Quantum noise in SQUIDs

Danilov, V. V.; Likharev, K. K.; Zorin, A. B.

doi:10.1109/tmag.1983.1062489

Cited by 44 publications

(49 citation statements)

References 16 publications

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“…Alternatively, the generic type of Cooper-pair (Bloch-transistor [7]) electrometers made from superconductors [8,9] seems to be quite promising as regards matching with the Cooper pair box. Furthermore, similar to dc-SQUIDs [10] and in contrast to SETs, the resistively shunted Cooper-pair electrometer belongs to the category of perfect (quantumlimited) linear detectors [11,12] and, therefore, can perform continuous measurements of a quantum object [13].…”

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confidence: 99%

Cooper-pair qubit and Cooper-pair electrometer in one device

Zorin

2002

Physica C: Superconductivity

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An all-superconductor charge qubit enabling a radio-frequency readout of its quantum state is described. The core element of the setup is a superconducting loop which includes the single-Cooperpair (Bloch) transistor. This circuit has two functions: First, it operates as a charge qubit with magnetic control of Josephson coupling and electrostatic control of the charge on the transistor island. Secondly, it acts as the transducer of the rf electrometer, which probes the qubit state by measuring the Josephson inductance of the transistor. The evaluation of the basic parameters of this device shows its superiority over the rf-SET-based qubit setup.PACS numbers: 74.50.+r, 85.25.Na, 03.67.Lx Superconducting structures with small Josephson tunnel junctions serve as a basis for electronic devices operating on single Cooper pairs and possessing remarkable characteristics. The paper Ref.[1] has demonstrated the potential of the single-Cooper-pair box circuit [2] as a charge qubit and thus has attracted renewed attention to this field. The practical realization of the Cooper pair qubit is not, however, simple and the main problems here are the achievement of a reliable readout of the quantum state and an elongation of the decoherence time. For the most part, these two issues are interrelated because a charge detector coupled to the qubit presents the principal source of quantum decoherence.The qubit setup consisting of a Cooper pair box and a capacitively coupled single electron transistor (SET, including the rf-SET [3]) has been extensively explored [4,5,6]. Although a preliminary analysis shows that qubit states can, in principle, be measured in the snapshot regime [6], the 'mismatch' of the charge carriers in the setup components (namely, the incoherent nature of charges in the SET) might lead to an unaccounted enhancement of decoherence in the system. Alternatively, the generic type of Cooper-pair (Bloch-transistor [7]) electrometers made from superconductors [8,9] seems to be quite promising as regards matching with the Cooper pair box. Furthermore, similar to dc-SQUIDs [10] and in contrast to SETs, the resistively shunted Cooper-pair electrometer belongs to the category of perfect (quantumlimited) linear detectors [11,12] and, therefore, can perform continuous measurements of a quantum object [13].Recently, we have proposed an rf-driven single-Cooperpair electrometer [14] whose energy-resolution figure ǫ can approach the standard quantum limit of /2. The transducer of this electrometer is a Bloch transistor inserted into a superconducting loop. The magnitude of the supercurrent circulating in the loop depends on the polarization charge (quasicharge) on the transistor island induced via a coupling capacitance by the charge source, e.g., the qubit.In this paper we present a circuit in which the electrometer's transducer takes over the function of the Cooper pair box (qubit) as well. The device's core el- ement is a superconducting loop including a mesoscopic double Josephson junction with a capacitive gate (transis...

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Cooper-pair qubit and Cooper-pair electrometer in one device

Zorin

2002

Physica C: Superconductivity

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“…Applying the resistively shunted junction (RSJ) model [17], the junction is characterized by its critical current , shunt capacitance , and shunt resistance . A flux-biased SQUID with total magnetic flux enclosed in the loop is analogous to a "flux" particle of mass (where is the flux quantum) moving in a potential.…”

Section: Energy Level Of the Coupled Rf Squid Qubitsmentioning

confidence: 99%

Quantum Entanglement and Controlled Logical Gates Using Coupled SQUID Flux Qubits

Zhou

Chu

Han

2005

IEEE Trans. Appl. Supercond.

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Abstract-We present an approach to realize universal two-bit quantum gates using two SQUID flux qubits. In this approach the basic unit consists of two inductively coupled SQUIDs with realistic device parameters. Quantum logical gates are implemented by applying resonant microwave pulse to the qubits. This procedure is demonstrated by realizing a controlled-NOT (CNOT) gate and the maximally entangled states of the coupled qubits through highly accurate numerical solution of the time-dependent Schrödinger equation of the system. This coupling scheme is simple and can be readily extended to many-qubit circuits required for scalable quantum information processing.Index Terms-Coupled SQUID flux qubits, quantum computing, quantum entanglement, two-bit gates.

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“…The corresponding value forE was approximately li, but it should be emphasized that quantum mechanics does not impose any precise lower limit onE 82 • A number of SQUIDs have obtained noise energies of 311 or less, but there is no evidence as yet that a SQUID has attained quantum limited performance as an amplifier.…”

Section: Voltmetersmentioning

confidence: 99%

SQUIDs: Principles, Noise, and Applications

Clarke

1990

Superconducting Devices

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Quantum noise in SQUIDs

Cited by 44 publications

References 16 publications

Cooper-pair qubit and Cooper-pair electrometer in one device

Cooper-pair qubit and Cooper-pair electrometer in one device

Quantum Entanglement and Controlled Logical Gates Using Coupled SQUID Flux Qubits

SQUIDs: Principles, Noise, and Applications

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