Exotic braids for cold atoms: simulating anyons in the lab
We present a method to simulate quantum computations by the creation,
transport, and fusion of anyonic particles in a planar system of cold
atoms in an optical lattice. Particles in our three-dimensional world
come in two fundamentally different types depending on their
statistical properties, determined by their behaviour under exchange,
or braiding, of a pair of particles: Fermions, building blocks of
matter, and bosons, carriers of interactions (or forces.) In planar
materials, however, quantum mechanics allows for anyons, particles
with exotic braiding and statistical properties, which in some cases
may be used to perform universal quantum computations. Accordingly,
some planar systems with anyons have been proposed as architectures
for a fault-tolerant quantum computer, where operations are carried
out by creating, moving, and fusing these particles. In this paper we
show how these operations can be performed on a suitable planar system
simulated in a gas of cold atoms trapped in an optical lattice, using
atoms of a different species as "moving heads" to control and drive
the computations. Our method requires putting together already
demonstrated experimental techniques, which makes a simulation of
anyonic properties feasible in the lab.
***
LX10901
MAGNETIC TURBULENT WAVES
Wave turbulence describes the statistical behavior of a set of
randomly interacting waves and therefore has been applied to a great
variety of systems (ocean surface waves, plasma waves in solar wind,
spin waves in solids…), but few laboratory experiments have been
performed so far. Now, in a paper appearing in Physical Review
Letters, François Boyer and Eric Falcon of the University Paris
Diderot, France, report the first observation of magnetic wave
turbulence on the surface of a ferrofluid submitted to a magnetic
field, a regime that has not yet been envisaged in theoretical studies.
When wave amplitudes are high enough, the wave turbulence theory
predicts a nonlinear resonant process between waves which generates
smaller wavelengths. In a ferrofluid (a fluid with a suspension of
nanometric magnetic particles), the dispersion relation of surface
waves was known to be tuned by a magnetic field. This leads the
authors to the first observation of a magnetic wave turbulence
regime. The existence domains of gravity and capillary wave
turbulence are also documented as well as a triple point of
coexistence of these three regimes: these new results are understood
using dimensional analysis. Such an experimental system where the
dispersion relation is tuned by the operator from a non dispersive to
a dispersive system is thus of primary interest to test the wave
turbulence theory.
***
BUR1096B
Surface Superconductivity and Paramagnetic Meissner Effect in a
conventional type-II Superconductor
New results pertaining to the simultaneous observation of surface
superconductivity and paramagnetic Meissner effect in a high purity single
crystal of elemental Nb have revealed the sequence of steps leading to an
eventual stabilization of ordered flux line lattice in a type-II
superconductor. Superconductors are materials with zero electrical
resistance and are classified into two categories (type-I and type-II) on
the basis of their magnetic characteristics. Using a mean field
description, Abrikosov had predicted the nucleation of quantized flux
lines below the upper critical field (Hc2) in type-II superconductors. The
temperature dependence of the shielding response of a small superimposed
ac field has elucidated that superconductivity nucleates first near the
surface of a Nb sphere at a threshold field value (Hc3), much larger than
Hc2. As the super currents flow, the magnetic flux gets compressed within
the sphere, resulting in a positive dc magnetization signal (often termed
as Paramagnetic Meissner Effect) for field values lying in between Hc3 and
Hc2. Diamagnetic signal appears when the quantized flux lines get formed
below Hc2. Initially this vortex array is disordered (amorphous), however,
the spatial order among these vortices emerges (Abrikosov vortex lattice)
below the onset temperature of peak effect phenomenon in the critical
current density of weakly (collective) pinned array of vortices.
Accordingly we can define a new (H,T) phase diagram in a realistic
specimen of a high purity type-II superconductor, as given in the figure.
