Local-oscillator-agnostic squeezing detection

We address the problem of measuring nonclassicality in continuous-variable bosonic systems without having access to a known reference signal. To this end, we construct broader classes of criteria for nonclassicality which allow us to investigate quantum phenomena regardless of the quantumness of selected subsystems. Such witnesses are based on the notion of partial normal ordering. This approach is applied to balanced homodyne detection using arbitrary, potentially nonclassical local oscillator states, yet only revealing the probed signal’s quantumness. Our framework is compared to standard techniques, and the robustness and advanced sensitivity of our approach is shown. Therefore, a widely applicable framework, well-suited for applications in quantum metrology and quantum information, is derived to assess the quantum features of a photonic system when a well-defined coherent laser as a reference state is not available in the physical domain under study.

💡 Research Summary

The paper tackles a fundamental limitation of balanced homodyne detection: the need for a well‑characterized coherent local oscillator (LO) as a phase reference. In many realistic scenarios—such as integrated photonic platforms, space‑based experiments, or situations where a laser source is unavailable—the LO may be unknown, non‑Gaussian, or even quantum‑enhanced. Conventional squeezing criteria that rely on full normal ordering of the measured photocurrent difference can mistakenly attribute non‑classicality to the signal when the LO itself is non‑classical, leading to false‑positive results.

To overcome this, the authors develop a resource‑theoretic framework based on partial normal ordering. They consider a bipartite system composed of a signal mode A and an auxiliary mode B (the LO). Classical states are defined as statistical mixtures of product states where the signal is a coherent state |α⟩ and the auxiliary mode is arbitrary. Importantly, the definition of non‑classicality is independent of the auxiliary subsystem: a state is non‑classical if it cannot be written as a mixture of such product states, regardless of the quantum properties of B.

The key technical tool is the operator \