Hanif, Farhan; (2025) Quantum measurement induced disturbance of non-classical systems. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis concerns the exhibition of the non-classical properties of quantum mechanics in physical systems. Throughout, I will focus on two measurement based-signatures of a specific form of non-classicality, known as macrorealism, which characterises the apparent classical worldview of macroscopic systems in terms of their definite properties and behaviours under observation. These signatures, known respectively as the nondisturbance condition (NDC) [1, 2] and Leggett-Garg inequalities (LGIs) [3, 4, 5], are probed for violations in a variety of system and measurement setups. First, it is shown that the NDC and two-time LGIs retain their logical status as tests of macrorealism under the effects of random measurement errors for a multilevel spin system. Further, if the measurements deployed in experimental tests of these conditions are imprecise in this random sense, the associated violations of macrorealism either diminish in magnitude but persist, or otherwise vanish entirely, with a worsening degree of imprecision as quantified by the standard deviation of the error probability distribution. It is argued that particular choices of time intervals between measurements, as motivated by optimisation in the ideal case of precise measurements, play a critical role in the robustness of the signature against a systematic form of the imprecision, whereas no such pattern is revealed for errors of an independent type. Finally, the diminishing quantities associated with the non-classicality violations are shown to have their effects compounded by the presence of mixedness in a pure state initialisation. Next, the long standing question of the quantum nature of the gravitational field of a mass in quantum superposition is considered through the lens of the measurement induced disturbance. For the purposes of this study, the NDC is shown to be violated by the action of a measurement of the gravitational field of the source in a quantum superposition, where the interactions occur exclusively through gravity. It is argued that this is only possible for a non-classical gravitational field, thus this work represents a test for the quantum nature of gravity. An analysis of the effects of decoherence on the detectability of the violation reveal the persistence of the violation for high levels of decoherence, offering a significant advantage over entanglement-based tests. In the final chapter, a particular measurement scheme is presented for testing the non-classicality of an ensemble of spin qubits collectively through use of an electromagnetic cavity field. Although the qubit ensemble and the electromagnetic field are both initially prepared in classicallike states, the action of sequential measurements allows for a violation of the classical notion of macrorealism (MR) for arbitrarily large j. This implies the possibility of detection of quantum effects deep into the macroscopic regime. As the measurement protocol proposed has already seen practical implementation in other contexts, the work shown in this thesis also offer a viable methodology for practical realisation.

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