
High-Fidelity Quantum Logic Gates - gk1
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.117.060504
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doener
"Oxford team achieves a quantum logic gate with record-breaking 99.9%
precision, reaching the benchmark required to build a quantum computer"

[https://www.reddit.com/r/science/comments/4wghmo/oxford_team...](https://www.reddit.com/r/science/comments/4wghmo/oxford_team_achieves_a_quantum_logic_gate_with/)

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TylerE
I find it interesting that 99.9% (e.g. a failure rate of 1/1000) is considered
"good".

On a traditional CPU, if it gave the wrong answer 1/1,000,000,000 that would
probably result in a recall.

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mathgenius
The 99% threshold is that required for error correction protocols to work. So
imagine a thousand of these 99.9% error rate gates with some error correction
routine on top of that so that the net effect is one very reliable gate. Where
"very" depends on how many of these unreliable gates you use. But below 99%
error rate none of this stuff works and so the whole scheme (quantum
computers) can't get off the ground.

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adrianN
Any constant bound suffices to make error correction work. It's just that the
number of extra bits that you need goes up.

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mathgenius
Unfortunately this is not at all true for quantum error correction protocols.
In the quantum world error correction makes things _worse_ below the error
threshold. Source: me doing a PhD on the subject at the moment.

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Aelinsaar
It's a good thing of course, but it's also trapped-ion qubits, so this is not
exactly something that can freely scale or be widely manufactured. It is in
short, probably not the future of quantum computing, it's just another
important step on the way.

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gji
Disclaimer: I used to work on trapped-ion systems.

I hear this a lot about trapped-ion qubits, but in their current state, no
technology is scalable. The usual suspects are superconducting, quantum dots,
or diamond NV-based qubits, but each technology has their own scalability
problems. Superconducting qubits suffer from either requiring massive (vacuum
tube sized) cavities or a ton of crosstalk. Good luck isolating
superconducting LC circuits from one another on one circuit board - a reason
why these are limited to only a few qubits. Quantum dots have pretty awful
decoherence issues and I'm not sure they can be implanted deterministically.
Moreover, I don't think people have demonstrated non-photon mediated
entanglement, which is not particularly scalable. NVs have the same
implantation and entanglement problems, though at least they can be used at
room-temperature. Superconducting and quantum dot technologies require
million-dollar dilution refrigerators and large amounts of (expensive!)
helium-3.

Obviously, these technologies also have their advantages over trapped-ion
qubits. But trapped-ion proponents also have their own roadmaps to scalability
(surface traps created using lithiography).

