QSUM: Quantum Science with Ultracold Molecules

Lead Research Organisation: Durham University
Department Name: Physics

Abstract

For over a century, scientists have been fascinated, and at times mystified, by quantum mechanics, the theory that governs atoms, molecules and, indeed, all matter at a microscopic level. Central to this theory are two concepts: (1) Wave-particle duality - the idea that particles, such as electrons in an atom, can behave like waves and that light waves can behave like particles, and (2) entanglement - the concept that once two (or more) particles have interacted, they cannot be treated as independent entities no matter how far apart they are. These inherently quantum phenomena are at the heart of a wide range of physical effects, but their role is often extremely difficult to elucidate. For example, in solid materials, where every atom interacts with many other atoms, it is very challenging to predict and understand how the quantum behaviour will manifest itself, and yet it leads to effects, such as high-temperature superconductivity and special forms of magnetism. Our Programme will advance the understanding of these complex quantum systems by studying the behaviour of molecules cooled to very low temperatures where we can isolate their quantum behaviour. In this respect, the use of molecules is crucial. Their rich internal structure means they couple strongly to electric and microwave fields, and interact with each other over a much greater distance compared with atoms. In advancing our understanding of the quantum science of molecules, we will also learn how to harness their properties to build new devices, including sensors of exceptional sensitivity, computers capable of solving previously unsolvable problems, and simulators that can design new materials, magnets and superconductors.

To study the quantum science of molecules in a controlled and systematic way, we need to develop the ability to manipulate the quantum properties of individual molecules. The first step towards this goal is to remove the thermal motion that normally hides their quantum behaviour. We have already developed methods to achieve this both using molecules in the solid state and in the gas-phase. In the solid state, we have demonstrated that certain organic dye molecules, when embedded in a suitable solid cooled to cryogenic temperatures, behave as near-ideal two-level quantum systems. Such molecules have the perfect properties to act as interfaces between quantum light and quantum matter - an essential building block of many future quantum devices. We will learn how to exploit these properties to generate single photons on demand, control individual photons, and store quantum information. In the gas phase, we have extended the methods of laser cooling and developed new techniques to cool molecules to within a millionth of a degree above absolute zero. In this quantum regime, it is possible to exert complete control over the internal state and motion of the molecules. With this control we can learn how to couple molecules to microwave and optical waveguides, to trap molecules on chips, to assemble ordered arrays of molecules that replicate the crystalline structure of real materials, and to explore how the interactions between molecules govern the behaviour of the many-particle system.

These ambitious goals calls for radical advances, which we will deliver through a set of interconnected experiments intimately linked to state-of-the-art theory. With isolated molecules we will develop the control of single molecules and their coupling to single photons; with small arrays of interacting molecules we will control interactions and entanglement in simple geometries; and with two- and three-dimensional lattices we will understand the complex behaviour of strongly interacting many-particle systems. Through these projects, our Programme will lay the foundations for a broad range of future scientific advances and technological applications based on the quantum control of molecules.

Planned Impact

QSUM will address fundamental questions in quantum science. The primary impacts will be new knowledge, new capability, the training of people, and improved public engagement. The stimulus we provide to new quantum technologies will benefit the economy and society in the longer term.

Knowledge:
We will learn how to use molecules to build and control strongly interacting many-body quantum systems, and how to handle and exploit complexity in such systems. We will learn to entangle molecules and explore their applications in quantum information processing. We will couple molecules to nanophotonic and microwave circuits and study how this capability might be used to make devices. The scientific knowledge and new techniques we generate will feed into a chain of research and development leading to new capability and technology across a range of disciplines, including condensed-matter physics, quantum technology and metrology. Our work will impact on two of the "Physics Grand Challenges" identified by EPSRC, namely "Emergence and Physics Far From Equilibrium" and "Quantum Physics for New Quantum Technologies".

