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Experiments with single photons in the optical wavelength are by now classic (e.g. Alain Aspect 1980's). The sources for these photons are single atoms or molecules.

Q: were single photons observed with cm, m (and larger) wavelength?

Remark: It is probably hard to detect such single photons. The detection techniques usually rely on photons triggering some macroscopic process (photomultipliers, creation of avalanches), which are not available in the energy range in question. But still such detection is probably not impossible in principle.

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    $\begingroup$ 1m wavelength? The energy of one "photon" (if it even is meaningful to speak of photons with such long wavelength) would be approximately $1.24\times{}10^{-6}$ electron volts. I think you were right on-target with your observation that it "probably is hard to detect such single photons." In fact, never mind detecting the photon. How will you distinguish it from the background noise at that wavelength? $\endgroup$
    – Solomon Slow
    Commented Nov 15 at 19:03

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Single photon counters are commercially available in IR up to wavelengths of about 2 $\mu$m. Anything beyond that will typically only be found in a research lab.

The reason commercial sensors stop there is not fundamental, but rather because that is what current applications demand. In particular, quantum key distribution networks want to make use of the telecom wavelength at 1550 nm. So detectors deeper in the IR are possible but there is not demand at the moment.

Deep into the IR you enter the "Terahertz gap", where detection is generally difficult. Detectors and sources of any kind in this area are an active area of research.

Emerging on the microwave side of the gap, as you remarked, there are detection difficulties in arbitrarily going to longer wavelengths since the photons become less and less energetic. This is an active area of research. Here is a paper from earlier this year which demonstrates a microwave single photon detector (sensitive to wavelengths around 4 cm).

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