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Quantum physics

The light conductor

Munich, 11/29/2013

Immanuel Bloch uses batteries of lasers and sophisticated control circuitry to create artificial crystals with which he can simulate and study physical phenomena at the quantum level.

“As we speak, the acoustic energy of the sound waves we produce is enough to cause the frequency of the laser to drift,” says Immanuel Bloch. Source: Jan Greune
“As we speak, the acoustic energy of the sound waves we produce is enough to cause the frequency of the laser to drift,” says Immanuel Bloch. Source: Jan Greune

When Immanuel Bloch draws back the room-high, opaque blinds that screen the optical table from ambient light, a strange sight comes into view – a bewildering array of hundreds of lenses and mirrors, switches, glass-fiber cables and other components. It reminds one of an overcrowded model railway, except that what moves around this layout are not trains, but light beams. Emanating from no less than 12 lasers, they seem to shoot every which way, tracing out zigzag paths across the tabletop, which covers an area of several square meters in Bloch’s physics laboratory on Munich’s Schellingstrasse.

And light’s journeys through this network take us into the quantum world, with its apparently bizarre laws. With the aid of light, Bloch penetrates into the depths of this world to tease out its secrets. Using clever tricks and high-precision control systems, he constructs model systems in which he can probe its weirdest properties. He exploits laser radiation to build cages out of light, in which so-called quantum gases are held captive. At ultralow temperatures, these atomic gases behave in accordance with the laws of quantum mechanics. So Bloch creates crystalline lattices with lasers and uses radiation pressure to cool atoms to temperatures within nanokelvins of absolute zero, colder than interstellar space. He can even use light to set up vortices in the gases, just as one does when one stirs one’s coffee.

Conducting an orchestra of atoms

It is a disconcerting world that one enters here, although physicists take a rather more sober view. “With this set-up, we are able to study, with unprecedented precision, the behavior of ultracold atoms in a quantum gas at temperatures close to absolute zero,” says quantum physicist Bloch. The atomic densities in such gases are around ten orders of magnitude lower than those in a typical solid. That one can use a gas to study phenomena that are otherwise observed only in the solid state, in which the particles interact strongly and the electrons are packed close together, is quite amazing. “For us, quantum gases are wonderful model systems, on which we can perform precision measurements – of solid-state physics,” says Bloch. “Condensed-matter systems are often so complex that it is not possible to construct a perfect microscopic model that accounts for their properties.”

It was long regarded as inconceivable that one could ever study quantum phenomena in such complex systems, until the American physicist Richard Feynman formulated a visionary idea – without regard for its practical applicability. In 1981, he gave a talk at the Massachusetts Institute of Technology, pointing out that normal computers are inherently incapable of testing theoretical models of phenomena such as special quantum magnets at the atomic level. Even future supercomputers could not solve the relevant quantum mechanical equations. An entirely different class of computing machines was required, which he called quantum simulators. As Bloch explains, Feynman recognized “that one could use an artificial system with readily adjustable parameters to mimic the behavior of the real system one wished to study.”

In 1981 that was a daring proposition. “Only now can we realize this vision in the lab,” says Bloch. But it is no simple task, for the atoms in a quantum gas are not easily tamed. “Think of an orchestra in which each musician plays what he likes – the result is complete chaos.” But ultralow temperatures impose order on atomic systems. “Then each atom plays the same note, so to speak, and we can specify its quality.” The physicist can now conduct the orchestra of atoms. “We need to be able to control the conditions in the system in order to study how they influence the interactions between the atoms,” Bloch explains. The crucial question then is how to relate observations of interactions at the atomic level to macroscopic properties of matter like electrical conductivity or magnetism.

Bloch carries out his delicate and intricate experiments in laboratories at two different institutions. For he is both Professor of Experimental Physics at LMU, and a Scientific Director at the Max Planck Institute (MPI) for Quantum Optics in Garching, near Munich. “What motivates me is the desire to observe phenomena that no one has ever seen before. The greatest reward comes when one achieves something that nobody has done before. And the 40-year-old Bloch has already experienced several such moments. Indeed, he is rated as a pioneer in the field of quantum simulation. He has won important awards, among them the Leibniz Prize conferred by the Deutsche Forschungsgemeinschaft and, most recently, the Körber European Science Prize, and his research group has an enviable reputation. His work on model systems for solid-state physics is very highly regarded by his peers worldwide, as his many publications in leading journals, like Science and Nature, testify. Hubert Filser, translation: ph

The complete article on Professor Bloch’s research will appear in the upcoming issue of insightLMU, LMU’s international newsletter.

Prof. Dr. Immanuel Bloch
Professor of Experimental Physics at LMU and Director of the Max Planck Institute for Quantum Optics in Garching. Born in 1972, Bloch studied Physics at Bonn University and obtained his doctorate with Theodor Hänsch, who would later win the Nobel Prize. He held a Chair in Physics at Mainz University from 2003 until 2008, when he returned to Munich. Bloch’s awards include the Leibniz Prize conferred by the Deutsche Forschungsgemeinschaft (2004), and in 2013 he won a Synergy Grant from the European Research Council (ERC).

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