WHAT IS QUANTUM THEORY ABOUT?

Schrödinger's Cat

He wrote:

'One can even set up quite ridiculous cases. A cat is penned up in a steel chamber, along with the following device (which must be secured against direct interference by the cat): in a Geiger counter there is a tiny bit of radioactive substance, so small that perhaps in the course of an hour one of the atoms decays, but also, with equal probability, perhaps none (decay); if it happens, the counter tube discharges and through a relay releases a hammer which shatters a small flask of hydrocyanic acid.'

'If one has left this entire system to itself for an hour, one would say the cat still lives if meanwhile no atom has decayed. The psi-function ( the wave describing the state) of the entire system would express this by having in it the living and dead cat (pardon the expression) mixed or smeared out in equal parts.' In other words, while it is in the closed box Schrödinger's cat is in an indeterminate state. After an hour this state is described by the sum of two waves in equal proportions - one for the 'alive' state of the cat and the other for its 'dead' state. This indeterminacy of the state of the cat arises because the cat's state is entangled with the state of a radioactive nucleus - if the nucleus is undecayed the cat is alive, if it is decayed the cat is dead. And the state of the nucleus really is indeterminate.

Once one opens the chamber, of course, it is easy to see that the cat is either alive or dead. As soon as this observation is made the indeterminacy in the state of the cat is removed: the wave describing the state of the cat collapses to either the 'alive' wave or the 'dead' wave.

The problem here is how, and at what point, the wave describing the indeterminate state collapses to a wave describing a well determined state. We don't really expect the cat to exhibit quantum indeterminacy, but at what stage does the process cease being a quantum process and begin to behave classically? This is a problem that is not completely resolved. It seems likely that when enough atoms or other small particles become involved in the magnification process, transforming the decay of the nucleus to the breaking of the flask of hydrocyanic acid, the quantum behaviour is gradually washed out. However at present nobody knows the criteria which decide when or how this occurs. Indeed careful experiments with quite large objects can show quantum behaviour. Interference experiments have been carried out with bucky balls which are football-shaped clusters of 60 carbon atoms. And recently electric currents in a superconducting ring have been seen in an indeterminate state in which clockwise and counterclockwise flow are combined like the live and dead states of the cat! (See this Physics Web newsflash).

The strange indeterminacy and entanglement of quantum states is the basis of exciting applications of quantum optics. Just as the state of Schrödinger's Cat was entangled with the state of a radioactive nucleus, the states of two photons may be entangled. This is exploited in new methods of cryptography (quantum cryptography) and may eventually allow us to build and use quantum computers.