Dougal Main

Abstract:

We present the first demonstration of velocity-selective pumping in an atomic vapour to preserve light-matter coherence. Control is illustrated by a subsequent demonstration of an atomic frequency comb quantum memory realised in the vapour.

Abstract:

Distributed quantum computing (DQC) combines the computing power of multiple networked quantum processing modules, ideally enabling the execution of large quantum circuits without compromising performance or connectivity. Photonic networks are well-suited as a versatile and reconfigurable interconnect layer for DQC; remote entanglement shared between matter qubits across the network enables all-to-all logical connectivity via quantum gate teleportation (QGT). For a scalable DQC architecture, the QGT implementation must be deterministic and repeatable; until now, no demonstration has satisfied these requirements. In this thesis, we report the first experimental demonstration of distributed quantum computing across a quantum network of two photonically interconnected trapped-ion modules.

Our experimental apparatus comprises two trapped-ion modules, separated by ∼ 2 m, each co-trapping one ⁴³Ca⁺ ion and one ⁸⁸Sr⁺ ion. The ⁴³Ca⁺ ions provide a robust quantum memory for the modules, while the ⁸⁸Sr⁺ ions provide an interface to the optical quantum network, enabling the remote entanglement of ⁸⁸Sr⁺ ions in separate modules. Using local mixed-species operations, we demonstrate the enhanced functionality of the mixed-species trapped-ion quantum network by creating more complex remotely entangled states. These include remote ⁸⁸Sr⁺- ⁴³Ca⁺ and ⁴³Ca⁺-⁴³Ca⁺ entanglement, as well as 3- and 4-qubit mixed-species GHZ states. Additionally, we demonstrate the storage of remote entanglement in the ⁴³Ca⁺ ions for up to 10 s.

Utilising these techniques, we demonstrate the distribution of quantum computations across the mixed-species trapped-ion quantum network. Using heralded remote entanglement between the ⁸⁸Sr⁺ ions, we deterministically teleport controlled Z gates between two ⁴³Ca⁺ ions in separate modules, achieving a fidelity of 86 %. We then implement distributed iSWAP and SWAP circuits, compiled with 2 and 3 instances of QGT, respectively, demonstrating the ability to distribute arbitrary two-qubit operations. Finally, we execute Grover’s search algorithm – the first implementation of a distributed quantum algorithm comprising multiple non-local two-qubit gates – and measure a success rate of 71 %. These results represent a significant step towards realising a large-scale distributed quantum computer.