Current-induced noise thermometer

Abstract

A current-sensing noise thermometer includes: a sensor resistor thermally coupled to a target; a superconducting coil; a superconducting thermal circuit breaker located between the two ends of the sensor resistor and the two ends of the superconducting coil; and a superconducting flow sensor; wherein the sensor resistor, the superconducting coil, and the superconducting thermal circuit breaker form a loop inductively coupled to the superconducting flow sensor. A noise filter may be present between the sensor resistor and the superconducting coil.

Description

[0001] Cross-reference to related applications

[0002] This application claims priority to UK application 2007785.5 entitled “Current-Induced Noise Thermometer”, filed on 26 May 2020, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to apparatus and methods for measuring extremely low temperatures, such as those in the range of a few milliklvin. Background Technology

[0004] The conventional method for measuring temperatures below approximately 4 K and down to the milliKelvin range is based on the use of a current-sensing noise thermometer as described in US6357912B1. Figure 1 of the accompanying drawings is a schematic diagram of such a device. As shown, a grounded sensor resistor Ro is connected in series with a superconducting coil L and to a SQUID input coil Lo. Grounding the sensor resistor allows it to be cooled to temperatures of a few microKelvin. To avoid introducing any additional Johnson noise, Lo is inductively coupled to the SQUID, and all connections to the sensor resistor are preferably made of a superconducting material. Temperature measurements are performed by measuring the noise current density and calibrated based on a known temperature at which a change in coil inductance is observed due to the Meisner effect.

[0005] In the prior art, the heat leakage into the sensor resistor R0 is on the order of 10 pW. This heat leakage creates a temperature gradient between the thermometer and the target of interest. To achieve an acceptable temperature gradient and obtain the lowest achievable temperature, only sensors with low resistance values ​​can be used. Using low-value resistors to meet these constraints in the prior art results in a lower bandwidth system and therefore a longer measurement time. Summary of the Invention

[0006] The drawback of traditional current-sensing noise thermometers is that parasitic heat leakage into the device limits its operating speed at the lowest temperatures.

[0007] An improved thermometer is desired that can measure temperatures below about 4K, and preferably in the milliKelvin range.

[0008] According to the present invention, a current-sensing noise thermometer is provided, comprising:

[0009] The sensor resistance that is thermally coupled to the target under test;

[0010] Superconducting coils;

[0011] A superconducting thermal circuit breaker located between the two ends of the sensor resistor and the two ends of the superconducting coil; and

[0012] Superconducting flow sensor;

[0013] The sensor resistor, superconducting coil, and superconducting thermal circuit breaker form a loop that is inductively coupled to the flow sensor.

[0014] The embodiments of the present invention can achieve heat leakage of less than 1fW. Attached Figure Description

[0015] Exemplary embodiments of the present invention are described below with reference to the accompanying drawings, wherein:

[0016] Figure 1 is a schematic diagram of a current-sensing noise thermometer of the prior art;

[0017] Figure 2 This is a schematic diagram of a current-sensing noise thermometer based on the first implementation scheme;

[0018] Figure 3 This is a schematic diagram of a current-sensing noise thermometer according to the second embodiment; and

[0019] Figure 4 This is a graph of temperature data measured using an embodiment of the present invention.

[0020] In the figures, similar parts are indicated by similar reference numerals. Detailed Implementation

[0021] Figure 2 This is a schematic diagram of an ultra-low heat leakage, fast current-sensing noise thermometer. The sensing resistor R0 is electrically connected to the target object at or near its midpoint. The sensing resistor R0 is connected to the input coil L0 via superconducting thermal circuit breakers SC0 and SC1 and a filter F0. This superconducting input coil L0 is inductively coupled to a flow sensor 10, such as a SQUID or HyQUID, which is electrically coupled to a readout circuit 11. The sensing resistor R0, superconducting thermal circuit breakers SC0 and SC1, filter F0, and input coil L0 form a single loop, which is directly inductively coupled to the flow sensor 10. This enables a large bandwidth, for example, about 1 MHz, and therefore allows for fast measurement. It is desirable that no transformer is provided between the sensor loop and the flow sensor.

