Waveguide quantum electrodynamics at the onset of spin-spin correlations

Abstract

We explore the competition between light-mediated and intrinsic matter-matter interactions in waveguide quantum electrodynamics. For this, we couple a superconducting transmission line to a model magnetic material, made of organic free radical molecules with a spin $S=1/2$ and a $g_S$ factor very close to that of a free electron. The microwave transmission has been measured in a wide range of temperatures ($0.013$ K $\le T\le 2$ K), magnetic fields ($0\le B\le 0.5$ T) and frequencies ($0\le \omega /2\pi \le 14$ GHz). We find that molecules belonging to one of the two crystal sublattices form one-dimensional spin chains. Temperature then controls the intrinsic correlations along these chains in a continuous and monotonic way. In the paramagnetic region ($T>0.7$ K), the microwave transmission shows evidences for the collective coupling of quasi-identical spins to the propagating photons, with coupling strengths that reach values close to the dissipation rates. As $T$ decreases, the growth of spin correlations, combined with the anisotropy in the spin-spin exchange constants, tend to suppress the collective spin-photon coupling. In this regime, the spin visibility in transmission reflects also a gradual change in the nature of the dominant spin excitations, from single spin flips to bosonic magnons.

10 pages, 4 figures (main) and 25 pages, 21 figures (supp. inf.)

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