### Quantum Computers: Flipping Coins and Killing Cats

In October, following leaks, secrecy and scepticism, a short paper was finally released online by the journal Nature. In it, researchers at Google claimed to have achieved “quantum supremacy.” They stated they had performed a computation that would take 10,000 years to complete with the world’s best supercomputer, but which only took them a few minutes on a new type of device called a quantum computer. To achieve this, the team at Google had to imprint a tiny network of superconductors on a computer chip, cool it to thousandths of a degree above the lowest temperature physically possible, and manipulate every component of the device with minimal error in concert with the rest of the chip. This was no mean feat. In the Nature paper where this achievement is recorded, Google christened the device “Sycamore.”

## A Chat With a Quantum Superstar

A few weeks ago, one of the heads of Google’s quantum research, John Martinis, was discussing ideas for Sycamore’s future with some of the professors and postdocs working in the same research group as me. I had no practical reason for being there, but you don’t just give up the opportunity to meet a celebrity in your field, so I managed to convince the professor in charge to let me sit in on the meeting. Meeting Martinis was somewhat anticlimactic – my interaction with him was limited to a handshake, after which I was seated in the corner, listening lamely as incomprehensible terms were being flung across the coffee table. But the bits I picked up were enough to paint a picture of the Google research effort and of the man Martinis himself.

He’s a tall, thin figure with a mop of thick, grey hair slapped rather chaotically onto his head, allowing him to nicely reflect our shared picture of a genius physicist. He came across as an affable and enthusiastic character who, perhaps as a consequence of the Californian start-up culture, oozed an easy-going confidence. It was the tacit assumption that Sycamore was the best in the world at everything it was capable of doing. And the things it couldn't do yet? Well it'll get there pretty shortly.

This portrait of poise and smiles was a little undercut, however, whenever rival players in the quantum game were mentioned. You see, Google isn't the only company investing in quantum computing. IBM, Microsoft, Lockheed Martin and a raft of others are collectively sinking millions into the field, all wanting to develop the most powerful quantum machines. At one point in the conversation, when discussing the limited access that some other companies provide to their own devices, Martinis asked conspiratorially “I mean, what are they hiding?”

Clearly there is a little bad air between the quantum companies. This was probably exacerbated by a recent paper released by IBM researchers, which turned out to be a bit of a thorn in the side of Google’s quantum supremacy claims. The idea of quantum supremacy is that if you have a large enough quantum computer that you have sufficient control over, you can do things that would take an enormous amount of time on a normal computer. When this length of time becomes ridiculous, people say that we have achieved quantum supremacy. Google claimed that, for the first time, their quantum computer performed a task that would take the world’s best supercomputer 10,000 years to complete. IBM, meanwhile, claimed that it would actually only take 2.5 days on that supercomputer. Of course Google, in turn, struck back with their own rebuttal. However the fact that IBM's paper came out before Google’s original was even officially released, somewhat took the wind out of the sails of this result.
Beyond these debates of quantum supremacy, there are two points that lie at the centre of the quantum race. The first is the concept of benchmarking – the varied set of approaches to testing how much control you have over your quantum device. Essentially, if a quantum computer is an extremely slippery bar of soap, benchmarking provides an answer to the question: how well can you hold onto it? Benchmarking allows one company to make a quantitative claim that their quantum computer is “better” than all the others. The other concept is “quantum error correction,” which is sometimes thought of as the holy grail of quantum computing – the achievement that will allow these devices to revolutionise the world. Whoever is able to implement quantum error correction in a large-scale manner will unlock the powers that quantum computers promise us.

In these articles, I want to explore the concepts that surround this burgeoning field. In this post and the ones to come, I'm going to be illustrating the ideas behind quantum algorithms, benchmarking and error correction, as I think this will provide an understanding of the mind-boggling and controversial world of quantum computing research. However to start off with, we’ll have to understand the principles on which these devices are based: the physics of quantum mechanics. This topic alone is enough to fill textbooks and completely fry your brain, so it will be enough to fill up this first article. But after establishing the ground work, we'll be able to turn to the other topics sparking debates and excitement. The second post will give an intuitive picture of how quantum computers work, while the next will explain why discussions of benchmarking happening outside of the media spotlight could be much more important for the field than minor scraps over supercomputer run times. Finally, I’ll address what all the fuss is about with quantum error correction. We’ll look at how it works, and how it will push the bounds of quantum computer capabilities and of currently-established science. At the end I’ll touch on how it might even provide insight into the fundamental nature of space and time.

But first, we’ll start more modestly, with a look at the brain-frying microscopic world of quantum mechanics.