Suppressing relaxation in superconducting qubits by quasiparticle pumping

Abstract

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. In this work, 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.Since Hahn's invention of the spin-echo in 1950 [1], coherent control techniques have been crucial tools for reducing errors, improving control fidelity, performing noise spectroscopy and generally extending coherence in both natural and artificial spin systems. All of these methods are similar: they correct for dephasing errors by reversing unintended phase accumulations due to a noisy environment through the application of a sequence of control pulses, thereby improving the dephasing time T 2 . However, such coherent control techniques cannot correct for irreversible processes that reduce the relaxation time T 1 , where energy is lost to the environment. Improving T 1 requires either reducing the coupling between the spin system and its noisy environment, reducing the noise in the environment itself [2], or implementing full quantum error correction.We demonstrate a pumping sequence that dynamically reduces the noise in the environment and improves T 1 of a superconducting qubit through an irreversible pumping process. The sequence contains the same type of control pulses common to all dynamical-decoupling sequences, but instead of coherently and deterministically controlling the qubit time evolution, the sequence is designed to shape the noise stochastically via inelastic energy exchange with the environment. Similar methods have been used to extend T 2 of spin qubits by dynamic nuclear polarization [3], and irreversible control techniques are commonly used to prepare systems into well-defined quantum states through optical pumping [4,5] and sideband cooling [6], but outside of quantum error correction, to our knowledge no dynamic enhancement of T 1 has been previously reported.We implement the pumping sequence in a superconducting flux qubit, with the aim of reducing the population of unpaired electrons or quasiparticles in close vicinity to the device. As a superconducting circuit is cooled well below its critical temperature, the quasiparticle density via BCS theory is expected to be exponentially suppressed, but a number of experimental groups have reported higher-than-expected values in a wide variety of systems [7][8][9][10]. Although...

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