Charge-Parity Switching and Quasiparticles in Transmons

Short answer: Quasiparticles are excitations in a superconductor that can tunnel across a Josephson junction and disturb a qubit, and charge-parity switching is the signature of single-charge tunnelling. A 2013 Nature Communications paper reported that quasiparticle tunnelling does not presently limit transmon coherence, and a 2022 paper traced parity switching to photon-assisted and trapped-quasiparticle processes. A 2019 paper showed a device that was free of quasiparticles 97 percent of the time.

What is parity switching?

The 2013 Nature Communications abstract describes tunnelling of quasiparticles across Josephson junctions as an intrinsic decoherence mechanism for qubit degrees of freedom. The 2022 PRX Quantum abstract calls single-charge tunnelling a decoherence mechanism, and says the origin of the excess quasiparticle excitations responsible for it is not fully understood. When a quasiparticle tunnels, the charge parity of a small superconducting island flips, which can be detected.

What did the 2013 experiment find?

The authors integrate high-fidelity qubit readout and feedback control in circuit quantum electrodynamics to turn a transmon into its own real-time charge-parity detector. They directly measure quasiparticle tunnelling across the single junction and separate its contribution to qubit relaxation and dephasing without relying on theory. The measured timescales are milliseconds, and they conclude that quasiparticle tunnelling does not presently bottleneck transmon coherence, leaving room for another order of magnitude increase. This is a statement about the device generation studied in 2013.

What did the 2019 device show?

A 2019 Physical Review B paper presents a superconducting device with a clear parity effect in electron number, even when on average a single nonequilibrium quasiparticle is present and parity switches at rates of about 100 Hz. The authors monitor the charge state of a superconducting island connected to normal leads by tunnel junctions in real time. The quasiparticles are created by Cooper pairs breaking on the island at a rate of a few kilohertz, and the authors demonstrate that the pair breaking is caused by the backaction of the single-electron transistor used as the charge detector. With sufficiently low probing currents, the island is free of quasiparticles 97 percent of the time.

What did the 2022 paper add?

The authors measure the flux dependence of charge-parity switching in an offset-charge-sensitive transmon to identify the contributions of photon-assisted parity switching and quasiparticle generation. The switching rate shows a qubit-state-dependent peak in its flux dependence, which they say indicates a cold distribution of excess quasiparticles predominantly trapped in the low-gap film of the device. By fitting a model that includes photon-assisted switching and interfilm quasiparticle dynamics, they find that the photon-assisted process contributes significantly to both parity switching and generation of excess quasiparticles.

How to read this

The three papers study different devices, and the 2013 conclusion about coherence does not automatically apply to later or different qubits. This post is based on abstracts and visible summary text and full texts were not checked. It makes no claim about products.

FAQ

What does parity switching tell us?

It marks single-charge tunnelling, caused by quasiparticles, which affects superconducting qubits.

Did quasiparticles limit transmon coherence in 2013?

The authors say tunnelling did not then bottleneck coherence, with room for about another order of magnitude.

Where do excess quasiparticles go?

The 2022 authors report they are predominantly trapped in the low-gap film of their device.

Can detectors disturb the experiment?

In the 2019 paper, the charge detector's backaction broke Cooper pairs, and lower probing currents left the island quasiparticle-free 97 percent of the time.