Cryogenic temperature sensor and measurement method

Abstract

The present specification describes a quantum computing system comprising a superconducting material sample, a cryogenic temperature sensor including a current source configured to pass a current through the superconducting material sample, and a voltage sensor configured to sense a voltage across the superconducting material sample, and a control unit including: controlling the current source to time-vary a current through the superconducting material sample while the voltage sensor senses a voltage across the superconducting material sample; detecting the change in the sensed voltage; and calculating a temperature of the superconducting material sample based on the value of the current upon detection of the change in the sensed voltage.

Description

[Technical Field]

[0001] CRYOGENIC TEMPERATURE SENSOR AND METHOD FOR MEASURING THE TEMPERATURE OF A SUPERCONDUCTING MATERIAL SAMPLE FIELD OF THE INVENTION The present invention relates to a cryogenic temperature sensor and method particularly suited for quantum computing and other cryogenic electronics applications. [Background technology]

[0002] Providing a stable, cryogenic environment is critical to the successful operation of current quantum processors. At temperatures above a few Kelvin (K), the effects on which quantum processors depend for their operation, including the preservation of stable quantum states on quantum bits (qubits) for information storage, rapidly decrease. To control the temperature of cryogenic environments for quantum computing applications, it is important to be able to accurately sense the temperature of these environments, which requires temperature sensors that can measure temperatures close to absolute zero. Summary of the Invention [Problem to be solved by the invention]

[0003] Most current technologies for measuring the temperature of quantum computing systems rely on "off-chip" temperature sensors, i.e., temperature sensors implemented as hardware physically separate from the quantum processor and located alongside the processor in the cryogenic enclosure in which the processor resides. A drawback of this approach is that, because the temperature sensor is physically separate from the processor, it does not directly measure the temperature of the processor itself. This is important because localized heating due to power dissipation in the processor's electronic components can cause the temperature of one part of the processor to deviate significantly from the temperature of the surrounding cryogenic environment. Thus, current technologies are limited in their ability to accurately measure the temperature of quantum processors. Furthermore, known cryogenic temperature measurement techniques, such as Coulomb blockade thermometry, have not been successfully adapted to provide rapid, real-time, and direct measurement of quantum processor temperature and require complex electronics for their implementation. More specifically, standard Coulomb blockade thermometry requires multiple tunnel junctions in series, which requires complex electronic structures for its implementation, and relies on direct current measurements, which limits its speed. While radio frequency Coulomb blockade thermometry techniques have been developed, they rely on complex off-chip electronics such as cryoamplifiers and directional couplers, making them unsuitable for directly measuring the temperature of components on the chip. The present invention aims to provide a method for measuring the temperature of components in a quantum processor that overcomes these limitations. [Means for solving the problem]

[0004] A first aspect of the present invention provides a cryogenic temperature sensor comprising: a superconducting material sample, a current source configured to pass a current through the superconducting material sample, a voltage sensor configured to sense a voltage across the superconducting material sample, and a control unit configured to: control the current source to time-vary a current through the superconducting material sample while the voltage sensor senses a voltage across the superconducting material sample; detect a change in the sensed voltage; and calculate a temperature of the superconducting material sample based on the value of the current upon detection of the change in the sensed voltage.

[0005] This temperature sensor measures the critical current I of a superconducting material sample. c This is the maximum current the sample can carry while remaining in a superconducting (i.e., zero resistance) state. c It should be noted that the value of is specific to a particular sample and is sensitive to its shape and dimensions. While a superconducting material sample is in the superconducting state, its resistance remains zero (hence no potential difference develops across the sample), so changes in current have no effect on the voltage measured across it. However, if the current I increases to I c When the superconducting material sample reaches a critical current, I, the superconducting material sample undergoes a step change in its resistance from zero to a finite, non-zero value. An abrupt change in voltage across the sample, proportional to the current and the resistance of the sample, is then observed. The detected voltage change therefore indicates that the time-varying current has exceeded the critical current at which the superconducting material sample has a non-zero resistance, and the critical current I c Because the temperature dependence of the value of is known, the temperature of the sample can be calculated based on the value of the current at which the change in voltage occurred. For example, calculating the temperature may include comparing the value that the current had at the time the change in voltage was detected with calibration data representing measurements of the critical current as a function of temperature of the superconducting material sample, thereby estimating the temperature of the superconducting material sample. A detailed description of the critical current, its temperature dependence, and examples of methods for calculating the temperature based thereon are provided below.

