductor material sample 903, causing the critical current of the superconductor material sample 903 to be exceeded. The superconductor material sample thereby undergoes a phase transition from a first, superconducting state to a second state in which it has a non-zero resistance. The phase transition is detected in step S803 by observing a large change in the voltage across the superconductor material sample 903 sensed by the voltage sensor 909. For example, the phase transition may be determined to have taken place when the voltage exceeds a predetermined threshold value.

[0072] In the mode of use of the Fig. 3 apparatus described above with reference to Figs. 4-8, the phase transition of the superconductor material sample was detected based on the change in the proportion of the test signal that was reflected before and after the phase transition. Other ways of detecting the phase transition using the circuit shown in Fig. 3 are possible, however, and one such alternative is as follows. If the capacitance of the coupling capacitor 309c is chosen to be large, such that the impedance, Z, of the LC resonator for the test signal is small relative to the line impedance, Zo, of the probe 309 (which is typically about 50 Q) while the superconductor material sample 303 is in the superconducting state. For example, the capacitance of the coupling capacitor 309c may be such that |Z0| / |Z| is at least 10, preferably at least 100, while the superconductor material sample is in the superconducting state. The LC resonator can be described as “overcoupled” in this configuration. In this situation, while the superconductor material sample is in the superconducting state (where |Z| « |Z0|), the reflection coefficient, F, of the LC resonator has a value of about -1 , since:

[0073] When the superconductor material sample 303 undergoes the phase transition, its resistance becomes large relative to the input impedance of the probe 309. Therefore, after the phase transition, the reflection coefficient becomes:

[0074] The reflection coefficient F therefore goes from a value of about -1 before the phase transition to +1 after the phase transition. In both cases, the proportion of the signal that is reflected is high due to the mismatch between the line impedance of the probe 309 and the impedance of the LC resonator. However, the sign of F (which is a complex number) has changed from -1 to +1 . This is effectively twice the change in the reflection coefficient that was achieved in the previous example, where the LC resonator was configured to produce resonance with the test signal (and hence effectively completely absorb the test signal) while the superconductor material was in the superconducting state (such that in this state |F| ~ 0), as shown by line 401 in Fig. 4, and completely reflect the test signal after the phase transition, as shown by line 402 in Fig. 4. The measurement speed achieved by this approach is a factor of four greater than that achieved by the approach shown in Fig. 4 (since the signal to noise ratio of the measured change is proportional to the square of the reflection coefficient, meaning that the same quality of measurement can be obtained in one quarter of the time relative to the Fig. 4 approach).

[0075] In the description of the Fig. 3 spin measurement apparatus above, it was explained that the value of the base current was chosen such that, when the current signal from the SET passes through the superconductor material sample 303, causing a phase transition from the first, superconducting state to a second state. The probe 309 was thus configured to sense this phase transition. This configuration implements the first aspect of the invention. To implement the second aspect, the current source 311 can be configured to provide a base current with a value such that the sum of the base current and the current ISET is less than the critical current of the superconductor material sample 303 and configuring the probe 309 to detect a change in the kinetic inductance of the superconductor material sample 303 (e.g. by sensing a change in the reflection coefficient of the LC resonator at a fixed frequency when the current ISET passes through the LC resonator). This apparatus may thus be used to implement the method shown in Fig. 10. In this method, a base current is passed through the superconductor material sample in step S1001. While the base current is being passed through the superconductor material sample, the current from the SET 301 is passed through the superconductor material sample. This causes a change in the kinetic inductance of the superconductor material sample 303, which is detected by the probe in step S1003. Since the base current is such that, even when the current ISET is present, the critical current lcof the sample 303 is not exceeded, the sample remains in the superconducting state throughout these steps.

[0076] The invention may be further understood with reference to the following clauses.

[0077] Clause 1. An electronic signal sensing apparatus for a quantum computing system, the apparatus being configured to receive a current signal from an electronic measurement device of the quantum computing system, wherein the apparatus comprises: a superconductor material sample configured such that the received current signal passes through the superconductor material sample; a current source configured to pass a base current through the superconductor material sample, wherein the base current is less than a critical current of the superconductor material sample; and a sensor configured to detect a phase transition of the superconductor material sample from a first state, which is a superconducting state, to a second state that occurs when the total current through the superconductor material sample increases from below the critical current to above the critical current.

[0078] Clause 2. The apparatus of any preceding clause, wherein the superconductor material sample is configured as an inductor and is electrically connected to a capacitor such that the superconductor material sample and the capacitor together form an LC resonator, whereby the LC resonator has a first reflection coefficient when the superconductor material sample is in the first state and a second reflection coefficient when the superconductor material sample is in the second state, and wherein the sensor is configured to detect the phase transition based on the change in the reflection coefficient that occurs when the superconductor material sample undergoes the phase transition.

