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What is the largest wavelength of electromagnetic radiation that can produce quantum effects that we can detect? Radio, microwave?

What are the constraints to measure photons of larger wavelengths?

Edit: I'm not looking for radiation you can measure with an antenna. But measured with an system that may measure a single photon at a time.

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    $\begingroup$ it is possible to detect radio photons? yes, you need an antenna. If you tune correctly you can even enjoy it. $\endgroup$
    – physicopath
    Mar 7, 2018 at 15:45
  • $\begingroup$ Depends on what you mean by a "quantum effect." For instance, there's a spectral line of atomic hydrogen at a wavelength of 21cm, in the microwave band; this is actually how we locate cool hydrogen clouds in the Milky Way. $\endgroup$
    – probably_someone
    Mar 7, 2018 at 15:48
  • $\begingroup$ Does tuning an oscillator to produce quantum absorption behavior (in say, cesium or rubidium) count? That gets you to ~3 cm. $\endgroup$
    – The Photon
    Mar 7, 2018 at 17:14
  • $\begingroup$ magnetic resonance imaging uses 64MHz when the field is around 1.5T $\endgroup$
    – hyportnex
    Mar 7, 2018 at 18:10
  • $\begingroup$ In Pieter's answer to the question he points out, the longest wavelength measured that way is 90$\mu$m. Radio is from kHz to low GHz, so kilometers to centimeters, so still quite far off from single photon detection. $\endgroup$
    – Jon Custer
    Mar 7, 2018 at 18:48

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