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Overview of research goals

This experiment aims to explore new avenues of quantum science with ultracold polar molecules by forming single RbCs molecules in optical tweezers by associating individual Rb and Cs atoms.

Controllable arrays of ultracold molecules offer an exciting new platform for quantum experiments. For example, such arrays may be used for quantum simulation of problems ubiquitous to condensed matter physics such as lattice spin models. Alternatively, with precise single-site control, the molecules may be independently moved around and merged together in miniature particle colliders, allowing for the study of ultracold chemistry on a single particle level where quantum effects dominate.

The aim of this experiment is to form such an interacting quantum system of ultracold RbCs molecules using individual Rb and Cs atoms confined in optical tweezers. An optical tweezer is a tightly focused beam of light with a typical beam waist of around 1 μm. The tightly focused beam enables light assisted collisions to reduce the trap occupancy to no more than one atom, allowing experimental manipulation on the single particle level.

By trapping and cooling individual Rb and Cs atoms in optical tweezers, we will assemble single molecules by merging the tweezers confining Rb and Cs into a single trap containing both atoms. By preparing both atoms in the lowest motional state of the trap, the atom pair may be associated into a weakly bound molecular state by performing magnetoassociation, a technique performed by ramping an applied magnetic field over an interspecies Feshbach resonance. Subsequently, we will transfer this weakly-bound molecule into its rovibrational ground state using STIRAP (stimulated Raman adiabatic passage).

Detection of the associated molecules will be achieved by dissociating the remaining molecules back into atoms by reversing the formation process described above. The remaining atoms may then be detected using fluorescence imaging.

Using an acousto-optic deflector (AOD) to create multiple tweezers will enable the trapping of many Rb and Cs atoms which will allow us to scale up our experiment to create several ground state molecules, each in a different optical tweezer trap. These molecule-filled traps may then be rearranged to create the aforementioned controllable arrays, allowing a new realm of physics to be explored.

The group has recently realised the trapping and imaging of single Rb and Cs atoms in individual tweezers. Currently, the group is working on the cooling of the individual Rb and Cs atoms trapped in optical tweezers, a necessary step before the atoms can be brought together and magnetoassociated into a weakly-bound molecule. This cooling will be implemented using Raman sideband cooling, a technique which allows the motional quantum number in the tweezer to be reduced by coherently driving transitions between different motional levels.

