Rol up, roll up: This quantum computer chip is for public use (Image: University of Bristol)

THE record-smashing quantum computer reminds me of Prince of Persia. A dizzying array of lenses and prisms that stretch across the room, it looks rather like the light-directing puzzles common in such video games. I long to twist the lenses and shoot laser beams everywhere.

That wouldn’t make me popular here. A quiet stillness pervades the Centre for Nanoscience and Quantum Information, part of the University of Bristol. Because quantum states are fragile, the building’s design dampens vibrations and even filters the power supply to remove electrical noise. Each part of the machine spread before me is carefully aligned so that mixing a pair of light beams carries out a specific calculation. Now, it’s set to turn 21 into 3 and 7, the two prime numbers that it is divisible by. It is the biggest number a quantum computer has ever broken into primes using the famous quantum protocol, Shor’s algorithm. Still, I can do the same thing in my head – so what’s the big deal?

The answer is clearer at my next stop, where I see a wafer of 20 or so chips, each a few centimetres long and made of silicon dioxide. Although not yet as capable as the behemoth I first encountered, these chips are the next stage in the lab’s attempt to build quantum computers that outperform even the best non-quantum machines.


Information on an ordinary computer is stored as bits, which can be either a 1 or a 0. Quantum bits, or qubits, are both at once, so a large array could process a great deal more information. But assembling even a handful of qubits is tough because of their fragility, so the best way to scale up is to scale down. “You could potentially start doing bigger and more complicated experiments,” says my guide, physicist Graham Marshall. “But can you make it so that it doesn’t feel the presence of the moon, or the movement of tectonic plates? There is a limit to how well you can stabilise something on that scale.”

That’s where the chips come in. Instead of using glass prisms to mix photons, channels filled with silicon nitride are etched into the chips’ surface in patterns that I can just make out. The channels confine and steer photons, guiding them so that they become “entangled” – a quantum property needed for computation. This should lead to computers that are easier to stabilise and so can scale up.

A similar chip (see image) is already hooked up to the internet, making history as the first quantum processor available to the public. Still, the device doesn’t incorporate a photon source or detector – these components spill out across another bench.

My third and final stop represents the lab’s most recent efforts. Made from pure silicon, as in ordinary computers, this chip is capable of bending light around sharp turns, so it can be much smaller – it is half the size of my thumbnail. The channels are too small to see but green and purple shades dance across its bright surface.

Crucially, this chip can generate its own photons as well as entangle them. Detection still takes place in a small chamber at the other end of the lab, which must be cooled to a few degrees above absolute zero (arxiv.org/abs/1304.1490) but Marshall says they are working on less chilly detectors that could be added to the chip.

“One vision I like is you buy a laptop or a desktop and it has ‘quantum inside’,” he says. It’s not clear whether these quantum guts would find uses in video games: quantum processors won’t make all calculations faster, but they should speed up database searches and simulations of molecules. At least I would be able to command my own quantum light beams.

This article appeared in print under the headline “Honey, I shrunk the quantum computer”