Displaced Janus States: Tunable Non-Gaussianity and Exact Higher-Order Coherences for Quantum Advantage

Abstract: Non-Gaussian states are essential for achieving a quantum advantage in continuous-variable (CV) information processing. Among these, superpositions of squeezed states offer a rich phenomenology, yet a complete analytical understanding of their higher-order quantum statistics-such as the transition from extreme bunching ($g^{(k)} \to \infty$) to suppression ($g^{(k>2)} \to 0$)-has remained elusive. In this work, we introduce and provide an exact solution for the displaced Janus state-a coherent superposition of two squeezed coherent states. We develop a powerful analytical framework built upon a new family of Generalized Squeezing Polynomials that yields closed-form expressions for its arbitrary-order coherence functions, Wigner function, and quantum Fisher information. This enables a full characterization of the state's tunable non-Gaussianity, revealing how quantum interference can transform the extreme photon bunching of its components into strong antibunching or perfect multiphoton suppression. We identify parameter regimes that generate Wigner negativity and can be harnessed for achieving Heisenberg-limited metrological precision. Our work provides a foundational toolkit for engineering non-Gaussian states, establishing the displaced Janus state as a key resource for hybrid quantum protocols and fault-tolerant CV computation.