Low- and high-frequency noise from coherent two-level systems

Recent experiments indicate a connection between the low- and high-frequency noises affecting superconducting quantum systems. We explore the possibilities that both noises can be produced by one ensemble of microscopic modes, made up, e.g., by sufficiently coherent two-level systems (TLS's). This implies a relation between the noise power in different frequency domains, which depends on the distribution of the parameters of the TLS's. We show that a distribution, natural for tunneling TLS's, with a log-uniform distribution in the tunnel splitting and linear distribution in the bias, accounts for experimental observations.     

Direct observation of Josephson capacitance

The effective capacitance has been measured in the split Cooper-pair box (CPB) over its phase-gate bias plane. Our low-frequency reactive measurement scheme allows us to probe purely the capacitive susceptibility due to the CPB band structure. The data are quantitatively explained using parameters determined independently by spectroscopic means. In addition, we show in practice that the method offers an efficient way to do nondemolition readout of the CPB quantum state.     

Dephasing of qubits by transverse low-frequency noise

We analyze the dissipative dynamics of a two-level quantum system subject to low-frequency, e.g., 1/f noise, motivated by recent experiments with superconducting quantum circuits. We show that the effect of transverse linear coupling of the system to low-frequency noise is equivalent to that of quadratic longitudinal coupling. We further find the decay law of quantum coherent oscillations under the influence of both low-and high-frequency fluctuations, in particular, for the case of comparable rates of relaxation and pure dephasing.     

Quantum zeno effect in Cooper-pair transport through a double-island Josephson system

Motivated by recent experiments, we analyze transport of Cooper pairs through a double-island Josephson qubit. At low bias in a certain range of gate voltages, coherent superpositions of charge states play a crucial role. Analysis of the evolution of the density matrix allows us to cover a wide range of parameters, including situations with degenerate levels, when dissipation strongly affects the coherent eigenstates. At high noise levels, the so-called Zeno effect can be observed, which slows down the transport. Our analysis explains certain features of the I-V curves, in particular, the visibility and shape of resonant peaks and lines.     

Non-adiabatic geometric phases and dephasing in an open quantum system

We analyze the influence of a dissipative environment on geometric phases in a quantum system subject to non-adiabatic evolution. We find dissipative contributions to the acquired phase and modification of dephasing, considering the cases of both weak short-correlated noise and slow quasi-stationary noise. Motivated by recent experiments, we find the leading non-adiabatic corrections to the results, known for the adiabatic limit.     

Oxidative dimerization of methane: Kinetics,mathematical modeling,and optimization with La/Ce catalysts

Kinetic features of oxidative dimerization of methane to ethane and ethylene, catalyzed by a mixture of lanthanum and cerium oxides deposited onto a magnesium oxide support, were examined. A kinetic model was proposed, and the process optimization was performed for a plug-flow reactor in the quasi-homogeneous approximation.