There is some ambiguity, with how I see other sources defining “quantum superposition”. From what I can extract, If there is a quantity of particles, whose combined wave-function is of a mixed nature between two other wave-functions, and if single particles are thought to emerge from that quantity, it can happen that the state of each particle is unknown, with a probability function between the two, mixed states. In that case, the particle can be superposed, as if having properties belonging to both states.
I think that some public writing fails to distinguish between the quantum superposition, and a possible, simple mixing of the properties of particles, whose states may be distinct.
(Edited 02/21/2018 :
In any case, if a particle is superposed, then one category of phenomena which may follow, is that its state may be “witnessed”,
at which point it is no longer superposed. But while its state is superposed, without collapsing this superposition, its superposed states can have an effect on whether it can be witnessed or not. Specifically, if the wave-functions of the two states cancel out, then the presence of the particle cannot be detected, and therefore, its state can also not be witnessed. )
(As of 02/21/2018 :
As a result of a recent experiment, I’m learning to modify my vocabulary to some extent. As I now have it, the collapse of superposition of states is not always possible. But, a state can nevertheless be witnessed as belonging to one out of two entangled photons, in which case it will either be equal or opposite, depending on the case, to the corresponding state of the other particle, of that entangled pair of particles.
Whether this state is defined by the superposition of a separate pair of states, may not be relevant, to whether it can be witnessed.
My experiment did not involve entanglement. But, I’m inferring ideas from it anyway, and this would be a hypothetical example:
- A type of entanglement is possible, that affects linear polarization.
- A type of entanglement is possible, that affects circular polarization.
Example (1) should lead to matching, while example (2) should lead to opposite states. More specifically, Example (1) should lead to an inversion along an arbitrary axis.
I should add a detail which most people already know:
- When a particle is witnessed as having a defined state, that state also changes.
End of Edit, 02/21/2018. )
I think that some experiments with entangled particles have as their basis, to use such cancellation, to reduce the rate at which some particles are witnessed in one beam, in an attempt to communicate this event to a second beam, whose particles should be entangled with the particles of the first beam.
The only part of this that really interests me for the moment, is the fact that light could be plane-polarized, and that at the same time, its photons could be in some superposed state. If a Polarizing Mirror next tries to extract light from that beam, which is polarized perpendicularly to the original direction, then the wave-function of this sought direction would be zero – even if this is due to cancellation. And then, not only would the amplitude of the derived beam be zero, but the state of the particles in the original beam, would also not be witnessed.
This would be, because a polarizing mirror actually ‘does something’, when a photon has the selected combination of properties. In contrast, if the linear polarizer is a Gel-Block, it ‘does something’, when photons have the opposite of the selected combination of properties – it absorbs them. Thus, for a gel-block not to witness the particles, it needs to be oriented parallel to the direction of polarization of the original beam.