People:
Our programme spans disciplines from quantum optics to condensed matter to chemistry, couples experiment and theory, and unites world-leading researchers in a truly collaborative effort. We will attract exceptional researchers, who will develop cutting-edge methods of quantum science and the capacity to become future leaders in Quantum Science and Technology. The postgraduate and postdoctoral researchers involved in this research will acquire expertise in electronics, numerical modelling, data analysis, laser science, optics, imaging, device physics, microwave engineering, software development, and statistics. These are all areas of importance for the UK economy and we expect to supply people to high-tech industry and numerically intensive commercial sectors, such as, finance, software modelling and consultancy.

Economy & Society:
In the short term, we expect to drive advances in high-tech equipment which can be exploited by UK manufacturers, because our research often requires us to extend the capabilities of commercially available equipment and to develop new devices.
In the long term, quantum science with molecules has the potential to generate a new wave of quantum technologies with social and economic benefits. Molecular sensors offer enhanced sensitivity to electric and microwave fields with applications in surface science, field calibration and the detection of weak signals. Molecules coupled to microwave and photonic circuits will provide single photons on demand to quantum networks, and be used as quantum non-demolition photon detectors and memories. Quantum simulation with molecules will address important problems in many-body physics, such as understanding high-temperature superconductors, and will aid the design of new materials and devices. Through our Advisory Group and links with the UK academic community we will engage with the UK National Quantum Technology Programme to identify promising technological applications of our research at an early stage.

Public engagement:
Our outreach activities will help ensure that the public is fully engaged with science and recognises its importance to society. We will present simple explanations of our work to schools and the wider community, and will work with a recognised artist to develop innovative approaches to communicating the subtleties of quantum science to a general audience. We will also promote our research to policy-makers in the UK and at the European level to help maintain the support of physical sciences research.

Publications

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Aldegunde J (2017) Hyperfine structure of alkali-metal diatomic molecules in Physical Review A

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Blackmore J (2018) Ultracold molecules for quantum simulation: rotational coherences in CaF and RbCs in Quantum Science and Technology

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Brookes SGH (2022) Interaction Potential for NaCs for Ultracold Scattering and Spectroscopy. in The journal of physical chemistry. A

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Brooks R (2022) Feshbach spectroscopy of Cs atom pairs in optical tweezers in New Journal of Physics

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Burdekin P (2020) Single-Photon-Level Sub-Doppler Pump-Probe Spectroscopy of Rubidium in Physical Review Applied

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Caldwell L (2020) Long Rotational Coherence Times of Molecules in a Magnetic Trap. in Physical review letters

 
Description We developed the methods to produce ultracold molecules by direct laser cooling, eventually reaching a temperature of 5 uK, and the methods for controlling the rotational and hyperfine states. We developed the theoretical tools for analyzing these cooling methods, and those tools have been incorporated (by others) into software that is freely available to the community. We developed new techniques to trap neutral atoms and molecules, including a microwave trap. Using theory and experiment, we showed that molecules can have long rotational coherence times when trapped. We studied collisions between laser-cooled molecules and atoms through a combination of experiment and theory. We worked out how trapped molecules can be cooled to the ground-state of motion, how they can be used for quantum simulation and quantum computing, and how these applications can be enhanced by interfacing molecules and Rydberg atoms.

We have developed a deeper understanding of the diatomic molecules in applied fields and used this to enhance the control of molecules. We have quantified the molecular hyperfine structure in a magnetic field and developed coherent rotational state control with microwave fields. We learned how to trap the molecules optically and performed a detailed study of the AC Stark effect in the trap. We demonstrated Ramsey interferometry using rotational state superpositions and used this to quantify and minimise decoherence effects. This is important for applications in quantum simulation, many of which exploit the rotational degree of freedom in the molecule. We have demonstrated the coherent manipulation of the internal state of the molecule using multiple microwave fields, driving transitions up to the 6th rotational level and laying the groundwork for the development of synthetic dimensions using molecules. We have also made seminal contributions to the understanding of ultracold collisions between molecules, investigating 'sticky' collisions and establishing that molecules may be lost from an optical trap due to photo-excitation of long-lived collision complexes. We have shown how to eliminate decoherence due to both magnetic field noise and the AC Stark shift for molecules in the rotational ground state, leading to the demonstration of robust storage qubits. We have developed a magic trap for RbCs molecules that supports long-lived rotational coherences. Using this trap we have observed second-scale coherent rotational superpositions for the first time. This has allowed us to detect dipole-dipole interactions between the molecules.

