Understanding superposition and entanglement

In summary, the conversation discusses the concepts of quantum mechanics and entanglement. The speaker, Marty, is seeking verification on some assertions, including the impact of measuring entangled particles and the no communication theorem. The responder clarifies that measurements terminate superposition and that determining if a particle is in superposition would violate the no-communication theorem. Marty then proposes a hypothetical situation, which the responder explains is not possible due to the relationship between non-commuting observables and entanglement. The conversation concludes with the clarification that a random sequence cannot be used to send a signal.
  • #1
martyscholes
4
0
I am a newbie here, just an enthusiast with an above average understanding of math and physics, but exposed to QM after I left college.

I have looked through the posts and did not see a concise summary to the following. Please forgive me if I overlooked some threads.

There are some assertions I have which I want to verify are generally accepted to be true.
A. Measuring entangled particle A only impacts particle B by causing both to decohere where measured and both to go into superposition on a noncommuting measurement.
B. The no communication theorem states that measuring one entangled particle cannot provide measurable information to the other entangled particle.
C. There is no way to measure whether a particle is in superposition for a particular measurement.

Do I understand all of this correctly?

Many thanks,
Marty
 
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  • #2
martyscholes said:
Do I understand all of this correctly?

Many thanks,
Marty

Welcome to PhysicsForums, Marty!

Yes, I would say that you have it stated pretty well. I would comment on your A. that a measurement terminates the superposition and sends the particles into a mixed state, at least for non-commuting observables. Occasionally, you can observe entanglement continuing for commuting operators.
 
  • #3
DrChinese said:
Welcome to PhysicsForums, Marty!

Yes, I would say that you have it stated pretty well. I would comment on your A. that a measurement terminates the superposition and sends the particles into a mixed state, at least for non-commuting observables. Occasionally, you can observe entanglement continuing for commuting operators.

Thanks for the fast reply!

Extending the above, if there were a way to determine whether or not an entangled particle was in superposition for a particular observable, then one could violate the no-communication theorem?

Thanks again,
Marty
 
  • #4
martyscholes said:
Extending the above, if there were a way to determine whether or not an entangled particle was in superposition for a particular observable, then one could violate the no-communication theorem?

Thanks again,
Marty

That's correct, you cannot determine if the superposition is still effective by a local operation. You must correlate results from both Alice and Bob to learn this after a series of operations, which of course defeats the objective of sending an FTL signal.
 
  • #5
DrChinese said:
That's correct, you cannot determine if the superposition is still effective by a local operation. You must correlate results from both Alice and Bob to learn this after a series of operations, which of course defeats the objective of sending an FTL signal.

Thanks again for staying with me on this and feel free to point out at any time where I am off base.

Suppose I had a device which produced two entangles particles A and B with non-commuting observables X and Y. In addition, suppose that X is not in superposition and is set to a predefined state (stay with me on this fantasy).

Now if we measure observable Y on particle A, then observable X on both particles is in superposition. Once observable X on partible B is measured, it will either have the expected state, which tells us nothing, or it is in the unexpected state, which means that particle A has had observable Y measured.

Where did I go wrong?

Thanks,
Marty
 
  • #6
martyscholes said:
Thanks again for staying with me on this and feel free to point out at any time where I am off base.

Suppose I had a device which produced two entangles particles A and B with non-commuting observables X and Y. In addition, suppose that X is not in superposition and is set to a predefined state (stay with me on this fantasy).

Now if we measure observable Y on particle A, then observable X on both particles is in superposition. Once observable X on partible B is measured, it will either have the expected state, which tells us nothing, or it is in the unexpected state, which means that particle A has had observable Y measured.

Where did I go wrong?

Thanks,
Marty

If X and Y are non-commuting, then they cannot have a relationship in which X is entangled and Y is not. Either they are both in an entangled state, or neither are.

Also: they cannot be in an entangled state with a known X or Y. X/Y must be unknown prior to the measurement. The result will therefore be random. You cannot send much of a signal using a random sequence.
 
  • #7
Ok.

Many thanks for the clarification.

Cheers,
Marty
 

FAQ: Understanding superposition and entanglement

What is superposition?

Superposition is the ability of a quantum system to exist in multiple states at the same time. This means that a particle can be in different positions or have different properties simultaneously.

How does superposition work?

Superposition is a fundamental principle of quantum mechanics and it works by allowing particles to simultaneously exist in multiple states until they are observed. When an observation is made, the superposition collapses and the particle takes on a single definite state.

What is entanglement?

Entanglement is a phenomenon that occurs when two or more particles become connected in such a way that the state of one particle is dependent on the state of the other, even if they are separated by a large distance.

How does entanglement occur?

Entanglement occurs when particles interact and become entangled, meaning their quantum states become linked. This can happen through various processes, such as collisions or interactions with a common environment.

What are the practical applications of superposition and entanglement?

Superposition and entanglement have many potential applications, including quantum computing, cryptography, and communication. They also allow us to better understand and manipulate the behavior of particles at the quantum level, leading to advancements in technology and scientific research.

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