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Coherence Time: Survival of a Quantum State

Cartoon depiction of coherence time. The sphere surrounded by a bubble represents an isolated quantum state. Environmental disruptions cause a quantum superposition to dissipate. Here the final system has two distinguishable states, represented by blue and yellow poles and is no longer in a coherent quantum superposition. Credit: Edwards/JQI

How long can a quantum superposition state survive? That length of time is called the coherence time, and depends on where a qubit lives. Quantum states are powerful for computing and exploring physics, but delicate when battling the environment. The key is to have a quantum state live longer than it takes to perform an operation or experiment. Physicists design complex containers that completely isolate quantum states from the surroundings, while still allowing for state manipulation.

Read more to learn more about quantum coherence record holders.

For example, in buckets made from either electric, magnetic, or optical fields, atoms are isolated in ultra-high vacuum chambers. The vacuum pressure is about 100 trillion times lower than atmospheric pressure. To visualize this, imagine light particles like hydrogen or nitrogen in a vacuum chamber. After special pumps remove most of the air, there are so few molecules left that before one molecule will collide with another, it will typically travel a distance comparable to the circumference of the earth. At atmospheric pressure, even though we can’t see them with our eyes, there are so many molecules floating about that they only travel about a hundredth the width of a human hair before they bump into a neighboring particle. 

In these isolated atomic systems, ion qubits are the record holders for longest coherence time, in some cases boasting times longer than 10 minutes. Here the qubit is formed from an ion's internal energy states. The qubit’s states are mostly insensitive to ambient disturbances such as magnetic fields, giving them the needed protection from the destructive environment.

While classical logic bits find their home in room-temperature semiconductors, materials full of interacting atoms and a sea of electrons can spell disaster for quantum bits. One way to remove the environmental disruptions from solid-state systems is to cool the system to near zero temperature. Chilly temperatures sort of freeze out much of the noisy surroundings. Now, as reported in Science Magazine, silicon hosts qubits that can live for 39 minutes at room temperature. This is a record for material systems, and while there are many challenges ahead (enabling qubit interactions, single qubit addressing, for example), such a long coherence time is expected to allow for new research possibilites in this platform. 

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 (http://jqi.umd.edu/physics-nobel-topological-exotic-matter) 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 (http://jqi.umd.edu/news/warm-welcome-weyl-physics) 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: http://arxiv.org/abs/1507.08482.

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