[0006] As will be explained with reference to the following examples, the temperature sensor defined above requires only relatively simple electronics for its implementation and can be constructed entirely "on-chip" as part of the chip containing the quantum processor (and indeed, in some preferred embodiments, components are located on the quantum computing chip, although it will be understood that some or all components, such as the control unit, may be off-chip). Thus, the temperature sensor can be placed very close to the components of the processor whose temperature is of interest, thereby achieving significantly more accurate temperature measurements than current technology with a less complex structure. Furthermore, the critical current can be repeatedly measured and output at high frequencies, allowing for rapid, real-time monitoring of temperature.

[0007] While the term "cryogenic" herein has its conventional meaning, i.e., temperatures below 120 K, it will be understood that embodiments of the cryogenic temperature sensor of the present invention may not necessarily be capable of measuring temperatures over the entire cryogenic temperature range up to 120 K. For example, some embodiments of the present invention may be suitable for sensing temperatures up to 1.2 K, which is a suitable range for many quantum computing applications.

[0008] The current source may, in a preferred embodiment, comprise a current digital-to-analog converter (IDAC), which can be controlled to generate a time-varying current of a desired profile based on a digital input, and thus can be conveniently controlled by a digital control unit to provide the required time-varying current.

[0009] In a preferred embodiment, the voltage sensor includes an amplifier configured to amplify the voltage across the superconducting material sample. This is because, in many implementations, the current I cBecause the voltage change detected when the critical current is reached is small (e.g., on the order of 100 μV), amplifying the magnitude of the voltage change improves detection. For example, the amplifier can be a negative-feedback transimpedance amplifier in which the superconducting material sample is arranged as a negative-feedback resistor. Furthermore, amplifiers (e.g., those in a negative-feedback transimpedance amplifier configuration, particularly operational amplifiers in such a configuration) can generate a voltage output, which is useful for monitoring the behavior of the superconducting material sample. Furthermore, in a particularly preferred embodiment, the voltage sensor further includes a comparator circuit (most preferably a Schmitt trigger) configured to receive the amplified voltage as an input and output a digital signal whose value changes in response to a change in voltage that occurs when the time-varying current exceeds the critical current, and the control unit detects the voltage change based on the change in the digital signal. In the Schmitt trigger example, the voltage comparator changes its binary output value in response to a sufficiently large change in the input voltage. Providing a digital (e.g., binary) output is useful because the critical current (and therefore the temperature) can be inferred from the value of the current at the time the digital signal changes.

[0010] Preferably, the current source and voltage sensor each include a pair of electrodes in electrical communication with the superconducting material sample, with the current through the superconducting material sample flowing between the electrodes of the current source and the sensed voltage being sensed between the electrodes of the voltage sensor; the electrodes of the voltage sensor being disposed between the electrodes of the current source. This configuration may be described as a "four-point" measurement setup and is advantageous because the portion of the circuit where the voltage is measured (i.e., between the locations of the two electrodes of the voltage sensor) does not include the electrodes of the current sensor, and as a result, the measured voltage does not include the voltage dropped at the point where the electrodes of the current sensor contact the circuit.

[0011] Preferably, the cryogenic temperature sensor further includes a magnetic field sensor arranged to measure the strength of a magnetic field at the location of the superconducting material sample, and the temperature calculation is further based on the strength of the magnetic field. Critical currents typically vary depending on the strength of the magnetic field experienced by the superconducting material sample, and in many quantum computing applications, the magnetic field in which the quantum processor is located varies over time. Therefore, taking the magnetic field strength into account for the temperature calculation improves the accuracy of the temperature measurement.

[0012] The superconducting material sample is preferably made of titanium nitride. This material has been found to be particularly suitable because samples of such material are easy to fabricate and its critical current has been found to exhibit good sensitivity to temperature in the temperature range of interest for quantum computing applications, particularly in the range of about 0.1 to 1.2 K. However, other superconducting materials can also be used.

