Quantum Sensing Exploits Critical Phenomena for Precision Beyond Classical Limits

Quantum sensing promises to revolutionise measurement precision, potentially exceeding the limits of classical technologies, but practical implementation faces significant hurdles. George Mihailescu from University College Dublin, Uesli Alushi and Roberto Di Candia from Aalto University, alongside Simone Felicetti and Karol Gietka et al., address this challenge by exploring a powerful new approach, critical quantum sensing. Their work introduces a comprehensive overview of how to harness the unique properties of systems undergoing phase transitions, where enhanced sensitivity and non-classical correlations naturally occur, to dramatically improve measurement accuracy. This tutorial guides readers through the fundamental concepts and increasingly complex strategies for critical sensing, focusing on optimising precision and bridging the gap between theoretical models and the realities of noisy, real-world systems, ultimately paving the way for future technological advancements in this exciting field.

Quantum phenomena offer the possibility of measuring physical quantities with precision beyond classical limits. However, current progress is constrained by scalability, environmental noise, and challenges in practical integration, highlighting the necessity for novel approaches. Researchers investigate critical quantum metrology, harnessing the enhanced susceptibility and nonclassical correlations naturally present in systems undergoing a quantum phase transition, to identify and quantify the resources enabling enhanced precision, specifically exploring long-range correlations and dynamical scaling near the critical point. This investigation demonstrates that critical systems exhibit a unique sensitivity to external perturbations, allowing for the development of sensors with significantly improved performance compared to conventional methods.

Precision Limits of Continuous Quantum Measurements

This body of work encompasses research in quantum physics, condensed matter physics, thermodynamics, and quantum metrology, focused on improving measurement precision using quantum effects. Researchers employ concepts such as the Fisher information, which quantifies measurement information, and the Cramér-Rao bound, which sets a fundamental limit on precision, exploring continuous measurements and Bayesian parameter estimation. The quantum Cramér-Rao sensitivity limit, representing the ultimate achievable precision, is a key focus. Another central theme is quantum thermodynamics, exploring the interplay between quantum mechanics and thermodynamics, including quantum work statistics and the design of quantum heat engines and refrigerators.

Researchers also investigate quantum fluctuations and non-equilibrium thermodynamics, examining systems not in thermal equilibrium, extending to the properties of materials at the quantum level, including the Kondo effect, the fractional quantum Hall effect, quantum criticality, Majorana fermions, and artificial quantum dots. The research also addresses open quantum systems, those interacting with their environment, and explores current fluctuations and full counting statistics, suggesting that electrical currents can be used to probe quantum systems and develop new types of quantum sensors. Key researchers contributing to this field include A. K.

Mitchell, G. T. Landi, M. T. Mitchison, and E. Sela. This collection of papers represents cutting-edge research focused on developing new technologies for precision measurement and quantum sensing, combining theoretical and experimental work to understand fundamental principles and solve real-world problems.

Quantum Parameter Estimation, Precision Limits and Trade-offs

This work details significant achievements in quantum parameter estimation, establishing fundamental limits and strategies for maximizing precision. Researchers rigorously analyze the quantum Fisher information matrix and the Cramér-Rao bound, demonstrating that the matrix must be invertible to define meaningful precision limits, as a singular matrix implies diverging variances and a vanishing signal-to-noise ratio. The study reveals a trade-off between precision in single and multi-parameter estimation. Single-parameter estimation infers one target parameter assuming others are known, while multi-parameter estimation simultaneously estimates several, resulting in generally lower attainable precision for any individual parameter.

This loss of precision is governed by correlations between unknown parameters, necessitating strategies that explicitly account for these correlations to balance precision across all parameters. Researchers explore these concepts through Ramsey interferometry, reinforcing abstract ideas and laying the groundwork for advanced schemes, revealing the defining features of critical systems as powerful platforms for sensing, including their geometric structure and diverging susceptibilities. The work establishes that the choice of measurement and available quantum resources profoundly impact the ensuing sensitivity to parameters being inferred.

Critical Metrology Enables Macroscopic Quantum Sensing

This work demonstrates the potential of critical metrology, a technique that leverages the enhanced sensitivity of quantum systems at phase transitions to improve measurement precision. Researchers explore how exploiting the properties of systems near critical points can offer advantages in scaling and robustness against noise, addressing limitations found in many existing quantum sensing platforms, detailing various critical sensing protocols applied to different systems, with a focus on how estimation precision scales with fundamental resources. The study highlights that critical metrology offers a promising pathway towards macroscopic quantum sensing, potentially overcoming challenges associated with decoherence and maintaining large-scale quantum correlations. While acknowledging that current technologies, such as atomic clocks, achieve high precision through classical averaging, this research suggests that harnessing nonclassical properties near critical points could unlock further improvements. The authors note that finite-size effects and the complexities of realistic, open quantum systems present ongoing challenges for the field, with future research directions including further exploration of ultrastrongly coupled light-matter systems and the development of protocols specifically tailored to dissipative regimes, ultimately aiming to translate these theoretical advances into practical sensing technologies.