A method for testing current of a low-temperature superconducting Josephson junction in a non-magnetic shielding environment

Abstract

This invention relates to a method for testing the current of a low-temperature superconducting Josephson junction in a non-magnetically shielded environment, comprising the following steps: placing the sample to be tested on a sample holder, and placing the sample holder at the bottom of the test rod; inserting the test rod into a Dewar filled with liquid helium to measure the temperature of the sample to be tested; and starting to measure the I of the sample to be tested when the temperature of the sample to be tested is lower than a preset temperature. cij Measure m times, and calculate the average value I based on the m measurements. ci‑av and uncertainty U i The sample to be tested is then heated to remove magnetic flux; it is determined whether the current test round has reached the set round number n. If not, the next round of testing is performed; if so, the average value I obtained from multiple pairs is used. ci‑av and uncertainty U i Construct a relational model; obtain U based on the relational model. i The average value I when = 0 ci‑av The average value I will be obtained. ci‑av As the critical current I in an unshielded environment c The empirical value. This invention solves the problems of high cost and low accuracy in existing low-temperature superconducting Josephson junction critical current measurement without parallel resistance.

Description

Technical Field

[0001] This invention relates to the field of superconducting electronics technology, and in particular to a method for testing the current of a low-temperature superconducting Josephson junction in an environment without magnetic shielding. Background Technology

[0002] Superconducting electronic circuit technology, characterized by high clock frequencies and minimal power consumption in bit operations, is a prime candidate for future high-performance superconducting computers. MIT Lincoln Laboratory (MIT LL) has made significant progress in the fabrication of superconducting fast single-throughput quantum (RSFQ) circuits, designing high-performance superconducting multi-bit processors for clock frequencies ranging from 10 GHz to 20 GHz for high-performance computing. Traditional processes typically use parallel Josephson junctions (i.e., resistors connected in parallel across a Josephson junction) as the basic unit for forming circuits. While these Josephson junctions (JJs) offer good noise immunity and circuit stability, they occupy a large area, limiting the integration density of JJ processes to typically below 10⁶ JJs / cm². 2 To increase the integration scale of the SFQ circuit to 10⁶ JJs / cm 2 To meet the requirements of large-scale integrated circuit applications, it is necessary to reduce the area occupied by junction boxes (JJs) and shunt resistors. JJs without parallel resistors significantly reduce the area per unit area. Therefore, JJs without parallel resistors have become potential candidates for high-integration-density, high-current-density, and high-clock-frequency superconducting Josephson circuits.

[0003] Compared to a conventional resistor-based junction box (JJ), a junction box without a parallel resistor is more sensitive to electromagnetic and magnetic noise from circuits, space, and other sources. The critical current (Ic) c As one of the important physical parameters of JJ, the critical current I is crucial for subsequent device and circuit design and process preparation. c The accuracy of measurements directly affects the margins of various electrical parameters in subsequent circuits. To obtain accurate measurements, testing should typically be conducted in a suitable shielded room. However, due to limitations in laboratory conditions, actual testing is usually performed in unshielded environments. This inevitably introduces various environmental noises into the JJ, resulting in significant uncertainty in the JJ current measurement results. Furthermore, compared to the environment of traditional computers, future superconducting computers will operate in noisy environments to achieve competitive applications with semiconductor computers. Therefore, it is necessary to conduct precise measurement studies of unparallel JJ currents in unshielded environments and to evaluate filtering techniques suitable for unshielded environments.

[0004] According to the Ambegaokar-Baratoff theory, the theoretical value of the critical current I c0 It can be calculated using the formula: I c0 =I g*π / 4. I g Defined as the current at which the junction enters its normal state, V g It is defined as the gap voltage of the Josephson junction. Due to the influence of complex noise effects, the actual measured I... c The value is usually less than the theoretical value I. c0 This severely affects the circuit performance of superconducting Josephson devices. Yasushi Ishikawa et al. analyzed the impact of magnetic noise on the IJ of superconductor / ordinary metal / superconductor (SNS) devices. c The influence of characteristics revealed that the Josephson current's dependence on the magnetic field is highly sensitive to the width of ordinary metals. Ohkubo et al. investigated the influence of I under overdamped conditions. c There are standard measurement methods, but there is a lack of research on underdamped (no parallel resistance) Nb / Al-AlOx / Nb junctions under unshielded conditions. Summary of the Invention

[0005] This invention provides a method for testing the current of a low-temperature superconducting Josephson junction in a non-magnetically shielded environment, solving the problem of current measurement in existing low-temperature superconducting junctions without parallel resistance. c The problem is that the measurement cost is high and the accuracy is low.

