A second laser pulse then pushes the radicals to recombine while they are in the middle of quantum beating. They use a laser pulse to create a radical pair within a large molecule. Steiner and his team have devised a method to directly observe quantum beats. (Concept image doesn’t fully represent the study molecule’s spin system) These oscillations were revealed using pump and push laser pulses to move an electron from one end of a donor-bridge-acceptor molecule to the other and back again. Eventually, the radicals recombine.Ĭoherent interconversion of the singlet and triplet states of radical pairs is normally impossible to observe. The oscillations last for less than 100 nanoseconds – a few beats – before they are dampened, and the spins end up in a statistical equilibrium. While quantum beats have been known since the 1970s, observing them directly has remained challenging. ‘This kind of quantum beat kinetics is rather exceptional because chemical kinetics is usually always monotonically approaching some equilibrium state,’ explains Ulrich Steiner from the University of Konstanz, who led the study together with Christoph Lambert from the University of Würzburg. The unpaired electrons interact with the tiny magnetic fields of surrounding nuclei, which causes them to flip between singlet and triplet states – a phenomenon known as quantum beating. But it doesn’t remain like that for long. When they create a radical pair, the pair is also singlet. Most molecules are singlets, although molecular oxygen is a notable everyday exception. The spins of two unpaired electrons can be either antiparallel (singlet) or parallel (triplet) to one another. Electrons behave like microscopic magnets, possessing a quantum mechanical property known as spin that gives them a magnetic moment. Being able to observe quantum oscillations directly could help scientists unravel how a migratory bird’s magnetic compass works, which is believed to be based on a radical pair mechanism.Ī radical pair is a system, two molecules for example, with two unpaired electrons. A method to detect quantum beats – extremely short-lived oscillations between spin states in a radical pair – has been invented by researchers in Germany and Russia.
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