Quantum networking at telecom wavelengths with Yb qubits

Arrays of single ultracold neutral atoms, each trapped in so-called optical tweezers are emerging as a powerful platform for quantum computing. By clever application of laser beams we can fully control and detect the quantum state of each individual atom. Alkali atoms with a single outer electron are often used for these experiments, but atoms with two outer electrons like ytterbium (Yb) provide exciting new possibilities, owing to the existence of multiple long-lived states that can each be used to store quantum information, so called qubits. Each qubit is manipulated using light of a different wavelength, which makes moving information between these different types of qubits a powerful tool to selectively perform operations on a subset of qubits. Meanwhile other information is hidden away in another qubit type and protected from the lasers used in the operation. These systems are currently being rapidly adopted internationally but are not yet established in the UK. This is partly because the added capabilities come at the cost of laser wavelengths that are less convenient to generate and traditionally required complicated and bulky setups. In this project, I will address three bottlenecks to the adoption and scaling of quantum computing with Yb lasers: complexity of laser systems, the footprint of devices and quantum networking. I will develop a complete laser system for control of Yb qubits based on a single laser technology. I will also investigate novel sources for ultracold Yb that are smaller and use less power than conventional designs. In achieving these objectives I will lower the barrier to the adoption of Yb by decreasing the complexity of the required setups. A key feature making quantum computing so powerful is entanglement. Entanglement generation between two qubits usually requires their close proximity. However, quantum networking and the creation of large computing systems consisting of many processing nodes require the generation of entanglement between spatially separated qubits. This is possible by generating entanglement between stationary qubits and photons as flying qubits. By performing measurements on a pair of photons we can then generate entanglement between the atoms that emitted these photons. I will develop an optical network interface for Yb and demonstrate the generation of entanglement between two Yb qubits over conventional optical fibres used in telecommunication networks. To efficiently collect single photons emitted from the Yb atoms I will place them into micro cavities formed by the machined and coated endfaces of optical fibres. The presence of such cavities causes the atoms to preferentially emit photons in their direction, so that the photons can be coupled into optical fibres with much larger efficiency than is possible otherwise. The specific structure of Yb atoms allows us to operate at a wavelength compatible with common optical telecommunication fibres and thus allows transmission over long distances. The development of a quantum networking interface that directly integrates with the promising Yb quantum computing platform is an important step to ensure the continued scaling of this technology by allowing the construction of distributed computing systems - analogous to conventional supercomputers consisting of many smaller processing units. Demonstrating a more compact and less complicated setup for working with ultracold Yb benefits scaling of quantum computers and removes barriers to commercialisation and field-deployed applications. Beyond quantum computing and networking this will benefit applications in quantum sensing and optical clocks.