**Quantum optics**deals with the interaction between light and matter and the very peculiar behaviour of light at very low intensities, where it appears only in wavepackets whose energy is a multiple of ℎω, where ω is the angular frequency of light and ℎ Planck's constant. These wavepackets - called photons - are indivisible and this fact leads to very surprising phenomena, for instance optically detecting an object without shining light on it, or quantum teleportation of the state of light (which is somewhat like "beaming" in Star Treck, though it wouldn't work with persons).

The matter usually consists of ensembles of atoms and molecules, but more general media (crystals, superconductors, nano-oscillators ..) become increasingly more important. In quantum optics the laws of quantum mechanics are used to manipulate the state of light or matter. For instance, one can laser cool atoms by shining light on it, or one can stop a light pulse and speed it up again in a gas of atoms.

The success of quantum optics as a research field is based on the excellent experimental control over the state of atoms and light. This has made it an ideal field to study the fundamental laws of quantum mechanics or to explore applications of quantum physics. Historically, quantum optics emerged from laser physics, which laid the foundations for modern tele-communication technology. However, it has also been used to improve gravitational wave detectors, for instance. The most recent applications of quantum optics are in

**Quantum Information**,

which deals with the direct exploitation of the laws of quantum mechanics in information technology. For instance, in quantum cryptography eavesdropping is made impossible because messages are sent using single photons. If the eavesdropper would measure the state of a photon he would inevitably change it (because of the reduction of the wave packet) and thus reveal his own presence. In quantum computing the superposition principle of quantum mechanics makes it possible to perform massive parallel computing that can in principle surpass the speed of any conceivable computer that is based on conventional technology.

**Quantum optics research at StFX**

Peter Marzlin's recent research has been focused on applications in quantum information and includes work on the reversible storage of single photons in matter and the creation of a strong photon-photon interaction for quantum logical gates. He also worked on possibilities how to minimize the effect of noise on quantum systems so that the performance of quantum logical gates can be optimized.

Department members working on quantum optics and quantum information:

Peter Marzlin

Karine Le Bris