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> Yes, I know that quantum key distribution is a potential replacement for public-key cryptography. But come on -- does anyone expect a system that requires spe

> Yes, I know that quantum key distribution is a potential replacement for public-key cryptography. But come on -- does anyone expect a system that requires specialized communications hardware and cables to be useful for anything but niche applications? I don't think there is any inherent reason why this would have to be done using "specialized communications hardware and cables". Didn't the Chinese just demonstrate entanglement sharing via satellite? I could imagine a post-quantum computing era where it is even possible to know, with certainty, if you have been hacked, or data breached, because of quantum cryptography. We are not any where near this kind of technology right now, but the physics is real.

ThePhysicist 8y ago
Yes, sounds a lot like the "I think there is a world market for maybe five computers" prediction by IBM's president in 1943. Practical quantum cryptography is actually not that far away IMHO as the theory is solid and there have been many successful test deployments in the field already. Sure the equipment is specialized but not much more than e.g. the electronics that you need for a mobile baseband station or other specialized applications. You mostly need a good single-photon source and some high-quality optical components and there's no fundamental obstacle to producing those in large quantities and (eventually) for an affordable price. Also, high-quality fiber-optical cables are expensive but not prohibitively so, hence intra-city quantum links seem absolutely feasible and even inter-city connections are doable (and as you pointed out even satellite or line-of-sight communication is possible). In my understanding the main issue that most cryptographers have with schemes like quantum key distribution is that they require classical cryptography in the bootstrapping process, so by their argument they can never be better than those classical schemes. They often forget to mention though that the adversary needs to be able to crack the classical cryptography during the key exchange process (i.e. within a few milliseconds to seconds), so the method is still much safer than any classical key exchange method except if you're facing a god-like adversary.
- posterboy 8y ago
  The "five computers" taken out of context might seem ridiculous but it is hard to believe that wouldnt have been obvious back then and so must have been understood differently, e.g. five mainframes to connect to per terminal and/or diversion to discourage competition. And it's not qualified over time, so it might have meant the very near future. ... or five architectures, five development boxes and the rest restricted implementations, ie. five different co-processors e.g. for time table scheduling which were selling by the time already.
  - lokedhs 8y ago
    The Wikipedia page on Watson discusses attempts at finding the source for this quote, but explains that there seems to be no consensus on this. https://en.wikipedia.org/wiki/Thomas_J._Watson https://en.wikipedia.org/wiki/Thomas_J._Watson It does quote Howard Aiken saying this in 1952: Originally one thought that if there were a half dozen large computers in this country, hidden away in research laboratories, this would take care of all requirements we had throughout the country Even though it's likely that Watson never made this particular statement, the sentiment that the statement itself carries was a real one, and certainly existed in the 40's. What would be interesting to know is when the shift happened from computers being seen as a niche tool to something that everybody would use.
- A2017U1 8y ago
  > Sure the equipment is specialized but not much more than e.g. the electronics that you need for a mobile baseband station Was under the impression that these computers were supercooled down to near absolute zero otherwise they are overwhelmed by decoherence, this problem has been solved? Achieving those temperatures is a fairly involved process.
  - ThePhysicist 8y ago
    For quantum cryptography you don't need qubits but entangled photons, and you need to be able to create pairs of these and measure the polarization of individual photons with high efficiency. Both of these things can be accomplished at room temperature (though there are technologies for photon detection that require cooling to very low temperatures).
    - A2017U1 8y ago
      Intriguing, thanks for this. Really enjoy the topic but find it hard to parse some claims that come out. D-wave a while ago made some bold statements that were shot down by many respected in the field, yet they seem to be progressing still and putting out papers. Simply measuring power in qubits seems to be not the entire story yet most reporting on the subject often boils down to that. I should read up more.
nabla9 8y ago
That amounts of specialized hardware. Strong lasers and receivers for each user, network of quantum repeater satellites. How do you connect to your bank in a cloudy day or when it rains?
- abdullahkhalids 8y ago
  Quantum key distribution is a method of securely exchanging a symmetric key from afar. Once you have that symmetric key, you encode your message with the key (say using a one-time pad), and send it via a classical channel as usual, and the other person decodes it. So what could happen is that that every month you could have a session with your bank to exchange a 100MB secure key using quantum key distribution. Then during the month, you and the bank could use regular internet to communicate securely using the key.
- smarky0x7CD 8y ago
  Fiber optic ines also work alright.
hannob 8y ago
> I could imagine a post-quantum computing era where it is even possible to know, with certainty, if you have been hacked, or data breached, because of quantum cryptography. How so? (Because none of that seems to even remotely have anything to do with quantum cryptography. It's a key exchange - and one with some very bad properties I might say, because it assumes a hell of a lot of things already existing like an authenticated channel. But it does nothing to detect breaches or whether you've been hacked.)

