TW202441410A - Cryogenic temperature sensor and measurement method - Google Patents

Abstract

This specification describes a cryogenic temperature sensor is comprising: a superconductor material sample; a current source configured to pass a current through the superconductor material sample; and a voltage sensor configured to sense a voltage across the superconductor material sample. The temperature sensor further comprises a control unit configured to: control the current source to time-vary the current through the superconductor material sample while the voltage sensor senses the voltage across the superconductor material sample; detect a change in the sensed voltage; and compute the temperature of the superconductor material sample based on the value of the current at the time of the detected change in the sensed voltage.

Description

Low temperature sensor and measurement method

The present invention relates to a low temperature sensor and a method for measuring the temperature of a superconducting material sample. The sensor and the method are particularly suitable for quantum computing and other low temperature electronic device applications.

Providing a stable cryogenic environment is critical to the successful operation of current quantum processors. At temperatures above a few degrees Kelvin (K), the effects that quantum processors rely on for their operation (including maintaining stable quantum states on quantum bits (qubits) for storing information) rapidly decrease. In order to control the temperature of cryogenic environments used for quantum computing applications, it is important to be able to accurately sense the temperature of these environments and to do this a temperature sensor capable of measuring temperatures close to absolute zero is required.

Most current techniques for measuring the temperature of quantum computing systems rely on “off-chip” temperature sensors—that is, temperature sensors that are implemented as hardware that is physically separate from the quantum processor, and are located alongside the processor inside the cryogenic enclosure in which the processor is mounted. One disadvantage 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 of the processor through the dissipation of power in its electronic components can cause the temperature of parts of the processor to deviate significantly from the temperature of surrounding parts of the cryogenic environment. Therefore, current techniques are limited in their ability to accurately measure the temperature of quantum processors. Additionally, known low temperature measurement techniques such as Coulomb barrier thermometry have not been successfully adapted to provide rapid, instantaneous and direct measurement of the temperature of a quantum processor, and their implementation requires complex electronics. More specifically, standard Coulomb barrier thermometry requires the provision of a large number of series tunnel junctions, which requires a complex electronic structure for its implementation and relies on direct current measurement which limits its speed. Although radio frequency Coulomb barrier thermometry has been developed, they rely on complex off-chip electronics such as low temperature lamps and directional couplers which make them unsuitable for directly measuring the temperature of portions of a chip. The present invention is directed to providing a method of measuring the temperature of a component of a quantum processor that overcomes these limitations.

A first aspect of the present invention provides a low temperature sensor, which includes: a superconducting material sample; a current source, which is configured to allow a current to pass through the superconducting material sample; a voltage sensor, which is configured to sense a voltage across the superconducting material sample; and a control unit, which is configured to: control the current source to make the current passing through the superconducting material sample time-varying when the voltage sensor senses the voltage across the superconducting material sample; detect a change in the sensed voltage; and calculate the temperature of the superconducting material sample based on the value of the current at the time of the detected change in the sensed voltage.

This temperature sensor exploits the temperature dependence of a sample of superconducting material's critical current _Ic , which is the maximum current that the sample can carry while remaining in a superconducting (i.e., zero resistance) state. It should be noted that the value of _Ic is specific to a particular sample and is sensitive to its shape and size. When a sample of superconducting material is in the superconducting state, changes in the current will not affect the voltage measured across the sample because its resistance remains zero (and therefore, there will be no potential difference across the sample). However, once the current I reaches _Ic , the sample of superconducting material undergoes a step change in its resistance from zero to a finite, non-zero value. Subsequently, a dramatic change in the voltage across the sample will be observed that is proportional to the current and the resistance of the sample. Therefore, the detected change in voltage means that the time-varying current has exceeded a critical current above which the superconducting material sample has a non-zero resistance, and since the temperature dependence of the value of the critical current _Ic is known, the temperature of the sample can be calculated based on the value of the current at which the change in voltage occurs. For example, calculating the temperature may include comparing the value of the current at the time of the detected change in voltage with calibration data representing the measurement of the critical current as a function of the temperature of the superconducting material sample, thereby inferring the temperature of the superconducting material sample. A detailed explanation of the critical current, its temperature dependence, and an example of a method for calculating the temperature based on the critical current will be given below.

