One of the more challenging features of the Quantum computing model is the dilemma caused by quantum entanglement. The term Entanglement is used to describe the theory in which particles can occupy two or more states but in observation can only be one of the states.
A well-known discussion of this dilemma began in the 1930′s, when Einstein and other physicists attempted to tackle the paradox of object states in quantum theory. As part of the debate, Schrödinger postulated a cat in a box as a good example of this duality. In Schrödinger’s cat hypothesis, a cat is said to exist in a closed chamber. A Geiger counter holds a small bit of radioactive element. If the element should decay, the Geiger would set off a hammer which would release poison and kill the cat. Since it is equally possible in any given instant that a radioactive particle has decayed or not decayed, the mathematical expression of this function would be that the cat is both alive and dead – in both states – until we have observed exactly which state the cat is in. While we know that it must as occupy only a single state, that state remains in duality until observation.
The simplest discussion that can arise from this dilemna is that upon opening the cage the state of the cat is immediately known to the observer. As part of the new state, the observer is now classically entwined to the observation. Now the cat, whose state is known, is entangled with the observer – whose knowledge of the state is due to the observation of the state of the cat. Hence entanglement exists between them.
In Quantum computing, entanglement is the state of particles such that as soon as the state of a particle is known, any particle which it is paired with is also known. Solving a problem that seems like a mental game, in fact becomes the basis for quantum computing.