Photonic Systems Brown Bag Seminar

Thursday, April 21, 2005, at 12 noon                                RLE Haus Room, 36-428

Quantum Communication: Why and How?

Mohsen Razavi

Quantum mechanics is the best known model for describing physical phenomena at microscopic scales.   In this model, a quantum state, represented mathematically by a vector in a Hilbert space, is associated with a physical system.   At any particular time, this quatum state fully characterizes the physical system.   Some future applications, such as the networking of quantum computers, require that a quantum state be transferred from one place to another.   This is what is known as quantum communication.   Quantum communication over long distances is extremely challenging.   Classical communication alone does not suffice to transmit a quantum state.   This is because the quantum measurement principle states that any observation of an unknown quantum state changes that state in a non-reversible fashion, and the no-cloning theorem states that it is impossible to duplicate - and hence characterize by a series of measurements - an unknown quantum state.   Physical transportation of a quantum system over long distances is highly impractical, at best, because decoherence arising from interaction with the environment will almost surely destroy the desired state.   There is a theoretically viable approach, however, for quantum communication.   It is known as teleportation, and it relies on entanglement (a quantum resource) together with quantum measurement and classical communication. This talk is an introduction to quantum communication.   It will describe the teleportation protocol for two-dimensional quantum states---qubits.   Some methods for implementing this protocol, as being developed by a team of researchers from MIT and Northwestern University will also be described.