Possibly. This page discusses several possible (gedanken-)experiments, involving fermions and photons, and tries to make deductions about entropy and the nature of measurement.

Probably not. One reason is that if this happened, then we would have a faster-than light signalling device. How is that? If the two devices were space-like separated by a large distance, then experimenter L could send messages to experimenter R by alternately switching in a hot or cold detector. Experimenter R would only have to watch which direction the temperature of their device went.

Another problem with this experiment is that there is some question of where the wave-function collapse actually took place. Did it occur while the fermion was in the magnetic field? Or did it only occur later, when the fermion collided with the film plate (or other particle detector)?

A better way to understand this type experiment might be by thinking about entropy. Entropy is usually defined in a classical sense, as the logarithm of the number of (thermodynamically accessible) states. In the following, it will be convenient to use the logarithm base-two, and talk entropy as being a number of bits.

One way to tie entropy to a quantum experiment is through the 'quantum teleportation' effect. There are two variants of this effect. In one variant, a quantum state in one location is destroyed, and reproduced in another location by conveying two bits of information and an entangled state to the second location. The other variant, the one of interest to us, chews up two bits in one location, and communicates an entangled states to the second location, at which the two bits are extracted. (Need diagrams here).

Leap: By considering the quantum teleportation experiment, it is tempting to split macroscopic state into two pieces. One is a set of classically-behaving 'bits', which contribute to the entropy and general thermodynamic behavior. The other piece is one big quantum-entangled state. The teleportation concept tells us that we can take bits and convert them into entangled states, and then pull them out again. A 'measurement' especially of the EPR type, is then really the statement that the two detectors have become (are) quantum correlated, and that we have also extracted bits from the state function. Experiments such as the 'quantum eraser' are 'just' a yanking of bits out, and then pushing them back into the quantum state.

What I find curious is that this view seems to indicate that detectors, e.g. a pair of stern-Gerlach detectors, become quantum-entangled with each other, and remain irrevocably so, even as we pull 'bits' out of the system that represent the actual measurements made. (Similarly, different grains of silver on a photographic film behind a two-slit arrangement become, and remain, quantum-entangled, even as bits are yanked out and mark the grains as 'exposed' and 'unexposed' grains).

We now come to the meta-questions: is the universe a pure state, or is it a mixture of disconnected states? If it is a mixture of disconnected states, how do we find them? If we find them, can we use quantum teleportation to 'shrink' one and 'grow' another? And what would the utility of that be?

- The original IBM quantum teleportation paper. (get citation)
- The quantum teleportation effect confirmed via photon experiment. (get citation)
- Quantum eraser (get citation)

August 2000

March 2001

Linas Vepstas linas@linas.org