Quantum mechanics is a nonlocal theory. The principle of locality states that the properties of one particle
can not be affected by another particle that is sufficiently
far away. Nonlocality in quantum mechanics, at least,
has two different aspects [1]. On of these aspects arises
in the Aharonov-Bohm effect. The Aharonov-Bohm effect is nonlocal in the sense that the electromagnetic field influences an electron in a region where the field is zero.
The other aspect of nonlocality in quantum mechanics
appears when one performs local measurements on spatially separated systems. These nonlocal effects result from the fact that the local measurements can alter the
overall state of a multipartite quantum system. Quantum
nonlocality usually refers to this aspect of nonlocality and
is studied often in the context of Bell inequalities.