Rydberg radio sensor

A Rydberg sensor 1 for detecting radio signals has a Rydberg vapour cell 3 and input and output units for probe P and control C laser beams, where the input and output units split respective input laser beams into respective arrays to intersect within the vapour cell. The input and output units each contain an array of beamsplitters 2 which have successively increasing splitter ratio along the array. The beam splitters may be graduated across the row or may have graduated optical filters 7. The beamsplitters may be prisms abutting and bonded to one another and to the vapour cell to provide resilience to vibration. Data from a respective array of photodetectors 5 are passed to a computer processor to measure and discriminate an incident radio signal, for example at a specific frequency and incident angle, in preference to others.