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arXiv:1806.05276 · Physical Review X

High-Efficiency Measurement of an Artificial Atom Embedded in a Parametric Amplifier

A. Eddins, John Mark Kreikebaum, D. M. Toyli, Eli M. Levenson-Falk, A. Dove, W. P. Livingston, B. A. Levitan, L. C. G. Govia, A. A. Clerk, I. Siddiqi

Published Jan 7, 2019• 36 citations
arXivHTMLPDF

Abstract

A crucial limit to measurement efficiencies of superconducting circuits comes from losses involved when coupling to an external quantum amplifier.Here, we realize a device circumventing this problem by directly embedding an artificial atom, comprised of a transmon qubit, within a flux-pumped Josephson parametric amplifier.This configuration is able to enhance dispersive measurement without exposing the qubit to appreciable excess backaction.Near-optimal backaction is obtained by engineering the circuit to permit high-power operation that reduces information loss to unmonitored channels associated with the amplification and squeezing of quantum noise.By mitigating the effects of off-chip losses downstream, the on-chip gain of this device produces end-to-end measurement efficiencies of up to 80%.Our theoretical model accurately describes the observed interplay of gain and measurement backaction and delineates the parameter space for future improvement.The device is compatible with standard fabrication and measurement techniques and, thus, provides a route for definitive investigations of fundamental quantum effects and quantum control protocols.

