Phonon downconversion to suppress correlated errors in superconducting qubits

TL;DR

This paper demonstrates that using normal metal reservoirs on superconducting qubit chips significantly reduces phonon-induced quasiparticle poisoning and correlated errors, enhancing quantum error correction effectiveness.

Contribution

The study introduces a novel phonon downconversion method with back-side metallization and a pump-probe scheme to suppress correlated errors in superconducting qubits.

Findings

Over 20-fold reduction in pair-breaking phonons.

Two orders of magnitude decrease in correlated quasiparticle poisoning.

Effective suppression of background radiation effects on qubits.

Abstract

Quantum error correction can preserve quantum information in the presence of local errors, but correlated errors are fatal. For superconducting qubits, high-energy particle impacts from background radioactivity produce energetic phonons that travel throughout the substrate and create excitations above the superconducting ground state, known as quasiparticles, which can poison all qubits on the chip. We use normal metal reservoirs on the chip back side to downconvert phonons to low energies where they can no longer poison qubits. We introduce a pump-probe scheme involving controlled injection of pair-breaking phonons into the qubit chips. We examine quasiparticle poisoning on chips with and without back-side metallization and demonstrate a reduction in the flux of pair-breaking phonons by over a factor of 20. We use a Ramsey interferometer scheme to simultaneously monitor quasiparticle parity on three qubits for each chip and observe a two-order of magnitude reduction in correlated poisoning due to background radiation.