3 ms·

I once asked one of the leading quantum computing experimentalists how many qubits he could completely control in the lab. He immediately and emphatically repl

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 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.

Locke1689 16y ago
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.
- bhrgunatha 16y ago
  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.
archgoon 16y ago
>He immediately and emphatically replied "none". This raises the question, what counts as "completely controlled"? What is typically meant when they say "Built X qubit quantum computer?"
dcosson 16y ago
Thanks for posting the link. The GHZ state they created is technically entangled (in the way that Einstein was worried about entanglement, where if you took one of the 14 qubits far away from the rest, you could measure it and instantly have knowledge of the other 13 qubits, which seemingly violates special relativity). However, even ignoring the fact that this was only to 50% fidelity, it is also only a superposition of two states. To run, say, the phase estimation algorithm (which is the heavy-lifting part of Shor's algorithm), you need an equal superposition of all 2^14 states, which is obviously a lot different. Anyway, I agree with your analysis - this is a great accomplishment, but we are still further from having true quantum computers than the news reports would make it seem.

Locke1689 16y ago

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.

bhrgunatha 16y ago
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.

archgoon 16y ago

>He immediately and emphatically replied "none". This raises the question, what counts as "completely controlled"? What is typically meant when they say "Built X qubit quantum computer?"

dcosson 16y ago

Thanks for posting the link. The GHZ state they created is technically entangled (in the way that Einstein was worried about entanglement, where if you took one of the 14 qubits far away from the rest, you could measure it and instantly have knowledge of the other 13 qubits, which seemingly violates special relativity). However, even ignoring the fact that this was only to 50% fidelity, it is also only a superposition of two states. To run, say, the phase estimation algorithm (which is the heavy-lifting part of Shor's algorithm), you need an equal superposition of all 2^14 states, which is obviously a lot different. Anyway, I agree with your analysis - this is a great accomplishment, but we are still further from having true quantum computers than the news reports would make it seem.