When a physical system goes through a phase transition its properties may change fundamentally. In a classical system such as a group of water molecules, the two phase transitions from melting to boiling progress through fundamentally different states of matter: solid, liquid, and gas. Microscopic temperature fluctuations drive these macroscopic changes. When the temperature of a system is lowered towards absolute zero, the possibility of a classical phase transition vanishes, but a quantum phase transition is allowed because of the Heisenberg uncertainty relation. Hot Paper #4 is all about an astonishing quantum switch from the superfluid state to an insulator. In classical terms this would be the equivalent of changing the electrical properties of copper to those of rubber.

Bose-Einstein condensate (BEC) is a state of matter at nanotemperatures in which atoms join into a single quantum state where they can flow without friction, thus becoming a superfluid. Experimentally this state of matter is realized with rubidium atoms in a magnetic trap, and the first researchers to do this won the Nobel Prize for physics in 2001. Now Immanuel Bloch’s BEC team (Ludwig-Maximilians University, Munich) have dramatically controlled the behavior of BEC using an optical lattice, and their achievements take us closer to the realization of quantum computers.

An optical lattice is an energy landscape created by several criss-crossing laser beams. Markus Greiner and his colleagues used six laser beams and some experimental wizardry to create a perfect cubic lattice of energy potential. From the point of view of individual rubidium atoms in the BEC, this landscape of energy mountains and valleys resembles an egg carton, with exactly the same number of atoms permitted at each potential well. For low potential depths in this lattice, the atoms behave as BEC, and through their coherent wavelike nature they create interference patterns of matter waves.

Hot Paper #4 describes what happens when the voltage is ramped up and then lowered. The increasing voltage reaches a point where the atoms stop behaving coherently. Suddenly they are trapped in the egg carton wells, and since they cannot move the ultracold gas behaves like an insulator. Like BEC itself, this is a special state of matter, in which the laser beams have created a lattice in free space. The key discovery in #4 is that the phase transition can be rapidly reversed: in just 14 ms of ramp-down, phase coherence is restored over the whole lattice.

Greiner’s work is exciting because the insulator state opens many new perspectives in quantum computing and precision atomic clocks. The creation of highly-entangled multi-particle states is a challenging goal for experimental quantum mechanics. The Munich group has recently demonstrated controlled collisions between rubidium atoms held in an optical lattice. This is a first step towards quantum computation because rubidium has a magnetic moment, and therefore two internal states that can serve as the 0 and 1 of a binary quantum bit. Atoms held in an optical lattice could provide the memory for a quantum computer.

The second newcomer this time, #9, reinforces the conclusions of #8, on the BOOMERANG balloon-borne observations of the CMB. The Degree Angular Scale Interferometer (DASI) is a 13-element array operated at the South Pole, where it measures the angular power spectrum of anisotropy in the CMB.

DASI adds to the whole raft of excellent observations on the components of the universe, which have created a "cosmic concordance." This manifesto sets baryonic matter at about 3 to 5%, cold dark matter 25 to 30%, and dark energy 65 to 75%. These conclusions have come from three different routes with very little cross talk: CMB observations, supernova cosmology, and studies of clusters of galaxies. What is exceptionally impressive is the way the science has been done, in the sense that observations of the evolving universe (cosmology) are confused by the evolution of the objects under study (stars, galaxies, clusters). The present golden age of observational cosmology has by chance brought together techniques that are orthogonal to each other, and so the cross correlations are placing narrow constraints on the concordance. 

Dr. Simon Mitton is the Senior Fellow of
St Edmund’s College, University of Cambridge, UK