De-multiplexer and method of separating modes of electromagnetic radiation

A mode converter for converting a mode of electromagnetic radiation from a first mode to a second mode, the mode converter comprising:

• an atomic ensemble comprising atomic valence electrons having a first state, a second state and a third state, wherein the second state has a higher energy than, and is linked to, the first state by one or more atomic transitions, and the third state has a higher energy than, and is linked to, the second state by an atomic transition;
• a signal source of electromagnetic radiation arranged to generate a first mode of electromagnetic radiation having a frequency corresponding to an off-resonant atomic transition between the second state and the third state of atomic valence electrons in the atomic ensemble, wherein the first mode of electromagnetic radiation from the signal source is arranged to be incident upon the atomic ensemble to stimulate off-resonant transitions of the atomic valence electrons in the atomic ensemble between the second state and the third state, and preferably wherein the signal source electromagnetic radiation has a bandwidth of greater than 1 GHz;
• one or more control sources of electromagnetic radiation each arranged to generate electromagnetic radiation having a frequency corresponding to an off-resonant atomic transition from the one or more atomic transitions linking the first state and the second state of atomic valence electrons in the atomic ensemble, wherein the electromagnetic radiation from the one or more control sources is arranged to be incident upon the atomic ensemble to stimulate off-resonant transitions of the atomic valence electrons in the atomic ensemble between the first state and the second state, and preferably wherein the electromagnetic radiation from each of the one or more control sources has a bandwidth of greater than 1 GHz;
• and
• wherein the mode converter is arranged such that on incidence of the first mode of electromagnetic radiation from the signal source and electromagnetic radiation from each of the one or more control sources to the atomic ensemble, a coherent excitation of the transition between the first state and the third state is created that stores the first mode of electromagnetic radiation from the signal source in the atomic ensemble;
• wherein the one or more control sources are each arranged to shape the electromagnetic radiation from each of the one or more control sources such that the subsequent incidence of electromagnetic radiation from each of the one or more control sources upon the atomic ensemble stimulates emission of a second mode of electromagnetic radiation from the atomic ensemble such that the first mode of electromagnetic radiation is converted in the second mode of electromagnetic radiation;
• and
• wherein the second mode of electromagnetic radiation has substantially the same frequency as the first mode of electromagnetic radiation and the second mode of electromagnetic radiation is a different functional mode than the first mode of electromagnetic radiation.