[0006] The technical solution adopted by this invention to solve its technical problem is: to provide a method for testing the current of a low-temperature superconducting Josephson junction in a non-magnetically shielded environment, comprising the following steps:

[0007] Place the sample to be tested on the sample holder, and place the sample holder at the bottom of the test rod;

[0008] Insert the test rod into a Dewar filled with liquid helium to measure the temperature of the sample to be tested.

[0009] When the temperature of the sample to be tested is lower than the preset temperature, the I value of the sample to be tested is measured. cij Measure m times, and calculate the average value I based on the m measurements. ci-av and uncertainty U i The sample to be tested is then heated to remove magnetic flux; where i represents the test round and j represents the sequence number of each test round.

[0010] Determine if the current test round has reached the set round number n. If not, return to the previous step and proceed to the next round. If it has, calculate the average value I obtained from multiple pairs. ci-av and uncertainty U i Construct the average value I ci-av and uncertainty U i Relational model;

[0011] Based on the aforementioned relational model, U is obtained. i The average value I when = 0 ci-av The average value I will be obtained.ci-av As the critical current I in an unshielded environment c Experience points.

[0012] Before placing the sample to be tested on the sample holder and placing the sample holder at the bottom of the test rod, the method further includes the step of wrapping the sample to be tested with aluminum foil.

[0013] When inserting the test rod into the Dewar filled with liquid helium, the sample to be tested is placed at a depth of more than 10 cm below the liquid surface.

[0014] The preset temperature is 4.3K.

[0015] The relational model is I. ci-av =A-Bexp(-U i / C), where A, B and C are fitting parameters.

[0016] Beneficial effects

[0017] Due to the adoption of the above technical solution, this invention has the following advantages and positive effects compared with the prior art: This invention conducts testing and research on non-parallel superconducting Josephson junctions in a non-magnetically shielded environment, and establishes the critical current I of the superconducting Josephson junction. c The test method is based on the critical current I under different demagnetization flux conditions. c The relationship between the value and uncertainty is used to extrapolate the critical current I of the Josephson junction under magnetically shielded conditions. c The value is determined to avoid or reduce the impact of the test environment and magnetic flux noise on the critical current I under actual experimental conditions. c The impact provides a reference and has guiding significance and application value for the subsequent research and application of SFQ circuits and superconducting large-scale digital integrated circuits. Attached Figure Description

[0018] Figure 1 This is a flowchart of the method for testing the current of a low-temperature superconducting Josephson junction under non-magnetic shielding conditions according to an embodiment of the present invention;

[0019] Figure 2 This is a relationship model curve diagram of sample s0506-juf3.1 in the embodiments of the present invention;

[0020] Figure 3 This is a comparison chart of the measurement results of sample s0506-juf3.1 with and without aluminum foil in an embodiment of the present invention. Detailed Implementation

[0021] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0022] The present invention relates to a method for testing the current of a low-temperature superconducting Josephson junction in a non-magnetically shielded environment. This method measures and obtains the test current I under non-magnetically shielded conditions. ci-av With uncertainty U i The relational model, based on which the extrapolation is obtained when U i I when = 0 ci-av The value is the critical current I in an unshielded environment. c Empirical values. For example... Figure 1 As shown, it includes the following steps:

[0023] Step 1: Place the sample to be tested on the sample holder and place the sample holder at the bottom of the test rod; before placing the sample to be tested on the sample holder, the sample to be tested can also be wrapped with aluminum foil.

