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Applied Physics Letters

Timing jitter measurement of single-flux-quantum pulse in Josephson transmission line

Hirotaka Terai, Zhen Wang, Yoshihito Hishimoto, Shinichi Yorozu, Akira Fujimaki, Nobuyuki Yoshikawa

Published Mar 12, 2004• 48 citations
Publisher

Abstract

Timing jitter is an important factor in determining the limit of speed and scale of single-flux-quantum circuits. We estimated the timing jitter by a simple method using 2-bit shift registers. It was estimated by observing the transition between 1- and 2-mode operations while changing the bias current to the clock line. A sharp transition was observed in the shift register with a smaller number of Josephson junctions in the clock and data lines, suggesting the existence of a finite timing jitter. The timing jitter per Josephson junction was estimated to be 0.09 ps at a designed bias current. The temperature dependence supported that the timing jitter comes from thermal fluctuation.

DOI
10.1063/1.1650912

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Timing jitter is an important factor in determining the limit of speed and scale of single-flux-quantum circuits. We estimated the timing jitter by a simple method using 2-bit shift registers. It was estimated by observing the transition between 1- and 2-mode operations while changing the bias current to the clock line. A sharp transition was observed in the shift register with a smaller number of Josephson junctions in the clock and data lines, suggesting the existence of a finite timing jitter. The timing jitter per Josephson junction was estimated to be 0.09 ps at a designed bias current. The temperature dependence supported that the timing jitter comes from thermal fluctuation.
Timing jitter is an important constraint for the performance of SFQ circuits. We focus on the timing jitter in a Josephson transmission line (JTL) that induces timing errors at logic gates. A proper data analysis of 2-bit shift registers gave a jitter value of about 0.08 ps for a single Josephson junction in a JTL powered at a designed bias current. The variations in timing parameters of logic cells also give rise to timing errors. We experimentally determined the timing parameters in several logic cells and found that the deviations between actual and nominal values are within 2 ps. The relationship between the gray zone and circuit scale was estimated by extrapolating the transition curves observed in 2-bit shift registers. The estimated circuit scale was 20,000-100,000 JJs and 1,000 JJs at an operating clock frequency of 20 GHz and 40 GHz, respectively.
We report on the timing jitter characterization of the superconducting single flux quantum (SFQ) coincidence circuit, which is an essential component of the superconducting coincidence photon counter. Two superconducting nanowire single photon detectors (SSPDs), each of which is irradiated with optically time-controlled photons, are connected to the SFQ coincidence circuit, and the timing jitter of the SFQ circuit is evaluated by changing the relative time delay between two input ports for the SFQ comparator unit. We successfully observe the transition curve of the probability of obtaining the signal from the SFQ coincidence circuit by sweeping photon arrival time to each SSPD and confirm that this curve shifts temporally upon changing the bias current to the Josephson transmission line (JTL) in the SFQ circuit. A systematic investigation reveals that the relation between time delay and the bias current to JTL can be estimated. The full width half maximum timing jitter of the SFQ circuit is 1.1 ps, which is sufficiently low so that it does not influence the entire timing jitter of the coincidence photon counter.
Digital superconducting systems based on Josephson junctions make generally use of the synchronous timing strategy. Today, one of its most promising applications is the high speed and high dynamic range signal sensing. A digital signal acquisition system should have a high resolution as well as a short sampling interval with a well known and constant time period. Short term clock fluctuations (clock jitter) induced by thermal noise can significantly disturb the system operation due to hazards of timing constraint violations. This uncertainty in the time period is currently a strong limitation for further improvements of fast signal sensing systems based on superconductive electronics. Recently, several theoretical and experimental studies describe the timing jitter in simple circuits, like Josephson transmission lines. In the presented work, we analyse different fabrication technologies for rapid single flux quantum (RSFQ) electronics and predict their timing jitter by the numerical solution of stochastic differential equations. We obtained a very good agreement between our simulation data and recent experimental results.
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A Josephson transmission line closed to form a ring was used to measure the time jitter produced by a single shunted Josephson junction. A toggle flip-flop cell was connected to a sampling oscilloscope, and was used to detect the flux quantum arrival time. Time jitter was determined from the variation of this time. The jitter was measured as a function of the number of cycles the flux quantum made in the ring. The reduced jitter produced by a single junction of 0.2 ps/spl radic/n (n-number of junctions passed by the quantum) was measured using Hypres 1 kA/cm/sup 2/ chip fabrication technology. It was found that jitter varies the distance between the quanta with the same rate of 0.2 ps/spl radic/n.