As will be illustrated with reference to the following examples, a temperature sensor as defined above requires only relatively simple electronics to implement and can be constructed entirely "on-chip" as part of a chip that includes a quantum processor (and indeed, in some preferred embodiments, the components are configured on a quantum computing chip - although it should be understood that some or all components (e.g., control units) may be off-chip). Thus, the temperature sensor can be located very close to the components of the processor whose temperature is of concern and thus achieve significantly more accurate temperature measurements than current technologies having a far less complex construction. It is also possible to monitor temperature rapidly, in real time, because the critical current can be measured repeatedly and output at a high frequency.

The term "cryogenic" has its conventional meaning herein (i.e., temperatures below 120 K), although it should be understood that embodiments of the cryogenic sensor of the present invention may not necessarily be capable of measuring temperatures across the entire cryogenic 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.

In a preferred embodiment, the current source may include a current digital-to-analog converter (IDAC). An IDAC can be controlled to generate a time-varying current of a desired profile based on a digital input, and can therefore be conveniently controlled by a digital control unit to provide the desired time-varying current.

In a preferred embodiment, the voltage sensor includes an amplifier configured to amplify the voltage across the superconducting material sample. This is beneficial because in many embodiments, the voltage change detected when the current reaches _Ic will be small (e.g., about 100 μV), so the detection of the voltage change is improved by amplifying its magnitude. For example, the amplifier can be a negative feedback transimpedance amplifier, in which the superconducting material sample is configured as the negative feedback resistor. In addition, an amplifier (e.g., one of a negative feedback transimpedance amplifier configuration, in particular, an operational amplifier in such a configuration) can produce a voltage output that facilitates monitoring the behavior of the superconducting material sample. Additionally, in particularly preferred embodiments, 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, the value of the digital signal changing in response to a change in voltage that occurs when the time-varying current exceeds the critical current, wherein the control unit detects the change in voltage based on a change in the digital signal. In the example of a Schmitt trigger, the voltage comparator changes its binary output value in response to a change in the input voltage of sufficient magnitude. Providing a digital (eg binary) output is useful because the critical current (and hence the temperature) can be inferred from the value of the current as changes in the digital signal occur.

Preferably, the current source and the voltage sensor each include a pair of respective electrodes in electrical communication with the sample of superconducting material, wherein current through the sample of superconducting material passes between the electrodes of the current source and the sensed voltage is sensed between the electrodes of the voltage sensor; wherein the electrodes of the voltage sensor are 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 across which 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 thus the measured voltage does not include any voltage drop across the points where the electrodes of the current sensor touch the circuit.

Preferably, the cryogenic sensor further comprises a magnetic field sensor configured to measure the strength of a magnetic field at the location of the superconducting material sample, wherein the calculation of temperature is further based on the strength of the magnetic field. The critical current typically varies 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 will vary over time. Therefore, taking into account the magnetic field strength when calculating temperature enhances the accuracy of the temperature measurement.

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

Preferably, the superconducting material sample is formed into a film preferably having a thickness in the range of 0.1 nanometers (nm) to 100 nm, more preferably 1 nm to 20 nm. Such a film can be easily fabricated on the semiconductor structure on which current quantum computing chips are based (for example, by known deposition techniques). Advantageously, the film can have a length in the range of 0.1 micrometers (μm) to 100 μm, preferably 10 μm to 50 μm in the direction of current flow through the superconducting material sample; and/or a width in the direction transverse to the direction of current flow through the superconducting material sample of 0.01 μm to 100 μm, preferably 0.36 μm to 2 μm. It is desirable that the cross-sectional area of the film along the direction in which the current travels be low, so that when the critical current is exceeded and the sample acquires a non-zero resistance, the resistance of the sample is large 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 region of the critical current if the critical current is not too small. The above dimensions have been found to achieve an optimal balance between these competing considerations.

