Physics Colloquium by Prasanna Venkatesh B

Time: 1500 HRS

Venue: A01-102

Title: Many emitters, one environment: Collective effects in quantum optics and quantum thermodynamics

Abstract: Quantum emitters that share a common electromagnetic environment radiate collectively. Their emission can be enhanced as a superradiant burst or suppressed in long-lived subradiant states. In the idealised limit where identical emitters couple identically to the field, the dynamics is invariant under permutations of the emitters, and this symmetry organises the collective states. These states also modify the optical and Casimir–Polder forces acting on emitter ensembles. On the other hand, permutation symmetry plays an equally central role in quantum thermodynamics, where it is known that identical heat engines can drive a common load to energies scaling super-extensively or identical battery cells can be charged faster and more efficiently. In this colloquium we will discuss two recent results, one from each setting.

Subradiant states are attractive for quantum memories and sensing, but their weak coupling to light makes them difficult to prepare. We show that measurements on a single emitter overcome this difficulty. In a permutation-symmetric ensemble, one projective measurement at a suitable time leaves a finite fraction of the population in a subradiant steady state. In generic arrays that lack this symmetry, repeated measurements on one emitter drive the others into steady states with large overlap with the subradiant subspace. When dipole–dipole interactions are strong, this requires far lower measurement rates than the Rabi frequency a local drive would need.

Turning to quantum thermodynamics, we consider batteries of N identical cells charged collectively. The fraction of stored energy that cannot be extracted as work vanishes at least as fast as 1/N, for cells of arbitrary finite dimension. This holds even for mixed steady states reached through a dephased charger. When the battery state becomes asymptotically pure, the convergence is faster, reaching a steep power law or even exponential decay in N². Purity of the battery state can therefore serve as a relevant criterion for designing charging protocols.