Experimental realization of a $\cos(2\varphi)$ transmon qubit

TL;DR

This paper demonstrates a $ ext{cos}(2 ext{phi})$ transmon qubit with enhanced charge parity protection, achieving coherent control and readout at low frequencies, and identifies flux noise as the main decoherence source.

Contribution

It experimentally realizes a $ ext{cos}(2 ext{phi})$ qubit with significantly reduced charge-induced errors, advancing superconducting qubit robustness.

Findings

Doublet of states split by 13.6 MHz observed

Charge matrix element suppressed by 100-fold

Coherence times limited by flux noise

Abstract

Superconducting circuits with embedded symmetries are good candidates to robustly protect quantum information from dominant error channels. The $\cos(2\varphi)$ qubit, consisting of an island shunted to ground through a tunneling element that selectively transmits pairs of Cooper pairs, leverages charge-parity symmetry to protect from charge-induced errors. In this experiment, we observe a doublet of states of opposite Cooper-pair parity split by $13.6~\mathrm{MHz}$. Operating in a soft-transmon regime, this splitting is two orders of magnitude smaller than in previous implementations, pushing charge-induced losses well beyond the measured coherence times. Despite the low transition frequency, we demonstrate coherent qubit control, single-shot readout, and resolve quantum jumps. Charge protection of the qubit is evidenced by a $100-$fold suppression of the island charge matrix element compared to the unprotected plasmon transition, placing dielectric loss limits above $10~\mathrm{ms}$. The measured $T_1 = 70~\mu\mathrm{s}$ and $T_2^\mathrm{echo}= 2.5~\mu\mathrm{s}$ are instead limited by $1/f$ flux noise in the tunnelling element's loop. This experiment shows that pushing Cooper-pair pairing in the transmon regime sets high limits on charge-induced losses while preserving coherent control and single-shot readout of the low-frequency qubit. We identify flux noise as the dominant remaining limitation, calling for gradiometric designs or novel $4e$-tunneling elements.