Article:

Using diamonds to make micro circuits that channel light, not electricity, could help realise the elusive promise of quantum computing.

The allure of quantum computing is that it is able to solve problems which are near-impossible for conventional computers to crack, such as breaking very long cryptography keys.

Quantum computers hold "qubits" of information stored in the quantum properties of a system, for example, a single photon or a cloud of gas atoms in a gas. Diamond can be used to generate single photons and create qubits by writing information into their quantum states.

What's more, diamond can shield the photons from external influences, preventing unwanted modification of the stored data. And unlike other qubit techologies, which require supercooling, diamond operates fine at room temperature.

"All the essential elements for a quantum computer have been demonstrated in diamond," says Steven Prawer of the University of Melbourne, Australia.

Just one problem

But there is a catch. So far it hasn't been possible to create diamond circuits that can manipulate qubits and perform operations on them. It's almost as if engineers had invented the transistor, but not the wires and other components needed to connect them up and make a chip.

Prawer and his colleague Francois Ladouceur, at the University of New South Wales, Sydney, have now invented diamond "wires" to direct photons. They ion-etched raised ridges of diamond 4.5 micrometers wide and 1.5 micrometers high on a flat diamond surface. This creates a channel along which light bounces, being reflected by the inside of the three raised walls. However, it can leak out of the bottom of the ridge into the bulk of the diamond.

To fix that leak, the team bombarded the ridges with energetic helium ions, which converted a layer of diamond underneath into graphite. The graphite was then etched away using acid to leave an airfilled void under the diamond ridge, sealing the light leak and turning the ridge into a near-perfect light guide.

The ridges can be up to several millimeters long, more than enough to connect up the components of a diamond quantum computer.

Combining cubits

The next step will be to make waveguides that split in two and then rejoin, providing a way to combine qubits. "A single photon will split itself between the two arms, and we can recombine [the resulting states] to make it interfere, [which] is the basis of quantum computing with photons," Ladouceur told New Scientist.

The diamond light "wires" could have other uses, he adds, for example, directing intense ultraviolet light which other materials cannot carry. This could be applied to sterilising food or medical equipment.

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