11,430 research outputs found
Separable states and the geometric phases of an interacting two-spin system
It is known that an interacting bipartite system evolves as an entangled
state in general, even if it is initially in a separable state. Due to the
entanglement of the state, the geometric phase of the system is not equal to
the sum of the geometric phases of its two subsystems. However, there may exist
a set of states in which the nonlocal interaction does not affect the
separability of the states, and the geometric phase of the bipartite system is
then always equal to the sum of the geometric phases of its subsystems. In this
paper, we illustrate this point by investigating a well known physical model.
We give a necessary and sufficient condition in which a separable state remains
separable so that the geometric phase of the system is always equal to the sum
of the geometric phases of its subsystems.Comment: 13 page
Two qubit copying machine for economical quantum eavesdropping
We study the mapping which occurs when a single qubit in an arbitrary state
interacts with another qubit in a given, fixed state resulting in some unitary
transformation on the two qubit system which, in effect, makes two copies of
the first qubit. The general problem of the quality of the resulting copies is
discussed using a special representation, a generalization of the usual Schmidt
decomposition, of an arbitrary two-dimensional subspace of a tensor product of
two 2-dimensional Hilbert spaces. We exhibit quantum circuits which can
reproduce the results of any two qubit copying machine of this type. A simple
stochastic generalization (using a ``classical'' random signal) of the copying
machine is also considered. These copying machines provide simple embodiments
of previously proposed optimal eavesdropping schemes for the BB84 and B92
quantum cryptography protocols.Comment: Minor changes. 26 pages RevTex including 7 PS figure
On the transport and thermodynamic properties of quasi-two-dimensional purple bronzes AMoO (A=Na, K)
We report a comparative study of the specific heat, electrical resistivity
and thermal conductivity of the quasi-two-dimensional purple bronzes
NaMoO and KMoO, with special emphasis on
the behavior near their respective charge-density-wave transition temperatures
. The contrasting behavior of both the transport and the thermodynamic
properties near is argued to arise predominantly from the different
levels of intrinsic disorder in the two systems. A significant proportion of
the enhancement of the thermal conductivity above in
NaMoO, and to a lesser extent in KMoO, is
attributed to the emergence of phason excitations.Comment: 8 pages, 6 figures, To appear in Physical Review
Quantum Chaos of Bogoliubov Waves for a Bose-Einstein Condensate in Stadium Billiards
We investigate the possibility of quantum (or wave) chaos for the Bogoliubov
excitations of a Bose-Einstein condensate in billiards. Because of the mean
field interaction in the condensate, the Bogoliubov excitations are very
different from the single particle excitations in a non-interacting system.
Nevertheless, we predict that the statistical distribution of level spacings is
unchanged by mapping the non-Hermitian Bogoliubov operator to a real symmetric
matrix. We numerically test our prediction by using a phase shift method for
calculating the excitation energies.Comment: minor change, 4 pages, 4 figures, to appear in Phys. Rev. Let
Smart Construction Objects
The primary aim of this research is to define smart construction objects (SCOs), the fundamental building blocks of future construction. SCOs are construction resources (e.g., machinery, device, and materials) that are made smart by augmenting them with technologies conferring autonomy, awareness, and the ability to interact with their vicinity. This smartness can enable better decision making in construction. Understanding of SCOs, however, is still in its infancy. Informed by theories on ubiquitous computing and general smart objects, this paper first defines the panoramic and interconnected properties that differentiate SCOs from conventional construction objects. Second, representative scenarios of the use of SCOs are given to illustrate the new workflow with enhanced smartness in the future. Next, using prefabrication construction as an example, this paper further elaborates SCOs using Industry Foundation Classes Extensible Markup Language and exploring their software/hardware representations. This is the first-ever research to articulate canonical SCOs and their core properties, computing applications, and representations. More specific and applicable SCOs are compellingly desired as the future study. Properly linked to building information modeling and Internet of Things, SCOs can enable a safer, greener, more efficient, and more effective construction system that has ever been seen.postprin
Quantum cloning and the capacity of the Pauli channel
A family of quantum cloning machines is introduced that produce two
approximate copies from a single quantum bit, while the overall input-to-output
operation for each copy is a Pauli channel. A no-cloning inequality is derived,
describing the balance between the quality of the two copies. This also
provides an upper bound on the quantum capacity of the Pauli channel with
probabilities , and . The capacity is shown to be vanishing if
lies outside an ellipsoid whose pole
coincides with the depolarizing channel that underlies the universal cloning
machine.Comment: 5 pages RevTeX, 3 Postscript figure
Optimal Eavesdropping in Quantum Cryptography. II. Quantum Circuit
It is shown that the optimum strategy of the eavesdropper, as described in
the preceding paper, can be expressed in terms of a quantum circuit in a way
which makes it obvious why certain parameters take on particular values, and
why obtaining information in one basis gives rise to noise in the conjugate
basis.Comment: 7 pages, 1 figure, Latex, the second part of quant-ph/970103
Spin-polarized transport through a single-level quantum dot in the Kondo regime
Nonequilibrium electronic transport through a quantum dot coupled to
ferromagnetic leads (electrodes) is studied theoretically by the nonequilibrium
Green function technique. The system is described by the Anderson model with
arbitrary correlation parameter . Exchange interaction between the dot and
ferromagnetic electrodes is taken into account {\it via} an effective molecular
field. The following situations are analyzed numerically: (i) the dot is
symmetrically coupled to two ferromagnetic leads, (ii) one of the two
ferromagnetic leads is half-metallic with almost total spin polarization of
electron states at the Fermi level, and (iii) one of the two electrodes is
nonmagnetic whereas the other one is ferromagnetic. Generally, the Kondo peak
in the density of states (DOS) becomes spin-split when the total exchange field
acting on the dot is nonzero. The spin-splitting of the Kondo peak in DOS leads
to splitting and suppression of the corresponding zero bias anomaly in the
differential conductance.Comment: 9 pages, 7 figure
Dependence of the decoherence of polarization states in phase-damping channels on the frequency spectrum envelope of photons
We consider the decoherence of photons suffering in phase-damping channels.
By exploring the evolutions of single-photon polarization states and two-photon
polarization-entangled states, we find that different frequency spectrum
envelopes of photons induce different decoherence processes. A white frequency
spectrum can lead the decoherence to an ideal Markovian process. Some color
frequency spectrums can induce asymptotical decoherence, while, some other
color frequency spectrums can make coherence vanish periodically with variable
revival amplitudes. These behaviors result from the non-Markovian effects on
the decoherence process, which may give rise to a revival of coherence after
complete decoherence.Comment: 7 pages, 4 figures, new results added, replaced by accepted versio
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