Distributing quantum resources such as entanglement and qubits over long distance fibre optic networks represents an enormous challenge. If we send single photons over 1000km, even at rates of 10GHz, we would need to wait hundreds of years to detect just one, due to loss in the fibre. Not very practical! Modern telecommunication overcomes this problem with amplifiers that boost the signal along the way. However, these would destroy the quantum characteristics of the photons such as entanglement, and, even in principle, this quantum information cannot be copied – we call this “no cloning”. Therefore, a quantum approach to overcome transmission loss is required – the quantum repeater.
There are many challenges facing the development of quantum repeaters as they are complex systems requiring many complex quantum (and classical) devices and sub-systems to function at the highest performance levels. Nonetheless, progress has been significant in recent years both from an engineering perspective but also with new approaches. As the performance of these systems continues to improve they will also be able to take advantage of the developments in QKD, and secure quantum communication in general, in terms of their integration in standard fibre optic networks. Even beyond the benefits for securing a European digital infrastructure, there are an increasing numbers of applications appearing that provide the vision for a future quantum internet.
The way a quantum repeater works is similar in concept to a classical repeater scheme with amplifiers – the transmission distance is broken up into segments where loss is not so great, as illustrated in the figure. We can separate and send entangled photons; one to a quantum memory and the other to a Bell State Measurement (BSM) – we saw this explained for teleportation. Indeed, rather than teleporting a qubit, here we teleport entanglement straight into the quantum memories.
The photon sent to the quantum memory is not measured and destroyed but stored while we wait for the next entangled link to be ready. We thus end up with longer fibre links with the entanglement now stored in these quantum memories. The quantum memories allow us to wait until the fibre links are ready. We can then play the same game, for example, and re-emit the photons to perform a BSM between these adjacent quantum memories, thus further extending the distance until we end up with entanglement shared between two distant parties Alice and Bob.
The quantum memories are clearly a critical element in this scheme and they are being developed in a wide range of technology platforms using either ensembles or in some cases, single, atoms and ions, in special traps or solid state crystals. Another key element not mentioned or illustrated is the need for special interfaces. The simplest example is an interface that converts the frequencies (the colour) of the photons. This is necessary as most quantum memories don’t operate at telecom wavelengths, so we need to convert them before they go into the fibre optic links.
Quantum memories are currently a major focus of many labs with a push to improve performance across a range of areas such as their storage time and the efficiency with which we can get the photons back. Key demonstrations have been made for these elementary links, like two entangled quantum memories, entanglement storage, and teleportation. Few groups bring together all the competencies needed to put all of this together and increasingly this requires a much larger collaborative effort. We can expect to see some demonstrations in fibre networks in the next few years but it is one of the visions of the quantum flagship to develop these technologies to enable an entanglement-based quantum communication network. In this sense one of the next big challenge will also be to start joining these elementary links together.