26 September 2014 ICFO in Scientific American

Entanglement between atoms with opposite spins give a zero net spin.

Determining spins of a “macroscopic spin singlet” by means of quantum nondemolition and entanglement In a recent experiment carried out 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, the team produced a “macroscopic spin singlet”, a new quantum state of matter thought to be responsible for high-temperature superconductivity and other exotic phenomena. Such is the importance of this experiment that the study, published in Physical Review Letters last month, was given further coverage in the recent issue of the renowned magazine Scientific American.

Although theoretical studies have predicted its existence, before now, no one had been able to see a macroscopic spin singlet. By using lasers, the team was able to cool down a cloud of rubidium atoms to 20 millionths of a degree above absolute zero. They were able to measure the direction in which the atoms\' angular momentum, or “spin”, was pointing by using pulses of laser light and a quantum nondemolition technique, a passive means of measuring a quantum system without altering its state. In their experiment, ICFO researchers were able to see that the total spin was so close to zero as to be nearly undetectable. In effect, these atoms cancel each other out to give a net spin equal to zero only but only when entangled.