A pair of two-level systems

N. N. Bogolyubov, A. S. Shumovskiǐ

V. Zaretskey, Baladitya Suri, S P Novikov +2 more

We report on the quadrupling of the transition spectrum of an Al/AlO${}_{x}$/Al Cooper-pair box (CPB) charge qubit in the $4.0--7.3$ GHz frequency range. The qubit was coupled to a quasilumped-element Al superconducting resonator and measured at a temperature of 25 mK. We obtained good matches between the observed spectrum and the spectra calculated from a model Hamiltonian containing two distinct low-excitation energy two-level systems (TLS) coupled to the CPB. In our model, each TLS has a charge that tunnels between two sites in a local potential and induces a change in the CPB critical current. By fitting the model to the spectrum, we have extracted microscopic parameters of the fluctuators including the well asymmetry, tunneling rate, and a surprisingly large fractional change ($30%--40%$) in the critical current (12 nA). This large change is consistent with a Josephson junction with a nonuniform tunnel barrier containing a few dominant conduction channels and a TLS that modulates one of them.

Ulrich Hohenester

Yun Fang

With time evolution operator,it is proved that the population of upper level is in circumstance of cyclic oscillation by laser pulses pair of Gaussian line shape in a homogeneous broadening two level system.The oscillatory time oxide is the delay time between the laser pulse pairs and the oscillatory frequency equals the one of transition between the lower and upper level.

Brian P. Anderson

Charles Andrew Downing, Elena del Valle, Antonio I. Fernández‐Domínguez

We study a driven-dissipative duo of two-level systems in an open quantum systems approach, modeling a pair of atoms or (more generally) meta-atoms. Allowing for complex-valued couplings in the setup, which are of both a coherent and incoherent character, gives rise to a diverse coupling landscape. We consider several points on this landscape, for example where the coupling between the two coupled two-level systems is dominated by coherent, incoherent, unsymmetrical, and even unidirectional interactions. Traversing the coupling terrain leads to remarkable features in the populations of the pair, correlations, and optical spectra. Most notably, the famous Mollow triplet spectrum for a single atom may be superseded for a pair by a Mollow quintuplet (or even by a spectral singlet) and the setup allows for population trapping to arise, all depending upon the precise nature of the coupling between the two-level systems.