Google claims to have demonstrated something called “quantum supremacy”, in a paper published in Nature. This would be mark as a significant milestone in the development of a new type of computer, known as a quantum computer, that could perform very difficult calculations much faster than anything possible on conventional “classical” computers. But a team from IBM has published their own paper claiming that they can reproduce the Google result on existing supercomputers. While Google vs IBM might make a good story, this disagreement between two of the world’s biggest technology companies rather distracts from the real scientific and technological progress behind both teams’ work.
Despite how it might sound, even exceeding the milestone of quantum supremacy wouldn’t mean quantum computers are about to take over. On the other hand, just approaching this point has exciting implications for the future of the technology. Quantum computers represent a new way of processing data. Instead of storing information in “bits” as 0s or 1s like classical computers do, quantum computers use the principles of quantum physics to store information in “qubits” that can also be in states of 0 and 1 at the same time. In theory, this is allows quantum machines to perform certain calculations much faster than classical computers.
In 2012, Professor John Preskill coined the term “Quantum supermacy” to describe that to point when quantum computers become powerful enough to perform some computational task that classical computers could not do in a reasonable timeframe. He deliberately didn’t require the computational task to be a useful one. Quantum supremacy is an intermediate milestone, something to aim for long before it is possible to build large, general-purpose quantum computers.
In its quantum supremacy experiment, the Google team performed one of these difficult but useless calculations, sampling the output of randomly chosen quantum circuits. They also carried out computations on the world’s most powerful classical supercomputer, Summit, and estimated it would take 10,000 years to fully simulate this quantum computation. IBM’s team have proposed a method for simulating Google’s experiment on the Summit computer, which they estimated would take only two days rather than 10,000 years.
Random circuit sampling has no known practical use, but there are very good mathematical and empirical reasons to believe it is very hard to replicate on classical computers. More precisely, for every additional qubit the quantum computer uses to perform the calculation, a classical computer would need to do double its computation time to do the same. The IBM paper does not challenge this exponential growth. What the IBM team did was find a way of trading increased memory usage for faster computation time.
They used this to show that how it might be possible to squeeze a simulation of the Google experiment onto to the Summit supercomputer, by exploiting the vast memory resources of that machine. (They estimate simulating the Google experiment would require memory equivalent to about 10m regular hard drives.) Larger quantum circuits could do the same calculations as huge amounts of classical computing memory.
The 53-qubit Google experiment is right at the limit of what can be simulated classically. IBM’s new algorithm might just bring the calculation within reach of the world’s biggest supercomputer. But add a couple more qubits, and the calculation will be beyond reach again. The Google paper anticipates this, stating: “We expect that lower simulation costs than reported here will eventually be achieved, but we also expect that they will be consistently outpaced by hardware improvements on larger quantum processors.”