DE-MULTIPLEXER AND METHOD OF SEPARATING MODES OF ELECTROMAGNETIC RADIATION

Abstract

A de-multiplexer (1) for separating two co-propagating modes of electromagnetic radiation includes a volume (2) having a path therethrough for receiving electromagnetic radiation, an input (8) for directing two co-propagating modes of electromagnetic radiation to be incident upon the volume, a control source (12) of electromagnetic radiation arranged to generate a time-dependent control field. The volume is arranged and the time-dependent control field is shaped such that, when the two co-propagating modes of electromagnetic radiation and the time-dependent control field are incident upon the volume contemporaneously, the time-dependent control field causes the volume to accept one of the two modes of electromagnetic radiation onto a mode of the volume without any parametric non-linear optical interaction taking place and to reflect or transmit the other of the two modes of electromagnetic radiation, so to spatially and/or temporally separate the two modes of electromagnetic radiation from each other.

Claims

1. A de-multiplexer for separating two co-propagating modes of electromagnetic radiation, wherein the electric fields of the two modes of electromagnetic radiation are orthogonal to each other, wherein the de-multiplexer comprises: a volume comprising a path therethrough for receiving electromagnetic radiation; an input for directing two co-propagating modes of electromagnetic radiation to be incident upon the volume, wherein the electric fields of the two modes of electromagnetic radiation are orthogonal to each other; a control source of optical or microwave electromagnetic radiation arranged to generate a time-dependent control field, wherein the time-dependent control field from the control source is arranged to be incident upon the volume and to overlap both contemporaneously and spatially with the two co-propagating modes of electromagnetic radiation; wherein the volume is arranged and the time-dependent control field is shaped such that, when the two co-propagating modes of electromagnetic radiation and the time-dependent control field are incident upon the volume contemporaneously, the time-dependent control field causes the volume to accept one of the two modes of electromagnetic radiation onto a mode of the volume without any parametric non-linear optical interaction taking place and to reflect or transmit the other of the two modes of electromagnetic radiation, so to spatially and/or temporally separate the two modes of electromagnetic radiation from each other.

2. A de-multiplexer as claimed in claim 1, wherein the two co-propagating modes of electromagnetic radiation comprise temporal field orthogonal modes.

3. A de-multiplexer as claimed in claim 1, wherein the two co-propagating modes of electromagnetic radiation have a frequency between 1 GHz and 400 THz.

4. A de-multiplexer as claimed in claim 1, wherein the mode of the volume is a stationary mode of the volume and the one of the two co-propagating modes of electromagnetic radiation is accepted onto the stationary mode of the volume.

5. A de-multiplexer as claimed in claim 1, wherein the control source is arranged to direct the control field in a direction collinear with the two co-propagating modes of electromagnetic radiation.

6. A de-multiplexer as claimed in claim 1, wherein the bandwidth of the control field is greater than the bandwidth of the two co-propagating modes of electromagnetic radiation.

7. A de-multiplexer as claimed in claim 1, wherein the time-dependent control field comprises a complex control field.

8. A de-multiplexer as claimed claim 1, wherein the control source is arranged to modulate the control field when the two co-propagating modes of electromagnetic radiation are incident upon the volume.

9. A de-multiplexer as claimed in claim 1, wherein the control source is arranged to shape and direct the time-dependent control field to be incident upon the volume to output the accepted mode of electromagnetic radiation from the volume.

10. A de-multiplexer as claimed in claim 1, wherein the volume comprises a cavity, and the cavity comprises an electro-optic modulator arranged to be controlled by the control field, wherein the control field and the two co-propagating modes of electromagnetic radiation are arranged to be incident upon the electro-optic modulator, and wherein the electro-optic modulator is arranged to modulate the two co-propagating modes such that one of the two modes of electromagnetic radiation is accepted into the cavity and the other of the two modes of electromagnetic radiation is reflected from or transmitted through the cavity.

