Dynamical control techniques with superconducting qubits

Presenting Author: Simon Gustavsson, Massachusetts Institute of Technology
Contributing Author(s): Fei Yan, Gianluigi Catelani, Jonas Bylander, Jeffrey Birenbaum, David Hover, Danna Rosenberg, Gabriel Samach, Steven J. Weber, Jonilyn L. Yoder, John Clarke, Andrew J. Kerman, Fumiki Yoshihara, Yasunobu Nakamura, Terry P. Orlando, William D. Oliver

Dynamical error suppression techniques are commonly used to improve coherence in quantum systems. They reduce dephasing errors by applying control pulses designed to reverse erroneous coherent evolution driven by environmental noise. However, such methods cannot correct for irreversible processes such as energy relaxation (T1). Here, we investigate a complementary, stochastic approach to reducing errors: instead of deterministically reversing the unwanted qubit evolution, we use control pulses to shape the noise environment dynamically. In the context of superconducting qubits, we implement a pumping sequence to reduce the number of unpaired electrons - quasiparticles - in close proximity to the device. We report a 70% reduction in the quasiparticle density, resulting in a threefold enhancement in qubit relaxation times, and a comparable reduction in coherence variability. In a separate experiment, we investigate qubit dephasing (T2) due to photon shot noise in a flux qubit transversally coupled to a coplanar microwave resonator. We have made the first quantitative spectroscopy of this noise for both thermal (i.e., radiation from higher temperature stages) and coherent photons (residual photons from the readout and control pulses), and we uniquely identify thermal shot noise as the dominant source of dephasing. Furthermore, by improving the filtering, we are able to reduce the residual photon population to 0.0004, resulting in T2 echo times approaching 100 us.

Read this article online: http://science.sciencemag.org/content/354/6319/1573?ijkey=aS%2520B1OEKGNuLt2&keytype=ref&siteid=sci, https://www.nature.com/articles/ncomms12964

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