6.1 Schrödinger’s cat

A famous statement of this idea was given by Erwin Schrödinger in his Schrödinger’s cat thought experiment. In that thought experiment, the fate of a cat in an enclosed box depends on the disintegration of a single radioactive nucleus, which, when it decays, will trigger the release of poison and thereby the death of the cat. The nucleus is supposed to be in the superposition state

If the possible states for the cat are and , then the total state for the system of cat and nucleus is

The cat is entangled with the nucleus, since there exists a correlation between the state of the nucleus and the state of the cat (the state written down in Equation 17 is an entangled state).

The point of the thought experiment is that it demonstrates that thinking of the nucleus + cat system as a superposition of those extreme states is absurd.

Surely it’s absurd to believe that a cat is well-described by a state vector ( or ), and surely the cat is in the definite state ‘alive’until it definitely dies (or better, until the experiment is stopped before the cat is killed).

The issue at hand is that a useful working quantum computer containing hundreds of thousands of qubits would be more akin to Schrödinger’s cat than a single quantum object, like a solitary microscopic superconducting circuit.