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While reading a published paper, I read that some photons were being detected at the arm of a Michelson interferometer which was intended and set to have approximately perfect deconstructive interference. This was ultimately attributed to imperfect alignment and optical equipment. The paper also said that the experimenters used HeNe laser alignment and other protocols to try and make as best a setup as possible. This got me wondering; just how difficult is it to perfectly align optical equipment to yield 100% constructive or deconstructive interference? While I mostly want the practical knowledge of how difficult the task is, any and all scientific explanations as to why it is difficult or easy are welcome.

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    $\begingroup$ What is an acceptable error bound for you? $\endgroup$
    – DanDan面
    Commented Apr 11, 2023 at 17:46
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    $\begingroup$ In practice one would design the experiment in such a way that there is a feedback loop that constantly adjusts the interferometer so that the detector sits at a dark spot. More precisely, there are usually two, three or more detectors to decide which way the optical path has to be tuned and by how much. If you want to see a practical implementation of this... LIGO is probably the most complex example of this type of interferometric zero measurement. $\endgroup$ Commented Apr 11, 2023 at 17:53
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    $\begingroup$ @onestrangequark can you please provide a reference for the paper you were reading? $\endgroup$
    – Jagerber48
    Commented Apr 11, 2023 at 18:01
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    $\begingroup$ 0% and 0.1% are definitely different beasts. 0% is unattainable by an human constructed device ever made. 0.1% is easy. In fact, 0.000001% is not uncommon. 0% is impossible. The particular study you would be interested in is called "metrology," and it is the study of how we make measurements. And one can make a living at it. $\endgroup$
    – Cort Ammon
    Commented Apr 11, 2023 at 18:12
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    $\begingroup$ it took a few years and millions of dollars to align the interfering lasers in the LIGO experiment ..... to detect gravity waves. $\endgroup$ Commented Apr 11, 2023 at 23:20

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It is very hard indeed. It requires mechanically positioning all the optical components to accuracies of order ~wavelength of light and holding them in those precise positions while a fork lift truck rumbles by 50 yards away from your test apparatus or when your football-scholarship research assistant passes gas in the lab.

For this reason, experiments of this type usually rely on a couple of tricks to make them feasible. The first is that for the case of recombining two light beams and looking for interference, you can calibrate the device to zero in on one interference case out of the unlimited number of possible cases which would result as the optical components are progressively shifted by integer wavelength amounts.

The second trick is to look for relative changes in the interference pattern as the experiment is conducted rather than absolute measurements.

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