Fri January 1, 2010

If you flip a hundred coins, you are unlikely to get exactly fifty heads and fifty tails; there is a statistical uncertainty in the outcome. Researchers at MIT have reduced the statistical uncertainty in the quantum mechanical equivalent of a coin toss. This quantum mechanical coin toss is more than a game: its uncertainty limits the precision of one of the world’s most sensitive measurement devices, the atomic clock. An atomic clock consists of tens of thousands of atoms, each of which can be in either of two states, much like a coin that can show either of two faces. Each atom is placed in a quantum superposition of the two states—each coin, as it were, suspended in mid-air with the potential to land on either face. The researchers at MIT use light to probe an ensemble of such atoms in a way that allows them to count how many atoms are “heads” without revealing the state of any individual atom—without disturbing the superposition. Thereafter, the laws of quantum mechanics demand that the count remain the same on any subsequent measurement. Thus, while each individual coin continues to tumble at random, the tumbling of the different coins is now choreographed: as one twists towards heads, another must turn towards tails. In the jargon of quantum mechanics, the states of the different atoms are now entangled. When one ultimately measures the states of the individual atoms—letting the coins land—the statistical uncertainty in the outcome is reduced. Just such a measurement is used to read out an atomic clock; if the clock is operated in an entangled state, its precision is no longer at the mercy of an ordinary coin toss.

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Thu January 1, 2009

Selectively addressing individual qubits in an array of ions is a vital task for scalable quantum computation with trapped ions. Traditionally, this is accomplished using complex optics for laser beam steering, but such schemes are not easily scalable. We have realized a scalable alternative which employs the advantages of microfabrication and cryogenic operation, and applies...

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Thu January 1, 2009

T. Hong, A. Gorshkov, D. Patterson, A. S. Zibrov, J. M. Doyle, M. D. Lukin, and M. G. Prentiss, Realization of coherent optically dense media via buffer-gas cooling, Phys. Rev. A 79, 013806 (2009).

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Thu January 1, 2009

Our world is run by electrons. Whether we switch on a light, browse the internet or play music on the iPod, it is electrons moving through the wires, chips and headphones. But how do electrons actually get from A to B? After all, they have to get through a solid, a crystal maze of countless atoms. On their way through the solid, electrons push and pull nearby atoms around, attracting positive charges and repelling negative ones. Its like an espalier, with arms flying high wherever the electron goes. These distortions in the crystal lattice thus closely follow the electron, and in fact the electron and the lattice deformations can be said to form a new entity or quasi-particle, called the polaron. Since the electron has to drag the lattice distortions with it, the polaron is heavier than an electron moving in empty space. That means a polaron is less inclined than a bare electron to change its speed or direction of motion if someone pulls on it. Polarons are ubiquitous in solid state materials, they are crucial for the understanding of colossal magnetoresistance, and they are responsible for conduction in fullerenes and polymers.

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Thu January 1, 2009

Ferromagnetism of delocalized (itinerant) fermions occurs due to repulsive interactions and the exchange energy which reduces the interaction energy for spin polarized domains due to the Pauli exclusion principle. At a critical interaction, given by the so-called Stoner criterion [1], they system spontaneously develops domains and becomes ferromagnetic. This, together with a suitable band structure...

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Thu January 1, 2009

Observation of Cold Collisions between Trapped Ions and Trapped Atoms. A. Grier, M. Cetina, F. Orucevic, and V. Vuletic, Phys. Rev. Lett. 102, 223201 (2009).

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Thu January 1, 2009

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Thu January 1, 2009

Detection of single spins is an important problem in quantum science and engineering. It plays a key role in the realization of quantum computation and communication as well as in quantum metrology and sensing. Working with single particles is important to take advantage of quantum mechanical features associated with these phenomena.

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Thu January 1, 2009

This is joint work with Vladan Vuletic.

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