Algorithms and resources for quantum technology, sensing and random number generation

Abstract

This dissertation presents theoretical results as well as proposed and conducted experiments in the areas of Quantum Sensing, Quantum State Engineering, Bound Entanglement, Quantum Contextuality and Quantum Random Number Generation. A novel detection scheme to improve Quantum Sensing by indirect sensing with the help of an ancillary quantum system is introduced. Sensing information is shown to be obtainable both by direct and indirect sensors, even though their quantum states are not cloned or explicitly transferred. The steps of sensing an external signal and the transfer of information to an ancillary Qubit are combined in one asymmetric pulse sequence. Squeezed spin states, which are a well-known resource for Quantum Sensing due to their robustness to Decoherence, are also discussed. Particularly, their creation in systems of Nitrogen-Vacancy Centers (NVs) in diamond and surrounding nuclear spins, as well as ensembles of such NVs is implemented with specifically tailored sequences. In terms of Quantum State Engineering, a method to purify unpolarized Qubits surrounding and coupled to a central spin is introduced. Repeated projective measurements are used to instil a Zeno-like effect, extendable to a general unpolarized spin bath. Given a suitable trajectory of measurement outcomes, whose crucial role is explored, said projections are shown to enable driving the quantum states of the surrounding nuclear spins towards pure entangled states. Sufficient generality of the approach is shown by applying it to both NVs and superconducting qubits as physical systems. A wide range of target states of the environmental spins can be reached, including pairwise correlated Singlet states, while maximal entanglement is reachable. Advantages for Quantum Sensing granted by specific states obtained by the introduced Purficiation method are described. Concerning Qudits or quantum systems with arbitrarily high dimensionality as a resource, the possibility for generation and measurement of Bound Entanglement with NVs is investigated and an experimental implementation is proposed. Additionally, an experimental violation of a KCBS Inequality, in order to demonstrate Quantum Contextuality with NVs is proposed and details of an implementation are discussed. It is moreover shown how Contextuality can be used as a resource towards Certified Quantum Random Number generation with NVs. Other approaches to Quantum Random Number Generation are also introduced, including a standard single-photon Ansatz using NVs as well as a scheme utilizing the period-doubling state of an Optical Parametric Oscillator.