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Bulletin of the American Physical Society

Calibration of flux crosstalk in flux qubit based quantum annealers with persistent current readout devices

Xi Dai, Antonio Martinez, Daniel Tennant, Denis Melanson, Muhammet Ali Yurtalan, Salil Bedkihal, Yongchao Tang, Alexander J. Melville, Bethany M. Niedzielski, Rabindra Nath Das, David K. Kim, Jonilyn Yoder, Steven J. Weber, Andrew J. Kerman, С. А. Новиков, Steven Disseler, James I. Basham, Jeffrey A. Grover, Evgeny Mozgunov, Daniel A. Lidar, Adrian Lupaşcu

Published Mar 5, 2020• 0 citations
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We demonstrate, for the first time, that a quantum flux parametron (QFP) is capable of acting as both isolator and amplifier in the readout circuit of a capacitively shunted flux qubit (CSFQ).By treating the QFP like a tunable coupler and biasing it such that the coupling is off, we show that T 1 of the CSFQ is not impacted by Purcell loss from its low-Q readout resonator (Q e = 760) despite being detuned by only 40 MHz.When annealed, the QFP amplifies the qubit's persistent current signal such that it generates a flux qubit-state-dependent frequency shift of 85 MHz in the readout resonator, which is over 9 times its linewidth.The device is shown to read out a flux qubit in the persistent current basis with fidelities surpassing 98.6% with only 80 ns integration, and reaches fidelities of 99.6% when integrated for 1 μs.This combination of speed and isolation is critical to the readout of high-coherence quantum annealers.
In the quest to reboot computing, quantum annealing (QA) is an interesting candidate for a new capability. While it has not demonstrated an advantage over classical computing on a real-world application, many important regions of the QA design space have yet to be explored. In IARPA's Quantum Enhanced Optimization (QEO) program, we have opened some new lines of inquiry to get to the heart of QA, and are designing testbed superconducting circuits and conducting key experiments. In this paper, we discuss recent experimental progress related to one of the key design dimensions: qubit coherence. Using MIT Lincoln Laboratory's qubit fabrication process and extending recent progress in flux qubits, we are implementing and measuring QA-capable flux qubits. Achieving high coherence in a QA context presents significant new engineering challenges. We report on techniques and preliminary measurement results addressing two of the challenges: crosstalk calibration and qubit readout. This groundwork enables exploration of other promising features and provides a path to understanding the physics and the viability of quantum annealing as a computing resource.
Magnetic flux tunability is an essential feature in most approaches to quantum computing based on superconducting qubits.Independent control of the fluxes in multiple loops is hampered by crosstalk.Calibrating flux crosstalk becomes a challenging task when the circuit elements interact strongly.We present a novel approach to flux crosstalk calibration, which is circuit model independent and relies on an iterative process to gradually improve calibration accuracy.This method allows us to reduce errors due to the inductive coupling between loops.The calibration procedure is automated and implemented on devices consisting of tunable flux qubits and couplers with up to 27 control loops.We devise a method to characterize the calibration error, which is used to show that the errors of the measured crosstalk coefficients are all below 0.17%.