We have developed a new experimental platform for the assembly of individual molecules in optical tweezers. In the course of this work we discovered a new method of molecule formation during the merging of pairs of optical tweezers (mergoassociation) and developed a quantitative theoretical understanding of the underlying physics. We have developed single-site addressing of molecules and the readout of multiple rotational states in a single iteration of the experiment.

We have also made a number of exciting contributions to the field of solid-state organic molecules used for quantum science and technology applications. We developed novel methods to fabricate nanocrystals of anthracene, as well as a technique to encapsulate them in a polymer, protecting them from sublimation and stabilising the fluorescence of larger molecules embedded inside. We also investigated a new guest-host organic molecule system of dibenzoterrylene-doped para-terphenyl, where we created nanocrystals and performed cryogenic microscopy and spectroscopy to determine molecular transitions in this system. Using similar techniques, we published a landmark paper in Phys. Rev. Lett. investigating phonon coupling to organic molecules in collaboration with Dara McCutcheon's theory group at the University of Bristol, showing excellent agreement between theory and experiment. Following on from this, we used a single organic molecule to prove that photon indistinguishability can be assessed using both continuous wave and pulsed excitation, showing the quantum optics community new ways to characterise coherence. We developed the capability to couple organic molecules to nanophotonic structures. We showed that molecular emission could be tuned using a piezoelectric substrate and that molecule emission could be coupled to hybrid plasmonic structures to enhance photon collection. We showed that organic molecules could be introduced to the middle of nanophotonic waveguides at cryogenic temperature, enabling coherent extinction measurements to assess the molecule-waveguide coupling. This work is broadly applicable to other solid state quantum systems and was published in Nature Communications.

We developed important new theoretical methods with general impact for the creation of ultracold molecules, both bosonic and fermionic. We developed the theory of molecular collisions shielded from destructive collisions with microwave fields, which has recently been crucial in the achievement of molecular Bose-Einstein condensation for NaCs. We developed an experiment/theory collaboration with the Prof. Kang-Kuen Ni at Harvard, and worked with her to achieve the first magnetoassociation of an ultracold molecule (NaCs) in an optical tweezer. We developed theoretical methods to handle atom-atom collisions in the presence of radiofrequency fields. We showed that it is possible to create Feshbach resonances that are suitable for molecule formation even in systems that do not have them naturally. We developed an experiment/theory collaboration with Prof. Chris Foot (Oxford) to understand mixed-isotope collisions of Rb in radiofrequency fields. We developed new methods to characterise Feshbach resonances that are far easier to use than older manual methods; we have made extensive use of the use methods in most of our subsequent work. We also explored the important topic of time delays ("sticky collisions") in ultracold collisions, which are very important for understanding the stability of ultracold molecules. We developed and published new versions of the MOLSCAT, BOUND and FIELD codes for quantum-mechanical calculations of molecular collisions and bound states. These were the first new versions published since 1996, so incorporated all the advances we had made over a 20-year period, including basic features crucial to ultracold molecules (inclusion of magnetic fields, calculation of scattering lengths, etc.) as well as the new methods described above.

We developed new methods to characterise the lifetimes of ultracold Feshbach molecules. We established an experiment/theory collaborations with Prof Jun Ye (JILA, University of Colorado) and with Zoran Hadzibabic (Cambridge) in which we used the these methods to understand the Efimov physics of 3-body collisions between K atoms. We developed the theory of ac Stark effects for ultracold molecules, and used it to control the states of RbCs molecules. We developed a new theory of "nonuniversal" lossy collisions between pairs of ultracold molecules, which we used to understand losses for RbCs in collisions with itself and with atoms and of CaF in collisions with Rb. We established an experiment/theory collaboration with Prof. Dajun Wang (Hong Kong) to develop an new interaction potential for Na+Rb and understand Lee-Huang-Yang effects in quantum droplets. We developed the theory of the bound states that can be formed between ultracold atom and molecules, and established an experiment/theory collaboration with Prof. Jian-Wei Pan (Shanghai) to understand resonances in ultracold collisions of NaK with K. We developed theoretical methods to understand collisions of alkali-metal atoms with Sr and Yb atoms in metastable states, which offer important new possibilities for the creation of ultracold molecules with magnetic as well as electric dipoles. We developed an experiment/theory collaboration with Prof. Hanns-Christoph Naegerl (Innsbruck) that has led to the first Bose-Einstein condensate of Cs in an excited hyperfine state. We developed theoretical methods to handle lossy collisions of polar molecules in static electric fields, and showed theoretically that shielding with a static field will be very effective in shielding ultracold CaF molecules from destructive collisions and allow evaporative cooling towards quantum degeneracy.
Exploitation Route Many of our findings are being adopted by other academic groups around the world.
Sectors Other

