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We performed a systematic study involving simulation and experimental techniques to develop induced-junction silicon photodetectors passivated with thermally grown SiO2 and plasma-enhanced chemical vapor deposited (PECVD) SiNx thin films that show a record high quantum efficiency. We investigated PECVD SiNx passivation and optimized the film deposition conditions to minimize the recombination losses at the silicon–dielectric interface as well as optical losses. Depositions with varied process parameters were carried out on test samples, followed by measurements of minority carrier lifetime, fixed charge density, and optical absorbance and reflectance. Subsequently, the surface recombination velocity, which is the limiting factor for internal quantum deficiency (IQD), was obtained for different film depositions via 2D simulations where the measured effective lifetime, fixed charge density, and substrate parameters were used as input. The quantum deficiency of induced-junction photodiodes that would be fabricated with a surface passivation of given characteristics was then estimated using improved 3D simulation models. A batch of induced-junction photodiodes was fabricated based on the passivation optimizations performed on test samples and predictions of simulations. Photodiodes passivated with PECVD SiNx film as well as with a stack of thermally grown SiO2 and PECVD SiNx films were fabricated. The photodiodes were assembled as light-trap detector with 7-reflections and their efficiency was tested with respect to a reference Predictable Quantum Efficient Detector (PQED) of known external quantum deficiency. The preliminary measurement results show that PQEDs based on our improved photodiodes passivated with stack of SiO2/SiNx have negligible quantum deficiencies with IQDs down to 1 ppm within 30 ppm measurement uncertainty.

We report proof-of-concept experiments on an optically driven Josephson voltage standard based on a mode-locked laser (MLL), a time-division multiplexer, and a cryogenic ultrafast photodiode driving an overdamped Josephson junction array (JJA). Our optical pulse pattern generator (PPG) concept builds on the capability of MLLs to produce trains of picosecond-wide optical pulses with little amplitude and temporal spread. Our present setup enables multiplication of the original 2.3 GHz pulse repetition frequency by a factor of 8. A commercial photodiode converts the optical pulses into about 25 ps wide electrical pulses in liquid helium several cm from the JJA. Using a custom-made MLL, we can drive a JJA with a low critical current of 360 μA at multiple Shapiro steps. We have performed experiments with pulse pairs whose time interval can be set freely without distorting the shapes of individual pulses. Experimental results are in qualitative agreement with theoretical simulations, and they demonstrate, e.g., crossover in the Shapiro step pattern when the time interval between the pulses is approximately equal to the inverse of the characteristic frequency of the JJA. However, there are quantitative discrepancies, which motivate an improved integration of photodiodes and JJAs to improve both the understanding and fidelity of Josephson Arbitrary Waveform Synthesizers. Considering future quantum technologies in a wider perspective, our optical approach is a potential enabler for fast and energy-efficient pulse drive without an expensive high-bandwidth electrical PPG and without high-bandwidth electrical cables that yield too high thermal conductance between cryogenic and room temperatures.

We developed an optical pulse-drive for the operation of the Josephson Arbitrary Waveform Synthesizer (JAWS) using a fast photodiode (PD) operated at 4 K, close to the JAWS chip. The optical pulses are transmitted to the PD by an easily removable optical fiber attached to it. A bare-lensed PD is mounted by flipchip technique to a custom-made silicon-carrier chip. This carrier chip is equipped with coplanar waveguides to transmit the electrical pulses from the PD to the JAWS chip mounted on a separate printed circtuit board (PCB). The main components of this optical setup are a laser source, a high-speed Mach-Zehnder modulator, and the modulator driver. The waveform pattern is supplied by a commercial pulse pattern generator providing up to 15 GHz electrical return-to-zero (RTZ)-pulses. Unipolar sinusoidal waveforms were synthesized. Using a JAWS array with 3000 junctions, an effective output voltage of 6.6 mV root mean square (RMS) at the maximum available clock-frequency of 15 GHz was achieved. Higher harmonics were suppressed by more than 90 dBc at laserbias operation margins of more than 1 mA. Index Terms-AC Josephson voltage standard, Josephson arbitrary waveform synthesizer, SNS Josephson junction, sigma-delta modulation, optical pulse-drive, flip-chip technology. I. INTRODUCTION A FTER many years since the first realization of a pulsedriven AC Josephson voltage standard [1], recent developments in increasing the effective output voltage to 1 V root mean square (RMS) or even more [2]-[5] show that the use of a pulse-driven Josephson voltage standard is an important approach for voltage metrology. This AC Josephson voltage standard is often called "Josephson Arbitrary Waveform Synthesizer" (JAWS) and it is already used in several NMIs for metrology applications [6]-[15]. For the application in JAWS, the Josephson junctions are operated by short current pulses to Manuscript