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Optical delay lines aid complex photonic devices

Posted: 02 Sep 2011     Print Version  Bookmark and Share

Keywords:delay line  data transmission  electrical resistance 

The proposal for building a simple passive optical delay line that could fit into a computer chip is hoped to lay the foundation for creating robust active circuit elements with photons, such as a transistor, according to researchers from the Joint Quantum Institute and Harvard University.

The research team, composed of Mohammad Hafezi and Jacob M. Taylor of Joint Quantum Institute, together with Harvard researchers Eugene A. Demler and Mikhail D. Lukin, say that delay lines added to postpone a photons arrival, are passive but critical in processing signals. Kilometres of glass fibre are easily obtained, but fabricating optical elements that can fit on a single chip creates defects that can lead to reduced transmission of information.

The proposed delay line, which harnesses sophisticated quantum effects, would help to protect signals from degradation and maybe lead to more complex photonic devices. Their research is described in an article titled, Robust optical delay lines via topological protection.

Quantum Hall physics is the remarkable phenomenon at the heart of this new approach. The Quantum Hall effect occurs in a two-dimensional sea of electrons under the influence of a large magnetic field. The electrons are allowed to travel along the edges of the material but do not have enough energy to permeate throughout the bulk or central regions. It is as if there are conduction highways along the edge of the material. Even if there are defects in the material, like potholes in the road, electrons still make it to their destination.

These highways, called edge states are open for transit only at specific values of the externally applied magnetic field. Because the routes are so robust against disorder and reliably allow for electron traffic, this effect provides a standard for electrical resistance.

In recent years, scientists have discovered that some materials can exhibit what is known as the Quantum Spin Hall Effect (QSHE), which depends on the spin attributes of the electron. Electrons not only carry charge, but also spin. Electrons can be thought of as tiny spinning tops that can rotate clockwise (in which case they are in a spin-up condition) or counter clockwise (spin-down). Notably, the robust edge states are present in the QSHE even without externally applied magnetic fields, making them amenable for developing new types of electronics.

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