Quantum devices are rapidly gaining momentum as a technology that will induce a paradigm shift in computing, communication and cryptography. Trapped ion qubits are one of the leading candidates for implementing a quantum computer, having previously demonstrated all of the required criteria. Local gate fidelities between ions exceed those for all other platforms, but the total number of ions in a trap is limited by unavoidable issues – one possibility for scaling the ion trap quantum processor is to create entanglement between ions in separate traps via single photons. The work in this thesis demonstrates the generation of remote entanglement between ions at high fidelity and rate, paving the way towards protocols using multiple entangled pairs for computations. We describe the construction from scratch of a twin-trap apparatus designed to entangle separated ions. We demonstrate the entanglement of strontium ions in traps separated by ∼ 2 m by swapping entanglement from single photons emitted by the 88Sr+ ions. A novel photon collection geometry is used, maximising the entanglement between the ion and photon without impeding optical access for standard ion trap laser beam geometries. The Bell state fidelity of the ion-photon state, which is itself a valuable entanglement resource for blind computation, is at least 97.70(12) % in either trap, with entangled pairs detected at a rate of at least 3.98 × 103 s−1. The remote ion-ion Bell state fidelity is 94.0(5) %, and is generated at a rate of 182 s−1, representing the highest fidelity remote entanglement reported in ions by a large margin, at a rate more than an order of magnitude faster than previous experiments. The two identical trap systems are designed with capabilities beyond those demonstrated here. We can co-trap 88Sr+ and 43Ca+ in a microfabricated trap suitable for local ion transport operations, opening up the possibility of performing distillation of remote entanglement in the same apparatus. Entanglement distillation can be used to make remote entanglement with similar fidelity to local operations, and would represent a significant step towards a fully scalable ion trap quantum computer architecture.
Quantum devices are rapidly gaining momentum as a technology that will induce a paradigm shift in computing, communication and cryptography. Trapped ion qubits are one of the leading candidates for implementing a quantum computer, having previously demonstrated all of the required criteria. Local gate fidelities between ions exceed those for all other platforms, but the total number of ions in a trap is limited by unavoidable issues – one possibility for scaling the ion trap quantum processor is to create entanglement between ions in separate traps via single photons. The work in this thesis demonstrates the generation of remote entanglement between ions at high fidelity and rate, paving the way towards protocols using multiple entangled pairs for computations. We describe the construction from scratch of a twin-trap apparatus designed to entangle separated ions. We demonstrate the entanglement of strontium ions in traps separated by ∼ 2 m by swapping entanglement from single photons emitted by the 88Sr+ ions. A novel photon collection geometry is used, maximising the entanglement between the ion and photon without impeding optical access for standard ion trap laser beam geometries. The Bell state fidelity of the ion-photon state, which is itself a valuable entanglement resource for blind computation, is at least 97.70(12) % in either trap, with entangled pairs detected at a rate of at least 3.98 × 103 s−1. The remote ion-ion Bell state fidelity is 94.0(5) %, and is generated at a rate of 182 s−1, representing the highest fidelity remote entanglement reported in ions by a large margin, at a rate more than an order of magnitude faster than previous experiments. The two identical trap systems are designed with capabilities beyond those demonstrated here. We can co-trap 88Sr+ and 43Ca+ in a microfabricated trap suitable for local ion transport operations, opening up the possibility of performing distillation of remote entanglement in the same apparatus. Entanglement distillation can be used to make remote entanglement with similar fidelity to local operations, and would represent a significant step towards a fully scalable ion trap quantum computer architecture.
Harnessing quantum mechanics to solve problems intractable on classical computers is one of the foremost goals of applied physics. Trapped ions make an excellent physical platform for such a quantum computer, as they have demonstrated unparalleled qubit operation fidelities and memory times, but significant challenges remain in scaling up these systems. A promising scalable architecture is a photonically connected network of mixed element trapped ion nodes, each containing a few ions of two species. Poor quality remote entanglement links between nodes can be tolerated, if entanglement between ions in the same node can be achieved with high fidelity. While local gates have been repeatedly demonstrated between ions of the same species, the fidelity of mixed element quantum gates have not previously surpassed the fault tolerant threshold, above which errors can in principle be corrected. In this thesis we demonstrate an entangling operation between 43Ca+, useful for its long lived qubits, and 88Sr+, well suited for remote entanglement, using a novel mechanism requiring only one laser to manipulate both species. This relatively simple scheme, which is independent of either qubit frequency, allows us to achieve Bell state fidelities of F = 99.8(1)%. We assess the operation of the gate further with state and process tomography, which achieve Bell state fidelity F = 98.1% and average gate fidelity F̅ = 99.05(6)% respectively, though they are hampered by experimental drifts during the long data collection. Finally, we use interleaved randomised benchmarking to get the best estimate of our gate errors, finding an average gate fidelity of F̅ = 99.62(3)% for a sequence of 60 interleaved gates. Along with a demonstration of entanglement between two 88Sr+ ions, achieved with fidelity F = 95.9(4)%, this work paves the way towards demonstrating an elementary mixed species quantum network in future experiments.
