Quantum Metrology Enhances Force Sensing with Squeezed Light States.

Researchers demonstrate enhanced force sensitivity using non-Gaussian quantum states in continuous variable systems. Number-squeezed Schrödinger cat states outperform conventional Fock states under realistic loss and control limitations, achieving optimal performance in spin-boson models and offering improvements for both massive and massless systems.

The precise measurement of force is fundamental to many areas of physics, from detecting gravitational waves to probing the quantum realm. Researchers continually seek methods to surpass the limitations imposed by quantum noise and environmental disturbances. A new theoretical study details an approach to optimise force sensing using specifically engineered quantum states of light or mechanical oscillators. By exploiting non-Gaussian states – those with probability distributions that deviate from the familiar bell curve – and accounting for the inevitable effects of signal loss and imperfect control, the team demonstrate a pathway to enhanced sensitivity. This work, detailed in a paper by Piotr T. Grochowski and Radim Filip, both of Palacký University, is titled ‘Optimal phase-insensitive force sensing with non-Gaussian states’.

Precision force sensing benefits from utilising quantum states beyond those describable by classical Gaussian statistics. This research demonstrates that non-Gaussian states – specifically Fock states and number-squeezed Schrödinger cat states – improve the precision of force detection in continuous variable systems, even when subjected to realistic noise, such as photon loss. The enhancement arises from employing excitation-number-resolving measurements, a technique that directly determines the quantum state’s photon number, and offers a pathway to improved precision measurement technologies.

Researchers initially investigated the performance of Fock states – quantum states characterised by a definite number of photons – in noisy environments via numerical simulation. These simulations revealed superior performance compared to standard Gaussian states, approaching the theoretical limit of sensitivity achievable in such systems. Gaussian states are those where probability distributions are normal, and are commonly used as a baseline for quantum experiments due to their relative ease of creation and characterisation.

However, the research identifies a shift in optimal state selection when considering practical experimental limitations. Number-squeezed Schrödinger cat states become advantageous when accounting for photon loss and constraints on system control. Schrödinger cat states are superpositions of two distinct quantum states, exhibiting both quantum and classical characteristics. ‘Number squeezing’ refers to a specific type of state preparation where the uncertainty in photon number is reduced.

Researchers employed optimal control techniques – utilising a tailored reward function based on the Fisher information – to identify cat states that maximise force sensitivity within a minimal spin-boson model. The Fisher information is a measure of how much information a random variable carries about an unknown parameter, in this case, the applied force. This suggests a complex relationship between state selection and the specific characteristics of the experimental platform. The findings expand the possibilities for optimising force sensing beyond simple Fock states, opening new avenues for research and development.

The study details the methodology, utilising numerical simulations performed with the QuTiP Python library and the Quantum Optimal Control Suite. Control pulses were optimised to maximise the Fisher information, and convergence was verified by employing a minimal Fock space size, ensuring the reliability and validity of the findings. A Fock space is a mathematical construct used to describe the possible states of a quantum system, based on the number of particles present.