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New photonic router works by flipping reflective atom's lid

Look ma! No electronics!

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Quantum boffins from Israel's Weizmann Institute have demonstrated a two-input/two-output router that works entirely with single photons – no electronics required.

It's not the first time a photonic router has been demonstrated, but what's different about the Israeli work is that everything is photonic: the device has two optical inputs, two optical outputs, and can route single photons from either input to either output based solely on a control provided by another single photon.

There are two important differences between this and the current “state of the art”.

On the one hand, there's a big body of work in designing systems that use electrical controls to route photons. The Register has discussed such kit in the past, and noted that it demands a lot of work integrating electro and optical components on the same chip.

On the other, while all-optical switches exist (Australia's CUDOS has been recognised for such work), they operate at the beam level rather than on individual photons.

The latest work, published in Science (abstract here, pre-press here), sets about to resolve a conundrum in quantum information processing: because photons don't normally interact with each other, they're ideal for communications; but because photons don't normally interact, it's hard to create all-photonic gates (by way of counter-example: electrons interact strongly, so it's easy to use an electrical signal to control a gate, but their interaction makes electrons interfere with each others' communications).

What the Weizmann Institute of Science group settled on as their solution was to use single photons to change the state of a single Rubidium atom at the centre of a matrix of four optical fibres (two in, two out). The atom is flipped by a photon, between acting as a mirror or being (effectively) transparent.

Photonic router schematic

The atom in the middle is the key. Image: Dayan et al

For example, if the “control atom” is presenting its reflective state to input 1, a photon arriving at that input will appear at output 1. In that state, the atom will transmit a photon from input 2 to output 1. When the atom's state is flipped, both inputs will appear at output 2, either by transmission (input 1) or reflection (input 2)*.

The researchers say that the state-flip can be accomplished with a single photon (although losses and nonlinearities mean that in practise, as many as three photons might be required).

The key technologies enabling this demonstration are laser cooling (which helps the trapping of the single Rubidium atom), and the high-quality chip-based optical resonators that couple directly to the optical fibres.

“The device we constructed demonstrates a simple and robust system, which should be applicable to any future architecture of such computers. In the current demonstration a single atom functions as a transistor – or a two-way switch – for photons, but in our future experiments, we hope to expand the kinds of devices that work solely on photons, for example new kinds of quantum memory or logic gates”, says Dr Barak Dayan, head of the Weizmann Institute’s Quantum Optics group. ®

*Bootnote – TL;dr version: What's going on at the quantum level is, of course, a little more complex than “reflect or transmit”. What happens is that the control atom absorbs an incoming photon: in the “reflective” state, it re-emits that photon back towards the input fibre, while in its “transmission” state, it emits a photon towards the output fibre.

“The atom radiates in both directions, and the destructive interference with the incoming [light] probe in the forward direction leads to reflection of the probe backwards”, the paper states. ®

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