 
Description Dilute Quantum Fluids Beyond the Mean-Field
Amount £805,212 (GBP)
Funding ID EP/T015241/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 03/2020 
End 03/2025
 
Description ORQUID - ORganic QUantum Integrated Devices
Amount £323,145 (GBP)
Funding ID EP/R044031/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 02/2018 
End 01/2021
 
Title An association sequence suitable for producing ground-state RbCs molecules in optical lattices 
Description The data that support the findings of this study are uploaded here. All the data files are self-explanatory. They contain individual column names.  The experimental data for Fig. 4(a) are in Fig4a_experimental_data.csv under the folders Fig_4>Fig_4a. To convert to binding energy there is a fitting algorithm in the Python code Feshbach_Fit.py . 
Type Of Material Database/Collection of data 
Year Produced 2023 
Provided To Others? Yes  
URL https://zenodo.org/doi/10.5281/zenodo.7777261
 
Title Data from R. C. Schofield et al., "Polymer-encapsulated organic nanocrystals for single photon emission" Opt. Mater. Express 10, 1586-1596 (2020). 
Description Raw data files to accompany R. C. Schofield et al., "Polymer-encapsulated organic nanocrystals for single photon emission" Opt. Mater. Express 10, 1586-1596 (2020), and a Mathematica notebook which can be used to import the data files. 
Type Of Material Database/Collection of data 
Year Produced 2020 
Provided To Others? Yes  
URL https://zenodo.org/record/3784729
 
Title Data to accompany C. Clear et al., "Phonon-induced optical dephasing in single organic molecules," Phys. Rev. Lett. (2020) 
Description Data to accompany C. Clear, R. C. Schofield, K. D. Major, J. Iles-Smith, A. S. Clark, and D. P. S. McCutcheon "Phonon-induced optical dephasing in single organic molecules," Phys. Rev. Lett. 124, 153602 (2020). The raw data for single molecule spectra, second-order correlation functions, fluorescence excitation spectra, and time-correlated single photon counting, can be imported using the included Mathematica notebook which also contains instructions. 
Type Of Material Database/Collection of data 
Year Produced 2020 
Provided To Others? Yes  
URL https://zenodo.org/record/3727562
 
Title Diatomic-py: A Python module for calculating the rotational and hyperfine structure of 1S molecules 
Description We present a computer program to calculate the quantised rotational and hyperfine energy levels of 1S diatomic molecules in the presence of dc electric, dc magnetic, and off-resonant optical fields. Our program is applicable to the bialkali molecules used in ongoing state-of-the-art experiments with ultracold molecular gases. We include functions for the calculation of space-fixed electric dipole moments, magnetic moments and transition dipole moments. 
Type Of Material Database/Collection of data 
Year Produced 2022 
Provided To Others? Yes  
URL https://data.mendeley.com/datasets/3yfxnh5bn5/1
 
Title Diatomic-py: A Python module for calculating the rotational and hyperfine structure of 1S molecules 
Description We present a computer program to calculate the quantised rotational and hyperfine energy levels of 1S diatomic molecules in the presence of dc electric, dc magnetic, and off-resonant optical fields. Our program is applicable to the bialkali molecules used in ongoing state-of-the-art experiments with ultracold molecular gases. We include functions for the calculation of space-fixed electric dipole moments, magnetic moments and transition dipole moments. 
Type Of Material Database/Collection of data 
Year Produced 2022 
Provided To Others? Yes  
URL https://data.mendeley.com/datasets/3yfxnh5bn5
 
