Characterization of a Dual Josephson Impedance Bridge Fr´ ed´ eric Overney 1 , Nathan E. Flowers-Jacobs 2 , Blaise Jeanneret 1 , Alain R¨ ufenacht 2 , Anna E. Fox 2 , Paul D. Dresselhaus 2 and Samuel P. Benz 2 1 Federal Institute of Metrology METAS, Lindenweg 50, 3003 Bern-Wabern, Switzerland 2 National Institute of Standards and Technology, Boulder, CO 80305, USA Abstract—This paper describes a dual Josephson impedance bridge capable of comparing any two impedances, that is, with any amplitude ratio and relative phase, over a wide range of frequency. A new, more compact, design has been achieved by mounting the two Josephson Arbitrary Waveform Synthesizers (dual JAWS) side-by-side in a single cryoprobe. We measured the crosstalk between the two JAWS sources and show that, by interchanging the impedances within the bridge, the effect of the crosstalk between JAWS sources can be reduced to a negligible level. Index Terms—Impedance comparison, AC Josephson voltage standard, Josephson Arbitrary Waveform Synthesizer, AC coaxial bridge. I. I NTRODUCTION The comparison of impedance standards using transformer- based bridges is widely used in national metrology institutes (NMIs) and allows the calibration of impedances with the highest accuracy in the audio frequency range [1], [2]. The major drawback of such bridges is that impedances can be compared only at a set of specific predefined magnitude ratios (typically 1:1 or 1:10) and relative phases (typically 0 ◦ or ±90 ◦ ). This limitation has been overcome by replacing the trans- former with two ac Josephson voltage standards (also known as dual Josephson Arbitrary Waveform Synthesizers or dual JAWS sources), which generate accurate and stable voltages with a completely programmable magnitude ratio and relative phase. The first realization of an impedance bridge based on two JAWS circuits demonstrated the capability to compare any two kinds of impedances over the whole audio frequency range [3], [4]. Since the first realization of this Dual Josephson Impedance Bridge (DJIB) in 2016 [3], the system has been further optimized: the electronics [5] have been modified to make the system more compact [6] and the JAWS design has been improved to fit both sources in the same cryoprobe, which is cooled in a single helium Dewar. The preliminary description of this new dual source and the characterization of the new DJIB are presented in this summary. II. DUAL JAWS SETUP Fig. 1 represents schematically the wiring of the two JAWS sources located side-by-side in the cryoprobe. For clarity, the microwave circuitry is omitted. Each JAWS is composed of four arrays of 12 810 Josephson junctions (JJs) with critical currents of 10 mA. The JJs in each JAWS are driven by a single Fig. 1. Schematic representation of the low-frequency wiring of the two JAWS sources inside the cryoprobe. The grounding scheme of the circuit can be changed by connecting the grounding plugs shown near the V - outputs on the right side of the figure. pulse generator channel; a four-way split is accomplished on- chip using two layers of Wilkinson dividers [6]. For each JAWS, the inner conductor of each of two coaxial cables brings the audio-frequency quantum-accurate Josephson voltage (V + and V − ) from the chip to the top of the cryoprobe. The outer conductors of these coaxial cables are isolated from the cryoprobe and connected together near the chip. Each JAWS also has a pair of twisted wires connected in parallel with the coaxial cables for system testing (colored wires in Fig. 1). Four supplementary twisted-pair wires are connected from DB-25 connectors at the top of the cryoprobe to the four JJ arrays of each JAWS to provide the compensation current needed to generate voltages > 200 mV. When the dual JAWS sources are implemented in the DJIB, the V − outputs of the two JAWS circuits are connected together and shorted to ground using the grounding plugs shawn in Fig. 1. The V + outputs supply the quantum-accurate voltages V top and V bot to the bridge. III. CROSSTALK As described above, the two JAWS sources are located side by side. Although this configuration allows a more compact design, the question of crosstalk between the two JAWS circuits must be investigated in detail. A. Effect of crosstalk on the DJIB Fig. 2 shows schematically the effect of JAWS crosstalk on the DJIB. Once the bridge is balanced, i.e., no current is U.S. Government work not protected by U.S. copyright