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 dawn of the quantum era of computing. (Hint: a long time.)

What is quantum supremacy?

There’s not much that’s simple about quantum computers. They 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 yet

We’re getting there. Researchers are trying to build quantum computers with a sufficient number of qubits to reach quantum supremacy. That is when quantum machines can do calculations that existing computers aren’t capable of processing, which is rather the point — if we just wanted to do what we can do now, that wouldn’t be particularly exciting. Those currently incalculable calculations include cracking Shor’s algorithm, which is the basis of much modern cryptography. When researchers manage to make a machine with enough qubits, cryptography will fall and we’ll enter an age of computers with incredible powers.

How far off is that?

Educated guesses suggest cracking cryptography will require a 100-million-qubit system, or that’s the latest prediction from a paper in Nature by researchers at Google’s Quantum AI Laboratory. However, reaching quantum supremacy — topping existing machines — is expected to arrive around the much 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 very 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 be able to keep up.

And how many qubits are we at now?

Last summer at the 4th 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 successfully made a 51-qubit machine. If those claims are true — they’ve yet to be peer reviewed — we’ve potentially hit quantum supremacy, but we’re still a long way off the full potential of quantum computing.

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Why are so many qubits needed?

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

And when will this all happen?

We’re in the early days of quantum computing. 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 predict companies will start seeing returns on their investment into quantum within five years. We won’t have perfectly working quantum computing, but some of the ideas and processing boosts that the research leads to may well be useful before quantum supremacy truly arrives.