4 ms·

Technically there's not really such a thing as "Not being able to receive the signal at all", the signal is there it is just (many thousands of times) lower tha

by WJW 3y ago

Technically there's not really such a thing as "Not being able to receive the signal at all", the signal is there it is just (many thousands of times) lower than the noise floor and almost impossible to recover. There are information theoretic tricks you can do with the shape of the signal to still fit a few bits/second in there but it is going to be pretty low. GPS and other satellite navigation systems are probably the most prevalent systems that do this; the satellites don't have all that much power but receivers make it work even with very small antennas.

cozzyd 3y ago
There is surely a quantum limit here. For example, try a THz signal through a wall...
- WJW 3y ago
  I would think even a Thz signal through a wall is still trillions of times above any quantum limit. Still, now you've got me curious and I'd be grateful for some proper boffins to elucidate us further: if you have an transmitter emitting single photons in an isotropic antenna pattern, could they still be received by a receiver even though there is no "wave" to speak of? Even if part of the information is going in another direction? There is some proper double slit-type weirdness going on there.
  - mlyle 3y ago
    As to detection: thermal limits dominate. Optically we make photon counters out of cooled avalanche photodiodes or photomultiplier tubes that will repeatedly trip for each incoming photon (at high probability). But they also randomly trip at a rate because of internal thermal noise. As to emission, yes, you can make (close to) omnidirectional single photon emitters. But the odds of it interacting with your antenna or detector are low.
  - cycomanic 3y ago
    Coherent free-space optical systems are limited by shot-noise which is essentially quantum limited. Similarly unrepeatered (links without amplification) links are also shot-noise limited. So quantum limits are not really weird. Also the quantum limits can still be treated entirely classical, no need to think about "quantum weirdness"

by WJW 3y ago

cozzyd 3y ago

There is surely a quantum limit here. For example, try a THz signal through a wall...

WJW 3y ago
I would think even a Thz signal through a wall is still trillions of times above any quantum limit. Still, now you've got me curious and I'd be grateful for some proper boffins to elucidate us further: if you have an transmitter emitting single photons in an isotropic antenna pattern, could they still be received by a receiver even though there is no "wave" to speak of? Even if part of the information is going in another direction? There is some proper double slit-type weirdness going on there.
- mlyle 3y ago
  As to detection: thermal limits dominate. Optically we make photon counters out of cooled avalanche photodiodes or photomultiplier tubes that will repeatedly trip for each incoming photon (at high probability). But they also randomly trip at a rate because of internal thermal noise. As to emission, yes, you can make (close to) omnidirectional single photon emitters. But the odds of it interacting with your antenna or detector are low.
- cycomanic 3y ago
  Coherent free-space optical systems are limited by shot-noise which is essentially quantum limited. Similarly unrepeatered (links without amplification) links are also shot-noise limited. So quantum limits are not really weird. Also the quantum limits can still be treated entirely classical, no need to think about "quantum weirdness"