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Ring around the resonator

This image, depicting light racing through and around ring resonators, is featured on the cover of Nature Photonics, December 2013 issue. The caption from the cover: "The electrical properties of topological electronic systems are robust to deformation. Now, scientists have created an optical equivalent of such systems — a room-temperature silicon chip in which infrared light travels around its edge, unaffected by defects (depicted here as melting). This design paves the way for the miniaturization of optical systems through exploiting electronic analogues." Image : E. Edwards/JQI

Ring resonators are circular waveguides that are used as optical cavities. They look like tiny racetracks and are often fabricated from silicon. Photons can enter and exit a resonator and even move to neighboring waveguides through evanescent coupling. The micro-rings only let light waves circulate-- “resonate”-- if they have the right wavelength (the circumference of the ring equaling an integer number of wavelengths). For an off-resonance condition less light will inhabit the ring. This image, featured on the cover of the December 2013 issue of Nature Photonics, depicts an array of ring resonators designed to be a photonic analog to electrons experiencing quantum Hall physics.

Read more to learn more about these micro-racetracks.

Coaxing light to behave like electrons is important for hybridizing photonic devices with existing electronics. Straight optical waveguides (like fiber) are the photonic analog of current carrying wire. Because of the circulation restrictions, resonators act as good filters, another element for information processing. In general, miniaturizing optical elements to a scale comparable to their electronic counterparts remains challenging. A recent result harnesses micro-resonators to create the photonic version of a material system that exhibits the electronic quantum Hall effect. This technology potentially could be used to miniaturize optical delay lines.

This team built a structure to guide infrared light over the surface of a room temperature, silicon-on-insulator chip.  Amazingly, they directly observed light racing around the boundary, impervious to defects. These photonic “edge states” are directly analogous to the quantum Hall effect for electrons.  

Briefly, a magnetic interaction is key for realizing quantum Hall states. The question here to ask is how researchers can design a material where photons---massless, charge-free, packets of energy--- flow as if they are being manipulated by a super-strong magnet. 

The secret is in the design of the resonator lattice, which determines the criteria for light hopping along the edge of the array rather than through the bulk/central region. The photons pile into an edge state only when the light has a particular color. A neat feature: along these ring resonator edge highways photons will skirt around defects, unimpeded. They cannot do a U-turn upon encountering a defect because they do not have the appropriate light frequency, which is their ticket to enter the backwards-moving path.

What else can ring resonators be used for? One example is sensors: scientists can detect changes in the cavity resonance, caused by nearby objects like molecules.

Recent Quantum Bits

October 17, 2016

Check out the second half of our feature story on Weyl semimetals and Weyl fermions, new materials and particles that have become a major focus for condensed matter researchers around the world. Part two looks at some of the theoretical work going on at JQI and CMTC. If you missed part one, it's not too late to catch up on the series. And if you missed our roundup of the research that led to last week's Nobel Prize in Physicsresearch that is closely related to Weyl materialswe encourage you to take a look.

JQI is also happy to congratulate Karina Jiménez-García on receiving a 2016 L'Oréal-UNESCO For Women in Science fellowship. "This is a recognition that I owe to all those that have guided and inspired me and those who have supported me throughout my professional career, especially my family," Jiménez-García said. We wrote a short story on how she plans to use the fellowship funds. It links to stories about the research she worked on while visiting JQI.

October 6, 2016

This year's Nobel Prize in Physics was awarded to three researchers who helped bring topology into physics. It's an innovation that has propelled condensed matter physics for the past three decades, leading recently to the discovery of several exotic materials.

We put together a roundup ( of the research that led to the prize and offered our take on topology. (Yes, we resorted to pastries.)

This year's prize is timely, too, as today we published part one ( of a two-part series on Weyl semimetals, topological materials with a long history. That history is due, in part, to this year's laureates: David Thouless, Duncan Haldane and Michael Kosterlitz.

Part one focuses on the history and basic physics of Weyl materials. Part two, which will appear next week, focuses on some of the research being explored by physicists at JQI and the Condensed Matter Theory Center at the University of Maryland.

September 15, 2016

From self-driving cars and IBM’s Watson to chess engines and AlphaGo, there is no shortage of news about machine learning, the field of artificial intelligence that studies how to make computers that can learn. Recently, parallel to these advances, scientists have started to ask how quantum devices and techniques might aid machine learning in the future.

To date, much research in the emerging field of quantum machine learning has attacked choke points in ordinary machine learning tasks, focusing, for example, on how to use quantum computers to speed up image recognition. But Vedran Dunjko and Hans Briegel at the University of Innsbruck, along with JQI Fellow Jake Taylor, have taken a broader view. Rather than focusing on speeding up subroutines for specific tasks, the researchers have introduced an approach to quantum machine learning that unifies much of the prior work and extends it to problems that received little attention before. They also showed how to increase learning performance for a large group of problems. The research has been accepted for publication in Physical Review Letters.

Quantum-enhanced machine learning. V. Dunjko, J. M. Taylor and H. J. Briegel, Physical Review Letters, to appear. arXiv:

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