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Design of an RSFQ control circuit to observe MQC on an rfSQUID

Roberto Rey-de-Castro, Mark F. Bocko, Andrea M. Herr, Cesar A. Mancini, M. J. Feldman

Published Jan 1, 2001• 16 citations
OpenAlex

Abstract

Abstract—We believe that the best chance to observe macroscopic quantum coherence (MQC) in an rf-SQUID qubit is to use on-chip RSFQ digital circuits for preparing, evolving and reading out the qubit’s quantum state. This approach allows experiments to be conducted on a very short time scale (sub-nanosecond) without the use of large bandwidth control lines that would couple environmental degrees of freedom to the qubit, thus contributing to its decoherence. In this paper we present our design of an RSFQ digital control circuit for demonstrating MQC in an rf-SQUID. We assess some of the key practical issues in the circuit design including the achievement of the necessary flux bias stability. We present an "active" isolation structure to be used to increase coherence times. The structure decouples the SQUID from external degrees of freedom, and then couples it to the output measurement circuitry when required, all under the active control of RSFQ circuits. Index Terms—superconducting qubit, macroscopic quantum coherence, RSFQ control.

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Design of an RSFQ control circuit to observe MQC on an rf-SQUID

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Superconductor Science and Technology•Nov 21, 2001•7 citations
Abstract—We believe that the best chance to observe macroscopic quantum coherence (MQC) in an rf-SQUID qubit is to use on-chip RSFQ digital circuits for preparing, evolving and reading out the qubit’s quantum state. This approach allows experiments to be conducted on a very short time scale (sub-nanosecond) without the use of large bandwidth control lines that would couple environmental degrees of freedom to the qubit, thus contributing to its decoherence. In this paper we present our design of an RSFQ digital control circuit for demonstrating MQC in an rf-SQUID. We assess some of the key practical issues in the circuit design including the achievement of the necessary flux bias stability. We present an "active" isolation structure to be used to increase coherence times. The structure decouples the SQUID from external degrees of freedom, and then couples it to the output measurement circuitry when required, all under the active control of RSFQ circuits. Index Terms—superconducting qubit, macroscopic quantum coherence, RSFQ control.
We believe that the best chance to observe macroscopic quantum coherence (MQC) in an rf-SQUID qubit is to use on-chip RSFQ digital circuits for preparing, evolving and reading out the qubit's quantum state. This approach allows experiments to be conducted on a very short time scale (sub-nanosecond) without the use of large bandwidth control lines that would couple environmental degrees of freedom to the qubit, thus contributing to its decoherence. In this paper we present our design of an RSFQ digital control circuit for demonstrating MQC in an rf-SQUID. We assess some of the key practical issues in the circuit design including the achievement of the necessary flux bias stability. We present an "active" isolation structure to be used to increase coherence times. The structure decouples the SQUID from external degrees of freedom, and then couples it to the output measurement circuitry when required, all under the active control of RSFQ circuits.
One of the main difficulties in observing quantum coherence (MQC) in rf-SQUID based systems is extremely short coherence times. Quantum coherence in macroscopic system is rapidly destroyed by interaction with the environment. This precludes any MQC experiments from using high-speed data acquisition that allows direct evidence of coherent evolution of linear combination of rf-SQUID eigenstates. In order to perform such experiments, isolation structures that effectively shield MQC experimental elements from small-signal interaction with data acquisition circuitry on chip and at the same time allow large signals such as RSFQ pulses to propagate through. We consider design and simulation of series of Josephson junctions with varied parameters as an example of such isolation filters.
We present the design of an on-chip time domain experiment to demonstrate macroscopic quantum coherence in an rf-SQUID, a superconducting loop interrupted by a Josephson junction. If suc- cessful the SQUID will serve as a for a superconducting quantum computer. The chip is currently in fabrication. odinger' s parable, a clear demonstration of rf-SQUID MQC would dramatically challenge our concepts of the quantum / classical boundary. The rf-SQUID MQC state was proposed as a candidate to serve as the for a possible superconducting quantum computer (2). Very recently, spectroscopic experiments showed the energy level structure expected for MQC phenomena in an rf-SQUID (3) and in the very similar persistent current qubit (4). It is our intention to conduct a time domain experiment in which the macroscopic quantum coherent oscillations will be observed directly as they evolve. The Hamiltonian of the rf-SQUID system, under the assumption of no dissipation, is
Our aim was to develop the concepts and perform the experiments to establish a clear path towards useful quantum coherent computation using superconducting flux-based qubits. We have concentrated upon using superconducting single flux quantum (SFQ) circuitry integrated on the qubit chip to act as the interface between a quantum circuit and laboratory electronics. We have developed SFQ circuits for initialization and for read-out of the quantum state of a qubit; for precise timing of duration and exact biasing all with picosecond-scale precision. We developed a set of techniques to design such qubit control circuitry while preserving proper isolation (not significantly contributing to decoherence). We also designed an SFQ experiment to investigate quantum behavior using foundry-fabricated chips at higher temperatures. We acquired and custom set-up a He3 refrigerator, and demonstrated SFQ circuit functionality at 300 mK for the first time. We have investigated a new method for quantum control both analytically and numerically, using isolated SFQ pulses instead of resonant excitation, and shown that the minimum operation time can be dramatically decreased ii some cases. However, the complete lack of custom fabrication has prevented us from making progress towards our experimental goals.
We report on the development and test of an integrated system composed of a flux qubit and a rapid single-flux quantum (RSFQ) circuit that allows qubit manipulation. The goal is to demonstrate the feasibility of control electronics integrated on the same chip as the qubit, in view of the application in quantum computation with superconducting devices. RSFQ logic relies on the storage and transmission of magnetic flux quanta and can be profitably used with superconducting qubits because of the speed, scalability, compatibility with the qubit fabrication process and low temperature environment. While standard RSFQ circuitry is well assessed, the application to quantum computing requires a complete rescaling of parameter values, in order to preserve the qubit coherence and reduce the power dissipation. In the system presented in this paper, the qubit role is played by a superconducting loop interrupted by a small dc SQUID, usually called a double SQUID, which behaves as a tunable rf-SQUID. Its energy potential has the shape of a double well, with the barrier between the wells controlled by magnetic flux applied to the inner dc SQUID. Here for the first time we report measurements at a base temperature of 370 mK in which flux control pulses with desired characteristics were supplied by a RSFQ circuit fabricated using non-standard parameters in the same chip as the qubit.
The observation and study of macroscopic quantum coherence (MQC), i.e. the coherent superposition and evolution of macroscopically distinct quantum states, is of great importance for both our understanding of nature and for quantum engineering, for example in quantum computing. Recently a first indication of MQC in a system of SQUIDs has been indirectly observed by means of spectroscopic techniques. Applications like quantum computing, however, require the manipulation and the observation of the time evolution of the MQC states. We have realized a system of SQUIDs for the study of MQC. Our system, based on trilayer Nb/AlOx/Nb technology, consists of a double rf-SQUID, source of the MQC state, inductively coupled to a non-hysteretic dc-SQUID magnetometer and to a hysteretic dc-SQUID threshold detector, used for the read-out of the magnetic flux state. The magnetometer is used to test and monitor the rf-SQUID behaviour, while the hysteretic dc-SQUID is used to perform fast measurements with a reduced back action on the rf-SQUID. We present measurements performed at 4.2 K and at 20 mK to characterize the single devices and their overall behaviour, and a scheme for the use of a set of SQUIDs as a qubit system.