© Queen’s Printer and Controller of HMSO, 2014. 10977/0814 www.npl.co.uk Abstract The optoelectronic coupling for a pulse-driven Josephson junction array (JJA) for use as a quantum voltage synthesizer has been designed. Since the drive system is electrically isolated from the JJAs, they can be connected in series, leading to a higher output voltage. A thin flm circuit, including interdigitated capacitors to couple a photodiode to the JJAs has been designed and tested and shown to meet the design goals. EMRP project Q-wave: A quantum standard for sampled electrical measurements To meet the demand from industry to provide ac voltage calibration of high performance A/D and D/A converters, a high frequency, real-time quantum voltage digitizer, is being developed as part of the European Metrology Research Program (EMRP) Q-WAVE project (see CPEM poster P15, Wednesday). The quantum voltage digitizer uses a delta sigma control loop to measure an arbitrary voltage waveform in terms of the JJA output. The comparator output is used to generate a pulse stream which drives a Mach-Zehnder modulator, creating a GHz rate pulse stream of optical pulses which provide the JJA drive via a photodiode located inside the cryostat. The resultant output (fast, quantized-area voltage pulses) are fed back around the loop via a low pass flter. Pulse-driven Josephson junction arrays Quantum accurate sinusoidal voltage waveforms have been demonstrated using programmable, binary segmented JJAs up to kHz frequencies [1]. However, this technology is not suitable for higher frequencies since the electronics and the interaction of the junctions require a fnite time to stabilize after switching. Pulse-driven JJAs, used here, can operate at higher frequencies with hundreds of kHz being demonstrated (for a review see [2]). An arbitrary current pulse applied to the JJA generates a voltage pulse of quantized area (e.g. [3]). The desired quantum accurate voltage output is generated by applying an appropriate sequence of pulses to the JJA. Array Design JJAs are fabricated by Physikalisch-Technische Bundesanstalt (PTB), Germany. They consist of SNSNS junctions, where S is superconductor (Nb) and N is normal metal (NbSi), which are linked in a “double-stacked” design. These JJAs operate at 4.2 K in a liquid helium cryostat. Low pass fltering of the output is provided on-chip. Novel optoelectronic coupling An optoelectronic input, using a photodiode (PD) can be used to drive the JJA [4]. Several JJAs can then be connected in series to provide a larger (industrial level) output voltage whilst being driven in parallel. Optoelectronic coupling also reduces the electrical noise transmitted from the room temperature electronics. Photodiode Circuit Design The electrical connections to the PD are made using a custom made chip carrier. The capacitor must be large enough to prevent signifcant change in PD bias and the circuit must have low inductance. On-chip interdigitated capacitors (C) were designed (based on reference [5]), fabricated and measured to have a capacitance of 71 pF and 72 pF for design A and 71 pF and 63 pF for design B. This exceeded the design goal (> 10 pF). Preliminary Data The temporal response of a commercial InGaAs photodiode was characterised at room temperature under a variety of conditions (spot size, spot position, over-flled mode, under- flled mode, power levels, and frequency) and was found to be suitable for driving the JJAs. The next stage of the project is to drive the JJAs using the PD with the PD at room temperature and then at 4.2 K. Acknowledgements This work was co-funded by the European Union within the European Metrology Research Program (EMRP) joint research project SIB59 Q-WAVE and by the UK National Measurement Ofce Electromagnetics and Time Program. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union. References [1] J. M. Williams, D. Henderson, J. Pickering, R. Behr, F. Muller and P. Scheibenreiter, “Quantum-referenced voltage waveform synthesizer,” IET Sci. Meas. Technol., Vol. 5, no. 5, pp. 163 – 174 (2011). [2] R. Behr, O. Kieler, J. Kohlmann, F. Müller and L. Palafox, “Development and metrological applications of Josephson arrays at PTB,” Meas. Sci. Technol., Vol. 23, no. 12, 124002 (19 pp.) (2012). [3] S. P. Benz, P. D. Dresselhaus, A. Rüfenacht, N. F. Bergren, J. R. Kinard and R. Landim, “Progress toward a 1 V pulse-driven AC Josephson voltage standard,” IEEE Trans. Instrum. Meas., Vol. 58, no. 4, pp. 838 – 843 (2009). [4] J. M. Williams, T. J. B. M. Janssen, L. Palafox, D. A. Humphreys, R. Behr, J. Kohlmann and F. Muller, “The simulation and measurement of the response of Josephson junctions to optoelectronically generated short pulses”, Supercond. Sci. Technol., Vol. 17, no. 6, pp. 815 – 818 (2004). [5] K. C. Gupta, R. Garg and I. J. Bahl, Microstrip lines and slotlines, Norwood: Artech House, 1979, p 261 AN OPTOELECTRONIC COUPLING FOR PULSE-DRIVEN JOSEPHSON JUNCTION ARRAYS Jane Ireland 1 , Dale Henderson 1 , Jonathan Williams 1 , Oliver Kieler 2 , Johannes Kohlmann 2 , Ralf Behr 2 , Jarle Gran 3 , Helge Malmbekk 3 , Kåre Lind 3 and Chi Kwong Tang 3 1 National Physical Laboratory, Queens Road, Teddington, Middlesex, TW11 0LW, UK 2 Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany 3 Justervesenet, P.O. Box 170, NO-2027 Kjeller, Norway jane.ireland@npl.co.uk