Coherent control of atomic and molecular scattering relies on the preparation of colliding particles in superpositions of internal states, establishing interfering pathways that can be used to tune the outcome of a scattering process. However, the incoherent addition of different partial wave contributions to the integral cross-sections, commonly encountered in systems with complex collisional dynamics, poses a significant challenge, often limiting the control. This work demonstrates that time-reversal symmetry can be used to overcome these limitations by constraining the relation between the S-matrix elements. For example, the preparation of a superposition of two states related by the time-reversal superposition can provide extensive control for transitions to a time-reversal invariant final state, such as the J = 0, M = 0. Using the example of ultracold O 2 –O 2 scattering, we show that for such states coherent control is robust against short-range dynamical complexity. Furthermore, the time-reversal symmetry also protects the control against a distribution of collisional energies. Beyond the ultracold regime, we observe significant differences in the controllability of crossed-molecular beam vs trap experiments with complete control achievable in the former case at any temperature, emphasizing the cooperative role of time-reversal and permutation symmetries in maintaining control at any temperature. These results open new avenues for the coherent control of complex inelastic collisions and chemical reactions both in and outside of the ultracold regime.
Coherent control of atomic and molecular scattering relies on the preparation of colliding particles in superpositions of internal states, establishing interfering pathways that can be used to tune the outcome of a scattering process. However, the incoherent addition of different partial wave contributions to the integral cross-sections, commonly encountered in systems with complex collisional dynamics, poses a significant challenge, often limiting the control. This work demonstrates that time-reversal symmetry can be used to overcome these limitations by constraining the relation between the S-matrix elements. For example, the preparation of a superposition of two states related by the time-reversal superposition can provide extensive control for transitions to a time-reversal invariant final state, such as the J = 0, M = 0. Using the example of ultracold O 2 –O 2 scattering, we show that for such states coherent control is robust against short-range dynamical complexity. Furthermore, the time-reversal symmetry also protects the control against a distribution of collisional energies. Beyond the ultracold regime, we observe significant differences in the controllability of crossed-molecular beam vs trap experiments with complete control achievable in the former case at any temperature, emphasizing the cooperative role of time-reversal and permutation symmetries in maintaining control at any temperature. These results open new avenues for the coherent control of complex inelastic collisions and chemical reactions both in and outside of the ultracold regime.
We show that quantum interference-based coherent control is a highly efficient tool for tuning ultracold molecular collision dynamics that is free from the limitations of commonly used methods that rely on external electromagnetic fields. By varying the relative populations and phases of initial coherent superpositions of degenerate molecular states, we demonstrate complete coherent control over integral scattering cross sections in the ultracold s-wave regime of both the initial and final collision channels. The proposed control methodology is applied to ultracold O_{2}+O_{2} collisions, showing extensive control over s-wave spin-exchange cross sections and product branching ratios over many orders of magnitude.
Quantum coherent control of bimolecular collisions beyond the ultracold regime can face a major challenge due to the incoherent addition of different partial wave contributions to the total scattering cross section. These contributions become increasingly numerous as the collision energy increases, leading to a loss of overall control. Here, we overcome this limitation by leveraging the recently discovered Partial Wave Phase Locking (PWPL) effect, which synchronizes the oscillations of all partial wave contributions. By using rigorous quantum scattering calculations, we demonstrate that PWPL enables coherent control of spin exchange in ion-atom collisions, far outside the ultracold regime, even with as many as 5000 partial wave contributions. The predicted extent of control is sufficient to be measurable in cold atom-ion hybrid experiments.
We explore coherent control of Penning and associative ionization in cold collisions of metastable He∗(23S) atoms via the quantum interference between different states of the He2∗ collision complex. By tuning the preparation coefficients of the initial atomic spin states, we can benefit from the quantum interference between molecular channels to maximize or minimize the cross sections for Penning and associative ionization. In particular, we find that we can enhance the ionization ratio by 30% in the cold regime. This work is significant for the coherent control of chemical reactions in the cold and ultracold regimes.
We consider the coherent control of ultracold molecule-molecule scattering, impacted by a dense set of rovibrational resonances. To characterize the resonance spectrum, a rudimentary model based on multichannel quantum defect theory has been used to study the control of the scattering cross section and the reaction rate. Complete control around resonance energies is shown to be possible, but thermal averaging over a large number of resonances significantly reduces the extent of control of reaction rates related to the random distribution of optimal control parameters between resonances. We show that measuring the extent of coherent control could be used to extract meaningful information about the relative contribution of direct scattering versus collision complex formation, as well as about the statistical regime.
Ultracold chemistry operates in a regime where thermal motion is suppressed to the microkelvin scale, allowing quantum mechanics to dictate every aspect of molecular interaction. This paper reviews the theoretical foundations and experimental breakthroughs that now make coherent control of ultracold reactions possible. It first outlines the separation of short- and long-range physics through quantum defect theory, emphasizing why short-range dynamics remain intractable and why universal loss emerges naturally. It then examines how long-range potentials, resonances, and quantum interference enable tunable scattering behavior. Two major milestones anchor this discussion: phase-coherent atom–atom–molecule coupling, which confirms the transfer and control of reaction phase, and the experimental resolution of long-range atom–molecule Feshbach resonances using a Fabry–P´erot interferometer model. Finally, it highlights the recent observation of a magnetically tunable molecule–molecule Feshbach resonance in NaLi, overturning long-held assumptions about the impossibility of resonance control in complex molecular systems. Together, these results demonstrate that ultracold molecules can be engineered to undergo fully coherent, interference-driven reactions, establishing a framework for quantum-controlled chemistry and laying the groundwork for applications in quantum simulation and computation.