[0013] Preferably, the superconducting material sample is formed as a film having a thickness preferably in the range of 0.1 to 100 nanometers (nm), more preferably 1 to 20 nm. Such films can be easily fabricated by known deposition techniques, for example, on semiconductor structures of the type on which current quantum computing chips are based. Advantageously, the film has a length along the direction of current flow through the superconducting material sample in the range of 0.1 to 100 micrometers (μm), preferably 10 to 50 μm; and / or a width transverse to the direction of current flow through the superconducting material sample in the range of 0.01 to 100 μm, preferably 0.36 to 2 μm. A low cross-sectional area of ​​the film along the direction of current travel is desirable, so that when the critical current is exceeded and the sample acquires a non-zero resistance, the sample's resistance is high and the change in voltage is correspondingly large. However, the value of the critical current increases with increasing cross-sectional area, and it is easier to control the value of the current in the critical current region if the critical current is not too small. The above dimensions have been found to achieve an optimal balance between these competing considerations.

[0014] Advantageously, the control unit may be configured to increase the current from below the critical current to above the critical current while the voltage sensor senses the voltage across the superconducting material sample. c The critical current at which a sample of superconducting material acquires a finite resistance in the presence of an increasing current (approaching I c is not the same as (and is larger than) the current at which the sample resumes its superconducting behavior as the current is decreased. Thus, the sample exhibits hysteresis: starting at a low current, as the current I c When the current reaches I c Superconductivity resumes only when the current drops below a threshold (the "retrapping current") that is significantly less than I c This is due in part to the fact that the sample experiences resistive heating due to the current above I. The transition from superconducting to non-superconducting behavior (as increasing current increases from below I c The increasing current transition can be detected more reliably and accurately because the increasing current transition (occurring when crossing the threshold) exhibits a much more abrupt change in resistance than the lower threshold at which the sample returns to the superconducting state as the current is decreased. Calculating the temperature based on this transition therefore provides a more accurate and reliable temperature measurement.

[0015] The cryogenic temperature sensor may further include a magnetic field source configured to generate a magnetic field at the location of the superconducting material sample, and the control unit may be configured to control the generated magnetic field based on the target temperature value. In particular, the generated magnetic field may be controlled so that the sensitivity of the critical current to changes in temperature has a maximum value at the target temperature value. The critical current and its sensitivity to temperature changes depend on the magnetic field experienced by the superconducting material sample. Therefore, it is advantageous to be able to control the magnetic field to a value that maximizes the sensitivity of the critical current for future measurements. As shown with reference to the following examples, the value of the critical current and its sensitivity to temperature changes can be calibrated, and such calibration data can be used to determine the magnetic field needed to maximize the temperature sensitivity of the critical current at the target temperature. As an alternative to providing the magnetic field source as part of the cryogenic temperature sensor, the control unit may be configured to control a magnetic field generator (e.g., a solenoid) separate from the cryogenic temperature sensor and positioned to control the magnetic field at the location of the superconducting material sample (and possibly also the magnetic field across the quantum computing chip whose temperature is being sensed). Such a magnetic field generator may be provided as part of a quantum computing system in which the cryogenic temperature sensor and the chip are included.

[0016] The present invention also provides a quantum computing system including: a quantum computing chip on which a quantum processor is disposed; and a cryogenic temperature sensor as defined above. Preferably, a current source, a voltage sensor, and a superconducting material sample are disposed on the quantum computing chip to measure the temperature of the quantum processor. A control unit may also be disposed on the chip, although it will be understood that this is not required. This provides a quantum computing chip with an "on-chip" temperature sensor. The temperature sensor may be disposed on the chip close to a component whose temperature is of interest (e.g., the quantum processor), thereby providing an accurate measurement of the component's temperature. The quantum computing system may include multiple cryogenic temperature sensors, each of which has a current source, a voltage sensor, and a superconducting material sample positioned to measure the temperature of a different respective region of the quantum computing chip. This allows a thermal "image" of the chip to be generated, with each temperature sensor providing a measurement of the chip's temperature at a respective location. At least some of the multiple temperature sensors may be configured to measure different temperature ranges—this may be achieved, for example, by providing different temperature sensors in which the superconducting material samples have different dimensions and / or are formed from different superconducting material samples. This increases the range of temperatures that can be measured. In this case, at least some of the temperature sensors configured to measure different temperature ranges may be positioned to measure the temperature at the same location on the chip.

[0017] The present invention also provides a cryogenic system comprising: a cryogenic enclosure configured to generate cryogenic conditions in its interior space; and a cryogenic temperature sensor as defined above, wherein the cryogenic temperature sensor is disposed in the interior space of the cryogenic enclosure. The cryogenic enclosure may be any device, such as a cryostat, configured to generate cryogenic conditions in its interior space.

