Quantum computers have come a step closer thanks to the efforts of physicists at Bristol University.

The scientists claim to have demonstrated the world's smallest optically controlled NOT gate using a pair of photons.

Dr Jeremy O'Brien, PhD student Alberto Politi and colleagues were able to fabricate the controlled logic gate from silica wave-guides on a silicon chip.

A quantum computer relies on the fact that quantum particles, such as photons, can exist in a superposition (two states at the same time) whereas transistors in a PC can exist only in one state at a time, i.e. 0 or 1.

The team generated pairs of photons which each encoded a quantum bit of information. They coupled these photons into and out of the controlled NOT chip using optical fibres.

The researchers also proved that entanglement, one of the strangest phenomena of the quantum world, was achieved on-chip.

Quantum entanglement of two particles means that the state of either of the particles is not defined, but only their collective state. On-chip entanglement has important applications in quantum metrology.

"As well as quantum computing and quantum metrology, on-chip photonic quantum circuits could have important applications in quantum communication, since they can be easily integrated with optical fibres to send photons between remote locations," said Politi.

Photons are a good choice for quantum technologies because they are relatively noise free, information can be moved around at the speed of light and manipulating single photons is relatively easy.

However, making two photons talk to each other in a controlled NOT gate is much harder.

Photons must also talk to each other to realise the ultra-precise measurements that harness the laws of quantum mechanics, i.e. quantum metrology.

Quantum optical circuits have typically relied on large optical elements with photons propagating in air, and consuming a square metre of optical table. This has made them hard to build and difficult to scale up.

So for several years, O'Brien and his team have been working towards building NOT gates and other circuit elements on a chip.

The team has published its findings in a paper entitled Silica-on-Silicon Waveguide Quantum Circuits.