109 research outputs found
Heralded quantum entanglement between two crystals
Quantum networks require the crucial ability to entangle quantum nodes. A
prominent example is the quantum repeater which allows overcoming the distance
barrier of direct transmission of single photons, provided remote quantum
memories can be entangled in a heralded fashion. Here we report the observation
of heralded entanglement between two ensembles of rare-earth-ions doped into
separate crystals. A heralded single photon is sent through a 50/50
beamsplitter, creating a single-photon entangled state delocalized between two
spatial modes. The quantum state of each mode is subsequently mapped onto a
crystal, leading to an entangled state consisting of a single collective
excitation delocalized between two crystals. This entanglement is revealed by
mapping it back to optical modes and by estimating the concurrence of the
retrieved light state. Our results highlight the potential of rare-earth-ions
doped crystals for entangled quantum nodes and bring quantum networks based on
solid-state resources one step closer.Comment: 10 pages, 5 figure
Entanglement of spin waves among four quantum memories
Quantum networks are composed of quantum nodes that interact coherently by
way of quantum channels and open a broad frontier of scientific opportunities.
For example, a quantum network can serve as a `web' for connecting quantum
processors for computation and communication, as well as a `simulator' for
enabling investigations of quantum critical phenomena arising from interactions
among the nodes mediated by the channels. The physical realization of quantum
networks generically requires dynamical systems capable of generating and
storing entangled states among multiple quantum memories, and of efficiently
transferring stored entanglement into quantum channels for distribution across
the network. While such capabilities have been demonstrated for diverse
bipartite systems (i.e., N=2 quantum systems), entangled states with N > 2 have
heretofore not been achieved for quantum interconnects that coherently `clock'
multipartite entanglement stored in quantum memories to quantum channels. Here,
we demonstrate high-fidelity measurement-induced entanglement stored in four
atomic memories; user-controlled, coherent transfer of atomic entanglement to
four photonic quantum channels; and the characterization of the full
quadripartite entanglement by way of quantum uncertainty relations. Our work
thereby provides an important tool for the distribution of multipartite
entanglement across quantum networks.Comment: 4 figure
Effects of decoherence and errors on Bell-inequality violation
We study optimal conditions for violation of the Clauser-Horne-Shimony-Holt
form of the Bell inequality in the presence of decoherence and measurement
errors. We obtain all detector configurations providing the maximal Bell
inequality violation for a general (pure or mixed) state. We consider local
decoherence which includes energy relaxation at the zero temperature and
arbitrary dephasing. Conditions for the maximal Bell-inequality violation in
the presence of decoherence are analyzed both analytically and numerically for
the general case and for a number of important special cases. Combined effects
of measurement errors and decoherence are also discussed.Comment: 18 pages, 5 figure
Towards Quantum Repeaters with Solid-State Qubits: Spin-Photon Entanglement Generation using Self-Assembled Quantum Dots
In this chapter we review the use of spins in optically-active InAs quantum
dots as the key physical building block for constructing a quantum repeater,
with a particular focus on recent results demonstrating entanglement between a
quantum memory (electron spin qubit) and a flying qubit (polarization- or
frequency-encoded photonic qubit). This is a first step towards demonstrating
entanglement between distant quantum memories (realized with quantum dots),
which in turn is a milestone in the roadmap for building a functional quantum
repeater. We also place this experimental work in context by providing an
overview of quantum repeaters, their potential uses, and the challenges in
implementing them.Comment: 51 pages. Expanded version of a chapter to appear in "Engineering the
Atom-Photon Interaction" (Springer-Verlag, 2015; eds. A. Predojevic and M. W.
Mitchell
Exact results on decoherence and entanglement in a system of N driven atoms and a dissipative cavity mode
We solve the dynamics of an open quantum system where N strongly driven
two-level atoms are equally coupled on resonance to a dissipative cavity mode.
Analytical results are derived on decoherence, entanglement, purity, atomic
correlations and cavity field mean photon number. We predict decoherence-free
subspaces for the whole system and the N-qubit subsystem, the monitoring of
quantum coherence and purity decay by atomic populations measurements, the
conditional generation of atomic multi-partite entangled states and of cavity
cat-like states. We show that the dynamics of atoms prepared in states
invariant under permutation of any two components remains restricted within the
subspace spanned by the completely symmetric Dicke states. We discuss examples
and applications in the cases N=3,4.Comment: 7 pages, 4 figures, accepted in EPJ
Reduction of heating rate in a microfabricated ion trap by pulsed-laser cleaning
Laser-cleaning of the electrodes in a planar micro-fabricated ion trap has
been attempted using ns pulses from a tripled Nd:YAG laser at 355nm. The effect
of the laser pulses at several energy density levels has been tested by
measuring the heating rate of a single 40Ca+ trapped ion as a function of its
secular frequency. A reduction of the electric-field noise spectral density by
~50% has been observed and a change in the frequency dependence also noticed.
