789 research outputs found
Direct estimation of functionals of density operators by local operations and classical communication
We present a method of direct estimation of important properties of a shared bipartite quantum state, within the "distant laboratories" paradigm, using only local operations and classical communication. We apply this procedure to spectrum estimation of shared states, and locally implementable structural physical approximations to incompletely positive maps. This procedure can also be applied to the estimation of channel capacity and measures of entanglement
Quantum networks for elementary arithmetic operations
Quantum computers require quantum arithmetic. We provide an explicit construction of quantum networks effecting basic arithmetic operations: from addition to modular exponentiation. Quantum modular exponentiation seems to be the most difficult (time and space consuming) part of Shor's quantum factorising algorithm. We show that the auxiliary memory required to perform this operation in a reversible way grows linearly with the size of the number to be factorised
Analysis and interpretation of high transverse entanglement in optical parametric down conversion
Quantum entanglement associated with transverse wave vectors of down
conversion photons is investigated based on the Schmidt decomposition method.
We show that transverse entanglement involves two variables: orbital angular
momentum and transverse frequency. We show that in the monochromatic limit high
values of entanglement are closely controlled by a single parameter resulting
from the competition between (transverse) momentum conservation and
longitudinal phase matching. We examine the features of the Schmidt eigenmodes,
and indicate how entanglement can be enhanced by suitable mode selection
methods.Comment: 4 pages, 4 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
Robust Multi-Partite Multi-Level Quantum Protocols
We present a tripartite three-level state that allows a secret sharing
protocol among the three parties, or a quantum key distribution protocol
between any two parties. The state used in this scheme contains entanglement
even after one system is traced out. We show how to utilize this residual
entanglement for quantum key distribution purposes, and propose a realization
of the scheme using entanglement of orbital angular momentum states of photons.Comment: 9 pages, 2 figure
Quantum Cryptography with Coherent States
The safety of a quantum key distribution system relies on the fact that any
eavesdropping attempt on the quantum channel creates errors in the
transmission. For a given error rate, the amount of information that may have
leaked to the eavesdropper depends on both the particular system and the
eavesdropping strategy. In this work, we discuss quantum cryptographic
protocols based on the transmission of weak coherent states and present a new
system, based on a symbiosis of two existing ones, and for which the
information available to the eavesdropper is significantly reduced. This system
is therefore safer than the two previous ones. We also suggest a possible
experimental implementation.Comment: 20 pp. Revtex, Figures available from the authors upon request, To be
published in PRA (March 95
Quantum cryptography based on qutrit Bell inequalities
We present a cryptographic protocol based upon entangled qutrit pairs. We analyze the scheme under a symmetric incoherent attack and plot the region for which the protocol is secure and compare this with the region of violations of certain Bell inequalities
Using of small-scale quantum computers in cryptography with many-qubit entangled states
We propose a new cryptographic protocol. It is suggested to encode
information in ordinary binary form into many-qubit entangled states with the
help of a quantum computer. A state of qubits (realized, e.g., with photons) is
transmitted through a quantum channel to the addressee, who applies a quantum
computer tuned to realize the inverse unitary transformation decoding of the
message. Different ways of eavesdropping are considered, and an estimate of the
time needed for determining the secret unitary transformation is given. It is
shown that using even small quantum computers can serve as a basis for very
efficient cryptographic protocols. For a suggested cryptographic protocol, the
time scale on which communication can be considered secure is exponential in
the number of qubits in the entangled states and in the number of gates used to
construct the quantum network
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