Qubit parity symmetry protects against dielectric loss

Protecting Superconducting Qubits with Parity Symmetry

Researchers at the Laboratoire de Physique de l’École Normale Supérieure, in collaboration with Google Quantum AI, have demonstrated a superconducting qubit design that dramatically reduces dielectric loss. By encoding information in states of opposite Cooper‑pair parity, the device’s charge matrix element is more than 100 × smaller than that of an unprotected plasmon transition, pushing the dielectric‑loss‑limited lifetime above 10 ms.

A Soft‑Transmon with Cos (2φ) Dynamics

The qubit operates at 13.6 MHz in a “soft‑transmon” regime, where the Hamiltonian contains a cos (2φ) term that enforces parity selection rules. An interferometer of Josephson junctions and superinductances shunts the island to ground, favoring paired Cooper‑pair tunneling while suppressing single‑Cooper‑pair processes that couple to dielectric loss. A modest 3 % asymmetry in the KITE element provides the residual coupling needed for control and single‑shot readout.

Measured Performance and Remaining Limits

Experimental data show a relaxation time \(T1\) of 70 µs, matching predictions based on 1/f flux noise in the interferometer loop. Dephasing is currently limited by the same flux noise, with an echo coherence time \(T{2E}\) of 2.5 µs. The dielectric‑loss contribution is effectively eliminated, confirming that parity symmetry shields the qubit from this dominant error channel. Single‑shot readout fidelity reaches 83 % with a 5 µs integration window, and the qubit initializes at an effective temperature of 400 µK.

Pathways to Millisecond‑Scale Coherence

The authors identify two clear routes to further improvement:

1. Flux‑Noise Mitigation – Reducing the interferometer loop area, employing gradiometric layouts, or locking the fluxoid can lower the flux‑noise spectral density.
2. Enhanced Superinductance – Increasing the inductance of the loop should suppress flux sensitivity without compromising the parity protection.

Future work may also explore novel materials, such as twisted‑layer superconductors, that could enable quartet tunneling without a flux loop, potentially combining the best features of heavy‑fluxonium and Cooper‑pair‑box architectures.

Significance for Scalable Quantum Computing

By demonstrating that dielectric loss can be systematically removed through symmetry engineering, this work adds a powerful tool to the superconducting‑qubit toolbox. While flux noise remains the bottleneck, the clear experimental roadmap suggests that millisecond‑scale coherence—an essential ingredient for fault‑tolerant quantum processors—could soon be within reach.

The study is available as an arXiv pre‑print (see source link).