State Creation, Preparation, & Distillation

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Entanglement is the defining phenomenon of quantum mechanics, and a critical resource in many quantum information protocols. We start with a source of tunable entanglement ["Entanglement Sources"], and using various waveplates and decoherers we can prepare a large array of states (in fact, we recently worked out how to prepare every possible quantum polarization state of two photons). For example we have created high fidelity Maximally Entangled Mixed States (MEMS), which have the greatest physically allowed entanglement (measured using the "tangle") for given state purity (quantified by the linear entropy). The creation of these novel states is in itself interesting and has lead to new ways to think about how we describe states ["Mixed state sensitivity of several quantum information benchmarks"]. They also facilitate the investigation of entanglement distillation, used to combat the deleterious effects of decoherence, which often leads to undesirable reduced entanglement and increased entropy in physical quantum systems ["Maximally Entangled Mixed States: Creation and Concentration"]. Specifically, we study "Procrustean" distillation in which an ensemble of partially entangled mixed states are filtered using partial polarizers to give a fewer number of more highly entangled lower entropy states Another use of entanglement is for 'remote state preparation', a quantum communication protocol by which a sender Alice can "teleport" or transmit a known state to a receiver Bob by sharing an entangled pair and a single bit of classical communication. Using our entangled photon source and generalized partial polarization measurements, we have been able to remotely prepare arbitrary quantum states with very high accuracy ["Remote state preparation: arbitrary remote control of photon polarization"].