**6. Nonlocality**

Two central concepts of quantum mechanics are Heisenberg's uncertainty principle and nonlocality. Nonlocality plays a fundamental role in quantum information science.

Whereas the quantum entanglement, which can be traced back to the Einstein, Boris Podolsky and Nathan Rosen (EPR) paradox in 1935 (they argued that the description of physical reality provided by quantum mechanics was *incomplete*). This argument gave rise to the discussions on the foundations of quantum mechanics related to **reality** and **locality**. This plays crucial roles in quantum information processing.

Quantum Theory can predict certain patterns of correlation among spatially separated events correctly. This manifests **non-local** influences between some of these events. This is a remarkable feature of the microscopic world prescribed by quantum theory. This idea of nonlocality was described by Albert Einstein rather dismissively as "spooky action at a distance" that was mentioned above.

For example, if a pair of *electrons* is created together, one will have clockwise *spin* and the other will have anticlockwise *spin* (*spin* is a particular property of particles mentioned above). The most important point is that there are two possible states and that the entire *spin* of a quantum system must always cancel out to zero.

However, it is claimed that the two *electrons* can be considered to simultaneously have *spins* clockwise-anticlockwise and anticlockwise-clockwise respectively, under *quantum theory*, and if *superposition* is possible, If the pair are then separated by any distance (without observing and thereby **decohering** (see below) and then later checked, the second particle can be seen to instantaneously take the opposite *spin* to the first, so that the pair maintains its zero total *spin*, no matter how far apart they may be.
