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Read more: 鈥�Tricks of the light: Nine fabulous photon spin-offs鈥�

We know it best as the story of Schr枚dinger鈥檚 notorious moggie: a cat fated to be both dead and alive at the same time. That was an intentional absurdity, but for a photon, existence in a quantum 鈥渟uperposition鈥� of different states is a fact of life. Its electric field, for example, can be simultaneously both horizontally and vertically polarised.

This is an inordinately powerful property. While the processing muscle of today鈥檚 supercomputers is limited because electron currents can only ever be on or off, a photon in a superposition represents a 鈥渜uantum bit鈥�, or qubit, that is both on and off at the same time. of Harvard University estimates that a quantum computer of just 150 qubits would have the processing power of all today鈥檚 supercomputers combined.

鈥淎 quantum computer of just 150 qubits would have the power of all today鈥檚 supercomputers鈥�

The reality is as yet less impressive. Photonic qubits are leading contenders for quantum computation since their polarisation is easily manipulated and quantum cryptography provides experience in processing them, but methods to store light are still at an early stage (see 鈥淟ight: Putting the brakes on the universal speedster鈥�). Quantum states in general are also prone to 鈥渄ecohere鈥� 鈥� lose their quantum properties when disturbed 鈥� meaning a quantum computer needs built-in error correction.

Still, slow progress is being made. In 2010, a team of American and Australian researchers including Aspuru-Guzik manipulated the polarisation and entanglement of a pair of photons to make a minimalist two-qubit computer. After many iterations, the computer calculated the energy levels of a hydrogen molecule to within a few parts per million ().