- #1
Marilyn67
- 100
- 20
- TL;DR Summary
- Presentation of two identical experiments : One with "pure" photons and one with entangled photons.
Hello,
The Mach-Zehnder interferometer makes it possible to highlight the corpuscular and undulatory aspect of light, (in particular using single photons).
By using the "continuous" beam from a coherent source, one is able to visualize directly on two screens E1 and E2, an interference pattern or a diffuse pattern by means of two lenses L1 and L2, depending on whether the light borrows the two paths through the mirrors M1 and M2 or an obstacle is placed on one of the two paths.
I did this fairly simple experiment a few weeks ago as said in my previous thread, and I have some photos and a short video if you're interested![Wink :wink: :wink:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
In the following diagram, light takes two paths :
The source is a 20 mW green laser diode at 532 Nm, and thanks to the BE beam expander, the fine tuning of the interferometer is facilitated, and the appearance of the patterns is relatively easy to obtain.
This tricky step was much more difficult without a beam expander to properly "match" too small "spots" on the BS1 and BS2 beamsplitters.
We observe very clearly with the naked eye in E1 and E2, pretty interference fringes like these (several millimeters of inter-fringes) :
By placing an obstacle on one of the two paths, these fringes disappear and are replaced by a diffuse pattern like this :
So far, no mystery.
Now, let's modify the source as shown in the following diagram :
This time, a beam is exclusively composed of entangled photons (all members of the same subset) at 810 Nm (near infrared) and its power is about a million times weaker, or even a little less.
The downstream device has not changed, except that the two screens E1 and E2 have been replaced by two CCD screens (cameras), to make it possible to "see" patterns invisible to the naked eye (near infrared spectrum and low power detectable with peltier-cooled cameras).
This device is of course in total darkness (box), and sheltered from any parasitic emission.
The upstream device is made up of a violet "pump" laser diode of 100 mW at 405 Nm, the beam of which passes through a BBO crystal (type I), after being polarized, stripped of its sub-harmonics (blue), then a birefringent crystal like a properly tilted quartz plate, for the phase matching needed for the SPDC.
The main beam is completely stopped and absorbed after passing through the BBO crystal, and the two entangled beams at 810 Nm each move away by 3° from the axis of the main beam.
So one of the two beams is sent into my Mach-Zehnder interferometer.
I am "Alice" and I perform exactly the same experiments as before, with the previous green diode.
The second beam is sent "elsewhere" (say to "Bob"), and I don't care.
I know, and we all know, that whatever I do with my interferometer can in no way be used to communicate "FTL" information to Bob, we agree (it is'nt my question).
My question is the following (I couldn't find it anywhere explicitly) :
By carrying out the same experiments as before (with the green diode), on my side only, (Alice) will the experimental results be (theoretically) the same here ? (always on my side, Alice), that is to say :
- One of my paths is closed and I directly observe my two diffuse patterns...(?)
- The light follows my two paths and I directly observe my two interference patterns...(?)
In other words, does the presence of an obstacle on one of my two paths always give (in theory) a difference in pattern on my two screens, indicating to me that the obstacle is present or not at my place ?![Confused :oldconfused: :oldconfused:](https://www.physicsforums.com/styles/physicsforums/xenforo/smilies/oldschool/confused.gif)
What answer does quantum mechanics provide to this question ?![Head Bang :headbang: :headbang:](https://www.physicsforums.com/styles/physicsforums/xenforo/smilies/oldschool/HeadBanger.gif)
In advance, thank you for your informed answers.
Cordially,
Marilyn
The Mach-Zehnder interferometer makes it possible to highlight the corpuscular and undulatory aspect of light, (in particular using single photons).
By using the "continuous" beam from a coherent source, one is able to visualize directly on two screens E1 and E2, an interference pattern or a diffuse pattern by means of two lenses L1 and L2, depending on whether the light borrows the two paths through the mirrors M1 and M2 or an obstacle is placed on one of the two paths.
I did this fairly simple experiment a few weeks ago as said in my previous thread, and I have some photos and a short video if you're interested
In the following diagram, light takes two paths :
This tricky step was much more difficult without a beam expander to properly "match" too small "spots" on the BS1 and BS2 beamsplitters.
We observe very clearly with the naked eye in E1 and E2, pretty interference fringes like these (several millimeters of inter-fringes) :
By placing an obstacle on one of the two paths, these fringes disappear and are replaced by a diffuse pattern like this :
So far, no mystery.
Now, let's modify the source as shown in the following diagram :
This time, a beam is exclusively composed of entangled photons (all members of the same subset) at 810 Nm (near infrared) and its power is about a million times weaker, or even a little less.
The downstream device has not changed, except that the two screens E1 and E2 have been replaced by two CCD screens (cameras), to make it possible to "see" patterns invisible to the naked eye (near infrared spectrum and low power detectable with peltier-cooled cameras).
This device is of course in total darkness (box), and sheltered from any parasitic emission.
The upstream device is made up of a violet "pump" laser diode of 100 mW at 405 Nm, the beam of which passes through a BBO crystal (type I), after being polarized, stripped of its sub-harmonics (blue), then a birefringent crystal like a properly tilted quartz plate, for the phase matching needed for the SPDC.
The main beam is completely stopped and absorbed after passing through the BBO crystal, and the two entangled beams at 810 Nm each move away by 3° from the axis of the main beam.
So one of the two beams is sent into my Mach-Zehnder interferometer.
I am "Alice" and I perform exactly the same experiments as before, with the previous green diode.
The second beam is sent "elsewhere" (say to "Bob"), and I don't care.
I know, and we all know, that whatever I do with my interferometer can in no way be used to communicate "FTL" information to Bob, we agree (it is'nt my question).
My question is the following (I couldn't find it anywhere explicitly) :
By carrying out the same experiments as before (with the green diode), on my side only, (Alice) will the experimental results be (theoretically) the same here ? (always on my side, Alice), that is to say :
- One of my paths is closed and I directly observe my two diffuse patterns...(?)
- The light follows my two paths and I directly observe my two interference patterns...(?)
In other words, does the presence of an obstacle on one of my two paths always give (in theory) a difference in pattern on my two screens, indicating to me that the obstacle is present or not at my place ?
![Confused :oldconfused: :oldconfused:](https://www.physicsforums.com/styles/physicsforums/xenforo/smilies/oldschool/confused.gif)
What answer does quantum mechanics provide to this question ?
![Head Bang :headbang: :headbang:](https://www.physicsforums.com/styles/physicsforums/xenforo/smilies/oldschool/HeadBanger.gif)
In advance, thank you for your informed answers.
Cordially,
Marilyn