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Superconductor Science and Technology

Using a Josephson junction as an effective on-chip temperature sensor

Paolo Durandetto, A. Sosso

Published Jan 18, 2021• 8 citations
Publisher

Abstract

Abstract Temperature sensing and control are essential in experiments running in cryocooled systems, as with the case of liquid helium-free superconducting devices. A Josephson junction as on-chip temperature sensor operated in ac provides the highest sensitivity and minimal power loading to the cryogenic environment, thanks to the noise rejection of lock-in detection. To demonstrate the advantages of on-chip sensing, we tested it with a Josephson voltage standard array in cryocooler and compared with the conventional case of a sensor on the cold surface of the refrigerator, showing that the power dissipated within the chip may further increment the device temperature up to some tenths of kelvin. An ac Josephson junction sensor is proven to be capable of directly stabilizing the temperature of the superconductive circuit from fluctuations of dissipated power during operation. Reliability issues related to flux trapping are discussed and solutions are proposed suited to different applications. Overall, on-chip control with ac Josephson temperature sensing has the advantage of avoiding the complexities in minimization of cryogenic thermal links, virtually reducing to zero the contact resistance and keeping the operating temperature of the superconductor constant, independently of instantaneous operating power.

DOI
10.1088/1361-6668/abdcc4

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Abstract Temperature sensing and control are essential in experiments running in cryocooled systems, as with the case of liquid helium-free superconducting devices. A Josephson junction as on-chip temperature sensor operated in ac provides the highest sensitivity and minimal power loading to the cryogenic environment, thanks to the noise rejection of lock-in detection. To demonstrate the advantages of on-chip sensing, we tested it with a Josephson voltage standard array in cryocooler and compared with the conventional case of a sensor on the cold surface of the refrigerator, showing that the power dissipated within the chip may further increment the device temperature up to some tenths of kelvin. An ac Josephson junction sensor is proven to be capable of directly stabilizing the temperature of the superconductive circuit from fluctuations of dissipated power during operation. Reliability issues related to flux trapping are discussed and solutions are proposed suited to different applications. Overall, on-chip control with ac Josephson temperature sensing has the advantage of avoiding the complexities in minimization of cryogenic thermal links, virtually reducing to zero the contact resistance and keeping the operating temperature of the superconductor constant, independently of instantaneous operating power.
In this paper we show how an overdamped Josephson junction can be exploited as temperature sensor in a closed-loop circuit to control the operating point of a cryocooled device, e.g. a quantum standard. The resistance of a Josephson junction is measured at the desired temperature with a commercial ac resistance meter, that then uses this value to regulate the chip temperature via its own PID control functionality. Experimental setup and measurement methods are described here. Moreover, the temperature sensitivity of an ideal junction has been determined for different bias-current values.
Abstract Providing radio frequency (RF) signals to cryogenic circuits requires bulky and expensive cabling interfacing specialized RF electronics at room temperature. Superconducting-normal metal-superconducting Josephson junction arrays (JJAs) can change this paradigm by placing the RF source and detector on-chip. In this work, we demonstrate that DC-biased JJAs can emit signals in the C-band frequency spectrum and beyond, with a minimal linewidth of 106.5 ± 0.1 MHz and maximum power of 11.9 fW. We fabricate JJAs comprised of amorphous MoGe or NbTiN superconducting islands and metallic Au weak links. Temperature, magnetic field, applied current, and device design are explored to control the RF sources, while we identify important features that affect the ideal source behavior. Combined with the ability of these JJAs to detect microwave radiation, these sources allow us to propose a fully DC-operated cryogenic on-chip measurement platform that is an alternative to the high-frequency circuitry currently required for quantum applications.
Abstract The paper describes a dry-cooled AC quantum voltmeter system operated up to kilohertz frequencies and 7 V rms. A 10 V programmable Josephson voltage standard (PJVS) array was installed on a pulse tube cooler (PTC) driven with a 4 kW air-cooled compressor. The operating margins at 70 GHz frequencies were investigated in detail and found to exceed 1 mA Shapiro step width. A key factor for the successful chip operation was the low on-chip power consumption of 65 mW in total. A thermal interface between PJVS chip and PTC cold stage was used to avoid a significant chip overheating. By installing the cryocooled PJVS array into an AC quantum voltmeter setup, several calibration measurements of dc standards and calibrator ac voltages up to 2 kHz frequencies were carried out to demonstrate the full functionality. The results are discussed and compared to systems with standard liquid helium cooling. For dc voltages, a direct comparison measurement between the dry-cooled AC quantum voltmeter and a liquid-helium based 10 V PJVS shows an agreement better than 1 part in 10 10 .
