Schrodingers cat

Schrödinger's cat is a [thought experiment]? that attempts to illustrate the incompleteness of the theory of quantum mechanics.

Said cat is placed in a sealed box. Attached to the box is an apparatus containing a radioactive nucleus and a canister of poison gas. When the nucleus decays, it emits a particle that triggers the apparatus, which opens the canister and kills the cat. At the start of the experiment and the the system exists in a mixed superposition of the states. When the box is opened the experimenter sees only a "decayed nucleus/dead cat" or a "undecayed nucleus/living cat." The question is when does the cat stop becoming a mixture of states and become one or the other? The purpose of the experiment is to illustrate that quantum mechanics is incomplete without some rules to describe when the wave function collapses and the cat becomes dead or alive instead of a mixture of both.

While the box is closed, no observation has taken place. According to the Copenhagen Interpretation, the system exists in a mixed superposition of the states "decayed nucleus/dead cat" and "undecayed nucleus/living cat." Only when the box is opened and an observation performed does the wave function collapse into one of the two states. In the Everett many-worlds interpretation of quantum mechanics, which does not single out observation as a special process, both states persist, but decohere?. When an observer opens the box, he becomes entangled with the cat, so observer-states corresponding to the cat being alive and dead are formed, and each can have no interaction with the other.

Curiously, all of this has some practical use in [quantum cryptography]?. It is possible to send down a fiber optic cable, light that is in a mixture of states. If one places a wiretap in the middle of the cable which intercepts the transmission and retransmits it, one conducts an observation which causes the light to fall into one state or another. By doing statistics at the other end of the cable, one can tell if one is received light that is in a mixture of states or if the light has been already observed and retransmitted. This allows one to develop communication systems that cannot be tapped without being noticed at the other end. This experiment (which one can do) also illustrates that the "observation" in quantum mechanics has nothing to do with consciousness in that a perfectly unconscious wiretap will cause the statistics at the end of the wire to be different.