11. A de-multiplexer as claimed in claim 10, wherein the bandwidth of the electro-optic modulator is greater than the bandwidth of the two co-propagating modes of electromagnetic radiation.

12. A de-multiplexer as claimed in claim 10, wherein the cavity comprises two electro-optic modulators arranged in an interferometer, and wherein the control source of electromagnetic radiation is arranged to generate two time-dependent control fields, wherein the two time-dependent control fields are arranged to be incident upon the two electro-optic modulators respectively and to overlap contemporaneously and spatially with the two co-propagating modes of electromagnetic radiation.

13. A de-multiplexer as claimed in claim 1, wherein the volume comprises an atomic ensemble, wherein the atomic ensemble comprises 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 an atomic transition; wherein one of the two co-propagating modes is arranged to stimulate transitions of the atomic valence electrons in the atomic ensemble between the first state and the second state or between the second state and the third state and the control field is arranged to be incident upon the atomic ensemble to stimulate transitions of the atomic valence electrons in the atomic ensemble between the second state and the third state or between the first state and the second state; and wherein the atomic ensemble is arranged such that on incidence of the one of the two co-propagating modes of electromagnetic radiation and the control field to the atomic ensemble, a coherent excitation of the transition between the first state and the third state is created, such that the atomic ensemble accepts one of the two co-propagating modes of electromagnetic radiation into the atomic ensemble and transmits the other of the two modes of electromagnetic radiation through the atomic ensemble.

14. A de-multiplexer as claimed in claim 13, wherein the atomic ensemble comprises rubidium atoms; the first state is the 5S state of rubidium, the second state is the 5P state of rubidium and the third state is the 4D state of rubidium.

15. A method of separating two co-propagating modes of electromagnetic radiation, wherein the electric fields of the two modes of electromagnetic radiation are orthogonal to each other, wherein the method comprises: directing two co-propagating modes of electromagnetic radiation to be incident upon a volume, wherein the electric fields of the two modes of electromagnetic radiation are orthogonal to each other and the volume comprises a path therethrough for receiving electromagnetic radiation; shaping and directing a time-dependent control field of optical or microwave electromagnetic radiation to be incident upon the volume and to overlap both contemporaneously and spatially with the two co-propagating modes of electromagnetic radiation; wherein the volume is arranged and the time-dependent control field is shaped such that, when the two co-propagating modes of electromagnetic radiation and the time-dependent control field are incident upon the volume contemporaneously, the time-dependent control field causes the volume to accept one of the two modes of electromagnetic radiation onto a mode of the volume without any parametric non-linear optical interaction taking place and to reflect or transmit the other of the two modes of electromagnetic radiation, so to spatially and/or temporally separate the two modes of electromagnetic radiation from each other.

16-21. (canceled)

22. A method as claimed in claim 15, comprising modulating the control field when the two co-propagating modes of electromagnetic radiation are incident upon the volume.

23. A method as claimed in claim 15, comprising shaping and directing the time-dependent control field to be incident upon the volume to output the accepted mode of electromagnetic radiation from the volume.

24. A method as claimed in claim 15, wherein the volume comprises a cavity, and the cavity comprises an electro-optic modulator, wherein the method comprises directing the control field to be incident upon the electro-optic modulator to control the electro-optic modulator, directing the two co-propagating modes of electromagnetic radiation be incident upon the electro-optic modulator, and shaping the control field to control the electro-optic modulator to modulate the two co-propagating modes such that one of the two modes of electromagnetic radiation is accepted into the cavity and the other of the two modes of electromagnetic radiation is reflected from or transmitted through the cavity.

