THE record-smashing quantum computer reminds me of . 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鈥檛 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鈥檚 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鈥檚 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鈥檚 algorithm. Still, I can do the same thing in my head 鈥� so what鈥檚 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鈥檚 attempt to build quantum computers that outperform even the best non-quantum machines.
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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. 鈥淵ou could potentially start doing bigger and more complicated experiments,鈥� says my guide, physicist . 鈥淏ut can you make it so that it doesn鈥檛 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.鈥�
That鈥檚 where the chips come in. Instead of using glass prisms to mix photons, channels filled with silicon nitride are etched into the chips鈥� surface in patterns that I can just make out. The channels confine and steer photons, guiding them so that they become 鈥渆ntangled鈥� 鈥� 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鈥檛 incorporate a photon source or detector 鈥� these components spill out across another bench.
My third and final stop represents the lab鈥檚 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 鈥� 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 () but Marshall says they are working on less chilly detectors that could be added to the chip.
鈥淥ne vision I like is you buy a laptop or a desktop and it has 鈥榪uantum inside鈥�,鈥� he says. It鈥檚 not clear whether these quantum guts would find uses in video games: quantum processors won鈥檛 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 鈥淗oney, I shrunk the quantum computer鈥�
