I How does a process of a Raman transition break quantum entanglement?

[tade]

I'm reading this article about quantum entanglement, and the author writes about a process in a Raman transition which would break the entanglement, and I'm interested about how it breaks the entanglement.

So the passage which I'm interested in begins with: "The answer is to do an operation that we would describe in words", and the author also writes: "otherwise leave it alone".

And so what I'm wondering is, if a particle happens to be "left alone", since its being "left alone", and not interacting with the laser at all, as the author describes it, why does this act still break the entanglement?

 

[DrClaude]

It is hard to know exactly what the author means because it is written with words rather than math. But the Raman transition is basically a measurement, as by monitoring whether a photon was absorbed or not, you have measured the state of the particle.

 

[tade]

It is hard to know exactly what the author means because it is written with words rather than math. But the Raman transition is basically a measurement, as by monitoring whether a photon was absorbed or not, you have measured the state of the particle.

and so after the Raman measurement is done, does it break the correlation between Alice and Bob

 

[DrClaude]

and so after the Raman measurement is done, does it break the correlation between Alice and Bob

Basically, yes.

 

[tade]

Basically, yes.

oh i see, so something about it is different from the type of measurement which allows for correlations right

 

[PeterDonis]

something about it is different from the type of measurement which allows for correlations

No. Any measurement will break entanglement. (More precisely, anything that involves decoherence will break entanglement; measurement involves decoherence.) And as @DrClaude said in post #2, the Raman transition involves a measurement.

 

[Twigg]

Raman transitions are just one way to implement a spin flip on an ion qubit. Spin flips are a coherent process and do not break entanglement. As others have already said, it's measurement that breaks entanglement.

In a typical ion quantum computer, measurement is performed by fluorescence: you drive the qubit's ground (or excited) state to a short-lived higher energy state and see if any photons are spontaneously emitted. If you detect a photon, then the qubit state you probed (ground or excited) was populated prior to your measurement. In this case, decoherence arises from the fact that the spontaneous emission process is driven by the vacuum electromagnetic field.