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How many photons are received per bit transmitted from Voyager 1?

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Re: How many photons are received per bit transmitted from Voyager 1?

#91

Post by opwieurposiu »

superposeur wrote: post #82
londons_explore wrote: post #71

Earlier quoted context omitted.

The main challenge is the earth to probe comms for distant probes, since the earth is often very close (in an angular sense) to the sun from the probes perspective, and the sun gives out a lot of black body radiation. However, due to the shape of the black body radiation curve, the sun gives out relatively less microwave radiation than it does visible light, which might outweigh the advantages of more directionality…

Ok what about using a maser instead of a laser?

The big dish antennas do use ruby masers, but not to transmit. The maser is used as the LNA on the receive side. Check out the picture on page 41 of the pdf, clearly this a flux capacitor, mislabeled to deceive us ;)

https://descanso.jpl.nasa.gov/monograph/series10/03_Reid_cha...

https://www.rfcafe.com/references/popular-electronics/amazin...

Re: How many photons are received per bit transmitted from Voyager 1?

#93

Post by cycomanic »

ramraj07 wrote: post #51
cycomanic wrote: post #43

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…

Can’t you calculate the CRLB for any given distribution if you wanted? That’s what my lab did for microscopy anyway. Saying you’re beating the Shannon limit is like saying you’re beating the second law of thermodynamics to me.. but I could be wrong.

Shannon theory assumes Gaussian noise, however in the very low power regime that's just not true. I agree it's unintuitive. Have a look at the Gordon paper I posted earlier.

Re: How many photons are received per bit transmitted from Voyager 1?

#94

Post by stracer »

aptitude_moo wrote: post #64
cycomanic wrote: post #43

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…

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.

Re: How many photons are received per bit transmitted from Voyager 1?

#95

Post by cycomanic »

Strilanc wrote: post #61
cycomanic wrote: post #52

Earlier quoted context omitted.

Subsea cables don't use repetition codes (they are very much suboptimal), but typically use large overhead (20%) LDPC codes (as do satellite comms systems for that matter (the dvb-s2 standard is a good example). Generally to get anywhere close to Shannon we always need sophisticated coding. Regarding the sensitivity of Subsea systems they are still significantly above 1 photon/bit, the highest sensitivity experiments…

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.

Re: How many photons are received per bit transmitted from Voyager 1?

#96

Post by adrian_b »

aptitude_moo wrote: post #64
cycomanic wrote: post #43

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…

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...

As also explained in the conclusion of the paper linked by you, "photon-counting" detectors are possible only when the energy of one photon is high enough, which happens only for infrared light or for higher frequencies.

"Photon-counting" methods cannot be implemented at frequencies so low as used in 5G networks or in any other traditional radio communications.

Re: How many photons are received per bit transmitted from Voyager 1?

#97

Post by resters »

Strilanc wrote: post #41

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.

Re: How many photons are received per bit transmitted from Voyager 1?

#98

Post by cycomanic »

superposeur wrote: post #65

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.

All space agencies have optical comms in their road maps. Largely they are thinking about inter satellite communications (the atmosphere causes significant issues when going back to earth). So the main application is to have some relay satellite that can then transmit to earth via RF. The application is not mainly deep space ropes but Leo or meo satellites, the typically only have very short transit times over the ground stations, so can't get all their measurement data down. By using e.g. a geo relay they can transmit lots of data optically and the geo relay can more slowly transmit the data to earth until the leo satellite comes back in view.

Re: How many photons are received per bit transmitted from Voyager 1?

#99

Post by somat »

An interesting thing about photons (which may not be true, I just enjoy this stuff amateurishly, that is, without the effort or rigor to actually understand it.) is that they might not exist. the em field is not quantized, or at least is not quantized at the level of photons. A "photon" only exists where the em field interacts with matter, where the electrons that create the disturbance can only pulse in discrete levels.

https://www.youtube.com/watch?v=ExhSqq1jysg

Not that this changes anything, we can only detect or create light with matter. but it does make me curious about single photon experiments and what they are actually measuring.

Re: How many photons are received per bit transmitted from Voyager 1?

#100

Post by resters »

stracer wrote: post #89
cycomanic wrote: post #43

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.

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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