One of the hard things about dealing with this, capacity wise in telecom, is that nothing single photon based works with known forms of cwdm or dwdm. It's not feasible to dedicate a dark fiber pair to every possible inter-site communication. For very important stuff that can pay for it, sure, but go price an inter city dark fiber IRU vs an inter city 100GbE...
Cambridge launches UK’s first quantum network
21–30 of 33 posts
Re: Cambridge launches UK’s first quantum network
#22Re: Cambridge launches UK’s first quantum network
#23Earlier quoted context omitted.
Having a bunch of entangled particles doesn't actually let you communicate. You still need to exchange particles with your interlocutor. If you can't communicate classically, you can't communicate with quantum states either- this is the no-communication theorem. Quantum mechanics allows for superdense coding, where you get a bit more than one bit per particle, but it's probably not really helpful.
But you can then twiddle with these photons after exchange right? Use them to communicate after? Otherwise what's the point?
The theorem says that you specifically can't communicate with entanglement alone. It's spooky, but just non-spooky enough to forbid building an ansible. If you poke one part of an entangled pair, you can't actually force which state you will get. You will get a measurement, and the other can be measured as well and they will both be observed in some state- but you can't push one leftwards to get the other to go rightwards.
If you could, high speed traders would not bother with microwave links.
Quantum networks are (in theory) impossible to snoop on without detection, so they are useful for distributing cryptographic keys. They might be useful to pass quantum states around between quantum computers if they're reliable enough.
Re: Cambridge launches UK’s first quantum network
#24This is using parts of Cambridge University’s 25-year-old dark fibre network, the GBN, which has a fun tube-style map: https://help.uis.cam.ac.uk/service/devices-networks-printing...
Re: Cambridge launches UK’s first quantum network
#25This is using parts of Cambridge University’s 25-year-old dark fibre network, the GBN, which has a fun tube-style map: https://help.uis.cam.ac.uk/service/devices-networks-printing...
Heh - I recall back in 94 when I was one of the first handful of webmasters at BT we also had a tube map centered round Martlesham (Think UK Bell Labs) with of shoots to LSEC TSEC(my lonely I486 that ran occasionally when I don't need all the horse power of my pc) AND BSEC
Re: Cambridge launches UK’s first quantum network
#26I wonder how intelligence services around the world are planning to combat the rise of these allegedly "unhackable" communication networks.
> allegedly "unhackable" communication networks they are unhackable using classical methods. So the hacks would be QM based as well (so DefCon will be like QM physicists symposium). Like something along the lines of entangling with (yet not measuring) the signal and performing "shadow" ("counterfactual quantum communication") read of a that signal (entanglement transitivity/swapping, especially if the read/measuremen…
No, this kind of thing is ruled out by the protocol. There's a theorem called "monogamy of entanglement" which means that two maximally entangled qubits can't also be entangled (or even correlated) with a third. This means that Alice and Bob can guarantee that the key they're agreeing on wasn't intercepted. The theoretical argument is airtight. It doesn't rely on any assumptions about the adversary's computing power, or on whether the adversary has access to a quantum computer. The only way it fails is if physics works significantly differently to how we think it does.
Of course there can still be implementation errors or attacks at the endpoints. The strength of the chain is the strength of the weakest link. But the abstract protocol itself is a very strong link.
Re: Cambridge launches UK’s first quantum network
#27Can someone with more knowledge on this topic speak to whether this is likely to take off, or is it just a research toy?
It's not likely to take off. Do you want pre-shared secrets and the expense of a quantum network to, in theory, have infinite security (in theory!), or are you okay with tried and tested RSA which gives you >1000000000 years security at a much cheaper cost, that actually works in practice?
Well not RSA, that will be broken by quantum computers. But there are some classical encryption methods that quantum computers are not known to break.
Re: Cambridge launches UK’s first quantum network
#28This is using parts of Cambridge University’s 25-year-old dark fibre network, the GBN, which has a fun tube-style map: https://help.uis.cam.ac.uk/service/devices-networks-printing...
Heh - I recall back in 94 when I was one of the first handful of webmasters at BT we also had a tube map centered round Martlesham (Think UK Bell Labs) with of shoots to LSEC TSEC(my lonely I486 that ran occasionally when I don't need all the horse power of my pc) AND BSEC
We are going back to our ‘BT Labs’ brand soon :)
Re: Cambridge launches UK’s first quantum network
#29quantum computers arent real though are they
The amount of quantum technology needed for this protocol is much less than a full quantum computer. In fact all you need is fibre optic cable and polarising filters, both of which already exist.
Re: Cambridge launches UK’s first quantum network
#30I wonder how intelligence services around the world are planning to combat the rise of these allegedly "unhackable" communication networks.
Here's a start (hint: they're not "unhackable" nor are they actually "guaranteed by the laws of physics") https://twitter.com/hashbreaker/status/689115480267816960 https://arxiv.org/abs/1803.04520
Everything that paper says is true, but it's also incredibly obvious. It just points out that it's impossible to rule out side-channel attacks at the endpoints. If Eve has powerful enough equipment then she can just read the secret out of Alice's brain before Alice even types it into her computer. I don't think anyone saying or hearing the claim that quantum protocols are "guaranteed by the laws of physics" understands it to mean that the protocol protects against that kind of attack.