12,907 research outputs found
Fast optimization of parametrized quantum optical circuits
Parametrized quantum optical circuits are a class of quantum circuits in
which the carriers of quantum information are photons and the gates are optical
transformations. Classically optimizing these circuits is challenging due to
the infinite dimensionality of the photon number vector space that is
associated to each optical mode. Truncating the space dimension is unavoidable,
and it can lead to incorrect results if the gates populate photon number states
beyond the cutoff. To tackle this issue, we present an algorithm that is orders
of magnitude faster than the current state of the art, to recursively compute
the exact matrix elements of Gaussian operators and their gradient with respect
to a parametrization. These operators, when augmented with a non-Gaussian
transformation such as the Kerr gate, achieve universal quantum computation.
Our approach brings two advantages: first, by computing the matrix elements of
Gaussian operators directly, we don't need to construct them by combining
several other operators; second, we can use any variant of the gradient descent
algorithm by plugging our gradients into an automatic differentiation framework
such as TensorFlow or PyTorch. Our results will find applications in quantum
optical hardware research, quantum machine learning, optical data processing,
device discovery and device design.Comment: 21 pages, 10 figure
A New Light Particle in B Decays?
We investigate the possibility whether the tensions with SM expectations
observed in several b -> sll transitions, including hints for lepton flavour
non-universality, could be due to the decay of B into a new light resonance. We
find that qualitative agreement with the data can be obtained with a light
vector resonance dominantly decaying invisibly. This scenario predicts a shift
in the muon anomalous magnetic moment that could explain the long-standing
discrepancy. The most stringent constraint comes from searches for B decays
with missing energy. A striking prediction is a strong q^2 dependence of the
lepton flavour universality ratios R_K and R_K* that should allow to clearly
confirm or rule out this possibility experimentally. We also comment on the
possible connection of the invisible decay product with Dark Matter.Comment: 5 pages, 2 figures. v2: typos corrected, references and
clarifications adde
An end-to-end-construction for singly periodic minimal surfaces
We show the existence of various families of properly embedded singly
periodic minimal surfaces in R^3 with finite arbitrary genus and Scherk type
ends in the quotient. The proof of our results is based on the gluing of small
perturbations of pieces of already known minimal surfaces.Comment: 49 page
Full characterization of the quantum spiral bandwidth of entangled biphotons
Spontaneous parametric down-conversion has been shown to be a reliable source of entangled photons. Among the wide range of properties shown to be entangled, it is the orbital angular momentum that is the focus of our study. We investigate, in particular, the bi-photon state generated using a Gaussian pump beam. We derive an expression for the simultaneous correlations in the orbital angular momentum, l, and radial momentum, p, of the down-converted Laguerre-Gaussian beams. Our result allows us, for example, to calculate the spiral bandwidth with no restriction on the geometry of the beams: l, p, and the beam widths are all free parameters. Moreover, we show that, with the usual paraxial and collinear approximations, a fully analytic expression for the correlations can be derived
Hamiltonians for one-way quantum repeaters
Quantum information degrades over distance due to the unavoidable
imperfections of the transmission channels, with loss as the leading factor.
This simple fact hinders quantum communication, as it relies on propagating
quantum systems. A solution to this issue is to introduce quantum repeaters at
regular intervals along a lossy channel, to revive the quantum signal. In this
work we study unitary one-way quantum repeaters, which do not need to perform
measurements and do not require quantum memories, and are therefore
considerably simpler than other schemes. We introduce and analyze two methods
to construct Hamiltonians that generate a repeater interaction that can beat
the fundamental repeaterless key rate bound even in the presence of an
additional coupling loss, with signals that contain only a handful of photons.
The natural evolution of this work will be to approximate a repeater
interaction by combining simple optical elements.Comment: 8 pages, 3 figure
Simultaneous intensive photometry and high resolution spectroscopy of Delta Scuti stars. V. The high--degree modes in the pulsational content of BV Circini
We discuss here the pulsation properties of the Scuti star BV
Circini on the basis of data obtained during a simultaneous photometric and
spectroscopic campaign in 1996 and a spectroscopic one in 1998, and taking also
advantage of the previous photometric observations by Kurtz (1981). Nine
pulsation modes were detected from photometry and thirteen from spectroscopy;
five of them are in common to both techniques. The spectroscopic data give
ample evidence of dramatic amplitude variations in some modes, in particular
the strongest spectroscopic mode in 1998 was not detectable in 1996 data. The
two dominant photometric modes (6.33 and 7.89 c\d) are observed on both
seasons. The typing of the modes was performed by means of a simultaneous model
fit of line profile and light variations. The 6.33 c\d photometric term is
probably the fundamental radial mode, while the 7.89 c\d is a nonradial mode
with mi different than 0. There are six high-degree prograde modes with an
azimuthal order m ranging from -12 to -14, and also a retrograde mode with m~7.
These modes combined with the identification of the 6.33 c\d mode allowed us to
estimate i~60 deg for the value of the inclination of the rotation axis. An
accurate evaluation of the main stellar physical parameters is also proposed as
a result of the pulsational analysis.Comment: 12 pages (in A&A style), 9 ps figures (Fig. 7 in colour) Accepted for
A&A Main Journa
Noise-robust quantum sensing via optimal multi-probe spectroscopy
The dynamics of quantum systems are unavoidably influenced by their
environment and in turn observing a quantum system (probe) can allow one to
measure its environment: Measurements and controlled manipulation of the probe
such as dynamical decoupling sequences as an extension of the Ramsey
interference measurement allow to spectrally resolve a noise field coupled to
the probe. Here, we introduce fast and robust estimation strategies for the
characterization of the spectral properties of classical and quantum dephasing
environments. These strategies are based on filter function orthogonalization,
optimal control filters maximizing the relevant Fisher Information and
multi-qubit entanglement. We investigate and quantify the robustness of the
schemes under different types of noise such as finite-precision measurements,
dephasing of the probe, spectral leakage and slow temporal fluctuations of the
spectrum.Comment: 13 pages, 14 figure
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