  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.
