Optically operated pulse-driven Josephson junction arrays and range extension using voltage dividers and buffer amplifiers

Abstract

Voltage dividers and buffer amplifier combinations can be used to transfer and extend traceability in voltage and electrical power metrology between a wide range of instruments. For voltages up to 1000 V, resistive voltage dividers are mostly preferred as their operating current vary little in the range 10 Hz to 1 MHz, compared to inductive and capacitive dividers. A buffer amplifier is often used to maintain the divider’s division ratio, by fixing the output impedance of the divider, and supplying enough current to drive both low-resistive and high-capacitive loading instruments. Through several design iterations, a buffer amplifier has been developed (chapter 2), with a near unity gain in an entire decade of applied voltages and frequencies up to 1 MHz. It has been used together with a 10:1 resistive voltage divider, which has also been developed (chapter 2) as a part of this work. This combination has been characterized for applied voltages up to 50 V for frequencies up to 100 kHz, using thermal transfer standards.

Quantum-accurate alternating current voltage waveforms can be synthesized from Josephson junction arrays, by applying a calculated pattern of fast current pulses of up to tens of gigapulses-per-second. By using photodiodes, placed together with the Josephson arrays, in liquid helium, an optical pulse-drive scheme can be used to realize the Josephson effect. The photodiode and Josephson array become an electrically floating unit, which makes it simpler to couple multiple, parallel-driven Josephson arrays in series. By splitting the optical pulse-drive, using fiber splitters, the number of photodiodes that a single optical pulse-source can drive is increased, which constitutes an inexpensive method of increasing the number of parallel-operated Josephson arrays.

In this thesis, a cryogenically operable packaging technique of a photodiode has been developed, and used to generate both uni- and bipolar current pulses in liquid helium. The unipolar module has also been used to drive various Josephson junction arrays consisting of up to 3000 junctions to synthesize both direct current voltage and unipolar alternating current voltage waveforms. This part of the thesis (chapter 3) mainly focuses on the requirements and abilities of the laser-pulsation sources and the photodiode packages to produce current pulses that can realize the Josephson effect in these arrays. By investigating methods of increasing the pulsation bit rate, and operation of multiple photodiodes from a single source, waveforms with a higher amplitude can be synthesized in these arrays.

Mach-Zehnder modulation of a continuous-wave laser has been performed to generate photo-current pulse widths (full-width-at-half-maximum) as short as 62 ps in the bipolar module, and peak heights up to 16mA for wider pulses in the unipolar module. A modelocked laser has been used to generate photo-current pulse widths as short as 37 ps in the bipolar module, and with peak heights up to 6.34mA. The unipolar photodiode module has been used to operate a single array of 3000 Josephson junctions to synthesize a unipolar waveforms with 18.6mV peak height at 1.875 kHz, and a 92 dBc suppression of higher harmonics.