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.
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.
Neutral atoms trapped and manipulated by laser light provide experimentally well accessible quantum systems allowing for a high degree of control over external and internal degrees of freedom. Arrays of dipole traps in which the atoms are confined individually in a configurable geometry constitute a versatile platform for quantum simulation and information applications. By exciting these atoms into Rydberg states, interactions of variable strength and range can be introduced into the system, allowing for the implementation of entangling gate operations or spin Hamiltonians. A crucial requirement for these schemes to function in a reliable way is the ability to create defect-free arrays of single atoms. This is generally a challenge in these types of systems, as common atom loading schemes are limited to roughly 50% probability of filling each site. In this work, a technique for the rearrangement of atoms within a scalable architecture based on micro-optical lens arrays was developed and implemented, resulting in the creation of uniformly filled regions containing more than 100 atoms, which represent the largest defect-free structures realized so far in systems of this kind. This was accomplished by filling the empty traps in a pre-defined pattern with an atom one by one using an optical tweezer. Thus, structures with up to 5 x 5 atoms could be rendered defect-free in more than 99% of attempts. Although the success rate drops below unity for larger clusters, a value of 3.1% for a 100-atom structure is still viable for experiments working with post-selection methods. The filling fraction of even the largest examined structures was observed to be higher than 88%, surpassing common loading schemes by a significant margin. The measurements presented in this thesis build on a region of the array containing 361 sites, being limited by available laser power. In contrast, the addressable range of the optical tweezer includes more than 1500 sites and microlens arrays with up to a million lenses are commercially available. By implementing coherent Rydberg excitation of this assembled atom array, significant progress toward a universal quantum computer or flexible quantum simulator has been made. Using a two-photon excitation scheme, coherent dynamics between the ground and Rydberg state could be observed simultaneously in a 5 x 5 region of the array, with two-photon Rabi frequencies on the order of Omega = 2Pi x 500kHz measured for a Rydberg laser beam waist of w_0,B = 18.7(10) µm. The choice of an appropriate Rydberg state and interatomic spacing led to the presence of strong nearest-neighbor interactions and allowed for the demonstration of the Rydberg blockade effect by observing a collective enhancement of the Rabi frequency consistent with the expected scaling ~ sqrt(N) as well as the suppression of multiple excitations. This mechanism represents the fundamental constituent of a two-qubit gate operation. The architecture introduced in this work offers a scalability unique among quantum simulation platforms and the presented results underpin its potential to propel the atom-optical approach for quantum information processing beyond the threshold of quantum supremacy. Different approaches for scaling up the system have been explored, indicating that defect-free structures of more than 1000 atoms are within range with feasible experimental improvements. Through a detailed analysis of the factors limiting the coherence of the observed dynamics, strategies for future experimental improvements have been developed. Among these, increasing the coupling strength to the Rydberg state into the megahertz regime by increasing laser power and implementing single-site addressing represents the most straight-forward and promising approach.
We present a new technique for achieving high levels of squeezing in strontium optical lattice clocks using the long-range van der Waals interactions between high-lying Rydberg states. This technique requires only one additional laser and could provide over 10 dB of squeezing in large atomic ensembles, in just a few microseconds, without significantly affecting the subsequent clock interrogation. We outline this spin-squeezing technique and report on our experimental progress towards demonstrating this method. Significant experimental milestones include development of a versatile UV laser system for demonstrating this technique, and the first observation of the Rydberg blockade effect in a divalent atom.
Strontium optical lattice clocks at JILA recently demonstrated record-high accuracy and stability. These advances were enabled by an ultrastable laser with fractional frequency stability of 1x10-16 at 1 second. This laser allows us to study systematic shifts of the 1S0 to 3P0 clock transition with unprecedented precision. Density-dependent frequency shifts represent an unavoidable perturbation for clocks based on many atoms. Our studies of atomic interactions in an optical lattice clock system uncover the nature of these interactions and reveal important many-body atomic correlation effects in this open quantum system. By extending our measurements to all ten nuclear-spin sublevels of the clock states, we observe the first direct evidence of SU(N) symmetric interactions in alkaline earth(-like) atoms. Using the techniques we developed in these studies, we also demonstrate a novel technique for measuring the frequency noise spectrum of an ultrastable laser. We discuss designs for the future direction of our experiment which will place 87Sr atoms within a high finesse cavity that is resonant on the 1S0 to 3P1 transition. We will use this system to study collective effects in cavity quantum electrodynamics. Most notably, strong atom-cavity coupling can enhance the spectroscopic sensitivity of our clock by creating spin-squeezed states via quantum non-demolition measurements. As a precursor to future work studying cavity-mediated collective behavior, we use the unique atomic structure of 88Sr to investigate free-space retarded dipolar coupling in an optically thick sample of atoms.
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.
Circular Rydberg atoms, with their long lifetimes and adjustable strong dipolar interactions, are promising candidates for the implementation of quantum simulations in regimes that were previously out of reach. A new quantum simulator paradigm, based on qubits encoded in circular states, would implement an XXZ spin-1/2 Hamiltonian with dynamically adjustable parameters over a wide range of values. This thesis addresses the preliminary steps in constructing such a quantum simulator. First, we present the realization of two-dimensional arrays of optically-trapped circular Rydberg atoms with arbitrary geometries. For this purpose, we prepare arrays of ultra-cold rubidium atoms, individually trapped in optical tweezers. An additional beam with adjustable position allows us to rearrange the atoms within the array to implement the desired geometry in a deterministic manner. We then excite the atoms to the circular state with principal quantum number n=52, while trapping them in hollow beams using the ponderomotive force. These experimental capabilities allowed us to control and characterize, for the first time, the dipolar interactions between circular states, within pairs of individually trapped atoms. We showed the good agreement of these measurements with theoretical expectations and highlighted their dependence on the system's geometry. This experimental study is coupled to an investigation of the motion of the circular atoms within their traps.
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.