29 research outputs found
Entanglement witnesses with variable number of local measurements
We present a class of entanglement identifiers which has the following
experimentally friendly feature: once the expectation value of the identifier
exceeds some definite limit, we can conclude the state is entangled, even if
not all measurements defining the identifier have been performed. These
identifiers are in the form of sums of non-negative functions of correlations
in a quantum state, mostly squares of correlations, and we illustrate their use
and strengths on various examples.Comment: 6 pages, 1 figur
Detecting genuine multipartite entanglement of pure states with bipartite correlations
Monogamy of bipartite correlations leads, for arbitrary pure multi-qubit
states, to simple conditions able to indicate various types of multipartite
entanglement by being capable to exclude the possibility of k-separability.Comment: journal versio
Entanglement and communication-reducing properties of noisy N-qubit states
We consider properties of states of many qubits, which arise after sending
certain entangled states via various noisy channels (white noise, coloured
noise, local depolarization, dephasing and amplitude damping). Entanglement of
these states is studied and their ability to violate certain classes of Bell
inequalities. States which violate them allow for higher than classical
efficiency of solving related distributed computational tasks with constrained
communication. This is a direct property of such states -- not requiring their
further modification via stochastic local operations and classical
communication such as entanglement purification or distillation procedures. We
identify novel families of multi-particle states which are entangled but
nevertheless allow local realistic description of specific Bell experiments.
For some of them, the "gap" between the critical values for entanglement and
violation of Bell inequality remains finite even in the limit of infinitely
many qubits.Comment: new version, more results adde