Alice and Bob in Quantumland

Robert Gilmore

Ruth E. Kastner

Robert Gilmore

1. Into Quantumland 2. The Heisenberg Bank 3. The Mechanic's Institute 4. The Copenhagen School 5. The Fermi-Bose Academy 6. Virtual Reality 7. Atoms in the Void 8. Castle Rutherford 9. The Particle MASSquerade 10. The Experimental Physics Phun Phair

Hazem Zohny

Tal Mor

The use of polarized photons in cryptography holds the promise of secure cryptographic quantum key distribution schemes. In theory, quantum key distribution provides a key with unconditional security. In practice, however, the implemented schemes are often operated in a regime which excludes unconditional security, or even in a regime in which insecurity can be proven. We shall describe here the basics of quantum key distribution, provide an overview of the main ideas that lead to a proof of its unconditional security, and discuss the insecurity of practical quantum key distribution to long distances. The above review will be undertaken with a special emphasis on the role of the polarization of photons. Finally, we shall briefly touch on the importance of photons and their polarization degree of freedom in recent proposals for small-scale optical quantum computing.

Vlatko Vedral

Abstract This chapter discusses some of the ways in which quantum entanglement can be used for quantum communication. Suppose Alice and Bob share an entangled state: they can use it for dense coding, in which Alice sends Bob just one qubit of an entangled pair, which delivers two bits of classical information. Alice and Bob can also use an entangled state for teleportation, in which a qubit in a unknown quantum state is teleported from Alice to Bob when Alice sends Bob two classical bits. Entanglement swapping is an example of a tripartite communication protocol. If Alice and Bob share entanglement, and Bob and Charlie share entanglement, entanglement swapping can be used to entangle Alice and Charlie. Even if Alice and Bob are entangled, Alice cannot communicate with Bob without sending him classical bits or qubits, which prevents Alice and Bob from breaking the laws of relativity. This chapter also discusses Pauli matrices and the extension of Hilbert spaces.