37 research outputs found
Creation and counting of defects in a temperature quenched Bose-Einstein Condensate
We study the spontaneous formation of defects in the order parameter of a
trapped ultracold bosonic gas while crossing the critical temperature for
Bose-Einstein Condensation (BEC) at different rates. The system has the shape
of an elongated ellipsoid, whose transverse width can be varied to explore
dimensionality effects. For slow enough temperature quenches we find a
power-law scaling of the average defect number with the quench rate, as
predicted by the Kibble-Zurek mechanism. A breakdown of such a scaling is found
for fast quenches, leading to a saturation of the average defect number. We
suggest an explanation for this saturation in terms of the mutual interactions
among defects.Comment: 9 pages, 10 figure
Observation of Solitonic Vortices in Bose-Einstein Condensates
We observe solitonic vortices in an atomic Bose-Einstein condensate after
free expansion. Clear signatures of the nature of such defects are the twisted
planar density depletion around the vortex line, observed in absorption images,
and the double dislocation in the interference pattern obtained through
homodyne techniques. Both methods allow us to determine the sign of the
quantized circulation. Experimental observations agree with numerical
simulations. These solitonic vortices are the decay product of phase defects of
the BEC order parameter spontaneously created after a rapid quench across the
BEC transition in a cigar-shaped harmonic trap and are shown to have a very
long lifetime.Comment: 7 pages, 7 figure
Solitonic Vortices in Bose-Einstein Condensates
We analyse, theoretically and experimentally, the nature of solitonic
vortices (SV) in an elongated Bose-Einstein condensate. In the experiment, such
defects are created via the Kibble-Zurek mechanism, when the temperature of a
gas of sodium atoms is quenched across the BEC transition, and are imaged after
a free expansion of the condensate. By using the Gross-Pitaevskii equation, we
calculate the in-trap density and phase distributions characterizing a SV in
the crossover from an elongate quasi-1D to a bulk 3D regime. The simulations
show that the free expansion strongly amplifies the key features of a SV and
produces a remarkable twist of the solitonic plane due to the quantized
vorticity associated with the defect. Good agreement is found between
simulations and experiments.Comment: 6 pages, 4 figure
Dynamics and interaction of vortex lines in an elongated Bose-Einstein condensate
We study the real-time dynamics of vortex lines in a large elongated
Bose-Einstein condensate (BEC) of sodium atoms using a stroboscopic technique.
Vortices are spontaneously produced via the Kibble-Zurek mechanism in a quench
across the BEC transition and then they slowly precess keeping their
orientation perpendicular to the long axis of the trap as expected for
solitonic vortices in a highly anisotropic condensate. Good agreement with
theoretical predictions is found for the precession period as a function of the
orbit amplitude and the number of condensed atoms. In configurations with two
or more vortex lines, we see signatures of vortex-vortex interaction in the
shape and visibility of the orbits. In addition, when more than two vortices
are present, their decay is faster than the thermal decay observed for one or
two vortices. The possible role of vortex reconnection processes is discussed.Comment: 4 pages, 4 figure
Probing multipulse laser ablation by means of self-mixing interferometry
In this work, self-mixing interferometry (SMI) is implemented inline to a
laser microdrilling system to monitor the machining process by probing the
ablation-induced plume. An analytical model based on the Sedov-Taylor blast
wave equation is developed for the expansion of the process plume under
multiple-pulse laser percussion drilling conditions. Signals were acquired
during laser microdrilling of blind holes on stainless steel, copper alloy,
pure titanium, and titanium nitride ceramic coating. The maximum optical path
difference was measured from the signals to estimate the refractive index
changes. An amplitude coefficient was derived by fitting the analytical model
to the measured optical path differences. The morphology of the drilled holes
was investigated in terms of maximum hole depth and dross height. The results
indicate that the SMI signal rises when the ablation process is dominated by
vaporization, changing the refractive index of the processing zone
significantly. Such ablation conditions correspond to limited formation of
dross. The results imply that SMI can be used as a nonintrusive tool in laser
