
Ask HN: Can anyone explain how DWave was able to scale to 1,800 qubits? - arthurcolle
I have a bunch of questions about the recent Nature article and about the underlying simulation itself.<p>1) how are the individual qubits arranged&#x2F;&quot;built&quot;<p>2) what is specifically meant by the topological phenomenon observed, and how does this relate to the quantum simulator accurately predicting what the classical model said what occur?<p>3) what is the interplay between &quot;performing a computation&quot; and running a quantum simulator? not really seeing the immediate connection there<p>4) last, not sure how far DWave had come previously to achieving such a large-scale computation with this approach, but it does seem like a pretty huge number of interacting qubits compared to other highly-entangled systems<p>Thanks<p>Link to Nature article in question: 
https:&#x2F;&#x2F;www.nature.com&#x2F;articles&#x2F;s41586-018-0410-x
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abdullahkhalids
Even though I work in quantum information, this stuff is a little far from
what I do (quantum optics), but I will do my best.

1\. The qubits are superconducting in nature. Each qubit is a superconducting
loop, in which the current can flow clockwise or anti-clockwise, which can be
called 0 and 1 respectively. There is superposition possible in the direction
of current flow, so you have qubits. They arrange the 1800 qubits into a
lattice. There are also current carrying wires near groups of qubits, which
allows them to change the state of these qubits by changing the magnetic field
experienced by these qubits.

2\. What they are doing is simulating some particular types of physical
systems, called Ising models. These systems consist of a lattice of particles
which interact with each other. If you change either the shape of the lattice
or the interaction between the particles, the equilibrium state of the system
changes. Usually we are interested in finding out various properties of the
system when at equilibrium. Doing this usually requires simulating the Ising
model in some way. Some of these Ising models - at least - can be efficiently
simulated classically and these guys have tried to simulate some of these
systems using their quantum simulator. They do so successfully, showing that
their very complicated machine's answers agree with those of a classical
computer. So they haven't shown a new phenomena or done something which hasn't
been done by classical computers. They have just shown that their machine
gives the right answers.

3\. I am not sure where you are getting these quotes from. But its a
simulation in the sense that it tells you various properties of some system.
You can call simulations just a particular type of computation.

4\. Their is no discussion of speed in the above, which is central to the race
towards quantum computers, and for good reason. Dwave's machine is not a
quantum computer i.e.\ cannot simulate BQP problems efficiently. Their
machines can't address individual qubits, or do arbitrary quantum gates on
these qubits, hence can't get the promised speedup of quantum computers.
Therefore, they are going a different route. They essentially want to make the
equivalent of Lisp machines for physical systems. Lisp machines were designed
to run one language really fast. Dwave want to make these machines that can
simulate some physical systems polynomially faster than classical computers.

When you see Dwave news in the future, unless you see otherwise, just assume
that they have these fancy classical computers, not quantum computers and you
are good.

~~~
quickthrower2
Thanks. So silly laymen question here, but I assume the 1800 qubits are not
all entangled together. Are any of them entangled at all?

~~~
abdullahkhalids
Most definitely not. I haven't looked too closely at the paper but I think
1800 are divided into groups of 4, where they can do some entangling
operations amongst each group, and weaker entangling operations between pairs
of groups.

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quickthrower2
I would like to add more questions to authorcolle's:

How many qubits are for error correction, and how many usable qubits would you
get?

Is it commercially viable to buy one of these machines, or are you better off
just using Google cloud or similar to get the equivalent amount of
computational power to solve problems that are appropriate for quantum
computing?

~~~
abdullahkhalids
See my answer here
[https://news.ycombinator.com/item?id=17844341](https://news.ycombinator.com/item?id=17844341)
They are not doing any sort of error correction. For the foreseeable future,
it is indeed much cheaper (by orders of magnitude) to buy commercial cloud
computing.