[0022] Positioning the electrical connection to the target at the midpoint of the sensing resistor R0 reduces the temperature gradient within the device. It is desirable that this connection be as close to the midpoint as possible, for example, about 10% of the resistance length, because the benefit diminishes with distance from the midpoint. Ideally, the resistance of the connection between the sensing resistor and the target should be much smaller than the resistance of the sensing resistor itself. The resistance of the connection can be on the order of tens of mΩ to nΩ.

[0023] The sensor resistor R0, superconducting thermal circuit breakers SC0 and SC1, and filter F0 are located in the environment of the target temperature and can be considered to be at the measurement temperature T in equilibrium. The input coil L0 and flow sensor 10 only need to remain superconducting at sufficiently low temperatures, such as below about 4K or about 1K if aluminum HyQUID is used as the flow sensor. The readout circuit 11 operates at room temperature.

[0024] The sensing resistor R0 can have a resistance value ranging from 20 mΩ to 100 Ω, with a desired value of 100 mΩ to 5 Ω and an optimal value of 200 mΩ to 2 Ω. The optimal resistance depends on the temperature being measured; lower resistance values ​​are more suitable for lower temperatures but require longer measurement times. The sensing resistor R0 can be made of platinum-tungsten alloy (PtW) or other materials such as BeCu or brass. The sensing resistor R0 can be in the form of wire, for example, with a diameter of 50 μm and a length on the order of approximately 0.1 mm to 10 mm. Wire resistors made using materials with relatively high resistivity can be smaller than those made using copper foil resistors in some prior art designs. The wire resistor can be made freestanding, for example, supported only by a connection to the target and / or a superconducting thermal circuit breaker. Making the sensor resistor freestanding improves its isolation from parasitic thermal inputs. The sensing resistor R0 must not be superconducting. It is desirable for the sensing resistor R0 to have a weakly temperature-dependent resistivity, which is true for most alloys. It is also desirable that the material of the sensing resistor R0 has low magnetoresistivity, enabling the device to be used in high external magnetic fields (e.g., 0.1T to 10T). The sensing resistor R0 can be spot-welded or wire-bonded to superconducting thermal circuit breakers SC0 and SC1. The sensing resistor R0 can also be part of another device (e.g., a qubit) whose temperature will be measured.

[0025] It is desirable that the connection between the sensor resistor R0 and the target under test be achieved using a wire with high thermal conductivity. For example, copper or gold wire can be used. This direct connection between the target and the sensor resistor R0 allows for direct measurement of the temperature of a specific component, rather than measuring the more macroscopic environment of the component as in existing designs. Furthermore, faster measurements are possible, in some cases on the order of less than one second. The wire connection also reduces or eliminates disturbances to the target under test. The conductive connection also allows for direct measurement of the temperature of the electron gas (e.g., two-dimensional electron gas), rather than inferring it from the temperature of the electron gas platform.

[0026] It is desirable that the superconducting thermal circuit breakers SC0 and SC1 be directly connected to the sensor resistor R0. In prior art devices, the resistive sensor is connected to the SQUID input coil via NbTi superconducting wire (typically 100 μm in diameter). In embodiments of the present invention, the superconducting thermal circuit breakers SC0 and SC1 have low thermal conductivity and can be formed using bare superconducting wire (e.g., aluminum or niobium wire) with a diameter of 10-50 μm. It is desirable that the superconducting thermal circuit breakers SC0 and SC1 have a thermal conductivity of less than or equal to 10 nW / K, more desirable less than or equal to 1 nW / K, and most desirable less than or equal to 0.5 nW / K. In one embodiment, the thermal conductivity is about 0.2 nW / K. Niobium is desirable in high-field applications. The superconducting circuit breakers can be wire-bonded or spot-welded to the resistor sample and / or the superconducting wire leads connected to the input coil L0. The superconducting thermal circuit breakers SC0 and SC1 reduce the heat input from the high-temperature (<4K) environment of the coil L0 and the flow sensor 10, thus enabling more accurate and faster temperature measurement.

