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Unforgeable Quantum Tokens Delivered over Fiber Network

spectrum.ieee.org

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Re: Unforgeable Quantum Tokens Delivered over Fiber Network

#21

Post by amluto »

yowayb wrote: post #15

How does a quantum state travel through fiber? Does it simply maintain state naturally during the journey?

Light is remarkably good at keeping its polarization state intact for long distances through single mode fiber. At least historically, the main issues with doing quantum computation with light is that’s it’s hard to store light and hard to get one photon to interact with another one in a controlled manner.

(Polarization of a photon is a two-state quantum system, otherwise known as a qubit.)

Re: Unforgeable Quantum Tokens Delivered over Fiber Network

#22

Post by xyzzy123 »

ano-ther wrote: post #20
xyzzy123 wrote: post #16

Is there any projected practical use for QKD apart from being a jobs program for researchers? (This is a thing I am fine with, research is research and it doesn't necessarily need a near-term practical outcome, but why is it "sold" to the public as though there is some useful capability coming just around the corner?). Who would use dedicated fiber to get secrets between point A and point B? Am I just insufficiently…

Large companies and governments go to some lengths to protect their internal communications between their sites. Cloud providers also have some dedicated fiber between their data centers.

Right but what are they going to do with the keys being exchanged? Load them into networked traditional computers?

If the computers are secure they can presumably do authenticated key agreement perfectly well and if they are not then I don't see how the QKD helps.

Security is nuanced and thinking in binaries is often a mistake - but I don't see how QKD meaningfully changes anyone's threat model in any plausible deployment scenario.

Re: Unforgeable Quantum Tokens Delivered over Fiber Network

#23

Post by westurner »

TachyonicBytes wrote: post #14
gus_massa wrote: post #10

Earlier quoted context omitted.

Once you put error correction, doenn't you lose all the nice properties of the non cloning theorem? If the protocol tolerates 30% of errors, doesn't it tolerate 30% of MITM? (60%??)

You don't need error correction for some crypto primitives. There are QKD networks deployed that don't have that kind of error correction, as far as I know.

How can QKD repeaters store and forward or just forward without collapsing phase state?

How does photonic phase state collapse due to fiber mitm compare to a heartbeat on a classical fiber?

There is quantum counterfactual communication without entanglement FWIU? And there's a difference between QND "Quantum Non-Demolition" and "Interaction-free measurement"

From https://news.ycombinator.com/item?id=41480957#41533965 :

>> IIRC I read on Wikipedia one day that Bell's actually says there's like a 60% error rate?(!)

> That was probably the "Bell test" article, which - IIUC - does indeed indicate that if you can read 62% of the photons you are likely to find a loophole-free violation

> [ "Violation of Bell inequality by photon scattering on a two-level emitter", ]

Bell test > Detection loophole: https://en.wikipedia.org/wiki/Bell_test#Detection_loophole :

> when using a maximally entangled state and the CHSH inequality an efficiency of η>2sqrt(2)−2≈0.83 is required for a loophole-free violation.[51] Later Philippe H. Eberhard showed that when using a partially entangled state a loophole-free violation is possible for

η>2/3≈0.67,[52] which is the optimal bound for the CHSH inequality. [53] Other Bell inequalities allow for even lower bounds. For example, there exists a four-setting inequality which is violated for η>(5−1)/2≈0.62 [54]

Isn't modern error detection and classical PQ sufficient to work with those odds?

> Historically, only experiments with non-optical systems have been able to reach high enough efficiencies to close this loophole, such as trapped ions, [55] superconducting qubits, [56] and nitrogen-vacancy centers. [57] These experiments were not able to close the locality loophole, which is easy to do with photons. More recently, however, optical setups have managed to reach sufficiently high detection efficiencies by using superconducting photodetectors, [30][31] and hybrid setups have managed to combine the high detection efficiency typical of matter systems with the ease of distributing entanglement at a distance typical of photonic systems. [10]

Re: Unforgeable Quantum Tokens Delivered over Fiber Network

#24

Post by amluto »

xyzzy123 wrote: post #22
ano-ther wrote: post #20

Earlier quoted context omitted.

Large companies and governments go to some lengths to protect their internal communications between their sites. Cloud providers also have some dedicated fiber between their data centers.

Right but what are they going to do with the keys being exchanged? Load them into networked traditional computers? If the computers are secure they can presumably do authenticated key agreement perfectly well and if they are not then I don't see how the QKD helps. Security is nuanced and thinking in binaries is often a mistake - but I don't see how QKD meaningfully changes anyone's threat model in any plausible deplo…

QKD will generate a session key, just like Diffie-Hellman or some of the post-quantum DH alternatives. If your threat model includes the risk that someone captures and stores ciphertext and subsequently gets access to a quantum computer and the ability to break whatever post-quantum scheme you’ve augmented with, then maybe QKD is useful. I agree that this is a bit of a stretch.

(Of course, one can also augment DH with symmetric crypto for the datacenter use case, with someone trustworthy literally carrying the key to the other end of the link, and I see no realistic usage of QKD that will outperform that unless one is worried about post-compromise recovery of a symmetric key stored in a piece of hardware. Plus, QKD has its own issues: security of QKD is subject to catastrophic failures if the single-photon source isn’t actually a single-photon source and possibly also if a malicious light source injected into the fiber causes the transmitter to stop being a single-photon source or the receiver to behave in a manner inconsistent with any possible single received photon. Think of these as side channel and fault attacks that are rather difficult to manage.)

Re: Unforgeable Quantum Tokens Delivered over Fiber Network

#25

Post by mystified5016 »

amluto wrote: post #21
yowayb wrote: post #15

How does a quantum state travel through fiber? Does it simply maintain state naturally during the journey?

Light is remarkably good at keeping its polarization state intact for long distances through single mode fiber. At least historically, the main issues with doing quantum computation with light is that’s it’s hard to store light and hard to get one photon to interact with another one in a controlled manner. (Polarization of a photon is a two-state quantum system, otherwise known as a qubit.)

Does that mean that an individual and unique photon travels the full distance from transmitter to receiver without interacting with anything?

That seems... really difficult. I'd always assumed fiber operates by continually absorbing and reemitting photons at low loss. Maybe I've fundamentally misunderstood optics.

Re: Unforgeable Quantum Tokens Delivered over Fiber Network

#26

Post by amluto »

mystified5016 wrote: post #25
amluto wrote: post #21

Earlier quoted context omitted.

Light is remarkably good at keeping its polarization state intact for long distances through single mode fiber. At least historically, the main issues with doing quantum computation with light is that’s it’s hard to store light and hard to get one photon to interact with another one in a controlled manner. (Polarization of a photon is a two-state quantum system, otherwise known as a qubit.)

Does that mean that an individual and unique photon travels the full distance from transmitter to receiver without interacting with anything? That seems... really difficult. I'd always assumed fiber operates by continually absorbing and reemitting photons at low loss. Maybe I've fundamentally misunderstood optics.

> Does that mean that an individual and unique photon travels the full distance from transmitter to receiver without interacting with anything?

Sort of. There will always be attenuation, which is more or less equivalent to losing some fraction of photons. And one can thing of the interaction of light with glass as the photons being absorbed and re-emitted, but thinking about that way doesn’t actually change the result. And, in any case, there isn’t really any such thing as an individual photon: photons are indistinguishable bosons.

In any case, the photons absolutely do interact on the way, but that interaction usually does not affect their polarization, and one of the many counterintuitive effects of quantum mechanics is that, if the polarization is intact, then no one learned it along the way.

Quantum mechanics is bizarre.

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