A team has measured forbidden vibrational transitions in a single trapped nitrogen molecular ion (N₂⁺) for the first time using quantum-logic spectroscopy. The technique, originally developed for trapped-ion atomic clocks, makes it possible to probe transitions that barely couple to light and are therefore inaccessible to conventional optical spectroscopy. The result opens a route to studying simple diatomic molecules with a precision previously reserved for atoms.

Forbidden vibrational transitions are those whose probability of absorption or emission is extremely low because of selection rules. In trapped molecular ions, detecting them requires an indirect method: instead of looking for the photon that excites the molecule, the vibrational state of N₂⁺ is coupled to the electronic state of an auxiliary ion (typically an atomic ion such as Ca⁺ or Be⁺) trapped in the same trap. The auxiliary ion acts both as a logic qubit and as a detector: its internal state reveals whether the molecule has changed vibrational state.

Quantum-logic spectroscopy works by measuring the state of the auxiliary ion after laser pulses attempt to drive the vibrational transition in N₂⁺. If the transition occurs, the Coulomb coupling between the two trapped ions modifies the frequency of the shared normal mode and, with it, the dynamics of the auxiliary ion. Repeated measurement of this qubit reconstructs the transition probability with a sensitivity that does not depend on the molecule emitting or absorbing appreciable light.

The appeal of N₂⁺ lies in its simplicity: it is a diatomic molecule with two identical nuclei, making it a clean testbed for molecular theory and for quantum electrodynamics in molecules. Measuring its vibrational transitions precisely allows ab initio calculations of molecular structure to be tested and possible variations of fundamental constants to be explored. The method is also transferable to other trapped molecular species, a step towards molecular clocks and quantum sensors based on molecular ions.