Ultracold single molecules have wide-ranging potential applications, such as ultracold chemistry, precision measurements, quantum simulation, and quantum computation. However, given the difficulty of achieving full control of a complex atom-molecule system, the coherent formation of single molecules remains a challenge. Here, we report an alternative route to coherently bind two atoms into a weakly bound molecule at megahertz levels by coupling atomic spins to their two-body relative motion in a strongly focused laser with inherent polarization gradients. The coherent nature is demonstrated by long-lived atom-molecule Rabi oscillations. We further manipulate the motional levels of the molecules and measure the binding energy precisely. This work opens the door to full control of all degrees of freedom in atom-molecule systems.
Ultracold single molecules have wide-ranging potential applications, such as ultracold chemistry, precision measurements, quantum simulation, and quantum computation. However, given the difficulty of achieving full control of a complex atom-molecule system, the coherent formation of single molecules remains a challenge. Here, we report an alternative route to coherently bind two atoms into a weakly bound molecule at megahertz levels by coupling atomic spins to their two-body relative motion in a strongly focused laser with inherent polarization gradients. The coherent nature is demonstrated by long-lived atom-molecule Rabi oscillations. We further manipulate the motional levels of the molecules and measure the binding energy precisely. This work opens the door to full control of all degrees of freedom in atom-molecule systems.
Ultracold Chemistry
The formation of ultracold molecules has already had a profound impact on many research areas of physics. However, conventional methods of producing such molecules are attainable only for a limited number of systems or they suffer for strong dephasing. He et al. sought to pair atoms, through coupling of their spins, to the two-body relative motion mediated by the inherent polarization gradients in a strongly focused trapping laser. They report a successful assembly of an ultracold 87Rb-85Rb molecule in an optical tweezer and observed coherent, long-lived atom-molecule Rabi oscillations. They further demonstrate the full control of the internal and external degrees of freedom in the atom-molecule system.
Science , this issue p. [331][1]
[1]: /lookup/doi/10.1126/science.aba7468
Molecules, with their rich spectrum of internal states as well as strong and tunable dipolar interactions, are promising candidates for a broad range of experiments including quantum information, quantum chemistry, precision measurement and probing physics beyond the Standard Model, and quantum simulation of interacting many-body systems. At the same time, the complexity that made them attractive also poses a significant challenge to achieve a high level of quantum control that is required in many of their potential applications. Multiple approaches are being pursued for full quantum control of molecules including association of atomic gases and direct cooling of molecules, yet it remains challenging to control molecules at single particle level due to the difficulty in tuning the atom-atom interaction and laser cooling of molecules. In this thesis, we achieved full control on a single weakly bound ground state molecule created by coherent optical association from single atoms. We first discuss the trapping and imaging of the atoms in the optical tweezer and the cooling into a single quantum state in order to achieve the quantum control of atoms. Combining this control with the resolution and flexibility of tweezers. we developed a versatile platform to study interactions between single atoms and properties of single molecules, using precise optical spectroscopy on pairs of atoms in the tweezer. Based on these studies, we proposed a promising pathway for coherent all-optical creation of molecules and experimentally demonstrated full control on the molecule including all degrees of freedom.
This thesis presents the development of a toolbox for the coherent control of ultracold polar molecules. Such systems of molecules promise the creation of long-lived, highly dipolar quantum gases with applications spanning the fields of quantum state controlled chemistry, quantum information, quantum simulation, and precision measurement. However, the addition of vibrational and rotational degrees of freedom leads to molecular systems being significantly more complex than their widely used atomic counterparts. In this work we demonstrate full control of the quantum state down to the hyperfine level of an optically trapped sample of ultracold bosonic 87Rb133 Cs molecules, and exploit that control to begin an investigation into the collision processes which take place in an ultracold molecular gas. We create a sample of up to ∼ 4000 optically trapped molecules in their rovibronic and hyperfine ground state. We characterise the molecules by measuring their temperature, binding energy, and molecule-frame electric dipole moment. We perform spectroscopy of the first rotationally excited state with hyperfine state resolution using microwaves to determine accurate values of rotational and hyperfine coupling constants. We use coherent π pulses to perform complete transfer population between selected hyperfine levels of the ground, first-excited, and second-excited rotational states. We investigate the effect of the off-resonant light of our optical dipole trap on the rotational and hyperfine structure of the molecules. Through a combination of high-resolution microwave spectroscopy and parametric heating measurements, we characterise the polarisability of the 87Rb133Cs molecule. We demonstrate that coupling between neighbouring hyperfine states manifests in rich structure with many avoided crossings in any rotational state other than the ground state. This coupling may be tuned by rotating the polarisation of the linearly polarised trapping light. Finally, we study the lifetime of polar bosonic 87Rb133Cs molecules in our 3D optical dipole trap. We examine the lifetime of the molecules as a function of dipole trap intensity, magnetic field, and hyperfine and rotational state. Despite the chemical stability of the 87Rb133 Cs molecule, we observe lifetimes of ∼1 s corresponding to 2-body decay rates close to the universal limit.
