9,536 research outputs found
A n-qubit controlled phase gate with superconducting quantum interference devices coupled to a resonator
We present a way to realize a -qubit controlled phase gate with
superconducting quantum interference devices (SQUIDs) by coupling them to a
superconducting resonator. In this proposal, the two logical states of a qubit
are represented by the two lowest levels of a SQUID. An intermediate level of
each SQUID is utilized to facilitate coherent control and manipulation of
quantum states of the qubits. It is interesting to note that a -qubit
controlled phase gate can be achieved with SQUIDs by successively applying
a Jaynes-Cummings pulse to each of the control SQUIDs before and
after a Jaynes-Cummings pulse on the target SQUID.Comment: 9 pages, 4 figures, 1 table, RevTeX, Resubmitted to Phys. Rev.
Estimating the central charge from the R\'enyi entanglement entropy
We calculate the von Neumann and R\'enyi bipartite entanglement entropy of
the model with a chemical potential on a 1+1 dimensional Euclidean
lattice with open and periodic boundary conditions. We show that the
Calabrese-Cardy conformal field theory predictions for the leading logarithmic
scaling with the spatial size of these entropies are consistent with a central
charge . This scaling survives the time continuum limit and truncations of
the microscopic degrees of freedom, modifications which allow us to connect the
Lagrangian formulation to quantum Hamiltonians. At half-filling, the forms of
the subleading corrections imposed by conformal field theory allow the
determination of the central charge with an accuracy better than two percent
for moderately sized lattices. We briefly discuss the possibility of estimating
the central charge using quantum simulators.Comment: 10 pages, 8 figures, 3 table
Quantum transport through a double Aharonov-Bohm-interferometer in the presence of Andreev reflection
Quantum transport through a double Aharonov-Bohm-interferometer in the
presence of Andreev reflection is investigated in terms of the nonequilibrium
Green function method with which the reflection current is obtained. Tunable
Andreev reflection probabilities depending on the interdot coupling strength
and magnetic flux as well are analysised in detail. It is found that the
oscillation period of the reflection probability with respect to the magnetic
flux for the double interferometer depends linearly on the ratio of two parts
magnetic fluxes n, i.e. 2(n+1)pi, while that of a single interferometer is 2pi.
The coupling strength not only affects the height and the linewidth of Andreev
reflection current peaks vs gate votage but also shifts the peak positions. It
is furthermore demonstrated that the Andreev reflection current peaks can be
tuned by the magnetic fluxes.Comment: 13 pages, 12 figur
Atomic Scale Sliding and Rolling of Carbon Nanotubes
A carbon nanotube is an ideal object for understanding the atomic scale
aspects of interface interaction and friction. Using molecular statics and
dynamics methods different types of motion of nanotubes on a graphite surface
are investigated. We found that each nanotube has unique equilibrium
orientations with sharp potential energy minima. This leads to atomic scale
locking of the nanotube.
The effective contact area and the total interaction energy scale with the
square root of the radius. Sliding and rolling of nanotubes have different
characters. The potential energy barriers for sliding nanotubes are higher than
that for perfect rolling. When the nanotube is pushed, we observe a combination
of atomic scale spinning and sliding motion. The result is rolling with the
friction force comparable to sliding.Comment: 4 pages (two column) 6 figures - one ep
Progress towards quantum simulating the classical O(2) model
We connect explicitly the classical model in 1+1 dimensions, a model
sharing important features with lattice gauge theory, to physical models
potentially implementable on optical lattices and evolving at physical time.
Using the tensor renormalization group formulation, we take the time continuum
limit and check that finite dimensional projections used in recent proposals
for quantum simulators provide controllable approximations of the original
model. We propose two-species Bose-Hubbard models corresponding to these finite
dimensional projections at strong coupling and discuss their possible
implementations on optical lattices using a Rb and K Bose-Bose
mixture.Comment: 7 pages, 6 figures, uses revtex, new material and one author added,
as to appear in Phys. Rev.
Dynamics of quantum dissipation systems interacting with Fermion and Boson grand canonical bath ensembles: Hierarchical equations of motion approach
A hierarchical equations of motion formalism for a quantum dissipation system
in a grand canonical bath ensemble surrounding is constructed, on the basis of
the calculus-on-path-integral algorithm, together with the parametrization of
arbitrary non-Markovin bath that satisfies fluctuation-dissipation theorem. The
influence functionals for both the Fermion or Boson bath interaction are found
to be of the same path-integral expression as the canonical bath, assuming they
all satisfy the Gaussian statistics. However, the equation of motion formalism
are different, due to the fluctuation-dissipation theories that are distinct
and used explicitly. The implications of the present work to quantum transport
through molecular wires and electron transfer in complex molecular systems are
discussed.Comment: 12page
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