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SUB-PICOSECOND MEASUREMENT OF TIME INTERVALS USING SINGLE ELECTRONICS

Peter A. Rosenthal

Published Jan 1, 1993• 0 citations
OpenAlex

Abstract

A single flux quantum (SFQ) pulse coincidence detector based on resistively shunted nonhysteretic Josephson junctions was designed and simulated. The coincidence detector generates an SFQ pulse when the delay between the arrival of SFQ pulses at its two inputs is less than the coincidence threshold. Simulations indicate that the minimum coincidence threshold time can be as short as 400 f 200 fs assuming Josephson junction characteristic voltages of 1 mV, overdamped dynamics and 4.2 K operating temperatures. Circuit architectures exploiting this gate are suggested. Estimates of the effects of thermal noise on resolution are presented indicating the potential for various time domain measurements with sub-picosecond resolution. I. INTRODUCTION The high speed properties of Josephson electronics offer unique possibilities for high time resolution metrology1 such as fast sampling, A-to-D conversion?^^ race arbiting? and time domain reflectometry5 In this paper I propose an ap- proach to time interval measurement that is based on a simple circuit consisting of several overdamped Josephson junctions. This circuit uses the picosecond pulses of Josephson junctions in a simple way that can be integrated naturally into a fre- quency vernier measurement apparatus? A junction with a nonhysteretic resistively shunted current-voltage characteristic can be induced to generate voltage pulses whose time integral is equal to the elementary flux quantum @pg = 2.07 mV-p~.~ In particular, the pulse width from an overdamped Josephson junction with a characteristic voltage VC = IcR~ is given by At = Q)/ VC and can be as short as 2 ps for a junction with a typical VC = 1 mV. The short widths of SFQ pulses combined with the high plasma frequencies possible with overdamped junctions can be exploited in a simple circuit that detects pulse coincidence from two inputs and has a coincidence threshold that can be as short as several hundred femtoseconds for junction IcR~ products around 1 mV. In the next section of this paper I describe the coincidence detection circuit operation, including the results of simulations estimating the potential- performance characteristics. The ef- fects of thermal fluctuations on resolution will be discussed. In Section I11 a time interval measurement system is de- scribed, based on the frequency vernier interpolation method. Using conventional electronics, the vernier method has at- tained time interval resolution limits of about 20 ps.* Our simulations show in principle that a carefully designed fre-

