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Durham University

Quantum Light and Matter

Cold and Ultracold Molecules

Controlled Cold and Ultracold Collisions

Cold and ultracold molecules have many potential applications. Three "grand challenges" that have been laid down for the field are quantum simulation, precision measurements and controlled chemistry.

Sympathetic cooling, in which molecules are cooled by contact with ultracold atoms, will be crucial in producing the low temperatures and high densities required to meet these challenges. To achieve it, it is essential to understand and control molecular collisions. The molecules are invariably held in traps formed with electric, magnetic or optical fields. Inelastic or reactive collisions cause trap loss, and must be prevented when loading optical lattices for applications such as quantum simulation. Collisions also cause uncertainties in precision measurements.

Understanding collisions between ultracold atoms has been essential to the whole field of ultracold atom physics. It will be even more important for ultracold molecules, where there are many more possible outcomes and much more scope for control by external fields and/or state selection. This is the domain of cold and ultracold chemistry.

At low temperatures, collisions occur over a small range of energies and a small number of angular-momentum states. As the temperature approaches zero, we can limit them to a single hyperfine state and a single partial wave. In the mK regime, collision energies become comparable to energy-level shifts caused by externally applied electromagnetic fields. The collision dynamics are very sensitive to reactant quantum state, even at the hyperfine level, and applied electromagnetic fields can lead to dramatic changes in reaction rates and branching ratios.

Our vision is to understand the dynamics of cold molecular collisions and build a toolkit to control their outcome. To control molecular collisions effectively, we need fine control over the internal state and translational motion of the reactants.

We will use magnetic fields and quantum-state selection at the hyperfine level, as well as state-of-the-art theory, to understand fundamental collision processes and mechanisms of control. We will apply this understanding to control whether collisions lead to reaction or to inelastic energy transfer. By suppressing reactive and inelastic collisions, we aim to achieve sympathetic cooling.

The new and improved understanding achieved through this project can be used to control collisions in more complex systems. Species of interest include triatomic molecules and polyatomic molecules with large hydrocarbon side-chains. With further developments in experiment and theory, the complexity can grow until the toolbox of control protocols is full.

Controlling Ground State RbCs Molecules

Ultracold polar molecules offer a wide range of exciting research directions owing to their long-range anisotropic dipole-dipole interactions and rich internal structure. Proposed applications span the fields of ultracold chemistry, precision measurement, quantum simulation and quantum computation.

In this experiment we produce ultracold RbCs molecules in the rovibrational and hyperfine ground state by associating ultracold Rb and Cs atoms using a two-step process. Starting from an atomic mixture with high phase-space density, we use magnetoassociation on a Feshbach resonance to produce weakly bound molecules. These molecules are then coherently transferred to a single deeply bound rovibrational state of the ground electronic state with near unity efficiency, using stimulated Raman adiabatic passage (STIRAP). The final temperature mirrors that of the original atomic gas, allowing us to exploit the established techniques of atomic cooling to bring molecules into the ultracold regime.

Coherent state control:

We have demonstrated the use of microwave fields to coherently transfer ~100% of the population between the lower lying rotational and hyperfine states. We are building on this to develop a "toolbox" of techniques for coherent state control.

Rabi oscillations between the two lowest rotationally excited states in RbCs. Each is driven as a single photon excitation.

Molecular collisions:

The lifetime of heteronuclear molecules such as RbCs is expected to be on the order of many seconds due to the lack of reactive and inelastic collisions. However, significant loss is seen after hundreds of microseconds, and this is an unexpected problem across all observed species of alkali-alkali hetronuclear molecules. The loss mechanism may be due to the proposed theory of "sticky collisions". We are invistigating into the loss processes and conditions, and working towards controlling the losses.

Funding:

"Understanding Collisions of Ultracold Polar Molecules" EPSRC EP/P008275/1 (February 2017 - February 2020)

"QSUM: Quantum Science with Ultracold Molecules" EPSRC EP/P01058X/1 (June 2017 - May 2022)

CsYb: Magnetic Polar Molecules

Ultracold dipolar molecules offer a wide range of potential applications in atomic and molecular physics, ranging from precision measurement of fundamental physics to ultracold quantum chemistry. The goal of this project is to create a quantum degenerate mixture of Yb and Cs confined in a 3D optical lattice and subsequently produce ultracold CsYb molecules. Such molecules possess both a magnetic moment and an electric dipole moment, and therefore offer intriguing possibilities for the quantum simulation of lattice spin models.

