Mitigation of quasiparticle poisoning in a quantum dot coupled to a superconductor

The density of quasiparticles in a superconductor should vanish exponentially as the temperature approaches zero. However, high-energy photons, cosmic rays, and radioactive materials can create a significant excess density of non-equilibrium quasiparticles, which can cause decoherence in superconducting, Andreev, or Majorana qubits. Improved shielding and sample design have led to superconducting qubits with parity lifetimes exceeding one second. However, in hybrid semiconductor-superconductor devices, parity lifetimes have remained limited to just a few milliseconds.

In this work, we use a charge sensor to measure the quasiparticle poisoning rates of a quantum dot coupled to a superconductor in an InSb nanowire proximitized by a thin film of aluminum. We observe that applying an external magnetic field perpendicular to the film extends the parity lifetime of the quantum dot from a few milliseconds to hundreds of milliseconds. This increase persists even after the magnetic field is removed. We attribute this suppression of poisoning rates to the formation of vortices in the superconducting film, which protect the quantum dot by acting as quasiparticle traps.

Our results demonstrate that quasiparticle poisoning will not be a limiting factor for Majorana qubits based on short Kitaev chains. We believe that our approach could be readily extended to other hybrid semiconducting-superconducting devices.