The thesis reports transferring the ultracold RbCs molecules to the absolute ground state and a trapping light that eliminates the differential AC Stark shift. We build a laser system to perform Stimulated Raman Adiabatic Passage (STIRAP) for the molecules trapped in tweezers, which transfer the molecules from the weakly- bounded state to the absolute ground state. We use the offset cavity stabilisation technique to stabilise the laser frequencies to a passive cavity to narrow the laser linewidths while keeping the laser frequency tunable. We then align the beams to the molecules. We demonstrate a “magic” laser frequency to eliminate the differential AC Stark shift of the rotational states in the ground vibrational state of RbCs molecule and to increase the rotational coherence time of multiple states. We study the AC Stark effect caused by the trapping light. We calculate the polarisabilities of the rotational states near a narrow electronic from X ^1Σ^+ state to b^3Π_0 state and found at certain laser frequency the polarisabilities become the same. We measure the transition frequencies to different vibrational states in the excited electronic state experimentally. We perform microwave spectroscopy to measure the light shift and show that the shift vanishes at the magic frequency. We confirmed the coherence in 1 ms timescale by measuring the Ramsey fringe contrast.
The thesis reports transferring the ultracold RbCs molecules to the absolute ground state and a trapping light that eliminates the differential AC Stark shift. We build a laser system to perform Stimulated Raman Adiabatic Passage (STIRAP) for the molecules trapped in tweezers, which transfer the molecules from the weakly- bounded state to the absolute ground state. We use the offset cavity stabilisation technique to stabilise the laser frequencies to a passive cavity to narrow the laser linewidths while keeping the laser frequency tunable. We then align the beams to the molecules. We demonstrate a “magic” laser frequency to eliminate the differential AC Stark shift of the rotational states in the ground vibrational state of RbCs molecule and to increase the rotational coherence time of multiple states. We study the AC Stark effect caused by the trapping light. We calculate the polarisabilities of the rotational states near a narrow electronic from X ^1Σ^+ state to b^3Π_0 state and found at certain laser frequency the polarisabilities become the same. We measure the transition frequencies to different vibrational states in the excited electronic state experimentally. We perform microwave spectroscopy to measure the light shift and show that the shift vanishes at the magic frequency. We confirmed the coherence in 1 ms timescale by measuring the Ramsey fringe contrast.
This thesis reports the creation and trapping of 87RbCs molecules in the absolute ground state with a temperature of 1 uK. We build a tunable narrow-linewidth laser system at 1550 nm and 980 nm, using a single high-finesse optical cavity as a reference for both colours. We use fibre-coupled electro-optic modulators to continuously tune both lasers. These allow a novel measurement of the free spectral range of the cavity to better than 1 part in 10^6. We perform one- and two-photon spectroscopy on 87RbCs Feshbach molecules and identify a suitable intermediate state for transfer to the molecular ground state. We measure the electric dipole moment of the molecular ground state as 1.225(3)(8) D, and demonstrate the highest lab-frame dipole moment of any ultracold molecular system at the time of measurement. We transfer the molecules to the electronic, rovibrational and hyperfine ground state using stimulated Raman adiabatic passage, with 88% efficiency. We measure the transition strengths and excited state linewidth for this transfer route. We develop a model for the transfer which includes the effect of laser linewidth, and find excellent agreement with experimental data. The molecular sample is trapped in an optical dipole potential, and has a lifetime of 0.89(6) s. We reference the STIRAP lasers to a novel design of frequency comb which uses difference frequency generation to cancel the carrier-envelope offset. We use this to measure the binding energy of the molecules as h x 114 268 135.24(4)(3) MHz. To our knowledge, this is the most precise determination of the dissociation energy of a molecule to date. Finally, we report progress toward loading the molecules into a 1D optical lattice at 1064 nm. We develop the tools and methods to characterise a lattice, and demonstrate trapping of Feshbach molecules in both a 1D optical lattice and a harmonic optical potential at 1064 nm.
This thesis reports the development of theoretical and experimental tools to understand and control the coupled A1Σ+ − b3Π0 system in ultracold 87Rb133Cs molecules, with applications in magic trapping and direct imaging of molecules. Building on detailed modelling of molecular structure and light–matter interactions, the work establishes conditions for long-lived rotational coherence and explores pathways towards quantum simulation and computation with RbCs. High-resolution spectroscopy of lowest vibrational levels in the coupled complex is performed, yielding vibrational splittings, rotational and hyperfine coupling constants, transition dipole moments and natural linewidths. Spectroscopy of a highly excited vibrational level of the complex is also performed, with the polarisation-dependent transition strengths and polarisabilities measured. These results demonstrate strong anisotropy of the initial state and enables direct detection of molecules. We then develop and validate a comprehensive description of rotationally magic trapping in RbCs. A general theory of dipole polarisability is applied near the transition from the rovibrational ground state in the X1Σ+ potential to the lowest vibrational states of the b3Π0+ potential, predicting rotationally magic frequencies at which differential polarisabilities between rotational states vanishes. Experimentally, we build and characterise a new magic-wavelength trapping system based on a 1145 nm laser. Measurements of magic frequencies for spinstretched superpositions, combined with long coherence times in the magic trap, confirm the model. Dephasing due to dipole–dipole interactions are observed, while long-lived rotational coherence is demonstrated with non-dipolar states. Extending the theory and experiments to non-stretched states by diagonalising the full AC Stark Hamiltonian shows nonlinear Stark shifts driven by laser-induced hyperfine couplings and identifies local magic conditions where intensity gradients vanish, enabling tailored coherence protection across a wide range of internal state.
