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Abstract:

Verifiable blind quantum computing enables a client to delegate computations while hiding their data and even the underlying algorithm from the cloud server. Because quantum information cannot be copied and measurements irreversibly change the quantum state, information stored in these systems can be protected with unconditional security, and incorrect operation of the server or attempted attacks can be detected. In this thesis, we report the first hybrid matter-photon implementation of verifiable blind quantum computing.

Our experimental platform consists of a trapped-ion quantum server and a client-side photonic detection system networked via a fibre-optic quantum link. We integrate a long-lived memory qubit into a trapped-ion quantum network node to enable simultaneous storage and manipulation of multiple entangled states across a network of quantum processors. We perform laser-based quantum gates between a ⁸⁸Sr⁺ and a ⁴³Ca⁺ ion with 0.961(2) fidelity, and between two ⁴³Ca⁺ ions with 0.985(5) fidelity. Ion-photon entanglement generated with a network qubit in ⁸⁸Sr⁺ is transferred to ⁴³Ca⁺ with 0.977(7) fidelity using error detection. We show that the fidelity of ion-photon entanglement decays ∼70 times slower on a memory qubit in ⁴³Ca⁺ than on the network qubit. Using dynamical decoupling, ion transport and sympathetic cooling, we further extend the storage duration; we measure an ion-photon entanglement fidelity of 0.81(4) after 10 s. We demonstrate that subsequent ion-photon entanglement generation with ⁸⁸Sr⁺ has no effect on the fidelity of ion-photon entanglement previously transferred to the memory. Our apparatus enables deterministic feedforward control (as required for measurement-based blind quantum computing), 91̽��ing fast switching of polarisation measurement basis by the client and deterministic logic gates between the network and the memory qubit in the server. We perform blind computations on linear cluster states and measure error rates surpassing a recently-discovered threshold for secure and robust verification. We quantify the privacy of this system at ⪝0.03 leaked classical bits per qubit in the cluster state. These results show a clear path to scalable and verified universal quantum computing in the cloud, which has wide-ranging applications in areas where confidentiality and verifiability are paramount, such as healthcare, finance, and defence.

Abstract:

We characterise an efficient optically-heated neutral atom source for ion trapping. We observe loading rates of up to 24(3) s−1 with heating powers below 85 mW, and demonstrate loading of a single ion in under 30 s with 41.4(4) mW of optical power in a room-temperature ion trap system with an ionisation probability of 1.50(5) × 10−5 . We calibrate a thermal model for the source’s internal temperature by imaging the fluorescence of a collimated flux of neutral calcium that effuses from the source at various optical heating powers. We show that the thermal performance of this source is mainly limited by radiative losses. We explore the effect of second-stage photo-ionisation laser power on the loading rate, and identify a path beyond the loading rates reported in this study. We predict that this source is also well-suited to a wide range of metals used in ion trapping.