232 research outputs found
Direct, Loss-Tolerant Characterization of Nonclassical Photon Statistics
We experimentally investigate a method of directly characterizing the photon
number distribution of nonclassical light beams that is tolerant to losses and
makes use only of standard binary detectors. This is achieved in a single
measurement by calibrating the detector using some small amount of prior
information about the source. We demonstrate the technique on a freely
propagating heralded two-photon number state created by conditional detection
of a two-mode squeezed state generated by a parametric downconverter.Comment: 5 pages, 2 figure
Precision metrology using weak measurements
Weak values and measurements have been proposed as means to achieve dramatic
enhancements in metrology based on the greatly increased range of possible
measurement outcomes. Unfortunately, the very large values of measurement
outcomes occur with highly suppressed probabilities. This raises three vital
questions in weak-measurement-based metrology, namely, (Q1) Does post-selection
enhance the measurement precision? (Q2) Does weak measurement offer better
precision than strong measurement? (Q3) Is it possible to beat the standard
quantum limit or to achieve the Heisenberg limit with weak measurement using
only classical resources? We analyse these questions for two prototypical, and
generic, measurement protocols and show that while the answers to the first two
questions are negative for both protocols, the answer to the last is
affirmative for measurements with phase-space interactions, and negative for
configuration space interactions. Our results, particularly the ability of weak
measurements to perform at par with strong measurements in some cases, are
instructive for the design of weak-measurement-based protocols for quantum
metrology.Comment: 5+5 pages, 2 figure
Quantum Enhanced Multiple Phase Estimation
We study the simultaneous estimation of multiple phases as a discretised
model for the imaging of a phase object. We identify quantum probe states that
provide an enhancement compared to the best quantum scheme for the estimation
of each individual phase separately, as well as improvements over classical
strategies. Our strategy provides an advantage in the variance of the
estimation over individual quantum estimation schemes that scales as O(d) where
d is the number of phases. Finally, we study the attainability of this limit
using realistic probes and photon-number-resolving detectors. This is a problem
in which an intrinsic advantage is derived from the estimation of multiple
parameters simultaneously.Comment: Accepted by Physical Review Letter
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
Extending electron orbital precession to the molecular case: Can orbital alignment be used to observe wavepacket dynamics?
The complexity of ultrafast molecular photoionization presents an obstacle to
the modelling of pump-probe experiments. Here, a simple optimized model of
atomic rubidium is combined with a molecular dynamics model to predict
quantitatively the results of a pump-probe experiment in which long range
rubidium dimers are first excited, then ionized after a variable delay. The
method is illustrated by the outline of two proposed feasible experiments and
the calculation of their outcomes. Both of these proposals use Feshbach 87Rb2
molecules. We show that long-range molecular pump-probe experiments should
observe spin-orbit precession given a suitable pump-pulse, and that the
associated high-frequency beat signal in the ionization probability decays
after a few tens of picoseconds. If the molecule was to be excited to only a
single fine structure state state, then a low-frequency oscillation in the
internuclear separation would be detectable through the timedependent
ionization cross section, giving a mechanism that would enable observation of
coherent vibrational motion in this molecule.Comment: 9 pages, 10 figures, PRA submissio
Continuous-Variable Quantum Computing in Optical Time-Frequency Modes using Quantum Memories
We develop a scheme for time-frequency encoded continuous-variable
cluster-state quantum computing using quantum memories. In particular, we
propose a method to produce, manipulate and measure 2D cluster states in a
single spatial mode by exploiting the intrinsic time-frequency selectivity of
Raman quantum memories. Time-frequency encoding enables the scheme to be
extremely compact, requiring a number of memories that is a linear function of
only the number of different frequencies in which the computational state is
encoded, independent of its temporal duration. We therefore show that quantum
memories can be a powerful component for scalable photonic quantum information
processing architectures.Comment: 5 pages, 6 figures, and supplementary information. Updated to be
consistent with published versio
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