Berry Groisman

Nonlocal variables with product-state eigenstates

An alternative proof for existence of ‘quantum nonlocality without entanglement’, i.e. existence of variables with product-state eigenstates which cannot be measured locally, is presented. A simple ‘nonlocal’ variable for the case of one-way communication is given and the limit for its approximate measurability is found.

Measurements of semilocal and nonmaximally entangled states

Consistency with relativistic causality narrows down dramatically the class of measurable observables. We argue that, by weakening the preparation role of ideal measurements, many of these observables become measurable. In particular, we show by applying entanglement assisted remote operations that all Hermitian observables of a (2×2)-dimensional bipartite system are measurable.

Reliable entanglement transfer between pure quantum states

The problem of the reliable transfer of entanglement from one pure bipartite quantum state to another using local operations is analyzed. It is shown that in the case of qubits the amount that can be transferred is restricted to the difference between the entanglement of the two states. In the presence of a catalytic state the range of the transferable amount broadens to a certain degree.

Optimal amount of entanglement to distinguish quantum states instantaneously

We introduce an aspect of nonlocality which arises when the task of quantum states distinguishability is considered under local operations and shared entanglement in the absence of classical communication. We find the optimal amount of entanglement required to accomplish the task perfectly for sets of orthogonal states and argue that it quantifies information nonlocality.

ENTANGLING AND DISENTANGLING CAPACITIES OF NOISY INTERACTIONS

Entangling and disentangling capacities are the key manifestation of the nonlocal content of a quantum operation. A lot of effort has been put recently into investigating (dis)entangling capacities of unitary operations, but very little is known about capacities of non-unitary operations. Here we investigate (dis)entangling capacities of unital CPTP maps acting on two qubits.

Instantaneous measurements of nonlocal variables

Abstract Relativistic causality imposes rigid restrictions on nondemolition (repeatable) measurements of nonlocal variables. We show that there are no causal restrictions on demolition (nonrepeatable) measurements: all Hermitian operators of multipartite quantum system can be measured instantaneously, provided unlimited supply of entanglement resources.