122 research outputs found
Experiments on the twisted vortex state in superfluid 3He-B
We have performed measurements and numerical simulations on a bundle of
vortex lines which is expanding along a rotating column of initially
vortex-free 3He-B. Expanding vortices form a propagating front: Within the
front the superfluid is involved in rotation and behind the front the twisted
vortex state forms, which eventually relaxes to the equilibrium vortex state.
We have measured the magnitude of the twist and its relaxation rate as function
of temperature above 0.3Tc. We also demonstrate that the integrity of the
propagating vortex front results from axial superfluid flow, induced by the
twist.Comment: prepared for proceedings of the QFS2007 symposium in Kaza
Dynamic Remanent Vortices in Superfluid 3He-B
We investigate the decay of vortices in a rotating cylindrical sample of
3He-B, after rotation has been stopped. With decreasing temperature vortex
annihilation slows down as the damping in vortex motion, the mutual friction
dissipation \alpha(T), decreases almost exponentially. Remanent vortices then
survive for increasingly long periods, while they move towards annihilation in
zero applied flow. After a waiting period \Delta t at zero flow, rotation is
reapplied and the remnants evolve to rectilinear vortices. By counting these
lines, we measure at temperatures above the transition to turbulence ~0.6T_c
the number of remnants as a function of \alpha(T) and \Delta t. At temperatures
below the transition to turbulence T \lesssim 0.55 T_c, remnants expanding in
applied flow become unstable and generate in a turbulent burst the equilibrium
number of vortices. Here we measure the onset temperature T_on of turbulence as
a function of \Delta t, applied flow velocity, and length of sample L.Comment: Submitted to the proceedings of the Quantum Fluids and Solids
Conference 2006 (to be published in Journal of Low Temperature Physics 2007)
New data are adde
The dynamics of vortex generation in superfluid 3He-B
A profound change occurs in the stability of quantized vortices in externally
applied flow of superfluid 3He-B at temperatures ~ 0.6 Tc, owing to the rapidly
decreasing damping in vortex motion with decreasing temperature. At low damping
an evolving vortex may become unstable and generate a new independent vortex
loop. This single-vortex instability is the generic precursor to turbulence. We
investigate the instability with non-invasive NMR measurements on a rotating
cylindrical sample in the intermediate temperature regime (0.3 - 0.6) Tc. From
comparisons with numerical calculations we interpret that the instability
occurs at the container wall, when the vortex end moves along the wall in
applied flow.Comment: revised & extended version. Journal of Low Temperature Physics,
accepted (2008
Vortex core contribution to textural energy in 3He-B below 0.4Tc
Vortex lines affect the spatial order-parameter distribution in superfluid
3He-B owing to superflow circulating around vortex cores and due to the
interaction of the order parameter in the core and in the bulk as a result of
superfluid coherence over the whole volume. The step-like change of the latter
contribution at 0.6Tc (at a pressure of 29bar) signifies the transition from
axisymmetric cores at higher temperatures to broken-symmetry cores at lower
temperatures. We extended earlier measurements of the core contribution to
temperatures below 0.2Tc, in particular searching for a possible new core
transition to lower symmetries. As a measuring tool we track the energy levels
of magnon condensate states in a trap formed by the order-parameter texture.Comment: 13 pages, 10 figures, submitted to proceedings of the QFS2010
conferenc
Quantum walks: a comprehensive review
Quantum walks, the quantum mechanical counterpart of classical random walks,
is an advanced tool for building quantum algorithms that has been recently
shown to constitute a universal model of quantum computation. Quantum walks is
now a solid field of research of quantum computation full of exciting open
problems for physicists, computer scientists, mathematicians and engineers.
In this paper we review theoretical advances on the foundations of both
discrete- and continuous-time quantum walks, together with the role that
randomness plays in quantum walks, the connections between the mathematical
models of coined discrete quantum walks and continuous quantum walks, the
quantumness of quantum walks, a summary of papers published on discrete quantum
walks and entanglement as well as a succinct review of experimental proposals
and realizations of discrete-time quantum walks. Furthermore, we have reviewed
several algorithms based on both discrete- and continuous-time quantum walks as
well as a most important result: the computational universality of both
continuous- and discrete- time quantum walks.Comment: Paper accepted for publication in Quantum Information Processing
Journa
On-chip generation of high-dimensional entangled quantum states and their coherent control
Optical quantum states based on entangled photons are essential for solving questions in fundamental physics and are at the heart of quantum information science1. Specifically, the realization of high-dimensional states (D-level quantum systems, that is, qudits, with D > 2) and their control are necessary for fundamental investigations of quantum mechanics2, for increasing the sensitivity of quantum imaging schemes3, for improving the robustness and key rate of quantum communication protocols4, for enabling a richer variety of quantum simulations5, and for achieving more efficient and error-tolerant quantum computation6. Integrated photonics has recently become a leading platform for the compact, cost-efficient, and stable generation and processing of non-classical optical states7. However, so far, integrated entangled quantum sources have been limited to qubits (D = 2)8, 9, 10, 11. Here we demonstrate on-chip generation of entangled qudit states, where the photons are created in a coherent superposition of multiple high-purity frequency modes. In particular, we confirm the realization of a quantum system with at least one hundred dimensions, formed by two entangled qudits with D = 10. Furthermore, using state-of-the-art, yet off-the-shelf telecommunications components, we introduce a coherent manipulation platform with which to control frequency-entangled states, capable of performing deterministic high-dimensional gate operations. We validate this platform by measuring Bell inequality violations and performing quantum state tomography. Our work enables the generation and processing of high-dimensional quantum states in a single spatial mode
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