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A single flux quantum (SFQ) pulse coincidence detector based on resistively shunted nonhysteretic Josephson junctions was designed and simulated. The coincidence detector generates an SFQ pulse when the delay between the arrival of SFQ pulses at its two inputs is less than the coincidence threshold. Simulations indicate that the minimum coincidence threshold time can be as short as 400 f 200 fs assuming Josephson junction characteristic voltages of 1 mV, overdamped dynamics and 4.2 K operating temperatures. Circuit architectures exploiting this gate are suggested. Estimates of the effects of thermal noise on resolution are presented indicating the potential for various time domain measurements with sub-picosecond resolution. I. INTRODUCTION The high speed properties of Josephson electronics offer unique possibilities for high time resolution metrology1 such as fast sampling, A-to-D conversion?^^ race arbiting? and time domain reflectometry5 In this paper I propose an ap- proach to time interval measurement that is based on a simple circuit consisting of several overdamped Josephson junctions. This circuit uses the picosecond pulses of Josephson junctions in a simple way that can be integrated naturally into a fre- quency vernier measurement apparatus? A junction with a nonhysteretic resistively shunted current-voltage characteristic can be induced to generate voltage pulses whose time integral is equal to the elementary flux quantum @pg = 2.07 mV-p~.~ In particular, the pulse width from an overdamped Josephson junction with a characteristic voltage VC = IcR~ is given by At = Q)/ VC and can be as short as 2 ps for a junction with a typical VC = 1 mV. The short widths of SFQ pulses combined with the high plasma frequencies possible with overdamped junctions can be exploited in a simple circuit that detects pulse coincidence from two inputs and has a coincidence threshold that can be as short as several hundred femtoseconds for junction IcR~ products around 1 mV. In the next section of this paper I describe the coincidence detection circuit operation, including the results of simulations estimating the potential- performance characteristics. The ef- fects of thermal fluctuations on resolution will be discussed. In Section I11 a time interval measurement system is de- scribed, based on the frequency vernier interpolation method. Using conventional electronics, the vernier method has at- tained time interval resolution limits of about 20 ps.* Our simulations show in principle that a carefully designed fre-
A single-flux-quantum (SFQ) pulse coincidence detector based on resistively shunted nonhysteretic Josephson junctions was designed and simulated. The coincidence detector generates an SFQ pulse when the delay between the arrival of SFQ pulses at its two inputs is less than the coincidence threshold. Simulations indicate that the minimum coincidence threshold time can be as short as 400+or-200 fs, assuming Josephson junction characteristic voltages of 1 mV, overdamped dynamics, and 4.2 K operating temperatures. Circuit architectures exploiting this gate are suggested. Estimates of the effects of thermal noise on resolution are presented, indicating the potential for various time-domain measurements with subpicosecond resolution.< >
An integrated circuit including all basic components of the recently suggested Resistive Single Flux Quantum (RSFQ) logic family has been designed, fabricated and tested successfully. The circuit includes a generator of periodic SFQ pulses, four buffer/amplifier stages for splitting, channeling and regeneration of the pulses, a detector/load stage, and universal RSFQ logic gate (here performing the NOT function). The 10μm design rule circuit employs 13 active Nb - Al2O3- Nb Josephson junctions with the critical current density ∼ 0.5 kA/cm2. externally shunted by Mo resistors with R 1 Ohm. The shunting provided critical damping of the junctions (\beta_{c}\lsim 1) and reasonable (\sim 500 \muV) IcR product. The circuit operation has been tested by measurement of dc voltages\bar{V}_{i}across various Josephson junctions as functions of the dc current through the pulse generator. Correct and stable operation of the circuit for the clock frequencies from 0 to ∼30 GHz has been demonstrated.
We demonstrate the operation of a coincidence photon detector with a short timing jitter consisting of two superconducting nanowire single photon detectors (SSPDs) and a single flux quantum (SFQ) circuit. By utilizing the timing discrimination capability of the SFQ coincidence circuit, the full width at half maximum timing jitter of the entire coincidence photon detector was evaluated as 32.3 ps, which is 36 ps less than that of the standard commercial time correlated single photon counting module, and the timing jitter of the SSPD was estimated as ∼15 ps. Owing to the short timing jitter characteristics, our coincidence photon detector could correctly capture the effect of pulse-width broadening by insertion of an optical bandpass filter. We have also demonstrated that our coincidence photon detection clearly shows Hong-Ou-Mandel interference with a weak coherent pulse. These results are a crucial step to realizing high timing resolution coincidence measurements, ushering in a technology for timing measurement based multi-photon quantum interference.
Rapid-Single-Flux-Quantum (RSFQ) digital circuits based on shunted Josephson junctions have been shown to be able to digitally process signals up to several hundreds GHz. The principle of operation is based on the generation, transport and processing of picosecond voltage pulses of 2.07 mV.ps quantized area, which corresponds to one quantum flux h/2e. Operation of RSFQ circuits can be performed with clock frequencies of the order of 20 to 50 GHz with present technology. We will present current developments and experimental results obtained so far, based on circuits developed in the JeSEF RSFQ process of IPHT Jena, using a 1 kA/cm2 shunted Nb/Al-AlOx/Nb junctions. Some particular potential applications for space will be emphasized
Superconductive Rapid Single Flux Quantum (RSFQ) logic and memory, in which ones and zeros are represented by the presence or absence within a timing window of quantized picosecond voltage pulse (/spl int/v(t)dt=h/2e=2.07 mV/spl middot/ps), corresponding to one SFQ, can be integrated into a digital computing system with an operating rate of several tens of GHz, based on the present Nb Josephson junction integrated circuit technology. It is the most promising technology beyond semiconductor transistors for low-power high-end computation. However, as the operating speed of circuits and systems increase, timing uncertainty from fabrication process variations makes global synchronization very hard. In this paper, we present a globally asynchronous, locally synchronous timing methodology for RSFQ digital design, which can solve the global synchronization problem. We also demonstrate the recent experimental results of some asynchronous circuits and systems implemented in RSFQ technology. Several key components such as a self-timed shift register, a self-timed demultiplexor, a Muller-C element, a completion detector, and a clock generator have been designed and tested. High speed operation has been confirmed up to 20 Gb/s for a prototype data buffer system, which consists two self-timed shift registers and an on-chip 5-38 GHz clock generator.
We study a single-shot single-flux-quantum (SFQ) readout circuit based on current-to-time conversion for superconducting flux qubits. Circulating currents representing states of a qubit were converted to propagation delay time of SFQ pulses, and then detected by a time discriminator. The key parameter which determines the availability of single-shot readout is current resolution. We evaluated the current resolution by measuring the gray zone which was defined as the transition width in the output probability of the current-to-time converter with 1000 trials for each value of an input current. For reducing unwanted effects on the qubit, shunt resistors are required to be removed for Josephson junctions in the readout circuit. We designed two readout circuits for comparison, the circuit composed of shunted junctions and that of unshunted junctions. Gray zones were measured to be 4.8μA\mu{\rm A}μAfor unshunted junctions and 2.2μA\mu{\rm A}μAfor the shunted ones at 4.2 K. These gray zones may originate from Johnson noise. Operation at a reduced temperature and employment of a likelihood decision circuit will lead to reduced gray zone, i.e., improved current resolution sufficient for single-shot readout.