24,585 research outputs found
Experimental study on Gaussian-modulated coherent states quantum key distribution over standard telecom fiber
In this paper, we present a fully fiber-based one-way Quantum Key
Distribution (QKD) system implementing the Gaussian-Modulated Coherent States
(GMCS) protocol. The system employs a double Mach-Zehnder Interferometer (MZI)
configuration in which the weak quantum signal and the strong Local Oscillator
(LO) go through the same fiber between Alice and Bob, and are separated into
two paths inside Bob's terminal. To suppress the LO leakage into the signal
path, which is an important contribution to the excess noise, we implemented a
novel scheme combining polarization and frequency multiplexing, achieving an
extinction ratio of 70dB. To further minimize the system excess noise due to
phase drift of the double MZI, we propose that, instead of employing phase
feedback control, one simply let Alice remap her data by performing a rotation
operation. We further present noise analysis both theoretically and
experimentally. Our calculation shows that the combined polarization and
frequency multiplexing scheme can achieve better stability in practice than the
time-multiplexing scheme, because it allows one to use matched fiber lengths
for the signal and the LO paths on both sides of the double MZI, greatly
reducing the phase instability caused by unmatched fiber lengths. Our
experimental noise analysis quantifies the three main contributions to the
excess noise, which will be instructive to future studies of the GMCS QKD
systems. Finally, we demonstrate, under the "realistic model" in which Eve
cannot control the system within Bob's terminal, a secure key rate of
0.3bit/pulse over a 5km fiber link. This key rate is about two orders of
magnitude higher than that of a practical BB84 QKD system.Comment: 21 pages, 9 figure
Glassy Dynamics in a Frustrated Spin System: Role of Defects
In an effort to understand the glass transition, the kinetics of a spin model
with frustration but no quenched randomness has been analyzed. The
phenomenology of the spin model is remarkably similiar to that of structural
glasses. Analysis of the model suggests that defects play a major role in
dictating the dynamics as the glass transition is approached.Comment: 9 pages, 5 figures, accepted in J. Phys.: Condensed Matter,
proceedings of the Trieste workshop on "Unifying Concepts in Glass Physics
Long range scattering effects on spin Hall current in -type bulk semiconductors
Employing a nonequilibrium Green's function approach, we examine the effects
of long-range hole-impurity scattering on spin-Hall current in -type bulk
semiconductors within the framework of the self-consistent Born approximation.
We find that, contrary to the null effect of short-range scattering on
spin-Hall current, long-range collisions do produce a nonvanishing contribution
to the spin-Hall current, which is independent of impurity density in the
diffusive regime and relates only to hole states near the Fermi surface. The
sign of this contribution is opposite to that of the previously predicted
disorder-independent spin-Hall current, leading to a sign change of the total
spin-Hall current as hole density varies. Furthermore, we also make clear that
the disorder-independent spin-Hall effect is a result of an interband
polarization directly induced by the dc electric field with contributions from
all hole states in the Fermi sea.Comment: 9 pages, 1 figur
New physics effects on top quark spin correlation and polarization at the LHC: a comparative study in different models
Extensions of the Standard Model often predict new chiral interactions for
top quark, which will contribute to top quark spin correlation and polarization
in production at the LHC. In this work, under the constraints from
the current Tevatron measurements, a comparative study of the spin correlation
and polarization is performed in three new physics models: the minimal
supersymmetric model without R-parity (RPV-MSSM), the third-generation enhanced
left-right model and the axigluon model. We find that the polarization
asymmetry may be enhanced to the accessible level in all these models while the
correction to the spin correlation may be detectable in the axigluon model and
the RPV-MSSM with couplings.Comment: Version in PRD (figs updated and discussions added
Phonon-drag effects on thermoelectric power
We carry out a calculation of the phonon-drag contribution to the
thermoelectric power of bulk semiconductors and quantum well structures for the
first time using the balance equation transport theory extended to the weakly
nonuniform systems. Introducing wavevector and phonon-mode dependent relaxation
times due to phonon-phonon interactions, the formula obtained can be used not
only at low temperatures where the phonon mean free path is determined by
boundary scattering, but also at high temperatures. In the linear transport
limit, is equivalent to the result obtained from the Boltzmann equation
with a relaxation time approximation. The theory is applied to experiments and
agreement is found between the theoretical predictions and experimental
results. The role of hot-electron effects in is discussed. The importance
of the contribution of to thermoelectric power in the hot-electron
transport condition is emphasized.Comment: 8 pages, REVTEX 3.0, 7 figures avilable upon reques
Shot noise of inelastic tunneling through quantum dot systems
We present a theoretical analysis of the effect of inelastic electron
scattering on current and its fluctuations in a mesoscopic quantum dot (QD)
connected to two leads, based on a recently developed nonperturbative technique
involving the approximate mapping of the many-body electron-phonon coupling
problem onto a multichannel single-electron scattering problem. In this, we
apply the B\"uttiker scattering theory of shot noise for a two-terminal
mesoscopic device to the multichannel case with differing weight factors and
examine zero-frequency shot noise for two special cases: (i) a single-molecule
QD and (ii) coupled semiconductor QDs. The nonequilibrium Green's function
method facilitates calculation of single-electron transmission and reflection
amplitudes for inelastic processes under nonequilibrium conditions in the
mapping model. For the single-molecule QD we find that, in the presence of the
electron-phonon interaction, both differential conductance and differential
shot noise display additional peaks as bias-voltage increases due to
phonon-assisted processes. In the case of coupled QDs, our nonperturbative
calculations account for the electron-phonon interaction on an equal footing
with couplings to the leads, as well as the coupling between the two dots. Our
results exhibit oscillations in both the current and shot noise as functions of
the energy difference between the two QDs, resulting from the spontaneous
emission of phonons in the nonlinear transport process. In the "zero-phonon"
resonant tunneling regime, the shot noise exhibits a double peak, while in the
"one-phonon" region, only a single peak appears.Comment: 10 pages, 6 figures, some minor changes, accepted by Phys. Rev.
Search for IR Emission from Intracluster Dust in A2029
We have searched for IR emission from the intracluster dust (ICD) in the
galaxy cluster A2029. Weak signals of enhanced extended emission in the cluster
are detected at both 24 and 70 micron. However, the signals are
indistinguishable from the foreground fluctuations. The 24 versus 70 micron
color map does not discriminate the dust emission in the cluster from the
cirrus emission. After excluding the contamination from the point sources, we
obtain upper limits for the extended ICD emission in A2029, 5 x 10^3 Jy/sr at
24 micron and 5 x 10^4 Jy/sr at 70 micron. The upper limits are generally
consistent with the expectation from theoretical calculations and support a
dust deficiency in the cluster compared to the ISM in our galaxy. Our results
suggest that even with the much improved sensitivity of current IR telescopes,
a clear detection of the IR emission from ICD may be difficult due to cirrus
noise.Comment: 5 pages, 4 figures, accepted by ApJ
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