209 research outputs found
Propagation of quantum correlations after a quench in the Mott-insulator regime of the Bose-Hubbard model
We study a quantum quench in the Bose-Hubbard model where the tunneling rate
is suddenly switched from zero to a finite value in the Mott regime. In
order to solve the many-body quantum dynamics far from equlibrium, we consider
the reduced density matrices for a finite number (one, two, three, etc.) of
lattice sites and split them up into on-site density operators, i.e., the mean
field, plus two-point and three-point correlations etc. Neglecting three-point
and higher correlations, we are able to numerically simulate the time-evolution
of the few-site density matrices and the two-point quantum correlations (e.g.,
their effective light-cone structure) for a comparably large number of lattice sites
Magnetically tuned spin dynamics resonance
We present the experimental observation of a magnetically tuned resonance
phenomenon resulting from spin mixing dynamics of ultracold atomic gases. In
particular we study the magnetic field dependence of spin conversion in F=2
87Rb spinor condensates in the crossover from interaction dominated to
quadratic Zeeman dominated dynamics. We discuss the observed phenomenon in the
framework of spin dynamics as well as matter wave four wave mixing. Furthermore
we show that the validity range of the single mode approximation for spin
dynamics is significantly extended in the regime of high magnetic field
Emergence of coherence in the Mott--superfluid quench of the Bose-Hubbard model
We study the quench from the Mott to the superfluid phase in the Bose-Hubbard
model and investigate the spatial-temporal growth of phase coherence, i.e.,
phase locking between initially uncorrelated sites. To this end, we establish a
hierarchy of correlations via a controlled expansion into inverse powers of the
coordination number . It turns out that the off-diagonal long-range order
spreads with a constant propagation speed, forming local condensate patches,
whereas the phase correlator follows a diffusion-like growth rate.Comment: 4 page
Cloning quantum entanglement in arbitrary dimensions
We have found a quantum cloning machine that optimally duplicates the
entanglement of a pair of -dimensional quantum systems. It maximizes the
entanglement of formation contained in the two copies of any
maximally-entangled input state, while preserving the separability of
unentangled input states. Moreover, it cannot increase the entanglement of
formation of all isotropic states. For large , the entanglement of formation
of each clone tends to one half the entanglement of the input state, which
corresponds to a classical behavior. Finally, we investigate a local
entanglement cloner, which yields entangled clones with one fourth the input
entanglement in the large- limit.Comment: 6 pages, 3 figure
Spatial entanglement of twin quantum images
We show that the spatial entanglement of two twin images obtained by parametric down conversion is complete, i.e., concerns both amplitude and phase. By considering a homodyne detection scheme, which allows comparison of field quadrature components of the two images pixel by pixel, Einstein-Podolsky Rosen correlations are shown to exist between symmetrical pixels of the two images. The best possible correlation is obtained by adjusting the phase profile of the local oscillator in the amplification area. The results for quadrature components hold even in the absence of any input image, i.e., for pure parametric fluorescence. In this case, they are not related to intensity and phase fluctuations
Birefringence measurements on polymer melts in an axisymmetric flow cell
The stress-optical rule relates birefringence to stress. Consequently, measurement of flow birefringence provides a non-intrusive technique of measuring stresses in complex flows. In this investigation we explore the use of an axisymmetric geometry to create a uniaxial elongational flow in polymer melts. In axisymmetric flows both birefringence and orientation angle change continuously along the path of the propagating light. Because polarimetry measurements provide the cumulative influence of the material's optical properties along the light's integrated path, this makes determination of local birefringence in the melt impossible. One can nevertheless use experimentally determined birefringence measurements to compare with predictions from computer simulations as a means of evaluating the ma! terials' governing constitutive stress equations. More specifically, in this investigation we compare the light intensity transmitted through the experimental set-up versus entry position, with the theoretically calculated transmitted intensity distribution as a means of comparing experiment and simulation. The main complication in our experiments is the use of a flowcell that necessarily consists of materials of different refractive indices. This introduces refraction and reflection effects that must be modelled before experimental results can be correctly interpreted. We describe how these effects are taken into account and test the accuracy of predictions against experiments. In addition, the high temperatures required to investigate polymer melts, mean that a further complication is introduced by thermal stresses present in the flowcell glass. We describe how these thermal-stresses are also incorporated ! nin the simulations. Finally, we present a few preliminary results and evaluate the success of the overall method
Secure Coherent-state Quantum Key Distribution Protocols with Efficient Reconciliation
We study the equivalence between a realistic quantum key distribution
protocol using coherent states and homodyne detection and a formal entanglement
purification protocol. Maximally-entangled qubit pairs that one can extract in
the formal protocol correspond to secret key bits in the realistic protocol.
More specifically, we define a qubit encoding scheme that allows the formal
protocol to produce more than one entangled qubit pair per coherent state, or
equivalently for the realistic protocol, more than one secret key bit. The
entanglement parameters are estimated using quantum tomography. We analyze the
properties of the encoding scheme and investigate its application to the
important case of the attenuation channel.Comment: REVTeX, 11 pages, 2 figure
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