Eigenstate-to-eigenstate evolution with no intermediate superposition?

• James MC
In summary, it seems that the only way that the state vector can evolve from one eigenstate of O to the next, is to rotate between the two eigenvectors so that intermediate state vectors are superpositions of the eigenstates of O. This question is still up for debate though, as there might be cases where values for a property cannot superpose, but still exist in some sense.
James MC
Eigenstates of some observable O are represented by orthonormal vectors in complex Hilbert space.

Is it true that the only possible way that the state vector can evolve from one eigenstate of O to the next, is to rotate between the two eigenvectors so that intermediate state vectors are superpositions of the eigenstates of O?

That is, are properties that (i) can vary (i.e. not constant) but (ii) cannot superpose, simply impossible to represent in quantum mechanics?

My suspicion is that you might be able to rotate the state vector through the complex plane in such a way that you go from one eigenstate to the next but avoiding superpositions of those eigenstates. But not sure.

So what is it in general? I mean let's say in the middle of the rotation through the complex plane that you suggested.

In my humble opinion I think the answer to the above question is yes in some sense, we must also consider that we are making measurements in the eigen-space of the operator!

Then what will you have to say about the state in some other non-commuting space? what if we measure that state?

I am just thinking loudly! Hope I made some sense!

James MC said:
That is, are properties that (i) can vary (i.e. not constant) but (ii) cannot superpose, simply impossible to represent in quantum mechanics?

What would be an example of a property that cannot superpose? As you say, these are orthogonal vectors in a vector space; so their linear combinations must also be vectors in that vector space.

Nugatory said:
What would be an example of a property that cannot superpose? As you say, these are orthogonal vectors in a vector space; so their linear combinations must also be vectors in that vector space.

Yes but it is not mandatory to treat their linear sums as representing anything in the world. Textbooks often say that if a system is associated with some vector space then any Hermitian operator on that space designates a property - as if that followed simply from being a Hermitian operator. But this is misleading. For if the dynamics is stipulated in such a way that the state vector cannot rotate into such a sum, then for that reason alone there is no reason to assume that such a vector is an eigenvector of some measureable property.

To answer your question, maybe the property of being conscious cannot superpose, as Wigner thought. Or as Penrose once thought (I think) the property of being a spacetime region cannot superpose. Then the question is whether such properties can be represented quantum mechanically (one might want to do this if one thought either of those properties collapses wave functions). This raises the question: if values for such properties are represented by orthonormal vectors, but superpositions are ruled out, can we make sense of time-evolution of their state vectors, or are such properties destined to never change...

Nugatory said:
What would be an example of a property that cannot superpose?
Well, there's charge. Also total angular momentum (e.g., you can't have a superposition of spin-1/2 and spin-1, iirc.)

I.e., any property for which superselection rules apply.

Of course, lots of reasonable Hamiltonians tend to commute with such operators, so the conundrum doesn't arise.

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James MC said:
Then the question is whether such properties can be represented quantum mechanically (one might want to do this if one thought either of those properties collapses wave functions). This raises the question: if values for such properties are represented by orthonormal vectors, but superpositions are ruled out, can we make sense of time-evolution of their state vectors, or are such properties destined to never change...
Let's take a step back...

What matters when constructing a quantum model of a system is the dynamical group. I.e., the largest group of transformations which maps solutions of the equations of motion into other solutions. Typically, the group has a continuous (identity-connected) subgroup, but there might also be discrete elements corresponding to parity, charge, and various other things.

Then one finds a maximal mutually-commuting subset of the generators of the group (usually containing the Hamiltonian), then we find the spectrum (by demanding that these operators be representable as self-adjoint operators on an abstract Hilbert space). The spectrum of the Casimir operators for the group then determines one set of superselection rules, i.e., different values for the Casimirs correspond to distinct concrete Hilbert spaces, and so on. The procedure for quantizing ordinary angular momentum is a simple example of this. (Are you familiar with the latter, as presented in, say, Ballentine's textbook?)

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1. What is eigenstate-to-eigenstate evolution with no intermediate superposition?

Eigenstate-to-eigenstate evolution with no intermediate superposition is a concept in quantum mechanics where a system evolves from one eigenstate to another without passing through any intermediate superpositions. This means that the system remains in a single, definite state throughout the evolution.

2. How is eigenstate-to-eigenstate evolution different from other quantum processes?

Eigenstate-to-eigenstate evolution is different from other quantum processes because it does not involve any intermediate superpositions. In other processes, the system may exist in a superposition of multiple states before collapsing into a definite state.

3. What are the implications of eigenstate-to-eigenstate evolution in quantum computing?

The concept of eigenstate-to-eigenstate evolution is crucial in quantum computing as it allows for precise control and manipulation of quantum states. This enables quantum computers to perform calculations and algorithms more efficiently than classical computers.

4. Can eigenstate-to-eigenstate evolution be observed in real-life systems?

Yes, eigenstate-to-eigenstate evolution has been observed in various quantum systems, such as atoms, photons, and superconducting circuits. These systems have been manipulated and controlled to undergo evolution from one eigenstate to another without any intermediate superpositions.

5. How does the concept of eigenstate-to-eigenstate evolution relate to the uncertainty principle?

The uncertainty principle states that it is impossible to know both the exact position and momentum of a particle simultaneously. In eigenstate-to-eigenstate evolution, the system remains in a definite state throughout the evolution, meaning that either the position or momentum is precisely known at any given time. This concept is closely related to the uncertainty principle, which describes the inherent uncertainty in measuring certain properties of a quantum system.

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