[0079] Clause 3. An electronic signal sensing apparatus for a quantum computing system, the apparatus being configured to receive a current signal from an electronic measurement device of the quantum computing system, wherein the apparatus comprises: a superconductor material sample configured such that the received current signal passes through the superconductor material sample; a current source configured to pass a base current through the superconductor material sample, wherein the base current is less than a critical current of the superconductor material sample; and a sensor configured to detect a change in the kinetic inductance of the superconductor material sample that occurs when the current through the superconductor material sample changes while the superconductor material is in a superconducting state.

[0080] Clause 4. The electronic signal sensing apparatus of clause 3, wherein the superconductor material sample is configured as an inductor and is electrically connected to a capacitor such that the superconductor material sample and the capacitor together form an LC resonator, whereby, when the superconductor material sample is in the superconducting state, the reflection coefficient of the LC resonator varies in dependence on the total current through the superconductor material sample, and wherein the sensor is configured to detect the change in kinetic inductance of the superconductor material sample based on the change in the reflection coefficient that occurs when the current signal passes through the superconductor material sample.

[0081] Clause 5. The apparatus of any preceding clause, wherein the superconductor material sample comprises a type-ll superconductor material.

[0082] Clause 6. The apparatus of any preceding clause, further comprising one or more inductors electrically connected to the superconductor material sample such that the base current passes through the or each inductor, wherein preferably the one or more inductors comprise a first inductor electrically connected to a first end of the superconductor material sample and a second inductor electrically connected to a second end of the superconductor material sample.

[0083] Clause 7. The apparatus of any preceding clause, wherein the sensor comprises a probe configured to deliver a test signal to the superconductor material sample and sense the proportion of the test signal that is reflected. Clause 8. A spin measurement apparatus for a quantum computing system, the spin measurement device comprising: the electronic signal sensing apparatus of any preceding clause; and an electronic spin measurement device configured to perform a spin to charge conversion on a qudit of the quantum computing system, thereby generating a current dependent on the spin of the qudit, and pass the generated current through the superconductor material sample.

[0084] Clause 9. The apparatus of clause 8, wherein the electronic spin measurement device is a single-electron transistor, a single-electron box or a quantum point contact.

[0085] Clause 10. A quantum computing chip comprising the spin measurement apparatus of clause 8 or clause 9, wherein the electronic signal sensing apparatus and the electronic measurement device are provided on the quantum computing chip.

[0086] Clause 11 . A method of detecting a current signal from an electronic measurement device in a quantum computing system, the method comprising: passing a base current through a superconductor material sample, wherein the base current is less than the critical current of the superconductor material sample; passing the current signal from the electronic measurement device through the superconductor material sample while the base current is being passed through the superconductor material sample, wherein the output current signal and the base current in combination exceed a critical current of the superconductor material sample, thereby causing the total current through the superconductor material sample to increase from below the critical current of the superconductor material sample to above the critical current of the superconductor material sample, whereby the superconductor material sample undergoes a phase transition from a first state, which is a superconducting state, to a second state; and detecting the phase transition. Clause 12. The method of clause 11 , wherein detecting the phase transition comprises delivering an electrical test signal to the superconductor material sample and detecting, based on the proportion of the test signal that is reflected, determining that the superconductor material sample is in a non-superconducting state.

[0087] Clause 13. The method of clause 11 or clause 12, further comprising, after detecting the phase transition, reducing the base current such that the superconductor material sample returns to the superconducting state.

[0088] Clause 14. A method of detecting a current signal from an electronic measurement device in a quantum computing system, the method comprising: passing a base current through a superconductor material sample, wherein the base current is less than the critical current of the superconductor material sample; passing the current signal from the electronic measurement device through the superconductor material sample while the base current is being passed through the superconductor material sample and the superconductor material sample is in a superconducting state, wherein the output current signal and the base current in combination are less than a critical current of the superconductor material sample such that the superconductor material sample remains in the superconducting state while the current signal is present, thereby causing the total current through the superconductor material sample to change, whereby the kinetic inductance of the superconductor material sample changes; and detecting the change in kinetic inductance.

[0089] Clause 15. The method of clause 14, wherein detecting the phase transition comprises delivering an electrical test signal to the superconductor material sample and detecting, based on the proportion of the test signal that is reflected, determining that the kinetic inductance of the superconductor material sample has changed.

Claims

CLAIMS1. An electronic signal sensing apparatus for a quantum computing system, the apparatus being configured to receive a current signal from an electronic measurement device of the quantum computing system, wherein the apparatus comprises: a superconductor material sample configured such that the received current signal passes through the superconductor material sample; a current source configured to pass a base current through the superconductor material sample, wherein the base current is less than a critical current of the superconductor material sample; and a sensor configured to detect a phase transition of the superconductor material sample from a first state, which is a superconducting state, to a second state that occurs when the total current through the superconductor material sample increases from below the critical current to above the critical current; wherein the superconductor material sample is configured as an inductor and is electrically connected to a capacitor such that the superconductor material sample and the capacitor together form an LC resonator, whereby the LC resonator has a first reflection coefficient when the superconductor material sample is in the first state and a second reflection coefficient when the superconductor material sample is in the second state, and wherein the sensor is configured to detect the phase transition based on the change in the reflection coefficient that occurs when the superconductor material sample undergoes the phase transition.