25-26. (canceled)

27. A method as claimed in claim 15, wherein the volume comprises an atomic ensemble, wherein the atomic ensemble comprises 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 an atomic transition; wherein the method comprises stimulating transitions of the atomic valence electrons in the atomic ensemble between the first state and the second state or between the second state and the third state by arranging one of the two co-propagating modes to be incident upon the atomic ensemble, and stimulating transitions of the atomic valence electrons in the atomic ensemble between the second state and the third state or between the first state and the second state by arranging the control field to be incident upon the atomic ensemble; and wherein the atomic ensemble is arranged such that on incidence of the one of the two co-propagating modes of electromagnetic radiation and the control field to the atomic ensemble, a coherent excitation of the transition between the first state and the third state is created, such that the atomic ensemble accepts one of the two co-propagating modes of electromagnetic radiation into the atomic ensemble and transmits the other of the two modes of electromagnetic radiation through the atomic ensemble.

28. (canceled)

29. 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.

30. (canceled)

Description

[0138] Certain preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

[0139] FIG. 1 shows schematically a de-multiplexer in accordance with an embodiment of the present invention:

[0140] FIG. 2 shows two co-propagating modes of electromagnetic radiation for inputting into and for separation by the de-multiplexer shown in FIG. 1;

[0141] FIG. 3 shows the time dependency of a control field used with the de-multiplexer shown in FIG. 1;

[0142] FIG. 4 shows schematically a de-multiplexer in accordance with another embodiment of the present invention;

[0143] FIGS. 5 and 6 show schematically a quantum memory de-multiplexer according to an embodiment of the present invention;

[0144] FIG. 7 shows an atomic level scheme used in an embodiment of the quantum memory de-multiplexer shown in FIGS. 5 and 6;

[0145] FIG. 8 shows schematically two co-propagating modes of electromagnetic radiation being separated by the quantum memory de-multiplexer shown in FIGS. 5 and 6;

[0146] FIG. 9 shows schematically a quantum memory de-multiplexer according to another embodiment of the present invention; and

[0147] FIG. 10 shows schematically a de-multiplexer used in combination with a mode converter according to another embodiment of the present invention.

[0148] In order to receive, detect and then decode telecommunication signals that have been encoded as multiple co-propagating modes, a de-multiplexer is required to separate the modes from each other, such that the modes are then able to be detected. A de-multiplexer according to an embodiment of the present invention will now be described that is able to separate two co-propagating modes of electromagnetic radiation where the electric fields of the two modes are temporally orthogonal to each other.

[0149] FIG. 1 shows schematically a de-multiplexer 1 in accordance with an embodiment of the present invention. The de-multiplexer 1 includes a mode-selective optical cavity 2, having a stationary cavity mode {circumflex over (B)}, (which is free from parametric classical non-linear optics). The cavity 2 has an input mirror 4 that incorporates an electro-optic modulator and two other mirrors 6 that define the cavity 2.

[0150] The de-multiplexer 1 has an input 8 for directing electromagnetic radiation from a signal source 9 into the cavity 2 through the input mirror 4 and an output 10 for receiving electromagnetic radiation that is output from the cavity 2. The input 8 is arranged to direct two (or more) co-propagating modes of electromagnetic radiation .sub.in generated by the signal source 9 into the cavity 2.

[0151] FIG. 2 shows two co-propagating modes of electromagnetic radiation for inputting into and for separation by the de-multiplexer 1 shown in FIG. 1. The two modes shown in FIG. 2, HG.sub.0 and HG.sub.1, are the zeroth and first order Hermite-Gauss functions. When these functions are used as the two co-propagating modes of electromagnetic radiation to be input into the cavity 2 for separation, this results in the two co-propagating modes being temporally field orthogonal to each other.

[0152] As shown in FIG. 1, the de-multiplexer 1 also includes a control source of electromagnetic radiation 12 which is arranged to generate a complex time-dependent control field R(t) to be incident upon the electro-optic modulator in the input mirror 4, in order to modulate the phase of the electromagnetic radiation incident upon the input mirror 4. Owing to the time dependency of the control field R(t), the electromagnetic radiation .sub.out output from the output 10 of the cavity comprises the two modes individually which are temporally separated, as will now be described.

[0153] Operation of the de-multiplexer will now be described with reference to FIGS. 1, 2 and 3.

