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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?

#131

Post by CobrastanJorji »

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.

I'm curious about the feasibility of combining the two problems of propulsion alway from Earth and communication with Earth into beam-powered propulsion aimed directly at Earth, pulsed for use as communication.

Probably infeasible for several reasons (only useful when accelerating DIRECTLY away from Earth, incoming light to power spaceship is probably coming from the sun and therefore likely also in the directly of Earth, so net zero acceleration at best from firing the photons back towards the sun), but it'd be pretty neat.

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

#132

Post by forgot-im-old »

stracer wrote: post #80
fsckboy wrote: post #79

Earlier quoted context omitted.

> How many photons are received per bit transmitted from Voyager 1? wouldn't you also want to know how many photons are transmitted and how many bits transmitted are received?

All transmitted bits are also received, at least when everything works as intended.

No, with error correction, not all transmitted bits are received, but the message bits can be recovered.., and if not they must retransmit later.

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

#133

Post by Sharlin »

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…

> Stuff like DRAM storing a 0 or 1 via the presence or absence of 40K electrons

I'd assume that these days it's a couple of orders of magnitude fewer than that (the cited source is from 1996). Incidentally, 40k e- is roughly the capacity of a single electron well ("pixel") in a modern CMOS image sensor [1] – but those 40k electrons are able to represent a signal of up to ~14 bits, around 10k distinct luminance values, depending on temperature and other noise sources.

[1] https://www.princetoninstruments.com/learn/camera-fundamenta...

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

#134

Post by IndrekR »

adrian_b wrote: post #96
aptitude_moo wrote: post #64

Earlier quoted context omitted.

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.

/at room temperature/

https://en.wikipedia.org/wiki/Johnson%E2%80%93Nyquist_noise

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

#135

Post by yyyfb »

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.

I guess not without a minimum bound on the communication speed.

If you have a way to reliably transmit N bits in time T using P photons, you can transmit N+1 bits in time 2 * T using also P photons. What you would do to transmit X0,X1,...Xn is:

- During the first time slot of duration T, transmit X1,... Xn if X0 = 0 and 0 otherwise (assuming absence of photons is one of the symbols, which we can label 0)

- During the second time slot of duration T, transmit 0 if X0=0 and X1,... Xn otherwise

This only uses P photons to transmit one more bit, but it takes twice as long. So if you're allowed to take all the time that you want to transmit, and have really good clocks, I guess that theoretically this is unbounded.

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

#136

Post by sebzim4500 »

aidenn0 wrote: post #86
sebzim4500 wrote: post #84

Earlier quoted context omitted.

On some level he's a victim of his own success. He invents information theory in the same paper that proves the most interesting results, so who else will work on it?

Even the practical work was done surprisingly early. I have a book on error correcting codes from the 1950s and it's missing very little (Most notably trellis codes and LDPC; the former being invented in the '70s and the latter in 1963).

There's been much more progress on compression though (Arithmetic Coding, ANS, etc.)

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

#137

Post by golergka »

somat wrote: post #99

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 lev…

Isn't that simply the principle of particle-wave duality? When particle/wave in field X interacts with field X, it behaves like a wave, but interactions with other fields are quantised.

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

#138

Post by dheera »

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…

I worked in quantum optics for a while. Our DARPA grant once had the "mission" to see how many bits of information could be theoretically crammed into 1 photon. It turns out to be an uninteresting question because you can theoretically cram infinite bits into one photon, encoded in the relative timing of the photon in a pulse train, limited only by the dispersion of your medium (in space, effectively zero).

Even dispersion is a boring question because it is possible to reverse dispersion by sending the light through a parametric amplifier to conjugate the phases and then running it through the dispersion medium a second time locally.

We later ended up working on other things.

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

#139

Post by nico »

sunk1st wrote: post #127
nico wrote: post #76

Earlier quoted context omitted.

Maybe I’m mixing Shannon’s limit with the sampling rate imposed by the Nyquist-Shannon Sampling theorem > Around 2004, Emmanuel Candès, Justin Romberg, Terence Tao, and David Donoho proved that given knowledge about a signal's sparsity, the signal may be reconstructed with even fewer samples than the sampling theorem requires.[4][5] This idea is the basis of compressed sensing … > However, if further restrictions are…

In so many words, Shannon gave a proof showing that in general the sample rate of a digital sensor puts an upper bound on the frequency of any signal that sensor is able to detect. Unlike the Nyquist-Shannon theory, compressed sensing is not generally applicable: it requires a sparse signal. As with many other optimization techniques, it’s a trade off between soundness and completeness.

Great way to explain it!

Loved this:

> As with many other optimization techniques, it’s a trade off between soundness and completeness

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

#140

Post by Strilanc »

Sharlin wrote: post #133
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…

> Stuff like DRAM storing a 0 or 1 via the presence or absence of 40K electrons I'd assume that these days it's a couple of orders of magnitude fewer than that (the cited source is from 1996). Incidentally, 40k e- is roughly the capacity of a single electron well ("pixel") in a modern CMOS image sensor [1] – but those 40k electrons are able to represent a signal of up to ~14 bits, around 10k distinct luminance values…

If you have a more modern estimate I'll take it. Very interesting about the CMOS sensors distinguishing +- 2 electrons (40K / 2^14).
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