December 26, 2011

Researchers should soon e able to cool atoms to 100 hundredths of a billionth of a degree, 100 times colder than best ever

In experiments with rubidium atoms in optical lattices,physicists successfully demonstrated they could remove entropy from atoms via orbital excitation blockade. In principle, they can reach temperatures 10-to-100-times colder than currently achieved, to temperatures of tenths-to-hundredths-of-a-billionth of a degree above absolute zero. However, they likely need lasers of longer wavelengths to do so in real life, said researcher Markus Greiner, a physicist at Harvard University.

This should enable exotic new states of matter and enable better quantum computers.

Nature - Orbital excitation blockade and algorithmic cooling in quantum gases

Cooling Process

1. the researchers cooled atoms of rubidium with lasers. When set up properly, these beams can force atoms to glow in a way that makes them emit more energy than they absorb, thus making them colder.

When the atoms gave off light as a result of being hit with the laser, this exerted a slight pressure on them. The scientists took advantage of that pressure to control the atoms, either keeping them in place or moving them around, sometimes creating collisions.

2. The researchers then made the atoms even colder with evaporative cooling, in which matter gets cooled in much the same way as a cup of coffee loses its warmth — the hottest atoms are allowed to evaporate, leaving behind the colder ones.

3. the researchers used webs of lasers known as "optical lattices." When two atoms are made to collide within the optical lattice, the excitations of one suppress the excitations of the other, a phenomenon called "orbital excitation blockade." The excited atoms are then removed from the system -- taking away entropy, the amount of energy available for work -- thus causing the remaining atoms to chill down.

Interaction blockade occurs when strong interactions in a confined, few-body system prevent a particle from occupying an otherwise accessible quantum state. Blockade phenomena reveal the underlying granular nature of quantum systems and allow for the detection and manipulation of the constituent particles, be they electrons, spins, atoms or photons. Applications include single-electron transistors based on electronic Coulomb blockade and quantum logic gates in Rydberg atoms. Here we report a form of interaction blockade that occurs when transferring ultracold atoms between orbitals in an optical lattice. We call this orbital excitation blockade (OEB). In this system, atoms at the same lattice site undergo coherent collisions described by a contact interaction whose strength depends strongly on the orbital wavefunctions of the atoms. We induce coherent orbital excitations by modulating the lattice depth, and observe staircase-like excitation behaviour as we cross the interaction-split resonances by tuning the modulation frequency. As an application of OEB, we demonstrate algorithmic cooling of quantum gases: a sequence of reversible OEB-based quantum operations isolates the entropy in one part of the system and then an irreversible step removes the entropy from the gas. This technique may make it possible to cool quantum gases to have the ultralow entropies required for quantum simulation of strongly correlated electron systems. In addition, the close analogy between OEB and dipole blockade in Rydberg atoms provides a plan for the implementation of two-quantum-bit gates in a quantum computing architecture with natural scalability.

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