Peter Drmota

Abstract:

The α-Fe₂O₃(11̅02) surface (also known as the hematite r-cut or (012) surface) was studied using low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), scanning tunneling microscopy (STM), noncontact atomic force microscopy (nc-AFM), and ab initio density functional theory (DFT)+U calculations. Two surface structures are stable under ultrahigh vacuum (UHV) conditions; a stoichiometric (1 × 1) surface can be prepared by annealing at 450 °C in ≈10^-6 mbar O₂, and a reduced (2 × 1) reconstruction is formed by UHV annealing at 540 °C. The (1 × 1) surface is close to an ideal bulk termination, and the undercoordinated surface Fe atoms reduce the surface bandgap by ≈0.2 eV with respect to the bulk. The work function is measured to be 5.7 ± 0.2 eV, and the VBM is located 1.5 ± 0.1 eV below E_F. The images obtained from the (2 × 1) reconstruction cannot be reconciled with previously proposed models, and a new "alternating trench" structure is proposed based on an ordered removal of lattice oxygen atoms. DFT+U calculations show that this surface is favored in reducing conditions and that 4-fold-coordinated Fe²⁺ cations at the surface introduce gap states approximately 1 eV below E_F. The work function on the (2 × 1) termination is 5.4 ± 0.2 eV.

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), supporting 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 report the first experimental demonstration of the odd-cycle game. We entangle two atoms separated by ∼ 2 m and the players use them to win the odd-cycle game with a probability ∼ 26σ above that allowed by the best classical strategy. The experiment implements the optimal quantum strategy, is free of loopholes, and achieves 97.8(3) % of the theoretical limit to the quantum winning probability. We perform the associated Bell test and measure a nonlocal content of 0.54(2) – the largest value for physically separate devices, free of the detection loophole, ever observed.