594 research outputs found
Discrimination of the binary coherent signal: Gaussian-operation limit and simple non-Gaussian near-optimal receivers
We address the limit of the Gaussian operations and classical communication
in the problem of quantum state discrimination. We show that the optimal
Gaussian strategy for the discrimination of the binary phase shift keyed (BPSK)
coherent signal is a simple homodyne detection. We also propose practical
near-optimal quantum receivers that beat the BPSK homodyne limit in all areas
of the signal power. Our scheme is simple and does not require realtime
electrical feedback.Comment: 7 pages, 4 figures, published versio
Exceeding classical capacity limit in quantum optical channel
The amount of information transmissible through a communications channel is
determined by the noise characteristics of the channel and by the quantities of
available transmission resources. In classical information theory, the amount
of transmissible information can be increased twice at most when the
transmission resource (e.g. the code length, the bandwidth, the signal power)
is doubled for fixed noise characteristics. In quantum information theory,
however, the amount of information transmitted can increase even more than
twice. We present a proof-of-principle demonstration of this super-additivity
of classical capacity of a quantum channel by using the ternary symmetric
states of a single photon, and by event selection from a weak coherent light
source. We also show how the super-additive coding gain, even in a small code
length, can boost the communication performance of conventional coding
technique.Comment: 4 pages, 3 figure
Detection-dependent six-photon NOON state interference
NOON state interference (NOON-SI) is a powerful tool to improve the phase
sensing precision, and can play an important role in quantum sensing and
quantum imaging. However, most of the previous NOON-SI experiments only
investigated the center part of the interference pattern, while the full range
of the NOON-SI pattern has not yet been well explored.In this Letter, we
experimentally and theoretically demonstrate up to six-photon NOON-SI and study
the properties of the interference patterns over the full range.The
multi-photons were generated at a wavelength of 1584 nm from a PPKTP crystal in
a parametric down conversion process.It was found that the shape, the coherence
time and the visibility of the interference patterns were strongly dependent on
the detection schemes.This experiment can be used for applications which are
based on the envelope of the NOON-SI pattern, such as quantum spectroscopy and
quantum metrology.Comment: 5 pages, 3 figure
Implementation of projective measurements with linear optics and continuous photon counting
We investigate the possibility of implementing a given projection measurement
using linear optics and arbitrarily fast feedforward based on the continuous
detection of photons. In particular, we systematically derive the so-called
Dolinar scheme that achieves the minimum error discrimination of binary
coherent states. Moreover, we show that the Dolinar-type approach can also be
applied to projection measurements in the regime of photonic-qubit signals. Our
results demonstrate that for implementing a projection measurement with linear
optics, in principle, unit success probability may be approached even without
the use of expensive entangled auxiliary states, as they are needed in all
known (near-)deterministic linear-optics proposals.Comment: 11 pages, 2 figures, updated to the published versio
Experimental demonstration of quantum source coding
We report an experimental demonstration of Schumacher's quantum noiseless
coding theorem. Our experiment employs a sequence of single photons each of
which represents three qubits. We initially prepare each photon in one of a set
of 8 non-orthogonal codeword states corresponding to the value of a block of
three binary letters. We use quantum coding to compress this quantum data into
a two-qubit quantum channel and then uncompress the two-qubit channel to
restore the original data with a fidelity approaching the theoretical limit.Comment: 5 pages, 4 figure
Quantum channel of continuous variable teleportation and nonclassicality of quantum states
Noisy teleportation of nonclassical quantum states via a two-mode
squeezed-vacuum state is studied with the completely positive map and the
Glauber-Sudarshan -function. Using the nonclassical depth as a measure of
transmission performance, we compare the teleportation scheme with the direct
transmission through a noisy channel. The noise model is based on the coupling
to the vacuum field. It is shown that the teleportation channel has better
transmission performance than the direct transmission channel in a certain
region. The bounds for such region and for obtaining the nonvanished
nonclassicality of the teleported quantum states are also discussed. Our model
shows a reasonable agreement with the observed teleportation fidelity in the
experiment by Furusawa et al. [Science {\bf 282}, 706 (1998)]. We finally
mention the required conditions for transmitting nonclassical features in real
experiments.Comment: 16 pages, 4 figure
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