26 August 2014 A new state of matter

Artist’s representation of a gas of cold atoms with singlet states.

Macroscopic spin singlet as PRL’s Editor’s Suggestion A recent experiment by Naeimeh Behbood, Robert Sewell and co-workers in the Quantum information with cold atoms and non-classical light group led by ICREA Professor at ICFO Morgan Mitchell, has produced of a new quantum state of matter, thought to be responsible for high-temperature superconductivity and other exotic phenomena. The work was published in Physical Review Letters and chosen as Editor’s Suggestion.

The new state of matter is called a “macroscopic spin singlet,” and has been hypothesized to play a role in high-temperature superconductors, quantum information processing, and even some models of the Big Bang. Despite much theoretical speculation, up to now, no one had ever seen a macroscopic spin singlet. The group used lasers to trap one million atoms and cool them to just 20 millionths of a degree above absolute zero. They then probed the atom cloud with pulses of laser light, to measure in which direction the atoms\' angular momentum, or “spin”, was pointing. Remarkably, in some cases the spin appeared to point in no direction at all. The total spin was so close to zero as to be nearly undetectable. “The only possible explanation”, says Behbood, “is that the atoms were colluding to hide their spins, in effect agreeing, ‘I\'ll point up if you point down,’ and ‘I\'ll point left if you point right.’” This collusion among atoms, impossible in classical physics, is known as quantum entanglement, and the pair of colluding atoms is called a spin singlet. Measurements show the new state of matter consists mostly of entangled atoms; at least 500,000 atoms joined to form singlets. The work opens the way to study other singlet phenomena with cold atoms, including quantum magnetism, even more exotic states known as spin liquids, and ultimately high-temperature superconductivity.