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arXiv:cond-mat/0702104 · New Journal of Physics

Verification of stable operation of rapid single flux quantum devices with frequency-dependent dissipation

Juha Hassel, Leif Grönberg, P Helistö

Published May 30, 2007• 2 citations
arXivHTMLPDF

Abstract

It has been suggested that rapid single flux quantum (RSFQ) devices could be used as the classical interface of superconducting qubit systems. One problem is that the interface acts as a dissipative environment for a qubit. Recently, ways to modify the RSFQ damping to reduce the dissipation have been introduced. One of the solutions is to damp the Josephson junctions by a frequency-dependent linear circuit instead of the plain resistor. The approach has previously been experimentally tested with a simple SFQ comparator. In this paper we perform experiments with a full RSFQ circuit, and thus conclude that in terms of stable operation the approach is applicable to scalable RSFQ circuits. Realization and optimization issues are also discussed.

DOI
10.1088/1367-2630/9/5/157

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It has been suggested that rapid single flux quantum (RSFQ) devices could be used as the classical interface of superconducting qubit systems. One problem is that the interface acts as a dissipative environment for a qubit. Recently, ways to modify the RSFQ damping to reduce the dissipation have been introduced. One of the solutions is to damp the Josephson junctions by a frequency-dependent linear circuit instead of the plain resistor. The approach has previously been experimentally tested with a simple SFQ comparator. In this paper we perform experiments with a full RSFQ circuit, and thus conclude that in terms of stable operation the approach is applicable to scalable RSFQ circuits. Realization and optimization issues are also discussed.
We propose and demonstrate the functioning of a special Rapid Single Flux Quantum (RSFQ) circuit with frequency-dependent damping. This damping is achieved by shunting individual Josephson junctions by pieces of open-ended RC transmission lines. Our circuit includes a toggle flip-flop cell, Josephson transmission lines transferring single flux quantum pulses to and from this cell, as well as DC/SFQ and SFQ/DC converters. Due to the desired frequency-dispersion in the RC line shunts which ensures sufficiently low noise at low frequencies, such circuits are well-suited for integrating with the flux/phase Josephson qubit and enable its efficient control.
Within the classical Rapid Single-Flux Quantum (RSFQ) digital designs, the superconductive phase dropping necessary for the optimal SFQ data processing is realized with short sections of transmission lines serving as lumped inductances. This approach meets several hard restrictions about some promising future RSFQ applications requiring a drastic reduction of the junctions' critical currents. In this paper, we analyse these restrictions, and present an alternative and experimentally verified technique for superconductive phase dropping, which is compatible with any RSFQ fabrication technology. This technique provides compact and robust RSFQ digital circuits with highly improved fabrication yield.
Superconducting single flux quantum (SFQ) circuits have been shown to offer huge advantages that are ideal for high performance computing applications. These circuits can operate at very high frequencies (tens of GHz) but consume very low power (from nanowatts to microwatts per gate). Energy-efficient SFQ (ERSFQ) circuits, evolved from traditional rapid SFQ (RSFQ) circuits, were invented with the goal of elimination of static power dissipation. When compared with RSFQ circuits, ERSFQ circuits require a new biasing network that includes biasing Josephson junction(s), biasing inductor(s), and feeding Josephson transmission line(s). Some general suggestions on how to design the network have been offered, but a detailed analysis and optimization scenario for the network is still to be provided. In this paper, we first optimize the previously reported DC-biased two-Josephson junction per bit RSFQ shift register circuit, which can be used to provide a more robust, energy-efficient, and high memory capacity storage unit. The simulated power dissipation of 16-bit RSFQ shift register is 0.45 μW per bit when operating at 21.7 GHz. We then convert the RSFQ-based shift register into an ERSFQ structure and investigate the effects of the ERSFQ bias network on the margins of the ERSFQ circuit. Finally, we propose a more comprehensive optimization method for the ERSFQ biasing network.
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 designed a 30 A/cm2RSFQ-qubit control circuit and tested operation at 4 K. The integration of RSFQ technology and qubits monolithically requires great attention to thermal budgeting and electromagnetic compatibility. These issues are of primary concern when developing an RSFQ-qubit experiment. The interface of an SFQ circuit to a qubit requires tunable coupling since RSFQ inherently limits logical elements to single-flux storage. By measuring the quantum-tunneling rate of a phase qubit, we can determine the effect of changing the coupling strength between the classical and quantum systems. By looking at the same system, we can diagnose the impact, whether thermal or electromagnetic, of an active digital circuit. These tests would set an upper bound on the deleterious effects of monolithic integration.