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Conference on Precision Electromagnetic Measurements

On-chip Josephson junction-based sensor for temperature control of a cryocooled quantum standard

Paolo Durandetto, A. Sosso

Published Aug 1, 2020• 0 citations
Publisher

Abstract

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.

DOI
10.1109/cpem49742.2020.9191717

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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 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.
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.
Optimization of the performance of Nb-based overdamped Josephson junctions for applications to superconducting electronics is a major issue today. The use of cryocoolers, thus avoiding liquid helium refrigeration, with junctions working at temperatures higher than 4.2 K, could greatly increase the use of these devices in applications such as fundamental metrology and measurement sensors. The question we address here is how to maximize the characteristic voltage and minimize the temperature influence on the current-voltage curves of overdamped junctions enabling their use at T > 4.2K. To optimize the properties of a single Josephson heterostructure, we propose a type of overdamped junction that consists of a relatively thin insulating layer and a thick (tens of nm) normal-metal film between two superconducting electrodes. Measurements of the dependence of the critical current and characteristic voltage as function of temperature for dierent electrodes configurations show how it is possible to improve the design of these junctions.
This paper presents a cryocooled Josephson arbitrary waveform synthesizer (JAWS) developed at the National Institute of Metrology (NIM), China. A 2-volt Josephson junction chip is mounted on the second stage cold head of a cryostat. We develop a temperature controller to measure the output voltage of the temperature sensor, and control the temperature of the second-stage cold head. The controller is composed of a current source of high stability, relays, a preamplifier, a low pass filter and a 24-bit analog to digital converter, and a microcontroller unit. Preliminary results show that the cooling power at 4.2 K is about 279 mW and the temperature fluctuation is within 40 mK.
Improvements in fabrication and packaging of our 10 V Programmable Josephson Voltage Standard circuits have enabled the successful operation of these devices on a cryocooler. Limited cooling power and temperature oscillations at the chip must not compromise the accuracy of the quantized voltage steps. Flat-spot measurements of quantized voltages produced with all of the sub-arrays when the chip is biased at a temperature of 4.35 K produced a step width of 0.78 mA. This result was obtained with a cryostat that uses a modest 1.3 kW air-cooled compressor that generates 180 mW of cooling power at the chip.
One of the most promising approaches towards large-scale quantum computation uses devices based on many Josephson junctions. Yet, even today, open questions regarding the single junction remain unsolved, such as the detailed understanding of the quantum phase transitions, the coupling of the Josephson junction to the environment or how to improve the coherence of a superconducting qubit. Here we design and build an engineered on-chip reservoir connected to a Josephson junction that acts as an efficient bolometer for detecting the Josephson radiation under non-equilibrium, that is, biased conditions. The bolometer converts the a.c. Josephson current at microwave frequencies up to about 100 GHz into a temperature rise measured by d.c. thermometry. A circuit model based on realistic parameter values captures both the current-voltage characteristics and the measured power quantitatively. The present experiment demonstrates an efficient, wide-band, thermal detection scheme of microwave photons and provides a sensitive detector of Josephson dynamics beyond the standard conductance measurements.