Bringing quantum networks to life
Control over the interaction between photons with atoms promises quantum systems to serve as transmitters, receivers, and memory elements for information in a future quantum network.
How can information encoded in atoms or molecules be exchanged with single photons without corruption or loss? That is the key problem with which laser physicist Professor Gerhard Rempe, a director of the Max Planck Institute for Quantum Optics (MPQ; Garching, Germany), and his research group are concerned. In their efforts to solve it, they have designed optical resonators made up of pairs of mirrors placed in diametrically opposite positions in an ultrahigh-vacuum chamber (see Fig. 1). The volume enclosed by such an optical cavity is much less than a cubic millimeter, but the reflectivity of the mirrors is extremely high. Once inside a resonator, a photon will be reflected on the order of 100,000X before it is transmitted through one of the mirrors.
In their experiments, the researchers prepared a single atom, which has been cooled to a temperature of much less than 1 mK and place it in a resonator or at the center of two crossed resonators positioned orthogonally to each other (see Fig. 2). If a photon were to interact with the atom only once, the interaction would be very weak. However, because each photon makes the round trip between the cavity’s mirrors many times, the interaction is correspondingly amplified.
Rempe and his team have used this configuration as the basis for the construction of novel light sources that generate a stream of single-photon bits, and even quantum mechanically entangled photons, on demand.1 In addition, the setup can be used to transfer quantum information encoded in a single photon to the atom in a controlled manner.2 Furthermore, the group has shown that the presence or absence of a single photon can be nondestructively measured without altering the information encoded in the particle. That’s roughly analogous to checking whether there is a letter in the post office box without reading the content of that letter. However, it’s much more difficult to verify the arrival of a quantum letter without reading the quantum content.3 Experiments such as these are being performed in several of the group’s laboratories.
Rempe is one of the initiators in the field of cavity quantum electrodynamics. In 1987, he was the first person to demonstrate the repeated absorption and re-emission of a single microwave photon by a single atom. He went on to study how optical photons behaved when introduced into cavities formed by highly reflective mirrors. With this work, he became one of the pioneers of cavity quantum electrodynamics in the optical frequency range.