The nature and boundary of the floating phase in a dissipative Josephson junction array

We study the nature of correlations within, and the transition into, the floating phase of dissipative Josephson junction arrays. Order parameter correlations in this phase are long-ranged in time, but only short-ranged in space. A perturbative RG analysis shows that, in {\it arbitrary} spatial dimension, the transition is controlled by a continuous locus of critical fixed points determined entirely by the \textit{local} topology of the lattice. This may be the most natural example of a line of critical points existing in arbitrary dimensions.Comment: Parts rewritten, typos correcte

Adjoint master equation for multi-time correlators

The quantum regression theorem is a powerful tool for calculating the muli-time correlators of operators of open quantum systems which dynamics can be described in Markovian approximation. It enables to obtain the closed system of equation for the multi-time correlators. However, the scope of the quantum regression theorem is limited by a particular time order of the operators in multi-time correlators and does not include out-of-time-ordered correlators. In this work, we obtain an adjoint master equation for multi-time correlators that is applicable to out-of-time-ordered correlators. We show that this equation can be derived for various approaches to description of the dynamics of open quantum systems, such as the global or local approach. We show that the adjoint master equation for multi-time correlators is self-consistent. Namely, the final equation does not depend on how the operators are grouped inside the correlator, and it coincides with the quantum regression theorem for the particular time ordering of the operators.Comment: 11 page

Controlling Purity, Indistinguishability and Quantum Yield of Incoherently Pumped Two-Level System by Spectral Filters

Dephasing processes significantly impact the performance of deterministic single-photon sources. Dephasing broadens the spectral line and suppresses the indistinguishability of the emitted photons, which is undesirable for many applications, primarily for quantum computing. We consider a light emitted by a two-level system with a pulsed incoherent pump in the presence of the spectral filter. The spectral filter allows control of the second-order autocorrelation function, indistinguishability, and quantum yield. We show that narrow spectral filters can increase the indistinguishability of the emitted light while undermining the quantum yield. The influence of the spectral filter on the second-order correlation function depends on the duration of the pump. When the pumping pulse is long compared to the lifetime of the two-level system, the narrow spectral filters lead to a rapid increase in the second-order autocorrelation function. In this limit, the statistics of the light from the two-level system inherit the statistics of the incoherent pump. In the case of the short duration of the pump pulse, it is possible to preserve single-photon properties to some degree for the sub-lifetime width of the spectral filter. Moreover, when the light emitted by the single-photon source is used to control a quantum system, e.g., cavity, the single-photon properties of the light manifest themselves differently, depending on the response time of the quantum system. In particular, in the case of long response time, the spectral filter with sub-lifetime width can provide the near-zero second-order autocorrelation function

Strong Electron Tunneling through a Small Metallic Grain

Electron tunneling through mesoscopic metallic grains can be treated perturbatively only provided the tunnel junction conductances are sufficiently small. If it is not the case, fluctuations of the grain charge become strong. As a result (i) contributions of all -- including high energy -- charge states become important and (ii) excited charge states become broadened and essentially overlap. At the same time the grain charge remains discrete and the system conductance $e$-periodically depends on the gate charge. We develop a nonperturbative approach which accounts for all these features and calculate the temperature dependent conductance of the system in the strong tunneling regime at different values of the gate charge.Comment: revtex, 8 pages, 2 .ps figure

A cluster algorithm for resistively shunted Josephson junctions

We present a cluster algorithm for resistively shunted Josephson junctions and similar physical systems, which dramatically improves sampling efficiency. The algorithm combines local updates in Fourier space with rejection-free cluster updates which exploit the symmetries of the Josephson coupling energy. As an application, we consider the localization transition of a single junction at intermediate Josephson coupling and determine the temperature dependence of the zero bias resistance as a function of dissipation strength.Comment: 4 page