This thesis details the development of the pre-requisite experimental tools to create a proof-of-principle spin-squeezed atomic clock based upon an array of individual strontium atoms using Rydberg-dressed interactions. We experimentally and theoretically study Rydberg-dressing in a strontium narrow-line MOT, demonstrating that it is possible to coherently admix a Rydberg state into the narrow intercombination transitions of strontium. This work is based upon a quantitative semi-classical Monte-Carlo model of a strontium narrow-line MOT, where the combination of a quantum treatment of the light scattering process with a Monte-Carlo simulation of the atomic motion leads to a quantitative description of the spatial, thermal and temporal dynamics of the narrow-line MOT. By performing calculations of the dynamic polarisability of all the states relevant to laser cooling strontium, we have designed and constructed a new experimental apparatus to facilitate the creation of a microtrap of strontium. We observe and characterise the frst known microtrap of strontium and outline the next steps towards the creation of an array of single atoms. Due to the creation of Rydberg atoms in the strontium microtrap, understanding ionisation and interaction mechanisms may be of signifcant importance. We therefore study Rydberg ionisation mechanisms in a thermal beam of strontium atoms using simultaneous measurements of Rydberg EIT and spontaneously created ions or electrons. By connecting the optical and electrical signals using the optical Bloch equations, we are able to determine the dominant ionisation mechanisms of Rydberg atoms in the thermal beam. We also report the frst observations of optical and electrical bistability, which may shed further light onto the origin of bistability in atom vapours.
This thesis details the development of the pre-requisite experimental tools to create a proof-of-principle spin-squeezed atomic clock based upon an array of individual strontium atoms using Rydberg-dressed interactions. We experimentally and theoretically study Rydberg-dressing in a strontium narrow-line MOT, demonstrating that it is possible to coherently admix a Rydberg state into the narrow intercombination transitions of strontium. This work is based upon a quantitative semi-classical Monte-Carlo model of a strontium narrow-line MOT, where the combination of a quantum treatment of the light scattering process with a Monte-Carlo simulation of the atomic motion leads to a quantitative description of the spatial, thermal and temporal dynamics of the narrow-line MOT. By performing calculations of the dynamic polarisability of all the states relevant to laser cooling strontium, we have designed and constructed a new experimental apparatus to facilitate the creation of a microtrap of strontium. We observe and characterise the frst known microtrap of strontium and outline the next steps towards the creation of an array of single atoms. Due to the creation of Rydberg atoms in the strontium microtrap, understanding ionisation and interaction mechanisms may be of signifcant importance. We therefore study Rydberg ionisation mechanisms in a thermal beam of strontium atoms using simultaneous measurements of Rydberg EIT and spontaneously created ions or electrons. By connecting the optical and electrical signals using the optical Bloch equations, we are able to determine the dominant ionisation mechanisms of Rydberg atoms in the thermal beam. We also report the frst observations of optical and electrical bistability, which may shed further light onto the origin of bistability in atom vapours.
This thesis describes Rydberg spectroscopy and dressing experiments in an ultracold strontium gas. The strontium atoms are cooled to sub-uK temperatures in a narrowline magneto-optical trap, where Rydberg atoms are created using a two-photon excitation scheme. This required the development of a high-power ultraviolet laser system at 319 nm. The laser has a large tuning range for access to triplet Rydberg states from principal quantum numbers of 35 to > 300. By performing Rydberg spectroscopy in a magneto-optical trap, we show that narrow spectra can be obtained where the line centre is determined to ~ 10 kHz. A frequency comb is then employed to make absolute frequency measurements of optical transitions to accuracies of < 2 MHz. Techniques are outlined for improving the accuracy further, showing that overall uncertainties on the order of 10 kHz can be obtained. The reliable measurement of Rydberg levels is important for studying the variation in quantum defect across Rydberg series, and hence improving the accuracy of atomic models. In this work we also develop a novel system in which to realise Rydberg dressing. By resonantly coupling the excited state of the cooling transition to a high lying Rydberg state, we create a Rydberg-dressed magneto-optical trap. We demonstrate that the atoms acquire Rydberg properties, while undergoing continuous cooling at ~ 1 uK. The lifetime of the trap is proved to be sufficiently long to observe interactions, however the interaction strength is currently limited by the non-uniform spatial profile of the dressing beam. Straightforward methods to overcome this limitation are presented. As such this work should lead to future experiments, whereby tuneable long-range interactions can be observed in the dynamics of the dressed magneto-optical trap.
