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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

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 kind of number for a transistor switching (haven't even decided on the number for that one yet), etc. A key reason quantum computing is so hard is that by default repetition makes things worse instead of better, because every repetition is another chance for an unintended measurement. So protecting a qubit tends to require special physical properties, like the energy gap of a superconductor, or complex error correction strategies like surface codes. A surface code can easily use 1000 physical qubits to store 1 logical qubit [2], and I wanted to contrast that with the sizes of implicit repetition codes used in classical computing. 1: https://web.mit.edu/rec/www/dramfaq/DRAMFAQ.html https://web.mit.edu/rec/www/dramfaq/DRAMFAQ.html 2: https://arxiv.org/abs/1208.0928 https://arxiv.org/abs/1208.0928

nico 2y ago
Very cool. It’s interesting to realize that at some level, every system is a quantum system if you “zoom in” enough
- Ringz 2y ago
  I would spontaneously respond that you are right and at the same time have no problem if someone explains to me that it is not so.
- empyrrhicist 2y ago
  I think the point is the model though - if a system's behavior can be modeled/described classically, it's a bit silly to to call it a "quantum" system in the same way that it's reductive to say Biology is just applied particle physics. Sure, but that's not a very useful level of abstraction.
  - jessriedel 2y ago
    If you want to understand the transition between a fundamental theory and its effective description in some limiting regime, you need to be able to describe a system in the limiting regime using the fundamental theory. It's not "silly" to talk about an atom having a gravitational field even if it's unmeasurably small (currently).
    - empyrrhicist 2y ago
      Lol, so you've got a working theory to bridge the quantum and classical worlds? That is, you've figured out how to make general relativity and quantum mechanics emerge from a more fundamental theory? Somebody get this person a Nobel! We're at the phase where we know the world is quantum, but we also simply don't have the ability to bridge that set of observable phenomena to what we know about macroscopic things. That's what makes this exercise "silly".
      - nico 2y ago
        No need to be sarcastic. We are trying to develop ideas and have a conversation We are not trying to prove who is right and who is wrong Regardless of whatever the mainstream agreement in physics might be regarding a preferred model for reality, everyone experiences reality directly, without any need for science or math. And they can express those experiences and ideas in their own way If you don’t agree with someone else’s ideas, please just explain why politely, and also maybe even try to understand their point of view. What would things look like if they were actually right? How might theirs be a good idea? Thank you
        empyrrhicist 2y ago
        I'm not trying to be mean, it just struck me as kind of a funny position. "If you want to understand the transition between a fundamental theory and its effective description in some limiting regime, you need to be able to describe a system in the limiting regime using the fundamental theory" Like, that's well and good, but in general we just can't do that without hand-waving, period. This is true all over the place (biology/physics, psychology/neuroscience/physics). It's sort of true, but not in a useful way.
- fsckboy 2y ago
  > at some level, every system is a quantum system if we consider "quantum" to mean our quantum theory, at the level of general relativity, gravity is not a quantum system. and the qualifier "yet" is also not known.
  - ngcc_hk 2y ago
    I wonder very much about that statement. So far it is not merging into one theory. Hence one may try to help the OP every physical system if zoom in will be better modelled asking the quantum … But quantum field actually is a universe model like GR and hence you have this issue of sort of action in a distance by superimposition. Hence zoom in is not helping.
grog454 2y ago
> 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.
- ziofill 2y ago
  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).
- Strilanc 2y ago
  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".
  - Kerbonut 2y ago
    If there's interference, could you do something like when using 7 repetition for each bit, take whatever 5 of 7 is, e.g. 1111100 is 1 and 1100000 is 0.
  - nomel 2y ago
    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.
cycomanic 2y ago
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 have been done for optical space comms (look e.g. for the work from Mit Lincoln Labs, David Geisler, David Kaplan and Bryan Robinson are some of the people to look for.
- Strilanc 2y ago
  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.
  - cycomanic 2y ago
    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.
    - jessriedel 2y ago
      > 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 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 the Shannon entropy of its spectrum, and no one says "we shouldn't call that the Shannon entropy because Shannon originally intended to apply it to macroscopic signals on a communication line".
fsckboy 2y ago
> 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?
- stracer 2y ago
  All transmitted bits are also received, at least when everything works as intended.
  - forgot-im-old 2y ago
    No, with error correction, not all transmitted bits are received, but the message bits can be recovered.., and if not they must retransmit later.
    - stracer 2y ago
      I meant the message bits carried by the emission, but you're right, the way things are done, more bits are prepared at first, to make the message more resilient against noise, and only then all the bits are modulated onto the EM emission. So while not all emitted bits do come correctly, all data bits can be reconstructed. Which makes me wonder, whether this way of achieving resilience against changes along the way is necessary. Maybe the data bits can be modulated directly onto analog current, and error resilience can be achieved by remodulating it on the analog level. Then, the emission would have as many bits as there are in the message, error-resilience (modulating, demodulating) would be done strictly using analog electric circuits.
      - forgot-im-old 2y ago
        By adding additional degrees of freedom in the transmission through 'analog modulation' you increase the transmission rate. This is certainly possible, for example FM is more robust than AM, but at the expense of bandwidth. In the presence of a thermal (Gaussian) limited noise model, the Shannon capacity always applies regardless of the modulation.
resters 2y ago
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.
- jessriedel 2y ago
  > 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.
  - eru 2y ago
    > 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?
s1dev 2y ago
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?"
Sharlin 2y ago
> 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-fundamentals/full-well-capacity-pixel-saturation https://www.princetoninstruments.com/learn/camera-fundamenta...
- Strilanc 2y ago
  If you have a more modern estimate I'll take it. Very interesting about the CMOS sensors distinguishing +- 2 electrons (40K / 2^14).
  - lebed2045 2y ago
    not 2 but around 10 seems possible https://www.mdpi.com/2079-9292/9/5/757 https://www.mdpi.com/2079-9292/9/5/757
dheera 2y ago
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.
foobar1962 2y ago
> 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.

