63 research outputs found
Experimental Purification of Single Qubits
We report the experimental realization of the purification protocol for
single qubits sent through a depolarization channel. The qubits are associated
with polarization encoded photon particles and the protocol is achieved by
means of passive linear optical elements. The present approach may represent a
convenient alternative to the distillation and error correction protocols of
quantum information.Comment: 10 pages, 2 figure
Reduced randomness in quantum cryptography with sequences of qubits encoded in the same basis
We consider the cloning of sequences of qubits prepared in the states used in
the BB84 or 6-state quantum cryptography protocol, and show that the
single-qubit fidelity is unaffected even if entire sequences of qubits are
prepared in the same basis. This result is of great importance for practical
quantum cryptosystems because it reduces the need for high-speed random number
generation without impairing on the security against finite-size attacks.Comment: 8 pages, submitted to PR
Entanglement enhanced classical capacity of quantum communication channels with correlated noise in arbitrary dimensions
We study the capacity of d-dimensional quantum channels with memory modeled
by correlated noise. We show that, in agreement with previous results on Pauli
qubit channels, there are situations where maximally entangled input states
achieve higher values of mutual information than product states. Moreover, a
strong dependence of this effect on the nature of the noise correlations as
well as on the parity of the space dimension is found. We conjecture that when
entanglement gives an advantage in terms of mutual information, maximally
entangled states saturate the channel capacity.Comment: 10 pages, 5 figure
Optimal purification of single qubits
We introduce a new decomposition of the multiqubit states of the form
and employ it to construct the optimal single qubit
purification procedure. The same decomposition allows us to study optimal
quantum cloning and state estimation of mixed states.Comment: 4 pages, 1 figur
Optimal eavesdropping in cryptography with three-dimensional quantum states
We study optimal eavesdropping in quantum cryptography with three-dimensional
systems, and show that this scheme is more secure than protocols using
two-dimensional states. We generalize the according eavesdropping
transformation to arbitrary dimensions, and discuss the connection with optimal
quantum cloning.Comment: 4 pages, 2 figure
Transition of D- Level Quantum Systems Through Quantum Channels with Correlated Noise
Entanglement and entanglement-assisted are useful resources to enhance the
mutual information of the Pauli channels, when the noise on consecutive uses of
the channel has some partial correlations. In this Paper, we study
quantum-communication channels in -dimensional systems and derive the mutual
information of the quantum channels for maximally entangled states and product
states coding with correlated noise. Then, we compare fidelity between these
states. Our results show that there exists a certain fidelity memory threshold
which depends on the dimension of the Hilbert space and the properties of
noisy channels. We calculate the classical capacity of a particular correlated
noisy channel and show that in order to achieve Holevo limit, we must use
particles with degrees of freedom. Our results show that entanglement is a
useful means to enhance the mutual information. We choose a special
non-maximally entangled state and show that in the quasi-classical depolarizing
and quantum depolarizing channels, maximum classical capacity in the higher
memory channels is given by the maximally entangled state. Hence, our results
show that for high error channels in every degree of memory, maximally
entangled states have better mutual information.Comment: 15 pages, 5 figures, PHYSICAL REVIEW A 75, 042301 (2007
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
Trace distance from the viewpoint of quantum operation techniques
In the present paper, the trace distance is exposed within the quantum
operations formalism. The definition of the trace distance in terms of a
maximum over all quantum operations is given. It is shown that for any pair of
different states, there are an uncountably infinite number of maximizing
quantum operations. Conversely, for any operation of the described type, there
are an uncountably infinite number of those pairs of states that the maximum is
reached by the operation. A behavior of the trace distance under considered
operations is studied. Relations and distinctions between the trace distance
and the sine distance are discussed.Comment: 26 pages, no figures. The bibliography is extended, explanatory
improvement
Lower Bounds for Attainable Fidelity in Entanglement Purification
We derive lower bounds for the attainable fidelity of standard entanglement
purification protocols when local operations and measurements are subjected to
errors. We introduce an error parameter which measures the distance between the
ideal completely positive map describing a purification step and the one in the
presence of errors. We derive non--linear maps for a lower bound of the
fidelity at each purification step in terms of this parameter.Comment: 11 page
A security proof of quantum cryptography based entirely on entanglement purification
We give a proof that entanglement purification, even with noisy apparatus, is
sufficient to disentangle an eavesdropper (Eve) from the communication channel.
In the security regime, the purification process factorises the overall initial
state into a tensor-product state of Alice and Bob, on one side, and Eve on the
other side, thus establishing a completely private, albeit noisy, quantum
communication channel between Alice and Bob. The security regime is found to
coincide for all practical purposes with the purification regime of a two-way
recurrence protocol. This makes two-way entanglement purification protocols,
which constitute an important element in the quantum repeater, an efficient
tool for secure long-distance quantum cryptography.Comment: Follow-up paper to quant-ph/0108060, submitted to PRA; 24 pages,
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