Quasiparticle Poisoning of Superconducting Qubits from Resonant Absorption of Pair-breaking Photons

TL;DR

This paper demonstrates that millimeter-wave photons resonantly absorbed by superconducting qubits generate quasiparticles, causing decoherence, and suggests that qubit design can be optimized to mitigate this effect for more robust quantum computing.

Contribution

It identifies resonant absorption of high-energy photons as a key mechanism for quasiparticle poisoning in superconducting qubits and demonstrates how the qubit structure acts as an antenna for these photons.

Findings

Qubit structure acts as a resonant antenna for millimeter-wave radiation.

Absorption of photons leads to increased quasiparticle density.

Photon-induced quasiparticles cause discrete charge shifts and decoherence.

Abstract

The ideal superconductor provides a pristine environment for the delicate states of a quantum computer: because there is an energy gap to excitations, there are no spurious modes with which the qubits can interact, causing irreversible decay of the quantum state. As a practical matter, however, there exists a high density of excitations out of the superconducting ground state even at ultralow temperature; these are known as quasiparticles. Observed quasiparticle densities are of order 1~$\mu$m$^{-3}$, tens of orders of magnitude larger than the equilibrium density expected from theory. Nonequilibrium quasiparticles extract energy from the qubit mode and induce discrete changes in qubit offset charge, a potential source of dephasing. Here we show that a dominant mechanism for quasiparticle poisoning in superconducting qubits is direct absorption of high-energy photons at the qubit junction. We use a Josephson junction-based photon source to controllably dose qubit circuits with millimeter-wave radiation, and we use an interferometric quantum gate sequence to reconstruct the charge parity on the qubit island. We find that the structure of the qubit itself acts as a resonant antenna for millimeter-wave radiation, providing an efficient path for photons to generate quasiparticle excitations. A deep understanding of this physics will pave the way to realization of next-generation superconducting qubits that are robust against quasiparticle poisoning and could enable a new class of quantum sensors for dark matter detection.