Mesoscopic fluctuations in driven artificial atoms

M. J. SÁNCHEZ, A. FERRÓN AND D. DOMINGUEZ

Workshop; ICTP Workshop on Coherent Phenomena in Disordered Optical Systems; 2014

ICTP

Population inversion (PI) is among the most interesting phenomena inmaser and laser physics. The usual mechanism to get PI, i.e. the most highly populated state isan excited state, requires driven quantum systems with three or more energy levels.Interesting systems to study PI are `artificial atoms´ made with mesoscopic Josephson devices.Among them, is the flux qubit (FQ), which for millikelvin temperatures exhibitsquantized energy levels that are sensitive to an external magnetic field.The FQ is among the most exhiting experimental proposal forPopulation inversion was observed in the FQ for ac amplitudes that excite to the third and fourenergy levels through Landau-Zener transitions.I will show that an even more notable effect is awaiting to be observed in thesesystems if the ac driving pulses are applied for longer time scales : PI could be observable evenfor ac amplitudes where only the two lowest levels of the FQ participate, thus solely mediated bythe environmental bath. Furthermore, the degree of population inversion can be controlled bytailoring a resonant frequency Ω in the environmental bath.In this talk I will describe: (i) how this type of PI can occur and (ii) how the occurrence and thedegree of PI can be tailored by changing the structure of the environmental bath.In addition we investigate the effect of time reversal symmetry broken in artificial atoms in contactwith an enviromental bath. Using as a test system the FQ driven by a biharmonic magnetic signalof period τ with a phase lag, it is possible to establish a direct analogy between interference effectsat avoided level crossings and scattering events in mesoscopic disordered media. As we show, thephase lag acts as effective time reversal control parameter that enables to study mesoscopicsignatures in the transition rate, like weak localization and fluctuactions effects