This thesis develops and exploits a room-temperature platform for quantum simulation with optically trapped circular Rydberg states of rubidium. The core of the work is a single-element hybrid architecture in which circular states encode the data degree of freedom, while optically accessible low-l Rydberg ancillae enable local addressing, conditional operations, and readout. These capabilities rely on operation near a Förster resonance. A first part of the thesis addresses a major scalability bottleneck of wide-field tweezer arrays: optical aberrations that degrade trap quality away from the optical axis. We present an atom-based method to calibrate a field-dependent wavefront-distortion model. This model is a central component of an array-generation algorithm developed previously within the team, enabling site-wise aberration correction at the programmed tweezer positions. Using the commercial aspheric lens integrated in our experimental setup, we obtain homogeneous, near-diffraction-limited traps over a 150 × 342 µm² field of view, ultimately limited by optical clipping. Building on these tools, we realize hybrid arrays combining bottle-beam traps for circular atoms and Gaussian tweezers for ancilla atoms. We then observe coherent resonant dipole-dipole spin-exchange (flip-flop) dynamics between two circular atoms prepared in |53C;54C⟩. The ancilla layer intervenes at three stages: it enables conditional preparation of the circular-atom pair state, contributes to a post-selection signal that identifies successful runs, and enables readout of the circular data-atom state. Because the ancilla is optically accessible, it provides local control and measurement of otherwise optically dark circular states. These results constitute a proof-of-principle for ancilla-enabled quantum simulation with circular Rydberg atoms in a hybrid platform. They motivate near-term upgrades that directly target the identified limitations, in particular cryogenic operation to access the long intrinsic lifetime of circular states, and efficient motional cooling to prepare atoms close to the motional ground state. By extending coherence times and reducing motion-induced damping, these upgrades would expand the accessible parameter regime and could enable the observation of hundreds of exchange oscillations with near-unit contrast. Combined with the site-resolved local control and non-destructive readout enabled by the ancilla layer, these upgrades would enable larger programmable arrays to probe slow many-body dynamics. With further improvements, for instance in fast and high-fidelity imaging, they would also pave the way for ancilla-assisted mid-circuit measurements.
This thesis develops and exploits a room-temperature platform for quantum simulation with optically trapped circular Rydberg states of rubidium. The core of the work is a single-element hybrid architecture in which circular states encode the data degree of freedom, while optically accessible low-l Rydberg ancillae enable local addressing, conditional operations, and readout. These capabilities rely on operation near a Förster resonance. A first part of the thesis addresses a major scalability bottleneck of wide-field tweezer arrays: optical aberrations that degrade trap quality away from the optical axis. We present an atom-based method to calibrate a field-dependent wavefront-distortion model. This model is a central component of an array-generation algorithm developed previously within the team, enabling site-wise aberration correction at the programmed tweezer positions. Using the commercial aspheric lens integrated in our experimental setup, we obtain homogeneous, near-diffraction-limited traps over a 150 × 342 µm² field of view, ultimately limited by optical clipping. Building on these tools, we realize hybrid arrays combining bottle-beam traps for circular atoms and Gaussian tweezers for ancilla atoms. We then observe coherent resonant dipole-dipole spin-exchange (flip-flop) dynamics between two circular atoms prepared in |53C;54C⟩. The ancilla layer intervenes at three stages: it enables conditional preparation of the circular-atom pair state, contributes to a post-selection signal that identifies successful runs, and enables readout of the circular data-atom state. Because the ancilla is optically accessible, it provides local control and measurement of otherwise optically dark circular states. These results constitute a proof-of-principle for ancilla-enabled quantum simulation with circular Rydberg atoms in a hybrid platform. They motivate near-term upgrades that directly target the identified limitations, in particular cryogenic operation to access the long intrinsic lifetime of circular states, and efficient motional cooling to prepare atoms close to the motional ground state. By extending coherence times and reducing motion-induced damping, these upgrades would expand the accessible parameter regime and could enable the observation of hundreds of exchange oscillations with near-unit contrast. Combined with the site-resolved local control and non-destructive readout enabled by the ancilla layer, these upgrades would enable larger programmable arrays to probe slow many-body dynamics. With further improvements, for instance in fast and high-fidelity imaging, they would also pave the way for ancilla-assisted mid-circuit measurements.
Among the thriving quantum computation and quantum simulation platforms based on arrays of Rydberg atoms, those using circular Rydberg atoms are particularly promising. These atoms uniquely combine the strong dipole-dipole interactions typical of Rydberg states with long lifetimes. However, low-angular-momentum ( ℓ ) laser-accessible Rydberg levels have been so far mostly used, because circular Rydberg atoms have no optical transitions, hindering their individual detection and manipulation. We remove this limitation with a hybrid platform, combining an array of data laser-trapped circular Rydberg atoms of rubidium with an auxiliary array of Rb ancilla atoms transiently excited to a low- ℓ Rydberg level. We perform a quantum nondemolition detection of the data qubit with the ancilla, through the blockade of the ancilla optical excitation induced by a Förster resonance. Conversely, we locally manipulate the data qubit through the excitation of the ancilla. This dual-Rydberg platform is highly promising for quantum computation and simulation. It adds to the circular-atom toolbox the midcircuit measurements, essential for error correction. More strikingly, it gives access to time correlations in long-term quantum simulations, uniquely accessible to circular Rydberg atoms.
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.
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.
Quantum simulation offers interesting perspectives for the understanding of complex systems that are out of reach for numerical approaches. In this line, a new quantum simulation platform has recently been proposed. A spin-1/2 XXZ hamiltonian is thus realized in which the parameters can be controlled over a large range. The spin states are encoded by circular Rydberg states coupled through dipole-dipole interaction. These states benefit from longer lifetimes compared to the low angular momentum states used for current quantum simulations. In addition, they can be laser-trapped through the ponderomotive effect, and their lifetime can be further enhanced by spontaneous emission inhibition. We present in this thesis the first step towards the implementation of the proposed quantum simulator. We first prepare an array of rubidium 87 atoms trapped individually in optical tweezers. These atoms are subsequently excited to the circular Rydberg state n = 52 and transferred to hollow bottle-beam traps. We thus demonstrate the ponderomotive trapping of these states over durations of several milliseconds. We take advantage of the optical detection of the states thus prepared with various experiments. We characterize the trapping potential by measuring its transverse frequency. We finally conclude with the coherent manipulation of these trapped circular states through Rabi oscillations and Ramsey interferometry to determine their coherence.
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 an optical tweezer array of 87 Rb atoms housed in an cryogenic environment that successfully combines a 4-K cryopumping surface, a <50-K cold box surrounding the atoms, and a room-temperature high-numerical-aperture objective lens. We demonstrate a 3000-s atom-trap lifetime, which enables us to optimize and measure losses at the 10 − 4 level that arise during imaging and cooling, which are important to array rearrangement. We perform both ground-state qubit manipulation with an integrated microwave antenna and two-photon coherent Rydberg control, with the local electric field tuned to zero via integrated electrodes. We anticipate that the reduced blackbody radiation at the atoms from the cryogenic environment, combined with future electrical shielding, should decrease the rate of undesired transitions to nearby strongly interacting Rydberg states, which cause many-body loss and impede Rydberg gates. This low-vibration, high-optical-access cryogenic platform can be used with a wide range of optically trapped atomic or molecular species for applications in quantum computing, simulation, and metrology.