Entanglement Generated by the Dispersive Interaction: The Dressed Coherent State

TL;DR

This paper demonstrates that in the dispersive regime of qubit-cavity systems, entanglement persists in the lab frame through dressed coherent states, impacting measurement protocols and system dynamics.

Contribution

It introduces the dressed coherent state as the fundamental description in the lab frame, revealing entanglement generation in the dispersive regime and analyzing its effects on qubit evolution and measurement.

Findings

Entanglement exists in the lab frame via dressed coherent states.

Qubit evolution depends on amplitude and phase of the dressed state.

Measurement contrast is affected by the dressed coherent state properties.

Abstract

In the dispersive regime of qubit-cavity coupling, classical cavity drive populates the cavity, but leaves the qubit state unaffected. However, the dispersive Hamiltonian is derived after both a frame transformation and an approximation. Therefore, to connect to external experimental devices, the inverse frame transformation from the dispersive frame back to the lab frame is necessary. In this work, we show that in the lab frame the system is best described by an entangled state known as the dressed coherent state, and thus even in the dispersive regime, entanglement is generated between the qubit and the cavity. Also, we show that further qubit evolution depends on both the amplitude and phase of the dressed coherent state, and use the dressed coherent state to calculate the measurement contrast of a recently developed dispersive readout protocol.