[0018] A second aspect of the present invention provides a method for measuring the temperature of a superconducting material sample, the method comprising: passing a time-varying current through the superconducting material sample while sensing a voltage across the superconducting material sample; detecting a change in the sensed voltage; and calculating a temperature of the superconducting material sample based on the value of the current upon detection of the change in the sensed voltage. This method achieves the advantages of the temperature sensor of the first aspect of the present invention discussed above.

[0019] In this method, the superconducting material sample is preferably placed on a quantum computing chip containing a quantum processor, again representing an "on-chip" arrangement where measurement of the temperature of the superconducting material sample provides an accurate measurement of the temperature in the vicinity of components near the sample. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 shows the behavior of a sample of superconducting material carrying a varying current. [Figure 2] FIG. 2 shows measurements of the critical current and retrapping current of a superconducting material sample as a function of temperature. [Figure 3] FIG. 3 shows the critical current measurements shown in FIG. 2 plotted on a theoretical model of critical current as a function of temperature. [Figure 4] FIG. 4 shows a plot of measurements of the critical current of a superconducting material sample as a function of the magnetic field experienced by the superconducting material sample, along with a theoretical model of the critical current as a function of magnetic field. [Figure 5] FIG. 5(a) shows the theoretical phase diagram of a Type I superconducting material, and FIG. 5(b) shows the empirically measured phase diagram for a sample of the same superconducting material. [Figure 6]Figure 6(a) shows the theoretical critical current of a superconducting material sample as a function of temperature for different magnetic field values. Figure 6(b) shows the sensitivity of the critical current to temperature for each of the theoretical curves shown in Figure 6(a). Figure 6(c) shows the change in voltage across a superconducting material sample corresponding to the critical current values ​​shown in Figure 6(a). Figure 6(d) shows the sensitivity of voltage to temperature for each of the theoretical curves shown in Figure 6(d). [Figure 7] FIG. 7 illustrates a schematic diagram of an example of a cryogenic temperature sensor according to an embodiment of the present invention. [Figure 8] FIG. 8 shows the arrangement of the superconducting material sample in the temperature sensor of FIG. DETAILED DESCRIPTION OF THE INVENTION

[0021] The theory underlying the temperature sensor of the present invention will be described in detail with reference to FIGS. 1 to 5(b).

[0022] Critical current of superconducting material sample I c is the maximum current the sample can carry while remaining in the superconducting (i.e., zero resistance) state. The critical current can be found by sensing the voltage across the sample while passing an increasing current I through it. Initially, as the current is swept up from zero, there is no voltage across the sample because its resistance in the superconducting state is zero, and the voltage is I c It remains zero for I. c When this is reached, the superconducting material sample acquires a finite resistance and a step change in voltage across the sample is observed.

[0023] ​Figure 1 is a graph showing the voltage sensed across a sample of superconducting material in millivolts (mV) as a function of the current passing through it. Line 101, annotated with a solid arrow, shows the voltage as the magnitude of the current increases from zero. The critical current for this sample is approximately 1.2 μA (shown at 103), at which point the voltage increases sharply from 0 mV to approximately 2.2 mV (or ~2.2 mV for negative currents, as shown on the left side of the graph). As the magnitude of the current decreases from above the critical current, the current I c It should be noted that the superconducting material sample does not instantly return to a superconducting state when the current drops below . The sample's resistance (and therefore the sensed voltage) gradually decreases as the current decreases (as indicated by line 102 annotated with a dashed arrow) until the current reaches a "retrapping current" value, approximately 0.7 μA in this example. Once the current falls below this value, the sample returns to a superconducting state. The retrapping current is more difficult to observe than the critical current because it exhibits a more gradual change in resistance than the gradual change associated with the critical current.

[0024] The critical current of a superconducting material sample depends on its temperature. In creating the present invention, the inventors recognized that by observing the current that results in an abrupt change in voltage across a superconducting material sample (indicating that a critical current has been reached, causing a transition from a superconducting state to a non-superconducting state with non-zero resistance), the temperature of the sample can be inferred. This is the principle on which the temperature sensor of the present invention operates.

[0025] The temperature dependence of the critical current can be fitted to a physically motivated equation based on the Bardeen formula, which is of the form

number

[0026] where T is the temperature, T c is the critical temperature of the sample (i.e., the highest temperature at which the sample can exhibit superconductivity in the absence of current), I c(T) is the critical current as a function of temperature, and I c (0) is the critical current at T=0K.

