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IEEE Transactions on Applied Superconductivity

SQUID systems for macroscopic quantum coherence and quantum computing

P. Carelli, Maria Gabriella Castellano, F. Chiarello, C. Cosmelli, Roberto Leoni, G. Torrioli

Published Mar 1, 2001• 17 citations
Publisher

Abstract

Among the various devices proposed as elements of a quantum computer, the rf-SQUID is a very promising candidate. In fact, systems based on this element can be adjusted in situ, can be coupled by means of superconducting transformers, can be prepared individually and measured with superconducting electronics. Moreover, many progresses were made in these years which showed quantum effects in this system. The present paper describes a complete device developed in order to get a direct measurement of the quantum coherent oscillation. The knowledge of this time, together with its limiting factors, is a prerequisite for fabricating a qubit based on rf-SQUIDs.

DOI
10.1109/77.919321

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•Jan 1, 2001•16 citations
Among the various devices proposed as elements of a quantum computer, the rf-SQUID is a very promising candidate. In fact, systems based on this element can be adjusted in situ, can be coupled by means of superconducting transformers, can be prepared individually and measured with superconducting electronics. Moreover, many progresses were made in these years which showed quantum effects in this system. The present paper describes a complete device developed in order to get a direct measurement of the quantum coherent oscillation. The knowledge of this time, together with its limiting factors, is a prerequisite for fabricating a qubit based on rf-SQUIDs.
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
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.
The growing amount of theoretical interest in the area of quantum computing have stimulated in recent years research with the aim of developing a corresponding technology. Superconducting Josephson systems appear to be among the most promising candidates. We present the characterization of a fully integrated Josephson device consisting of an RF SQUID coupled to a readout system based on a dc SQUID sensor. We report measurements of the dc SQUID performances showing a high intrinsic responsivity and a low flux noise, giving a good signal to noise ratio in the small signal mode. In the classical regime data on the decay rate from the metastable flux states of RF SQUID are also reported. The low dissipation level and the good insulation of the probe from the external noise are encouraging in view of macroscopic quantum experiments. Work is in progress to improve the design of the device to increase its application capability toward the quantum regime.
At low temperature and low damping, the magnetic flux of a rf Superconducting QUantum Interference Device (SQUID) acts as a macroscopic quantum variable whose dynamic is described by a double well potential. A rf SQUID consists of a superconducting loop interrupted by a thin insulating barrier, known as a Josephson junction. The modified rf SQUID design discussed in this thesis has independent, in situ, controls that allow varying the barrier height between the potential wells and their relative tilt. This high level of control of the potential together with the capability to implement an experimental setup optimized to reduce coupling to external sources of noise, made this system ideal not only for tests of quantum transition at a macroscopic level, but also as a scalable solid state device for quantum computation. A variety of measurements of quantum phenomena are reported in this thesis, including microwave level spectroscopy, coherent oscillations, and macroscopic resonant tunneling (MRT). These measurements are compared with theoretical models to study the possible sources of decoherence in a rf SQUID qubit. The analysis confirmed that the main source of decoherence is not due to coupling to the external environment, but it can be linked to intrinsic low frequency flux noise. Work from many groups over the last several years has shown that the performance of a superconductive qubit is limited by coupling to spurious defects in the materials used for the device fabrication. In this work we analyze the possibility of using the MRT as a tool for future studies of effects of materials and fabrication process in the realization of improved qubits, and we also explore the feasibility for measurements of quantum phenomena that involve fast pulsing of the potential barrier height.
We present different types of integrated planar SQUID devices in view of Macroscopic Quantum Coherence (MQC) experiments. These devices include an rf SQUID as a probe to test the MQC, coupled to a readout system based on a dc SQUID sensor. Both dc SQUID and flux transformer are in a gradiometric configuration to minimize the electromagnetic noise from environment. We report data on dc SQUID performances as well as preliminary results on the decay from the metastable flux state for rf SQUID-based devices above the crossover temeperature (classical limit).
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.