Ultracold polar molecules have become the subject of burgeoning fields of research in recent years owing to their prospects in quantum simulation, quantum computation and precision measurement. Their relatively large ground state dipole moment, coupled with a complex internal structure forms an incredibly powerful toolbox in which to perform quantum science experiments. Unfortunately, spatial confinement using light, which is necessary for many of the aforementioned applications, causes differential ac Stark shifts between quantum states, limiting coherence times and stifling their applicability. By accessing a so-called magic condition, this effect can be eliminated. This thesis presents work towards producing a quantum simulator based on a bulk gas of 87Rb133Cs molecules. We develop a magic wavelength trap by exploring nominally forbidden transitions from the X1Σ + ground state to b 3Π0 states. By tuning our trapping laser between transitions to different vibrational states of b3Π0, we can arbitrarily tune the difference in polarisability between pairs of rotational states and engineer second-scale coherence times. When we trap our molecules in a superposition of rotational states that exhibit a dipole moment in the laboratory frame, we observe the effects of long-range dipole-dipole interactions between molecules. These dipole-dipole interactions form the basis for quantum simulation and computation applications, observations of which marks an important milestone for realising a quantum simulator using 87Rb133Cs. We then demonstrate a route to ground state 87Rb133Cs molecules that is compatible with a protocol for loading Feshbach molecules into an optical lattice, developed by researchers at the University of Innsbruck. This method can be combined with a magic wavelength trap to produce a sample of 87Rb133Cs molecules in a magic lattice. Finally, we engineer synthetic dimensions that simulate simple single-particle Hamiltonians by coupling multiple rotational states in 87Rb133Cs. This forms the foundations for utilising rotational states of diatomic molecules as a platform for exploring synthetic dimensions
RbCs molecules in the electronic, rovibrational and hyperfine ground state, using stimulated Raman adiabatic passage to transfer the molecules from a weakly bound Feshbach state. We measure one-way transfer efficiencies of 92(1)% and fully characterise the strengths and linewidths of the transitions used. We model the transfer, including a Monte Carlo simulation of the laser noise, and find this matches well with both the transfer efficiency and our previous measurements of the laser linewidth and frequency stability.
Polar molecules offer a new platform for quantum simulation of systems with long-range interactions, based on the electrostatic interaction between their electric dipole moments. Here, we report the development of coherent quantum state control using microwave fields in 40 Ca 19 F and 87 Rb 133 Cs molecules, a crucial ingredient for many quantum simulation applications. We perform Ramsey interferometry measurements with fringe spacings of ∼1 kHz and investigate the dephasing time of a superposition of N = 0 and N = 1 rotational states when the molecules are confined. For both molecules, we show that a judicious choice of molecular hyperfine states minimises the impact of spatially varying transition-frequency shifts across the trap. For magnetically trapped 40 Ca 19 F we use a magnetically insensitive transition and observe a coherence time of 0.61(3) ms. For optically trapped 87 Rb 133 Cs we exploit an avoided crossing in the AC Stark shifts and observe a maximum coherence time of 0.75(6) ms.
Coherence and Collisions in Ultracold 87Rb133Cs Molecules
This thesis presents work towards the development of a quantum simulator based on 87Rb133Cs molecules using a bulk sample of up to ~4000 molecules at temperatures of ~1 µK. We demonstrate coherent control over the molecules' internal state using resonant microwave fields. We test this coherence by performing high resolution Ramsey spectroscopy of the first rotational transition and observing how the contrast of the spectroscopic fringes decay. We are able to affect the coherence of the superposition using an external laser field, as the two component states have a significant differential AC Stark shift. We extend this microwave control by including an additional microwave field, demonstrating the Autler-Townes effect with hyperfine state resolution and transfer between two hyperfine states in N=0. We study the internal structure of the molecule in the presence of external fields focussing on controlling the differential AC Stark shift between the rotational ground state and the first excited state. We investigate the effect of the off-resonant light on the internal structure in static magnetic and electric fields. In the DC electric field we use two models to describe the molecular structure, demonstrating that at modest values of the electric field the nuclear spin angular momentum is decoupled from the rotational angular momentum of the molecule, this enables us to construct a set of optimum trapping parameters for states with MN=0. Through careful measurements of the molecular lifetime we are able to determine that molecular losses are limited by two-body collisions. By introducing a time dependent intensity modulation to our optical trap we are further able to determine that the dominant loss process in bulk samples of ultracold 87Rb133Cs molecules is the laser excitation of collision complexes. Finally, we investigate the collisional properties of 87Rb133Cs with 87Rb and 133Cs atoms. We find that the loss is caused by single molecule-single atom collisions and sub-universal over a wide range of magnetic fields.