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.
Modularity underpins classical computing; as quantum processors encounter limits on fabrication yield, reliability, and size, they will also need it acutely. The bottleneck to linking modules is producing shared entanglement at sufficient rate, density, and fidelity. Trapped ions hold the best demonstrated photonic links, yet they rely on bulky collection optics that cap how densely links can be packed, and remote entanglement operations trail local gates by two orders of magnitude in rate and fidelity. We synthesize several enabling results $\unicode{x2014}$ single-photon heralding, coherent recoil correction, projective distillation, and trap-integrated photonics $\unicode{x2014}$ into one comprehensive architecture that substantially narrows this gap. Single-photon heralding leads to linear scaling of success probability with detection efficiency, allowing compact integrated photonics to saturate the entanglement rate at a local-operation limit in dense, easy-to-parallelize channels. Addressing its inherent error mechanisms at their source, we project a Bell-pair fidelity of 99.9% at rates and densities compatible with fault-tolerant operations. Remote entanglement then need not remain the bottleneck for modular trapped-ion computing; the limit shifts to the local operations that must improve regardless.
Quantum computers have wide-ranging potential applications, many of which will require thousands or even millions of quantum bits to be useful. Current state-of-the-art universal quantum computers, on the other hand, contain only several tens of qubits, and scaling to larger system sizes remains one of the primary challenges. Among current quantum computing platforms, trapped ions are a leading hardware option. One proposal for scaling such systems consists of a modular architecture. The architecture consists of multiple nodes, each with an ion trap containing a communication qubit (138Ba+) and a memory qubit (171Yb+). The communication qubit is responsible for generating photons that link the remote nodes together via entanglement swapping while the memory qubits are used for storing information and performing local computations. We report progress towards demonstration of the remote entanglement of two barium ions. The creation of this link is a probabilistic process and fails much more often than it succeeds. The success rate does not impact the fidelity of the resulting entangled state but imposes significant constraints on the utility of this protocol. We examine the current limitations on both the fidelity of the resulting entangled state and the success probability. In addition to the two-node experiment, we have designed and built a new ion trap system that should yield much higher photon collection rates. This design represents a significant shift from previous systems because of the inclusion of optical elements inside the vacuum chamber and their resulting proximity to the ions. We incorporate two objective lenses with a numerical aperture of 0.8, each of which can collect twice as much light as the objectives used for the remote entanglement experiment. We present preliminary results characterizing the performance of this system and discuss how it could be incorporated into a three-node network, which has not yet been demonstrated using trapped ions.
We demonstrate remote entanglement of trapped-ion qubits via a quantum-optical fiber link with fidelity and rate approaching those of local operations. Two ^{88}Sr^{+} qubits are entangled via the polarization degree of freedom of two spontaneously emitted 422 nm photons which are coupled by high-numerical-aperture lenses into single-mode optical fibers and interfere on a beam splitter. A novel geometry allows high-efficiency photon collection while maintaining unit fidelity for ion-photon entanglement. We generate heralded Bell pairs with fidelity 94% at an average rate 182 s^{-1} (success probability 2.18×10^{-4}).
Trapped ions are one of the leading platforms for quantum computation and networking, with long coherence times and high fidelity qubit operation. We exploit these attributes to build a quantum network with the use of three trapped ion modules connected via photonic links. In each of these systems, we have ions of different species for use as memory or communication qubits. Our newest module contains two high NA=0.8 objectives, each covering 20% of the solid angle of photons emitted from the ion, making it suitable for use as the middle node of the network. Photons are collected from each chamber, and we generate remote entanglement between each pair of nodes by interfering the photons on a beamsplitter. This heralds entanglement of the communication ions in the pair of network nodes, which we then swap onto the memory qubits. I will discuss our progress towards creating a Greenberger–Horne–Zeilinger (GHZ) state between the memory ions in the three nodes.