Recent publications

	Preparation of 87Rb and 133Cs in the motional ground state of a single optical tweezer S. Spence, R. V. Brooks, D. K. Ruttley, A. Guttridge, and S. L. Cornish arXiv We report simultaneous Raman sideband cooling of a single 87Rb atom and a single 133Cs atom held in separate optical tweezers at 814 nm and 938 nm, respectively. Starting from outside the Lamb-Dicke regime, after 45 ms of cooling we measure probabilities to occupy the three-dimensional motional ground state of 0.86(−0.04)(+0.03) for Rb and 0.95(−0.04)(+0.03) for Cs. Our setup overlaps the Raman laser beams used to cool Rb and Cs, reducing hardware requirements by sharing equipment along the same beam path. The cooling protocol is scalable, and we demonstrate cooling of single Rb atoms in an array of four tweezers. After motional ground-state cooling, a 938 nm tweezer is translated to overlap with a 814 nm tweezer so that a single Rb and a single Cs atom can be transferred into a common 1064 nm trap. By minimising the heating during the merging and transfer, we prepare the atoms in the relative motional ground state with an efficiency of 0.81(−0.08)(+0.08). This is a crucial step towards the association of arrays of single RbCs molecules confined in optical tweezers.
	Feshbach Spectroscopy of Cs Atom Pairs in Optical Tweezers R. V. Brooks, A. Guttridge, M. D. Frye, D. K. Ruttley, S. Spence, J. M. Hutson, and S. L. Cornish arXiv We prepare pairs of 133Cs atoms in a single optical tweezer and perform Feshbach spectroscopy for collisions of atoms in the states (f=3,mf=±3). We detect enhancements in pair loss using a detection scheme where the optical tweezers are repeatedly subdivided. For atoms in the state (3,−3), we identify resonant features by performing inelastic loss spectroscopy. We carry out coupled-channel scattering calculations and show that at typical experimental temperatures the loss features are mostly centred on zeroes in the scattering length, rather than resonance centres. We measure the number of atoms remaining after a collision, elucidating how the different loss processes are influenced by the tweezer depth. These measurements probe the energy released during an inelastic collision, and thus give information on the states of the collision products. We also identify resonances with atom pairs prepared in the absolute ground state (f=3,mf=3), where two-body radiative loss is engineered by an excitation laser blue-detuned from the Cs D2 line. These results demonstrate optical tweezers to be a versatile tool to study two-body collisions with number-resolved detection sensitivity.
	Preparation of one 87Rb and one 133Cs atom in a single optical tweezer R. V. Brooks, S. Spence, A. Guttridge, A. Alampounti, A. Rakonjac, L. A. McArd, J. M. Hutson, and S. L. Cornish New J. Phys. 23 065002 (2021) We report the preparation of exactly one 87Rb atom and one 133Cs atom in the same optical tweezer as the essential first step towards the construction of a tweezer array of individually trapped 87Rb133Cs molecules. Through careful selection of the tweezer wavelengths, we show how to engineer species-selective trapping potentials suitable for high-fidelity preparation of Rb + Cs atom pairs. Using a wavelength of 814 nm to trap Rb and 938 nm to trap Cs, we achieve loading probabilities of 0.508(6) for Rb and 0.547(6) for Cs using standard red-detuned molasses cooling. Loading the traps sequentially yields exactly one Rb and one Cs atom in 28.4(6)% of experimental runs. Using a combination of an acousto-optic deflector and a piezo-controlled mirror to control the relative position of the tweezers, we merge the two tweezers, retaining the atom pair with a probability of 0.99(-0.02)(+0.01). We use this capability to study hyperfine-state-dependent collisions of Rb and Cs in the combined tweezer and compare the measured two-body loss rates with coupled-channel quantum scattering calculations.

Current team

	Prof. Simon Cornish Principal Investigator s.l.cornish@durham.ac.uk ORCID | ResearcherID | ResearchGate | Google Scholar
	Dr Alexander Guttridge Postdoctoral research assistant alexander.guttridge@durham.ac.uk ORCID
	Stefan Spence PhD student s.j.spence@durham.ac.uk ORCID
	Daniel Ruttley PhD student daniel.k.ruttley@durham.ac.uk ORCID
	Ce Li MRes student ce.li@durham.ac.uk
	Albert Tao MRes student li.tao@durham.ac.uk
	Imogen Forbes MSci student

Lab photographs and images

Coming soon!

Links and collaborators

This project is part of the QSUM: Quantum Science with Ultracold Molecules collaboration between Durham University, Imperial College London, and the University of Oxford.

We frequently work with with the Durham Cold Molecules Theory research group.

Funding

"QSUM: Quantum Science with Ultracold Molecules" EPSRC EP/P01058X/1 (June 2017 - August 2023)

Former team members

	Dr Lewis McArd Postdoctoral research assistant (2018-2021) ORCID
	Dr Alexandros Alampounti Postdoctoral research assistant (2019-2021) ORCID
	Dr Rahul Sawant Postdoctoral research assistant (2017-2019) ORCID
	Dr Ana Rakonjac Postdoctoral research assistant (2014-2019) ORCID
	Vincent Brooks PhD student (2017-2022) ORCID
	Mitch Walker MPhys student (2020-2021)
	Samuel White MSci student (2020-2021)
	Oliver Powell MPhys student (2019-2020)
	Kevin Roice Summer student (2021)