14,047 research outputs found
Quantum quench dynamics of the Bose-Hubbard model at finite temperatures
We study quench dynamics of the Bose-Hubbard model by exact diagonalization.
Initially the system is at thermal equilibrium and of a finite temperature. The
system is then quenched by changing the on-site interaction strength
suddenly. Both the single-quench and double-quench scenarios are considered. In
the former case, the time-averaged density matrix and the real-time evolution
are investigated. It is found that though the system thermalizes only in a very
narrow range of the quenched value of , it does equilibrate or relax well in
a much larger range. Most importantly, it is proven that this is guaranteed for
some typical observables in the thermodynamic limit. In order to test whether
it is possible to distinguish the unitarily evolving density matrix from the
time-averaged (thus time-independent), fully decoherenced density matrix, a
second quench is considered. It turns out that the answer is affirmative or
negative according to the intermediate value of is zero or not.Comment: preprint, 20 pages, 7 figure
Heat capacity of the quantum magnet TiOCl
Measurements of the heat capacity C(T,H) of the one-dimensional quantum
magnet TiOCl are presented for temperatures 2K < T < 300K and magnetic fields
up to 5T. Distinct anomalies at 91K and 67K signal two subsequent phase
transitions. The lower of these transitions clearly is of first order and seems
to be related to the spin degrees of freedom. The transition at 92K probably
involves the lattice and/or orbital moments. A detailed analysis of the data
reveals that the entropy change through both transitions is surprisingly small
(~ 0.1R), pointing to the existence strong fluctuations well into the
non-ordered high-temperature phase. No significant magnetic field dependence
was detected.Comment: 4 pages, 2 figure
Potential Errors in a Scheme of Universal Quantum Gates in Kane's Model
We re--investigate a plausible proposal for universal quantum gates in Kane's
model, in which the authors assumed that electron spin is always downward under
a background magnetic field and the value of controlling parameters is varied
instantaneously. We demonstrate that a considerable error appears, for example,
in the X rotation. As result, the controlled operations don't work. Such a
failure is caused by improper choice of the computational bases; actually, the
electron spin is not always downward over time during quantum operations.Comment: 11 pages, 3 figures; improved with typos corrected; Accepted for
publication in Phys. Rev.
A method to quantitatively evaluate Hamaker constant using the jump-into-contact effect in Atomic Force microscopy
We find that the jump-into-contact of the cantilever in the atomic force
microscope (AFM) is caused by an inherent instability in the motion of the AFM
cantilever. The analysis is based on a simple model of the cantilever moving in
a nonlinear force field. We show that the jump-into-contact distance can be
used to find the interaction of the cantilever tip with the surface. In the
specific context of the attractive van der Waals interaction, this method can
be realized as a new method of measuring the Hamaker constant for materials.
The Hamaker constant is determined from the deflection of the cantilever at the
jump-into-contact using the force constant of the cantilever and the tip radius
of curvature, all of which can be obtained by measurements. The results have
been verified experimentally on a sample of cleaved mica, a sample of Si wafer
with natural oxide and a silver film, using a number of cantilevers with
different spring constants. We emphasize that the method described here is
applicable only to surfaces that have van der Waals interaction as the
tip-sample interaction. We also find that the tip to sample separation at the
jump-into-contact is simply related to the cantilever deflection at this point,
and this provides a method to exactly locate the surface.Comment: 11 pages, 4 figures, 1 tabl
Existence Criterion of Genuine Tripartite Entanglement
In this paper, an intuitive mathematical formulation is provided to
generalize the residual entanglement for tripartite systems of qubits [Phys.
Rev. A 61, 052306 (2000)] to the tripartite systems in higher dimension. The
spirit lies in the tensor treatment of tripartite pure states [Phys. Rev. A 72,
022333 (2005)]. A distinct characteristic of the present generalization is that
the formulation for higher dimensional systems is invariant under permutation
of the subsystems, hence is employed as a criterion to test the existence of
genuine tripartite entanglement. Furthermore, the formulation for pure states
can be conveniently extended to the case of mixed states by utilizing the
Kronecker product approximate technique. As applications, we give the analytic
approximation of the criterion for weakly mixed tripartite quantum states and
consider the existence of genuine tripartite entanglement of some weakly mixed
states.Comment: 6 pages, 2 figure
Minimum-error discrimination between mixed quantum states
We derive a general lower bound on the minimum-error probability for {\it
ambiguous discrimination} between arbitrary mixed quantum states with given
prior probabilities. When , this bound is precisely the well-known
Helstrom limit. Also, we give a general lower bound on the minimum-error
probability for discriminating quantum operations. Then we further analyze how
this lower bound is attainable for ambiguous discrimination of mixed quantum
states by presenting necessary and sufficient conditions related to it.
Furthermore, with a restricted condition, we work out a upper bound on the
minimum-error probability for ambiguous discrimination of mixed quantum states.
Therefore, some sufficient conditions are obtained for the minimum-error
probability attaining this bound. Finally, under the condition of the
minimum-error probability attaining this bound, we compare the minimum-error
probability for {\it ambiguously} discriminating arbitrary mixed quantum
states with the optimal failure probability for {\it unambiguously}
discriminating the same states.Comment: A further revised version, and some results have been adde
Pressure-induced insulator-to-metal transition in low-dimensional TiOCl
We studied the transmittance and reflectance of the low-dimensional
Mott-Hubbard insulator TiOCl in the infrared and visible frequency range as a
function of pressure. The strong suppression of the transmittance and the
abrupt increase of the near-infrared reflectance above 12 GPa suggest a
pressure-induced insulator-to-metal transition. The pressure-dependent
frequency shifts of the orbital excitations, as well as the pressure
dependences of the charge gap and the spectral weight of the optical
conductivity above the phase transition are presented.Comment: 4 pages, 6 figure
Unsupervised Domain Adaptation for 3D Keypoint Estimation via View Consistency
In this paper, we introduce a novel unsupervised domain adaptation technique
for the task of 3D keypoint prediction from a single depth scan or image. Our
key idea is to utilize the fact that predictions from different views of the
same or similar objects should be consistent with each other. Such view
consistency can provide effective regularization for keypoint prediction on
unlabeled instances. In addition, we introduce a geometric alignment term to
regularize predictions in the target domain. The resulting loss function can be
effectively optimized via alternating minimization. We demonstrate the
effectiveness of our approach on real datasets and present experimental results
showing that our approach is superior to state-of-the-art general-purpose
domain adaptation techniques.Comment: ECCV 201
Quantum Computing with Continuous-Variable Clusters
Continuous-variable cluster states offer a potentially promising method of
implementing a quantum computer. This paper extends and further refines
theoretical foundations and protocols for experimental implementation. We give
a cluster-state implementation of the cubic phase gate through photon
detection, which, together with homodyne detection, facilitates universal
quantum computation. In addition, we characterize the offline squeezed
resources required to generate an arbitrary graph state through passive linear
optics. Most significantly, we prove that there are universal states for which
the offline squeezing per mode does not increase with the size of the cluster.
Simple representations of continuous-variable graph states are introduced to
analyze graph state transformations under measurement and the existence of
universal continuous-variable resource states.Comment: 17 pages, 5 figure
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