An electromagnetic frequency converter includes an atomic ensemble; one or more first sources (6, 8) of electromagnetic radiation (P, R) to be incident upon the atomic ensemble to excite atomic valence electrons from a ground state to a first Rydberg state; one or more second sources (6, 14) of electromagnetic radiation (A, C) to be incident upon the atomic ensemble to excite atomic valence electrons from an excited state to a second Rydberg state; a first input (20) and/or output (26) for electromagnetic radiation (L) to be incident upon the atomic ensemble from the first input or received from the atomic ensemble at the first output; and a second input (14) and/or output (24) for electromagnetic radiation (M) to be incident upon the atomic ensemble from the second input or received from the atomic ensemble at the second output.