[0027] Equation 1 is based on the Bardeen-Cooper-Schrieffer (BCS) theory, a microscopic theory that models superconductivity based on the condensation of Cooper pairs and is valid at all temperatures. However, while Equation 1 assumes that the sample is a thin wire, in preferred embodiments of the present invention, samples in the form of thin films are preferred. This equation can be adapted to the thin film case by replacing the 3 / 2 power in Equation 1 with a parameter i, which is determined based on measurements of the sample in question.

number

[0028] Measurements of the critical current (plotted by circles 201) and retrapping current (plotted by diamonds 202) of a sample of superconducting material in the form of a thin film are shown in Figure 2, where the temperature is the T c and the current is normalized to the value of I c The critical current is always larger than the retrapping current, but is normalized to T (T = 0). c (above this temperature the sample is no longer superconducting). The superconducting material sample on which these measurements were performed was a titanium nitride (TiN) film 50 μm long (along the direction in which the current was passed) and 0.36 μm wide (transverse to the direction in which the current was passed).

[0029] 3 shows a fit (plotted as a dashed curve 301 extending from the upper left to the lower right of the graph) of a function of the form Equation 2 to the critical current measurement 302 shown in FIG. 2, where the best fit is for parameters i=0.63, T c = 1.18K, and I c (T=0)=1.37 μA. This empirical fit of Equation 2 to the critical current values ​​measured for the sample gives I cGiven a measurement of (T) (obtained by passing an increasing current through the sample while monitoring the voltage and recording the value of the current at which a step change in voltage occurs), the temperature can be calculated by solving equation 2 for T, allowing the sample to be used with a temperature sensor.

[0030] The sensitivity of the temperature sensor described above (i.e., the rate of change of its critical current with respect to temperature) is given by:

number

[0031] The sensitivity derived from the empirical fit shown in Figure 3 is plotted on the graph (as the dashed line 303 extending from the bottom left to the top right of the graph). The sensitivity is T = T c increases with temperature.

[0032] In a temperature sensor based on the principles described above, it is the voltage across a sample of superconducting material that is sensed. The sensed voltage V at the critical current sense (the point at which the voltage goes from zero to a finite value) is the critical current I c The voltage change sensed at the critical current transition is the product of the current (T) and the resistance R of the sample at the point where it stops superconducting. sense The sensitivity of to temperature (i.e., its rate of change with temperature) is given by:

number

[0033] I c (0) and R depend on the dimensions of the superconducting material sample, which can be selected at the time of sample preparation.

[0034] In addition to being temperature dependent, the critical current I of a superconducting material sample c also depends on the strength of the magnetic field B applied to it. For a magnetic field applied in a plane perpendicular to a sample of superconducting material in the form of a thin film, the critical current obeys a Kim-type formula:

number

[0035] FIG. 4 shows measurements of the critical current as a function of B (normalized to the critical current at zero magnetic field) and the fit of Equation 6 to these measurements (plotted as a dashed line). Critical temperature T c also has a magnetic field dependence and follows the formula:

number

[0036] Therefore, whether a superconducting material sample is in a superconducting state depends on its temperature, magnetic field, and the current it is carrying. Figure 5(a) is a phase diagram showing the boundary between the superconducting and non-superconducting states of a Type I superconductor as a function of temperature and magnetic field (in the absence of current). In region 501, the sample is superconducting, and in region 502, it is non-superconducting (and therefore has non-zero resistance). Figure 5(b) is a graph showing measurements of the resistance of this Type I superconducting material sample plotted as a function of temperature and magnetic field. The data points plotted as circles represent the measurements taken, and the shaded region of the graph (set on the scale to the right) represents the measured resistance normalized to the maximum measured resistance (observed at temperature and magnetic field values ​​well outside the superconducting region). Curve 503 shows the theoretical boundary between the superconducting and non-superconducting states depicted in Figure 5(a). It was found that for values ​​of temperature and magnetic field within the theoretical superconducting region, the measured resistance was always approximately zero, while in the region at the theoretical boundary, the resistance increased rapidly (but not instantaneously) with increasing temperature and magnetic field up to a maximum value.

