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.
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.
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.
Increasing the number of qubits that can be controlled in a quantum system represents an essential challenge to the field of quantum computing. Quantum networks consisting of nodes for local information processing and photonic channels to distribute entanglement between different nodes represent a promising modular approach to achieve this scaling. Trapped atomic ions are an ideal candidate for quantum network nodes, with long-lived identical qubit memories that can be locally entangled through their Coulomb interaction and remotely entangled through photonic channels. In this work I will first discuss our established toolkit for using 171Yb+ and 138Ba+ ions individually or together within a quantum node. Next I will show how the 138Ba+ toolkit has been extended to allow for quantum operations in the 52D3/2 manifold. I will then demonstrate how we can generate ion-photon entanglement as a resource to connect separate nodes with a focus on some important improvements which will allow us to implement it as part of a larger network. These improvements include first the use of separate memory (171Yb+ ) and photon generating (138Ba+ ) ions. Additionally, the use of separate atomic lines within 138Ba+ for excitation and collection allows us to preserve integrity of this photonic interface by ensuring the purity of the single photons that are produced. To this end I demonstrate a single-photon source for quantum networking based on a trapped 138Ba+ ion with a single photon purity of g2(0) = (8.1 ± 2.3) × 10−5 without background subtraction. Trade-offs between the photonic generation rate and the memory-photon entanglement fidelity for the case of polarization photonic qubits are also examined and optimized by tailoring the spatial mode of the collected light. These techniques should be useful in constructing larger ion-photon networks.
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 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 computing promises solutions to some of the world's most important problems that classical computers have failed to address. The trapped-ion-based quantum computing platform has a lot of advantages for doing so: ions are perfectly identical and near-perfectly isolated, feature long coherent times, and allow high-fidelity individual laser-controlled operations. One of the greatest remaining obstacles in trapped-ion-based quantum computing is the issue of scalability. The approach that we take to address this issue is a modular architecture: separate ion traps, each with a manageable number of ions, are interconnected via photonic links. To avoid photon-generated crosstalk between qubits and utilize advantages of different kinds of ions for each role, we use two distinct species - ¹⁷¹Yb⁺ as memory qubits and ¹³⁸Ba⁺ as communication qubits. The qubits based on ¹⁷¹Yb⁺ are defined within the hyperfine "clock" states characterized by a very long coherence time while ¹³⁸Ba⁺ ions feature visible-range wavelength emission lines. Current optical and fiber technologies are more efficient in this range than at shorter wavelengths. We present a theoretical description and experimental demonstration of the key elements of a quantum network based on the mixed-species paradigm. The first one is entanglement between an atomic qubit and the polarization degree of freedom of a pure single photon. We observe a value of the second-order correlation function g⁽²⁾(0) = (8.1 ± 2.3)⨉10⁻⁵ without background subtraction, which is consistent with the lowest reported value in any system. Next, we show mixed-species entangling gates with two ions using the Mølmer-Sørensen and Cirac-Zoller protocols. Finally, we theoretically generalize mixed-species entangling gates to long ion chains and characterize the roles of normal modes there. In addition, we explore sympathetic cooling efficiency in such mixed-species crystals. Besides these developments, we demonstrate new techniques for manipulating states within the D₃⸝₂-manifold of zero-nuclear-spin ions - a part of a protected qubit scheme promising seconds-long coherence times proposed by Aharon et al. in 2013. As a next step, we provide a detailed description of the protocols for three- and four-node networks with mixed species, along with a novel design for the third trap with in-vacuum optics to optimize light collection.
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.