Robustness of contrast enhancement under dissipation and transmon-level leakage

Determine whether the contrast enhancement produced by collective coupling in the laboratory-frame anti-Jaynes–Cummings model with a squeezed vacuum survives when weak cavity loss and qubit dephasing are included, and when the transmon Hilbert space is extended to include the |2⟩ level.

Background

The paper studies a closed-system laboratory-frame anti-Jaynes–Cummings model in which a squeezed vacuum is coupled either to one emitter or to a collective Dicke ensemble. Collective coupling substantially increases the first-turning-point contrast at large residual detuning; for example, eight emitters produce a contrast of 0.575 at f=5, compared with 0.140 for one emitter.

The authors note that realistic circuit-QED implementations introduce effects omitted from the calculation. Weak cavity loss and qubit dephasing could reduce the reported contrast, while a Kerr strength comparable to the coupling may invalidate the spin-1/2 approximation for a transmon by admixing the |2⟩ state. The paper leaves unresolved whether the reported contrast gain remains under these dissipative and multilevel corrections.