

Largest quantum computer yet: 14 qubits - SoftwarePatent
http://www.physorg.com/news/2011-04-quantum-bits-physicists-limits.html

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michael_nielsen
I once asked one of the leading quantum computing experimentalists how many
qubits he could completely control in the lab. He immediately and emphatically
replied "none". I always think of this when I see headlines touting
achievements such as a 14-qubit quantum computer, or stories about controlled
entanglement of 14 qubits. These make good narrative hooks for an article, but
they can also hide a lot.

EDIT: A free pdf of a draft of the paper is at:
<http://arxiv.org/PS_cache/arxiv/pdf/1009/1009.6126v2.pdf>. A quick skim
suggests that at 14 qubits the state they actually prepare in the lab is,
indeed, not very similar to the state they intend to prepare, with a reported
fidelity of about 50%. That's the same fidelity they'd get if they just
prepared an all |0> state. While the paper reports terrifically interesting
work, this and many other details in the paper suggest quite a subtle picture.

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Locke1689
While 14 qubits is an impressive achievement, one of the biggest problems now
seems to be maintaining coherence for more than a few nano or microseconds.

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bhrgunatha
Yes the post title is misleading. They haven't produced a 14 qubit quantum
computer able to do any computation, but they have produced a single quantum
register of 14 qubits with (according to the PDF linked by michael_nielsen) <
50% coherence - about 50% fidelity. Less impressive sounding but still pretty
amazing.

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SoftwarePatent
You should care about quantum computers because they can factor numbers in
polynomial time, which breaks RSA public-key encryption.

<http://en.wikipedia.org/wiki/Shor%27s_algorithm>

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younata
Ok, forgive my ignorance, but why is it that quantum algorithms can ONLY run
on quantum computers? Is it the fact that qubits can have three states? If
that is the case (which it likely isn't), why is base 3 better for this work
than base 2?

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vmind
Basically: Qubits (sort of) encode all possible states that a standard string
of bits can take at the same time as a superposition, such that when you
measure them, you have a possibility of observing each possible setting of the
bits.

You can manipulate the possibility of observing certain states by performing
operations on the bits (which are effectively interference). So a quantum
calculation is more a probabilistic restriction on which state you want,
rather than a direct calculation. In order to be sure of a result, you need to
repeat the calculation to get a desired confidence (or just check the answer
directly if that would be faster).

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dcosson
> So a quantum calculation is more a probabilistic restriction on which state
> you want, rather than a direct calculation.

Not necessarily, some quantum algorithms give an answer with 100% probability
(like the Deutch-Josza algorithm). You're right in that the two most
interesting ones (Grover's and Shor's algorithms) are probabilistic, though.

