Magnetic strong coupling in a spin-photon system and transition to classical regime

TL;DR

This paper investigates the transition from quantum to classical behavior in a spin-photon system, analyzing energy levels, Rabi splittings, and experimental evidence of magnetic strong coupling relevant for quantum computing.

Contribution

It provides a detailed analysis of the energy level structure and Rabi frequency distribution in a spin-photon system, identifying a sharp transition at a critical photon number and presenting experimental validation.

Findings

Sharp transition in Rabi frequency distribution at photon number ~ spin system size N

Divergence of Rabi frequency spectrum width as sqrt(N) at transition point

Experimental demonstration of magnetic strong coupling at room temperature

Abstract

We study the energy level structure of the Tavis-Cumming model applied to an ensemble of independent magnetic spins $s=1/2$ coupled to a variable number of photons. Rabi splittings are calculated and their distribution is analyzed as a functin of photon number $n_{\rm max}$ and spin system size $N$. A sharp transition in the distribution of the Rabi frequency is found at $n_{\rm max}\approx N$. The width of the Rabi frequency spectrum diverges as $\sqrt{N}$ at this point. For increased number of photons $n_{\rm max}>N$, the Rabi frequencies converge to a value proportional to $\sqrt{n_{\rm max}}$. This behavior is interpreted as analogous to the classical spin resonance mechanism where the photon is treated as a classical field and one resonance peak is expected. We also present experimental data demonstrating cooperative, magnetic strong coupling between a spin system and photons, measured at room temperature. This points towards quantum computing implementation with magnetic spins, using cavity quantum-electrodynamics techniques.