Locality constrains the flow of information between different parts of many-body quantum systems. In quantum computers, this affects the ability to perform arbitrary interactions for quantum information processing tasks. A crucial challenge for scalable quantum architectures is thus to minimize the overheads due to locality constraints. Additionally, locality constraints affect the way information and entanglement can be spread in many body quantum systems, and our ability to make predictions about such systems.
One way to implement arbitrary interactions in locality-constrained architectures is by permuting qubits, also known as performing quantum routing. In the first part of this thesis, we investigate quantum routing strategies using emerging architectural capabilities beyond swap gates. We first investigate the use of mid-circuit measurements and classical feedback, which enable quantum teleportation. We prove upper bounds on the speedup afforded by the use of quantum teleportation for routing. We also design architectural connectivities which can exploit teleportation-based speedups. We then consider quantum routing by continuous-time Hamiltonian evolution, designing a routing algorithm that achieves a polynomial speedup in routing time over gate-based routing methods in architectures with a bottleneck, which is an intermediate region that constrains interactions between the rest of the device. We improve the lower bound on the time taken for routing in such systems, and develop a matching upper bound on the closely related task of generating and distributing entanglement.
In the second part of this thesis, we investigate the behavior of finite-sized systems evolving by local Hamiltonians for a long duration. A major direction of research in condensed matter physics is the investigation of whether isolated systems thermalize (i.e., reach a state of thermal equilibrium). Various theoretical proposals such as information scrambling, entanglement between subsystems, and the eigenstate thermalization hypothesis have attempted to explain the conditions under which systems equilibrate. At the same time, notable counter-examples to thermalization have been found, such as systems that are integrable, exhibit many-body localization, or quantum many-body scars. We approach this question from a complexity-theoretic perspective. We show that the problem of deciding whether or not observables equilibrate to a given value is PSPACE-complete. We also show that deciding whether observables relax to the thermal equilibrium value is contained in PSPACE, and is PSPACE-hard under quantum polynomial time reductions. These problems are thus intractable even for a quantum computer. Our results indicate that the proposed conditions leading to thermalization are either difficult to decide, or are not generic indicators of thermalization.
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