Molecular fragments originating from interstellar space interact with the Earth’s atmosphere. These interactions are key but have historically been investigated via ensemble-averaged experiments that mask the underlying dynamics of each scattering event at the quantum level. Quantum-state resolved observation of single molecular collisions represents a key experimental hurdle. An approach identifies individual rotationally inelastic collisions between single molecular ions and neutral atoms using quantum-logic methodologies. Individual trapped N+ 2 ions were prepared in their rotational ground state, then exposed to a beam of helium atoms, enabling detection of isolated collisions.

Nitrogen Ion Helium Collisions Reveal Individual Rotational Excitation Events

Researchers from University of Basel, collaborating with Instituto de Fisica Fundamental (IFF-CSIC), have achieved quantum-state resolved detection of single molecular collisions; they determined the ratio of elastic to rotationally inelastic scattering for nitrogen ions colliding with helium atoms, identifying rotational excitation in *43 out of* 187 collision events. This represents an improvement over previous methods that averaged data from many impacts and lacked this level of precision, previously preventing observation of individual instances on a quantum scale.

By employing quantum logic techniques alongside a radiofrequency ion trap coupled to a molecular beam source, the team established an experimental framework allowing non-destructive readout of internal states following each impact. They prepared individual nitrogen ions in their rotational ground state before exposing them to a helium beam, then used optical forces dependent on rotation to reveal any subsequent alterations to that initial condition after each collision attempt.

State Resolved Molecular Collisions via Quantumlogic Spectroscopy

Quantumlogic-spectroscopic measurements show how molecular states change upon collision enabling determination of the probability of rotationally inelastic scattering. Comparisons with both quantum and classical scattering calculations yield detailed insights into underlying collision dynamics. Combining single-collision detection with non-destructive readout of the molecular quantum state establishes a new experimental framework for investigating molecular scattering beyond conventional ensemble measurements.

Our understanding of molecule interactions and chemical reactions relies on studying collisions between them, but typically relevant observables, such as collision rates, final-quantum-state distributions and product branching ratios, are averages over parameters including initial quantum states, kinetic energies, impact parameters, and relative orientations. This averaging obscures dynamic pathways, hampers characterisation of competing processes and obstructs investigation of fine details within collision processes. Therefore, experiments allowing study of individual molecular-collision events at the quantum level are required to achieve the precision needed for detailed insights into molecular scattering dynamics.

Precise investigations require both detection of isolated scattering events *and* control/readout of internal quantum states in colliding partners; sophisticated experimental tools developed within the areas of quantum information and atomic clocks probe and manipulate these internal states of single atomic ions confined in radiofrequency traps. These techniques were recently adapted to studies of single atom collisions yielding detailed insights into their collision dynamics demonstrating the scientific potential of this approach, molecules present greater challenges than atoms when it comes to control.

Advances in manipulating single molecular ions now enable non-destructive readout of their quantum states, a technique dubbed quantumlogic spectroscopy (QLS) where the state of a trapped molecule is mapped onto an atomic “logic” ion for read out.

Because the molecular state remains unchanged during readout, repeated interrogation becomes possible with high sensitivity; building on previous work, they demonstrate here a method based on QLS to probe individual rotationally inelastic collisions between single trapped molecules and neutrals thus introducing quantum-logic techniques to molecular collision studies. The team initiated these collisions by passing a beam of The atoms through an ion trap containing state-selected N+ 2 ions coupled to Ca+ atomic ions, this approach complements experiments trapping neutral molecules in optical tweezers which have also reached single particle levels. Individual scattering events were detected by monitoring spatial rearrangement of the ions in the trap.

Before and after each collision, the molecule’s state was probed using a QLS scheme; correlating collisions with changes in state determined the ratio of elastic to rotationally inelastic collisions. This opens up a new model for studying molecular collisions where individual scattering events are sensitively probed at the quantum level using quantum-logic techniques, a general method is introduced for studying inelastic processes involving molecular ions relevant in atmospheric, plasma and interstellar environments, data limited due to lack of widely applicable experimental methods so far.