[0037] 6(a) and 6(c) show theoretical plots of the critical current and the voltage change at the critical current (which is the critical current multiplied by the resistance of the sample in its non-superconducting state) as a function of temperature for different magnetic field values ​​based on Equation 2, Equation 5, and Equation 6, respectively. In some embodiments of a cryogenic temperature sensor according to the present invention, calculation of temperature based on the sensed voltage change can be performed based on a calibration of the critical current versus temperature and magnetic field as represented in Figures 6(a) and 6(c) - either a theoretical curve as shown therein, or, more preferably, a curve fitted to empirical measurements of the particular superconducting material sample incorporated into the temperature sensor.

[0038] The temperature sensitivity of the critical currents plotted in Figures 6(a) and 6(c) and the corresponding voltage change equations are shown in Figures 6(b) and 6(d), respectively. The equations for these sensitivity curves are given by the magnetic field-dependent critical temperature T c (B) Equation (6) is the critical current I c The sensitivity formula (Equation 3) and the voltage change V sensed at the critical current sense (Equation 4) is derived from the critical current and voltage curves to give Equations 7 and 8 below.

number

number

[0039] As can be seen in Figures 6(b) and 6(d), at any given temperature, the sensitivity of the critical current and voltage change to temperature varies for different magnetic field strengths. Therefore, in some embodiments, the sensitivity of the temperature sensor can be maximized by applying an appropriate magnetic field to the superconducting material sample. Temperature sensors incorporating samples that form the basis of the functions plotted in Figures 6(a) through 6(d) can therefore achieve excellent sensitivity over a temperature range of 0.1 to 1.2 K.

[0040] 7 is a circuit diagram that schematically illustrates an example of a cryogenic temperature sensor according to an embodiment of the present invention. SNS , which is varied over time by a control unit (not shown) configured to control the current source in operation. Preferably, the time variation of the current is such that the current always approaches the critical current from below (thus ensuring that the observed voltage change corresponds to the critical current and not a re-trapping current)—this can be achieved, for example, by varying the current in a periodic “sawtooth” pattern, where in each cycle the current increases successively before returning to zero in a step change.

[0041] current I SNS is input to a voltage generating circuit 710, where it passes through a superconducting material sample 711. The voltage generating circuit 710 includes an amplifier 712 in a negative feedback transimpedance amplifier configuration, with the superconducting material sample arranged as a negative feedback resistor (connected to the negative input and output terminals of the amplifier 712). In this example, the amplifier 712 is an operational amplifier. A constant reference voltage V REF is input to the positive terminal of amplifier 712. As outlined above, when a superconducting material sample is in its superconducting state, it has zero resistance and therefore no voltage is dropped across it. As a result, when in the superconducting state, the voltage sensed at the negative terminal of amplifier 712 is equal to the voltage V at the output terminal of amplifier 712. TSNS The amplifier detects that the voltage at its negative terminal is equal to V REF The voltage at the output terminals, V, is equal to TSNS To control this, when sample 711 is superconducting, V TSNS =V REF However, once the critical current of the superconducting material sample 711 is exceeded, its resistance becomes non-zero and a voltage I SNS *R SNS The amplifier then increases the output voltage V to compensate for this voltage drop. TSNS must be adjusted, and V TSNS is V TSNS =VREF -I SNS *R SNS Therefore, the output voltage V TSNS is the current that passes through the superconducting material sample 711 as it is increased from below to the critical current, I SNS *R SNS Only the value changes.

[0042] Output voltage V TSNS is input to a voltage comparator circuit 720, which in this example includes a Schmitt trigger 721. The purpose of this circuit is to detect changes in the output voltage and output a digital signal indicative of these changes. The Schmitt trigger 721 in this example outputs a binary signal (0 or 1). The value of the output is determined by (i) the voltage V TSNS increases above an upper threshold (in which case the output switches to 0) or decreases below a lower threshold (which falls below the upper threshold, in which case the output switches to 1). The thresholds are selected so that a voltage change on the order of that expected when the critical current is exceeded results in the output value switching. The output of comparator circuit 720 is therefore a binary signal indicating whether superconducting material sample 711 is in a superconducting or non-superconducting state.

[0043] The output of the voltage comparator 720 is output to a control unit which calculates the temperature of the superconducting material sample 711 based on the value of the current at the time a change in the digital signal from 1 to 0 (indicating a transition from a superconducting state to a non-superconducting state) is detected. As previously mentioned, this calculation can be based on calibration data, such as a function fitted to empirical measurements of the critical current of the sample 711 at different temperatures (and optionally different magnetic field strengths).

