Landau-Zener transitions in a superconducting flux qubit

Johansson, Jan; Amin, M. H. S.; Berkley, A. J.; Bunyk, P.; Choi, Vicky; Harris, Richard E.; Johnson, Mark W.; Lanting, T.; Lloyd, Seth; Rose, Geordie

doi:10.1103/physrevb.80.012507

Cited by 31 publications

(37 citation statements)

References 23 publications

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“…An open system with T ¼ 0 has been theoretically predicted to behave similarly to a closed system for such an anticrossing 10,[34][35][36] . In this experiment, it was infeasible to reduce T below B20 mK.…”

Section: Resultsmentioning

confidence: 99%

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Thermally assisted quantum annealing of a 16-qubit problem

et al. 2013

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Efforts to develop useful quantum computers have been blocked primarily by environmental noise. Quantum annealing is a scheme of quantum computation that is predicted to be more robust against noise, because despite the thermal environment mixing the system's state in the energy basis, the system partially retains coherence in the computational basis, and hence is able to establish well-defined eigenstates. Here we examine the environment's effect on quantum annealing using 16 qubits of a superconducting quantum processor. For a problem instance with an isolated small-gap anticrossing between the lowest two energy levels, we experimentally demonstrate that, even with annealing times eight orders of magnitude longer than the predicted single-qubit decoherence time, the probabilities of performing a successful computation are similar to those expected for a fully coherent system. Moreover, for the problem studied, we show that quantum annealing can take advantage of a thermal environment to achieve a speedup factor of up to 1,000 over a closed system.

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Section: Resultsmentioning

confidence: 99%

“…Of key importance are the energy scales D i (s) and E(s) in equation (1), which can be calculated from independently calibrated device parameters (see, for example, Harris et al 33 and Johansson et al 34 ). Results for the 16 qubits used in this study are plotted in Fig.…”

Section: Resultsmentioning

confidence: 99%

Thermally assisted quantum annealing of a 16-qubit problem

et al. 2013

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“…For those whose interests lie in developing hardware for AQO or forms of GMQC that likewise encode the logical basis into the flux basis, it is far more convenient to have a set of tools for characterizing quantum-mechanical properties that require only low bandwidth bias controls. Such methods, some appropriate in the coherent regime 51,52 and others in the incoherent regime, 36,47,53 have been reported in the literature. We have made use of such low-frequency methods as our apparatuses typically possess 128 low bandwidth bias lines to facilitate the adiabatic manipulation of a large number of devices.…”

Section: Qubit Propertiesmentioning

confidence: 99%

“…53 In principle, this method should be applicable in both the coherent and incoherent regimes. In practice, we have found it only possible to probe the device to modestly larger ⌬ q than we can reach via LOMRT purely due to the low bandwidth of our bias lines.…”

Section: Qubit Propertiesmentioning

confidence: 99%

Experimental demonstration of a robust and scalable flux qubit

et al. 2010

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A rf-superconducting quantum interference device ͑SQUID͒ flux qubit that is robust against fabrication variations in Josephson-junction critical currents and device inductance has been implemented. Measurements of the persistent current and of the tunneling energy between the two lowest-lying states, both in the coherent and incoherent regimes, are presented. These experimental results are shown to be in agreement with predictions of a quantum-mechanical Hamiltonian whose parameters were independently calibrated, thus justifying the identification of this device as a flux qubit. In addition, measurements of the flux and critical current noise spectral densities are presented that indicate that these devices with Nb wiring are comparable to the best Al wiring rf SQUIDs reported in the literature thus far, with a 1 / f flux noise spectral density at 1 Hz of 1.3 −0.5 +0.7 ⌽ 0 / ͱ Hz. An explicit formula for converting the observed flux noise spectral density into a freeinduction-decay time for a flux qubit biased to its optimal point and operated in the energy eigenbasis is presented. I. MOTIVATIONExperimental efforts to develop useful solid-state quantum information processors have encountered a host of practical problems that have substantially limited progress. While the desire to reduce noise in solid-state qubits appears to be the key factor that drives much of the recent work in this field, it must be acknowledged that there are formidable challenges related to architecture, circuit density, fabrication variation, calibration, and control that also deserve attention. For example, a qubit that is inherently exponentially sensitive to fabrication variations with no recourse for in situ correction holds little promise in any large-scale architecture, even with the best of modern fabrication facilities. Likewise, a qubit that requires an inordinate number of custom-tuned time-dependent control signals to be launched onto the chip, in order to correct for fabrication variations or to compensate for unintended on-chip crosstalk, would also not be useful in a large-scale processor. Thus, a qubit designed in the absence of information concerning its ultimate use in a larger-scale system may prove to be of little utility in the future. In what follows, we present an experimental demonstration of a superconducting flux qubit 1 that has been specifically designed to address several issues that pertain to the implementation of a large-scale quantum information processor. While noise is not the central focus of this paper, we nonetheless present experimental evidence that, despite its physical size and relative complexity, the observed flux noise in this flux qubit is comparable to the quietest such devices reported on in the literature to date.It has been well established that rf superconducting quantum interference devices ͑SQUIDs͒ can be used as qubits given an appropriate choice of device parameters. Such devices can be operated as a flux-biased phase qubit using two intrawell energy levels 2 or as a flux qubit using...

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“…Such a procedure was proved successful in providing accurate description of the experimental data for flux qubits in Ref. 40.…”

Section: A Tunneling Between Ground Statesmentioning

confidence: 99%

Non-Markovian incoherent quantum dynamics of a two-state system

Amin

Brito

2009

Phys. Rev. B

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We present a detailed study of the non-Markovian two-state system dynamics for the regime of incoherent quantum tunneling. Using perturbation theory in the system tunneling amplitude ∆, and in the limit of strong system-bath coupling, we determine the short time evolution of the reduced density matrix and thereby find a general equation of motion for the non-Markovian evolution at longer times. We relate the nonlocality in time due to the non-Markovian effects with the environment characteristic response time. In addition, we study the incoherent evolution of a system with a double-well potential, where each well consists several quantized energy levels. We determine the crossover temperature to a regime where many energy levels in the wells participate in the tunneling process, and observe that the required temperature can be much smaller than the one associated with the system plasma frequency. We also discuss experimental implications of our theoretical analysis.

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Landau-Zener transitions in a superconducting flux qubit

Cited by 31 publications

References 23 publications

Thermally assisted quantum annealing of a 16-qubit problem

Thermally assisted quantum annealing of a 16-qubit problem

Experimental demonstration of a robust and scalable flux qubit

Non-Markovian incoherent quantum dynamics of a two-state system

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