Realizing an efficient quantum optical interface for multi-qubit systems would enable their quantum network integration and the scaling-up of the qubit number via distributed quantum computing. This thesis presents the building blocks for integrating a quantum processor based on a Rydberg atom array with a nanophotonic cavity as an optical interface. In our approach, the integration would be achieved by coherently transporting tweezer-trapped atoms between the cavity near-field and free space. To that end, we first characterize the strong coupling between single photons and atoms enabled by the nanophotonic device. Two 87Rb atoms are trapped in optical tweezers and brought to the cavity near-field, while the cavity is probed in reflection. We observe Purcell enhancement and cavity-mediated interactions between the atoms, corresponding to a single atom cooperativity of C = 70 in the mode center. The qubits are encoded in atomic hyperfine ground states, which have good coherence properties even when the atoms are trapped in the cavity near-field. We create an atomic Bell state by pairing up the coherent manipulation of atomic qubits with a quantum nondemolition measurement, implemented via a single photon reflection from the cavity. Crucial to the integration, we show that single atom coherence and two-atom entanglement can be preserved as the atoms are moved away from the cavity. The nanophotonic dielectric surface can host charges which affect highly polarizable Rydberg states, and this influence is studied by measuring the n = 70 Rydberg state coherence at different distances from the cavity. Our measurements suggest that a Rydberg array may be placed ∼ 200 μm away from a nanophotonic device while retaining high quantum gate fidelities. Our methods can be applied to other atomic species and device geometries, paving the path to quantum networks of Rydberg array processors.
Realizing an efficient quantum optical interface for multi-qubit systems would enable their quantum network integration and the scaling-up of the qubit number via distributed quantum computing. This thesis presents the building blocks for integrating a quantum processor based on a Rydberg atom array with a nanophotonic cavity as an optical interface. In our approach, the integration would be achieved by coherently transporting tweezer-trapped atoms between the cavity near-field and free space. To that end, we first characterize the strong coupling between single photons and atoms enabled by the nanophotonic device. Two 87Rb atoms are trapped in optical tweezers and brought to the cavity near-field, while the cavity is probed in reflection. We observe Purcell enhancement and cavity-mediated interactions between the atoms, corresponding to a single atom cooperativity of C = 70 in the mode center. The qubits are encoded in atomic hyperfine ground states, which have good coherence properties even when the atoms are trapped in the cavity near-field. We create an atomic Bell state by pairing up the coherent manipulation of atomic qubits with a quantum nondemolition measurement, implemented via a single photon reflection from the cavity. Crucial to the integration, we show that single atom coherence and two-atom entanglement can be preserved as the atoms are moved away from the cavity. The nanophotonic dielectric surface can host charges which affect highly polarizable Rydberg states, and this influence is studied by measuring the n = 70 Rydberg state coherence at different distances from the cavity. Our measurements suggest that a Rydberg array may be placed ∼ 200 μm away from a nanophotonic device while retaining high quantum gate fidelities. Our methods can be applied to other atomic species and device geometries, paving the path to quantum networks of Rydberg array processors.
Platforms consisting of neutral atoms individually trapped in arrays of optical tweezers have achieved important milestones in quantum simulation and quantum computation. With access to programmable Rydberg interactions, individual qubit control, long coherence times, and naturally identical qubit transitions, neutral atoms are a promising building-block for the use in future quantum machines. However, they face the same challenge as other quantum platforms in scaling the number of qubits needed to perform algorithms with a provable advantage over classical computers. An avenue toward scaling systems of neutral atoms may be to use a quantum network that connects individual systems to construct a distributed computing architecture. Key to this approach is for the atoms to have access to an efficient optical interface, where the quantum information can be mapped to a single photon and sent across a quantum network. This thesis explores using a nanophotonic crystal cavity to act as this interface. With our device, we have demonstrated high-cooperativity coupling of two atoms to the photonic mode, generated entanglement at the device using a projective measurement, and coherently transported an entangled pair of atoms away from the device. Furthermore, we have taken the first steps toward integrating this device with Rydberg-excited atoms. We have found that the nanoscale dielectric device creates a point-charge electric field that minimally perturbs ground-Rydberg qubit coherence at distances greater than 200 microns. Using the Rydberg blockade effect, we have created an entangled pair of atoms to perform entanglement-assisted sensing of the electric field to better understand its control. Overall, these experiments show how this nanophotonic device is a promising candidate to use as an optical interface for a Rydberg atom-based quantum computing node.
