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Cold Atmospheric Molecules on a Chip

Periodic Reporting for period 4 - CAtMolChip (Cold Atmospheric Molecules on a Chip)

Reporting period: 2020-12-01 to 2021-11-30

The objectives of this project were to study collisions and decay processes of atoms and molecules in highly excited quantum states known as Rydberg states, and in this process develop new experimental techniques appropriate for achieving this task. The atoms and molecules of interest were those of importance in the chemistry and physics of Earth's atmosphere. This work was carried out in a controlled laboratory environment. It exploited, and involved the development of, state-of-the art experimental techniques to trap and isolate the species of interest so that they could be studied on timescales > 1 ms. The results of this work are relevant to attaining a complete understanding of the physical and chemical processes taking place in the environment in which live. The experimental methods developed through the project are expected to also be applicable in other research areas, ranging from fundamental physical chemistry and physics to sensing, studies of antimatter, and the development of quantum technologies.
The work performed in the project included:

(1) The development of a cryogenically cooled (30 K) experimental apparatus for preparing cold, electrostatically trapped Rydberg atoms and molecules. This apparatus was used for laser spectroscopic studies of long-lived Rydberg states of nitric oxide (NO). It was also used in experiments to guide, accelerate, decelerate and trap molecules in these long-lived Rydberg states. Systematic studies of the decay processes of the trapped molecules were performed. The results of this work provided new insights in the role of rotational and vibrational channel interactions, and vibrational excitation on the slow decay of these Rydberg states in NO.

(2) The implementation of a new 2D spectroscopy technique, in which laser photoexcitation to Rydberg states was combined with detection by pulsed electric field ionisation in a slowly-rising electric field, to allow direct identification of Rydberg series in small molecules to which individual excited states belong. This method was used, e.g. to identify and characterise long-lived Rydberg states of NO prior to deceleration and electrostatic trapping.

(3) First experiments to observe quantum-state-resolved resonant energy transfer in collisions of Rydberg atoms with polar molecules, and the first demonstration of control over this energy transfer processes using electric fields. This work was initially performed with Rydberg helium atoms in pulsed supersonic beams and room temperature (300 K) gases of ammonia. It was then extended to temperatures below 100 mK in an intra beam collision apparatus to allow the first observation of effects of van der Waals interactions on the process of resonant energy in this type of collision system.

(4) Experimental and theoretical studies of quantum mechanical tunnelling processes that lead to ionisation of Rydberg atoms and molecules in strong electric fields. This work represented a new way to study tunnel ionisation in atoms and molecules and allowed the refinement of Rydberg-state-selective electric field ionisation methods used to detect excited species throughout the project.

(5) The realisation of new experimental tools for controlling the motion of atoms and molecules in Rydberg states using time-varying inhomogeneous electric fields. The approaches developed in this work allowed atoms and molecules in a wider range of excited states to be confined and guided, than was previously possible. These tools have applications, e.g. in studies of collisions and decay processes involving Rydberg atoms and molecules, including resonant energy transfer.

(6) The development, and first demonstration, of a method to perform matter-wave interferometry with atoms in Rydberg states by exploiting the quantum-state-dependent forces that can be exerted on them using inhomogeneous electric fields. This new and original experimental technique has applications in the study of weak long-range interactions of Rydberg atoms with polar molecules, studies of quantum mechanical geometric phases for particles with large electric dipole moments, and measurements of acceleration due to gravity for antimatter.

(7) The first demonstration of a cavity-enhanced Ramsey spectroscopy technique to probe and manipulate Rydberg atoms located above chip-based superconducting microwave resonators. The methodology and experimental techniques used in this work open new opportunities for high-sensitivity non-destructive detection of Rydberg atoms or molecules, and studies of long-range interactions between Rydberg atoms/molecules and surfaces.
The outcomes associated with each work area above have moved this research program well beyond the prior state of the art.

The apparatus constructed for deceleration and trapping cold Rydberg molecules contains a state-of-the-art decelerator which had only previously been operated at room temperature. Its operation at 30 K is a significant technical advance that opens a range of exciting research opportunities. In this apparatus, we trapped nitric oxide (NO) molecules for the first time, made the first measurements of quantum-state-dependent decay rates of long-lived Rydberg states in NO, and identified contributions, on the order of ~1 kHz from intramolecular interactions, to these decay rates.

The new 2D spectroscopy technique developed through the project allowed the identification and characterisation of long-lived Rydberg states of nitric oxide molecules in a direct and unambiguous way that was previously not possible.

The quantum-state-resolved and electric-field controlled resonant energy transfer processes studied in collisions of Rydberg atoms with polar molecules had not previously been observed in such detail and with such high sensitivity. These energy transfer reactions involve coupling the nuclear degrees of freedom in the ground-state molecules with the electronic degrees of freedom in the Rydberg atoms. The experiments performed allowed the interplay between resonant dipole-dipole and van der Waals interactions between the atoms and the molecules to be identified for the first time.

The studies of ionisation dynamics and quantum mechanical tunnel ionisation rates that were carried out as part of the project exceed the precision with which these processes were investigated previously, and highlighted limitations of the theoretical methods used to calculate these rates, and timescales on which these processes occur.

The new experimental tools developed to control the motion of highly-excited atoms and molecules using time-varying inhomogeneous electric fields did not previously exist, but now allow the motion of samples in a wider range of quantum state than previously possible to be controlled.

The approach to Rydberg-atom matter-wave interferometry, using sequences of microwave and electric-field-gradient, pulses that we have developed, is groundbreaking. It has opened a new era of coherent Rydberg atom/molecule optics, and many exciting possibilities for studies of atom-molecule and molecule-molecule interactions at low temperatures, and new tests of quantum mechanics.

The cavity-enhanced Ramsey spectroscopy experiments performed as part of this project to study Rydberg atoms located close to chip-based superconducting microwave resonators were the first of their kind and opened new possibilities for highly sensitive non-destructive detection of atoms and molecules.
Rydberg laboratory on a chip