Quantum-enhanced metrology relies on entanglement to achieve sensitivities beyond the standard quantum limit. While remarkable progress has been made in generating highly entangled many-body states, extracting their metrological advantage remains a central challenge because the encoded information is often inaccessible to realistic measurements. A key development of the past decade has been the realization that many-body interactions can play a dual role: they can be used not only to generate entanglement, but also to decode it. This idea underlies interaction-based readout and time-reversal protocols, in which controlled non-linear dynamics transform weakly encoded signals into experimentally accessible observables. Cavity quantum electrodynamics (QED) provides a particularly powerful setting for these approaches because it combines collective enhancement, tunable interactions, and controllable reversibility within a single platform. In this review, we discuss the emergence of time-reversal protocols in cavity QED, from their conceptual roots in Loschmidt echoes to modern implementations of signal amplification through a time-reversed interaction (SATIN), scrambling-enhanced metrology, and more general interaction-based readout schemes. We examine the physical mechanisms that enable reversible many-body dynamics, review key experimental demonstrations, and discuss future directions involving complex entangled states, nonlinear decoding, and emerging quantum platforms. Together, these developments suggest that the ability to decode quantum information may become as important as the ability to generate it, establishing reversible many-body dynamics as a central resource for quantum-enhanced sensing.Quantum-enhanced metrology relies on entanglement to achieve sensitivities beyond the standard quantum limit. While remarkable progress has been made in generating highly entangled many-body states, extracting their metrological advantage remains a central challenge because the encoded information is often inaccessible to realistic measurements. A key development of the past decade has been the realization that many-body interactions can play a dual role: they can be used not only to generate entanglement, but also to decode it. This idea underlies interaction-based readout and time-reversal protocols, in which controlled non-linear dynamics transform weakly encoded signals into experimentally accessible observables. Cavity quantum electrodynamics (QED) provides a particularly powerful setting for these approaches because it combines collective enhancement, tunable interactions, and controllable reversibility within a single platform. In this review, we discuss the emergence of time-reversal protocols in cavity QED, from their conceptual roots in Loschmidt echoes to modern implementations of signal amplification through a time-reversed interaction (SATIN), scrambling-enhanced metrology, and more general interaction-based readout schemes. We examine the physical mechanisms that enable reversible many-body dynamics, review key experimental demonstrations, and discuss future directions involving complex entangled states, nonlinear decoding, and emerging quantum platforms. Together, these developments suggest that the ability to decode quantum information may become as important as the ability to generate it, establishing reversible many-body dynamics as a central resource for quantum-enhanced sensing.

quant-pharXiv:2607.02320Jul 3, 20260 citations

Quantum sensing and metrology with entangled states

Vladimir S. Malinovsky

Optical and Quantum Sensing and Precision Metrology IIMar 9, 20220 citations

Matters Arising: Time-reversal-based quantum metrology with many-body entangled states

Liam Paul McGuinness

In their paper "Time-reversal-based quantum metrology with many-body entangled states" Nature Physics (2022), Colombo et. al. claim to measure both an unknown phase and an oscillating magnetic field with a precision that cannot be achieved using independent particles - a limit known as the standard quantum limit. By entangling an ensemble of $\sim300$ atoms, Colombo et. al. measure an angle of rotation away from a known initial state and additionally measure a magnetic field oscillating at 290 Hz. The authors report an experimental precision approximately a factor of 4 beyond what is possible with the same number of independent atoms (12.8 dB and 11.8 dB for these tasks respectively). These claims are incorrect. Colombo et. al. do not surpass the precision bound for 300 independent particles, nor do they even surpass the precision bound for a single particle. Colombo et. al. cite several experiments that surpass the standard quantum limit using entanglement. Each and every paper cited performs incorrect, incomplete or misleading comparisons of the type that we highlight here. The consequence being that the single particle precision bound has never been experimentally surpassed with entanglement.In their paper "Time-reversal-based quantum metrology with many-body entangled states" Nature Physics (2022), Colombo et. al. claim to measure both an unknown phase and an oscillating magnetic field with a precision that cannot be achieved using independent particles - a limit known as the standard quantum limit. By entangling an ensemble of $\sim300$ atoms, Colombo et. al. measure an angle of rotation away from a known initial state and additionally measure a magnetic field oscillating at 290 Hz. The authors report an experimental precision approximately a factor of 4 beyond what is possible with the same number of independent atoms (12.8 dB and 11.8 dB for these tasks respectively). These claims are incorrect. Colombo et. al. do not surpass the precision bound for 300 independent particles, nor do they even surpass the precision bound for a single particle. Colombo et. al. cite several experiments that surpass the standard quantum limit using entanglement. Each and every paper cited performs incorrect, incomplete or misleading comparisons of the type that we highlight here. The consequence being that the single particle precision bound has never been experimentally surpassed with entanglement.

arXiv:2208.14816Aug 31, 20220 citations

An application of two-photon entangled states to quantum metrology

Marco Genovese

Journal of Modern OpticsOct 15, 20001 citation

Entangled Quantum Metrology

ScienceMay 3, 20071 citation

Time-reversal protocols for quantum-enhanced metrology

Simone Colombo

I discuss the time reversal of nonlinear many-body Hamiltonians and its application to the fields of quantum metrology and quantum information science. I present the experimental realization of such a protocol in an optically engineered many-body spin Hamiltonian. We used time-reversed Hamiltonian approach to achieve metrological gains well beyond the limit of the readout scheme. Moreover, with the same protocol, we experimentally elucidate the intimate relation between quantum information scrambling, out-of-time-order correlators, and metrological gain. In particular, I will highlight the robustness of time reversal protocols, and how these render many fast-scrambling and complex Hamiltonians useful for quantum-enhanced metrology.I discuss the time reversal of nonlinear many-body Hamiltonians and its application to the fields of quantum metrology and quantum information science. I present the experimental realization of such a protocol in an optically engineered many-body spin Hamiltonian. We used time-reversed Hamiltonian approach to achieve metrological gains well beyond the limit of the readout scheme. Moreover, with the same protocol, we experimentally elucidate the intimate relation between quantum information scrambling, out-of-time-order correlators, and metrological gain. In particular, I will highlight the robustness of time reversal protocols, and how these render many fast-scrambling and complex Hamiltonians useful for quantum-enhanced metrology.

Optical and Quantum Sensing and Precision Metrology IIMar 8, 20230 citations

\sim300

atoms, Colombo et. al. measure an angle of rotation away from a known initial state and additionally measure a magnetic field oscillating at 290 Hz. The authors report an experimental precision approximately a factor of 4 beyond what is possible with the same number of independent atoms (12.8 dB and 11.8 dB for these tasks respectively). These claims are incorrect. Colombo et. al. do not surpass the precision bound for 300 independent particles, nor do they even surpass the precision bound for a single particle. Colombo et. al. cite several experiments that surpass the standard quantum limit using entanglement. Each and every paper cited performs incorrect, incomplete or misleading comparisons of the type that we highlight here. The consequence being that the single particle precision bound has never been experimentally surpassed with entanglement.