41,050 research outputs found
Entanglement production due to quench dynamics of an anisotropic XY chain in a transverse field
We compute concurrence and negativity as measures of two-site entanglement
generated by a power-law quench (characterized by a rate 1/tau and an exponent
alpha) which takes an anisotropic XY chain in a transverse field through a
quantum critical point (QCP). We show that only the even-neighbor pairs of
sites get entangled in such a process. Moreover, there is a critical rate of
quench, 1/tau_c, above which no two-site entanglement is generated; the entire
entanglement is multipartite. The ratio of the two-site entanglements between
consecutive even neighbors can be tuned by changing the quench rate. We also
show that for large tau, the concurrence (negativity) scales as sqrt{alpha/tau}
(alpha/tau), and we relate this scaling behavior to defect production by the
quench through a QCP.Comment: 5 pages including 4 figures; added a figure on multipartite
entanglement and some references -- this is the published versio
Effective g-factor in Majorana Wires
We use the effective g-factor of subgap states, g*, in hybrid InAs nanowires
with an epitaxial Al shell to investigate how the superconducting density of
states is distributed between the semiconductor core and the metallic shell. We
find a step-like reduction of g* and improved hard gap with reduced carrier
density in the nanowire, controlled by gate voltage. These observations are
relevant for Majorana devices, which require tunable carrier density and g*
exceeding the g-factor of the proximitizing superconductor. Additionally, we
observe the closing and reopening of a gap in the subgap spectrum coincident
with the appearance of a zero-bias conductance peak
New Consequences of Induced Transparency in a Double-Lambda scheme: Destructive Interference In Four-wave Mixing
We investigate a four-state system interacting with long and short laser
pulses in a weak probe beam approximation. We show that when all lasers are
tuned to the exact unperturbed resonances, part of the four-wave mixing (FWM)
field is strongly absorbed. The part which is not absorbed has the exact
intensity required to destructively interfere with the excitation pathway
involved in producing the FWM state. We show that with this three-photon
destructive interference, the conversion efficiency can still be as high as
25%. Contrary to common belief,our calculation shows that this process, where
an ideal one-photon electromagnetically induced transparency is established, is
not most suitable for high efficiency conversion. With appropriate
phase-matching and propagation distance, and when the three-photon destructive
interference does not occur, we show that the photon flux conversion efficiency
is independent of probe intensity and can be close to 100%. In addition, we
show clearly that the conversion efficiency is not determined by the maximum
atomic coherence between two lower excited states, as commonly believed. It is
the combination of phase-matching and constructive interference involving the
two terms arising in producing the mixing wave that is the key element for the
optimized FWM generation. Indeed, in this scheme no appreciable excited state
is produced, so that the atomic coherence between states |0> and |2> is always
very small.Comment: Submitted to Phys. Rev. A, 7 pages, 4 figure
Generation of N-qubit W state with rf-SQUID qubits by adiabatic passage
A simple scheme is presented to generate n-qubit W state with
rf-superconducting quantum interference devices (rf-SQUIDs) in cavity QED
through adiabatic passage. Because of the achievable strong coupling for
rf-SQUID qubits embedded in cavity QED, we can get the desired state with high
success probability. Furthermore, the scheme is insensitive to position
inaccuracy of the rf-SQUIDs. The numerical simulation shows that, by using
present experimental techniques, we can achieve our scheme with very high
success probability, and the fidelity could be eventually unity with the help
of dissipation.Comment: to appear in Phys. Rev.
Spatial Coherence of a Polariton Condensate
We perform Young's double-slit experiment to study the spatial coherence
properties of a two-dimensional dynamic condensate of semiconductor microcavity
polaritons. The coherence length of the system is measured as a function of the
pump rate, which confirms a spontaneous buildup of macroscopic coherence in the
condensed phase. An independent measurement reveals that the position and
momentum uncertainty product of the condensate is close to the Heisenberg
limit. An experimental realization of such a minimum uncertainty wave packet of
the polariton condensate opens a door to coherent matter-wave phenomena such as
Josephson oscillation, superfluidity, and solitons in solid state condensate
systems
Technological innovations at the onset of the Mid-Pleistocene Climate Transition in high-latitude East Asia
The interplay between Pleistocene climatic variability and hominin adaptations to diverse terrestrial ecosystems is a key topic in human evolutionary studies. Early and Middle Pleistocene environmental change and its relation to hominin behavioural responses has been a subject of great interest in Africa and Europe, though little information is available for other key regions of the Old World, particularly from Eastern Asia. Here we examine key Early Pleistocene sites of the Nihewan Basin, in high-latitude northern China, dating between ∼1.4 to 1.0 million years ago (Ma). We compare stone tool assemblages from three Early Pleistocene sites in the Nihewan Basin, including detailed assessment of stone tool refitting sequences at the ∼1.1 Ma-old site of Cenjiawan. Increased toolmaking skills and technological innovations are evident in the Nihewan Basin at the onset of the Mid-Pleistocene Climate Transition (MPT). Examination of the lithic technology of the Nihewan sites, together with an assessment of other key Palaeolithic sites of China, indicates that toolkits show increasing diversity at the outset of the MPT and in its aftermath. The overall evidence indicates the adaptive flexibility of early hominins to ecosystem changes since the MPT, though regional abandonments are also apparent in high-latitudes, likely owing to cold and oscillating environmental conditions. The view presented here sharply contrasts with traditional arguments that stone tool technologies of China are homogeneous and continuous over the course of the Early Pleistocene.Introduction Results - Stone-tool-knapping skills recorded in the Cenjiawan assemblage - Technological comparisons of the Nihewan Basin assemblages Discussio
Efficient quantum cryptography network without entanglement and quantum memory
An efficient quantum cryptography network protocol is proposed with
d-dimension polarized photons, without resorting to entanglement and quantum
memory. A server on the network, say Alice, provides the service for preparing
and measuring single photons whose initial state are |0>. The users code the
information on the single photons with some unitary operations. For preventing
the untrustworthy server Alice from eavesdropping the quantum lines, a
nonorthogonal-coding technique (decoy-photon technique) is used in the process
that the quantum signal is transmitted between the users. This protocol does
not require the servers and the users to store the quantum state and almost all
of the single photons can be used for carrying the information, which makes it
more convenient for application than others with present technology. We also
discuss the case with a faint laser pulse.Comment: 4 pages, 1 figures. It also presented a way for preparing decoy
photons without a sinigle-photon sourc
Shortest-Path Fractal Dimension for Percolation in Two and Three Dimensions
We carry out a high-precision Monte Carlo study of the shortest-path fractal
dimension \dm for percolation in two and three dimensions, using the
Leath-Alexandrowicz method which grows a cluster from an active seed site. A
variety of quantities are sampled as a function of the chemical distance,
including the number of activated sites, a measure of the radius, and the
survival probability. By finite-size scaling, we determine \dm = 1.130 77(2)
and in two and three dimensions, respectively. The result in 2D
rules out the recently conjectured value \dm=217/192 [Phys. Rev. E 81,
020102(R) (2010)].Comment: 5 pages, 4 figure
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