787 research outputs found
Optimal superdense coding of entangled states
We present a one-shot method for preparing pure entangled states between a
sender and a receiver at a minimal cost of entanglement and quantum
communication. In the case of preparing unentangled states, an earlier paper
showed that a 2n-qubit quantum state could be communicated to a receiver by
physically transmitting only n+o(n) qubits in addition to consuming n ebits of
entanglement and some shared randomness. When the states to be prepared are
entangled, we find that there is a reduction in the number of qubits that need
to be transmitted, interpolating between no communication at all for maximally
entangled states and the earlier two-for-one result of the unentangled case,
all without the use of any shared randomness. We also present two applications
of our result: a direct proof of the achievability of the optimal superdense
coding protocol for entangled states produced by a memoryless source, and a
demonstration that the quantum identification capacity of an ebit is two
qubits.Comment: Final Version. Several technical issues clarifie
The states of W-class as shared resources for perfect teleportation and superdense coding
As we know, the states of triqubit systems have two important classes:
GHZ-class and W-class.
In this paper, the states of W-class are considered for teleportation and
superdense coding, and are generalized to multi-particle systems. First we
describe two transformations of the shared resources for teleportation and
superdense coding, which allow many new protocols from some known ones for
that. As an application of these transformations, we obtain a sufficient and
necessary condition for a state of W-class being suitable for perfect
teleportation and superdense coding. As another application, we find that state
can be used to
transmit three classical bits by sending two qubits, which was considered to be
impossible by P. Agrawal and A. Pati [Phys. Rev. A to be published]. We
generalize the states of W-class to multi-qubit systems and multi-particle
systems with higher dimension. We propose two protocols for teleportation and
superdense coding by using W-states of multi-qubit systems that generalize the
protocols by using proposed by P. Agrawal and A. Pati. We obtain an
optimal way to partition some W-states of multi-qubit systems into two
subsystems, such that the entanglement between them achieves maximum value.Comment: 10 pages, critical comments and suggestions are welcom
Hyperdense coding and superadditivity of classical capacities in hypersphere theories
In quantum superdense coding, two parties previously sharing entanglement can
communicate a two bit message by sending a single qubit. We study this feature
in the broader framework of general probabilistic theories. We consider a
particular class of theories in which the local state space of the
communicating parties corresponds to Euclidean hyperballs of dimension n (the
case n = 3 corresponds to the Bloch ball of quantum theory). We show that a
single n-ball can encode at most one bit of information, independently of n. We
introduce a bipartite extension of such theories for which there exist dense
coding protocols such that log_2 (n+1) bits are communicated if entanglement is
previously shared by the communicating parties. For n > 3, these protocols are
more powerful than the quantum one, because more than two bits are communicated
by transmission of a system that locally encodes at most one bit. We call this
phenomenon hyperdense coding. Our hyperdense coding protocols imply
superadditive classical capacities: two entangled systems can encode log_2
(n+1) > 2 bits, even though each system individually encodes at most one bit.
In our examples, hyperdense coding and superadditivity of classical capacities
come at the expense of violating tomographic locality or dynamical continuous
reversibility.Comment: Expanded discussion in response to referee comments. Accepted for
publication in New Journal of Physic
Generalized remote state preparation: Trading cbits, qubits and ebits in quantum communication
We consider the problem of communicating quantum states by simultaneously
making use of a noiseless classical channel, a noiseless quantum channel and
shared entanglement. We specifically study the version of the problem in which
the sender is given knowledge of the state to be communicated. In this setting,
a trade-off arises between the three resources, some portions of which have
been investigated previously in the contexts of the quantum-classical trade-off
in data compression, remote state preparation and superdense coding of quantum
states, each of which amounts to allowing just two out of these three
resources. We present a formula for the triple resource trade-off that reduces
its calculation to evaluating the data compression trade-off formula. In the
process, we also construct protocols achieving all the optimal points. These
turn out to be achievable by trade-off coding and suitable time-sharing between
optimal protocols for cases involving two resources out of the three mentioned
above.Comment: 15 pages, 2 figures, 1 tabl
Experimental implementation of a NMR entanglement witness
Entanglement witnesses (EW) allow the detection of entanglement in a quantum
system, from the measurement of some few observables. They do not require the
complete determination of the quantum state, which is regarded as a main
advantage. On this paper it is experimentally analyzed an entanglement witness
recently proposed in the context of Nuclear Magnetic Resonance (NMR)
experiments to test it in some Bell-diagonal states. We also propose some
optimal entanglement witness for Bell-diagonal states. The efficiency of the
two types of EW's are compared to a measure of entanglement with tomographic
cost, the generalized robustness of entanglement. It is used a GRAPE algorithm
to produce an entangled state which is out of the detection region of the EW
for Bell-diagonal states. Upon relaxation, the results show that there is a
region in which both EW fails, whereas the generalized robustness still shows
entanglement, but with the entanglement witness proposed here with a better
performance
Delayed commutation in quantum computer networks
In the same way that classical computer networks connect and enhance the
capabilities of classical computers, quantum networks can combine the
advantages of quantum information and communications. We propose a
non-classical network element, a delayed commutation switch, that can solve the
problem of switching time in packet switching networks. With the help of some
local ancillary qubits and superdense codes we can route the information after
part of it has left the network node.Comment: 4 pages. 4 figures. Preliminar versio
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