# Observation of perfect absorption in hyperfine levels of molecular spins with hermitian subspaces

## TL;DR

The paper shows how perfect absorption of radiation can be achieved in a quantum system using molecular spins and a microwave resonator, without relying on PT-symmetry.

## Contribution

The study introduces a new mechanism for perfect absorption using non-Hermitian Hamiltonians and Hermitian subspaces, beyond the PT-symmetry paradigm.

## Key findings

- Perfect absorption is achieved in a system of molecular spins and a microwave resonator at milliKelvin temperatures.
- Hermitian subspaces are linked to perfect absorption and can be engineered via resonator-spin detuning.
- The results suggest potential applications in single-photon switches and modulators.

## Abstract

We investigate Perfect Absorption (PA) of radiation, in which incoming energy is entirely dissipated, in a system consisting of molecular spin centers coherently coupled to a planar microwave resonator operated at milliKelvin temperature and in the single photon regime. This platform allows us to fine tune the spin-photon coupling and to control the effective dissipation of the two subsystems towards the environment, thus giving us the opportunity to span over a wide space of parameters. Our system can be effectively described by a non-Hermitian Hamiltonian exhibiting distinct Hermitian subspaces. We experimentally show that these subspaces, linked to the presence of PA, can be engineered through the resonator-spin detuning, which controls the composition of the polaritons in terms of photon and spin content. In such a way, the required balance between the feeding and the loss rates is effectively recovered even in the absence of PT-symmetry. We show that Hermitian subspaces influence the overall aspect of coherent spectra of cavity QED systems and enlarge the possibility to explore non-Hermitian effects in open quantum systems. We finally discuss how our results can be potentially exploited for applications, in particular as single-photon switches and modulators.

Perfect absorption has been achieved in passive open quantum systems and is inherently linked to Parity-Time symmetry. Here, the authors experimentally and theoretically demonstrate a version beyond this paradigm using a system described by a non-Hermitian Hamiltonian, and providing a connection between Perfect Absorption and Hermitian subspaces.