40 research outputs found
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
Entanglement transformation with no classical communication
We present an optimal scheme to realize the transformations between single
copies of two bipartite entangled states without classical communication
between the sharing parties. The scheme achieves the upper bound for the
success probabilities [PRA 63, 022301 (2001), PRL 83, 1455 (1999)] of
generating maximally entangled states if applied to entanglement concentration.
Such strategy also dispenses with the interaction with an ancilla system in the
implementation. We also show that classical communications are indispensable in
realizing the deterministic transformations of a single bipartite entangled
state. With a finite number of identical pairs of two entangled bosons, on the
other hand, we can realize the deterministic transformation to any target
entangled state of equal or less Schmidt rank through an extension of the
scheme.Comment: published versio
Optimum measurement for unambiguously discriminating two mixed states: General considerations and special cases
Based on our previous publication [U. Herzog and J. A. Bergou, Phys.Rev. A
71, 050301(R) (2005)] we investigate the optimum measurement for the
unambiguous discrimination of two mixed quantum states that occur with given
prior probabilities. Unambiguous discrimination of nonorthogonal states is
possible in a probabilistic way, at the expense of a nonzero probability of
inconclusive results, where the measurement fails. Along with a discussion of
the general problem, we give an example illustrating our method of solution. We
also provide general inequalities for the minimum achievable failure
probability and discuss in more detail the necessary conditions that must be
fulfilled when its absolute lower bound, proportional to the fidelity of the
states, can be reached.Comment: Submitted to Journal of Physics:Conference Series (Proceedings of the
12th Central European Workshop on Quantum Optics, Ankara, June 2005
Optimum unambiguous discrimination of two mixed quantum states
We investigate generalized measurements, based on positive-operator-valued
measures, and von Neumann measurements for the unambiguous discrimination of
two mixed quantum states that occur with given prior probabilities. In
particular, we derive the conditions under which the failure probability of the
measurement can reach its absolute lower bound, proportional to the fidelity of
the states. The optimum measurement strategy yielding the fidelity bound of the
failure probability is explicitly determined for a number of cases. One example
involves two density operators of rank d that jointly span a 2d-dimensional
Hilbert space and are related in a special way. We also present an application
of the results to the problem of unambiguous quantum state comparison,
generalizing the optimum strategy for arbitrary prior probabilities of the
states.Comment: final versio
Coherent states engineering with linear optics: Possible and impossible tasks
The general transformation of the product of coherent states
to the output state (
or ), which is realizable with linear optical circuit, is
characterized with a linear map from the vector
to
. A correspondence between the
transformations of a product of coherent states and those of a single photon
state is established with such linear maps. It is convenient to apply this
linear transformation method to design any linear optical scheme working with
coherent states. The examples include message encoding and quantum database
searching. The limitation of manipulating entangled coherent states with linear
optics is also discussed.Comment: 6 pages, 2 figure