DOI
10.1103/physrevx.9.011004

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A crucial limit to measurement efficiencies of superconducting circuits comes from losses involved when coupling to an external quantum amplifier.Here, we realize a device circumventing this problem by directly embedding an artificial atom, comprised of a transmon qubit, within a flux-pumped Josephson parametric amplifier.This configuration is able to enhance dispersive measurement without exposing the qubit to appreciable excess backaction.Near-optimal backaction is obtained by engineering the circuit to permit high-power operation that reduces information loss to unmonitored channels associated with the amplification and squeezing of quantum noise.By mitigating the effects of off-chip losses downstream, the on-chip gain of this device produces end-to-end measurement efficiencies of up to 80%.Our theoretical model accurately describes the observed interplay of gain and measurement backaction and delineates the parameter space for future improvement.The device is compatible with standard fabrication and measurement techniques and, thus, provides a route for definitive investigations of fundamental quantum effects and quantum control protocols.
A crucial limit to measurement efficiencies of superconducting circuits comes from losses involved when coupling to an external quantum amplifier. Here, we realize a device circumventing this problem by directly embedding an artificial atom, comprised of a transmon qubit, within a flux-pumped Josephson parametric amplifier. This configuration is able to enhance dispersive measurement without exposing the qubit to appreciable excess backaction. Near-optimal backaction is obtained by engineering the circuit to permit high-power operation that reduces information loss to unmonitored channels associated with the amplification and squeezing of quantum noise. By mitigating the effects of off-chip losses downstream, the on-chip gain of this device produces end-to-end measurement efficiencies of up to 80%. Our theoretical model accurately describes the observed interplay of gain and measurement backaction and delineates the parameter space for future improvement. The device is compatible with standard fabrication and measurement techniques and, thus, provides a route for definitive investigations of fundamental quantum effects and quantum control protocols.
We investigate the quantum measurement physics of a system in which a phase-sensitive parametric amplifier is used to directly measure a qubit to which it is coupled dispersively. By analytic calculation of the signal-to-noise ratio, we show that the amplification properties of the paramp can be exploited to achieve on-chip gain of the qubit readout signal, mitigating the damage done to the measurement rate by circulator added noise and losses in between the readout cavity and any following amplification chain. With a different choice of drive phase, we show that when added noise is not a concern, the squeezing properties of the paramp can be used to enhance the measurement rate relative to a standard dispersive readout by reducing the readout noise without corresponding reduction of qubit signal. We then proceed to calculate the back-action dephasing of the qubit by way of a phase-space technique, and find analytic results valid in the long-time limit while numerically handling the finite-time case. We show how in both operating modes, in the long-time limit, the back-action dephasing of the qubit can be made as small as is allowed by quantum mechanics by using using sufficiently many photons in the measurement, i.e.\@ by using a sufficiently strong coherent measurement drive. Crucially, we find that in the presence of experimentally realistic amounts of added noise, the on-chip gain afforded by such a device can be used to come significantly closer to the quantum limit than is possible with a standard dispersive measurement setup, allowing for efficiencies of ~ 75% under conditions that would yield efficiencies <~ 1 % when applying the usual scheme.
Superconducting qubits are a leading platform for scalable quantum computing and quantum error correction. One feature of this platform is the ability to perform projective measurements orders of magnitude more quickly than qubit decoherence times. Such measurements are enabled by the use of quantum-limited parametric amplifiers in conjunction with ferrite circulators-magnetic devices which provide isolation from noise and decoherence due to amplifier backaction. Because these nonreciprocal elements have limited performance and are not easily integrated on chip, it has been a long-standing goal to replace them with a scalable alternative. Here, we demonstrate a solution to this problem by using a superconducting switch to control the coupling between a qubit and amplifier. Doing so, we measure a transmon qubit using a single, chip-scale device to provide both parametric amplification and isolation from the bulk of amplifier backaction. This measurement is also fast, high fidelity, and has 70% efficiency, comparable to the best that has been reported in any superconducting qubit measurement. As such, this work constitutes a high-quality platform for the scalable measurement of superconducting qubits.
Broadband quantum-limited amplifiers play a critical role in the single-shot readout of superconducting qubits, but a popular implementation, the traveling wave parametric amplifier, involves a complex design and fabrication process. Here, we present a simple design for a Josephson parametric amplifier, using a lumped element resonator comprising a superconducting quantum interference device whose useful bandwidth is enhanced with an on-chip impedance-matching circuit. We demonstrate a flux-coupling geometry that maximizes the coupling to the Josephson loop and minimizes spurious excitation of the amplifier resonant circuit. The amplifier, which operates in a flux-pumped mode, is demonstrated with a power gain of more than 20 dB over a bandwidth of about 300 MHz, where approximate noise measurements indicate quantum-limited performance. A procedure is given for optimizing the bandwidth for this kind of amplifier, using a linearized circuit simulation while minimizing non-linearities.
High-fidelity quantum nondemolition qubit measurement is critical to error correction and rapid qubit feedback in large-scale quantum computing. Maximizing high-fidelity dispersive readout in superconducting qubits commonly requires passing a short and strong pulse through the qubit's readout resonator, which is then processed by a sufficiently high bandwidth, high saturation power, and quantum-limited amplifier. We have developed an amplifier design pipeline that combines time-domain simulation of the untruncated device Hamiltonian, fabrication constraints, and maximization of saturation power. We have realized an amplifier based on a modified trilayer (Nb-Al-AlOx{\mathrm{Al}\mathrm{O}}_{x}AlOx​-Nb) fabrication suite at NIST, which utilizes an array of 25 rf superconducting quantum interference devices embedded within a low-QQQ resonator powered by a high-power voltage pump delivered via a diplexer on the signal port. We show that, despite the intensity of the pump, the device is quantum efficient and capable of high-fidelity measurement limited by state transitions in the transmon. We present experimental data demonstrating up to $\ensuremath{-}91.2\phantom{\rule{0.2em}{0ex}}\mathrm{dBm}$ input saturation power with 20-dBdBdB gain, up to 28-MHz instantaneous bandwidth, and phase-preserving qubit measurements with $62\mathrm{%}$ quantum efficiency.
We provide a thorough theoretical analysis of qubit state measurement in a setup where a driven, parametrically-coupled cavity system is directly coupled to the qubit, with one of the cavities having a weak Kerr nonlinearity. Such a system could be readily realized using circuit QED architectures. We demonstrate that this setup is capable in the standard linear-response regime of both producing a highly amplified output signal while at the same time achieving near quantum-limited performance: the measurement backaction on the qubit is near the minimal amount required by the uncertainty principle. This setup thus represents a promising route for performing efficient large-gain qubit measurement that is completely on-chip, and that does not rely on the use of circulators or complex non-reciprocal amplifiers.