
Quantum computing: First two-qubit logic gate in silicon - vermilingua
http://www.engineering.unsw.edu.au/news/quantum-computing-first-two-qubit-logic-gate-in-silicon
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IIAOPSW
A bit misleading. Waveguides in Silicon have been used to make photon qubit
gates for a while now. It isn't the first time Silicon has been used.

The real takeaway is "Here we present a two-qubit logic gate, which uses
single spins in isotopically enriched silicon".

Paper found here:
[http://www.nature.com/nature/journal/vaop/ncurrent/pdf/natur...](http://www.nature.com/nature/journal/vaop/ncurrent/pdf/nature15263.pdf)

Using spin qubits is nothing new, but normally its done in an ion trap. Doing
it in Silicon is novel. I've barely skimmed the paper so I don't really know
what their decoherence properties are.

~~~
zamalek
You seem to know a lot more about this than I do: in your opinion how much of
a leap is this? Could this be the Chicago Pile of quantum computing or just
another D-Wave?

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IIAOPSW
So I did ctrl+f for "coherence time" and found this:

A marked improvement in coherence times has been observed by defining the
quantum dots in silicon, which can be isotopically purified, such that quantum
dots with single-spin fidelities above the threshold of surface codes can be
realized

Meaning they get a bit less than a 10% error per operation.

 _the_ thing that will make quantum computing practical is quantum error
correction. I describe it as "the hospital problem". Suppose there's a car
accident. An ambulance gets sent to pick up the survivors and fix it. But an
ambulance is also a vehicle and so there is a chance it will also get in an
accident. You can send more ambulances to fix the situation, but those
ambulances are also vehicles and could also get into accidents. The layout of
the roads, frequency of hospitals, and probability of accident per meter will
determine on average how many problems get fixed per ambulance. If on average
an ambulance gets into less than one accident then it is worth it. Otherwise
ambulances cause more accidents than they fix. The point where ambulances fix
exactly as many accidents as they create is called the threshold. Once you
pass the threshold, with sufficient hospitals you can make the expected number
of accidents arbitrarily low (aka the probability of successful computation
arbitrarily high).

When people talk about surface codes, they mean the broad class of error
correction schemes which look a bit like the toric code. The toric code has a
threshold value around 10% which is great! However it is literally a lattice
of atoms (or other spin systems) embedded on a torus. Construction is
difficult. It also doesn't scale. A toric code with a few dozen qubits works
as well as one with a few hundred. In fact several no-go theorems show that
any transitionally invariant lattice in less than four dimensions won't scale.
My research is actually in getting around these theorems by throwing out the
lattice idea entirely.

This research is important, but the real Chicago Pile of QC will be passing
the threshold of an easy to implement correction scheme.

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greeneggs
"When people talk about surface codes, they mean the broad class of error
correction schemes which look a bit like the toric code. The toric code has a
threshold value around 10% which is great! However it is literally a lattice
of atoms (or other spin systems) embedded on a torus. Construction is
difficult. It also doesn't scale. A toric code with a few dozen qubits works
as well as one with a few hundred. In fact several no-go theorems show that
any transitionally invariant lattice in less than four dimensions won't scale.
My research is actually in getting around these theorems by throwing out the
lattice idea entirely."

I'm not sure what you are trying to say here, but it doesn't seem to be
standard thought.

~~~
IIAOPSW
When you lookup "surface code" the first thing that comes up is "toric code".
Off hand there's the toric code, the 3d toric code, welded toric code, color
codes, and the surface code itself (which is a lattice of "measure qubits"
next to a lattice of "data qubits"). The basic idea on all of them is the same
as Kitaev's original insight of topologically protected data. IMO its
completely fair to lump these together as "surface codes".

If you disagree with my choice of words then I'm happy to concede and
rephrase. If something else I said doesn't sound kosher please by all means
lmk.

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selimthegrim
No, he's correct. In addition I would add that passive error correction is
taking leaps and bounds lately, making dephasing essentially a non issue in
multi-qubit systems (scalability is yet to be fully manifested)

~~~
IIAOPSW
correct in what respect? I'm not trying to be rude here but it is unclear what
exactly I'm being called out on. Do you disagree with my use of the word
"surface codes" or do you disagree with the way I've lumped a bunch of QEC's
together under one name? Or is the disagreement something else entirely.

And yeah I'm aware of the advances in passive QEC. Exciting time to be alive!

~~~
selimthegrim
The he I meant was you, I'm not calling you out.

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Beltiras
A quote from a consultation I did this morning: "A QC capable of Shor's
Algorithm is decades away, if at all possible." My reasoning was that you can
do convoluted physics experiments simulating degree 1 qbits (2, 3 or 4) but
raising the order of magnitude might be close to impossible given the physics
we know. If we could manage to do 5-10 (2 degrees) in silicone, we should have
engineering to do 6 degrees since we know a lot about the material. I was
asked who was the largest stakeholder in QC. I said it probably was not the
NSA since that's the obvious one. I've since had the thought that the NSA
might be an anti-stakeholder, that they would be better off if QC _isn 't_
realized. They have pretty good leakage from current crypto systems. If the
world is forced to move away from it, they have to break a whole new suite of
cyphersystems.

~~~
selimthegrim
If the National Security state is willing to piss away umpteen billions of
dollars on shit like the F-35, the Osprey, the M2 Bradley, et al, you think a
few billion on quantum computing just to be safe is even an eye blink?

