Quantum Measurement: the Roll of the Screen

Most recent answer: 02/05/2015

Q:
Why does a photon remain in a state or superposition until it is "measured" by a screen? Why isn't it "measured" by the surrounding environment as it travels to the screen, causing "collapse" of the wave function? The screen seems to have a special place in the double-slit experiment, which doesn't make sense.
- John (age 55)
VT
A:

What's special about the screen here is that if you were to solve for its quantum state after the particle (photon or whatever) arrives, you'd get a state with components representing large-scale different situations involving lots of particles. The spread-out single particle wave turns into a superposition of different states representing absorption by different parts of the screen. Once that happens, there's no way of steering the cat back into the bag, getting all those many particles in the screen and those coming out of it back to the same positions regardless of which part of the screen was hit. That means that there won't be interference between those different possibilities. There is, to put it mildly, some disagreement as to whether that means that all possibilities propagate on into the future as distinct worlds or whether somehow the state collapses to just one such possibility. The former types of interpretations seem to follow from the equations describing how the waves behave, the latter seem more in tune with our intuition.

By is there anything else special about the screen? could some other sort of detectors also "collapse" the wave? They certainly can. The requirement to do so is always the same- that the "detector" be sufficiently coupled to the outside world to transmit out some information as to which way the particle is taking. This process of losing interference is called "decoherence". There's always a background of interference due to coupling between the simple part of the quantum system (e.g. that one particle) and everything else. In many cases, the sources of decoherence are well understood. It's a topic of both fundamental interest and of practical importance, since decoherence destroys the interference needed for quantum computation.  People look for simple quantum states that have little decoherence yet can couple to some control variable for use as the bits in quantum computers.

Mike W.

 

Here's some other answers on the topic.

https://van.physics.illinois.edu/qa/listing.php?id=17688&t=how-far-does-quantum-interference-reach
https://van.physics.illinois.edu/qa/listing.php?id=21288

 

 


 


(published on 02/05/2015)