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Superconductor Science and Technology

Characterization of superconducting pulse discriminators based on parallel NbN nanostriplines

Mikkel Ejrnaes, A. Casaburi, Roberto Cristiano, Nadia Martucciello, F. Mattioli, Alessandro Gaggero, Roberto Leoni, J-C Villégier, S. Pagano

Published Jan 20, 2011• 9 citations
Publisher

Abstract

A superconducting pulse discriminator based on a cascade switch to the normal state of parallel ultrathin NbN nanostrips has been fabricated and carefully investigated. Correct operation was achieved using 1 ns input pulses with amplitudes down to 15 µA. The discriminator had a peak current gain of 12 and an FWHM timing jitter of 80 ps, limited by our measurement instrument resolution. These characteristics, together with simple on-chip integration, small area and low dissipation, make this device suitable for applications such as readout of fast cryogenic detectors and the output stage of superconducting digital circuits.

DOI
10.1088/0953-2048/24/3/035018

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A superconducting pulse discriminator based on a cascade switch to the normal state of parallel ultrathin NbN nanostrips has been fabricated and carefully investigated. Correct operation was achieved using 1 ns input pulses with amplitudes down to 15 µA. The discriminator had a peak current gain of 12 and an FWHM timing jitter of 80 ps, limited by our measurement instrument resolution. These characteristics, together with simple on-chip integration, small area and low dissipation, make this device suitable for applications such as readout of fast cryogenic detectors and the output stage of superconducting digital circuits.
We describe a new superconductive three-terminal device that acts as a fast pulse amplifier and discriminator. The device is based on a parallel nano-wire configuration. The nano-wires are properly biased near their critical current. When a current pulse of sufficient amplitude is injected into one of the wires, a transition to the normal state of all the wires occurs, thus generating a large output current pulse. This device could be used as a pulse amplifier for signals generated by nano-wire detectors or by RSFQ circuits. The parallel nano-wire structure can be realized with the same fabrication process used for NbN Superconducting Single Photon Detector (SSPD), so its integration into the existing technology is straightforward. Here we present numerical simulations of the device, with special attention to circuit requirement for the correct operation.
Ultrathin NbN nanowires is the material of choice for superconducting single photon detectors (SSPD) due to the good efficiency, dark count rate and timing jitter at 1550 nm wavelength obtained. These performance parameters are achieved using nanowires a few nanometers thick and 100 nm wide patterned into a meander shape in order to achieve area coverage. The meander shape effectively makes the SSPD of a single very long nanowire in turn giving it a significant inductance which limits the maximum count rate of the detector. Recently, we demonstrated how one can exploit a cascade switch to the normal state of nanowires connected in parallel to significantly reduce the SSPD inductance and increase the signal amplitude. Here we present how one can configure SSPDs that uses multiple cascade switches to the normal state. We show how this principle can be used to expand the SSPD coverage area with a very limited increase in detector inductance with area. Finally we discuss our first results obtained with SSPD based on the multiple cascade switch principle, showing correct operation, increased operational bias range and increased signal pulse amplitudes.
Ultrathin NbN nanowires is the material of choice for superconducting single photon detectors (SSPD) due to the good efficiency, dark count rate and timing jitter at 1550 nm wavelength obtained. These performance parameters are achieved using nanowires a few nanometers thick and 100 nm wide patterned into a meander shape in order to achieve area coverage. The meander shape effectively makes the SSPD of a single very long nanowire in turn giving it a significant inductance which limits the maximum count rate of the detector. Recently, we demonstrated how one can exploit a cascade switch to the normal state of nanowires connected in parallel to significantly reduce the SSPD inductance and increase the signal amplitude. Here we present how one can configure SSPDs that uses multiple cascade switches to the normal state. We show how this principle can be used to expand the SSPD coverage area with a very limited increase in detector inductance with area. Finally we discuss our first results obtained with SSPD based on the multiple cascade switch principle, showing correct operation.
We investigate the timing jitter in parallel superconducting NbN-nanowire single photon detectors based on a cascade switch mechanism. The measured timing jitter is asymmetric and has an oscillatory dependence on bias current. At the highest bias current the full width at half maximum was 1.5 times larger than an on-chip reference meander NbN nanowire. A physical model of the dynamics occurring during cascade switch is developed, that quantitatively accounts for our observations as a consequence of different nanowire critical currents within the detector.
Scalable superconducting nanowire single photon detector (SNSPDs) arrays require cryogenic digital circuits for multiplexing the output detection pulses. Among existing superconducting digital devices, superconducting nanowire cryotron (nTron) is a three-terminal device with an ultra-compact size, which is promising for large scale monolithic integration. In this report, in order to evaluate the potential and possibility of using nTrons for reading and digitizing SNSPD signals, we characterized the grey zone, speed, timing jitter and power dissipation of a proper designed nTron. With a DC bias on the gate, the nTron can be triggered by a few μA high and nanoseconds wide input signal, showing the nTron was capable of reading an SNSPD pulse at the same signal level. The timing jitter depended on the input signal level. For a 20 μA high and 5 ns wide input pulse, the timing jitter was 33.3 ps, while a typical SNSPD's jitter was around 50 ps. With removing the serial inductors and operating it in an AC bias mode. The nTron was demonstrated to be operated at a clock frequency of 615.4 MHz, which was faster than the maximum counting rate of a typical SNSPD. In additional, with a 50 Ω bias resistor and biased at 17.6 μA, the nTron had a total power dissipation of 19.7 nW. Although RSFQ circuits are faster than nTrons, for reading SNSPD or other detector arrays that demands less operation speed, our results suggest a digital circuit made from nTrons could be another promising alternative.