The ability to control particles in the quantum regime at the individual particle level is a challenge at the frontier of quantum science and engineering. Of the many quantum objects developed in pursuit of this goal, ultracold molecules host rich internal states and unique inter-particle interactions that could lend themselves useful to a broad range of applications including quantum computing and simulation, precision measurements and quantum chemistry. In this thesis we present a bottom-up approach to creating single molecules in individual optical tweezers. In particular, single atoms of two different species are first trapped and cooled in individual optical tweezers then brought together to form a single molecule coherently. Using this approach, we have successfully created an array of fully quantum-state-controlled polar molecules. The quantum state – including both internal and external states – of the resulting molecules are known to us, and is the first time single neutral molecules have been under full quantum state control. This platform offers exciting new opportunities to harness the rich properties of molecules in many quantum science applications.
The creation of a gas of ultracold polar molecules with a high phase space density brings new possibilities beyond experiments with ultracold atomic gases. In particular, long-range, anisotropic, and tunable dipole-dipole interactions open the way for novel quantum gases, with applications including strongly correlated many-body systems, and ultracold chemistry. This thesis will present the final steps to complete control over both internal and external degrees of freedom of the molecule which allows us to control, and even completely suppress, the chemical reactions between molecules. First, the control over internal states has been achieved through coherent state transfer to the ro-vibronic ground state and coherent manipulations of the hyperfine and rotational states with microwave radiation. Second, external degrees of freedom are controlled by loading the gas into an optical lattice. With the molecules loaded into a one-dimensional lattice, the orientation of the molecular collisions is controlled by manipulating both internal (hyperfine states) and external (motional states in the direction of tight confinement) degrees of freedom. Most striking is that by preparing the molecules all in the lowest band of the lattice in the same internal state, the molecular collisions can only occur in a side-by-side" orientation, where the chemical reaction rate is suppressed by the repulsive dipole-dipole interactions. The chemical reaction can be suppressed completely by further constraining the motion in the trap in a strong 3D lattice. Here we see lifetimes longer than 20 s, limited by o-resonant light scattering. Finally, the ac polarizability of the molecules is explored and controlled. The different rotational states of the molecule have different polarizabilities and will experience a different trapping force in both the optical dipole trap or lattice. We show that there is a magic angle" between the quantization axis and the polarization of the trapping laser at which the polarizabilities of two different rotational states can be matched, eliminating dephasing and allowing for coherent manipulations between rotational states.
Abstract Ultracold polar molecules combine a rich structure of long-lived internal states with access to controllable long-range anisotropic dipole–dipole interactions. In particular, the rotational states of polar molecules confined in optical tweezers or optical lattices may be used to encode interacting qubits for quantum computation or pseudo-spins for simulating quantum magnetism. As with all quantum platforms, the engineering of robust coherent superpositions of states is vital. However, for optically trapped molecules, the coherence time between rotational states is typically limited by inhomogeneous differential light shifts. Here we demonstrate a rotationally magic optical trap for 87 Rb 133 Cs molecules that supports a Ramsey coherence time of 0.78(4) s in the absence of dipole–dipole interactions. This is estimated to extend to >1.4 s at the 95% confidence level using a single spin-echo pulse. In our trap, dipolar interactions become the dominant mechanism by which Ramsey contrast is lost for superpositions that generate oscillating dipoles. By changing the states forming the superposition, we tune the effective dipole moment and show that the coherence time is inversely proportional to the strength of the dipolar interaction. Our work unlocks the full potential of the rotational degree of freedom in molecules for quantum computation and quantum simulation.