[0027] Filter F0 is a noise filter configured to prevent high-frequency noise from propagating to the sensor resistor R0 and thereby introducing heat. Filter F0 is a desired low-pass filter, for example, with a cutoff frequency of approximately 100 MHz or higher. It is desirable to configure filter F0 to prevent electromagnetic energy (e.g., GHz noise in the SQUID, which serves as flow sensor 10) from propagating to the sensor resistor R0. It is desirable that the temperature of filter F0 be as close as possible to the target measurement temperature to reduce thermal noise coupled from the normal metallic components of the filter into the circuit.

[0028] An advantageous form of filter that can be used in this invention is a threaded filter, for example, comprising superconducting twisted-pair wires wound around a threaded conductive core and embedded in a metal-doped epoxy resin material. The length of the superconducting wire in the threaded filter can be from about 10 cm to 1 m, with shorter lengths being more suitable for lower temperatures. The threaded walls shield adjacent turns of the superconducting wire from each other. The core can be made of pure silver or copper and is preferably not superconducting so that it can be cooled more easily. The metal-doped epoxy resin material is desired to have a high metal particle density but is non-conductive. The metal particles are preferably non-ferrous metals, such as silver, copper, platinum, or gold. The inventors have determined that, in some cases, it is desirable to control the curing temperature of the epoxy resin to ensure that the metal-doped epoxy resin is non-conductive. Before winding the twisted pair, the metal-doped epoxy resin material can be coated onto the core, and then more metal-doped epoxy resin material can be coated after winding to completely surround the twisted pair. An outer sheath of the same material as the core can also be provided to shield the filter from external fields.

[0029] The flow sensor 10 is inductively coupled to a coil L0 and can be a SQUID (ideally a DC SQUID, i.e., a superconducting loop containing two Josephson junctions) or a HyQUID (a superconducting loop interrupted by a normal conducting segment and having an interferometer connected to the normal conducting segment – ​​see, for example, WO 2012 / 007736A1). The function of the flow sensor 10 is to pick up and amplify the Johnson noise (also known as Johnson-Nyquist noise or thermal noise) in the sensor resistor R0 so that the Johnson noise can be measured via the readout circuit 11. Because the power density of the Johnson noise is linearly related to temperature via the Boltzmann constant, the temperature of the sensor resistor can be determined. The noise thermometer can be calibrated using the superconducting transition of a superconductor in a reference device (e.g., a superconducting thermal circuit breaker SC0, SC1).

[0030] Figure 3 Another embodiment of the invention is described, which may be referred to as a helium immersion current-sensing noise thermometer. Figure 3 Implementation plan and Figure 2 The same parts of the embodiments are indicated by the same reference numerals, and for the sake of brevity, they will not be described further below.

[0031] exist Figure 3 In one embodiment, the sensor resistor R0 is electrically isolated from the target under test, but is instead thermally coupled to the target via one or more wires encased in a silver sintered material and immersed in a liquid helium (e.g., helium-3 (3He)) bath 12. Two wires may be immersed in one bath. The sensor resistor may also be located in a liquid helium bath. The silver sintered material increases thermal conductivity between the helium and the wires by increasing the contact surface area. Using two wires to couple the sensor resistor to the target provides a balanced system, similar to the midpoint connection in the embodiment of Figure 1.

[0032] Embodiments of the present invention can reduce heat leakage to within the fW range, which allows the use of sensor resistors with greater resistance, increases bandwidth, and thus reduces the required measurement time. Figure 4 A series of measurements performed using a thermometer according to an embodiment of the invention are depicted. Measurements can be taken approximately every 15 seconds, showing the shape of the sample's response to an applied thermal pulse. Furthermore, the sample's heat capacity can be determined by the distance between horizontal lines, which represent the average temperature before (solid lines) and after (dashed lines) the application of the thermal pulse. Such measurements are not possible in prior art designs, which typically provide measurements representing average temperatures over time intervals of several minutes.

[0033] Embodiments of the present invention can produce a smaller device than some prior art devices, making it easier to install the device in available space, reducing the heat capacity of the thermometer, and allowing multiple thermometers to be connected.