Advantageously, the control unit may be configured to increase the current from below the critical current to above the critical current when the voltage sensor senses the voltage across the superconducting material sample. Typically, the critical current _Ic at which the superconducting material sample acquires a finite resistance in the presence of an increasing current (which approaches _Ic from below) is different from (and greater than) the current at which the sample resumes its superconducting behavior as the current is reduced. Thus, the sample exhibits a hysteresis: starting from a low current, it ceases to superconduct as soon as the current reaches _Ic , but resumes superconductivity only when the current is reduced below a critical value (the "retrapping current") that is significantly less than _Ic . This is due in part to the fact that once the current has exceeded _Ic , the sample undergoes resistive heating by the current. The transition from superconducting to non-superconducting behavior (occurring when an increasing current passes below _Ic ) exhibits a more dramatic change in resistance than the lower threshold at which the sample returns to the superconducting state when the current is reduced, so the increasing current transition can be detected more reliably and accurately. Calculating the temperature based on this transition therefore provides a more accurate and reliable temperature measurement.

The low temperature sensor may further include a magnetic field source configured to generate a magnetic field at the location of the superconducting material sample, wherein the control unit is configured to control the generated magnetic field based on a target temperature value. In particular, the generated magnetic field can 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 changes in temperature depend on the magnetic field to which the superconducting material sample is subjected. Therefore, it is advantageous that the magnetic field can be controlled to a value that maximizes the sensitivity of the critical current for future measurements. As we will show with reference to the following example, the value of the critical current and its sensitivity to changes in temperature can be calibrated, and such calibration data can be used to determine the magnetic field required to maximize the temperature sensitivity of the critical current at the target temperature. As an alternative to providing a magnetic field source as part of the cryogenic sensor, the control unit may be configured to control a magnetic field generator (e.g., a solenoid) that is separate from the cryogenic sensor and configured to control the magnetic field at the location of the superconducting material sample (and the magnetic field across the quantum computing chip whose temperature is sensed). Such a magnetic field generator may be provided as part of a quantum computing system that includes the cryogenic sensor and the chip.

The present invention also provides a quantum computing system, comprising: a quantum computing chip on which a quantum processor is configured; and the low temperature sensor defined above. Preferably, the current source, the voltage sensor and the superconducting material sample are configured on the quantum computing chip for measuring the temperature of the quantum processor. It should be understood that the control unit can also be configured on the chip, but this is not necessarily the case. This provides an "on-chip" temperature sensor for a quantum computing chip. The temperature sensor can be configured on the chip, close to a component whose temperature is of concern (such as the quantum processor), thereby providing an accurate measurement of the temperature of the component. The quantum computing system may include a plurality of such cryogenic sensors, the current source, voltage sensor and superconducting material sample of each of the plurality of cryogenic sensors being configured 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, wherein each temperature sensor provides a measurement of the temperature of the chip at its respective location. At least some of the plurality of temperature sensors may be configured to measure different temperature ranges - for example, this may be achieved by providing different temperature sensors wherein the superconducting material sample has different sizes and/or is formed from a different superconducting material sample. This increases the range of measurable temperatures. In this case, at least some of the temperature sensors configured to measure different temperature ranges may be configured to measure the temperature of the same location on the chip.

The present invention also provides a cryogenic system, comprising: a cryogenic housing configured to generate a cryogenic condition in an internal space thereof; and the cryogenic sensor defined above, wherein the cryogenic sensor is disposed in the internal space of the cryogenic housing. The cryogenic housing may be any device configured to generate a cryogenic condition in the internal space, such as a cryostat.

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

In this method, the superconducting material sample is preferably arranged on a quantum computing chip including a quantum processor. This again represents an "on-chip" arrangement, wherein the measurement of the temperature of the superconducting material sample provides an accurate measurement of the temperature of components near the sample.

With reference to FIG. 1 to FIG. 5( b ), we will now explain in detail the theory underlying the temperature sensor of the present invention.

The critical current _Ic of a sample of superconducting material is the maximum current that 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. Initially, as the current sweeps upward from zero, there will be no voltage across the sample because its resistance in the superconducting state is zero, and the voltage remains zero for I < _Ic . Once the value of I reaches _Ic , the sample of superconducting material acquires a finite resistance and a step change in the voltage across the sample will be observed.

