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Old Apr 17th 2015, 11:49 PM   #1
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Question Related To The Delayed Choice Quantum Eraser Experiment

I came across this article on Wikipedia:

The experimental setup, described in detail in Kim et al.,[1] is illustrated in Fig 2. An argon laser generates individual 351.1 nm photons that pass through a double slit apparatus (vertical black line in the upper left hand corner of the diagram).
An individual photon goes through one (or both) of the two slits. In the illustration, the photon paths are color-coded as red or light blue lines to indicate which slit the photon came through (red indicates slit A, light blue indicates slit B).
So far, the experiment is like a conventional two-slit experiment. However, after the slits, spontaneous parametric down conversion (SPDC) is used to prepare an entangled two-photon state. This is done by a nonlinear optical crystal BBO (beta barium borate) that converts the photon (from either slit) into two identical, orthogonally polarized entangled photons with 1/2 the frequency of the original photon. The paths followed by these orthogonally polarized photons are caused to diverge by the Glan-Thompson Prism.
One of these 702.2 nm photons, referred to as the "signal" photon (look at the red and light-blue lines going upwards from the Glan-Thompson prism) continues to the target detector called D0. During an experiment, detector D0 is scanned along its x-axis, its motions controlled by a step motor. A plot of "signal" photon counts detected by D0 versus x can be examined to discover whether the cumulative signal forms an interference pattern.
The other entangled photon, referred to as the "idler" photon (look at the red and light-blue lines going downwards from the Glan-Thompson prism), is deflected by prism PS that sends it along divergent paths depending on whether it came from slit A or slit B.
Somewhat beyond the path split, the idler photons encounter beam splitters BSa, BSb, and BSc that each have a 50% chance of allowing the idler photon to pass through and a 50% chance of causing it to be reflected. Ma and Mb are mirrors.

Figure 3. x axis: position of D0. y axis: joint detection rates between D0 and D1, D2, D3, D4 (R01, R02, R03, R04). R04 is not provided in the Kim article, and is supplied according to their verbal description.

Figure 4. Simulated recordings of photons jointly detected between D0 and D1, D2, D3, D4 (R01, R02, R03, R04)
The beam splitters and mirrors direct the idler photons towards detectors labeled D1, D2, D3 and D4. Note that:
If an idler photon is recorded at detector D3, it can only have come from slit B.
If an idler photon is recorded at detector D4, it can only have come from slit A.
If an idler photon is detected at detector D1 or D2, it might have come from slit A or slit B.
The optical path length measured from slit to D1, D2, D3, and D4 is 2.5 m longer than the optical path length from slit to D0. This means that any information that one can learn from an idler photon must be approximately 8 ns later than what one can learn from its entangled signal photon.
Detection of the idler photon by D3 or D4 provides delayed "which-path information" indicating whether the signal photon with which it is entangled had gone through slit A or B. On the other hand, detection of the idler photon by D1 or D2 provides a delayed indication that such information is not available for its entangled signal photon. Insofar as which-path information had earlier potentially been available from the idler photon, it is said that the information has been subjected to a "delayed erasure".
By using a coincidence counter, the experimenters were able to isolate the entangled signal from photo-noise, recording only events where both signal and idler photons were detected (after compensating for the 8 ns delay). Refer to Figs 3 and 4.
When the experimenters looked at the signal photons whose entangled idlers were detected at D1 or D2, they detected interference patterns.
However, when they looked at the signal photons whose entangled idlers were detected at D3 or D4, they detected simple diffraction patterns with no interference.

If i understood the experiment properly, idler photons that reach either D4 or D3 detectors are photons for which the "which path" is known and would consequently lead to their entangled signal photons at D0 not causing any interference 8ns BEFORE the signal photons are detected at either D3 or D4.

Which puzzled me about a few things...

a) What would happen if we did NOT detect an interference at D0 8ns prior to the entangled idler photons reaching D3 or D4 and decide to remove the BSa and BSb beam splitters really really fast such that the idler photons would travel to either D1 or D2 instead, hence the "which path" is not known and therefore we should actually see an interference pattern at D0.

But we already got the "no interference" result 8ns prior, so something should stop us from doing this. Would the universe cause us to have a heart attack to keep us from messing with it?

(Possibly we could arrange the experiment in a way which would give us more than 8ns to remove BSb and BSa if the time-interval is too short, through the use Fiber optics cables, etc)

b) If we moved either the mirror Mb or Ma just a tiny little bit from it's position, such that the red or blue path for the photon would be a bit different in length, wouldn't then we be able to tell via the time it took to reach either D2 or D1 detectors which of the two slits it came from?

Does this experiment really rely that much on getting mirror Ma and Mb at EXACTLY the right positions, for the paths to be EXACTLY the same, such that if the blue/red path differ only by 1 nano-meter the universe would "register" that and would "know" the "which path" can be extracted from that?

Last edited by Jeronimus; Apr 18th 2015 at 12:05 AM.
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Old Apr 18th 2015, 07:47 PM   #2
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additional question:

c) If we replaced D0 with another double split, with the red and blue path each pointed at one of the two slits. Would in the case of the idler photon reaching D3/D4, the signal photon choose exactly one of the slits, hence not interfere with itself?
Therefore, in case of the signal photon hitting an area of the screen it could not possibly hit when interfering with itself, we would know for certain that the which path is known 8ns beforehand with just a single photon pair?
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