Resonant optical bistability in support of enhanced Rydberg-atom sensors

Radio frequency antennas based on highly excited Rydberg atom vapors can in principle reach sensitivities beyond those of any conventional wire antenna, especially at lower frequencies where very long wires are needed to accommodate the growing wavelength. Conventional Rydberg sensors are based on individual atom response, with increased signal resolution relying on the $O({10}^{3})$ electric dipole moment enhancement, scaling as the square of the Rydberg-state principal quantum number $N\ensuremath{\sim}50$. However, despite more than 30 years of steady advances, beyond-classical signal sensitivity has yet to be demonstrated. More recently, the optical bistability effect, a many-body nonequilibrium phase transition occurring at somewhat higher vapor densities, has been exploited for an order of magnitude or more increased sensitivity for some setups through tuning into the critical region. However, the results fall significantly short of those using more advanced ``conventional'' Rydberg sensor setups---which achieve even greater enhancement by exploiting resonant interaction between a pair of nearby Rydberg levels. This paper seeks to combine the many-body and resonant enhancement effects by extending the bistable phase analysis to include a pair of resonant Rydberg levels, supported by an exact treatment of the atom velocity thermal average. A dynamic linear response formalism is developed as well to explore the trade-off between measurement sensitivity and finite-bandwidth signals. We demonstrate regions of the phase diagram which could be exploited for record breaking, beyond-classical sensitivity. Of course, only a limited number of vapor parameters are under full experimental control, and experiments will be needed to quantitatively constrain the effective mean-field interaction parameters appearing in the theory and thereby define the accessible regions of the phase diagram.