2. An electronic signal sensing apparatus for a quantum computing system, the apparatus being configured to receive a current signal from an electronic measurement device of the quantum computing system, wherein the apparatus comprises: a superconductor material sample configured such that the received current signal passes through the superconductor material sample;a current source configured to pass a base current through the superconductor material sample, wherein the base current is less than a critical current of the superconductor material sample; and a sensor configured to detect a change in the kinetic inductance of the superconductor material sample that occurs when the current through the superconductor material sample changes while the superconductor material is in a superconducting state; wherein the superconductor material sample is configured as an inductor and is electrically connected to a capacitor such that the superconductor material sample and the capacitor together form an LC resonator, whereby, when the superconductor material sample is in the superconducting state, the reflection coefficient of the LC resonator varies in dependence on the total current through the superconductor material sample, and wherein the sensor is configured to detect the change in kinetic inductance of the superconductor material sample based on the change in the reflection coefficient that occurs when the current signal passes through the superconductor material sample.

3. The apparatus of any preceding claim, wherein the superconductor material sample comprises a type-ll superconductor material.

4. The apparatus of any preceding claim, further comprising one or more inductors electrically connected to the superconductor material sample such that the base current passes through the or each inductor, wherein preferably the one or more inductors comprise a first inductor electrically connected to a first end of the superconductor material sample and a second inductor electrically connected to a second end of the superconductor material sample.

5. The apparatus of any preceding claim, wherein the sensor comprises a probe configured to deliver a test signal to the superconductor material sample and sense the proportion of the test signal that is reflected.

6. A spin measurement apparatus for a quantum computing system, the spin measurement device comprising: the electronic signal sensing apparatus of any preceding claim; andan electronic spin measurement device configured to perform a spin to charge conversion on a qudit of the quantum computing system, thereby generating a current dependent on the spin of the qudit, and pass the generated current through the superconductor material sample.

7. The apparatus of claim 6, wherein the electronic spin measurement device is a single-electron transistor, a single-electron box or a quantum point contact.

8. A quantum computing chip comprising the spin measurement apparatus of claim 6 or claim 7, wherein the electronic signal sensing apparatus and the electronic measurement device are provided on the quantum computing chip.

9. A method of detecting a current signal from an electronic measurement device in a quantum computing system, the method comprising: passing a base current through a superconductor material sample, wherein the base current is less than the critical current of the superconductor material sample; passing the current signal from the electronic measurement device through the superconductor material sample while the base current is being passed through the superconductor material sample, wherein the output current signal and the base current in combination exceed a critical current of the superconductor material sample, thereby causing the total current through the superconductor material sample to increase from below the critical current of the superconductor material sample to above the critical current of the superconductor material sample, whereby the superconductor material sample undergoes a phase transition from a first state, which is a superconducting state, to a second state, wherein the superconductor material sample is configured as an inductor and is electrically connected to a capacitor such that the superconductor material sample and the capacitor together form an LC resonator, whereby the LC resonator has a first reflection coefficient when the superconductor material sample is in the first state and a second reflection coefficient when the superconductor material sample is in the second state; anddetecting the phase transition based on the change in the reflection coefficient that occurs when the superconductor material sample undergoes the phase transition.

10. The method of claim 9, wherein detecting the phase transition comprises delivering an electrical test signal to the superconductor material sample and detecting, based on the proportion of the test signal that is reflected, determining that the superconductor material sample is in a non-superconducting state.11 . The method of claim 9 or claim 10, further comprising, after detecting the phase transition, reducing the base current such that the superconductor material sample returns to the superconducting state.

12. A method of detecting a current signal from an electronic measurement device in a quantum computing system, the method comprising: passing a base current through a superconductor material sample, wherein the base current is less than the critical current of the superconductor material sample; passing the current signal from the electronic measurement device through the superconductor material sample while the base current is being passed through the superconductor material sample and the superconductor material sample is in a superconducting state, wherein the output current signal and the base current in combination are less than a critical current of the superconductor material sample such that the superconductor material sample remains in the superconducting state while the current signal is present, thereby causing the total current through the superconductor material sample to change, whereby the kinetic inductance of the superconductor material sample changes, wherein the superconductor material sample is configured as an inductor and is electrically connected to a capacitor such that the superconductor material sample and the capacitor together form an LC resonator, whereby, when the superconductor material sample is in the superconducting state, the reflection coefficient of the LC resonator varies in dependence on the total current through the superconductor material sample; anddetecting the change in kinetic inductance based on the change in the reflection coefficient that occurs when the current signal passes through the superconductor material sample.

13. The method of claim 12, wherein detecting the phase transition comprises delivering an electrical test signal to the superconductor material sample and detecting, based on the proportion of the test signal that is reflected, determining that the kinetic inductance of the superconductor material sample has changed.

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