Quantum Bomb Detection

[Bill S]

For the Elitzur–Vaidman experiment, I need to check that I have the right idea about the interferometer, and the process of splitting photons, but one step at a time.

 

My understanding of the interferometer is that it has two paths (1 & 2) from the emitter (E) to the detectors (d1 & d2), that identify the path(s) taken by the photon, and that there must be another detector (D) that registers the resulting interference pattern after recombination.

 

It would be wise to check this first, as, if my impression is wrong, it might influence my need for further questions.

 

Happy New Year to everyone.

 

[swansont]

There are two detectors, one for each possible path through the Mach-Zehnder interferometer.

 

https://en.wikipedia.org/wiki/Elitzur–Vaidman_bomb_tester

"The interferometer is aligned so that the interference is constructive at C and destructive at D."

[Bill S]

There are two detectors, one for each possible path through the Mach-Zehnder interferometer.

 

 

So, nothing to observe any interference pattern after recombination?

[swansont]

So, nothing to observe any interference pattern after recombination?

You have the two detectors. What do you expect the interference would look like?

[Bill S]

My thinking was that each detector detected light from only one path, so how/where would the recombination happen?

[swansont]

My thinking was that each detector detected light from only one path, so how/where would the recombination happen?

The detectors are after the path recombination. If there's no interaction, one detector will always trigger, and the other one never does. Interference means the other one can trigger.

[Bill S]

So far, so good.

 

If a single photon is sent through, and a photon is a quantum, how can it be “split”?

 

My guess is it is linked to the fact that the “photon” travels as a wave, but explanations talk of a photon being split. I thought a quantum was as small as one could go.

[swansont]

So far, so good.

 

If a single photon is sent through, and a photon is a quantum, how can it be “split”?

 

My guess is it is linked to the fact that the “photon” travels as a wave, but explanations talk of a photon being split. I thought a quantum was as small as one could go.

From the obseved behavior of interference patterns arising when a single photon is in the system, the conclusion is that the photon, in essence, travels both paths (as long as you can't determine which path information). Saying it is split is a colloquial expression to describe this.

[Bill S]

From the obseved behavior of interference patterns arising when a single photon is in the system, the conclusion is that the photon, in essence, travels both paths (as long as you can't determine which path information). Saying it is split is a colloquial expression to describe this.

 

 

This was, more or less, the point I had reached, but doubts crept in when I read Michael Brooks’ explanation: “To visualise what is going on, think of a photon entering the interferometer and taking one path while a ghostly copy of itself goes down the other. In Elitzur and Vadman’s thought experiment, half the time there is a photon-triggered bomb blocking one path” ……. “Only the real photon can trigger the bomb, so if it is the ghostly copy that gets blocked by the bomb, there is no explosion – and nor is there an interference pattern at the other end.

 

I appreciate that this is an analogy, and that as such, nit-picking its details is not very helpful, but it does seem to say there is a basic difference between the “entities” that follow each path. How can this be, if “they” are the same photon?

[swansont]

If the bomb is triggered, you know what path the photon took.

[Bill S]

If the bomb is triggered, you know what path the photon took.

 

 

I get that part.

 

Perhaps I’m trying to understand something that comes into the “shut up, and calculate” area of QM, but the bit that bugs me is the idea that if it is a single photon that takes both paths, its manifestation on each path can be different, and “it” can do different things on different paths.