This thesis details the first measurements of Rydberg dipole blockade in a cold ensemble of divalent atoms. Strontium atoms are cooled and trapped in a magneto-optical trap and coherently excited to Rydberg states in a two-photon, three-level ladder scheme. Owing to the divalent nature of strontium, one electron can be excited to the Rydberg state, whilst the other lower-lying electron is available to undergo resonant optical excitation to autoionising states, which ionise in sub-nanosecond timescales. The remaining ions that are recorded on a micro-channel plate are proportional to the number of Rydberg atoms. The development of a narrow linewidth laser system necessary for an additional stage of cooling is explained and characterised. Two frequency stabilisation schemes are discussed: one to address the short-term laser frequency instabilities based on the Pound-Drever-Hall technique; the other to address the long-term laser frequency instabilities based on Lamb-dip spectroscopy in an atomic beam. The cooling dynamics on the narrow cooling transition is studied experimentally and modelled via theoretical simulations.
Optical tweezer arrays of neutral atoms have emerged as a versatile and powerful platform for quantum simulation and quantum information processing. In this thesis, I present the realization of an experimental platform for quantum simulation based on individually trapped strontium atoms in optical tweezer arrays. The experiment combines precise laser cooling, optical control, and high-resolution imaging to enable the manipulation and high fidelity detection of single atoms. I developed and optimized a two-stage three-dimensional magneto-optical trap (3D-MOT) that produces cold and dense samples of strontium atoms from a thermal beam. In the f irst stage, atoms are captured and precooled using the dipole-allowed 1S0 → 1P1 transition at 461nm (which has a linewidth of !=2ω ↑ 32MHz). The second stage employs the 1S0 → 3P1 narrow intercombination transition at 689nm (with a linewidth of !=2ω ↑7.5kHz) to further cool and compress the atomic cloud. By optimizing the trapping parameters, I achieved clouds with temperatures around 5μK and densities above 1010cm→3, providing ideal initial conditions for e!cient loading into optical tweezers. A 3↑3 array of 360μK deep optical tweezers is generated by spatially modulating an 813.4nm laser beam using two crossed acousto-optic deflectors. The deflected beams are relayed and focused into the science chamber using a high–numericalaperture objective, which simultaneously serves as the collection optics for atomic f luorescence. The fluorescence light emitted by the atoms is imaged onto a scientific qCMOS camera, enabling site-resolved detection of individual atoms. I implemented and optimized the in-trap cooling dynamics to prevent atom loss during imaging and to minimize residual thermal motion. In addition, by precisely tuning lightassisted collisions, multiple atoms occupancies are removed from the traps and a single-atom regime is reached in the array, with a filling fraction of about 50%. I have demonstrated a single-atom detection fidelity of 99,986(1)% and a survival probability of 97(2)% between subsequent image acquisition, comparable to other current state-of-the-art experiments. To characterize the platform, I carried out release-and-recapture experiments, from which it is possible to measure a temperature for the individually trapped atoms of 12.95(5)μK. These results demonstrate the successful implementation of a robust and high-fidelity platform for optical trapping and imaging of single strontium atoms. The apparatus provides the foundation for future quantum simulation experiments with alkalineearth atoms excited to the Rydberg state, o"ering precise control over individual particles and the ability to scale towards larger, programmable atomic arrays.
