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arXiv:1702.02253 · Physical Review Applied

Tunable Superconducting Qubits with Flux-Independent Coherence

M. D. Hutchings, J. B. Hertzberg, Y. Liu, Nicholas T. Bronn, G. A. Keefe, Markus Brink, Jerry M. Chow, B. L. T. Plourde

Published Oct 12, 2017• 190 citations
arXivHTMLPDF

Abstract

As superconducting-qubit architectures progress to more complex layouts, frequency crowding in qubit arrays increasingly hinders computing performance. Frequency-tunable qubits can assuage this problem, but are subject to the magnetic-flux noise that is ubiquitous in such circuitry. The authors exploit asymmetric Josephson junctions in transmon qubits to reduce sensitivity to flux noise, while preserving sufficient tunability to avoid frequency collisions. In their most asymmetric qubit, dephasing due to flux noise is reduced to the point where its rate is independent of the flux bias.

DOI
10.1103/physrevapplied.8.044003

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As superconducting-qubit architectures progress to more complex layouts, frequency crowding in qubit arrays increasingly hinders computing performance. Frequency-tunable qubits can assuage this problem, but are subject to the magnetic-flux noise that is ubiquitous in such circuitry. The authors exploit asymmetric Josephson junctions in transmon qubits to reduce sensitivity to flux noise, while preserving sufficient tunability to avoid frequency collisions. In their most asymmetric qubit, dephasing due to flux noise is reduced to the point where its rate is independent of the flux bias.
Superconducting qubits are building blocks for quantum processors, but they suffer from frequency collisions, which hinder the construction of large qubit lattices. This study introduces weakly tunable qubits, whose frequency can be tuned over a small range via external magnetic flux. Through theoretical derivation and experimental demonstration, the authors show that these transmonlike qubits---without losing the desired key properties---can be used to avoid frequency collisions in large qubit systems while minimizing the sensitivity to flux noise. They can also be applied to operate flux-tuned qubit gates.
Scaling up superconducting quantum processors with optimized performance requires a sufficient flexibility in the choice of operating points for single and two qubit gates to maximize their fidelity and cope with imperfections. Flux control is an efficient technique to manipulate the parameters of tunable qubits, in particular to activate entangling gates. At flux sensitive points of operation, the ubiquitous presence of 1/f flux noise however gives rise to dephasing by inducing fluctuations of the qubit frequency. We show how two-tone modulation of the flux bias, a bichromatic modulation, gives rise to a continuum of dynamical sweet spots where dephasing due to slow flux noise is suppressed to first order for a wide range of time-averaged qubit frequencies. The qubits can be operated at these dynamical sweet spots to realize protected entangling gates and to avoid collisions with two-level-system defects.
The ability to non-dissipatively tune the Josephson coupling energy of Josephson junctions is a useful tool in frequency-tunable qubits. This is typically done by threading magnetic flux through two junctions connected in a loop, a geometry that exposes the qubit to magnetic environmental noise. In this paper, we show that by biasing a junction with supercurrent from a separate pair of superconducting leads coupled to the device, the Josephson energy can be tuned without the need for a flux loop. Our multiterminal device may enable the realization of a frequency-tunable qubit with greatly reduced susceptibility to flux noise.
In this work, we study a series of tunable flux qubits inductively coupled to a coplanar waveguide resonator fabricated on a sapphire substrate. Each qubit includes an asymmetric superconducting quantum interference device, which is controlled by the application of an external magnetic field and acts as a tunable Josephson junction. The tunability of the qubits is typically $\ifmmode\pm\else\textpm\fi{}3.5\phantom{\rule{0.2em}{0ex}}\mathrm{GHz}$ around their central gap frequency. The measured relaxation times are limited by dielectric losses in the substrate and can attain ${T}_{1}\ensuremath{\sim}8\phantom{\rule{0.2em}{0ex}}\text{\ensuremath{\mu}}\mathrm{s}$. The echo dephasing times are limited by flux noise even at optimal points and reach ${T}_{2E}\ensuremath{\sim}4\phantom{\rule{0.2em}{0ex}}\text{\ensuremath{\mu}}\mathrm{s}$, almost an order of magnitude longer than state of the art.
Split transmons have been widely used in making quantum processors for building scalable and fault-tolerant superconducting quantum computers. With tunable external flux threading the superconducting quantum interference devices (SQUIDs) of qubits, split transmons have a degree of freedom for tuning frequencies but introduce flux noise as an important dephasing factor at the same time. Here, an improved version of a transmon is proposed and is called a ``triple-Josephson-junction transmon'' (tri-JJ transmon). A tri-JJ transmon is obtained by shunting another Josephson junction to the SQUID of a split transmon, which can produce much more different local minima and maxima of qubit frequencies tuned by external flux as ``sweet spots'' than those of split transmons. The first-order effect of the flux noise at the sweet spots can be eliminated and the corresponding pure-dephasing times are estimated as about $200\phantom{\rule{4pt}{0ex}}\textmu{}\mathrm{s}$ by considering only critical current noise. Tri-JJ transmons are compatible with the process and the protocol of control and measurement of split transmons and can be competitive candidates for constructing a high-performance superconducting quantum processor.
Abstract Superconducting qubits are a promising platform for building a larger-scale quantum processor capable of solving otherwise intractable problems. In order for the processor to reach practical viability, the gate errors need to be further suppressed and remain stable for extended periods of time. With recent advances in qubit control, both single- and two-qubit gate fidelities are now in many cases limited by the coherence times of the qubits. Here we experimentally employ closed-loop feedback to stabilize the frequency fluctuations of a superconducting transmon qubit, thereby increasing its coherence time by 26% and reducing the single-qubit error rate from (8.5 ± 2.1) × 10−4 to (5.9 ± 0.7) × 10−4. Importantly, the resulting high-fidelity operation remains effective even away from the qubit flux-noise insensitive point, significantly increasing the frequency bandwidth over which the qubit can be operated with high fidelity. This approach is helpful in large qubit grids, where frequency crowding and parasitic interactions between the qubits limit their performance.