[0034] In particular, epoxy resin threaded filters with doped metals and superconducting wires serve as low-pass filters (F0) to filter out potential high-frequency noise that may heat the sensor resistor.

[0035] Furthermore, the increased bandwidth means that temperature can be measured in higher magnetic field environments (because noise caused by magnetic field vibrations no longer dominates the spectrum, and this noise can be ignored during fitting to extract the temperature).

[0036] Therefore, this invention provides a fast, compact, high-field, low-power precision noise thermometer that can be used in various fields, particularly for measurements in many quantum technologies, such as determining the temperature of components (e.g., qubits) in quantum information systems. By achieving rapid measurements with high precision (e.g., 1% accuracy within 1 second), new types of measurements are made possible, such as heat capacity measurements at extremely low temperatures. This invention is particularly suitable for studying the fractional quantum Hall effect by accurately measuring the temperature of a two-dimensional electron gas. This invention can be used for the calibration of other devices, such as other forms of cryogenic thermometers. Embodiments of this invention can operate in magnetic fields up to 10 Tesla. Embodiments of this invention can measure temperatures below about 9 K, with a desire to measure temperatures below 4 K and, more importantly, temperatures in the range of 0.1 mK to 100 mK.

[0037] Having described embodiments of the invention, it will be understood that the foregoing embodiments are exemplary rather than prescriptive. Variations may be made within the scope of the invention, as defined by the appended claims.

Claims

1. A current-sensing noise thermometer, comprising: The sensor resistance that is thermally coupled to the target under test; Superconducting coils; A superconducting thermal circuit breaker is located between the two ends of the sensor resistor and the two ends of the superconducting coil. The superconducting thermal circuit breakers SC0 and SC1 have a thermal conductivity of less than or equal to 10 nW / K. as well as Superconducting flow sensor; The sensor resistor, superconducting coil, and superconducting thermal circuit breaker form an inductively coupled loop with the superconducting flow sensor.

2. The current-sensing noise thermometer according to claim 1, wherein the superconducting thermal circuit breaker comprises wire, and the wire is niobium or aluminum wire.

3. The current-sensing noise thermometer according to claim 1, wherein the superconducting thermal circuit breaker is connected to the sensor resistor by wire bonding or spot welding.

4. The current-sensing noise thermometer according to claim 1, wherein the superconducting thermal circuit breakers SC0 and SC1 have a thermal conductivity of less than or equal to 1 nW / K.

5. The current-sensing noise thermometer according to claim 1 further includes a noise filter located between the sensor resistor and the superconducting coil.

6. The current-sensing noise thermometer according to claim 5, wherein the noise filter is a low-pass filter having a cutoff frequency of 100MHz or higher.

7. The current-sensing noise thermometer according to claim 5, wherein the noise filter is a threaded filter with superconducting wire, and the superconducting wire is embedded in an epoxy resin doped with non-ferrous metals.

8. The current-sensing noise thermometer according to claim 1, wherein the midpoint of the sensor resistor is electrically coupled to the target.

9. The current-sensing noise thermometer according to claim 1, wherein the sensor resistor is electrically isolated from the environment.

10. The current-sensing noise thermometer of claim 9, wherein the sensor resistor is thermally coupled to the target via a wire immersed in liquid helium.

11. The current-sensing noise thermometer according to claim 10, wherein the wire is coated with silver sintered material.

12. The current-sensing noise thermometer of claim 1, wherein the sensor resistor is incorporated into another device.

13. The current-sensing noise thermometer according to any one of claims 1 to 12, wherein, The current-sensing noise thermometer is used to measure the temperature of a target in a strong magnetic field.

14. The current-sensing noise thermometer according to any one of claims 1 to 12, wherein, The current-sensing noise thermometer is used to measure the temperature of an electronic gas.

15. The current-sensing noise thermometer according to any one of claims 1 to 12, wherein, The current-sensing noise thermometer is used for calibrating another device.

16. The current-sensing noise thermometer according to claim 1, wherein the superconducting thermal circuit breakers SC0 and SC1 have a thermal conductivity of less than or equal to 0.5 nW / K.

Images