Characterizing decoherence rates of a superconducting qubit by direct microwave scattering

We experimentally investigate a superconducting qubit coupled to the end of an open transmission line, in a regime where the qubit decay rates to the transmission line and to its own environment are comparable. We perform measurements of coherent and incoherent scattering, on- and off-resonant fluorescence, and time-resolved dynamics to determine the decay and decoherence rates of the qubit. In particular, these measurements let us discriminate between non-radiative decay and pure dephasing. We combine and contrast results across all methods and find consistent values for the extracted rates. The results show that the pure dephasing rate is one order of magnitude smaller than the non-radiative decay rate for our qubit. Our results indicate a pathway to benchmark decoherence rates of superconducting qubits in a resonator-free setting.

Electromagnetically induced transparency in a rectangular quantum dot on a single electron

In this paper, we investigated the realization of electromagnetically induced transparency (EIT) in a rectangular quantum dot (QD) with a single electron in the presence of probe and control laser fields. The lowest three levels of the confined electron that form ladder and V configuration were chosen. We discussed the dependence of density matrix elements for ladder configuration and for V configuration on detunings of the probe field for various values of quantum dot dimensions. This dependence is discussed for both cases, at cryogenic temperatures when spontaneous emission dominates the relaxation mechanism and at room or higher temperatures when dephasing rate cannot be neglected.

Effects of initial coherence on distinguishability of pure/mixed states and chiral stability in an open chiral system

We examine how initial coherences in open chiral systems affect distinguishability of pure versus mixed states and purity decay. Interaction between a system and an environment is modeled by a continuous position measurement and a two-level approximation is taken for the system. The resultant analytical solution is explored for various parameters, with emphasis on the interplay of initial coherences of the system and dephasing rate in determining the purity decay and differences in the time evolution of pure versus mixed initial states. Implications of the results for several fundamental problems are noted.

Spatial dependence of moving three-level atoms interacting with a three-laser beam

We investigate the dynamical behavior of a three-level moving atom interacting with a laser field. We show that the sensitivity of atomic dynamics can be considerably improved by exploiting the spatial dependence of moving atoms within the framework of the homotopy perturbation method. The dynamics of the system is analyzed as well as the effect of various parameters including detuning and dephasing rate. The spatial dependence approach has the same properties as the time-dependent approach, but it is preferable when the time-scale is very small. The very sensitive dependence and the role of the off-resonant interaction are highlighted.

Spin squeezing of one-axis twisting model in the presence of phase dephasing

We present a detailed analysis of spin squeezing of the one-axis twisting model with a many-body phase dephasing, which is induced by external field fluctuation in a two-mode Bose-Einstein condensates. Even in the presence of the dephasing, our analytical results show that the optimal initial state corresponds to a coherent spin state $|\theta_{0}, \phi_0\rangle$ with the polar angle $\theta_0=\pi/2$. If the dephasing rate $\gamma\ll S^{-1/3}$, where $S$ is total atomic spin, we find that the smallest value of squeezing parameter (i.e., the strongest squeezing) obeys the same scaling with the ideal one-axis twisting case, namely $\xi^2\propto S^{-2/3}$. While for a moderate dephasing, the achievable squeezing obeys the power rule $S^{-2/5}$, which is slightly worse than the ideal case. When the dephasing rate $\gamma>S^{1/2}$, we show that the squeezing is weak and neglectable.