Tunable Non-Gaussianity and Exact Higher-Order Coherences for Quantum Advantage

TL;DR

This paper introduces the displaced Janus state, a new non-Gaussian quantum state, providing an analytical framework for higher-order statistics, and demonstrates its potential for quantum advantage in metrology and computation.

Contribution

It develops a complete analytical toolkit for the displaced Janus state, enabling precise engineering of non-Gaussian features and quantum Fisher information analysis.

Findings

Exact higher-order statistics derived for displaced Janus states

Interference effects enable sub-Poissonian and multiphoton suppression

Quadratic parameter encoding achieves Heisenberg-limited scaling

Abstract

Non-Gaussian states are essential for achieving a quantum advantage in continuous-variable (CV) information processing. Among these, coherent superpositions of squeezed states are a foundational resource. While exact higher-order statistics are available in the undisplaced case, a complete and analytically tractable treatment with a common displacement has been missing. We introduce and solve the displaced Janus state-a coherent superposition of two squeezed coherent states that share the same displacement-and develop an analytical framework, based on a family of Generalized Squeezing Polynomials, that yields closed-form expressions for arbitrary-order factorial moments and coherence functions \(g^{(k)}(0)\), the full Wigner function, and the quantum Fisher information. The analysis shows how interference at a fixed mean, driven by a mismatch of the component covariances rather than by mean separation, can be precisely engineered to transform the extreme photon bunching of the constituents into strong sub-Poissonian and even perfect multiphoton suppression. We further provide a rigorous quantum Fisher information analysis, proving that parameters encoded by linear generators (for example, the number operator) are bounded by the standard quantum limit, whereas parameters encoded by quadratic generators (for example, squeezing transformations) achieve Heisenberg-limited scaling. Together, these results furnish a complete analytical toolkit for a versatile class of non-Gaussian states, establishing the displaced Janus state as a key primitive for hybrid quantum protocols, quantum metrology, and fault-tolerant continuous-variable computation.