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arXiv:cond-mat/0510189 · Applied Physics Letters

Rapid single flux quantum devices with selective dissipation for quantum information processing

Juha Hassel, P. Helistö, Heikki Seppä, J. Kunert, Ludwig Fritzsch, H.‐G. Meyer

Published Oct 30, 2006• 12 citations
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Abstract

The authors study the frequency dependent damping in rapid single flux quantum (RSFQ) circuits as means to reduce dissipation and consequent decoherence in RSFQ/qubit circuits. They show by simulations and experiments that stable RSFQ operation can be achieved by shunting the Josephson junctions with an RC circuit instead of a plain resistor. The authors derive criteria for the stability of such an arrangement and discuss the effect on decoherence.

DOI
10.1063/1.2382733

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The authors study the frequency dependent damping in rapid single flux quantum (RSFQ) circuits as means to reduce dissipation and consequent decoherence in RSFQ/qubit circuits. They show by simulations and experiments that stable RSFQ operation can be achieved by shunting the Josephson junctions with an RC circuit instead of a plain resistor. The authors derive criteria for the stability of such an arrangement and discuss the effect on decoherence.
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.
Rapid single-flux-quantum (RSFQ) technology has been proposed as control electronics for superconducting quantum bits because of the material and working temperature compatibility. In this work, we consider practical aspects of RSFQ circuit design for low noise low power operation. At the working temperature of 20 mK and operational frequency of 2 GHz, dissipated power per junction is reduced to 25 pW by using 6 µA critical current junctions available at the Hypres and VTT low J c fabrication process. To limit phonon temperature to 30 mK, a maximum of 40 junctions can be placed on a 5 mm × 5 mm chip. Electron temperature in resistive shunts of Josephson junctions is minimized by use of cooling fins, giving minimum electron temperatures of about 150 mK for the Hypres process and 70 mK for the VTT process.
A new family of dc-powered Josephson junction digital devices, the Rapid Single Flux Quantum (RSFQ) logic, is described. The devices use overdamped Josephson junctions and two-junction interferometers to store, pass and process the digital information presented in form of single flux quanta. We have carried out extensive numerical simulation of the dynamics of the RSFQ logic gates and of some more complex circuits including serial full adder and reversible shift register, within the standard microscopic-theory ("Werthamer") description of Josephson junctions. The minimum clock cycles of the basic RSFQ circuits turn out to be as small as 2.5 ps. The most promising ways to use the RSFQ logic circuits at the present stage of development of the Josephson junction digital technology are discussed.
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.
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