# Direct Dispersive Monitoring of Charge Parity in Offset-Charge-Sensitive Transmons

## TL;DR

This paper demonstrates a method for direct, high-fidelity dispersive readout of charge parity in offset-charge-sensitive transmons, enabling improved detection of charge fluctuations and quasiparticle dynamics with enhanced device coherence.

## Contribution

The authors introduce a novel dispersive measurement technique for charge parity in OCS transmons, significantly extending charge-parity lifetime and improving qubit coherence.

## Key findings

- Charge-parity lifetime extended by two orders of magnitude.
- Achieved high-fidelity dispersive readout of charge and plasmon states.
- Enhanced energy relaxation time T1 to approximately 200 microseconds.

## Abstract

A striking characteristic of superconducting circuits is that their eigenspectra and intermode coupling strengths are well predicted by simple Hamiltonians representing combinations of quantum circuit elements. Of particular interest is the Cooper-pair-box Hamiltonian used to describe the eigenspectra of transmon qubits, which can depend strongly on the offset-charge difference across the Josephson element. Notably, this offset-charge dependence can also be observed in the dispersive coupling between an ancillary readout mode and a transmon fabricated in the offset-charge-sensitive (OCS) regime. We utilize this effect to achieve direct, high-fidelity dispersive readout of the joint plasmon and charge-parity state of an OCS transmon, which enables efficient detection of charge fluctuations and nonequilibrium-quasiparticle dynamics. Specifically, we show that additional high-frequency filtering can extend the charge-parity lifetime of our device by two orders of magnitude, resulting in a significantly improved energy relaxation time $T_1\sim200~\mu\mathrm{s}$.