Posted: March 27, 2008 |
Silicon chips for optical quantum technologies |
(Nanowerk News) A team of physicists and engineers has demonstrated exquisite control of single particles of light – photons – on a silicon chip to make a major advance towards the long sought after goal of a super-powerful quantum computer.
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Dr Jeremy O’Brien, his PhD student Alberto Politi, and their colleagues at Bristol University have demonstrated the world’s smallest optical controlled-NOT gate – the building block of a quantum computer.
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The team were able to fabricate their controlled-NOT gate from silica wave-guides on a silicon chip, resulting in a miniaturised device and high-performance operation.
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“This is a crucial step towards a future optical quantum computer, as well as other quantum technologies based on photons,” said Dr O’Brien.
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The team reports its results in the March 27 2008 Science Express – the advanced online publication of the journal Science.
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Quantum technologies with photons
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Quantum technologies aim to exploit the unique properties of quantum mechanics, the physics theory that explains how the world works at very small scales.
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For example a quantum computer relies on the fact that quantum particles, such as photons, can exist in a “superposition” of two states at the same time – in stark contrast to the transistors in a PC which can only be in the state “0” or “1”.
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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.
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Making two photons “talk” to each other to realise the all-important controlled-NOT gate is much harder, but Dr O’Brien and his colleagues at the University of Queensland demonstrated this back in 2003 [Nature 426, 264].
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Photons must also “talk” to each other to realise the ultra-precise measurements that harness the laws of quantum mechanics – quantum metrology.
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Last year Dr O’Brien and his collaborator Professor Takeuchi and co-workers at Hokkaido University reported such a quantum metrology measurement with four photons [Science 316, 726].
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Silica-on-silicon wave-guide quantum circuits
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“Despite these and other impressive demonstrations, 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,” said Alberto Politi.
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“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,” added Dr O’Brien.
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The team’s chips, fabricated at CIP Technologies, have dimensions measured in millimetres.
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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.
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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.
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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.
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This on-chip entanglement has important applications in quantum metrology.
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“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.
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