[0044] Figure 8 shows a schematic example of how a superconducting material sample 711 in a temperature sensor may be arranged. In this example, the superconducting material sample 711 is formed as a thin film disposed on a shallow trench isolation (STI) layer 803 on a silicon wafer 804. Preferably, the dimensions of this film are: thickness (z-direction) in the range of 1 to 20 nanometers (nm); length (x-direction through which current flows in use) in the range of 0.1 to 100 micrometers (μm), more preferably 10 to 50 μm; and width (direction perpendicular to x and z) in the range of 0.01 to 100 μm, more preferably 0.36 to 2 μm.

[0045] A contact layer 801 is disposed on top of the superconducting material sample 711 (optionally with an intermediate layer 802 between them). SNS is passed through the sample 711 between two current electrodes 811, 812. Two voltage electrodes 821, 822 are positioned between the current electrodes 811, 812. One voltage electrode 821 is electrically connected to the negative terminal of the amplifier 712, and the other electrode 822 is connected to the negative terminal of the amplifier 712, which outputs an output voltage V TSNS is connected to the output of amplifier 712. This arrangement is a "four-point" measurement configuration, in which the electrodes that sense the voltage are separate from and located between the electrodes that deliver current to the sample. This arrangement improves the accuracy of sensing the voltage across the sample 711, as it ensures that the voltage dropped at the contact points between the current electrodes 811, 812 and the sample is not sensed.

[0046] The temperature sensor described above may be incorporated into a quantum computing chip that is co-located with a quantum processor. The temperature sensor may be part of a cryogenic system that is located within the interior space of a cryogenic enclosure (e.g., a cryostat) configured to generate cryogenic conditions in the interior space.

[0047] The present invention may be further understood by reference to the following clauses.

[0048] <Article 1> a cryogenic temperature sensor including: a superconducting material sample; a current source configured to pass a current through the superconducting material sample; a voltage sensor configured to sense a voltage across the superconducting material sample; and a control unit configured to: control the current source to time-vary a current through the superconducting material sample while the voltage sensor senses a voltage across the superconducting material sample; detect a change in the sensed voltage; and calculate a temperature of the superconducting material sample based on the value of the current at the time of detecting the change in the sensed voltage. <Clause 2> The cryogenic temperature sensor of the preceding clause, wherein the voltage sensor includes an amplifier configured to amplify the voltage across the superconducting material sample. <Article 3> In the cryogenic temperature sensor of clause 2, the voltage sensor further includes a comparator circuit, preferably a Schmitt trigger, configured to receive the amplified voltage as an input and to output a digital signal whose value changes in response to a change in voltage that occurs when the time-varying current exceeds a critical current, and the control unit detects the change in voltage based on the change in the digital signal. <Article 4> The cryogenic temperature sensor of the preceding clause further includes a magnetic field sensor positioned to measure a strength of a magnetic field at the location of the superconducting material sample, the calculation of the temperature being further based on the strength of the magnetic field. <Article 5> In the cryogenic temperature sensor of the preceding clause, the superconducting material sample is made of titanium nitride. <Article 6> In the cryogenic temperature sensor of the preceding clause, the superconducting material sample is formed as a film having a thickness preferably in the range of 0.1 to 100 nanometers (nm), more preferably 1 to 20 nm. <Article 7> In the cryogenic temperature sensor of clause 6: the film has a length in the range of 0.1 to 100 micrometers (μm), preferably 10 to 50 μm, along the direction in which current is passed through the superconducting material sample; and / or a width in the range of 0.01 to 100 μm, preferably 0.36 to 2 μm, transverse to the direction in which current is passed through the superconducting material sample. <Article 8> In the cryogenic temperature sensor of the preceding clause, the current source and voltage sensor each include a pair of electrodes in electrical communication with the superconducting material sample, wherein a current through the superconducting material sample flows between the electrodes of the current source and a sensed voltage is sensed between the electrodes of the voltage sensor; the electrodes of the voltage sensor are disposed between the electrodes of the current source. <Article 9> In the cryogenic temperature sensor of the preceding clause, the control unit is configured to increase the current from below the critical current to above the critical current while the voltage sensor senses the voltage across the superconducting material sample. <Article 10> The cryogenic sensor of the preceding clause further includes a magnetic field source configured to generate a magnetic field at the location of the superconducting material sample, the control unit configured to control the generated magnetic field based on the target temperature value. <Article 11> A quantum computing system comprising: a quantum computing chip having a quantum processor disposed thereon; and the cryogenic temperature sensor of the preceding clause. <Article 12> In the quantum computing system of clause 11, a current source, a voltage sensor and a superconducting material sample are disposed on the quantum computing chip to measure the temperature of the quantum processor. <Article 13> The quantum computing system of clause 12 includes a plurality of cryogenic temperature sensors, each of the plurality of cryogenic temperature sensors having a current source, a voltage sensor, and a superconducting material sample positioned to measure the temperature of a different respective region of the quantum computing chip. <Article 14> 1. A method for measuring a temperature of a superconducting material sample, the method comprising: passing a time-varying current through a superconducting material sample while sensing a voltage across the superconducting material sample; detecting a change in the sensed voltage; and calculating a temperature of the superconducting material sample based on the value of the current upon detection of the change in the sensed voltage. <Article 15> In the method of clause 14, the superconducting material sample is disposed on a quantum computing chip including a quantum processor.