The two main challenges to operating a programmable Josephson voltage standard (PJVS) on a cryocooler are the available cooling power and the temperature oscillations of the cold head. We overcame these challenges and successfully operated a PJVS circuit on a cryocooler by employing one supercritical helium buffer that damps the temperature oscillations, developing a new cryogenic package that increases the thermal conductivity between the chip and the cold head, and increasing overall device performance with fabrication improvements. A 1.32 mA step width of the quantized voltage produced with all of the subarrays of the PJVS circuit biased was achieved at an operating temperature of 4.3 K. The quantum accuracy of the PJVS is maintained at temperatures up to 4.8 K. This result was obtained with a cryocooler that employs a 3 kW water-cooled compressor to produce at the chip about 270 mW of net cooling power at 4.3 K.
Low energy dissipation and ability to operate at low temperatures provide for Josephson junction circuits a niche as a support for low temperature devices. With high speed operation (Chen W. et al., 1999) capability the Josephson junction circuits make a prime candidate for applications which are difficult to engineer with existing CMOS technology. The development of Josephson junction technology took a major turn for the better with the invention of the Rapid-Single-Flux-Quantum (RSFQ) devices (Likharev K.K. et al., 1991), an improvement over voltage biased Josephson Junctions logic which were plagued with the junction switching and reset problems. The modern applications of SFQ circuits extend to a larger range of temperature operation and the applications vary from low temperature magnetic sensor, to high speed mixed signal circuits, voltage and current standards (Turner C.W. et al., 1998), and auxiliary components for quantum computing circuits. Most of the SFQ circuits are fabricated with Niobium, but Aluminum based circuits are being used for quantum gates (Nielsen M.A. et al., 2000) and qubit operations. SFQ circuits based on Magnesium di-Boride junctions are being developed for higher temperature operations (Tahara S. et al., 2004). Predominantly most of the Josephson junction circuits today are operated at around 4K. All the circuits are optimized usually for liquid helium temperatures, so the circuits operated in helium bath Dewars or cryostat's do not experience any temperature gradients or drift effects which can affect the operating margins. With the improvement in the fabrication technology and soft-wares for SFQ circuit technology, the designing complex circuits have become easier. Complex Circuits with over 20K junctions such digital synthesizer and digital RF Trans-receiver have already been demonstrated (Oleg M.A. et al., 2011). Development of circuits over 100K junctions are actively under progress. However, with enormously large circuits, power requirements also increase. Looking at the range of applications and complexity of the problems of energy minimization, we try to look at the problem in two approaches. One for large complex circuits, we try to reduce the power bias itself, or the overall load of current that is supplied
Superconducting single flux quantum (SFQ) technology is well known for digital logic operations below 4.2 K, which are compatible with the Josephson junction qubit. Operation speed and signal sensitivity of circuits in SFQ are also sufficient to control qubits. Therefore, the SFQ circuit is a candidate technology for multiple qubit control. However, the self-heating effect of the SFQ circuit is a potential barrier to development of an SFQ-supported multiple qubit system because the qubit operating temperature is lower than the conventional 4.2 K. The multi-chip approach is a possible solution because the qubit chip can be thermally isolated from the SFQ circuitry. We report on recent progress in SFQ circuit and packaging technology for operation at sub-Kelvin temperatures. As for multi-chip module technology, a test sample of the multi-chip module shows good reliability of electrical connections between chips, and each solder bump has a bandwidth wide enough for signal transmission. We have designed lower power SFQ circuits for sub-Kelvin temperatures. The critical currents of Josephson junctions were scaled down, inductances were scaled up, and bias voltage was reduced to make power consumption lower. Based on our fabrication technology, critical current densities were optimized by considering trade-off relations between fabrication availability and operation speed. We successfully confirmed its correct operations at around 0.35 K. The combination of the multi-chip module and optimized SFQ circuits is very effective way of controlling qubit.