FIG. 1 is a graph showing the voltage (in millivolts (mV)) sensed across a sample of superconducting material as a function of the current (in microamperes (µA)) passed through it. Line 101, annotated with a solid arrow, shows the voltage as the magnitude of the current increases from zero. The critical current in this sample is about 1.2 µA (indicated by 103), at which point the voltage increases dramatically from 0 mV to about 2.2 mV (or -2.2 mV in the case of a negative current, as shown in the left side of the graph). It should be noted that as the magnitude of the current decreases from above the critical current, the sample of superconducting material does not immediately return to the superconducting state when the current drops below _Ic . As the current is reduced, the resistance of the sample (and therefore the sensed voltage) gradually decreases (as indicated by line 102 annotated with a dashed arrow) until the current reaches a "retrapping current" value, which in this example is about 0.7 µA. When the current drops below this value, the sample returns to the 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 step change associated with the critical current.

The critical current in a sample of superconducting material depends on its temperature. In making the present invention, the inventors have realised that by observing a sudden change in current across a voltage across a sample of superconducting material (indicating that the critical current has been reached to cause a transition from a superconducting state to a non-superconducting state having non-zero resistance), the temperature of the sample can be inferred. This is the principle of operation of the temperature sensor of the present invention.

The temperature dependence of the critical current can be fitted to a physical excitation equation based on the Bardeen equation. The original Bardeen equation is in the form: Equation (1) where T is temperature; _Tc is the critical temperature of the sample (i.e., the highest temperature at which the sample can exhibit superconductivity in the absence of any current); _Ic (T) is the critical current as a function of temperature; and _Ic (0) is the critical current at T = 0 K.

Equation 1 is based on the Bardeen-Cooper-Schrieffer (BCS) theory, a microscopic theory that models superconductivity based on condensation of Cooper pairs and is valid at all temperatures. However, Equation 1 assumes that the sample is a thin wire, whereas in the preferred embodiment of the present invention, a sample in the form of a thin film is preferred. By replacing Λ3/2 in Equation 1 with a parameter i to be determined based on measurements of the sample in question, the formula can be applied to the case of a thin film: Equation (2)

FIG2 shows measurements of the critical current (plotted as circles 201) and the re-trapping current (plotted as diamonds 202) of a sample of superconducting material in the form of a thin film. Here, the temperature is normalized to the value _Tc of the sample and the current is normalized to _Ic (T = 0). It can be seen that the critical current is always greater than the re-trapping current, but both drop to zero as T approaches _Tc (beyond _Tc , the sample no longer exhibits superconductivity). The sample of superconducting material on which these measurements were performed was a titanium nitride (TiN) film having a length (in the direction of current flow) of 50 µm and a width (in the direction transverse to the current flow) of 0.36 µm.

FIG3 shows a function in the form of Equation 2 that fits the critical current measurement 302 shown in FIG2 (plotted as the dashed curve 301 extending from the upper left to the lower right of the graph). Here, the best fit is obtained with parameters i = 0.63; _Tc = 1.18 K and _Ic (T = 0) = 1.37 µA. This experimental fit of Equation 2 to the critical current values measured for the sample enables the sample to be used in a temperature sensor because, given a measurement of _Ic (T) (obtained by passing an increasing current through the sample while monitoring the voltage across the sample 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.

The sensitivity of the temperature sensor just described (i.e., the rate of change of its critical current with respect to temperature) is given by: Equation (3)

The sensitivity derived from the experimental fit shown in Figure 3 is plotted on the graph (as the dashed line 303 extending from the lower left to the upper right of the graph). The sensitivity increases with temperature up to T = _Tc .

In a temperature sensor based on the above principle, the voltage across a sample of superconducting material will be sensed. The sensed voltage V _sense at the critical current (voltage from zero to a finite value) will be the product of the critical current I _c (T) and the resistance R at which the sample stops superconducting. The sensitivity of the voltage change sensed at the critical current transition V _sense to temperature (i.e., its rate of change relative to temperature) is given by: Equation (4)

I _c (0) and R depend on the size of the superconducting material sample that can be chosen when fabricating the sample.

