Remember that photon

摘要

Storing single photons in atomic memories, and releasing them at a later time, is a required step on the way to quantum repeaters and long-distance quantum cryptography networks. This step has now been taken. The basic unit of quantum information, the quantum bit or qubit, can be encoded in various physical quantities, such as the polarization states of photons, or the spin states of atomic nuclei. To make qubits practically useful, random coupling of them with the external world — an effect known as decoherence — must at all costs be avoided or corrected. This makes photons (the quanta of light) particularly suitable for qubit transmission, as they can travel over very long distances with very little decoherence. For qubit storage, encoders such as atoms come into their own: they can be kept in 'traps' for long periods, again avoiding deleterious decoherence effects from outside.