By Peter van Loock, University of Mainz
The first and most common approach to quantum communication across large distances, circumventing the effect of an optical transmission loss exponentially growing with distance, is the quantum repeater. However, the standard quantum repeater based on local quantum memories and two-way classical communication is extremely slow, producing low rates and requiring long-lasting memories.
An obvious remedy here is to replace quantum error detection (as employed in a standard quantum repeater in the form of entanglement purification) by quantum error correction. We shall first give an overview over recent proposals for such an encoded quantum repeater, with a particular emphasis on its ultrafast manifestation using quantum codes against photon losses and one-way classical communication. We will then discuss the possibility of implementing such ultrafast long-distance quantum communication with linear optics. For this purpose, we propose a projection measurement onto encoded Bell states with a static network of linear optical elements.
By increasing the size of the quantum error correction code, both Bell measurement efficiency and photon-loss tolerance can be made arbitrarily high at the same time. As a result, we can show that all-optical quantum communication over large distances with communication rates similar to those of classical communication is possible solely based on local state teleportations using optical sources of encoded Bell states, fixed arrays of beam splitters, and photon detectors. In other words, nonlinear effects are only needed for the generation of the encoded qubits, but not at all for their local processing including error syndrome identification, correction, and state recovery.
We also discuss an extension of our scheme in order to deal in addition with various depolarizing errors, e.g. caused by faulty detectors and resource states, paving the way for ultrafast fault-tolerant long-distance quantum communication with static linear optics.