In the mountains, there is a saddle — a point where the trail from one valley crosses over into another. Standing exactly on it, you take a step and are already descending the new slope. In chemistry, such a place is called the transition state: a brief moment when atoms are frozen on the threshold of transformation. For nearly a hundred years, physicists were sure that in the quantum world this pass disappears. The reason seemed ironclad: to catch the moment, you need to know simultaneously where the atom is and where it is moving, and the uncertainty principle forbids such precision.
However, the authors of the study found a loophole. They showed that quantum equations themselves draw in phase space (where the axes are particle coordinates and velocities) two surfaces — stable and unstable. Their intersection gives a single closed trajectory. This is the quantum transition state — not a smeared probability but a sharp boundary. If you draw an imaginary plane through it, the probability current flows through it in only one direction, without reverse vortices.
Thus, the chemical saddle gains a second life. Previously considered a purely classical approximation, it now stands on a solid quantum foundation. The practical payoff is the ability to calculate reaction rates more accurately and control them with light or electric fields, relying not on intuition but on a strict one-way flow.
The idea of the transition state was born in the 1930s when Henry Eyring and Michael Polanyi described a reaction as climbing an energy pass. Their theory still underpins calculations in the chemical industry.