Josephson voltage standard circuit operation with a pulse tube cooler

For a more wide-spread use of Josephson voltage standards, cryogen-free operation, by means of an appropriate closed-cycle refrigerator, is highly desirable. In this work we present a low-noise pulse tube cooler (PTC) that is capable to cool the voltage standard circuits to temperatures below 4 K. As a low loss dielectric waveguide with a very small thermal conductivity a Teflon strip was used for the microwave transmission. The matched transition from the WR12 rectangular waveguide to the Teflon strip and vice versa was made using special exponential tapers. Programmable 1 Volt 14 bit SINIS and 10 Volt SIS arrays assembled in the PTC operated well, i.e., demonstrated a similar performance to that measured in liquid helium. The Josephson junction arrays were integrated in coplanar strips instead of the commonly used microstriplines for the microwave transmission lines. The 1 Volt SINIS array contained 8192 Josephson junctions (JJs). It generated an inherently stable Josephson voltage step of 1.19 V at a driving microwave frequency of 70 GHz. The step width was 150 /spl mu/A and the critical current of the junctions was 550 /spl mu/A. Similar results were obtained under liquid helium cooling conditions. In another experiment a 10 Volt voltage standard chip with 19700 SIS JJs was installed. It was possible to generate stable Shapiro steps at the 10 V level.

I. Introduction

The cryogen-free operation of Josephson voltage standard circuits, first described by Yi-Hua Tang et al. [1], requires a low electromagnetic interference (EMI) cryocooler that provides a cooling power of about 100 mW at 4.2 K. Due to its intrinsic low level of mechanical vibrations a 4 K pulse tube cooler (PTC) is suitable for such applications. Here we report on the performance of a two-stage 4 K PTC that was constructed for the operation of Josephson voltage circuits [2]. To reduce the heat transfer to the 4 K stage a Teflon strip was used as a low loss dielectric waveguide for the microwave transmission in the PTC. The nearly perfect transition from the WR12 rectangular waveguide to the Teflon strip and vice versa was made using special exponential tapers. The microwave attenuation of the complete waveguide, including a 50 Teflon foil for the vacuum seal, was about 1.2 dB in the 70 GHz frequency range. A similar dielectric waveguide with other dimensions and tapers was first suggested for Josephson voltage standards by Clark A. Hamilton [3].