The critical current _Ic of a superconducting material sample depends not only on the strength B of the magnetic field applied to it, but also on the temperature. For a magnetic field applied perpendicular to a plane of a superconducting material sample in the form of a thin film, the critical current follows a Kim-Type formula: Equation (5)

FIG. 4 shows measurements of the critical current (normalized to the critical current at zero magnetic field) as a function of B and a fit of Eq. 6 to these measurements (plotted as a dashed line).

The critical temperature T _C is also magnetic field dependent and follows the following equation: Equation (6)

Therefore, whether a sample of superconducting material is in the superconducting state depends on its temperature, the magnetic field, and the current it carries. 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 any current). In region 501, the sample is superconducting, while in region 502 it is non-superconducting (and therefore has a non-zero resistance). Figure 5(b) is a graph showing measurements of the resistance of a sample of this type-I superconducting material plotted as a function of temperature and magnetic field. The data points plotted as circles represent the measurements made and the toned region of the graph (determined on the scale on the right) represents the measured resistance, normalized to the maximum resistance measured (which is observed at values of temperature and magnetic field outside the superconducting region). Curve 503 shows the theoretical boundary between the superconducting and non-superconducting states represented in Figure 5(a). At values of temperature and magnetic field in the theoretical superconducting region, the measured resistance is always approximately zero, and in the region of the theoretical boundary, the resistance is found to increase rapidly (but not instantaneously) to a maximum value as the temperature and magnetic field increase.

Figures 6(a) and 6(c) show theoretical graphs of the critical current and the change in voltage at the critical current (which is the critical current multiplied by the resistance of the sample in the non-superconducting state) as a function of temperature for different values of magnetic field, respectively, based on Equations 2, 5, and 6. In some embodiments of the cryogenic sensor according to the present invention, the calculation of the temperature based on the change in sensed voltage 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) - with theoretical curves as shown therein, or, more preferably, curves fitted to experimental measurements of samples of the particular superconducting material incorporated into the temperature sensor.

The sensitivity of the critical current to temperature plotted in Figures 6(a) and 6(c) and the corresponding voltage change formula are shown in Figures 6(b) and 6(d), respectively. By inserting the magnetic field dependence formula for the critical temperature _Tc (B) (Equation 6) into the sensitivity formula for the critical current _Ic (Equation 3) and the voltage change _Vsense sensed at the critical current (Equation 4), an expression for these sensitivity curves is derived from the critical current and voltage curves, which gives: Equation (7) and Equation (8)

It can be seen in Figures 6(b) and 6(d) that, at any given temperature, the critical current and voltage changes have different sensitivities to temperature for different strengths of the magnetic field. Thus, in some embodiments, the sensitivity of the temperature sensor can be maximized by applying a suitable magnetic field to the superconducting material sample. Thus, a temperature sensor incorporating the sample upon which the functions plotted in Figures 6(a) to 6(d) are based can achieve excellent sensitivity across a temperature range of 0.1 K to 1.2 K.

FIG7 schematically shows a circuit diagram of an example of a low temperature sensor according to an embodiment of the present invention. The temperature sensor comprises a current source 700, which in this case is a 6-bit digital-to-analog converter configured to output a current _ISNS (which in operation is varied over time by a control unit (not shown) configured to control the current source). Preferably, the time variation of the current is such that the current always approaches the critical current from below (thereby ensuring that the observed voltage variation corresponds to the critical current and not the sinking current) - this can be achieved by, for example, varying the current in a cyclical "sawtooth" pattern, in each cycle of which the current increases continuously before falling back to zero in a step change.

