Weak Measurement and Feedback in Superconducting Quantum Circuits

Murch, Kater; Vijay, R.; Siddiqi, Irfan

doi:10.1007/978-3-319-24091-6_7

Cited by 7 publications

(8 citation statements)

References 45 publications

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“…The rapid development of quantum information science in the superconducting domain [13] has seen an exponential increase in qubit coherence time within the past decade, leading to many scientific advances [14]. This technological progress has led to a wide variety of advances in quantum physics, as observed and controlled in these systems, including greater than 99% fidelity in single-qubit quantum gates [15], multiqubit-entanglement-generating gates [16], the violation of Bell's inequality [17], and quantum-process tomography [18].…”

Section: Introductionmentioning

confidence: 99%

Quantum Trajectories and Their Statistics for Remotely Entangled Quantum Bits

et al. 2016

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We experimentally and theoretically investigate the quantum trajectories of jointly monitored transmon qubits embedded in spatially separated microwave cavities. Using nearly quantum-noise-limited superconducting amplifiers and an optimized setup to reduce signal loss between cavities, we can efficiently track measurement-induced entanglement generation as a continuous process for single realizations of the experiment. The quantum trajectories of transmon qubits naturally split into low and high entanglement classes. The distribution of concurrence is found at any given time, and we explore the dynamics of entanglement creation in the state space. The distribution exhibits a sharp cutoff in the high concurrence limit, defining a maximal concurrence boundary. The most-likely paths of the qubits' trajectories are also investigated, resulting in three probable paths, gradually projecting the system to two even subspaces and an odd subspace, conforming to a "half-parity" measurement. We also investigate the most-likely time for the individual trajectories to reach their most entangled state, and we find that there are two solutions for the local maximum, corresponding to the low and high entanglement routes. The theoretical predictions show excellent agreement with the experimental entangled-qubit trajectory data.

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Section: Introductionmentioning

confidence: 99%

Quantum Trajectories and Their Statistics for Remotely Entangled Quantum Bits

et al. 2016

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“…Theories for open quantum systems and quantum measurements [1][2][3][4][5][6] developed during the past decades are now becoming standard tools in analysing and designing experiments for quantum technologies. Decoherence from uncontrollable or undetectable noise from the environment or measurement backaction is ubiquitous in current experiments in different platforms, such as in superconducting circuits [7][8][9], ion-trap experiments [10,11], and NV-centers [12]. However, given records from measurements that can be made on such systems, there can be different estimation strategies to best extract information about the systems.…”

Section: Introductionmentioning

confidence: 99%

Quantum state smoothing: why the types of observed and unobserved measurements matter

2019

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We investigate the estimation technique called quantum state smoothing introduced in (Guevara and Wiseman 2015 Phys. Rev. Lett. 115 180407), which offers a valid quantum state estimate for a partially monitored system, conditioned on the observed record both prior and posterior to an estimation time. The technique was shown to give a better estimate of the underlying true quantum states than the usual quantum filtering approach. However, the improvement in estimation fidelity, originally examined for a resonantly driven qubit coupled to two vacuum baths, was also shown to vary depending on the types of detection used for the qubit's fluorescence. In this work, we analyse this variation in a systematic way for the first time. We first define smoothing power using an average purity recovery and a relative average purity recovery, of smoothing over filtering. Then, we explore the power for various combinations of fluorescence detection for both observed and unobserved channels. We next propose a method to explain the variation of the smoothing power, based on multitime correlation strength between fluorescence detection records. The method gives a prediction of smoothing power for different combinations, which is remarkably successful in comparison with numerically simulated qubit trajectories. Gesellschaft which is a function of time difference τ and is symmetric under the interchange of the records, i.e.3 ss ss ss 2 using the steady-state correlators defined in equation (21)- (22). We note that the first argument,definition is the record that appear twice in the correlator. Thus we now have correlators in equations (23) and (26) defined with unit-less measurement results and can capture solely the correlation between any two records. We show in figures 4(a) and (b) the two-time and threetime correlators for all combinations of records JK and J d M , as functions of τ. There are only two out of six of the two-time correlators that are zero:  [ ] J J d , d X N 2 , and  [ ] J J d , d X Y 2 . For the three-time correlators, there are three out of nine, i.e.    [ ] [ ] [ ] in equation (23) are shown as functions of τ. The non-vanishing correlators are for the following:in equation (26) are shown as functions of τ, where  is chosen to be one Rabi period. The colour legend is read in the same way as in figure 3, but with dK and dM, representing any two types of records. The values of correlators here are used for the analysis in table 1. Time τ is presented in units of the Rabi period T Ω =2π/Ω. Therefore, we instead focus on a parameter-independent feature, which is the vanishing or non-vanishing property of the correlators. Some correlators, such as, are zero regardless of the values of  and τ. Those that do not vanish identically are non-zero for almost all values of  and τ.In order to predict the power of quantum state smoothing offered by different measurement unravelling combinations, we propose the following principles. Firstly, the stronger the correlation, the better the smoothing power. We quantify ...

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“…Efficient discrimination of quantum states is important for, e.g., fast qubit readout [1][2][3][4], rapid feedback and steering [5][6][7], preparation of nonclassical states of light [5,[8][9][10], and nanoscale magnetometry [11,12]. Given sufficient information about the statistics and dynamics of a physical readout apparatus, it is possible to speed up a readout through a real-time adaptive decision rule (described below) [1][2][3][4][5][6]10].…”

Section: Introductionmentioning

confidence: 99%

Maximal Adaptive-Decision Speedups in Quantum-State Readout

et al. 2016

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The average time T required for high-fidelity readout of quantum states can be significantly reduced via a real-time adaptive decision rule. An adaptive decision rule stops the readout as soon as a desired level of confidence has been achieved, as opposed to setting a fixed readout time t f . The performance of the adaptive decision is characterized by the "adaptive-decision speedup," t f =T. In this work, we reformulate this readout problem in terms of the first-passage time of a particle undergoing stochastic motion. This formalism allows us to theoretically establish the maximum achievable adaptive-decision speedups for several physical two-state readout implementations. We show that for two common readout schemes (the Gaussian latching readout and a readout relying on state-dependent decay), the speedup is bounded by 4 and 2, respectively, in the limit of high single-shot readout fidelity. We experimentally study the achievable speedup in a real-world scenario by applying the adaptive decision rule to a readout of the nitrogen-vacancy-center (NV-center) charge state. We find a speedup of ≈2 with our experimental parameters. In addition, we propose a simple readout scheme for which the speedup can, in principle, be increased without bound as the fidelity is increased. Our results should lead to immediate improvements in nanoscale magnetometry based on spin-to-charge conversion of the NV-center spin, and provide a theoretical framework for further optimization of the bandwidth of quantum measurements.

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Weak Measurement and Feedback in Superconducting Quantum Circuits

Cited by 7 publications

References 45 publications

Quantum Trajectories and Their Statistics for Remotely Entangled Quantum Bits

Quantum Trajectories and Their Statistics for Remotely Entangled Quantum Bits

Quantum state smoothing: why the types of observed and unobserved measurements matter

Maximal Adaptive-Decision Speedups in Quantum-State Readout

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