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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?

#171

Post by eru »

cycomanic wrote: post #95
Strilanc wrote: post #61

Earlier quoted context omitted.

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.

Oh, you can have multiple layers of error correcting coding.

Eg Google stores data internally with something like Reed-Solomon error correction, but they typically have two independent copies.

So they have repetition code at the classical nano-scale, then Reed-Solomon error correction at the next level, and at the highest level they apply repetition again.

There's nothing confusing about this, as long as you are careful to make sure that your listener knows which level you are talking about.

> Repetition codes have a very clearly defined meaning in communication theory, using them to mean something else is very confusing.

OP used them exactly with the orthodox meaning as far as I can tell.

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

#172

Post by eru »

imoverclocked wrote: post #161
thegabriele wrote: post #157

Earlier quoted context omitted.

> All classical phenomena are repetition codes Could you ELI5 this "bit"? Thanks

I think what they mean is that classical systems use more than a single packet of energy to represent a state. Our “digital systems” are actually analog systems in saturation states. Each time you want to set a bit in memory, you need enough energy to fill up a capacitor (or similar related concept) which is far more than a single electron. This methodology from the viewpoint of quantum systems is repetition/redundan…

Going off on a bit of a tangent:

Some people suggest that digital computing and neural networks are a bit fit, and that would should be using analog devices.

That sounds very appealing at first. But we have (at least) two problems:

First, our transistors dissipate almost no energy when they are either 'fully open' or 'fully closed'. Because either there's approximately no current, or approximately no resistance. Holding them partially open, like you'd do in analog processing, would produce a lot of heat.

The second problem: electrons are discrete, and thanks to miniaturisation and faster and faster clockspeeds, we are actually getting into realms where that makes a difference. So either you have to accept that the maximum resolution of activation of your analog neuron is fairly small (perhaps 10 bits or so?), which is not that much better than using your transistors in binary only; or you'll have to use much larger transistors in your neural chips.

Both problems together mean that analog computing for neural networks isn't really competitive with digital computing. (Outside of some very niche applications, perhaps.)

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

#173

Post by eru »

jessriedel wrote: post #110
resters wrote: post #97

Earlier quoted context omitted.

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.

> Isn't sending more than one photon always "repetition" in that sense? 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.

> And to be clear, you can absolutely send a classical signal with individual quanta.

Yes. Though how 'classical' your understanding of that system is stands to reason?

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

#174

Post by philipwhiuk »

superposeur wrote: post #81
Out_of_Characte wrote: post #72

Earlier quoted context omitted.

There's several projects for laser-based communication and research. It would also make it really difficult to aim since you can miss your target now. https://www.jpl.nasa.gov/news/nasas-deep-space-optical-comm-... https://en.m.wikipedia.org/wiki/Laser_Interferometer_Space_A...

Interesting about the JPL program and I’m amazed this prototype was only launched last year! Apparently the answer to comms laser use is “not yet but soon”.

Actually the answer is 'right now'. The Psyche spacecraft is actively using a laser to provide data on it's way to the Psyche asteroid.

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

#175

Post by eru »

codedokode wrote: post #158
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…

I am not sure if you can use 1 photon per bit because (as I understand) emitting and capturing photons is a probabilistic process and when you have 1 photon, there is a probability that it will not be captured by an antenna, but rather will be reflected or will turn into heat. Or am I wrong here?

In principle, you can send more than one photon per bit on average. Photons have a lot of ways they can encode additional bits, eg frequency, polarisation, timing.

You are right that you can randomly lose some photons. That's what error correcting codes are. See https://en.wikipedia.org/wiki/Error_correction_code

As an example, assume every photon can encode 10 bits without losses, but you lose 10% of your photons. Then with a clever error correcting code you can encode just shy of 9 bits per photon.

You can think of the error correcting code 'smearing' 9 * n bits of information over 10 * n photos, and as long as you collect 0.9 * n photons, you can recover the 9 * n bits of information.

It's the same reason your CD still plays, even if you scratch it. In fact, you can glue a paper strip of about 1 cm width on the bottom of your CD, and it'll still play just fine. Go wider, and it won't, because you'll be exceeding the error correcting capacity of the code they are using for CDs.

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

#176

Post by philipwhiuk »

HarHarVeryFunny wrote: post #45

The fact that we can communicate with Voyager, and in both directions, blows my mind. It's completely counter-intuitive. At least for Voyager->earth we can use giant radio telescopes to detect the faint signal, but how do we manage to focus on those few hundreds of photons per bit coming from a pinpoint source a light day away?! In the earth->Voyager direction it seems even less intuitive - sure we can broadcast a po…

Per the article we lose 99.9% of the photons sent. The antenna is as directional as possible but with a similar dish you could pick up the signal on the Moon at this point I expect (probably a smaller dish given the lack of atmospheric noise)

Equally we're broadcasting to the area of space Voyager's in. We're not able to to target it to the dish - 12' isn't the DSOC accuracy - it's the size required to pick up enough data given the signal diffusion.

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

#178

Post by philipwhiuk »

pcdoodle wrote: post #14

I am confused. I thought photons were just visible light but I guess these little buggers are everywhere. Also very surprised voyager is using 2.3ghz, that's crazy saturated on earth due to wifi. How these engineers make this all work, is magic to me.

> Also very surprised voyager is using 2.3ghz, that's crazy saturated on earth due to wifi

In addition to it being 2.4GHz, this is the reason for having the National Radio Quiet Zone near Green Bank

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

#179

Post by immibis »

nico wrote: post #76
cycomanic wrote: post #55

Earlier quoted context omitted.

I don't think compressed sensing is really extracting more information than Shannon, it simply exploits the fact that the signal we are interested in is sparse so we don't need to sample "everything". But this is somewhat outside my area of expertise so my understanding could be wrong.

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…

Consider the simple example of frequency aliasing. If you sample a 3.2MHz sine wave at a 1MHz sample rate, it looks the same as a 0.2MHz wave. But if you know a priori that the signal only has frequency components between 3 and 3.5 MHz, then you know the 0.2MHz you are measuring is actually 3.2MHz - you can fully reconstruct the original signal even though you are not sampling it fast enough.

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

#180

Post by immibis »

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…

https://physics.stackexchange.com/questions/752390/double-sl...
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