Noshir B. Dubash

Josephson output interfaces for RSFQ circuits

We have developed and demonstrated high bandwidth Josephson circuits to interface the output of RSFQ circuits to room temperature electronics. Asynchronous dc powered voltage driver circuits have been designed to amplify RSFQ signal levels to voltage outputs in the 2-4 mV range, in a wide bandwidth. These driver circuits have been characterized and tested for data rates up to 8 Gb/s. The bit error rate for one of these drivers has been measured up to 7 Gb/s for a (2/sup 31/-1) bit long pseudo-random bit sequence (PRBS). In order to match the data rate of Josephson circuits to slower room temperature electronics, we have developed clock-controlled shift registers which allow shift-in and shift-out of data at different frequencies. Complete functionality of these circuits integrated with the drivers has been demonstrated at low speed. Shift registers integrated with the voltage driver circuits have been tested at high-speed for data rates up to 6 Gb/s.

Superconducting single flux quantum 20 Gb/s clock recovery circuit

A clock recovery circuit has been successfully tested at frequencies up to 20 GHz. This cell is designed for a rapid-single-flux-quantum (RSFQ) telecommunication data switch. It serves to set the receiver clock in phase with the incoming digital signal. The circuit consists of a dc-to-SFQ converter, ring oscillator [(RO) is a closed-loop RSFQ Josephson transmission line], confluence buffer, and an 8-bit binary counter. The input signal transforms to SFQ pulses, and each pulse resets the phase of the ring oscillator, giving a locking time of 1 bit. Thus, the pull-in (capture) range and hold-in (tracking) range are the same, and strictly depend on the encoding of the input signal. This range is estimated to be about 1 GHz at frequency 20 GHz, if the sequence of consecutive ONEs or ZEROs does not exceed 20 bits. The quality factor QRO of ring oscillator is about 2000, which gives a jitter of 50 fs for a 35-junction RO. A sampling technique was used to demonstrate phase recovery (phase locking) with only one ...

Josephson effect gain and noise in SIS mixers

The authors report measurements of gain and noise in SIS mixers at 230 and 492 GHz. Measurements were made of relatively high gain and noise associated with Josephson currents that have not been previously reported. These measurements show that Josephson currents are increasingly important as operating frequencies are raised. The techniques used to make these measurements are discussed. Measurements made with hot and cold black-bodies are shown to be inaccurate at high frequencies. The problem is that SIS mixers do not always respond linearly to the signal power incident on them. This is particularly important when (1) very broad band mixers are used and (2) Josephson effect currents are important. Both of these circumstances are present in the quasioptical SIS mixers favored for 500 GHz and higher. Monochromatic signals were used to measure gain and noise to get around these problems. >

System demonstration of a multigigabit network switch

4/spl times/4 single-flux-quantum (SFQ) network switch has been packaged and successfully demonstrated in a hybrid closed-cycle refrigerator (CCR) system at multigigabit data rates. Full operation of the packaged switch, with self-routing of 1-Gb/s data packets, was demonstrated using a 1-GHz address header decode. The switch is packaged on a superconducting multichip module (MCM) mounted on the 4.5-K stage of the CCR. On-chip asynchronous Josephson drivers and cooled GaAs preamplifiers are used to amplify the SFQ outputs of the switch. The maximum operation bandwidth of the switch is currently limited by the asynchronous Josephson driver. New designs of the Josephson drivers, which have demonstrated operation at 10 Gb/s, are expected to enable switch operation at 10 Gb/s. The system is equipped with fiber-optical inputs and high-speed cryogenic photodetectors. The fiber-optic interfaces, RF packaging, and MCM packaging in the CCR system have demonstrated error-free operation at 10 Gb/s.

