So today D-Wave’s latest paper has been published in Nature. You can take a look at the abstract here (and the paper if you have a subscription!). The paper is entitled ‘Quantum annealing with manufactured spins’
So what is this new publication all about?
Everyone knows that when you observe a quantum computation, you destroy it, right? So how are you supposed to know if your quantum computer is working correctly? That’s what this latest Nature article from the scientists at D-Wave addresses. We’ve known for some time that the D-Wave quantum computers are performing computations, and we know that the answers they are giving us are correct, (they agree with our predictions).
But wouldn’t it be cool to be able to go further, to actually look INSIDE a quantum computer, with large numbers of qubits all interacting and computing, and catch the quantum mechanics in the act?
Not just a string of atoms
The first cool thing about this experiment is that the system under test this isn’t just the usual suspects found in quantum experiments – a string of atoms, a series of electron spins in a crystal, or a bunch of photons. It’s not a curiosity that scientists have found lurking in the natural world allowing them to observe some quantum mechanics. This is a processor! It is programmable – it actually solves problems, looks similar to the integrated circuits inside your laptop, and you can program it using Python! Anyway… I digress. What I mean to say is that it is very important to realise that these quantum effects are controllable. We’re no longer just looking at quantum systems – like atoms – and verifying their quantum nature. We’re taking those systems, and moulding and warping their energy levels, and controlling the way they interact with each other, so that we can use those quantum effects to help us compute.
Respecting the bigger picture
It’s fairly easy to isolate a single quantum bit and do some experiments on it to check that it is behaving quantum mechanically. It’s much harder to test that it’s STILL working quantum mechanically when it’s in the middle of an incredibly complex processor, connected to all kind of lines and electronics. It would be like designing a bridge that was able to support its own weight – but never considering what would happen when the bridge is used as it should be – with high volumes of traffic passing over it every day.
That’s the second cool thing about this result – during the experiment, the processor is operated in the same way as it is operated during problem solving. We didn’t have to do anything particularly esoteric to the qubits in order to watch them. We’re simply lifting the lid off the black box so we can take a peek at the quantum mechanics of the computation as it happens during normal problem solving.
In the experiment itself, this ‘black box’ is a subsection of the processor known as a unit cell. It is a fundamental block which is replicated and tiled together to form the larger processor. The unit cell tested contains 8 qubits, all linked together. There are 16 such unit cells in the current generation of D-Wave’s processors – known as the ‘Rainier’ architecture.
So how exactly do the scientists ‘watch’ the quantum mechanics? Well, the unit cell mentioned above is operated in the same way as it would be during a normal computation – running what is known as a quantum annealing algorithm. The difference is that at a certain point during the computation, the usually slow, careful annealing of the qubits is suddenly interrupted by a very fast signal. This signal causes the unit cell to ‘freeze’ in whatever state it was in at the time. If you repeat the computation lots of times, but each time apply your ‘freezing’ signal at a slightly different moment during the quantum computation, you can build up a series of ‘snapshots’, like stills on a movie reel. D-Wave scientists compiled all these snapshots to reveal exactly what is happening during the quantum computation.
The next step is to check that these results really do agree with what quantum mechanics tells us. So a theoretical model of the unit cell was set up, based on the predictions of quantum physics, and the model fits very well indeed. Even more interesting, a second model was set up, which captured how CLASSICAL physics predicts the processor should behave. The results were striking – the classical model wasn’t even close! There’s no way these results can be explained using classical physics.
This is a pretty awesome result for quantum computation in general. People have been worrying for a while that it may not be possible to ever build large scale quantum computing systems, that once we start putting those fragile qubits into a real processor environment that the quantum mechanics will be destroyed. The results from this latest paper reveal to us exactly the opposite – that quantum effects persist, and allow us to control them.
Maybe quantum mechanics isn’t so spooky after all. In fact, I’d say that the future of building large scale processors that operate using quantum mechanics looks more promising than ever.