[0024] Step 2: Insert the test rod into the Dewar filled with liquid helium, and position the sample to be tested 10 cm below the liquid helium surface, and measure the temperature of the sample to be tested;

[0025] Step 3: When the temperature of the sample to be tested is below 4.3K, begin measuring the I of the sample to be tested. cij Measure m times, and calculate the average value I based on the m measurements. ci-av and uncertainty U i The sample to be tested is then heated to remove magnetic flux; where i represents the test round and j represents the sequence number of each test round.

[0026] Step 4: Determine if the current test round has reached the set round number n. If not, return to the previous step to start the next round of testing. If it has reached the set round number n, calculate the average value I obtained from multiple pairs. ci-av and uncertainty U i Construct the average value I ci-av and uncertainty U i Relational model;

[0027] Step 5, calculate U based on the relational model. i The average value I when = 0 ci-av The average value I will be obtained. ci-av As the critical current I in an unshielded environment c Experience points.

[0028] The present invention will be further illustrated by a specific embodiment below.

[0029] In this embodiment, the above steps were used to actually measure the S0506-Juf3.1 sample, and I was obtained under unshielded conditions. c with I c Uncertainty relationship, the relationship is as follows Figure 2 As shown, its relational model is: I ci-av =A-Bexp(-U i / C), where A = 749μA, B = 193μA, and C = 20. It is worth noting that due to different testing environments, the fitting parameters A, B, and C all have a ±20% offset. Based on the above relationship model, the critical current I under shielding conditions can be extrapolated. c The empirical value is 556 μA, and the critical current I of the s0506-juf3.1 sample under shielded conditions is... c The measured value was 560 μA, and the deviation between the two was less than 1%.

[0030] The above steps are actually based on Nb / Al-AlO x In this embodiment, for Nb samples, the cases with and without aluminum foil were compared to obtain repeatability measurement results of the critical current with and without aluminum foil. (See attached image.) Figure 3 Based on Nb / Al-AlO x Test data for the Nb sample revealed that the average critical current I in the Nb cylinder wrapped in aluminum foil was... c The average critical current I of the uncoated aluminum foil sample is 349 μA. c The flux density (IA) was 299 μA, 16.7% higher with aluminum foil coating than without. Experiments demonstrate that wrapping the Nb cylinder with aluminum foil helps reduce magnetic leakage and increases the temperature gradient between the liquid helium and the sample, increasing the probability that magnetic flux is excluded from the superconducting layer and thus contributing to a higher critical current (IA). c The accuracy of the value measurement.

Claims

1. A method for testing the current of a low-temperature superconducting Josephson junction in a non-magnetically shielded environment, characterized in that, Includes the following steps: Place the sample to be tested on the sample holder, and place the sample holder at the bottom of the test rod; Insert the test rod into a Dewar filled with liquid helium to measure the temperature of the sample to be tested. Measurement of the sample begins when the temperature of the sample to be tested is lower than a preset temperature. ,Measurement Next, based on the measurement The average of the results is calculated. and uncertainty The sample to be tested is then subjected to a heating and demagnetization operation; wherein, Indicates the number of test rounds. Indicates the sequence number of each test round; Determine if the current test round has reached the set number of rounds. If the target is not reached, return to the previous step and perform the next round of testing; if the target is reached, calculate based on the average of the multiple pairs. and uncertainty Construct average and uncertainty Relational model; Based on the aforementioned relational model, obtain Average value when =0 The average value will be obtained. As the critical current in an unshielded environment Experience points; Wherein, the preset temperature is 4.3K, and the relationship model is... ,in, , and These are the fitting parameters.

2. The method for testing the current of a low-temperature superconducting Josephson junction in a non-magnetically shielded environment according to claim 1, characterized in that, Before placing the sample to be tested on the sample holder and placing the sample holder at the bottom of the test rod, the method further includes the step of wrapping the sample to be tested with aluminum foil.

3. The method for testing the current of a low-temperature superconducting Josephson junction in a non-magnetically shielded environment according to claim 1, characterized in that, When inserting the test rod into the Dewar filled with liquid helium, the sample to be tested is placed at a depth of more than 10 cm below the liquid surface.

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