Generation of a Population Inversion between Quantum States of a Macroscopic Variable

Abstract

A population inversion has been created between the two adjacent fluxoid wells (with energy difference $\varepsilon$) in an rf SQUID subject to weak radiation at 100 GHz. The states in the two wells have opposite currents, of magnitude ∼2 μA, circulating through an inductance of 0.2 nH. The steady state population of the upper well $n_u\left(\varepsilon \right)$ shows local maxima whose positions correspond to calculated photon absorption resonances between the ground state and an intermediate state. The maximum $n_u\left(\varepsilon \right)$ is 0.94 compared to the expected thermal equilibrium value of $n_u\left(\varepsilon \right)=2\mathrm{}\times {10}^{-9\mathrm{}}$.