Switching photons

Illustration of the photonic router the Weizmann Institute scientists created. At the center is the single atom (orange) that routes photons (yellow) in different directions. Credit: Weizmann Institute of Science

The photonics router. Credit: Weizmann Institute

300,000,000,000,000,000,000 (300 quintillion): this is the number of photons that a normal 100W light bulb emits every single second. You cannot even start to think about such a number. And yet scientists at the Weizmann Institute of Technology in Israel have demonstrated a device that can pick up each individual photon and route it to a given destination.

In telecommunications we have been using photons for several decades now, through optical fibres, and we have learnt how to switch streams of photons in optical add drop multiplexers. But when we "switch" we switch hundreds of billions of photons as a whole (that is the number of photons flowing through a fibre in a nanosecond..., that is fibres working at 1 Gbps). Even when using the fastest fibre we are still talking about billions of photons switched at a time. 

As one can see, there is a factor of a billion, even in the best situation between what we are doing today and what scientists at Weizmann have managed to do!

The "switch" is a single atoms that can be in two states. A single photon arrives on one of two optical fibres, let's say form the left or from the right (although this talk of left and right makes no sense from the point of view of an atom...). As response of its state the atom can reflect or transmit the incoming photon. As an example, if the atom is in state "1" than the arriving photon coming from the right will continue its path moving on the fibre to the left whereas a photon arriving from the left will be reflected backwards and will change the state of the atom to "0".  When the atom is in the "0" state photons coming from the left will continue in the same direction whilst any photon coming form the right will be reflected backwards flipping the atom state again. The atom switches through the push of a single photon, there is no other external signal required.

The photons, as it can be seen, are both carriers for the information flowing through and the signal for actuating the switching.

This trick is made possibile by using laser cooling and trapping of atoms and by fabrication of chip based ultra-hign quality optical resonators that are coupled directly with the fibre.

This experiment is yet another step towards the development of a quantum computer where scientists are looking for ways to communicate between quantum systems without disrupting them. Atoms interact with each other whilst photons don't (and they interact very weakly with other particles, hence the difficulty in making this experiment work). They are an ideal means of communications in quantum computers. 

We have seen much progress, many steps on the way towards a quantum computer but unfortunately as we move ahead it looks like the destination is also shifting so there is no clear finish line we can predict to reach in a definite time. Sometime in the next decade? May be.

Author - Roberto Saracco

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