Quantum Metrology in the Ultrastrong Coupling Regime of Light-Matter Interactions: Leveraging Virtual Excitations without Extracting Them

TL;DR

This paper demonstrates how virtual excitations in ultrastrong light-matter systems can be harnessed to significantly enhance quantum measurement precision without extracting these excitations, surpassing traditional limits.

Contribution

It introduces a novel method to utilize virtual excitations in the Dicke model for quantum metrology, achieving quadratic enhancement in estimation precision.

Findings

Virtual squeezing leads to exponential scaling of measurement precision.

Enhancement scales as exp(4ξ) compared to exp(2ξ) for real squeezed states.

Framework applicable to various ultrastrongly coupled quantum systems.

Abstract

Virtual excitations, inherent to ultrastrongly coupled light-matter systems, induce measurable modifications in system properties, offering a novel resource for quantum technologies. In this work, we demonstrate how these virtual excitations and their correlations can be harnessed to enhance precision measurements, without the need to extract them. Building on the paradigmatic Dicke model, which describes the interaction between an ensemble of two-level atoms and a single radiation mode, we propose a method to harness hybridized light-matter modes whose renormalized frequencies encode the effects of virtual excitations for quantum metrology. Remarkably, we find that for a fixed squeezing parameter $\xi$, exploiting virtual squeezing through oscillator frequency shifts yields a quadratic enhancement in estimation precision -- scaling as $\exp(4\xi)$ -- compared to the conventional $\exp(2\xi)$ scaling of real squeezed states. These results show that virtual excitations, though unobservable, can drive metrological performance beyond the standard quantum limit. Our approach establishes a broadly applicable framework for high-precision measurements across a wide class of ultrastrongly coupled quantum systems.