Quantum systems engineering

Abstract:

The existence of multistability in quantum systems beyond the mean-field approximation remains an intensely debated open question. Quantum fluctuations are finite-size corrections to the mean-field as the full exact solution is unobtainable and they usually destroy the multistability present on the mean-field level. Here, by identifying and using exact modulated dynamical symmetries in a driven-dissipative fermionic chain we exactly prove multistability in the presence of quantum fluctuations. Further, unlike common cases in our model, rather than destroying multistability, the quantum fluctuations themselves exhibit multistability, which is absent on the mean-field level for our systems. Moreover, the studied model acquires additional thermodynamic dynamical symmetries that imply persistent periodic oscillations, constituting the first case of a boundary time crystal,to the best of our knowledge, a genuine extended many-body quantum system with the previous cases being only in emergent single- or few-body models. The model can be made into a dissipative time crystal in the limit of large dissipation (i.e. the persistent oscillations are stabilized by the dissipation) making it both a boundary and dissipative time crystal.

Abstract:

Contextuality is a nonclassical feature of quantum systems---exhibited by data that is produced empirically or theoretically---which in the realm of sheaf theory is characterised by local consistency but global inconsistency. A large part of this thesis is concerned with studying how this signature of nonclassicality is apparent also when measurements are embedded in some causal structure, and so motivating the study of causal contextuality. We begin with temporal correlations, which occur when measurements are performed sequentially on the system, and in which the definition of nonclassicality becomes sensitive to memory resources with which the classical system is equipped. For certain types of such memory, we show that there exists a map from the temporal setup to a (appropriately defined) contextuality setup, such that every nonclassical temporal empirical model satisfying no-signalling constraints consistent with the memory function corresponds to a contextual empirical model on this constructed scenario---one can view this also as a simulation of a subset of the temporal correlations by the contextuality setup. The existence of such a map allows us to apply a result from Vorob'ev in order to say, for any temporal setup and choice memory function, whether nonclassical correlations can arise. We then study causal setups by employing the notion of strategy from game semantics. We in particular show how `playing off' Nature strategies, corresponding to adaptive hidden variables, against Experimenter strategies, which may also be adaptive, realises the classical correlations of certain causal setups from the literature. We show that adaptivity on the side of the Experimenter, by reducing the sets of measurements empirical data is obtained over, can remove the inconsistencies that are imperative for the observation of contextuality. In the second part of the thesis, we study spin Hamiltonians on random graphs, focusing on exact descriptions in the thermodynamic limit. By utilising the graphon, which is the limit object of a dense random graphs sequence, we are able to derive analytical results for certain graphons and certain choice of Hamiltonian. Our overarching result is that the equilibrium physics in the thermodynamic limit is described by a set of coupled equations containing the graphon, and which describes product, \ie unentangled, states