Description
Quantum sensing with undetected photons is a technique in which a sample is probed by photons of one wavelength, while information about the sample is extracted by measuring photons of a different wavelength that never interacts with it. This approach has enabled significant experimental advances in spectroscopy, microscopy, and bio-sensing. Despite this progress, a detailed theoretical treatment using the tools of quantum metrology has been lacking, leaving open two key questions: how close current schemes come to fundamental sensitivity limits, and what measurement strategies are optimal.
We address this by applying a multiparameter quantum estimation framework to the joint estimation of a sample's unknown transmission and phase shift — the two quantities of primary interest in high-sensitivity transmissive and dispersive sensing. Because these parameters must be estimated simultaneously, single-parameter quantum Fisher information bounds are generally unattainable, and a rigorous multiparameter treatment is essential to correctly characterise achievable precision. We derive the fundamental limits to this joint estimation problem and show that the optimal measurement scheme requires only a single controllable phase shift, making it straightforward to implement in existing experimental setups.
We further investigate multi-pass interactions, a common strategy for amplifying weak signals by passing the probe through the sample multiple times. We show that the optimal number of passes scales inversely with the logarithm of the sample's transmission, providing a simple design rule for maximising information gain while avoiding the signal degradation caused by absorption in low-transmission samples.
Together, these results clarify the metrological power of quantum sensing with undetected photons, establish how close existing schemes are to fundamental precision limits, and provide concrete, experimentally accessible guidance for designing future high-sensitivity sensing schemes based on this technique.
Our results can be found in - https://arxiv.org/abs/2604.19120
| I am the presenting author | Yes |
|---|