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VW Solves Quantum Chemistry Problems on a D-Wave Machine

spectrum.ieee.org

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Re: VW Solves Quantum Chemistry Problems on a D-Wave Machine

#2

Post by Havoc »

>For beginners, he says, an actual D-Wave device isn’t even necessary.

I find this somewhat surprising.

If you think of AI code designed for GPUs, there I can see "yeah you can practice on a CPU". It'll suck but it'll work.

For quantum tech the entire sales pitch is that it's fundamentally different...doing what's near impossible on conventional hardware.

Yes I realise he's talking about the library so annealing on a CPU I guess but still seems like a very strange comment in this context.

Re: VW Solves Quantum Chemistry Problems on a D-Wave Machine

#3

Post by zaroth »

I appreciated the rudimentary presentation of the capabilities and limitations of the machine and calling out the connectivity of the qubits versus a universal quantum machine which would have full connectivity between all the bits.

I’d be curious is there a simple formula for calculating the “effective universal qubits” of the D-Wave?

2,048 indeed sounds like a lot of qubits based on my extremely limited knowledge of quantum, but with only ~6k connections versus fully connected which would be n(n-1)/2 = ~2mil is it just a marketing gimmick?

Why is it useful to push the bit count so high if the connectivity is so limited?

Re: VW Solves Quantum Chemistry Problems on a D-Wave Machine

#4

Post by zaroth »

Havoc wrote: post #2

>For beginners, he says, an actual D-Wave device isn’t even necessary. I find this somewhat surprising. If you think of AI code designed for GPUs, there I can see "yeah you can practice on a CPU". It'll suck but it'll work. For quantum tech the entire sales pitch is that it's fundamentally different...doing what's near impossible on conventional hardware. Yes I realise he's talking about the library so annealing on a…

I think the performance of the actual device is so limited it’s like running an iPhone simulator to debug your software.

But if the expected performance of the quantum machine is theoretically going to increase exponentially every X months for the next 2 decades, combined with the theory that certain problems shift from exponential to polynomial time solutions, yes, eventually the “debugger” will not be useful to actually try running your solver.

Re: VW Solves Quantum Chemistry Problems on a D-Wave Machine

#5

Post by twtw »

... and couldn't get the correct answers for LiH. Interesting that the article didn't mention this.

From the paper:

> For lithium hydride, LiH, we were not able to reproduce closely the ground state energy with the currently available hardware. When accounting for 3 orbitals and using a scaling factor of r = 4, we already had to use 1558 qubits, which is a large fraction of available qubits. To summarize: the investigated method in general works, but it might be difficult to apply it to larger systems.

Re: VW Solves Quantum Chemistry Problems on a D-Wave Machine

#7

Post by klyrs »

zaroth wrote: post #3

I appreciated the rudimentary presentation of the capabilities and limitations of the machine and calling out the connectivity of the qubits versus a universal quantum machine which would have full connectivity between all the bits. I’d be curious is there a simple formula for calculating the “effective universal qubits” of the D-Wave? 2,048 indeed sounds like a lot of qubits based on my extremely limited knowledge o…

I don't know of a good way to compare their real-valued gates to complex gates; it's probably a scaling factor due to encoding those gates as gadgets.

As for full connectivity; this is a benefit over gate-model: they can simulate fully-connected logical qubits; on the order of 2sqrt(Q/2), or 64, in their existing hardware.

As for the high bit count: sparse, structured problems can make very good use of the existing connectivity. Simulations of a cubic lattice are nearly competitive with modern classical hardware, for example. IIRC they can factor 16-bit numbers, too --these problems are "quasiplanar" and have relatively low connectivity requirements.

Full connectivity actually brings some huge engineering challenges; DWave's strategy is more about "this is what's actually possible today" and not "we're going to make universal quantum computers, don't ask us about error correction or crosstalks or calibration of large-scale microwave circuits"

Re: VW Solves Quantum Chemistry Problems on a D-Wave Machine

#8

Post by jf- »

twtw wrote: post #5

... and couldn't get the correct answers for LiH. Interesting that the article didn't mention this. From the paper: > For lithium hydride, LiH, we were not able to reproduce closely the ground state energy with the currently available hardware. When accounting for 3 orbitals and using a scaling factor of r = 4, we already had to use 1558 qubits, which is a large fraction of available qubits. To summarize: the investi…

Presumably because that’s just a matter of scale. The calculation didn’t turn out wrong, the machine just isn’t powerful enough to calculate it. All of this work is proof of concept for more powerful devices down the road, the point isn’t that it’s better than a classical computer right now.
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