What is... Quantum supremacy?

The race to quantum computers is hotting up, with researchers claiming their machines will manage tasks beyond any modern rival

Researchers around the world are in a race to build a quantum computer that’s capable of calculations current machines can only dream of – and they’re getting close. That goal is quantum supremacy. Here’s what it means and how long we’ll have to wait for the quantum era of computing.

What is quantum supremacy?

Before we go there, some background. Quantum computers work via mind-bending qubits, which are the equivalent of bits but, rather than merely having an on or off position as our binary versions do, a quantum bit (qubit) can either be off, on or both (called superposition). Combined with the idea of entanglement – qubits interact with each other – this means the difference in processing power between a two-bit computer and a two-qubit quantum computer are on the exponential scale.

You haven’t mentioned supremacy.

I’m getting there! Researchers are trying to build quantum computers with a sufficient number of qubits to reach quantum supremacy. That’s when quantum machines can do calculations beyond existing computers’ reach. A prime example, if you’ll excuse the mathematical pun, is Shor’s algorithm, which is the basis of much modern cryptography. When a machine with enough qubits is made, cryptography will fall and we’ll enter a new age of computing power.

How far off is that?

Educated guesses suggest cracking cryptography will require a system with 100 million qubits – that’s the latest prediction from a paper in

Nature by researchers at Google’s Quantum AI Laboratory. But reaching quantum supremacy is expected to arrive at the more modest 50-qubit mark.

To be clear, that’s not the point at which quantum overtakes your desktop PC, but when it surpasses the best computer we can build. The fifth fastest supercomputer in the world is Berkeley Lab’s Cori, which earlier this year was used to simulate a 45-qubit machine. Beyond 50 qubits, not even supercomputers will keep up.

How many qubits are we at now?

This summer at the Fourth International Conference on Quantum Technologies, Google announced that it’s working on a 49-qubit machine, although its dramatic announcement was undercut by a team from Harvard University claiming it had made a 51-qubit machine. If those claims are true – they’ve yet to be peer reviewed, – we’ve potentially hit supremacy, but we’re still a long way off the full potential of quantum computing.

Why are so many qubits needed?

In classical computers, it’s easy to check for errors. Quantum computers work in such a way that error checking requires a lot of excess processing power to validate the answer. Experts are working on quantum machines that don’t bother with error checking, and they may be the first to arrive, but will have more limited uses.

And when will this all happen?

We’re still in the early days. Google’s 49-qubit machine hasn’t even been made yet: it’s merely crowing about the design, which has been tested on a nine-by-one circuit rather than the described seven-by-seven. That said, those same Google researchers say companies will start seeing returns on their quantum investment within five years. We won’t have perfectly working quantum computing, but some of the processing boosts that the research leads to may well be useful before quantum supremacy arrives.