Speaker: Robert McDermott
Host: Olivia Lanes, Ph.D.

Title: Correlated Charge Noise and Relaxation Errors in Superconducting Qubits

Papers:

Abstract: The central challenge in building a quantum computer is error correction. Unlike classical bits, which are susceptible to only one type of error, quantum bits (“qubits”) are susceptible to two types of error, corresponding to flips of the qubit state about the X- and Z-directions. While the Heisenberg Uncertainty Principle precludes simultaneous monitoring of X- and Z-flips on a single qubit, it is possible to encode quantum information in large arrays of entangled qubits that enable accurate monitoring of all errors in the system, provided the error rate is low. Another crucial requirement is that errors cannot be correlated. Here, we characterize a superconducting multiqubit circuit and find that charge fluctuations are highly correlated on a length scale over 600 microns; moreover, discrete charge jumps are accompanied by a strong transient suppression of qubit energy relaxation time across the millimeter-scale chip. The resulting correlated errors are explained in terms of the charging event and phonon-mediated quasiparticle poisoning associated with absorption of gamma rays and cosmic-ray muons in the qubit substrate. I discuss implications of this work for proposed quantum error correction schemes. If time permits, I will describe new insights into quasiparticle poisoning and the ultimate limits to qubit coherence.

BIO: Robert McDermott received the PhD degree in the group of John Clarke at UC-Berkeley in 2002. Following a postdoc at NIST, Boulder and UC Santa Barbara in the group of John Martinis, he joined the University of Wisconsin-Madison in 2006, where he is a Professor of Physics. He leads an experimental superconducting quantum computing group that is focused on the development of tools to enable the continued scaling of superconducting qubits towards fault-tolerance. Separate research directions include scalable qubit control and measurement, fundamental studies of decoherence, development of novel protected qubits, and quantum sensing for dark matter detection.

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The Quantum Computing Seminar Series is a deep dive into various academic and research topics within the quantum community. It will feature community members and leaders every Friday, 12 PM EDT.

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