101–110 of 205 posts
The overwhelming loss in this calculation is from the antenna’s radiated energy spreading out over a larger and larger area (despite the directional “gain” factor). I’m wondering: would a probe launched today instead employ a laser to communicate? This would seem to offer many orders of magnitude improvement in the directionality of the signal.
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I imagine it'd certainly employ some type of beamforming at the least.
Assuming you don't need fast steering, is a 3.7m transmitter array doing beamforming really better than a 3.7m dish transmitting at the same power? My intuition would have been that you are better off using a fairly standard transceiver and spending your engineering budget either increasing power or getting a bigger dish (either by launching on a wider rocket or with a folding design). Lasers might interesting for th…
Actually the limit predicted by Shannon can be significantly beaten, because Shannon assumes gaussian noise, but if we use photon counting receivers we need to use a poisson distribution. This is the Gordon-Holevo limit. To beat Shannon you need PPM formats and photon counters (single photon detectors). One can do significantly better than the numbers from voyager in the article using optics even without photon cpunt…
They send optical pulses in one of up to 128 possible time slots, thereby carrying 7 bits each. And each optical pulse on earth may only be received by 5-10 photons.
TLDR; 4e22 photons per second 2.6e22 per bit. For comparison, ~2e26 photons will be received through your iris in your life
That's how many are sent by Voyager. Only about 1500 or 400 photons per bit are actually received by the radio dish (depending on which frequency is being used).
Actually the limit predicted by Shannon can be significantly beaten, because Shannon assumes gaussian noise, but if we use photon counting receivers we need to use a poisson distribution. This is the Gordon-Holevo limit. To beat Shannon you need PPM formats and photon counters (single photon detectors). One can do significantly better than the numbers from voyager in the article using optics even without photon cpunt…
Is there some fundamental limit to the number of bits per photon that can be communicated via EM radiation? I think it does not exist, because photons aren't all equal, we can use very high frequency and X-ray quantum can probably carry much more information than RF quantum.
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Very interesting, I studied telecommunications and I thought the Shannon limit was the absolute limit. I wonder now if this Gordon Holevo limit is applicable for "traditional" telecommunications (like 5G) as opposed to photon counting a deep space probe EDIT: This paper seems to answer my question [1] [1] https://opg.optica.org/directpdfaccess/8711ab35-bbc2-4d51-8e...
Can you please post another link? This one does not work.
Earlier quoted context omitted.
Is there some fundamental limit to the number of bits per photon that can be communicated via EM radiation? I think it does not exist, because photons aren't all equal, we can use very high frequency and X-ray quantum can probably carry much more information than RF quantum.
this is called the Shannon limit. To discern signal from noise, a minimum sample rate of 2x the frequency of the signal is required. A signal is something that can be turned on or off to send a bit. Higher frequencies can carry more data as you infer but the engineering challenges of designing transmitters and receivers create tradeoffs in practical systems.
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I think you're picturing a different level of the network stack than I had in mind. Yes, above the physical level they will be explicitly using very sophisticated codes. But I think physically it is the case that messages are transmitted using pulses of photons, where a pulse will contain many photons and will lose ~5% of its photons per kilometer when travelling through fiber (which is why amplifiers are needed alon…
But we are classical, so I think it's wrong (or at least confusing) to talk about the many photons as repetition codes. Then we might as well start to call all classical phenomena repetition codes. Also how would you define SNR when doing this? Repetition codes have a very clearly defined meaning in communication theory, using them to mean something else is very confusing.
All classical phenomena are repetition codes (e.g., https://arxiv.org/abs/0903.5082 ). And this is perfectly compatible with the meaning in communication theory, except that the symbols we're talking about are the states of the fundamental physical degrees of freedom.
In the exact same sense, the von Neumann entropy of a density matrix is the Shannon entropy of its spectrum, and no one says "we shouldn't call that the Shannon entropy because Shannon originally intended to apply it to macroscopic signals on a communication line".
Wasn't expecting my question to hit top of HN. I guess I'll give some context for why I asked it. I work in quantum error correction, and was trying to collect interesting and quantitative examples of repetition codes being used implicitly in classical systems. Stuff like DRAM storing a 0 or 1 via the presence or absence of 40K electrons [1], undersea cables sending X photons per bit (don't know that one yet), some k…
Isn't sending more than one photon always "repetition" in that sense? Classical systems probably don't do that because of the engineering complexity of sending a single photon at a time -- we had oscillators and switches, not single photon emitters.
Yes. But regardless of whether its feasible to send single quanta in any given circumstance, the redundant nature of the signals is key to understanding its much higher degree of robustness relative to quantum signals.
And to be clear, you can absolutely send a classical signal with individual quanta.