About The Speaker

Dr. Bergou is Professor at the Department of Physics and Astronomy at Hunter College of the City University of New York. Prof. Bergou's research interests are in theoretical quantum optics and quantum information science with an eye on foundations. He is Fellow of the American Physical Society and Optica (formerly Optical Society of America). Current research activities include broadcasting of quantum states with applications in various quantum information protocols, linear optical implementations of quantum networks, and quantitative complementarity relations. An underlying theme over the years is the discrimination of quantum states which is a ubiquitous task in many different quantum communication and quantum computation schemes.

Event Details

The no-cloning theorem forbids the distribution of an unknown state to more than one receiver. However, quantum entanglement assisted with measurements provides various pathways to communicate information to parties within a network. For example, if the sender knows the state, and the state is chosen from a restricted set of possibilities, a procedure known as remote state preparation can be used to broadcast a state. In this talk, we first examine a remote state preparation protocol that can be used to send the state of a qubit, confined to the equator of the Bloch sphere, to an arbitrary number of receivers. The entanglement cost is less than that of using teleportation to accomplish the same task. We also present variations on this task: probabilistically sending an unknown qubit state to two receivers, sending different qubit states to two receivers, sending qutrit states to two receivers, and discuss some applications of these protocols. Next, we generalize this basic broadcasting protocol to broadcast product and multi-partite entangled quantum states in a network where, in the latter case, the sender can remotely add phase gates or abort distributing the states. The generalization allows for multiple receivers and senders, an arbitrary basis rotation, and adding and deleting senders from the network. We also discuss the case where a phase to be applied to the broadcast states is not known in advance but is provided to a sender encoded in another quantum state. Applications of broadcasting product states include authentication and three-state quantum cryptography. Then, we study the distribution of a single multiqubit state shared among several receivers entangled with multi-qubit phase gates, giving the graph states as an example. We show that by coordinating with the sender, the receivers can assist in performing remote, distributed measurement-based quantum computation with the Pauli X basis measurement alone. As another application of this, we discuss the distribution of the multi-qubit GHZ state. We close by a discussion of the capabilities and limitations of implementations using linear optical quantum networks.