UMD CMNS Physics S1 Color

Ultra-cold atoms may wade through quantum friction

Theoretical physicists studying the behavior of ultra-cold atoms have discovered a new source of friction, dispensing with a century-old paradox in the process. Their prediction, which experimenters may soon try to verify, was reported recently in Physical Review Letters.

The friction afflicts certain arrangements of atoms in a Bose-Einstein Condensate (BEC), a quantum state of matter in which the atoms behave in lockstep. In this state, well-tuned magnetic fields can cause the atoms to attract one another and even bunch together, forming a single composite particle known as a soliton.

Solitons appear in many areas of physics and are exceptionally stable. They can travel freely, without losing energy or dispersing, allowing theorists to treat them like everyday, non-quantum objects. Solitons composed of photons—rather than atoms—are even used for communication over optical fibers.

Studying the theoretical properties of solitons can be a fruitful avenue of research, notes Dmitry Efimkin, the lead author of the paper and a former JQI postdoctoral researcher now at the University of Texas at Austin. “Friction is very fundamental, and quantum mechanics is now quite a well-tested theory,” Efimkin says. “This work investigates the problem of quantum friction for solitons and marries these two fundamental areas of research.”

Efimkin, along with JQI Fellow Victor Galitski and Johannes Hofmann, a physicist at the University of Cambridge, sought to answer a basic question about soliton BECs: Does an idealized model of a soliton have any intrinsic friction?

Prior studies seemed to say no. Friction arising from billiard-ball-like collisions between a soliton and stray quantum particles was a possibility, but the mathematics prohibited it. For a long time, then, theorists believed that the soliton moved through its cloudy quantum surroundings essentially untouched.

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Dormant Black Hole Eats Star, Becomes X-ray Flashlight

A team of University of Michigan and University of Maryland researchers, including Physics' Lixin Dai, is the first to catch x-ray echoes of a tidal disruption event. Their paper, “Relativistic Reverberation in the Accretion Flow of a Tidal Disruption Event,” is published in  Nature.

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Gravitational Waves Detected from Second Pair of Colliding Black Holes

Both of the twin Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors—located in Livingston, Louisiana, and Hanford, Washington—detected the gravitational wave event, named GW151226. The LIGO Scientific Collaboration (LSC) and the Virgo Collaboration used data from the twin LIGO detectors to make the discovery, which is accepted for publication in the journal Physical Review Letters.

Gravitational waves carry information about their origins and about the nature of gravity that cannot otherwise be obtained. Physicists on the LIGO and Virgo teams concluded that the final moments of a black hole merger produced the gravitational waves observed on December 26, 2015.

LIGO’s historic first detection on September 14, 2015 resulted from a merger of two black holes 36 and 29 times the mass of the sun. In contrast, the black holes that created the second event were relative flyweights, tipping the scales at 14 and eight times the mass of the sun. Their merger produced a single, more massive spinning black hole that is 21 times the mass of the sun, and transformed an additional sun’s worth of mass into gravitational energy.

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Tiny Diamonds Could Enable Huge Advances in Nanotechnology

Nanomaterials have the potential to improve many next-generation technologies. They promise to speed up computer chips, increase the resolution of medical imaging devices and make electronics more energy efficient. But imbuing nanomaterials with the right properties can be time consuming and costly. A new, quick and inexpensive method for constructing diamond-based hybrid nanomaterials could soon launch the field forward.

University of Maryland researchers developed a method to build diamond-based hybrid nanoparticles in large quantities from the ground up, thereby circumventing many of the problems with current methods. The technique is described in the June 8 issue of the journal Nature Communications.

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Disorder Grants a Memory to Quantum Spins

Nature doesn’t have the best memory. If you fill a box with air and divide it in half with a barrier, it’s easy to tell molecules on the left from molecules on the right. But after removing the barrier and waiting a short while, the molecules get mixed together, and it becomes impossible to tell where a given molecule started. The air-in-a-box system loses any memory of its initial conditions.

The universe has been forgetting its own initial state since the Big Bang, a fact linked to the unrelenting forward march of time. Systems that forget where they started are said to have thermalized, since it is often—but not always—an exchange of heat and energy with some other system that causes the memory loss. For example, a melting ice cube forgets its orderly arrangement of water molecules when heat from its surroundings splits the cube’s crystal bonds. In some sense, the initial information about the ice cube—the structure of the crystal, the distance between molecules, etc.—leaks away.

The opposite case is localization, where information about the initial arrangement sticks around. Such a situation is rare, like an ice cube that never melts, but one example is Anderson localization, in which particles or waves in a crystal are trapped near impurities. They tend to bounce off defects in the crystal and scatter in random directions, yielding no net movement. If there are enough impurities in a region, the particles or waves never escape.

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