Scientists have directly watched angular momentum move through a crystal for the very first time — and discovered a bizarre twist along the way. Using ultra-powerful terahertz laser pulses, researchers triggered tiny atomic rotations inside a quantum material and found that the direction of rotation can unexpectedly flip as momentum is transferred. The strange reversal happens because of the crystal’s underlying symmetry, creating an almost impossible-sounding effect where two rotations combine into one spinning the opposite way.

For two decades, physicists have predicted the existence of a remarkable family of exotic molecules: giant atoms bound to ordinary atoms, with an electron so distant from its nucleus that it sculpts the pair into bizarre and diverse shapes. Reported in Physical Review Letters, the final member of this “quantum zoo” has been spotted. Led by Herwig Ott at RPTU University Kaiserslautern-Landau in Germany, a team of physicists has created and detected the “butterfly” molecule, completing a 20-year hunt for the elusive structure.

The molecules in this quantum zoo belong to a class known as ultralong-range Rydberg molecules. They form when an ordinary atom becomes bound to a Rydberg atom, whose outermost electron has been excited so far from the nucleus that the atom swells to thousands of times its normal size.

The orbital shapes traced out by these distant electrons give each molecule type its character, and its nickname. Some have elaborate lobed structures reminiscent of trilobites; others spread into the winged outline of a butterfly. These molecules are thousands of times more sensitive to electric fields than ordinary molecules, making them especially useful objects for probing the quantum world.

When a singer belts out a tune while a guitar player strums along, sound waves travel through the air, driving collective oscillations of the molecules within. Meanwhile, at the quantum level, something similar is going on. Atoms inside materials, everything from our bodies to metals and more, naturally jiggle around, creating tiny vibrational waves that ripple across the material. These vibrations are known as phonons: the quantum version of sound waves.

Now, physicists at Caltech and Stanford University have developed devices called nanoelectromechanical systems (NEMS) that allow phonons to exhibit their quantum behavior purely through the intrinsic properties of the material that makes up the device. Previously, it was not possible to observe such behavior without the help of an external quantum device, such as a superconducting qubit.

This means that, through this newly discovered mechanism, a solitary NEMS device can, for example, serve as a greatly simplified and very compact quantum sensor or qubit.