Due to photon losses its is increasingly difficult to generate entangled photons at more than a few hundred kilometres distance. Quantum memories are an essential technology to increase the distance of entangled particles in a future quantum repeater technology. The quantum memories used within this project consist of trapped single ions which have the advantage over many other quantum memories that their electronic quantum state can be precisely controlled by laser pulses, and detected with more than 99% efficiency. Even successive measurements in different bases are possible through quantum non-demolition (QND) measurements. Moreover, single trapped ions are ideal candidates for quantum memories, as they preserve the quantum state for up to several minutes. At the same time, single photons are best suited to act as carriers for transmitting quantum states and distributing entanglement over longer distances, and hence the ability to establish quantum correlations between single atoms and single photons is extremely important for distributing quantum information. The realization of heralded entanglement between distant atomic ensembles was amongst the first major achievements in this direction. More recently, single neutral atoms trapped at distant locations were entangled by first generating single atom-photon entanglement and then mapping the photonic state on the electronic state of the second atom.

QND detection and heralded absorption of photonic entanglement.
Figure 1: QND detection and heralded absorption of photonic entanglement. (a) Scheme for mapping a photonic entangled state of a down-conversion source into the electronic state of two atoms (b) The successful absorption of a polarisation qubit of a photon in the atom is detected by QND fluorescence detection on transition |g> to |a>. The stored electronic state can then be detected either by mapping the state back onto a photon or by direct detection within the atom.


Previously, we have demonstrated the heralded absorption of a single photon from a down-conversion source by a single ion. This is the first step towards storing and detecting photonic entanglement in an atomic ion memory. Also, we have demonstrated a scheme for entangling distant atoms based on quantum interference and the detection of a single photon scattered from two effectively one meter distant laser cooled and trapped atomic ions. The detection of this single photon heralds the entanglement of two internal states of the trapped ions with a high rate and with a fidelity limited mostly by atomic motion. The intention is to build on this work to investigate greater separations between atoms, as well as other schemes that will also allow for storing the photonic states in the atoms.


Below some of our previous key results on quantum optics and memories with trapped ions.

A completely list of publications from the Quantum Optics and Spectroscopy Group @University of Innsbruck can be found here.

Our previous work on single photon-single ion interaction at ICFO - The Institute of Photonics Sciences, Spain can be found here.