Resources Needed for Entangling Two Qubits through an Intermediate Mesoscopic System

One of the main challenges in designing large scale quantum processors is connecting separated qubits. In this thesis, we explore new opportunities that mesoscopic many-body systems provide for creating quantum correlation between isolated quantum systems. In particular, we study entangling two non-interacting qubits through an intermediate mesoscopic system consisting of identical two-level systems. Two uncoupled qubits can be entangled either by projectively measuring a joint property of them or by creating an indirect interaction between them. The focus of this thesis is on procedures that are based on joint measurement on the qubits. We propose a new method for entangling two non-interacting qubits by measuring their parity indirectly through an intermediate mesoscopic system. Indirect joint measurement scheme benefits from coherent magnification of the target qubits’ state in the collective state of the mesoscopic system; such that a low-resolution measurement on the mesoscopic system suffices to prepare post-selected entanglement on the target qubits. The protocol is designed to require only global control and course-grained collective measurement of the mesoscopic system along with local interactions between the target qubits and mesoscopic system. A generalization of the method measures the hamming weight of the qubits’ state and probabilistically produces an entangled state by post-selecting on hamming weight one. Our technique provides a new design element that can be integrated into quantum processor architectures and quantum measurement devices. We quantify the resources required for implementing the indirect joint measurement technique when the intermediate mesoscopic system consists of spin-1/2 particles with internal dipolar coupling. A mesoscopic spin system consisting of two non-interacting halves, each coupled to one of the target qubits is proved to provide a helpful geometry that allows implementing the coherent magnification process with experimentally available control tools. We show that the requirements on the amplified state of the target qubits and the mesoscopic spin system perfectly maps to the specifications of micro-macro entanglement between each target qubit and its nearby half of the mesoscopic spin system. In the light of this equivalence, the effects of experimental imperfections are explored; in particular, bipartite entanglement between the target qubits is shown to be robust to imperfect preparation of the mesoscopic spin system. Our analysis provides a new approach for using an intermediate spin system for connecting separated qubits. It also opens a new path in exploring entanglement between microscopic and mesoscopic spin systems