SFQ data communication switch

A new SFQ data communication switch has been designed and tested. Complete operation of a 4/spl times/4 switch circuit has been demonstrated with address decoding at low-speed. Transmission through a given path in the switch has been demonstrated for data rates up to 4 Gb/s. Circuit simulations show operation of the switch cells up to 30 Gb/s. The circuit was fabricated using HYPRESs standard 1 kA/cm/sup 2/ niobium process. The switch has a crossbar architecture with an RF SQUID based switch cell at each crosspoint. The address is decoded by means of RSFQ shift registers which are integrated into the switch matrix. The design enables high bit-rate, low crosstalk, non-blocking architecture, NRZ or RZ data format, and self routing of variable length data packets.

Underdamped long Josephson junction coupled to overdamped single-flux-quantum circuits

We report a circuit that integrates an underdamped long Josephson junction with overdamped single-flux-quantum (SFQ) circuits. We confirm that the resonant soliton modes in the long junction are not affected by SFQ cells coupled to the junction, and demonstrate that the radiation frequency and linewidth of the soliton resonances can be measured with SFQ T-flip-flops. Our experimental results also show that a 4π quantum mechanical phase leap at the end of the long junction, which is due to the reflection of a soliton, creates two single flux quanta propagating in the overdamped Josephson transmission line.

Photon induced noise in the SIS detector

The dominant source of noise in an SIS mixer is the noise in the photon-induced current. We have made accurate measurements of noise induced in SIS junctions by 95 GHz photons. The noise is measured at 1.5 GHz using a low-noise cryogenic measurement system. The measured photon-induced noise is compared to the noise predicted by Tuckers theory augmented by a vacuum/thermal noise term. For small to moderate rf powers, at which SIS mixers are operated, the measured noise is nearly perfectly predicted by this theory for all the devices measured. Measurements of series arrays of SIS junctions also agree with this theory showing that the noise of each SIS junction in the array is independent. At large rf powers, the measured noise was higher than the predicted noise, in devices with smaller capacitance. We also measured the noise in single junctions and arrays with no rf radiation. These measurements agreed very well with the predicted shot noise for most bias conditions. >

Shot noise and photon-induced correlations in 500 GHz SIS detectors

Photon-induced current correlations in SIS detectors can result in an output noise that is greater or less than shot noise. Evidence of these correlations had been observed for 100 GHz rf by accurate noise measurements as reported in our previous work. We now present a detailed analysis of these current correlations for frequencies between 100 and 500 GHz. We also report new measurements of photon-induced noise in a 490 GHz SIS mixer, and discuss the Gaussian beam techniques used to eliminate the thermal background radiation. For small 490 GHz rf power, the output noise is equal to shot noise. The results of the 100 and 490 GHz photon noise measurement are summarized in context to shot noise and the effect of the current correlations predicted by the theoretical model.<<ETX>>

Linewidth measurements and phase locking of Josephson oscillators using RSFQ circuits

We present a technique for linewidth measurement and phase-locking of Josephson oscillators using digital rapid single-flux-quantum (RSFQ) circuits. The oscillator consists of a resistively shunted 6 /spl mu/m/spl times/6 /spl mu/m Nb/AlO/sub x//Nb Josephson tunnel junction that is integrated with RSFQ input and output circuits. A cascade of RSFQ T flip-flops is used to directly monitor the output of the Josephson oscillator. Spectral characteristics have been measured directly for oscillator frequencies ranging from 10-50 GHz. The linewidth can be reduced by over 100 times by phase-locking the oscillator to an RSFQ pulse train generated by an external sinusoidal signal. These Josephson oscillators can be used as on-chip stable high frequency clocks for RSFQ circuits.

Single flux quantum components for packet switches

A superconducting front-end receiver operating in a wide frequency band from 10 to 40 Gb/s could increase the throughput of packet switches. Rapid Single Flux Quantum logic, which has the advantage of high-speed operation at medium integration levels, was used to build receiver components: a multiple bit-rate clock recovery circuit and a demultiplexer. Only 16 heading bits of the packet were required to read the clock frequency in a range from 22.5 to 45 Gb/s. Preliminary experiments showed single bit-rate clock recovery cell operation up to 35 GHz and /spl plusmn/17% bias current margins for a 1:2 demultiplexer. The interface between the receiver and semiconductor components is discussed.