Current _ISNS is input to a voltage generating circuit 710 where the current flows through a superconducting material sample 711. Voltage generating circuit 710 includes an amplifier 712 in a negative feedback transimpedance amplifier configuration where the superconducting material sample is configured as a negative feedback resistor (connected to the negative input terminal and output terminal of amplifier 712). In this example, amplifier 721 is an operational amplifier. A constant reference voltage _VREF is input to the positive terminal of amplifier 712. As outlined above, when the superconducting material sample is in its superconducting state, it has zero resistance and therefore no voltage drops across the sample. Therefore, when in the superconducting state, the voltage sensed at the negative terminal of amplifier 712 is equal to the voltage _VTSNS at the output terminal of amplifier 712. The amplifier controls the voltage V _TSNS at its output terminal so that the voltage at the negative terminal is equal to V _REF , so when the sample 711 is superconducting, V _TSNS = V _REF . However, when the critical current of the superconducting material sample 711 is exceeded, its resistance becomes non-zero and a voltage _ISNS R _SNS drops across the sample. The amplifier must then adjust the output voltage V _TSNS to compensate for this voltage drop, and then V _TSNS becomes V _TSNS = V _REF - _ISNS R _SNS . Therefore, when the current through the superconducting material sample 711 increases from below to the critical current, the output voltage V _TSNS changes _ISNS R _SNS .

The 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 such changes. In this example, the Schmitt trigger 721 outputs a binary signal (0 or 1). The value of the output is switched when (i) the voltage V _TSNS increases above an upper threshold value (in which case the output switches to 0) or decreases to a lower threshold value below the upper threshold value (in which case the output switches to 1). The threshold values are selected so that a change in voltage of approximately the expected voltage change when the critical current is exceeded causes a switch in the output value. Therefore, the output of the comparator circuit 720 is a binary signal indicating whether the superconducting material sample 711 is in a superconducting state or a non-superconducting state.

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 when a change from 1 to 0 of the digital signal is detected (indicating a transition from a superconducting state to a non-superconducting state). As previously described, this calculation can be based on calibration data, such as a function fitted to experimental measurements of the critical current of the sample 711 at different temperatures (and, if appropriate, different magnetic field strengths).

FIG8 schematically shows an 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 on a shallow trench isolation (STI) layer 803 arranged on a silicon wafer 804. Preferably, the dimensions of this film are a thickness in the range of 1 nanometer (nm) to 20 nanometers (in the direction z); a length in the range of 0.1 micrometer (µm) to 100 micrometers, more preferably 10 µm to 50 µm (in the direction x in which current flows in use); and a width in the range of 0.01 µm to 100 µm, more preferably 0.36 µm to 2 µm (in the directions perpendicular to x and z).

A contact layer 801 is disposed on a superconducting material sample 711 (optionally with an intermediate layer 802 therebetween). A current _ISNS passes through the sample 711 between two current electrodes 811, 812. Two voltage electrodes 821, 822 are disposed 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 output of the amplifier 712 where an output voltage _VTSNS is output. This configuration is a "four-point" measurement configuration where the electrodes that sense the voltage are separated from the electrodes that transmit the current through the sample and are disposed therebetween. This configuration improves the accuracy of sensing the voltage across sample 711 because it ensures that any voltage drops across the contact points between the current electrodes 811, 812 and the sample are not sensed.

The temperature sensor just described can be incorporated into a quantum computing chip on which it is configured together with a quantum processor. The temperature sensor can be part of a cryogenic system, wherein the temperature sensor is disposed inside an interior space of a cryogenic enclosure (e.g., a cryostat) configured to produce cryogenic conditions in the interior space.

The present invention may be further understood by referring to the following clauses.

Clause 1. A low temperature sensor comprising: a sample of superconducting material; a current source configured to pass a current through the sample of superconducting material; a voltage sensor configured to sense a voltage across the sample of superconducting material; and a control unit configured to: control the current source to time-variate the current through the sample of superconducting material when the voltage sensor senses the voltage across the sample of superconducting material; detect a change in the sensed voltage; and calculate a temperature of the sample of superconducting material based on the value of the current at the time of the detected change in the sensed voltage.

Clause 2. A cryogenic sensor as in any preceding clause, wherein the voltage sensor comprises an amplifier configured to amplify the voltage across the superconducting material sample.

Clause 3. A low temperature sensor as in clause 2, wherein the voltage sensor further comprises a comparator circuit, preferably a Schmitt trigger, configured to receive the amplified voltage as an input and output a digital signal, the value of the digital signal changing in response to a change in voltage occurring when the time-varying current exceeds the critical current, wherein the control unit detects the change in voltage based on a change in the digital signal.