The demonstrated methodology paves the way for state-to-state scattering studies on single molecule/single quantum-state levels allowing unprecedented detail when examining molecular interactions. The current experiments began with preparing N+ 2 ions in their rotational ground state then monitoring preservation or change of that state upon detection of The atom collision; illustrates key steps while setup details are shown schematically in ( Methods). The protocol consisted of three stages: initialising molecular states, attempting collisions and confirming no loss of state as illustrated.

An experimental cycle started by preparing laser-cooled N+ 2, Ca+ two-ion strings (inset in ); resonance-enhanced multiphoton ionization prepared the molecule in its rotational ground state followed by verification using QLS. The QLS interrogation relied on an optical dipole force dependent on rotation which caused motional excitation if the ion was initially grounded; motion detection occurred via Rabi sideband thermometry on Ca+. Calcium ions served multiple roles, coolant, collision probe *and* logic ion for quantum-logic state detection.

Collisions were initiated with a pulsed molecular beam containing He and N2 at a ratio of 7:1 passing through the trap; beam pulses limited overall observed probability to below 50% per cycle (Methods). A small fraction of N2 in the beam enabled initial generation of N+ 2 ions as outlined above.

After each exposure to the beam, rotational state of N+ 2 was probed alongside mass determination using resonant motional excitation, this discriminated between genuine collisions and spurious chemical reactions involving trapped N+ 2 with background gas. Individual scattering events detected kinetic energy imparted to trapped ions upon collision; collisional heating could cause ion position swaps after dissipation via laser cooling or affect Ca+ fluorescence due to Doppler effects.

Changes in positions on a camera () served as primary markers for identifying single collision events through their effects on trapped ion motion, while changes in molecular state were detected using quantum-logic spectroscopy ( Sec S1 SI). A similar approach previously determined collision rates of trapped atomic ions interacting with residual background gas; following Ref., a change in spatial arrangement occurs when kinetic energy exceeds trap reordering energy, undetected collisions must be considered during analysis.

From 187 total beam exposures, 43 resulted in an altered rotational state while 94 did not show expected motional excitation and 13 detected state changes occurred without corresponding position shifts were included in the data (Sec S1 B SI); it was assumed that collision probability for each pair correlated with its respective Langevin rate constant to determine contributions from N+ 2 + He system.

Rotational Inelasticity as Initial Focus for Single Molecule Collisions

While this demonstration of single-molecule collision detection marks an advance, current work focuses on rotationally inelastic events, those altering only rotational energy. This contrasts with earlier ensemble measurements which captured broader spectra including changes in electronic and vibrational states despite averaging; future research must address whether more complex interactions exhibit fundamentally different dynamics at quantum levels. Acknowledging initial experiments focus solely on rotations *and* don’t yet encompass full molecular interaction complexity is important.

Nevertheless, it demonstrates that detecting collisions between individual molecules using quantum precision is possible, previously limited to averaged data. The team and Instituto de Fisica Fundamental (IFF-CSIC) have detected individual molecular collisions while simultaneously measuring a change in quantum state for the first time, this was achieved by trapping nitrogen ions and monitoring their rotational energy after impacts with helium atoms. This approach moves beyond traditional experiments which average over many events obscuring vital details about each interaction at the atomic level; sensitive detection alongside non-destructive readout techniques were employed, utilising calcium ions as probes, establishing an experimental framework capable of revealing subtle dynamics previously hidden within ensemble measurements.

Detecting these isolated collision events, and simultaneously measuring changes in the molecular state, provides detail lost when averaging data from numerous interactions. From 187 beam exposures, researchers identified 43 altered rotational states, offering insights into how energy transfers during molecular impacts. The authors note that future work will focus on whether more complex molecular interactions exhibit different behavior at this level of precision.

👉 More information
🗞 Quantum-logic detection of rotationally inelastic collisions of single molecules
✍️ Mikolaj Roguski, Shimoni Patel, Richard Karl, Aleksandr Shlykov, Markus Meuwly and Stefan Willitsch (University of Basel); Octavio Roncero (Affiliation: Instituto de Fisica Fundamental (IFF-CSIC))
🧠 ArXiv: https://arxiv.org/abs/2611.15573

More like this