Quantum repeaters are likely to be needed in order to overcome existing bounds of quantum communications, but making practical devices remains challenging. For example, in quantum key distribution (QKD), there are fundamental rate–distance limits (related to the maximum secret key rate that is possible for a given transmission distance) and without quantum repeaters this issue remains a hurdle. Marco Lucamarini and a team at Toshiba Research Europe (Cambridge, UK) have now proposed a twin-field QKD approach that, without quantum repeaters, may enable 550 km QKD with acceptable noise using standard optical fibre; this would go beyond the typical ~200–340 km repeaterless bounds of QKD with standard optical fibre. Two light sources are required in twin-field QKD, creating the pulses for each user (‘Alice’ and ‘Bob’) that are phase-randomized and phase-encoded with secret bits. On interference of the pulses at a beam splitter, an eavesdropper ‘Charlie’ is able to infer whether the secrets bits of Alice and Bob are the same or different (that is, 00 or 11 versus 01 or 10), but is unable to determine the absolute values (0 or 1), making the scheme resistant to hacking.