Arrays of neutral atoms trapped in optical tweezers have emerged as a leading platform for quantum information processing and quantum simulation due to their scalability, reconfigurable connectivity, and high-fidelity operations. Individual atoms are promising candidates for quantum networking due to their capability to emit indistinguishable photons that are entangled with their internal atomic states. Integrating atom arrays with photonic interfaces would enable distributed architectures in which nodes hosting many processing qubits could be efficiently linked together via the distribution of remote entanglement. However, many atom array techniques cease to work in close proximity to photonic interfaces, with atom detection via standard fluorescence imaging presenting a major challenge due to scattering from nearby photonic devices. Here, we demonstrate an architecture that combines atom arrays with up to 64 optical tweezers and a millimeter-scale photonic chip hosting more than 100 nanophotonic cavities. We achieve high-fidelity ( ~ 99.2%), background-free imaging in close proximity to nanofabricated cavities using a multichromatic excitation and detection scheme. The atoms can be imaged while trapped a few hundred nanometers above the dielectric surface, which we verify using Stark shift measurements of the modified trapping potential. Finally, we rearrange atoms into defect-free arrays and load them simultaneously onto the same or multiple devices.
The realization of an efficient quantum optical interface for multi-qubit systems is an outstanding challenge in science and engineering. Using two atoms in individually controlled optical tweezers coupled to a nanofabricated photonic crystal cavity, we demonstrate entanglement generation, fast nondestructive readout, and full quantum control of atomic qubits. The entangled state is verified in free space after being transported away from the cavity by encoding the qubits into long-lived states and using dynamical decoupling. Our approach bridges quantum operations at an optical link and in free space with a coherent one-way transport, potentially enabling an integrated optical interface for atomic quantum processors.
Efficient interfaces between photons and memory qubits constitute fundamental build- ing blocks for quantum networking and large-scale quantum information processing. Our approach utilizes a photonic crystal cavity to realize such optical interfaces for atoms in optical tweezers. With this platform, we observe strong coupling between two atoms mediated by the cavity. Combining this observation with coherent manipulation and non-destructive measurements, we implement a protocol for generating Bell pairs that remain entangled when transported away from the cavity structure. These results present prospects for building integrated optical interconnects for neutral atom quan- tum information processors, adding capabilities such as rapid non-destructive readout and flexible connectivity to the rapidly-growing platform.
Scalable quantum computers and quantum networks require the combination of quantum processing nodes with efficient light-matter interfaces to distribute quantum information in local or long-distance quantum networks. Neutral-atom arrays have both been coupled to Rydberg states to enable high-fidelity quantum gates in universal processing architectures, and to optical cavities to realize interfaces to photons. However, combining these two capabilities and coupling atom arrays to highly excited Rydberg states in the mode of an optical cavity has been an outstanding challenge. Here we present a novel cavity-coupled Rydberg array that achieves this long-standing goal. We prepare, detect, and control individual atoms in a scalable optical tweezer array, couple them strongly to the optical mode of a high-finesse optical cavity and excite them in a controlled way to Rydberg states. We show that high-cooperativity coupling to an optical cavity -- demonstrated via the dispersive shift of the resonance of the cavity in presence of the atoms -- and strong Rydberg interactions -- demonstrated via the collective enhancement of Rydberg coupling in the atomic array -- can be achieved in our setup at the same spatial location. Our presented experimental platform opens the path to several new directions, including the realization of quantum network nodes, quantum simulation of long-range interacting, open quantum systems and photonic-state engineering leveraging high-fidelity Rydberg control.
Underlying matter and light are their building blocks of tiny atoms and photons. The ability to control and utilize matter-light interactions down to the elementary single atom and photon level at the nano-scale opens up exciting studies at the frontiers of science with applications in medicine, energy, and information technology. Of these, an intriguing front is the development of quantum networks where N >> 1 single-atom nodes are coherently linked by single photons, forming a collective quantum entity potentially capable of performing quantum computations and simulations. Here, a promising approach is to use optical cavities within the setting of cavity quantum electrodynamics (QED). However, since its first realization in 1992 by Kimble et al., current proof-of-principle experiments have involved just one or two conventional cavities. To move beyond to N >> 1 nodes, in this thesis we investigate a platform born from the marriage of cavity QED and nanophotonics, where single atoms at ~100 nm near the surfaces of lithographically fabricated dielectric photonic devices can strongly interact with single photons, on a chip. Particularly, we experimentally investigate three main types of devices: microtoroidal optical cavities, optical nanofibers, and nanophotonic crystal based structures. With a microtoroidal cavity, we realized a robust and efficient photon router where single photons are extracted from an incident coherent state of light and redirected to a separate output with high efficiency. We achieved strong single atom-photon coupling with atoms located ~100 nm near the surface of a microtoroid, which revealed important aspects in the atom dynamics and QED of these systems including atom-surface interaction effects. We present a method to achieve state-insensitive atom trapping near optical nanofibers, critical in nanophotonic systems where electromagnetic fields are tightly confined. We developed a system that fabricates high quality nanofibers with high controllability, with which we experimentally demonstrate a state-insensitive atom trap. We present initial investigations on nanophotonic crystal based structures as a platform for strong atom-photon interactions. The experimental advances and theoretical investigations carried out in this thesis provide a framework for and open the door to strong single atom-photon interactions using nanophotonics for chip-integrated quantum networks.