Controllable Non-Hermitianity in Continuous-Variable Qubits

TL;DR

This paper reveals that phase errors in photonic cat qubits cause asymmetric leakage, enabling the use of non-Hermitian Hamiltonians to control gain, loss, and phase transitions, thus turning dephasing into a resource.

Contribution

It introduces a controllable non-Hermitian physics platform using photonic cat qubits, demonstrating control over parity-time symmetry and entanglement phase transitions.

Findings

Asymmetric leakage from cat states can be harnessed for non-Hermitian physics.

Control of PT symmetry phase transition via amplitude adjustment.

Observation of entanglement phase transition at exceptional points.

Abstract

Pure dephasing is the dominant leak mechanism in photonic cat qubits because its phase errors disrupt the parity protection, rendering the qubit vulnerable to energy relaxation. In this manuscript, we reveal that this dephasing mechanism conceals an interesting physical phenomenon: it induces \textit{asymmetric leakage} from the cat-state subspace, where even- and odd-parity cat states decay at different rates. This leak asymmetry enables the dynamics of the system to be described by a non-Hermitian Hamiltonian, thereby transforming the cat qubit into a platform with controllable gain and loss for probing non-Hermitian physics. Within this platform, we demonstrate the possibility to control the parity-time symmetry phase transition in a single cat qubit by adjusting its amplitude. Moreover, we couple two cat qubits to realize an entanglement phase transition induced by the exceptional point. Our work constructs a controllable non-Hermitian system simulator, overturning the conventional paradigm that treats dephasing as harmful noise.