2,024 research outputs found
Reconnections of quantized vortex rings in superfluid He at very low temperatures
Collisions in a beam of unidirectional quantized vortex rings of nearly
identical radii in superfluid He in the limit of zero temperature (0.05
K) were studied using time-of-flight spectroscopy. Reconnections between two
primary rings result in secondary vortex loops of both smaller and larger
radii. Discrete steps in the distribution of flight times, due to the limits on
the earliest possible arrival times of secondary loops created after either one
or two consecutive reconnections, are observed. The density of primary rings
was found to be capped at the value independent of
the injected density. This is due to collisions between rings causing piling-up
of many other vortex rings. Both observations are in quantitative agreement
with our theory.Comment: 7 pages, 4 figures, includes supplementary materia
No Effect of Steady Rotation on Solid He in a Torsional Oscillator
We have measured the response of a torsional oscillator containing
polycrystalline hcp solid He to applied steady rotation in an attempt to
verify the observations of several other groups that were initially interpreted
as evidence for macroscopic quantum effects. The geometry of the cell was that
of a simple annulus, with a fill line of relatively narrow diameter in the
centre of the torsion rod. Varying the angular velocity of rotation up to
2\,rad\,s showed that there were no step-like features in the resonant
frequency or dissipation of the oscillator and no history dependence, even
though we achieved the sensitivity required to detect the various effects seen
in earlier experiments on other rotating cryostats. All small changes during
rotation were consistent with those occurring with an empty cell. We thus
observed no effects on the samples of solid He attributable to steady
rotation.Comment: 8 pages, 3 figures, accepted in J. Low Temp. Phy
Photonic Maxwell's demon
We report an experimental realisation of Maxwell's demon in a photonic setup.
We show that a measurement at the single-photon level followed by a
feed-forward operation allows the extraction of work from intense thermal light
into an electric circuit. The interpretation of the experiment stimulates the
derivation of a new equality relating work extraction to information acquired
by measurement. We derive a bound using this relation and show that it is in
agreement with the experimental results. Our work puts forward photonic systems
as a platform for experiments related to information in thermodynamics.Comment: 8 pages, 3 figure
Benchmarking of Gaussian boson sampling using two-point correlators
Gaussian boson sampling is a promising scheme for demonstrating a quantum
computational advantage using photonic states that are accessible in a
laboratory and, thus, offer scalable sources of quantum light. In this
contribution, we study two-point photon-number correlation functions to gain
insight into the interference of Gaussian states in optical networks. We
investigate the characteristic features of statistical signatures which enable
us to distinguish classical from quantum interference. In contrast to the
typical implementation of boson sampling, we find additional contributions to
the correlators under study which stem from the phase dependence of Gaussian
states and which are not observable when Fock states interfere. Using the first
three moments, we formulate the tools required to experimentally observe
signatures of quantum interference of Gaussian states using two outputs only.
By considering the current architectural limitations in realistic experiments,
we further show that a statistically significant discrimination between quantum
and classical interference is possible even in the presence of loss, noise, and
a finite photon-number resolution. Therefore, we formulate and apply a
theoretical framework to benchmark the quantum features of Gaussian boson
sampling under realistic conditions
Encoding a qubit into multilevel subspaces
We present a formalism for encoding the logical basis of a qubit into
subspaces of multiple physical levels. The need for this multilevel encoding
arises naturally in situations where the speed of quantum operations exceeds
the limits imposed by the addressability of individual energy levels of the
qubit physical system. A basic feature of the multilevel encoding formalism is
the logical equivalence of different physical states and correspondingly, of
different physical transformations. This logical equivalence is a source of a
significant flexibility in designing logical operations, while the multilevel
structure inherently accommodates fast and intense broadband controls thereby
facilitating faster quantum operations. Another important practical advantage
of multilevel encoding is the ability to maintain full quantum-computational
fidelity in the presence of mixing and decoherence within encoding subspaces.
