The Feynman Lectures of Physics states:

In fact, although we mentioned many frequencies, no phenomenon directly involving a frequency has yet been detected above approximately $10^{12}$ cycles per second. We only deduce the higher frequencies from the energy of the particles, by a rule which assumes that the particle-wave idea of quantum mechanics is valid.

The copyright on the lectures is from 1963. Have higher frequencies than 1THz been directly detected since then?

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    $\begingroup$ What would you say qualifies as a 'direct' detection? a waveform in an oscilloscope? $\endgroup$ – diffeomorphism Sep 25 '14 at 18:45
  • $\begingroup$ Honestly, I don't know. I'd use the same criteria Feynman did, if I knew what that was. $\endgroup$ – Doug Richardson Sep 25 '14 at 19:58
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    $\begingroup$ My guess is, that Feynman may have been wrong even in 1963, but I don't know the state of the art of non-linear optics at the time. Today one can surely say that time domain methods for generation and frequency counting have certainly been used for the 1e15Hz range (visible light to near UV), and if you take jilawww.colorado.edu/yelabs/sites/default/files/uploads/… as an indicator, that has already been expanded into the 1e16Hz range. $\endgroup$ – CuriousOne Sep 26 '14 at 4:08
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    $\begingroup$ I agree with @CuriousOne, I think since we can transfer up to something like ~255 Tbps now, that would qualify as being able to receive higher frequencies. I also found a petahertz optical oscilloscope here/PDF. So I think the answer to your question is yes. $\endgroup$ – honeste_vivere Oct 27 '14 at 13:11
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    $\begingroup$ Okay, though I think that counting days and measuring a change in the orbital position of Earth are just two different methods to measure the same thing (in your example). Part of the limitation, if we choose your constraint of counting cycles, will be the natural frequencies of materials. There will be a point where we can no longer "count cycles" in the strict sense because the materials used in the detector cannot respond fast enough. However, I do not think that using known quantum relationships or other properties of matter should be considered cheating. $\endgroup$ – honeste_vivere Oct 28 '14 at 11:21

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