Polariton-polariton coherent coupling in a molecular spin-superconductor chip

The ability to establish coherent communication channels is key for scaling up quantum devices. Here, we engineer interactions between distant polaritons, hybrid spin-photon excitations formed at different lumped-element superconducting resonators within a chip. The chip consists of several resonator pairs, slightly detuned in frequency to make them addressable, capacitively coupled within each pair and inductively coupled to a common readout line. They interact locally with samples of PTMr and Tripak$^{-}$ organic free radicals, deposited onto their inductors, which provide model $S = 1/2$, $g \simeq 2$ spin ensembles. Frequency-dependent microwave transmission experiments, performed at very low temperatures, measure polariton frequencies as a function of magnetic field in different scenarios. When only one resonator within a pair hosts a molecular sample, the results evidence that spins couple remotely to the empty LER as well as to the local cavity mode. If both resonators interact with a spin ensemble, the magnetic field tunes the polariton frequencies relative to each other, on account of the different spin-photon interactions at each LER. When polaritons are brought into mutual resonance, an avoided level crossing emerges that gives direct spectroscopic evidence for a coherent polariton-polariton interaction mediated by the circuit. Pump-probe experiments reveal that the excitation of a polariton within a connected pair is felt, thus it can be read out, by the other one. These observations, backed by model calculations, illustrate the control and detection of distant photon-photon and spin-spin correlations and entanglement in a scalable modular chip.

💡 Research Summary

The authors present a hybrid quantum platform that combines organic spin ensembles with superconducting lumped‑element resonators (LERs) to realize coherent interactions between distant polaritons—hybrid spin‑photon excitations. The chip contains seven pairs of NbTiN LERs, each pair capacitively coupled with a tunable coupling κij and all pairs inductively coupled to a common transmission line for readout. By varying the inter‑digitated capacitor lengths, the bare resonance frequencies of the two resonators in a pair are detuned by more than the inter‑resonator coupling, ensuring that each mode is spatially localized and can be addressed individually.

Two organic free‑radical molecules, PTMr and Tripak⁻, are deposited on the inductors of selected LERs. Each deposit forms a macroscopic spin ensemble (N≈10¹³–10¹⁴) with S = ½, g≈2, and a narrow electron‑spin resonance (γ≈7–12 MHz). Low‑temperature (≈11 mK) microwave transmission (S₂₁) measurements reveal the collective spin‑photon coupling strengths G ranging from 5 MHz up to 22 MHz, corresponding to cooperativities well above unity.

The first set of experiments investigates a single spin‑filled resonator (LER‑10) coupled to an empty partner (LER‑9). Transmission spectra show that the same spin ensemble couples not only to the local mode (G₁₀≈5.4 MHz) but also to the remote mode (G₉,₁₀≈2.5 MHz). This remote coupling is explained by a two‑cavity extension of the Jaynes‑Cummings model, where the effective interaction G₉,₁₀ = G₁₀ √