EPJ Plus Highlight - An interferometric approach to multi-parameter measurement

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Published on 21 October 2025

Novel interferometer setup enables multiple parameters of an optical network to be measured simultaneously, with a precision limited only by the laws of quantum mechanics

Quantum mechanics has vastly improved our ability to make precise measurements. By harnessing effects such as entanglement, squeezing and interference, researchers have surpassed the noise limits imposed on classical techniques – allowing for higher-resolution measurements of quantities including energy, time, and polarization. Over the past decade, it has become especially important for researchers to measure multiple physical parameters of a quantum system simultaneously. However, previous approaches have faced numerous challenges – including the constraints they impose on the values of unknown parameters.

Through new research published in EPJ Plus, an international team from the University of Bari, Italy, and the University of Portsmouth, UK, presents an interferometry-based quantum sensing scheme capable of simultaneously estimating multiple parameters of an optical network, with a sensitivity limited only by the fundamental laws of quantum mechanics. Their approach could help to improve the precision and scope of quantum measurements across applications ranging from biological imaging to gravitational wave detection.

In squeezed light, the uncertainty in one property of the light, such as its phase, can be reduced, while the uncertainty in the complementary property increases correspondingly. This is due to a fundamental quantum constraint: the Heisenberg’s uncertainty principle. Even vacuum states, which do not contain photons but still carry quantum fluctuations in the photon numbers, can be squeezed to reduce uncertainty along a given phase direction while increasing it along the orthogonal direction.

In the proposed setup, a squeezed vacuum state and a squeezed coherent state are injected into an interferometer-based optical network. The output beams carried complementary information about three unknown parameters in the network: the phase shifts imparted on the two optical modes, and the reflectivity of a beam splitter combining the two input states. By jointly measuring the output, they not only estimated these parameters simultaneously, but also did so at the highest level of quantum metrological precision scaling in the photon numbers, with its sensitivity holding regardless of the actual values of the parameters being estimated.