Quantum memory device

A method of storing and retrieving one or more modes of electromagnetic radiation in a quantum memory device, the quantum memory device 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 an atomic transition, and the third state has a higher energy than, and is linked to, the second state by one or more atomic transitions;
• the method comprising:
• storing one or more modes of electromagnetic radiation in the atomic ensemble by:
• stimulating off-resonant transitions of the atomic valence electrons in the atomic ensemble between the first state and the second state by arranging for one or more modes of electromagnetic radiation generated by a signal source of electromagnetic radiation to be incident upon the atomic ensemble, wherein the frequency of the electromagnetic radiation from the signal source corresponds to an off-resonant atomic transition between the first state and the second state of atomic valence electrons in the atomic ensemble, and wherein the signal source electromagnetic radiation has a bandwidth of greater than 1 GHz;
• and
• stimulating off-resonant transitions of the atomic valence electrons in the atomic ensemble between the second state and the third state by arranging for electromagnetic radiation generated by one or more control sources of electromagnetic radiation to each be incident upon the atomic ensemble, wherein the frequency of the electromagnetic radiation from the one or more control sources each corresponds to an off-resonant atomic transition from the one or more atomic transitions linking the second state and the third state of atomic valence electrons in the atomic ensemble, and wherein the electromagnetic radiation from each of the one or more control sources has a bandwidth of greater than 1 GHz;
• wherein the one or more modes of electromagnetic radiation from the signal source and the electromagnetic radiation from each of the one or more control sources are arranged to be incident upon the atomic ensemble such that a coherent excitation of the transition between the first state and the third state is created that stores the one or more modes of electromagnetic radiation from the signal source in the atomic ensemble;
• and
• retrieving the one or more modes of electromagnetic radiation that have been stored in the atomic ensemble by:
• further stimulating off-resonant transitions of the atomic valence electrons in the atomic ensemble between the second state and the third state by arranging for electromagnetic radiation generated by one or more control sources of electromagnetic radiation to each be incident upon the atomic ensemble after the one or more modes of electromagnetic radiation generated by the signal source have been stored in the atomic ensemble;
• wherein the electromagnetic radiation from each of the one or more control sources is arranged to be incident upon the atomic ensemble to stimulate emission of the one or more stored modes of electromagnetic radiation from the atomic ensemble.

A quantum memory device for storing one or more modes of electromagnetic radiation, the quantum memory device 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 an atomic transition, and the third state has a higher energy than, and is linked to, the second state by one or more atomic transitions;
• a signal source of electromagnetic radiation arranged to generate one or more modes of electromagnetic radiation to be stored having a frequency corresponding to an off-resonant atomic transition between the first state and the second state of atomic valence electrons in the atomic ensemble, wherein the one or more modes of electromagnetic radiation from the signal source are 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;
• 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 second state and the third 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 second state and the third state;
• wherein the quantum memory device is arranged such that on incidence of one or more modes 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 one or more modes of electromagnetic radiation from the signal source in the atomic ensemble, and subsequent incidence of electromagnetic radiation from each of the one or more control sources upon the atomic ensemble stimulates emission of the one or more stored modes of electromagnetic radiation from the atomic ensemble;
• and
• wherein the inverse of the residual Doppler linewidth owing to the incidence of the electromagnetic radiation generated by the signal source and the one or more control sources on the atomic valence electrons in the atomic ensemble is greater than half the lifetime of the coherent excitation of the transition between the first state and the third state.

A method of storing and retrieving one or more modes of electromagnetic radiation in a quantum memory device, the quantum memory device 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 an atomic transition, and the third state has a higher energy than, and is linked to, the second state by one or more atomic transitions;
• the method comprising:
• storing one or more modes of electromagnetic radiation in the atomic ensemble by:
• stimulating off-resonant transitions of the atomic valence electrons in the atomic ensemble between the first state and the second state by arranging for one or more modes of electromagnetic radiation generated by a signal source of electromagnetic radiation to be incident upon the atomic ensemble, wherein the frequency of the electromagnetic radiation from the signal source corresponds to an off-resonant atomic transition between the first state and the second state of atomic valence electrons in the atomic ensemble;
• and
• stimulating off-resonant transitions of the atomic valence electrons in the atomic ensemble between the second state and the third state by arranging for electromagnetic radiation generated by one or more control sources of electromagnetic radiation to each be incident upon the atomic ensemble, wherein the frequency of the electromagnetic radiation from the one or more control sources each corresponds to an off-resonant atomic transition from the one or more atomic transitions linking the second state and the third state of atomic valence electrons in the atomic ensemble;
• wherein the one or more modes of electromagnetic radiation from the signal source and the electromagnetic radiation from each of the one or more control sources are arranged to be incident upon the atomic ensemble such that a coherent excitation of the transition between the first state and the third state is created that stores the one or more modes of electromagnetic radiation from the signal source in the atomic ensemble;
• wherein the inverse of the residual Doppler linewidth owing to the incidence of the electromagnetic radiation generated by the signal source and the one or more control sources on the atomic valence electrons in the atomic ensemble is greater than half the lifetime of the coherent excitation of the transition between the first state and the third state;
• and
• retrieving the one or more modes of electromagnetic radiation that have been stored in the atomic ensemble by:
• further stimulating off-resonant transitions of the atomic valence electrons in the atomic ensemble between the second state and the third state by arranging for electromagnetic radiation generated by one or more control sources of electromagnetic radiation to each be incident upon the atomic ensemble after the one or more modes of electromagnetic radiation generated by the signal source have been stored in the atomic ensemble;
• wherein the electromagnetic radiation from each of the one or more control sources is arranged to be incident upon the atomic ensemble to stimulate emission of the one or more stored modes of electromagnetic radiation from the atomic ensemble.