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Thread: Double Slit Single Photon Interference - possible with analog modulation?

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    Default Double Slit Single Photon Interference - possible with analog modulation?

    A question not about how to create a laser show, but how to re-create one of the world's most famous physics experiments of all time.

    My RGB LP hasn't arrived yet, so I am left to killing time by learning LSX, browsing fabric stores, and dreaming of my first experiements to run when it finally does arrive...

    Anyway, the infamous double slit experiement (Young's Experiement), has always fascinated me to no end, and one of the many reasons why I became interested in lasers to begin with - well that and the 12 years spent mingling in the 90's San Francisco music scene...

    Anyway, I imagine it's fairly easy to do a simple double slit interference test - with a photon beam - you just have to get the slits the right width and the right distance from the laser - the hard part comes in lowering the intensity enough so that you can pretty much guarantee that single photons are firing from the laser.

    The question is, is this possible with the analog modulation boards used in our projectors? Is there a DIY method for measuring photon reception at the receiving end?

    There is a company called TeachSpin - that sells a complete single photon interference apparatus - I beleive it is around $7k... But how much fun is it to buy a ready-made apparatus?

    Anyone ever experimented around in this area?

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    I don't have personal experience, but single photon sources are an active field of development, and I don't think it's possible using a hobbyist system. I'd love to be proven wrong though :P
    Single photons are hard to detect as well. This is not a simple experiment.

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    Bluefang,

    You need to do a little research first. The photon flux in a 1W laser is on the order of 1 x 10^18/ sec. Even with a detector capable of detecting 1 x 10^6/ sec and projecting the beam through an OD 6 filter you would need to modulate down to 0.0001%. This is not practical.

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    Quote Originally Posted by planters View Post
    Bluefang,

    You need to do a little research first. The photon flux in a 1W laser is on the order of 1 x 10^18/ sec. Even with a detector capable of detecting 1 x 10^6/ sec and projecting the beam through an OD 6 filter you would need to modulate down to 0.0001%. This is not practical.
    You can purchase an apparatus off the street for $7k... I wouldn't call that impractical. I would say that it would not be a wise investment for a hobbiest, but it's not impractical.

    So let's just get rid of some of those zeros. Who said we have to use a 1W laser. Let's use a 100mW 640nm laser. There's a couple zeros right there. Next, let's not use a TTL driver, but an analog driver. This part I'm not sure how many zeros we can knock off - no idea... The off-the-shelf experiment uses quite a few other tricks to reduce intensity - going through several rounds of slits, along with most likely an OD filter of some sort...

    After doing "a little research", here is a nice reference for obtaining the actual numbers...

    https://www.berthold.com/en/bio/how-...ce-photon-flux

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    It is easy to make a source which only outputs a resonable photon flux, you just keep stacking attenuators until you are down to your desired photon flux (proving that your photons are quantized into particles in any fashion is a much trickier however!). More recent versions young's experiment use sources which can be configured to 'fire exactly one photon now, then turn off' which make convincing people that your results are actually demonstrating that a single photon acts as a wave, but this is not necessary so long as you argue your case on how you confirmed that there is a small probability of 2 photons coming out of your source will interact with eachother.

    Detecting them is tricky, although you should be able to get away with using an avalanche photodiode (APD) in 'gieger mode' where you bias it above the breakdown voltage and pray that you don't blow it up (especially since APDs are quite expensive compared to a normal pin diode!). You could also try with a PMT, but you will have a hard time dealing with the dark counts (they will be hard to deal with in either case without an exceptionally fancy detector such as the stuff sold by EG&G, thorlabs, etc)

    Then it is just a matter of setting your detector at various spots and counting for a long time...

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    This certainly can be done. However, the numbers are huge compared to typical hobbyist modulation techniques. You could assemble the equipment for something on the order of the $7K for the turn key experiment and probably for significantly less depending on your starting point as an amateur. The problem is not just to reduce the intensity. This could be achieved with a series of 6OD filters. You need to establish that the flux is quantized to several deviations greater than the photon transit interval to the target. You will need to count the photons and overwhelm the noise. I am not arguing not to pursue it. but that without some head start this is not a light weight way to pass the time waiting for a commercial RGB or some scrim. It is because you set up the situation to sound like idle curiosity that I stated it would be impractical.

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    Not to worry. It is indeed pretty much "idle curiosity" at the moment... I was wondering how sensitive the analog modulators actually are. I.E, what is their lower limit. It is indeed more of something along the lines of "how the heck do you emit single photons" and "how the heck do you detect single photons" type of question. With the experiment linked above, it seems they take the path of firing a significantly small photon flux such that statistically they are firing single photons, and not the path of absolutely firing single photons.
    Last edited by BlueFang; 01-23-2014 at 03:19.

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    how about a q switched laser and a polarizer? expand that beam say 20 times and then focus the very center core of the beam into a fiber. Only the most straight photons would enter. pass that through a polarizer before the fiber. That would act to sample the beam and then you would get rid of all but the most unpolarized photons. That should drop the number of photons dramatically and let you select more or less with the polarizer/rotator...
    Last edited by kecked; 01-23-2014 at 02:56.

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    For these quantum mechanical experiments EVERYTHING is going to be statistical. That's the nature of quantum mechanics. In order to "demonstrate" that the photon is interfering with its own wave function ie with itself you have to create a flux that is low enough that there is a sufficiently high probability that only one photon is present within the interval between the source and the detector. If you were to produce 100 photons per second within a laboratory scale experiment (easy with the above mentioned filtering techniques) this might seem to achieve the needed separation, but you have to demonstrate that you haven't generated all 100 in the first picosecond to test the quantum interference. You need to count photons.

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    You can perform several variations on the double-slit experiment without resorting to single-photon runs. It's slightly more complicated, but you can accomplish the experiment with nothing more advanced than a laser pointer and some polarizing filters, plus the two slits. Scientific American published a DIY article back in 2007 on how to do this. Here are a few links:

    http://www.scientificamerican.com/ar...uantum-eraser/
    http://homephysics.blogspot.com/2010...t-and-diy.html
    http://www.scientificamerican.com/ar...wave-particle/

    And a more advanced explanation: http://en.wikipedia.org/wiki/Delayed...quantum_eraser

    Adam

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