Hacker News Reader

Comment by ThePhysicist | Hacker News Reader

Regardless of whether the Martinis team actually demonstrated quantum supremacy, their achievement is undoubtedly worth the praise it received. I built and operated a superconducting two-qubit processor for my PhD thesis in 2010, so I have a good understanding of the challenges invovled in operating and tuning qubits. For me, seeing the 53 qubit chip, its wiring setup and the control electronics is just breathtaking.

I think John's team neatly demonstrated that they are far ahead of the competition (e.g. Rigetti, IBM) in terms of maturity and qubit control, and if that's any indication of future success I would strongly bet on their team for building the first actually working quantum computer.

View on HN

It looks so bizarre, not the computer as we know it: https://www.flickr.com/photos/ibm_research_zurich/4078696912...

Reminds me of the first transistor.

https://upload.wikimedia.org/wikipedia/commons/b/bf/Replica-...

It's a science experiment and unfinished proof of concept. Looks mad-sciencey.

These pictures are always really fun, but they're not really "the computer" -- most of what you see is the cooling system.

Hell, even the sight of complex mechanical computers like the Babbage difference engine will make me tremble.

https://upload.wikimedia.org/wikipedia/commons/thumb/8/8b/Ba...

See it in action on YouTube.

I feel like I get a similar feeling when I see really alien-looking wind turbines.

Most of that is the refrigeration system needed to keep the device cold enough, see https://en.wikipedia.org/wiki/Dilution_refrigerator

Could you recommend a few books that someone with no particular knowledge on the subject could read to get a grasp on what's going on?

Scott Aaronson's book, Quantum Computing since Democritus, seemed remarkably easy to understand when I read it.

https://uwaterloo.ca/institute-for-quantum-computing/sites/c... Quantum Cryptography School for Young Students

Michael Nielsen has a nice series of blog articles on the subject:

http://michaelnielsen.org/

(look for "The Quantum World")

What is a "first actually working quantum computer" if not this Sycamore device that Google used to demonstrate quantum supremacy?

Maybe something that solves a natural problem instead of an artificial one.

One with fault-tolerance.

I reckon one that has quantum error correction on qubits which present Boolean values. I.e. one which could also run a classical program. That is the quantum computer which is actually a quantum Turing machine and which can do speedups of e.g. factorization. Unfortunately, it is hypothesized that such a quantum Turing machine will require about ~1000 qubits of this machine per qubit with error correction, maybe more.

For what it's worth, the FAQ on his blog dances around this topic:

"Running Shor’s algorithm to break the RSA cryptosystem would require several thousand logical qubits. With known error-correction methods, that could easily translate into millions of physical qubits, and those probably of a higher quality than any that exist today."

"How to factor 2048 bit RSA integers in 8 hours using 20 million noisy qubits"

https://arxiv.org/abs/1905.09749

You could of course argue that it's already a quantum computer that they've built. It's not a general purpose quantum computer though, as it doesn't allow you to run arbitrary gate sequences on it due to limited coherence time and limited inter-qubit connectivity. So while you surely can do some quantum computations on it you can't yet run arbitrary quantum programs.

In terms of limited connectivity: mesh connectivity is theoretically sufficient. Putting all qubits in superposition in a preparation phase then allows you to do any arbitrary quantum computation by a sequence of measurements and one-qubit operations. You would be doing one-way measurement quantum computing, and not quantum circuits, but it's just as universal.

I'd love to see one that can compute energy levels for molecules that are larger than we can currently simulate on supercomputers. quantum chemistry simulations typically scale O(n7) where n = # of basis functions (usually larger than the number of atoms in the molecule).

I agree with you, but with a couple of corrections. The cheaper algorithm called Density Functional Theory is probably the most popular kind of quantum calculation on molecules, and scales as ~N^2.5 in practice. On the other hand, the number of basis functions is a more than the number of valence electrons in the molecule (if the calculation is at all sane), much larger than the number of atoms. So the exponent is often smaller than you said, but the N is also much bigger. So in the end you're absolutely right about the need for quantum computing for molecules.

Reply on news.ycombinator.com