***
EV10436
Making (One-way) Waves
We have constructed a unique mechanical arrangement of one-way
coupled oscillators that enables the endless propagation of solitary
waves, but only in one direction, and only if there is an odd number
of elements. Typical oscillators, such as the coiled springs in a
mattress, are coupled so that vibrating one vibrates the others, and
vice versa, a consequence of Newton's famous third law of action and
reaction. However, in our array, disturbances propagate in only one
direction. Each oscillator is an inverted pendulum restrained by
springs, whose left-right oscillation directs jets of falling water
onto the next oscillator. An even number of such oscillators reaches
a quiescent equilibrium where the pendulums alternate left-right in
pairs. However, for odd arrays, there is always one unpaired
oscillator that frustrates this quiescent equilibrium and propagates
indefinitely as a single left-right pendulum oscillation. Our first
prototype array was circular and made from K’NEX construction
pieces, while our final apparatus is linear with interconnected ends
and constructed from machined aluminum, PVC pipe, rubber hoses, and
other inexpensive materials. Our array realizes an extreme case of
wave propagation in anisotropic media and is a dramatic mechanical
example of one-way coupling.
***
LM11138E
A New View of Cavitation
Cavitation is a fascinating phenomenon in which a liquid forms bubbles
of vapour and gas as a result of a variation of pressure. The bubbles
themselves usually attract most attention: their collapse leads to huge
temperatures and pressures, making them suitable for use as tiny
chemical reactors. We have examined cavitation from a different point of
view: not as an evolution of the bubbles, but as an evolution of the
molecules of gas that actually comprise the bubbles.
Imagine that as soon as a gas molecule enters a cavitating bubble, we
colour it black. How long is it before all gas molecules in the vessel
are blackened? For how long do the gas molecules reside in cavitating
bubbles? To address such questions, we have measured, for the first
time, the dynamics of both the dissolved gas and the liquid in a fluid,
in which cavitation is caused by a strong acoustic field. We dissolved
an NMR-sensitive gas in water and used Magnetic Resonance Imaging to
trace its motion and, separately, the motion of the water. The motion of
the water and the dissolved gas can be very different, because the gas
jumps from its dissolved state into and out of the cavitating bubbles
and travels with them. The residence time for gas molecules inside the
bubble can be very short, on the order of two oscillation cycles.
Depending on the cavitation intensity, all molecules of dissolved gas in
the vessel can go through the "bubbly" state within a second.
***
YU10004
Link Maps – Helping first year physics students stay on course
A new type of concept map helps first year students without a physics
background achieve a whole letter grade better than their peers! Concept
maps are a well known study aid, but they have not been used extensively
in physics – until now. At the University of Sydney a new type of concept
map, dubbed Link Maps, focus on the central concepts covered in the first
year physics course, and the equations which show how they are related. In
the pilot study, first year students without background in physics were
invited to attend one extra tutorial per week, called Map Meetings. In
these, Link Maps were the central feature around which the relevant weekly
topic was covered and problems were solved. The Map Meetings were very
popular; weekly attracting around 20% of the population comprising 351
students. However, in addition to their popularity, students who
consistently attended Map Meetings achieved 9 marks out of 90 (translating
to a whole letter grade) better in the final exam than students with
comparable academic background who were unable to attend the Map Meetings.
***
LN11136
Evidence for magnetic proximity effect up to room
temperature in Fe(Ga,Mn)As interfaces
The control of the mutual interaction between magnetically
''active'' layers at the interface of heterostructures is
a major challenge in solid state physics and one of the
key points of spintronics research. Diluted magnetic
semiconductors (DMS), in which magnetic impurities are
artificially imbedded into a semiconductor host crystal
lattice may allow to integrate the spin degree of freedom
and semiconducting properties in a single material.
Although the correlation between magnetic and transport
properties in DMS is likely to be a crucial ingredient in
possible applications, the physical mechanisms underlying
the magnetic properties are still a matter of an intense
debate. We report experimental evidence of magnetic
coupling at the interface between a thin ferromagnetic Fe
film and a DMS (in the present case (GaMn)As). Our
results reveal the presence of a long range ferromagnetic
order, up to room temperature, in a (Ga,Mn)As region as
thick as 2 nm induced by the ferromagnetic order of the Fe
film. The Mn magnetization at the interface is aligned
antiparallel with respect to the magnetization direction
of Fe, and increases with the thickness of the Fe
overlayer
