I Intrinsic or non-intrinsic properties

[friend]

As I understand it, the properties of a particle exist in superposition in the wave function until a measurement forces a the wave function to "collapse" to one possibility, and the particle continues to have that value for the property until it interacts again with something else. But for someone who doesn't know that the particle has been previously measured and that it has not yet interacted again, then for that person, that property is still in superposition and is inherently unknowable until there is a measurement. So it seems there is two states of knowledge that seem to be in contradiction. For one person, the value is determined, for the other it is inherently unknowable. Both conditions for both observers seem to be able to exist at the same place at the same time. What's going on? Are properties inherently intrinsic to the particle or not? How do we know whether any particle we might ever measure has not already been measured by someone else or has not afterward interacted again?

 

[mikeyork]

You have put your finger on the central problem with the way QM is usually discussed. If a state has been previously determined (observed or prepared) then, unless it interacts again before a subsequent observation, the fundamental principle on which all science depends is that a subsequent observation must give a consistent result. It is entirely irrelevant whether the subsequent observer has incomplete information or not.

The key point about whether we should treat the state as determined or a superposition is that any observer will get a result that must be consistent with the information inherent in the state's creation. Think about the supposed non-locality issue with regard to entangled particles separated in space and compare it to the classical case of a pair of gloves sent to opposite sides of the world. The difference is that in the classical case the prepared pair of gloves is already separated into two gloves, pre-determined to be left and right respectively, when they are put in their boxes. So the state of each glove is information already available in the universe whether or not any observer is aware of it. But in the QM case, the prepared state is a only a composite state in which there is no information concerning the individual states other than that they are entangled with pre-determined composite quantum numbers. The information concerning which particle is in which state is not available to any observer until either is observed.

 

[friend]

As I recall, a quantum computer relies on the superposition of particle properties. But we don't know whether through some series of entanglements (perhaps) someone in the universes may already know what state the qubit is in. How then can the quantum computer work when someone knows the collapsed state of that superposition? Are you telling me that there is some proof that a measurement has never taken place on our systems? Just because you may not know something doesn't mean nobody knows?

 

[Nugatory]

As I understand it, the properties of a particle exist in superposition in the wave function until a measurement forces a the wave function to "collapse" to one possibility, and the particle continues to have that value for the property until it interacts again with something else.

Yes, that's a reasonable way of thinking about this particular question - the wave function interacted with the measuring device, it was changed by the interaction, and it changed into a state such that subsequent measurements of the same property will yield the same result. But...

But for someone who doesn't know that the particle has been previously measured and that it has not yet interacted again, then for that person, that property is still in superposition and is inherently unknowable until there is a measurement.

This is pretty much the same problem as if I toss a coin and then look at it while you aren't watching. I know whether it's heads or tails, and you won't know until you look.

 

[mikeyork]

As I recall, a quantum computer relies on the superposition of particle properties. But we don't know whether through some series of entanglements (perhaps) someone in the universes may already know what state the qubit is in. How then can the quantum computer work when someone knows the collapsed state of that superposition?

A determinate state (even if unknown) is just a limiting case of a superposition. A quantum computer may create a superposition and that superposition might be the limiting determinate case.

Are you telling me that there is some proof that a measurement has never taken place on our systems? Just because you may not know something doesn't mean nobody knows?

A genuine superposition such as in the tests of Bell's theorem or a double slit experiment will manifest as such. If a classical deterministic state is observed then it will have been prepared (or previously observed) as such. Note that "observation" in QM includes state preparation by the observational apparatus not just detection (always conditional on the preparation).

 

[friend]

A genuine superposition such as in the tests of Bell's theorem or a double slit experiment will manifest as such.

A genuine superposition... from whose perspective? The second person who has not measured the particle yet will still think it is in "genuine" superposition.

I guess I'm asking the old Bohr/Einstein debate about whether the properties of a particle objectively exist apart from anyone's observation or whether those properties don't really exist until observed. Here it seems both can actually be right.

But as I recall, an observation can collapse the wave function even after the superposition has been used in experiment. If one observes which slit a particle went through even from behind a screen where you would expect to observe fringes, this can collapse the wave function and the fringes will disappear. So can an observation be made of the qubit after a quantum computer calculation has been completed. If not, why not?

And by the way, how does one "prepare" a system into a superposition for a quantum computer? Any preparation would collapse the system out of superposition, right?

 

[Nugatory]

A genuine superposition... from whose perspective? The second person who has not measured the particle yet will still think it is in "genuine" superposition.

Sure, but what does what he thinks have to do with anything except what he thinks? The particle has been through an interaction that has led to decoherence so it's no longer in a superposition of the possible measurement outcomes, and this happened whether the second person knows about it or not.

I guess I'm asking the old Bohr/Einstein debate about whether the properties of a particle objectively exist apart from anyone's observation or whether those properties don't really exist until observed.

No, you are not asking that question. It doesn't even arise here, because you've already said that the particle has been observed. (And do remember that in quantum mechanics the words "observe" and "observation" don't mean what they do in ordinary English usage - you don't need an observer to have an observation).

And by the way, how does one "prepare" a system into a superposition for a quantum computer? Any preparation would collapse the system out of superposition, right?

No, because all states are always superpositions of something. For example, if I want to prepare a particle in a position of spin-up and spin-down (which makes for a fine two-state qubit) I prepare it with horizontal spin, in the left-hand direction for example. This state is not a superposition of spin-left and spin-right, but it is a superposition of spin-up and spin-down which is what I'm looking for.

 

[friend]

No, you are not asking that question. It doesn't even arise here, because you've already said that the particle has been observed. (And do remember that in quantum mechanics the words "observe" and "observation" don't mean what they do in ordinary English usage - you don't need an observer to have an observation).

It takes an interaction (with a measurement machine) to force a particle out of superposition and collapse the wave function into one of the alternatives. And it remains in that collapsed state until it interacts with something else. So funny thing. It take an interaction to force the wave function out of superposition, and it also takes an interaction to force the existence of a superposition. In both cases the particle that interacts (to force a collapse or superposition) gets entangled with the particle. I think the difference is that in measurement, the measuring device tells us the state of those entangled particles. So we know the state of the system. In the forced superposition, we do not know the state of those entangled interacting particles. But those entangled particles that force superposition could possibly be observed by someone at some time in the future. And we can never say that they won't ever be measured. So how do we know those particles that forced a superposition might one day be read and what we thought was a superposition becomes a collapsed, measured state?

No, because all states are always superpositions of something. For example, if I want to prepare a particle in a position of spin-up and spin-down (which makes for a fine two-state qubit) I prepare it with horizontal spin, in the left-hand direction for example. This state is not a superposition of spin-left and spin-right, but it is a superposition of spin-up and spin-down which is what I'm looking for.

Thank you. I see your point.