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Millisecond charge-parity fluctuations and induced decoherence in a superconducting qubit

Quasiparticle excitations adversely affect the performance of superconducting devices in a wide range of applications. They limit the sensitivity of photon detectors in astronomy, the accuracy of current sources in metrology, the cooling power of micro-refrigerators, and could break the topological protection of Majorana qubits. In superconducting circuits for quantum information processing, tunneling of quasiparticles across Josephson junctions constitutes a decoherence mechanism. As relaxation and pure dephasing times of transmon-type charge qubits now reach 100 microseconds, understanding whether quasiparticle tunneling may already bottleneck coherence is of high interest. We integrate recent advances in qubit readout and feedback control in circuit quantum electrodynamics to perform the first real-time observation of quasiparticle tunneling in a transmon qubit. We demonstrate quasiparticle-tunneling contributions to qubit relaxation and pure dephasing in the millisecond range. Thus, quasiparticle tunneling will not limit coherence for at least one order of magnitude beyond the state of the art.