How many photons are received per bit transmitted from Voyager 1? • Hacker News Board

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

#121

Post by nomel » Mon, Jun 03, 2024, 6:06 PM UTC

Strilanc wrote: post #67

grog454 wrote: post #50

Earlier quoted context omitted.

> by default repetition makes things worse instead of better Can you elaborate on this a bit? My intuition is that, by default, statistical models benefit from larger N. But I have no experience in quantum physics.

It's because unintended measurement is a type of error in a quantum computer. Like, if an electron passing near your qubit would get pushed left if your qubit was 0 and right if was 1, then you will see errors when electrons pass by. Repeating the 0 or 1 a thousand times just means there's 1000x more places that electrons passing by would cause a problem. That kind of redundancy makes that kind of error mechanism wor…

Is this the correct interpretation?

Classical systems: You measure some state, with the measurement containing some error. Averaging the measurement error usually gets closer to the actual value.

Quantum systems: Your measurement influences/can influence the state, which can cause an error in the state itself. Multiple measurements means more possible influence.

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

#122

Post by superposeur » Mon, Jun 03, 2024, 6:15 PM UTC

opwieurposiu wrote: post #91

superposeur wrote: post #82

Earlier quoted context omitted.

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...

Cool use of maser for receive.

Not having thought this through before, I see now that while a transmit maser may have efficiency advantages, it may not improve directionality relative to a standard parabolic radio transmitter. All methods of producing microwaves will have basically the same diffraction-limited gain for a given “aperture” (dish) size. That darn uncertainty principle! (However, an optical laser would still give way better directionality.)

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

#123

Post by im3w1l » Mon, Jun 03, 2024, 6:25 PM UTC

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.

Send three photons A B C. They arrive at times ta, tb, tc. Compute fraction (tc - tb) / (tb - ta). This can encode any positive real number with arbitrary precision. But clearly you need either very precise measurements or send the photons at a very slow rate.

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

#124

Post by s1dev » Mon, Jun 03, 2024, 6:27 PM UTC

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 believe that a classical radio receiver is measuring a coherent state. This is a much lower level notion than people normally think about in QEC since the physical DoF are usually already fixed (and assumed to be a qubit!) in QEC. The closest analogue might be different choices of qubit encodings in a bosonic code.

In general, I'm not sure that the classical information theory toolkit allows us to compare a coherent state with some average occupation number N to say, M (not necessarily coherent) states with average occupation number N' such that N' * M = N. For example, you could use a state that is definitely not "classical" / a coherent state or you could use photon number resolving measurements.

A tangential remark: The classical information theory field uses this notion of "energy per bit" to be able to compare more universally between information transmission schemes. So they would ask something like "How many bits can I transmit with X bandwidth and Y transmission power?"

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

#125

Post by s1dev » Mon, Jun 03, 2024, 6:31 PM UTC

Sanzig wrote: post #90

ramraj07 wrote: post #51

Earlier quoted context omitted.

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.

You are correct. People often say "Shannon limit" (the general case) when they are really referring to the "Shannon-Hartley Limit" (the simplified case of an additive white Gaussian noise channel). For example, MIMO appears to "break" the Shannon-Hartley limit because it does exceed the theoretical AWGN capacity for a simple channel. However, when you apply Shannon's theory to reformulate the problem for the case of…

I often wondered why MIMO was such an investigated topic. It would make sense if the Shannon limit is higher for this channel. Is there a foundational paper or review that shows this?

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

#126

Post by ethbr1 » Mon, Jun 03, 2024, 6:51 PM UTC

RetroTechie wrote: post #39

ethbr1 wrote: post #8

Earlier quoted context omitted.

Not my field, but assuming transmitting hardware (including beam forming) is constant and that atmosphere can mostly be ignored (see comments about it usually being a non-impact in the transmission frequencies), two approaches would suggest: 1. Increase the effective receiving dish size, to capture more of the signal. Essentially, this would be effective in direct proportion to beam spread (the more beam spread, the…

3. Use relays. Of course that would mean sending (a) giant receiver dish(es) in the general direction a probe is sent. On the flip side, if using a single relay it could travel at roughly 1/2 the speed of the probe. Note that signal strength weakens with distance^2. So if eg. you'd have 2 relays (1/3 and 2/3 between Earth & the probe), each relay would receive 9x stronger signal. No doubt the 'logistics' (trajectory,…

That'd be really cool! And definitely helped by the fact that you'd only need to head out at a fraction of Voyagers speeds to get the benefit.

There's some details on the Voyager gravity-assist mechanics here [0], but you'd also need the escape trajectory to be pointed in the Voyager direction which would further constrain...

That said, Earth-Jupiter-Saturn alignments don't seem that rare (on a decades scale).

[0] http://www.gravityassist.com/IAF3-2/Ref.%203-143.pdf

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

#127

Post by sunk1st » Mon, Jun 03, 2024, 6:52 PM UTC

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…

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.

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

#128

Post by sunk1st » Mon, Jun 03, 2024, 6:55 PM UTC

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.

Perhaps we should consider relaying the signal through a third satellite.

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

#129

Post by PeterCorless » Mon, Jun 03, 2024, 6:57 PM UTC

I just wanted to chime in with a reminder that though Voyager 1 is speeding away from Sol at a constant velocity because of the Earth's revolution around the sun it can be up to ±1 AU closer or further away, depending on the time of the year.

This article is for Voyager 2, but the issue is the same. For a brief moment every year we actually get closer to Voyager 1, then we pivot away in our revolution around the sun and the distance between Earth and Voyager 1 or 2 increases sharply. So distance, when plotted over time, looks like a wobbly line.

https://earthsky.org/space/voyager-spacecraft-getting-closer...

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

#130

Post by Dylan16807 » Mon, Jun 03, 2024, 7:02 PM UTC

lordnacho wrote: post #5

I love these kinds of questions. So what does that conclusion mean about when the probe will be so far away that we are below the Shannon limit? And can we beat the Shannon limit somehow, eg collect for longer, put the dish outside the atmosphere, and so on?

> And can we beat the Shannon limit somehow, eg collect for longer

If you turn on a faucet for longer, you're not beating the "gallon limit" of the system. The limit is not a fixed number, it's directly based on how much you do to improve the signal.

And they have already slowed down the transmission speed repeatedly.