2,172 research outputs found
Optimal experiment design revisited: fair, precise and minimal tomography
Given an experimental set-up and a fixed number of measurements, how should
one take data in order to optimally reconstruct the state of a quantum system?
The problem of optimal experiment design (OED) for quantum state tomography was
first broached by Kosut et al. [arXiv:quant-ph/0411093v1]. Here we provide
efficient numerical algorithms for finding the optimal design, and analytic
results for the case of 'minimal tomography'. We also introduce the average
OED, which is independent of the state to be reconstructed, and the optimal
design for tomography (ODT), which minimizes tomographic bias. We find that
these two designs are generally similar. Monte-Carlo simulations confirm the
utility of our results for qubits. Finally, we adapt our approach to deal with
constrained techniques such as maximum likelihood estimation. We find that
these are less amenable to optimization than cruder reconstruction methods,
such as linear inversion.Comment: 16 pages, 7 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
Detection of FeO towards SgrB2
We have observed the J=5-4 ground state transition of FeO at a frequency of
153 GHz towards a selection of galactic sources.
Towards the galactic center source SgrB2, we see weak absorption at
approximately the velocity of other features towards this source (62 km
s LSR).
Towards other sources, the results were negative as they were also for
MgOH(3-2) and FeC(6-5). We tentatively conclude that the absorption seen toward
SgrB2 is due to FeO in the hot ( 500 K) relatively low density absorbing
gas known to be present in this line of sight.
This is the first (albeit tentative) detection of FeO or any iron--containing
molecule in the interstellar gas. Assuming the observed absorption to be due to
FeO, we estimate [FeO]/[SiO] to be of order or less than 0.002 and
[FeO]/[H] of order . This is compatible with our negative
results in other sources.
Our results suggest that the iron liberated from grains in the shocks
associated with SgrB2 remains atomic and is not processed into molecular form.Comment: 1 postscrit figure,10 page
NonClassicality Criteria in Multiport Interferometry
Interference lies at the heart of the behavior of classical and quantum
light. It is thus crucial to understand the boundaries between which
interference patterns can be explained by a classical electromagnetic
description of light and which, on the other hand, can only be understood with
a proper quantum mechanical approach. While the case of two-mode interference
has received a lot of attention, the multimode case has not yet been fully
explored. Here we study a general scenario of intensity interferometry: we
derive a bound on the average correlations between pairs of output intensities
for the classical wavelike model of light, and we show how it can be violated
in a quantum framework. As a consequence, this violation acts as a
nonclassicality witness, able to detect the presence of sources with
sub-Poissonian photon-number statistics. We also develop a criterion that can
certify the impossibility of dividing a given interferometer into two
independent subblocks.Comment: 5 + 3 pages, published versio
Quantum phase estimation with lossy interferometers
We give a detailed discussion of optimal quantum states for optical two-mode
interferometry in the presence of photon losses. We derive analytical formulae
for the precision of phase estimation obtainable using quantum states of light
with a definite photon number and prove that maximization of the precision is a
convex optimization problem. The corresponding optimal precision, i.e. the
lowest possible uncertainty, is shown to beat the standard quantum limit thus
outperforming classical interferometry. Furthermore, we discuss more general
inputs: states with indefinite photon number and states with photons
distributed between distinguishable time bins. We prove that neither of these
is helpful in improving phase estimation precision.Comment: 12 pages, 5 figure
Simplified Quantum Process Tomography
We propose and evaluate experimentally an approach to quantum process
tomography that completely removes the scaling problem plaguing the standard
approach. The key to this simplification is the incorporation of prior
knowledge of the class of physical interactions involved in generating the
dynamics, which reduces the problem to one of parameter estimation. This allows
part of the problem to be tackled using efficient convex methods, which, when
coupled with a constraint on some parameters allows globally optimal estimates
for the Kraus operators to be determined from experimental data. Parameterising
the maps provides further advantages: it allows the incorporation of mixed
states of the environment as well as some initial correlation between the
system and environment, both of which are common physical situations following
excitation of the system away from thermal equilibrium. Although the approach
is not universal, in cases where it is valid it returns a complete set of
positive maps for the dynamical evolution of a quantum system at all times.Comment: Added references to interesting related work by Bendersky et a
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
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