Claims

1. a quantum computing chip having a quantum processor disposed thereon; a cryogenic temperature sensor including a superconducting material sample, a current source configured to pass a current through the superconducting material sample, and a voltage sensor configured to sense a voltage across the superconducting material sample; a control unit configured to control the current source to time-vary a current through the superconducting material sample while the voltage sensor senses a voltage across the superconducting material sample, detect the change in the sensed voltage, and calculate a temperature of the superconducting material sample based on a value of the current at the time of detecting the change in the sensed voltage; A quantum computing system wherein the superconducting material sample is disposed on the quantum computing chip to measure the temperature of the quantum processor.

2. 10. The quantum computing system of claim 1, wherein the voltage sensor comprises an amplifier configured to amplify the voltage across the superconducting material sample.

3. 3. The quantum computing system of claim 2, wherein the voltage sensor further includes a comparator circuit that is a Schmitt trigger and is configured to receive the amplified voltage as an input and to output a digital signal whose value changes in response to a change in voltage that occurs when the time-varying current exceeds a critical current, and wherein the control unit detects the change in voltage based on the change in the digital signal.

4. 4. The quantum computing system of claim 1, further comprising a magnetic field sensor positioned to measure a strength of a magnetic field at the location of the superconducting material sample, wherein the calculation of the temperature is further based on the strength of the magnetic field.

5. 5. The quantum computing system of claim 1, wherein the superconducting material sample is made of titanium nitride.

6. 6. A quantum computing system according to any one of claims 1 to 5, wherein the superconducting material sample is formed as a film having a thickness preferably in the range of 0.1 to 100 nanometers (nm), more preferably 1 to 20 nm.

7. 7. The quantum computing system of claim 6, wherein the film has a length in the range of 0.1 to 100 micrometers (μm), preferably 10 to 50 μm, along the direction in which current is passed through the superconducting material sample, and / or a width in the direction transverse to the direction in which current is passed through the superconducting material sample of 0.01 to 100 μm, preferably 0.36 to 2 μm.

8. the current source and the voltage sensor each include a pair of electrodes in electrical communication with the superconducting material sample, a current through the superconducting material sample flows between the electrodes of the current source, and a sensed voltage is sensed between the electrodes of the voltage sensor; 8. The quantum computing system of claim 1, wherein electrodes of the voltage sensor are disposed between electrodes of the current source.

9. 9. The quantum computing system of claim 1, wherein the control unit is configured to increase the current from below a critical current to above a critical current while the voltage sensor senses a voltage across the superconducting material sample.

10. 10. The quantum computing system of claim 1, further comprising a magnetic field source configured to generate a magnetic field at the location of the superconducting material sample, the control unit configured to control the generated magnetic field based on a target temperature value.

11. 11. The quantum computing system of claim 1, wherein the current source and the voltage sensor are disposed on the quantum computing chip to measure a temperature of the quantum processor.

12. 12. The quantum computing system of claim 1, including a plurality of the cryogenic temperature sensors, the current source, the voltage sensor, and the superconducting material sample of each of the plurality of the cryogenic temperature sensors positioned to measure the temperature of a different respective region of the quantum computing chip.

13. 1. A method for measuring the temperature of a sample of superconducting material disposed on a quantum computing chip including a quantum processor, comprising: passing a time-varying current through the superconducting material sample while sensing a voltage across the superconducting material sample; Detecting a change in the sensed voltage; and calculating a temperature of the superconducting material sample based on the value of the current upon detection of the change in sensed voltage.

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