The transitions of hydrogen molecules embedded in a crystal depend on the surroundings—a behavior that could be used to tailor molecular quantum dynamics.

In quantum physics, we often learn that the rules governing a system are set by its symmetry. These rules—known as selection rules—determine which transitions between quantum states are allowed and which are forbidden. For example, rotational symmetry constrains how an atom’s angular momentum can change. But what if those rules are not fixed? A recent study of hydrogen (H₂)—one of the simplest molecules in nature—showed that the allowed pathways between quantum states are determined not solely by the molecule’s internal symmetry but also by its surroundings. By embedding hydrogen molecules in different crystalline environments, Nathan McLane and colleagues from the University of Maryland, College Park, have demonstrated that the symmetry of the host material can selectively enable or suppress nuclear-spin transitions [1]. In doing so, the team revealed that quantum dynamics is not just an intrinsic property—it can be shaped by the environment.

H₂ is one of the simplest systems for exploring quantum behavior. Its two identical protons can align their spins in two different ways: In so-called orthohydrogen the nuclear spins are parallel, whereas in parahydrogen they are antiparallel. Although this difference is subtle, it leads to markedly different physical properties for the two forms. Crucially, transitions between them are highly constrained: In an isolated hydrogen molecule, the overall wave function is symmetric under exchange of the two protons, and this exchange symmetry forbids direct conversion between ortho and para states [2]. This restriction makes H₂ a textbook example of how symmetry governs quantum dynamics.

With a carefully designed experiment and a handful of tin atoms, University of Tennessee, Knoxville’s physicists have found a long-sought form of superconductivity, taking one more step toward creating custom quantum materials.

Scientists have known about superconductivity for more than a century. At low temperatures, resistance in certain materials vanishes and they carry electrical current without losing any energy. Superconductors are part of particle accelerators and magnetic resonance imaging machines. While they need extremely cool environments to work, the mechanism that drives them is quite well understood: electrons, which normally repel each other, form pairs and carry the current.

A research team led by Prof. Hao Ning of the Hefei Institutes of Physical Science of the Chinese Academy of Sciences, in collaboration with Anhui University and the University of Science and Technology of China, has identified two distinct types of unusual low-energy quasiparticle states in the kagome superconductor CsV₃Sb₅ using single-atom impurities as local “quantum probes” combined with scanning tunneling spectroscopy.

The study was recently published in Nature Physics.

CsV₃Sb₅ has attracted growing interest for its unusual crystal structure and complex quantum phenomena. Evidence for time-reversal symmetry breaking remains under debate, and the mechanism of its superconductivity is still not fully understood. Studying its response to single-atom impurities provides a promising way to address these questions.