Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

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

physics.stackexchange.com

Post reply on HN
Print view View original

61–70 of 205 posts

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

#61

Post by Strilanc »

cycomanic wrote: post #52
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…

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 along the way). In this case the "repetition code" is the number of photons in a pulse.

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

#63

Post by ziofill »

grog454 wrote: post #50
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…

> 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 actually depends how this sentence is intended. There exist quantum repetition codes: the Shor code is the simplest example that uses 9 physical qubits per logical qubit. Since the information is quantum it needs majority voting over two independent bases (hence 3x3=9 qubits to encode a logical one).

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

#64

Post by aptitude_moo »

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...

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

#65

Post by superposeur »

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.

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

#67

Post by Strilanc »

grog454 wrote: post #50
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…

> 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 worse instead of better.

There are ways of repeating quantum information that protect against accidental measurement errors. For example, if your logical 0 is |000> + |110> + |011> + |101> and your logical 1 is |111> + |001> + |100> + |010> then can recover from one accidental measurement. And there are more complex states that protect against both bitflip errors and accidental measurements simultaneously. They're just more complicated to describe (and implement!) than "use 0000000 instead of 0 and 1111111 instead of 1".

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

#69

Post by deelowe »

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 imagine it'd certainly employ some type of beamforming at the least.

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

#70

Post by nsguy »

ethbr1 wrote: post #8
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?

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…

TIL from another comment that the Shannon limit assumes Gaussian noise so it's not actually always the theoretical limit.

You can't work around the Shannon limit by using encoding. It's the theoretical information content limit. But you can keep reducing the bandwidth and one way of doing that is adding error correction. So intuitively I'd say yes to your question, the distance can go to infinity as long as you're willing to accept an increasingly low receive bit rate. What's less clear to me is whether error correction on its own can be used to approach the Shannon limit for a given S/N ratio - I think the answer is no because you're not able to use the entire underlying bandwidth. But you can still extract a digital signal from noise given enough of a signal...

EDIT: There is a generalization of the Shannon limit to non-white Gaussian noise here: https://dsp.stackexchange.com/a/82840

Post reply on HN