Entanglement is the physical property that marks the most striking deviation of the quantum from the classical world: highly – if not perfectly – correlated results of measurements performed on entangled particles that can, in principle, be arbitrarily far away. What is particularly puzzling about these correlations is that they cannot be explained by attributing properties to the individual particles that determine, irrespectively of what happens to the other particle, how each will respond to its measurement.
In addition to being an intriguing fundamental property, entanglement is also a key resource for quantum communication. For instance, Alice and Bob could perform measurements on their respective (entangled) particles and thereby establish a provable-secure key. Another use of entanglement is the faithful transfer of an unknown quantum state through quantum teleportation, or networking of two distant quantum processors. Unfortunately, photons – the particle of choice to distribute entanglement – are subject to loss and decoherence as they travel through optical fibres. Taking into account attenuation in realistic fibre links, which often include lossy splices and sharp bends, this limits the distribution of entanglement, and hence the extension of a quantum network for general quantum communication application, to metropolitan size. However, it is in principle possible to overcome these obstacles using so-called quantum repeaters. The basic idea of a quantum repeater is to divide a long quantum communication link into shorter segments (elementary links), to distribute entanglement in a heralded (announced) fashion between end nodes of these segments, and then to deterministically connect neighboring segments to create long-distance entanglement
In our group, we generate and analyze entanglement created via spontaneous parametric down-conversion (SPDC) in non-linear crystals, and we use it as a resource for applications such as quantum cryptography and quantum teleportation, and well as in future quantum repeaters, many of which we have demonstrated in a real-world setting using deployed telecommunication fibres. Towards this end, we currently develop a spectrally multiplexed source of entangled photon pairs using cavity-enhanced SPDC as well as a spectrally-multiplexed Bell-state measurement. Additional research—based on a similar interaction between light and matter as that underpinning the creating of entangled photons—is devoted to the optical interface between our quantum memories and NV centres, which are developed by our colleague Ronald Hanson.