Clause 4. A cryogenic sensor as in any preceding clause, further comprising a magnetic field sensor configured to measure the 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.

Clause 5. A cryogenic sensor as claimed in any preceding clause, wherein the superconducting material sample is made of titanium nitride.

Clause 6. A cryogenic sensor as in any preceding clause, wherein the superconducting material sample is formed into a film preferably having a thickness in the range of 0.1 nanometers (nm) to 100 nm, more preferably 1 nm to 20 nm.

Clause 7. A low temperature sensor as in clause 6, wherein: the film has a length in the direction of current passing through the superconductor material sample in the range of 0.1 micrometer (μm) to 100 micrometers, preferably 10 μm to 50 μm; and/or has a width in the direction transverse to the direction of current passing through the superconductor material sample in the range of 0.01 μm to 100 μm, preferably 0.36 μm to 2 μm.

Clause 8. A low temperature sensor as in any preceding clause, wherein the current source and the voltage sensor each comprise a pair of respective electrodes in electrical communication with the superconducting material sample, wherein a current passing through the superconducting material sample passes between the electrodes of the current source and a sensed voltage is sensed between the electrodes of the voltage sensor; wherein the electrodes of the voltage sensor are disposed between the electrodes of the current source.

Clause 9. A cryogenic sensor as in any preceding clause, wherein the control unit is configured to increase the current from below the critical current to above the critical current when the voltage sensor senses the voltage across the superconducting material sample.

Clause 10. A cryogenic sensor as in any preceding clause, further comprising a magnetic field source configured to generate a magnetic field at the location of the superconducting material sample, wherein the control unit is configured to control the generated magnetic field based on a target temperature value.

Clause 11. A quantum computing system comprising: a quantum computing chip on which a quantum processor is disposed; and a low temperature sensor as in any of the preceding clauses.

Clause 12. The quantum computing system of Clause 11, wherein the current source, the voltage sensor, and the superconducting material sample are arranged on the quantum computing chip for measuring the temperature of the quantum processor.

Clause 13. A quantum computing system as in clause 12, comprising a plurality of said cryogenic sensors, wherein the current source, the voltage sensor and the superconducting material sample of each of said plurality of cryogenic sensors are configured to measure the temperature of a different respective region of said quantum computing chip.

Clause 14. A method of measuring the temperature of a sample of superconducting material, the method comprising: passing a momentarily varying current through the sample of superconducting material while sensing a voltage across the sample of superconducting material; detecting a change in the sensed voltage; and calculating the temperature of the sample of superconducting material based on the value of the current at the time of the detected change in the sensed voltage.

Clause 15. The method of clause 14, wherein the superconducting material sample is disposed on a quantum computing chip comprising a quantum processor.

101: Line 102: Line 103: Line 201: Circle 202: Diamond 301: Dashed curve 302: Critical current measurement 303: Dashed line 501: Region 502: Region 503: Curve 700: Current source 710: Voltage generating circuit 711: Superconductor material sample 712: Amplifier 720: Voltage comparator circuit 721: Schmitt trigger 801: Contact layer 802: Intermediate layer 803: Shallow trench isolation (STI) layer 804: Silicon wafer 811: Current electrode 812: Current electrode 821: Voltage electrode 822: Voltage electrode

Figure 1 shows the behavior of a sample of a superconducting material carrying a varying electric current; Figure 2 shows the measurement of the critical current and the re-trapping current of a sample of a superconducting material as a function of temperature; Figure 3 shows the measurement of the critical current shown in Figure 2 plotted on a theoretical model of the critical current as a function of temperature; Figure 4 shows the measurement of the critical current of a sample of a superconducting material as a function of the magnetic field experienced by the sample of the superconducting material plotted together with a theoretical model of the critical current as a function of the magnetic field; Figure 5(a) shows a theoretical phase diagram of a type I superconducting material, and Figure 5(b) shows an experimentally measured phase diagram of a sample of the same superconducting material; FIG6(a) shows the theoretical critical current of a superconducting material sample as a function of temperature for different values of magnetic field; FIG6(b) shows the sensitivity of the critical current to temperature for each of the theoretical curves shown in FIG6(a); FIG6(c) shows the change in voltage across the superconducting material sample corresponding to the critical current value shown in FIG6(a); and FIG6(d) shows the sensitivity of the voltage to temperature for each of the theoretical curves shown in FIG6(d); FIG7 schematically shows an example of a low temperature sensor according to an embodiment of the present invention; and FIG8 shows the configuration of the superconducting material sample in the temperature sensor of FIG7.

