10,170 research outputs found
Quantum Cloning of Binary Coherent States - Optimal Transformations and Practical Limits
The notions of qubits and coherent states correspond to different physical
systems and are described by specific formalisms. Qubits are associated with a
two-dimensional Hilbert space and can be illustrated on the Bloch sphere. In
contrast, the underlying Hilbert space of coherent states is
infinite-dimensional and the states are typically represented in phase space.
For the particular case of binary coherent state alphabets these otherwise
distinct formalisms can equally be applied. We capitalize this formal
connection to analyse the properties of optimally cloned binary coherent
states. Several practical and near-optimal cloning schemes are discussed and
the associated fidelities are compared to the performance of the optimal
cloner.Comment: 12 pages, 12 figure
Near optimal discrimination of binary coherent signals via atom-light interaction
We study the discrimination of weak coherent states of light with significant
overlaps by nondestructive measurements on the light states through measuring
atomic states that are entangled to the coherent states via dipole coupling. In
this way, the problem of measuring and discriminating coherent light states is
shifted to finding the appropriate atom-light interaction and atomic
measurements. We show that this scheme allows us to attain a probability of
error extremely close to the Helstrom bound, the ultimate quantum limit for
discriminating binary quantum states, through the simple Jaynes-Cummings
interaction between the field and ancilla with optimized light-atom coupling
and projective measurements on the atomic states. Moreover, since the
measurement is nondestructive on the light state, information that is not
detected by one measurement can be extracted from the post-measurement light
states through subsequent measurements.Comment: 11 pages, 9 figure
Unambiguous quantum state filtering
In this paper, we consider the generalized measurement where one particular
quantum signal is unambiguously extracted from a set of non-commutative quantum
signals and the other signals are filtered out. Simple expressions for the
maximum detection probability and its POVM are derived. We applyl such
unambiguous quantum state filtering to evaluation of the sensing of decoherence
channels. The bounds of the precision limit for a given quantum state of probes
and possible device implementations are discussed.Comment: 7 pages, 5 figure
Discrimination of Optical Coherent States using a Photon Number Resolving Detector
The discrimination of non-orthogonal quantum states with reduced or without
errors is a fundamental task in quantum measurement theory. In this work, we
investigate a quantum measurement strategy capable of discriminating two
coherent states probabilistically with significantly smaller error
probabilities than can be obtained using non-probabilistic state
discrimination. We find that appropriate postselection of the measurement data
of a photon number resolving detector can be used to discriminate two coherent
states with small error probability. We compare our new receiver to an optimal
intermediate measurement between minimum error discrimination and unambiguous
state discrimination.Comment: 5 pages, 4 figure
Implementing Non-Projective Measurements via Linear Optics: an Approach Based on Optimal Quantum State Discrimination
We discuss the problem of implementing generalized measurements (POVMs) with
linear optics, either based upon a static linear array or including conditional
dynamics. In our approach, a given POVM shall be identified as a solution to an
optimization problem for a chosen cost function. We formulate a general
principle: the implementation is only possible if a linear-optics circuit
exists for which the quantum mechanical optimum (minimum) is still attainable
after dephasing the corresponding quantum states. The general principle enables
us, for instance, to derive a set of necessary conditions for the linear-optics
implementation of the POVM that realizes the quantum mechanically optimal
unambiguous discrimination of two pure nonorthogonal states. This extends our
previous results on projection measurements and the exact discrimination of
orthogonal states.Comment: final published versio
Zero-Error Attacks and Detection Statistics in the Coherent One-Way Protocol for Quantum Cryptography
This is a study of the security of the Coherent One-Way (COW) protocol for
quantum cryptography, proposed recently as a simple and fast experimental
scheme. In the zero-error regime, the eavesdropper Eve can only take advantage
of the losses in the transmission. We consider new attacks, based on
unambiguous state discrimination, which perform better than the basic
beam-splitting attack, but which can be detected by a careful analysis of the
detection statistics. These results stress the importance of testing several
statistical parameters in order to achieve higher rates of secret bits
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