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In the radio part of the electromagnetic spectrum it is really easy to make very clean and powerful RF waves using modern electronics, and it really isn't that hard to make arbitrary RF waves.

But this type of electronic RF generation doesn't really have an equivalent on the optical side of the spectrum. With optical light you basically have to resort to using a laser to get anything close to a coherent and well-controlled light signal. On the other hand, you can do really amazing things with lasers, from quantum entanglement, to squeezed light used in LIGO to detect gravitational waves

So I was wondering, what non-trivial practical benefits would there be in having a radio-frequency laser? To be clear, I am not asking about how to make an RF laser, just about its potential uses and what those uses require. I'm also aware the microwave "maser" was the first laser. As an example of a possible use case, I would imagine that an RF laser that could be operated in some "quantum-limited" state would be useful for superconducting quantum computers.

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    $\begingroup$ at first I thought this would be impossible, because of the mm to kilometer wavelengths of RF., but found out they are getting there pubs.acs.org/doi/abs/10.1021/cen-v046n003.p040, and then saw the date 1968, and this is 2019 and nothing comes up on google later?. Wikipedia says that masers go to RF too.but have only found the hydrogen 21 cm line maser. $\endgroup$
    – anna v
    Apr 28, 2019 at 7:19
  • $\begingroup$ Re, "you can do really amazing things with lasers" Not really. You do amazing things with the light that is emitted by a laser. What's magic about the laser itself is the efficiency with which it creates light that has such amazingly useful properties. Some of the things that we now do using lasers once were done using powerful incandescent lamps and spatial filters that threw away almost all of the light rays except for a select few, followed by color filters that threw away almost all of what made it through the spatial filter except for an extremely narrow band of wavelengths. $\endgroup$ Nov 18, 2020 at 2:47
  • $\begingroup$ @Solomon Slow, sure but that is obviously what I meant. A photon is a photon after all. $\endgroup$
    – KF Gauss
    Nov 18, 2020 at 2:55
  • $\begingroup$ @KFGauss, Maybe obvious to you. But maybe not obvious to everybody who reads this question and the answers yet to come. $\endgroup$ Nov 18, 2020 at 2:59

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I am not asking about how to make an RF laser.

That's OK. You don't need one. The radio waves emitted by an ordinary radio transmitter already are highly coherent. All of the energy is practically in one single frequency (assuming no modulation), and the transmitting antenna effectively is a single "point source" when you look at it on the scale of the emission's wavelength.

One thing you can easily do with an optical laser though that would be hard to do with a long-wavelength radio signal is to focus all of its energy into a narrow, "laser-like" beam. You can do it for RF energy, but you need structures (antennas, dishes, etc.) that are much larger than the wavelength. At microwave frequencies, that could be a dish that's only a few meters across, but at "medium wave" frequencies, you would need structures that were multiple kilometers across.

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  • $\begingroup$ Thank you! Unfortunately, that is what I was suspecting. Could you comment on the quantum aspects of this though? There are various quantum optical processes you can do with lasers that just aren't possible with ordinary sources (squeezed states, definite photon number, etc.). Or are you saying that even those things you can do with an ordinary radio sources that are readily available? $\endgroup$
    – KF Gauss
    Nov 18, 2020 at 5:00

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