
Cambridge launches UK’s first quantum network - ColinWright
https://www.cam.ac.uk/research/news/cambridge-launches-uks-first-quantum-network
======
fanf2
This 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...](https://help.uis.cam.ac.uk/service/devices-networks-
printing/network-services/infoinstitutions/fibre/gbn)

~~~
walshemj
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

~~~
joshuajeeson
We are going back to our ‘BT Labs’ brand soon :)

~~~
walshemj
I always thought that adastral branding was rubbish - have they demolished the
H blocks yet

~~~
joshuajeeson
still refurbishing

------
mepian
I wonder how intelligence services around the world are planning to combat the
rise of these allegedly "unhackable" communication networks.

~~~
trhway
> 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/measurement is
done after Alice&Bob had already got satisfied with the integrity of the
signal as there are also interesting possibilities with that "entanglement in
the future affecting the past". Or like this QM analog of NSA Google fiber
splitter [https://www.extremetech.com/extreme/156673-the-first-
quantum...](https://www.extremetech.com/extreme/156673-the-first-quantum-
entanglement-of-photons-through-space-and-time) : the photon 4 would be
injected back into the channel and Alice and Bob would never know the
difference [between the pair 1,4 and the original pair 1,2] while the photon 3
would go into Eve's pocket ).

~~~
OscarCunningham
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.

------
kingbirdy
Can someone with more knowledge on this topic speak to whether this is likely
to take off, or is it just a research toy?

~~~
Moodles
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?

~~~
OscarCunningham
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.

------
dan-robertson
So the quantum part of this network is that it is doing _quantum key
distribution_ [1]. The idea is that messages are sent in a predictably lossy
way and that further non-predicted losses correspond to the messages being
observed (by the environment or by physics).

In particular messages (bits) are sent as photons with certain polarisations.
The space of possible polarisations is roughly rays through the origin in the
plane (ignoring circular polarisation). The only way to observe polarisation
is to take two possible polarisations and ask which one the photon is. The
answer depends probabilistically on the photon’s state and the polarisations
chosen. For example a +45° polarisation when observed in the basis of {0°,
90°} corresponds to 50% chance of 0° and 50% chance of 90°. If a certain
polarisation _p_ is sent and then one observes with a basis of _p_ or _q_ ,
there is a 100% chance of observing state _p_. When a photon is observed it’s
polarisation becomes whatever polarisation was observed (this it can’t be
measured twice)

The algorithm used works roughly as:

0\. Four polarisations are known in advance, made of two orthogonal bases,
(but the four states are together orthogonal or linearly independent) and
parties can send any polarisation down some channel[2], and observe any photon
they receive with one of the bases, chosen before the photon arrives. A
0-state and 1-state is decided for each basis.

1\. Alice produces some random data to send to Bob

2\. For each bit she wishes to send, she chooses a basis at random and sends
the corresponding 0-state or 1-state

3\. For each photon he receives, bob chooses a basis at random and observes
the photon, recording his basis choice and whether he got the 0-state or the
1-state.

4\. Alice and bob compare notes on the bases they chose over a classical
insecure channel. They should have picked the same basis 50% of the time.

5\. Bob and Alice keep the bits where the basis matched. In these cases, bob
should have perfectly received the data.

6\. Some [random I think] subset is compared over the classical channel. Where
these don’t match up, the photons must have been observed (potentially
eavesdropped). An eavesdropper can get a 75% probability [I think] that they
were eavesdropped but not noticed by Bob/Alice as an eavesdropper would have
had to pick a basis and could have either got it right and sent on an
equivalent photon or got it wrong and had Bob probabilistically get the right
bit out.

7\. If the error rate is suitably low then Alice and bob assume their data was
safely transmitted and the rest of it can be used to make a random key.

Please correct me if I’m wrong on this as I’m not an expert and I’m not sure
I’ve understood it correctly.

[1]
[https://en.m.wikipedia.org/wiki/Quantum_key_distribution](https://en.m.wikipedia.org/wiki/Quantum_key_distribution)

[2] optical fibre is this channel. I don’t really understand the physics of it
but I thought reflection (inside the fibre) could cause polarisation to change
or just loss of photon. I don’t know this is dealt with but I’d love to find
out.

------
simpleAdam
quantum computers arent real though are they

~~~
OscarCunningham
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.

------
walrus01
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...

