
New silicon structure opens the gate to quantum computers - lainon
https://www.princeton.edu/news/2017/12/11/new-silicon-structure-opens-gate-quantum-computers
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ohiovr
I know nothing about this stuff but this caught my eye:

“The challenge is that it’s very difficult to build artificial structures
small enough to trap and control single electrons without destroying their
long storage times,” said David Zajac, a graduate student in physics at
Princeton and first-author on the study. “This is the first demonstration of
entanglement between two electron spins in silicon, a material known for
providing one of the cleanest environments for electron spin states.”

Electrides like Calcium Aluminate Electride can readily trap individual
electrons. And it is stable at room temperatures.

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chunky1994
I haven't touched this stuff in a while so take this with a grain of salt.

This is good progress since it's about an order of magnitude faster for
operations than previously constructed CNOT gates, however it still has many
caveats for a semiconducter-like leap towards real quantum computers:

a) Low temperature bound for the level of fidelity they want

b) Doesn't allow us to engineer quantum chips of more qubits efficiently since
the decoherence between quantum dots is still a major issue.

c) The science journalism title is a bit misleading. While this happens in a
silicon structure there is nothing new about it, what's new is that they
managed to use the driving resonance to control the dephasing during the gate
operation.

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jessriedel
FYI: This is for a very different substrate (quantum dots, which are basically
individual electrons trapped in special silicon defects) compared to the
substrates that have seen more attention for quantum computing
(superconducting circuits and trapped ions). They are behind in terms of raw
qubit counts -- which are not that meaningful -- and in terms of their overall
abilities, but they hope to catch up by building higher quality components
that cross the fault-tolerant threshold first.

If, like me, you don't already know a lot about quantum computing experiments,
this article shouldn't interest you. At best, this is a small step of many
thousands that will be taken on the way building a working computer.

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rgbrenner
the paper:
[https://arxiv.org/ftp/arxiv/papers/1708/1708.03530.pdf](https://arxiv.org/ftp/arxiv/papers/1708/1708.03530.pdf)

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mastazi
Thank you so much, I wasn't aware that the pre-print was on Arxiv.

PS the web abstract link (as opposed to the pdf link above) is
[https://arxiv.org/abs/1708.03530](https://arxiv.org/abs/1708.03530)

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xt00
Anybody who can read the actual PDF -- is this at room temp and no large
magnetic field? Those two things seem to be the big stumbling blocks for these
types of quantum computers..

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milesokeefe
Yep, still a major issue. This was run at 0.15 kelvin (-273c/-459.4f) and
they'd like to get it colder:

 _In these experiments, the electron temperature accounts for 10% of the
reduction in our visibility, and spin relaxation accounts for the remaining
5-10%. In our experiment we readout the qubits sequentially, reading the left
qubit first. Relaxation of the right qubit during the readout of the left
qubit is what leads to the asymmetry of the expected visibilities. Relaxation
contributions to the readout fidelity can be mitigated by using a faster
readout technique such as RF reflectometry or by incorporating a cold
amplifier into the readout circuit (9, 10). Further improvements can be made
by reducing the electron temperature or using cavity-based measurement
approaches (11)._

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propter_hoc
I did graduate research in an area of experimental physics that was quite
close to state of the art quantum computing research. It was very exciting,
but all the research was pretty much in extreme conditions that aren't really
usable for a realistic computing device (ultra-high vacuum, nanokelvin
temperatures, high magnetic fields or laser trap confinement, etc.). From the
looks of it this paper is in the same vein. Good progress, but the gate isn't
quite "open" yet.

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badspeler
That's true but once Science conquers something, engineering isn't very far
behind. The apparatus to build Bose-Einstein Condensates was a fucking monster
of vacuums and quantum optics but now we can fit a Bose-Einstein Condensate on
a microchip.
[https://arxiv.org/abs/0912.0553](https://arxiv.org/abs/0912.0553)

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techdragon
I Just read that paper and while It’s hardly just a microchip it’s damn
impressive considering the technology involved and the requirements it’s
fulfilling. It’s amazing that we can now build devices that can produce ultra
high vacuums smaller than a hotel minibar, and design systems that are capable
of active cooling down to <1K without the use of any cryogenic refrigeration
at all.

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tim--
My biggest pet peeve about quantum computing is that no one can answer me the
question of "what can quantum computing do for me?".

The answer I hear is that it helps solve the traveling salesman in record
time, and that's all great and everything, but how will quantum computing be
able to do things such as decrease the time it takes to train a RNN, or look
up data in a database?

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jcoffland
The traveling salesman problem is NP-complete and therefore not known to be
solvable in less time on a quantum computer.

[https://cstheory.stackexchange.com/questions/31084/travellin...](https://cstheory.stackexchange.com/questions/31084/travelling-
sales-man-with-quantum-computers)

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visarga
> The traveling salesman problem is NP-complete and therefore not known to be
> solvable in less time on a quantum computer.

That's only a problem if you need the exact solution. If you're OK with 99.9%
quality, approximate solutions are much cheaper. So the question is, is it
necessary to invest into that 1%?

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jcoffland
That has nothing to do with quantum vs. classical.

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visarga
Approximate solutions have to do with solving problems with limited resources
and scaling up. Quantum computers will be fast at solving certain types of
problems, while approximate solutions are fast at solving practical problems.

