Consecutive Measurements on One of an Entangled Pair

In summary, the conversation discusses a scenario in which two entangled particles are generated and one particle is measured along different axes while the other particle's spin is measured at different points in time. It is concluded that entanglement ceases after the first measurement on the first particle and any subsequent measurements on the second particle are not affected by the first measurement. Additionally, it is mentioned that there may still be a correlation between spin and position/momentum, but this is due to the nature of spin and its relationship with p/q.
  • #1
Q-reeus
1,115
3
I'd like some informed opinion on the following scenario, which in a certain article forms the basis of a controversial claim.
Suppose an entangled spin anti-correlated pair of particles are generated by some process. Particle A flies off along the +z direction, particle B along the -z direction. In particle A's path are placed three consecutively spaced spin detectors. The first measures say along the x axis, the second along the y axis, the third along the x-axis again. We can choose to measure particle B's spin either after the first, second, or third measurement performed on particle A. Is it the case that entanglement ceases after the first measurement on A, in which case particle B's spin will always be measured as the opposite of the first x-axis measurement on A, regardless of subsequent measurements on A. Or does entanglement survive until particle B is actually measured, in which case presumably it could have the anti-correlated value of particle A's second or third measurement, depending on just when measurement on B is performed?
 
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  • #2
There is "some" sense in which the A's entangled state survives after A's first measurement. But probably not in the fashion you might guess.

1. If you measure A's x spin first, then A is no longer spin entangled with B. It does not really matter whether A or B is measured first, the results are identical and cannot be distinguished.

2. If you measure A's x spin first, it is *possible* that A may remain position/momentum entangled with B thereafter. This is because spin commutes with p/q.
 
  • #3
DrChinese said:
There is "some" sense in which the A's entangled state survives after A's first measurement. But probably not in the fashion you might guess.

1. If you measure A's x spin first, then A is no longer spin entangled with B. It does not really matter whether A or B is measured first, the results are identical and cannot be distinguished.

2. If you measure A's x spin first, it is *possible* that A may remain position/momentum entangled with B thereafter. This is because spin commutes with p/q.
Thanks DrC. Point 1 answers my main query, falsifying the claim in the article I alluded to, which was that subsequent measurements on A could potentially force changes in the definite spin state of particle B remotely. Point 2 I am less clear on. So in general there will be jointly spin and position/momentum anti-correlation initially? Could you explain how spin commutes with p/q? I naively assumed they are unlinked quantities.
 

1. What is meant by "Consecutive Measurements on One of an Entangled Pair"?

Consecutive measurements on one of an entangled pair refers to the process of performing multiple measurements on one of the particles in an entangled pair, in order to observe and study its quantum properties.

2. How are entangled pairs created?

Entangled pairs are created through a process called entanglement, which involves manipulating the quantum states of two particles in such a way that their properties become correlated. This can be achieved through various methods such as photon polarization or superposition.

3. What is the significance of performing consecutive measurements on an entangled pair?

Consecutive measurements on an entangled pair allow scientists to study the relationship between the two particles in the pair and observe how their properties are interconnected. This can provide insights into the nature of quantum entanglement and its potential applications in fields such as quantum computing.

4. What are some potential applications of entangled pairs?

Entangled pairs have potential applications in quantum communication, cryptography, and computing. They could also be used in precision measurements and quantum teleportation.

5. Is it possible to perform consecutive measurements on one particle of an entangled pair without affecting the other?

Yes, it is possible to perform consecutive measurements on one particle of an entangled pair without affecting the other, as long as the particles remain isolated from external interference. This is known as quantum non-demolition measurement.

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