Washington, March 28 : A team of physicists and engineers has come a step closer to developing a super-powerful quantum computer by demonstrating exquisite control of single particles of light - photons - on a silicon chip.
The demonstration was made by Dr Jeremy O'Brien, his PhD student Alberto Politi, and their colleagues at Bristol University, who displayed the world's smallest optical controlled-NOT gate - the building block of a quantum computer.
The team was able to fabricate their controlled-NOT gate from silica wave-guides on a silicon chip, resulting in a miniaturized device and high-performance operation.
"This is a crucial step towards a future optical quantum computer, as well as other quantum technologies based on photons," said Dr O'Brien.
Quantum technologies aim to exploit the unique properties of quantum mechanics, the physics theory that explains how the world works at very small scales.
Photons are an excellent choice for quantum technologies because they are relatively noise free; information can be moved around quickly - at the speed of light; and manipulating single photons is easy.
According to Alberto Politi, 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.
"For the last several years the Centre for Quantum Photonics has been working towards building controlled-NOT gates and other important quantum circuits on a chip to solve these problems," said Dr O'Brien.
The team's chips, fabricated at CIP Technologies, have dimensions measured in millimetres.
This impressive miniaturisation was permitted thanks to the silica-on-silicon technology used in commercial devices for modern optical telecommunications, which guides light on a chip in the same way as in optical fibres.
The team generated pairs of photons which each encoded a quantum bit or qubit of information. They coupled these photons into and out of the controlled-NOT chip using optical fibres.
By measuring the output of the device, they confirmed high-fidelity operation.
In the experimental characterisation of the quantum chips the researchers also proved that one of the strangest phenomena of the quantum world, namely "quantum entanglement", 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.
This 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 Alberto Politi.