Quantum metrology: Problems, results, prospects

Evgenii B. Aleksandrov, Anton K. Vershovskii

This paper is an extension of a part of the talk delivered under the more general title Narrow spectral lines in fundamental metrology: state of the art, prospects, and problems' at the session of the 90th anniversary of Physics–Uspekhi. The talk reviewed past developments and the current status of the metrology of length, frequency/time, and magnetic fields. The measurement of these quantities currently relies on the high stability of energies of standard transitions between metastable atomic states. Because of space restrictions in the journal, all metrology topics other than the title one were omitted in the present review.

Carlton M. Caves

Vittorio Giovannetti, Seth Lloyd, Lorenzo Maccone

Ze-Zhou Zhang, Hong‐Gang Luo, Wei Wu

Equilibrium probes have been widely used in various noisy quantum metrology schemes. However, such an equilibrium-probe-based metrology scenario severely suffers from the low-temperature-error-divergence problem in the weak-coupling regime. To circumvent this limit, we propose a strategy to eliminate the error-divergence problem by utilizing the strong-coupling effects, which can be captured by the reaction-coordinate mapping. The strong couplings induce a noncanonical equilibrium state which greatly enhances the metrology performance. It is found that our metrology precision behaves as a polynomial-type scaling relation, which suggests the reduction of temperature can be used as a resource to improve the metrology performance. Our result is sharply contrary to that of the weak-coupling case, in which the metrology precision exponentially decays as the temperature decreases. Paving a way to realize a high-precision noisy quantum metrology at low temperatures, our result reveals the importance of the non-Markovianity in quantum technologies.

Прудников Олег Николаевич, D.N. Kapusta, Andrei Goncharov +2 more

Monika Schleier-Smith

These lecture notes discuss applications of atom-light interactions in cavities to quantum metrology, simulation, and computation. A focus is on nonlocally interacting spin systems realized by coupling many atoms to a delocalized mode of light. We will build up from the fundamentals: understanding how a cavity enables light to coherently imprint information on atoms and atoms to imprint information on the light, enabling quantum non-demolition measurements that constitute a powerful means of engineering nonclassical states. By extension, letting the intracavity light act back on the atoms enables coherent photon-mediated interactions. I start by discussing collective spin models, emphasizing applications in entanglement-enhanced metrology, before proceeding to richer many-body physics enabled by incorporating spatiotemporal control or employing multiple cavity modes. I will highlight opportunities for leveraging these tools for quantum simulations inspired by problems in condensed matter and quantum gravity. Along the way, I provide a pedagogical introduction to criteria for strong atom-light coupling, illustrate how the corresponding figure of merit -- the cooperativity -- sets fundamental limits on the coherence of atom-light interactions, and discuss prospects for harnessing high-cooperativity cavity QED in quantum simulation and computation.