# B Can superposition be attained for multiple states at once?

#### Grinkle

Gold Member
I hope my articulation makes sense.

Can I prepare a particle so that it has >1 states in superposition and resolve them at different times? I will make up states to try and illustrate my question better.

Prepare a particle so that spin up and spin down are in a state of superposition. Also, the states of spin left and spin right are in a state of superposition. I mean these states (up/down and left/right) to be independent of each other.

Then I measure up/down. Have I also fixed left/right even if don't observe it or can left/right still be said to be in a state of superposition for this particle?

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#### PeterDonis

Mentor
Prepare a particle so that spin up and spin down are in a state of superposition. Also, the states of spin left and spin right are in a state of superposition.
This is easily done, just prepare the particle in a state that is not an eigenstate of up/down or left/right. However, the particle still only has one state; it doesn't have multiple states. Viewing the state as being a "superposition" of up/down and left/right is a mathematical choice and doesn't affect the physics.

I mean these states (up/down and left/right) to be independent of each other.
I don't know what you mean by this. The particle only has one state.

Then I measure up/down. Have I also fixed left/right even if don't observe it
You can't measure up/down and left/right in the same measurement; those observables don't commute.

can left/right still be said to be in a state of superposition for this particle?
The state after the measurement will be either up or down, and neither of those is an eigenstate of left/right. So you could view the state after measurement as a superposititon of left and right. But as above, that's just a mathematical choice and doesn't affect the physics. The particle still has only one state.

#### Nugatory

Mentor
Can I prepare a particle so that it has >1 states in superposition and resolve them at different times?
A quantum system always has only one state. Superposition just means that it is mathematically possible to write the state in different ways. For example any state $|\psi\rangle$ can be written in the form $|\psi\rangle=\alpha|up\rangle+\beta|down=\gamma|left\rangle+\delta|right\rangle=\epsilon|45\rangle+\zeta|225\rangle$ - a superposition of up/down and also a superposition of left/right and of the two diagonal axes, and we could choose any other angle as well if we wanted. So yes, a state can be a superposition of more than one thing; in fact, it always is.

Prepare a particle so that spin up and spin down are in a state of superposition. Also, the states of spin left and spin right are in a state of superposition. I mean these states (up/down and left/right) to be independent of each other.
You can’t make them independent of one another, because there’s only one way of writing any particular state as (for example) a sum of up/down and as a sum of left/right. So once you’ve chosen your state $|\psi\rangle$ to be a particular superposition of up and down (that is, you've chosen particular values for $\alpha$ and $\beta$) you've determined the values of $\gamma$ and $\delta$ so in that sense they're not independent. However, all four of them can be non-zero so the state still a superposition of up/down and left/right and you cannot predict with certainty a measurement on either axis. So say you have prepared the particle in such a state and then....
Then I measure up/down. Have I also fixed left/right even if don't observe it or can left/right still be said to be in a state of superposition for this particle?
The measurement of up/down will collapse the wave function into either $|up\rangle$ or $|down\rangle$. Both of these states are superpositions of left and right; for example $|up\rangle=|left\rangle+|right\rangle$.

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#### Grinkle

Gold Member
Thanks @PeterDonis @Nugatory .

Looks like I was confusing state variables with the state itself in my thinking about superposition.

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"Can superposition be attained for multiple states at once?"

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