Quantum simulation offers the possibility to study quantum mechanical problems which are untraceable on classical computers. This thesis introduces a novel platform for quantum simulation and presents the first experimental realisation of single potassium atoms trapped in optical tweezers. Interactions between individual atoms are induced by strong energy shifts of atoms excited to states with large principal quantum number, so-called Rydberg states. Either direct excitation to Rydberg states or off-resonant dressing can be used to induce these interactions. We argue that potassium is well-suited for the implementation of Rydberg dressing, enabling simultaneous dressing of both ground states to engineer complex interactions. Using techniques to cool and trap cold atoms, single potassium atoms are prepared in arrays of optical tweezers. Raman sideband cooling reduces vibrational excitations and prepares the atoms close to the motional ground state. This cooling technique mitigates severe limitations for Rydberg dressing, arising from thermal broadening and inhomogeneous light shifts in the array of atoms. To directly excite atoms to Rydberg states, a high power laser setup with two cavity-enhanced frequency doubling stages is constructed, generating up to one Watt of ulta-violet light at 286 nm. By exciting atoms to Rydberg states we observe Rabi oscillations with Rabi frequencies of up to 1 MHz, demonstrating coherent control of Rydberg atoms. Finally, we combine these techniques to observe interactions in two ways: First, we create a so-called superatom of up to four individual atoms using direct excitation and Rydberg-blockade and observe coherent oscillations to this collective state. We measure the expected square root scaling of the effective Rabi frequency with the number of individual atoms, confirming the creation of a many-body entangled state. Secondly, we off-resonantly dress the atoms prepared in a one-dimensional chain and measure correlated interaction shifts over multiple sites. In summary, we have developed a platform for quantum simulation with single atoms in optical tweezers. The presented results show coherent control of single atoms and interactions induced by Rydberg states. We have thus demonstrated that the system is well-suited for quantum simulation of many-body systems.
Atomic Caesium clocks have made an astonishing development in the past few decades, with them gaining an order of magnitude in precision every decade. A new generation of atomic optical clocks is now emerging, which is moving to the forefront of timekeeping. By going from microwave to optical frequencies these clocks are now reaching fractional uncertainty levels in the 10\\(^-\\)\\(^1\\)\\(^8\\), opening a whole new range of exciting applications. This thesis presents the design and implementation of an experiment for laser cooling of strontium atoms, focusing on the mobility of the setup, opening a range of new applications as a mobile clock. The experimental apparatus applies a range of new techniques for strontium, such as the use of a two dimensional magneto-optical trap for pre cooling atoms, allowing for a compact and transportable system. A mobile laser system for laser cooling strontium atoms was realized. This includes a frequency doubling system for the main cooling transition. The constructed monolithic frequency doubling cavity has generated 300 mW of blue light and proven to be a stable setup suitable for a mobile system. The 3D-MOT was able to capture 4 * 10\\(^5\\) atoms with a long lifetime of 5.5 s, with further improvements for a significant increase in atom number currently being implemented.
For frequency metrology experiments, the fractional uncertainty is a key figure of merit: generating correlations between atoms allows this value to improve up to linearly with the number of atoms being interrogated. This has been demonstrated elsewhere using long-range Rydberg interactions in arrays of single strontium atoms and ensembles of caesium atoms. In our experiment, we are working towards demonstrating spin squeezing long-range Rydberg interactions to generate entanglement between strontium atoms in mesoscopic (N < 5) ensembles, trapped in clock-magic arbitrary tweezer arrays. We are interested in investigating how the fractional uncertainty of spin-squeezed clock measurements varies with atom number, in the case that tweezer sites are not restricted to containing a single atom at most. We hope to determine the optimum average number of atoms per site within the Rydberg blockade radius as a function of number of sites. This thesis presents progress towards this goal, which has been made in two ways. First, we have developed a novel technique for loading vertically large (100 micron) tweezer arrays with uniform and linearly-varying distributions of atom numbers in as fast as 20 milliseconds. We are able to load individual sites with average populations of between one and ten atoms, and show variations in atom number of up to a factor of two between sites in the array. We outline a theoretical model which successfully predicts our experimental results. Additionally, we show the first measurement of the 5s2 1S0 → 5s5p 3P0 clock transition in our experiment. We present experimental measurements of this transition, investigating how the linewidth and linecentre of the measured frequency spectrum varies with trap depth, atom temperature, probe beam intensity, and atomic density. We consider theoretical models of the clock transition based on the motional states of atoms in a tweezer and on collisional effects, and use these to study the effects of motional dephasing on the measured spectrum in our experiment. With these results in hand, we look ahead to the future of our experiment.