Google's quantum leap

53-qubit quantum computer outperforms classical computing, in a highly specific way.

Google recently announced that it had achieved what has widely been termed quantum supremacy, by building a computer, based on quantum interactions, that theoretically vastly outperforms an equivalent classical computer carrying out the same task. As impressive as this might sound, there are several caveats to bear in mind.

First, some background. Quantum theory is an attempt to explain the interactions of atomic and subatomic particles in the universe, to explain observations in the physical world that can't be accounted for any other way. Even the best physicists on the planet tend to find quantum theory / quantum mechanics weird, because it is. For example, according to quantum theory, a single particle can be in two places at once and two particles can be entangled together so that modifying the properties of one of them instantly modifies the other, even if they're at opposite ends of the universe.

The latter state, called entanglement, breaks the light-speed limit for the transmission of information. Einstein, a man not given to excessive superstition, called this "spooky action at a distance" and that's just one of many spooky things about quantum theory. Spooky or not, it happens and can be reliably tested under experimental conditions. In fact schoolchildren have been performing such experiments for decades: Thomas Young's double-slit physics experiment to demonstrate particle-wave duality also demonstrates the existence of quantum effects.

What's potentially useful from a computing point of view is superposition, in which a collection of entangled quantum particles exists in an undefined and unknown state, a state that only becomes apparent when it's observed. If that sounds logical enough, there's a twist: the superposition is potentially in any and all of the states at once, only 'collapsing' into a defined state when the observation happens.

By doing the equivalent of sneaking up on a quantum superposition and only glancing at it at the last minute out of the corner of one's eye, we can potentially create a computer that calculates multiple possible 'answers' to problems simultaneously, providing the correct one once the observation happens. Instead of a 'bit' in a classical computer that's either on or off, 1 or 0, a quantum bit, or qubit, could potentially hold a 1, a 0 and every partial number in between, doing simultaneous calculations until the correct result emerges.

The above paragraph is a huge simplification that would horrify most particle physicists but it helps explain why quantum computing has been an area of research - and increasing funding - for some time. A quantum computer could potentially outperform any classical computer on the planet, in fact all of them combined, if it had enough qubits. That's because quantum computing power increases exponentially as the number of qubits rises.

Quantum computers would be ideally suited to searching vast databases and creating - or cracking - encrypted communications, amongst many other applications that haven't yet been fully considered or tested. However, qubits are hard to make and hard to maintain. They interact with their environment in any number of ways, from heat to light to mechanical vibration, and so must be kept as isolated from their surroundings as possible. Qubits may be small but the equipment to maintain in their peaceful state them is massive.

Now, to Google's announcement. As impressive as it is that the company has built a quantum computer which - at least theoretically - outperforms anything in classical computing, it's not really a fair comparison. The task chosen was specifically designed to be something at which a quantum computer might excel. Even then, Google may have over-estimated the amount of time a classical computer might take to complete the same task. That would downgrade the quantum computer's performance from quantum supremacy to mere quantum superiority, but that's still impressive. Even so, it will be much harder to take a task that's routinely performed by classical computers, translate it into a form that makes sense to a quantum computer, and obtain a dramatically faster result. It's not impossible, but certainly very difficult.

This isn't to detract from the achievement of the Google team in any way. There are many steps on the road to a functioning quantum computer capable of handling the types of processing task that we use every day. This is one. It may not be a huge step, but it still takes us forward.

Quantum computers aren't going to be providing unbreakable encrypted communications in the near future, nor mining a fortune in cryptocurrency overnight as some observers have half-jokingly suggested. However, it's not too much of a stretch to think that in ten to twenty years' time, quantum computers will be doing useful processing work at a much more efficient rate than classical computers, of any size.