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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?

#161

Post by imoverclocked »

thegabriele wrote: post #157
jessriedel wrote: post #109

Earlier quoted context omitted.

> Then we might as well start to call all classical phenomena repetition codes All classical phenomena are repetition codes (e.g., https://arxiv.org/abs/0903.5082 ). And this is perfectly compatible with the meaning in communication theory, except that the symbols we're talking about are the states of the fundamental physical degrees of freedom. In the exact same sense, the von Neumann entropy of a density matrix is…

> 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/redundancy which provides robustness against stray electrons (eg: from induced current/interference.)

As we build systems that require less power, we are also building systems that use less electrons to represent a single bit.

Another fun tidbit in this space comes down to signal propagation in a lossy medium: we don’t actually use square waves for our clocks since a square wave is composed of many different frequencies. Since different frequencies propagate at different speeds, a square wave looks less and less like a square and also uses more power than a simple sine wave. If you remember your Fourier/Laplace knowledge, you probably remember that a sine is a single frequency and thus it will be coherent through a conductor and use the least amount of power to generate.

Edit: I’m talking about electrons here but the concept of many packets for a single state extends to most of our communication channels today … eg radio communications like we see to/from Voyager.

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

#162

Post by lupire »

thegabriele wrote: post #157
jessriedel wrote: post #109

Earlier quoted context omitted.

> Then we might as well start to call all classical phenomena repetition codes All classical phenomena are repetition codes (e.g., https://arxiv.org/abs/0903.5082 ). And this is perfectly compatible with the meaning in communication theory, except that the symbols we're talking about are the states of the fundamental physical degrees of freedom. In the exact same sense, the von Neumann entropy of a density matrix is…

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

From, the linked paper:

> This insight captures the essence of Quantum Darwin- ism: Only states that produce multiple informational off- spring – multiple imprints on the environment – can be found out from small fragments of E. The origin of the emergent classicality is then not just survival of the fittest states (the idea already captured by einselection), but their ability to “procreate”, to deposit multiple records – copies of themselves – throughout E

Basically, quantum information is unknown. All possible classical states are mixed together. Once you measure it, it decoheres into a classical system, where all measurements are highly correlated -- some states exist, and others don't exist. A quantum coin flip is heads or tails on top, and tails or heads on bottom. But once you measure it, it all decoheres at once. You get a classical coin flip, that is hrads on top and tails on bottom, or vice versa. You can't get heads on top, and tails or heads on bottom unsettled for a second bit of information. The same message is written twice, on the top and the bottom of the coin. You can look at the top of coin or the bottom and get the same result. If the top of the coin melts, you can still read the result of the flip from the bottom.

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

#163

Post by LegionMammal978 »

acchow wrote: post #154
im3w1l wrote: post #123

Earlier quoted context omitted.

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.

Special relativity will give limits on how aligned the clocks can be (which should be a function of the distance between the clocks). So there will be precision limits.

Unless the clocks are accelerating, that shouldn't change the ratio between the two time differences, right? If the two ends have a constant velocity relative to each other, then each should perceive the other as being off by a fixed rate.

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

#164

Post by foobar1962 »

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…

> Wasn't expecting my question to hit top of HN.

Thanks for asking it. Thanks too to the person that provided the thorough answer.

After reading that answer - seeing all the math equations and physics that cover several disciplines - I wonder how some people can just hand-wave "science" away as a conspiracy to fool the masses. They clearly have little idea the amount of knowledge that works together to get answers questions like this.

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

#166

Post by mr_toad »

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…

Existence is a difficult concept for something moving at the speed of light.

Light follows a null geodesic through space-time with zero (space-time) length and no proper time. Past, future, and causality have no meaning to a photon. We think of photons travelling through space because our symmetry is broken, we have mass, and we and experience time and space.

Observers like us will see light follow the same world-line from its source to its target. It cannot interact with anything else, and some might say it was only ever emitted so that it could interact with its target.

So from a certain point of view the “existence” of a photon is entirely bound up in its interaction with source and target, and it’s not really useful to speak of it in other terms. The quantised interaction is the photon.

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

#168

Post by aptitude_moo »

stracer wrote: post #94
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...

Can you please post another link? This one does not work.

Weird, it works for me. You can use any of the other published links in the sibling comments. The name of the paper is "Quantum Limits in Optical Communications" by Banaszek

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

#169

Post by jessriedel »

thegabriele wrote: post #157
jessriedel wrote: post #109

Earlier quoted context omitted.

> Then we might as well start to call all classical phenomena repetition codes All classical phenomena are repetition codes (e.g., https://arxiv.org/abs/0903.5082 ). And this is perfectly compatible with the meaning in communication theory, except that the symbols we're talking about are the states of the fundamental physical degrees of freedom. In the exact same sense, the von Neumann entropy of a density matrix is…

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

All the objective classical facts out there (e.g., the location of the moon, the existence of the dinosaurs, what I ate for breakfast) are recorded in many degrees of freedom (e.g., the photons scattered off the moon, the dinosaur bones, and the microscopic bits of bagel in the trash). By the no-cloning theorem, quantum information cannot be amplified, so this necessarily means the information we can access is classical. But these classical facts are the exception rather than the rule, as far as the full arena of quantum mechanics is concerned. There are always many more degrees of freedom that are not redundantly recorded, for the simple fact that you need to use one degrees of freedom to record another.

Even if the world were fundamentally classical (which it's not), the only things you could actually know about it would be the tiny subset that is amplified to macroscopic scales, necessarily producing many records in, if nothing else, the many atoms in the neurons in your brain.

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