ThePhysicist 8y ago

Yes, sounds a lot like the "I think there is a world market for maybe five computers" prediction by IBM's president in 1943. Practical quantum cryptography is actually not that far away IMHO as the theory is solid and there have been many successful test deployments in the field already. Sure the equipment is specialized but not much more than e.g. the electronics that you need for a mobile baseband station or other specialized applications. You mostly need a good single-photon source and some high-quality optical components and there's no fundamental obstacle to producing those in large quantities and (eventually) for an affordable price. Also, high-quality fiber-optical cables are expensive but not prohibitively so, hence intra-city quantum links seem absolutely feasible and even inter-city connections are doable (and as you pointed out even satellite or line-of-sight communication is possible). In my understanding the main issue that most cryptographers have with schemes like quantum key distribution is that they require classical cryptography in the bootstrapping process, so by their argument they can never be better than those classical schemes. They often forget to mention though that the adversary needs to be able to crack the classical cryptography during the key exchange process (i.e. within a few milliseconds to seconds), so the method is still much safer than any classical key exchange method except if you're facing a god-like adversary.

posterboy 8y ago
The "five computers" taken out of context might seem ridiculous but it is hard to believe that wouldnt have been obvious back then and so must have been understood differently, e.g. five mainframes to connect to per terminal and/or diversion to discourage competition. And it's not qualified over time, so it might have meant the very near future. ... or five architectures, five development boxes and the rest restricted implementations, ie. five different co-processors e.g. for time table scheduling which were selling by the time already.
- lokedhs 8y ago
  The Wikipedia page on Watson discusses attempts at finding the source for this quote, but explains that there seems to be no consensus on this. https://en.wikipedia.org/wiki/Thomas_J._Watson https://en.wikipedia.org/wiki/Thomas_J._Watson It does quote Howard Aiken saying this in 1952: Originally one thought that if there were a half dozen large computers in this country, hidden away in research laboratories, this would take care of all requirements we had throughout the country Even though it's likely that Watson never made this particular statement, the sentiment that the statement itself carries was a real one, and certainly existed in the 40's. What would be interesting to know is when the shift happened from computers being seen as a niche tool to something that everybody would use.
A2017U1 8y ago
> Sure the equipment is specialized but not much more than e.g. the electronics that you need for a mobile baseband station Was under the impression that these computers were supercooled down to near absolute zero otherwise they are overwhelmed by decoherence, this problem has been solved? Achieving those temperatures is a fairly involved process.
- ThePhysicist 8y ago
  For quantum cryptography you don't need qubits but entangled photons, and you need to be able to create pairs of these and measure the polarization of individual photons with high efficiency. Both of these things can be accomplished at room temperature (though there are technologies for photon detection that require cooling to very low temperatures).
  - A2017U1 8y ago
    Intriguing, thanks for this. Really enjoy the topic but find it hard to parse some claims that come out. D-wave a while ago made some bold statements that were shot down by many respected in the field, yet they seem to be progressing still and putting out papers. Simply measuring power in qubits seems to be not the entire story yet most reporting on the subject often boils down to that. I should read up more.

nabla9 8y ago

That amounts of specialized hardware. Strong lasers and receivers for each user, network of quantum repeater satellites. How do you connect to your bank in a cloudy day or when it rains?

abdullahkhalids 8y ago
Quantum key distribution is a method of securely exchanging a symmetric key from afar. Once you have that symmetric key, you encode your message with the key (say using a one-time pad), and send it via a classical channel as usual, and the other person decodes it. So what could happen is that that every month you could have a session with your bank to exchange a 100MB secure key using quantum key distribution. Then during the month, you and the bank could use regular internet to communicate securely using the key.
smarky0x7CD 8y ago
Fiber optic ines also work alright.

hannob 8y ago

> I could imagine a post-quantum computing era where it is even possible to know, with certainty, if you have been hacked, or data breached, because of quantum cryptography. How so? (Because none of that seems to even remotely have anything to do with quantum cryptography. It's a key exchange - and one with some very bad properties I might say, because it assumes a hell of a lot of things already existing like an authenticated channel. But it does nothing to detect breaches or whether you've been hacked.)