This is the first reported experiment where the "anomalous heating" phenomenon
has been reduced by removing the source as opposed to reducing its thermal
driving by cryogenic cooling. This technique may open the way to better control
of the electrode surface quality in ion microtraps
Entangling the free motion of a particle pair: an experimental scenario
The concept of dissociation-time entanglement provides a means of manifesting
non-classical correlations in the motional state of two counter-propagating
atoms. In this article, we discuss in detail the requirements for a specific
experimental implementation, which is based on the Feshbach dissociation of a
molecular Bose-Einstein condensate of fermionic lithium. A sequence of two
magnetic field pulses serves to delocalize both of the dissociation products
into a superposition of consecutive wave packets, which are separated by a
macroscopic distance. This allows to address them separately in a switched
Mach-Zehnder configuration, permitting to conduct a Bell experiment with simple
position measurements. We analyze the expected form of the two-particle wave
function in a concrete experimental setup that uses lasers as atom guides.
Assuming viable experimental parameters the setup is shown to be capable of
violating a Bell inequality.Comment: 9 pages, 3 figures; corresponds to published versio
Ion-trap quantum information processing: experimental status
Atomic ions trapped in ultra-high vacuum form an especially well-understood
and useful physical system for quantum information processing. They provide
excellent shielding of quantum information from environmental noise, while
strong, well-controlled laser interactions readily provide quantum logic gates.
A number of basic quantum information protocols have been demonstrated with
trapped ions. Much current work aims at the construction of large-scale
ion-trap quantum computers using complex microfabricated trap arrays. Several
groups are also actively pursuing quantum interfacing of trapped ions with
photons.Comment: review article for Frontiers of Physics replace corrupted TeX fil
Genetic Architecture of Hybrid Male Sterility in Drosophila: Analysis of Intraspecies Variation for Interspecies Isolation
Background: The genetic basis of postzygotic isolation is a central puzzle in evolutionary biology. Evolutionary forces causing hybrid sterility or inviability act on the responsible genes while they still are polymorphic, thus we have to study these traits as they arise, before isolation is complete. Methodology/Principal Findings: Isofemale strains of D. mojavensis vary significantly in their production of sterile F 1 sons when females are crossed to D. arizonae males. We took advantage of the intraspecific polymorphism, in a novel design, to perform quantitative trait locus (QTL) mapping analyses directly on F1 hybrid male sterility itself. We found that the genetic architecture of the polymorphism for hybrid male sterility (HMS) in the F1 is complex, involving multiple QTL, epistasis, and cytoplasmic effects. Conclusions/Significance: The role of extensive intraspecific polymorphism, multiple QTL, and epistatic interactions in HMS in this young species pair shows that HMS is arising as a complex trait in this system. Directional selection alone would be unlikely to maintain polymorphism at multiple loci, thus we hypothesize that directional selection is unlikely to be the only evolutionary force influencing postzygotic isolation
Widespread Over-Expression of the X Chromosome in Sterile F1 Hybrid Mice
The X chromosome often plays a central role in hybrid male sterility between species, but it is unclear if this reflects underlying regulatory incompatibilities. Here we combine phenotypic data with genome-wide expression data to directly associate aberrant expression patterns with hybrid male sterility between two species of mice. We used a reciprocal cross in which F1 males are sterile in one direction and fertile in the other direction, allowing us to associate expression differences with sterility rather than with other hybrid phenotypes. We found evidence of extensive over-expression of the X chromosome during spermatogenesis in sterile but not in fertile F1 hybrid males. Over-expression was most pronounced in genes that are normally expressed after meiosis, consistent with an X chromosome-wide disruption of expression during the later stages of spermatogenesis. This pattern was not a simple consequence of faster evolutionary divergence on the X chromosome, because X-linked expression was highly conserved between the two species. Thus, transcriptional regulation of the X chromosome during spermatogenesis appears particularly sensitive to evolutionary divergence between species. Overall, these data provide evidence for an underlying regulatory basis to reproductive isolation in house mice and underscore the importance of transcriptional regulation of the X chromosome to the evolution of hybrid male sterility
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