micromachining applications for monitoring the process quality in an indirect
way
Interplay between powder catchment efficiency and layer height in self-stabilized laser metal deposition
In laser metal deposition (LMD) the height of the deposited track can vary
within and between layers, causing significant deviations during the process
evolution. Previous works have shown that in certain conditions a
self-stabilizing mechanism occurs, maintaining a regular height growth and a
constant standoff distance between the part and the deposition nozzle. Here we
analyze the link between the powder catchment efficiency and the deposition
height stability. To this purpose, a monitoring system was developed to study
the deposition in different process conditions, using inline measurements of
the specimen weight in combination with the layer height information obtained
with coaxial optical triangulation. An analytical model was used to estimate
the deposition efficiency in real-time from the height monitoring and the
process parameters, which was verified by the direct mass measurements. The
results show that the track height stabilization is associated to a reduction
of the powder catchment efficiency, which is governed by the melt pool relative
position with respect to the powder cone and the laser beam. For a given set of
parameters, the standoff distance can be estimated to achieve the highest
powder catchment efficiency and a regular height through the build direction
Phase Noise in Real-World Twin-Field Quantum Key Distribution
We investigate the impact of noise sources in real-world implementations of
Twin-Field Quantum Key Distribution (TF-QKD) protocols, focusing on phase noise
from photon sources and connecting fibers. Our work emphasizes the role of
laser quality, network topology, fiber length, arm balance, and detector
performance in determining key rates. Remarkably, it reveals that the leading
TF-QKD protocols are similarly affected by phase noise despite different
mechanisms. Our study demonstrates duty cycle improvements of over 2x through
narrow-linewidth lasers and phase-control techniques, highlighting the
potential synergy with high-precision time/frequency distribution services.
Ultrastable lasers, evolving toward integration and miniaturization, offer
promise for agile TF-QKD implementations on existing networks. Properly
addressing phase noise and practical constraints allows for consistent key rate
predictions, protocol selection, and layout design, crucial for establishing
secure long-haul links for the Quantum Communication Infrastructures under
development in several countries.Comment: 18 pages, 8 figures, 2 table
Spontaneous creation of Kibble-Zurek solitons in a Bose-Einstein condensate
When a system crosses a second-order phase transition on a finite timescale,
spontaneous symmetry breaking can cause the development of domains with
independent order parameters, which then grow and approach each other creating
boundary defects. This is known as Kibble-Zurek mechanism. Originally
introduced in cosmology, it applies both to classical and quantum phase
transitions, in a wide variety of physical systems. Here we report on the
spontaneous creation of solitons in Bose-Einstein condensates via the
Kibble-Zurek mechanism. We measure the power-law dependence of defects number
with the quench time, and provide a check of the Kibble-Zurek scaling with the
sonic horizon. These results provide a promising test bed for the determination
of critical exponents in Bose-Einstein condensates.Comment: 7 pages, 4 figure
Coherent phase transfer for real-world twin-field quantum key distribution
Quantum mechanics allows distribution of intrinsically secure encryption keys by optical means. Twin-field quantum key distribution is one of the most promising techniques for its implementation on long-distance fiber networks, but requires stabilizing the optical length of the communication channels between parties. In proof-of-principle experiments based on spooled fibers, this was achieved by interleaving the quantum communication with periodical stabilization frames. In this approach, longer duty cycles for the key streaming come at the cost of a looser control of channel length, and a successful key-transfer using this technique in real world remains a significant challenge. Using interferometry techniques derived from frequency metrology, we develop a solution for the simultaneous key streaming and channel length control, and demonstrate it on a 206 km field-deployed fiber with 65 dB loss. Our technique reduces the quantum-bit-error-rate contributed by channel length variations to <1%, representing an effective solution for real-world quantum communications