Title MOLSCAT, BOUND and FIELD 
Description This dataset provides files for the packages MOLSCAT, BOUND and FIELD, which are referenced in the following CPC 50th Anniversary articles: MOLSCAT: A program for non-reactive quantum scattering calculations on atomic and molecular collisions - https://doi.org/10.1016/j.cpc.2019.02.014 BOUND and FIELD: Programs for calculating bound states of interacting pairs of atoms and molecules - https://doi.org/10.1016/j.cpc.2019.02.017 This dataset contains:- 1) Full documentation for the programs MOLSCAT, BOUND and FIELD in pdf form; 2) A directory source_code containing a) the Fortran source code; b) a GNUmakefile that can build the executables needed for the example calculations described in the documentation; c) a sub-directory input containing input files for the example calculations described in the documentation; d) a sub-directory output containing the corresponding output files; 4) A directory data containing auxiliary data files for some potential routines used in the example calculations; 5) A plain-text file README that gives information on changes that may be needed to adapt the GNUmakefile to a specific target computer. 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? Yes  
URL https://data.mendeley.com/datasets/rtzgf5mwpn
 
Title MOLSCAT, BOUND and FIELD 
Description This dataset provides files for the packages MOLSCAT, BOUND and FIELD, which are referenced in the following CPC 50th Anniversary articles: MOLSCAT: A program for non-reactive quantum scattering calculations on atomic and molecular collisions - https://doi.org/10.1016/j.cpc.2019.02.014 BOUND and FIELD: Programs for calculating bound states of interacting pairs of atoms and molecules - https://doi.org/10.1016/j.cpc.2019.02.017 This dataset contains:- 1) Full documentation for the programs MOLSCAT, BOUND and FIELD in pdf form; 2) A directory source_code containing a) the Fortran source code; b) a GNUmakefile that can build the executables needed for the example calculations described in the documentation; c) a sub-directory input containing input files for the example calculations described in the documentation; d) a sub-directory output containing the corresponding output files; 4) A directory data containing auxiliary data files for some potential routines used in the example calculations; 5) A plain-text file README that gives information on changes that may be needed to adapt the GNUmakefile to a specific target computer. 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? Yes  
URL https://data.mendeley.com/datasets/rtzgf5mwpn/1
 
Title Research data supporting "Pinpointing Feshbach Resonances and Testing Efimov Universalities in ³?K" 
Description The data for the measurements presented throughout the figures in the paper are provided, as well as the underlying measurements that are just summarized in the Tables. The readme files provided include the necessary information to interpret and use the data. 
Type Of Material Database/Collection of data 
Year Produced 2023 
Provided To Others? Yes  
URL https://www.repository.cam.ac.uk/handle/1810/353418
 
Description Theory collaboration on dipolar interactions 
Organisation Rice University
Department Department of Physics and Astronomy
Country United States 
Sector Academic/University 
PI Contribution Experimental measurement of coherence times of rotational state superpositions in a magic trap. We explored different superpositions to control the effective dipole-dipole interactions between molecules.
Collaborator Contribution Theoretical modelling of the coherence observed in the experiment using the Moving Average Cluster Expansion (MACE) method.
Impact Gregory, P.D., Fernley, L.M., Tao, A.L. et al. Second-scale rotational coherence and dipolar interactions in a gas of ultracold polar molecules. Nat. Phys. (2024). https://doi.org/10.1038/s41567-023-02328-5
Start Year 2023
 
Description Workshops for schools about atoms 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Schools
Results and Impact A workshop to introduce 8 to 10 year old children, along with their carers and teachers, to the beauty of atoms and their intricate interactions with light - brought to life with discussion, art, experiment, poetry and dance. The workshops were delivered in person and online. Outcomes and feedback can be found here: https://www.worldofatoms.com/.
Year(s) Of Engagement Activity 2018,2019,2020,2021