127 research outputs found
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 -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
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