nico 2y ago

Very cool. It’s interesting to realize that at some level, every system is a quantum system if you “zoom in” enough

Ringz 2y ago
I would spontaneously respond that you are right and at the same time have no problem if someone explains to me that it is not so.
empyrrhicist 2y ago
I think the point is the model though - if a system's behavior can be modeled/described classically, it's a bit silly to to call it a "quantum" system in the same way that it's reductive to say Biology is just applied particle physics. Sure, but that's not a very useful level of abstraction.
- jessriedel 2y ago
  If you want to understand the transition between a fundamental theory and its effective description in some limiting regime, you need to be able to describe a system in the limiting regime using the fundamental theory. It's not "silly" to talk about an atom having a gravitational field even if it's unmeasurably small (currently).
  - empyrrhicist 2y ago
    Lol, so you've got a working theory to bridge the quantum and classical worlds? That is, you've figured out how to make general relativity and quantum mechanics emerge from a more fundamental theory? Somebody get this person a Nobel! We're at the phase where we know the world is quantum, but we also simply don't have the ability to bridge that set of observable phenomena to what we know about macroscopic things. That's what makes this exercise "silly".
    - nico 2y ago
      No need to be sarcastic. We are trying to develop ideas and have a conversation We are not trying to prove who is right and who is wrong Regardless of whatever the mainstream agreement in physics might be regarding a preferred model for reality, everyone experiences reality directly, without any need for science or math. And they can express those experiences and ideas in their own way If you don’t agree with someone else’s ideas, please just explain why politely, and also maybe even try to understand their point of view. What would things look like if they were actually right? How might theirs be a good idea? Thank you
      - empyrrhicist 2y ago
        I'm not trying to be mean, it just struck me as kind of a funny position. "If you want to understand the transition between a fundamental theory and its effective description in some limiting regime, you need to be able to describe a system in the limiting regime using the fundamental theory" Like, that's well and good, but in general we just can't do that without hand-waving, period. This is true all over the place (biology/physics, psychology/neuroscience/physics). It's sort of true, but not in a useful way.
fsckboy 2y ago
> at some level, every system is a quantum system if we consider "quantum" to mean our quantum theory, at the level of general relativity, gravity is not a quantum system. and the qualifier "yet" is also not known.
- ngcc_hk 2y ago
  I wonder very much about that statement. So far it is not merging into one theory. Hence one may try to help the OP every physical system if zoom in will be better modelled asking the quantum … But quantum field actually is a universe model like GR and hence you have this issue of sort of action in a distance by superimposition. Hence zoom in is not helping.

grog454 2y ago

> 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.

ziofill 2y ago
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).
Strilanc 2y ago
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".
- Kerbonut 2y ago
  If there's interference, could you do something like when using 7 repetition for each bit, take whatever 5 of 7 is, e.g. 1111100 is 1 and 1100000 is 0.
- nomel 2y ago
  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.

cycomanic 2y ago

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 have been done for optical space comms (look e.g. for the work from Mit Lincoln Labs, David Geisler, David Kaplan and Bryan Robinson are some of the people to look for.

Strilanc 2y ago
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.
- cycomanic 2y ago
  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.
  - jessriedel 2y ago
    > 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 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 the Shannon entropy of its spectrum, and no one says "we shouldn't call that the Shannon entropy because Shannon originally intended to apply it to macroscopic signals on a communication line".

fsckboy 2y ago

> 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?

stracer 2y ago
All transmitted bits are also received, at least when everything works as intended.
- forgot-im-old 2y ago
  No, with error correction, not all transmitted bits are received, but the message bits can be recovered.., and if not they must retransmit later.
  - stracer 2y ago
    I meant the message bits carried by the emission, but you're right, the way things are done, more bits are prepared at first, to make the message more resilient against noise, and only then all the bits are modulated onto the EM emission. So while not all emitted bits do come correctly, all data bits can be reconstructed. Which makes me wonder, whether this way of achieving resilience against changes along the way is necessary. Maybe the data bits can be modulated directly onto analog current, and error resilience can be achieved by remodulating it on the analog level. Then, the emission would have as many bits as there are in the message, error-resilience (modulating, demodulating) would be done strictly using analog electric circuits.
    - forgot-im-old 2y ago
      By adding additional degrees of freedom in the transmission through 'analog modulation' you increase the transmission rate. This is certainly possible, for example FM is more robust than AM, but at the expense of bandwidth. In the presence of a thermal (Gaussian) limited noise model, the Shannon capacity always applies regardless of the modulation.

resters 2y ago

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.

jessriedel 2y ago
> 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.
- eru 2y ago
  > 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?

s1dev 2y ago

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?"

Sharlin 2y ago

> 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-fundamentals/full-well-capacity-pixel-saturation https://www.princetoninstruments.com/learn/camera-fundamenta...

Strilanc 2y ago
If you have a more modern estimate I'll take it. Very interesting about the CMOS sensors distinguishing +- 2 electrons (40K / 2^14).
- lebed2045 2y ago
  not 2 but around 10 seems possible https://www.mdpi.com/2079-9292/9/5/757 https://www.mdpi.com/2079-9292/9/5/757

dheera 2y ago

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.

foobar1962 2y ago

> 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.