The optical quantum entanglement experiments

F. J. Duarte

Genta Masada, Akira Furusawa

Abstract Entanglement is an essential feature of quantum theory and the core of the majority of quantum information science and technologies. Quantum computing is one of the most important fruits of quantum entanglement and requires not only a bipartite entangled state but also more complicated multipartite entanglement. In previous experimental works to demonstrate various entanglement‐based quantum information processing, light has been extensively used. Experiments utilizing such a complicated state need highly complex optical circuits to propagate optical beams and a high level of spatial interference between different light beams to generate quantum entanglement or to efficiently perform balanced homodyne measurement. Current experiments have been performed in conventional free‐space optics with large numbers of optical components and a relatively large‐sized optical setup. Therefore, they are limited in stability and scalability. Integrated photonics offer new tools and additional capabilities for manipulating light in quantum information technology. Owing to integrated waveguide circuits, it is possible to stabilize and miniaturize complex optical circuits and achieve high interference of light beams. The integrated circuits have been firstly developed for discrete‐variable systems and then applied to continuous‐variable systems. In this article, we review the currently developed scheme for generation and verification of continuous‐variable quantum entanglement such as Einstein‐Podolsky‐Rosen beams using a photonic chip where waveguide circuits are integrated. This includes balanced homodyne measurement of a squeezed state of light. As a simple example, we also review an experiment for generating discrete‐variable quantum entanglement using integrated waveguide circuits.

David S. Simon, Gregg S. Jaeger, Alexander V. Sergienko

John F. Stanton

This thesis develops an undergraduate level understanding of quantum entanglement by expressing its properties in three unique mediums: mathematical formalism, application in technology and experiment. The mathematical formalism of entanglement is developed by working through theoretical experiments that utilize the entangled polarization states of photons. Notation used to describe entangled photon states is then used to illustrate how other types of entangled quantum states can be used in real technology, such as is the case with quantum computing. Finally, the theoretical predictions associated with entanglement are discussed in reference to two quantum optics experiments.

F. J. Duarte

Hikida, Satoshi

Violations of Bell inequalities show that quantum entanglement produces nonclassical correlations between spatially separated systems. In quantum eraser experiments, interference is restored once distinguishability is removed. This highlights the essential role of distinguishability in quantum state transformations. However, it has not been recognized that a paradox can emerge using only conventional optical transformations of quantum states, and no experiment has been proposed to determine which of three mutually exclusive hypotheses outside the implicit assumptions of standard optical transformations and capable of resolving the paradox is realized in practice. Here we show that a paradox emerges in a configuration that reduces the distinguishability of an entangled state, and we propose an experiment capable of identifying which hypothesis is realized. Theory alone cannot determine which hypothesis is realized. Therefore, experimental verification is required. Regardless of the experimental outcome, the experiment would reveal an unrecognized property linking entanglement and distinguishability.