Measurement-induced generation of Schrödinger cat states in cavity QED

Generating Schrödinger cat states—coherent superpositions of macroscopically distinguishable states—is a fundamental challenge in continuous-variable quantum information processing. Traditional methods often require strong Kerr nonlinearities, photon subtraction, or complex reservoir engineering, which are experimentally demanding. This paper explores whether a simpler, measurement-based protocol can generate these nonclassical states in a cavity-QED system using only dispersive atom-cavity interactions and conditional atomic measurements.

The researchers propose a periodic protocol consisting of three steps: coherent driving of a cavity mode, dispersive interaction with a sequence of two-level atoms, and postselection of the atoms. The dispersive interaction entangles the cavity field with the atomic state, encoding which-way information about the cavity's coherent-state branches. By performing a projective measurement on the atom in a superposition basis, the protocol erases this which-way information, projecting the cavity field into a non-Gaussian superposition state. The authors use the Lindblad master equation to numerically simulate the system's evolution, accounting for cavity photon loss to evaluate the protocol's robustness.

The study demonstrates that conditional atomic measurements can successfully generate both two-component and multi-component Schrödinger cat states. Analytical results for the ideal lossless case confirm the emergence of Wigner negativity, a hallmark of nonclassicality. Numerical simulations show that these states remain robust against moderate cavity dissipation. However, as dissipation increases, the interference patterns in phase space become asymmetric, and the quantum coherence is progressively suppressed. The protocol is scalable, allowing for the generation of 2^N-component cat states through successive interaction cycles.