We present our experimental and theoretical work developing measurement-feedback implementations of precision sensors using cold atomic ensembles. We will focus on our work on spin-squeezed magnetometers.
Quantum metrology uses quantum features such as entanglement and squeezing to improve the sensitivity of quantum-limited measurements. Long established as a valuable technique in optical measurements such as gravitational-wave detection, quantum metrology is increasingly being applied to atomic instruments such as matter-wave interferometers, atomic clocks, and atomic magnetometers. Several of these new applications involve dual optical/atomic quantum systems, presenting both new challenges and new opportunities. Here we describe an optical magnetometry system that achieves both shot-noise-limited and projection-noise-limited performance, allowing study of optical magnetometry in a fully-quantum regime [1]. By near-resonant Faraday rotation probing, we demonstrate measurement-based spin squeezing in a magnetically-sensitive atomic ensemble [2-4]. The versatility of this system allows us also to design metrologically-relevant optical nonlinearities, and to perform quantum-noise-limited measurements with interacting photons. As a first interaction-based measurement [5], we implement a non-linear metrology scheme proposed by Boixo et al. with the surprising feature of precision scaling better than the 1/N “Heisenberg limit” [6].
In quantum metrology, entangled states of many-particle systems are investigated to enhance measurement precision of the most precise clocks and field sensors. Whereas single-parameter quantum metrology is well established, joint multiparameter estimation poses conceptual challenges and has been explored only theoretically. We experimentally demonstrated multiparameter quantum metrology with an array of entangled atomic ensembles. By splitting a spin-squeezed ensemble, we created an atomic sensor array featuring intersensor entanglement that can be flexibly configured to enhance measurement precision of multiple parameters jointly. Using an optimal estimation protocol, we achieved substantial gains over the standard quantum limit in key multiparameter estimation tasks, thus grounding the concept of quantum enhancement of field sensor arrays and imaging devices.
An atomic ensemble with many-body entanglement is desirable for precision measurement. As a type of such quantum state, the spin squeezed state has been pursued in both cold and warm atoms for applications of a quantum-enhanced atomic clock, interferometer, and magnetometer. Here, we report the numerical simulation of many-body dynamics in a Rydberg atomic array with dipole–dipole interaction, and evaluate the generation of spin squeezing. The method builds on the dissipative discrete truncated Wigner approximation, which combines the mean-field dynamics of a spin ensemble with Monte Carlo sampling. By taking into account experimental imperfections such as spin decoherence, we apply this approach to the dynamics in both strontium and rubidium Rydberg atomic arrays with the current available scale. This offers the possibility to accurately simulate the many-body dynamics of interacting quantum systems in achievable platforms for application of quantum simulation and quantum metrology.