Funding:

"MMQA: MicroKelvin Molecules in a Quantum Array" EPSRC EP/I012044/1 (December 2010 - November 2016)

"QSUM: Quantum Science with Ultracold Molecules" EPSRC EP/P01058X/1 (June 2017 - May 2022)

Fermionic Molecules of KCs

Ultracold polar molecules offer a wide range of exciting research directions owing to their long-range anisotropic dipole-dipole interactions. Proposed applications span the fields of ultracold chemistry, precision measurement, quantum simulation and quantum computation. The prospect of using quantum simulators to elucidate a range of intractable problems in condensed-matter physics has attracted particular attention. In many cases, the simulation protocol requires an ultracold gas of fermionic particles with long-range interactions confined in an optical lattice.

The goal of this project is to realise a gas of ultracold fermionic KCs molecules by associating pre-cooled atoms of K and Cs. This molecule has the advantage over other bi-alkali molecules of being stable against reactive collisions and offers both fermionic and bosonic isotopes. By confining the molecules in an array of two-dimensional pancake traps we will deliver a test platform for quantum simulation applications. This trap geometry is suited to the study of a large range of physical phenomena, including high-TC BCS-like interlayer super- fluidity, quantum magnetism and topological superfluid phases.To achieve this ambitious objective we propose to combine state-of-the-art experiments in synergy with world leading theoretical support into a transformative program of research that stands to cement the UK's position at the forefront of an exciting international field.

This project is also part of QSUM (Quantum Science with Ultracold Molecules), see https://www.qsum.org.uk for more information.

Funding

"A Stable Quantum Gas of Fermionic Polar Molecules" EPSRC EP/N007085/1 (January 2016 - December 2019)

"QSUM: Quantum Science with Ultracold Molecules" EPSRC EP/P01058X/1 (June 2017 - May 2022)

Molecules in Tweezers

The Molecules in Tweezers project is a new experiment at Durham University and is part of QSUM: Quantum Science with Ultracold Molecules. The aim of the project is to assemble an interacting quantum system one molecule at a time. By trapping individual Rb and Cs atoms in tightly focussed optical tweezer traps, we will assemble single molecules by merging the microtraps and associating the atoms, and we will subsequently transfer the molecules to their ground rovibrational state. This scheme will be scaled to several molecules, giving us a deterministically prepared array of ultracold molecules.

Funding

"QSUM: Quantum Science with Ultracold Molecules" EPSRC EP/P01058X/1 (June 2017 - May 2022)

Precision Interaction Potentials

Coming Soon....

Quantum Gas Microscope

Ultracold polar molecules promise and exciting new direction for quantum simulation. Their rich internal structure, which stems from their complex internal structure, permits long-range interactions between molecules in addition to strong coupling to electric and microwave fields. Arrays of polar molecules may exhibit strongly-interacting many-body quantum states which are central to a range of phenomena such as the fractional quantum Hall effect, high-temperature superconductivity, and exotic forms of magnetism. Understanding how these phenomena emerge is one of the great challenges of modern physics.

In a new experimental apparatus, we plan to create an array of ultracold polar molecules by association from a pre-cooled atomic mixture following a similar scheme used in our existing RbCs experiment. Once loaded into an optical lattice, a high resolution imaging system will be used to read out the quantum state and site occupation of the molecules with single-site resolution. Such imaging systems have been developed for both bosonic and fermionic atoms, and have proved to be invaluable in the study of many-body physics with ultracold atoms. The development of similar methods for ultracold molecules will be similarly critical, and enable the study of such systems in the presence of long range dipole-dipole interactions.

Funding:

This project is a part of the QSUM (Quantum Science with Ultracold Molecules) programme grant, funded by the Engineering and Physical Sciences Research Council (EPSRC).

"QSUM: Quantum Science with Ultracold Molecules" EPSRC EP/P01058X/1 (June 2017 - May 2022)

Theory of Weakly Bound States and Ultracold Scattering

Coming Soon....