700: Current source

710: Voltage generating circuit

711:Superconducting material samples

712:Amplifier

720: Voltage comparator circuit

721: Schmidt trigger

Claims (13)

A quantum computing system, comprising: a quantum computing chip, on which a quantum processor is configured; and a low temperature sensor, comprising: a superconducting material sample; a current source, which is configured to pass a current through the superconducting material sample; a voltage sensor, which is configured to sense a voltage across the superconducting material sample; and a control unit, which is configured to: control the current source to make the current through the superconducting material sample time-varying when the voltage sensor senses the voltage across the superconducting material sample; detect a change in the sensed voltage; and calculate the temperature of the superconducting material sample based on the value of the current at the time of the detected change in the sensed voltage; The superconducting material sample is arranged on the quantum computing chip to measure the temperature of the quantum processor. A quantum computing system as claimed in any preceding claim, wherein the voltage sensor comprises an amplifier configured to amplify the voltage across the superconducting material sample. A quantum computing system as claimed in claim 2, wherein the voltage sensor further includes a comparator circuit configured to receive the amplified voltage as an input and output a digital signal, preferably a Schmitt trigger, the value of the digital signal changing in response to a change in voltage occurring when the time-varying current exceeds the critical current, wherein the control unit detects the change in voltage based on a change in the digital signal. A quantum computing system as claimed in any preceding claim, further comprising a magnetic field sensor configured to measure the 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. A quantum computing system as claimed in any preceding claim, wherein the superconducting material sample is made of titanium nitride. A quantum computing system as claimed in any preceding claim, wherein the superconductor material sample is formed into a film preferably having a thickness in the range of 0.1 nanometers (nm) to 100 nm, more preferably 1 nm to 20 nm. A quantum computing system as claimed in claim 6, wherein: the film has a length in the direction of current passing through the superconductor material sample in the range of 0.1 micrometer (μm) to 100 micrometers, preferably 10 μm to 50 μm; and/or has a width in the direction transverse to the direction of current passing through the superconductor material sample in the range of 0.01 μm to 100 μm, preferably 0.36 μm to 2 μm. A quantum computing system as claimed in any of the preceding claims, wherein the current source and the voltage sensor each comprise a pair of respective electrodes electrically connected to the superconducting material sample, wherein a current passing through the superconducting material sample passes between the electrodes of the current source and a sensed voltage is sensed between the electrodes of the voltage sensor; wherein the electrodes of the voltage sensor are arranged between the electrodes of the current source. A quantum computing system as claimed in any preceding claim, wherein the control unit is configured to increase the current from below the critical current to above the critical current when the voltage sensor senses the voltage across the superconducting material sample. A quantum computing system as claimed in any preceding claim, further comprising a magnetic field source configured to generate a magnetic field at the location of the superconducting material sample, wherein the control unit is configured to control the generated magnetic field based on a target temperature value. A quantum computing system as claimed in any preceding claim, wherein the current source and the voltage sensor are disposed on the quantum computing chip for measuring the temperature of the quantum processor. A quantum computing system as claimed in any preceding claim, comprising a plurality of said cryogenic sensors, said current source, voltage sensor and superconducting material sample of each of said plurality of cryogenic sensors being arranged to measure the temperature of a different respective region of said quantum computing chip. A method for measuring the temperature of a superconducting material sample, wherein the superconducting material sample is disposed on a quantum computing chip, the quantum computing chip including a quantum processor, the method comprising: passing a momentarily 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 the temperature of the superconducting material sample based on the value of the current at the time of the detected change in the sensed voltage.