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I'm curious about the feasibility, both theoretically and experimentally, of tightly focusing (or spatially trapping) a single photon to guarantee its precise targeting. If it’s possible, isn’t it against the uncertainty principle?

For instance, is it feasible to guide/send a single photon towards precisely hitting a particular quantum dot (to excite it) within an array of quantum dots?

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  • $\begingroup$ See the 2012 Nobel prize winning research and experiment by Serge Haroche where a single photon was confined to a small region. See also this PSE post. $\endgroup$
    – joseph h
    Commented Mar 7 at 6:50
  • $\begingroup$ Thanks, the suggestion was helpful. But, my question is mostly related to hitting a specific target by a single photon. For example, you want to excite a specific quantum dot in an array of quantum dots. $\endgroup$
    – Omid
    Commented Mar 7 at 7:03
  • $\begingroup$ This may be helpful: en.wikipedia.org/wiki/Bessel_beam $\endgroup$
    – PM 2Ring
    Commented Mar 7 at 11:15
  • $\begingroup$ Thanks so much @PM2Ring for the link. I checked it out. But I couldn’t find any relation between Bessel beams and focusing single photons in the Wikipedia page. Would you kindly explain a little how they might be related? $\endgroup$
    – Omid
    Commented Mar 7 at 14:38
  • $\begingroup$ It would depend on the size of the dot ... and many of your photons would miss and many would be successful .... its probability. $\endgroup$
    – PhysicsDave
    Commented Mar 7 at 15:33

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Yes, you can focus a single photon to a small region. It is done all the time in photonic quantum information experiments. For example, to perform quantum state tomography on the spatial degrees of freedom of a single photon state, it is modulated by a spatial light modulator after which it is focussed onto the end of a single mode optical fibre (core radius of a few micron) which guides it to a detector. Of course you need to prepare the same single photon state over and over again the get all the information you need for the tomography.

The issues with the uncertainty relation is dictated by the Fourier relationship between the spatial mode and its angular spectrum. In that sense, it behaves in exactly the same way that you will get with a classic optical beam. The only reason why the issue is more prominent in quantum experiments is because you can only make one measurement for each single photon.

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If you try to do so, the single photon state will transform into a multi-photon state (in fact a coherent superposition of all sorts of different particle number eigenstates). This simply reflects the fact that a consistent relativistic one-particle quantum mechanical theory does not exist but relativistic quantum field theory provides the proper desription.

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    $\begingroup$ This is only an issue when considering fundamental interactions. Most of the time this is not relevant in quantum optics experiments. $\endgroup$
    – flippiefanus
    Commented Mar 7 at 7:21

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