QUROPE Quantum Information Processing and Communication in Europe

A. Reiserer, S. Ritter and G. Rempe
Science 342, 1349-1351 (2013)

A promising candidate architecture for quantum computing is photonics, in which quantum bits (qubits) are
encoded onto the states of optical fields. Quantum information can then be coherently manipulated and
transmitted over long distances at room temperature with almost no noise. However, a key challenge for
photonics is the requirement for conditional logic operations between qubits, which requires single optical
photons to interact with one another. Photons do not interact in vacuum, and photon-photon non-linearities
mediated by material systems are very weak.

In their work, Reiserer and colleagues break new ground by showing that a single atom trapped inside a
high-finesse optical cavity can interact so strongly with an incident field that the reflectivity of the cavity can
be switched by the internal spin state of the atom. The authors demonstrate how to use this interaction to
non-destructively infer the presence of a photon reflected from the cavity from fluorescence measurements
of the atomic state. In the future, this very strong atom-optical coupling could be used to allow one photon to
switch another photon, providing the conditional operation needed for universal photonic quantum
computing.