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

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