

The First Universal Quantum Processor - TravisLS
http://www.physorg.com/news177515046.html

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chrjozefharibo
A grid of coupled qubits can be used to simulate quantum systems. A quantum
system many physicists like to simulate is the Hubbard model
(<http://en.wikipedia.org/wiki/Hubbard_model>) . A simulation of the Hubbard
model can possibly lead to a better understanding of high temperature
superconductivity. This in turn can tell physicists where to look for room-
temperature superconductors, the discovery of which will drastically transform
the world as we know it.

This is in my opinion the most important application of a quantum computer.

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lionheart
I'm curious, what kinds of application ideas exist that are possible on
quantum computers that aren't on regular computers?

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rdtsc
Theoretically none. Quantum computers are universal Turing machines. The main
advantage of quantum computers is speedup.

5 years ago (when I took a couple of courses in QC) there were basically 2
quantum algorithms -- Shor's factorization and Grover's search. The former
speeds up integer factorization (and discrete logarithm) and brings it to
polynomial time, and the later speeds up search from O(n) to O(sqrt(n)).

So just being able to quickly factor integers breaks a lot of encryption
algorithms, and a faster search would really help with data processing.

One of the main difficulty with quantum computers is quantum de-coherence.
Qubits when they are entangled must be isolated from the outside environment
until the final result measurement is taken. That is what is preventing the
creation of large, multi-qubit computers. For example, it would be useful to
have a 1024 qubit machine to simultaneously represent all 1024 bit states.

Quantum error correction might solve the problem, but we are yet to see
practical quantum computers.

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amichail
Where does quantum cryptography fit into this?

<http://en.wikipedia.org/wiki/Quantum_cryptography>

Does this have anything to do with quantum computing?

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rdtsc
You are right, thank you. I embarrassingly forgot quantum cryptography.

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enki
you were right to leave it out - quantum cryptography and quantum computing
have only the quantum in common. (i've published on quantum cryptography
protocols)

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amichail
Could you elaborate on this?

Classical encryption can be done on a classical computer.

Is it the case that (some) quantum encryption cannot be done on a quantum
computer?

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enki
quantum cryptography isn't "encryption" in the sense that crypto algorithms
are involved. the encryption part is just XOR with a random one-use string
(the key). the hard part is distributing that key.

a better name is quantum key distribution. entangled photons are used to get
the random data to both parties in a way that eavesdroppers would introduce
error (because of something called quantum indeterminancy).

since no computation is involved, there's not a lot relation to quantum
computing. wikipedia's got the details!

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jerf
Disclaimer: I'm not quantum-anything researcher. But: Based on the description
of how "quantum encryption" works, I prefer to call it "quantum intrusion
detection". It allows you to verify that the channel is free of eavesdroppers
and get a stream of random bits that are guaranteed to only be agreed upon by
the two ends of the connection, which can then be used for any other purpose
that a stream of random bits only known by two parties can be used for. Using
it for a one-time pad is the easiest thing from a theoretical point of view,
but you can use it to feed a conventional encryption algorithm, too.

The real key isn't the "random", which is easily obtained from a wide variety
of other sources, including fully unpredictable ones like decay sources, nor
is it the "encryption" which is merely one application (though easily the
"killer app") of the system. The real key is the part where you can detect any
intrusion into the connection.

(Obviously, you, enki, know this, well, unless I'm wrong. I'm just
elaborating.)

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enki
hey, nice idea - but that's not how it works.

there's no intrusion detection - an attacker increases the error rate, and by
the error rate we can calculate the maximum probabilistic information an
attacker plausibly might have. i say can, because that number is actually
pretty uninteresting, since it isn't intrusion detection - we just now there's
something causing the error rate to go up - doesn't have to be an attacker. a
higher error rate in practice just means we gotta throw more bits away both
during error correction and during privacy amplification - which is kinda like
packetloss on a classical channel.

at some point the error rate is too high for error correction with a positive
yield (= more key lost in communication, than gained through said
communication) - thus the scheme stops working.

my personal favorite name is 'quantum key growing', because that's essentially
what happens - you turn an initial shared secret (= random key) into a longer
random key. (and while you're continually doing this, you're using up part of
the previous key to secure communication as you go along)

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jerf
Is there some interesting characteristic of the channel that you're looking
for, other than intruders, though? Merely detecting the error rate on a
channel is uninteresting; conventional algorithms have nailed that problem to
the wall. I'm not worried about exactly how it is done, I'm thinking about
what it's good for; take away the ability to detect intruders, even if that's
not the only thing it may be doing, and what's left?

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cool-RR
Waiting for Scott Aaronson, <http://scottaaronson.com/blog/> , to give his
verdict.

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Keyframe
Quantum Computing seems like voodoo to me, I have to read up more about it.
Anyone care enough to explain a bit to me how data compression can benefit
from it?