The formalism is developed in detail for single-qubit operations and
generalized for multiple qubits. As an illustrative example, we perform a
simulation of closed-loop optimal control of single-qubit operations for a
model multilevel system, and subsequently apply these operations at finite
temperatures to investigate the effect of decoherence on operational fidelity.Comment: IOPart LaTeX, 2 figures, 31 pages; addition of a numerical simulatio
Strategies for enhancing quantum entanglement by local photon subtraction
Subtracting photons from a two-mode squeezed state is a well-known method to
increase entanglement. We analyse different strategies of local photon
subtraction from a two-mode squeezed state in terms of entanglement gain and
success probability. We develop a general framework that incorporates
imperfections and losses in all stages of the process: before, during, and
after subtraction. By combining all three effects into a single efficiency
parameter, we provide analytical and numerical results for subtraction
strategies using photon-number-resolving and threshold detectors. We compare
the entanglement gain afforded by symmetric and asymmetric subtraction
scenarios across the two modes. For a given amount of loss, we identify an
optimised set of parameters, such as initial squeezing and subtraction beam
splitter transmissivity, that maximise the entanglement gain rate. We identify
regimes for which asymmetric subtraction of different Fock states on the two
modes outperforms symmetric strategies. In the lossless limit, subtracting a
single photon from one mode always produces the highest entanglement gain rate.
In the lossy case, the optimal strategy depends strongly on the losses on each
mode individually, such that there is no general optimal strategy. Rather,
taking losses on each mode as the only input parameters, we can identify the
optimal subtraction strategy and required beam splitter transmissivities and
initial squeezing parameter. Finally, we discuss the implications of our
results for the distillation of continuous-variable quantum entanglement.Comment: 13 pages, 11 figures. Updated version for publicatio
Ultrasonic irrigation flows in root canals:effects of ultrasound power and file insertion depth
Ultrasonic irrigation during root canal treatment can enhance biofilm disruption. The challenge is to improve the fluid flow so that the irrigant reaches areas inaccessible to hand instrumentation. The aim of this study is to experimentally investigate how the flow field and hydrodynamic forces induced by ultrasonic irrigation are influenced by the ultrasound power and file insertion depth. A root canal phantom was 3D printed and used as a mold for the fabrication of a PDMS channel. An ultrasonic instrument with a #15 K-file provided the irrigation. The flow field was studied by means of Particle Image Velocimetry (PIV). The time averaged velocity and shear stress distributions were found to vary significantly with ultrasound power. Their maximum values increase sharply for low powers and up to a critical power level. At and above this setting, the flow pattern changes, from the high velocity and shear stress region confined in the vicinity of the tip, to one covering the whole root canal domain. Exceeding this threshold also induces a moderate increase in the maximum velocities and shear stresses. The insertion depth was found to have a smaller effect on the measured velocity and shear stresses. Due to the oscillating nature of the flow, instantaneous maximum velocities and shear stresses can reach much higher values than the mean, especially for high powers. Ultrasonic irrigation will benefit from using a higher power setting as this does produce greater shear stresses near the walls of the root canal leading to the potential for increased biofilm removal
Attosecond sampling of arbitrary optical waveforms
Advances in the generation of ultrashort laser pulses, and the emergence of new research areas such as attosecond science, nanoplasmonics, coherent control, and multidimensional spectroscopy, have led to the need for a new class of ultrafast metrology that can measure the electric field of complex optical waveforms spanning the ultraviolet to the infrared. Important examples of such waveforms are those produced by spectral control of ultrabroad bandwidth pulses, or by Fourier synthesis. These are typically tailored for specific purposes, such as to increase the photon energy and flux of high-harmonic radiation, or to control dynamical processes by steering electron dynamics on subcycle time scales. These applications demand a knowledge of the full temporal evolution of the field. Conventional pulse measurement techniques that provide estimates of the relative temporal or spectral phase are unsuited to measure such waveforms. Here we experimentally demonstrate a new, all-optical method for directly measuring the electric field of arbitrary ultrafast optical waveforms. Our method is based on high-harmonic generation (HHG) driven by a field that is the collinear superposition of the waveform to be measured with a stronger probe laser pulse. As the delay between the pulses is varied, we show that the field of the unknown waveform is mapped to energy shifts in the high-harmonic spectrum, allowing a direct, accurate, and rapid retrieval of the electric field with subcycle temporal resolution at the location of the HHG
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