[0154] FIG. 3 shows the time dependency of the control field R(t) and its effect on the two co-propagating modes of electromagnetic radiation .sub.in that are directed through the input 8 of the de-multiplexer 1 to be incident upon the input mirror 4 of the cavity 2.

[0155] In operation of the de-multiplexer 1, two co-propagating modes of electromagnetic radiation .sub.in (e.g. that have been encoded and transmitted together as a telecommunications signal) that are desired to be separated for decoding, are directed through the input 8 of the de-multiplexer 1 to be incident upon the input mirror 4 of the cavity 2. At the same time, the control source 12 generates a time-dependent control field R(t) to be incident upon the input mirror 4 such that it overlaps both spatially and contemporaneously on the input mirror 4 with the two co-propagating modes of electromagnetic radiation .sub.in.

[0156] The incidence of the time-dependent control field R(t) on the input mirror 4 of the cavity 2 causes the cavity 2 to accept one of the co-propagating modes of electromagnetic radiation .sub.in (e.g. the mode HG.sub.1, as shown in FIG. 3) onto the stationary mode {circumflex over (B)} of the cavity 2. The other one of the co-propagating modes of electromagnetic radiation .sub.in (e.g. the mode HG.sub.0, as shown in FIG. 3) is reflected from the input mirror 4 of the cavity 2 and thus output through the output 10 of the de-multiplexer 1 as a single mode of electromagnetic radiation .sub.out.

[0157] After a period of time, during which the accepted mode of electromagnetic radiation is retained inside the cavity 2 (mapped onto the stationary mode {circumflex over (B)} of the cavity 2) by the input mirror 4 and the two other mirrors 6 that define the cavity 2, the control field R(t) is modulated such that the accepted mode is output through the input mirror 4 of the cavity 2 and thus output through the output 10 of the de-multiplexer 1 as a single mode of electromagnetic radiation Rout.

[0158] The temporally separated output modes of electromagnetic radiation .sub.out can then be detected and decoded as appropriate.

[0159] FIG. 4 shows schematically a de-multiplexer 21 in accordance with another embodiment of the present invention. The de-multiplexer 21 shown in FIG. 4 is similar to the de-multiplexer 1 shown in FIG. 1 (i.e. in that it is based on a cavity 22 which is free from parametric classical non-linear optics), except that instead of an electro-optic modulator incorporated into the input mirror, the de-multiplexer 21 shown in FIG. 4 includes an input mirror 24, an output mirror 25 and two further mirrors 36 that define a Mach-Zehnder interferometer. The input and output mirrors 24, 25 also function as beamsplitters in the Mach-Zehnder interferometer and as part of the cavity 22.

[0160] The Mach-Zehnder interferometer includes two electro-optic modulators .sub.1, .sub.2 that are positioned in the arms of the Mach-Zehnder interferometer. The de-multiplexer 1 also includes two control sources of electromagnetic radiation 32, 33 which are arranged to generate complex time-dependent control fields R.sub.1(t), R.sub.2(t) to be incident upon the two electro-optic modulators .sub.1, .sub.2 respectively. The control fields R.sub.1(t), R.sub.2(t) are used to control the phases of electromagnetic radiation passing through the two electro-optic modulators .sub.1, .sub.2 respectively.

[0161] A further difference from the cavity shown in FIG. 1 is that the cavity 22 shown in FIG. 4 has a lower mirror 27 that is leaky and so is arranged to provide an output 34 for a mode of electromagnetic radiation .sub.2. The de-multiplexer 21 also includes an output 30 that is arranged to receive the other mode of electromagnetic radiation .sub.1 from the output mirror 25.

[0162] Owing to the time dependency of the control fields R.sub.1(t), R.sub.2(t), the electromagnetic radiation .sub.1, .sub.2 output from the outputs 30, 34 of the cavity 2 respectively comprises the two modes individually which are spatially separated, as will now be described.

[0163] In operation of the de-multiplexer 21, two co-propagating modes of electromagnetic radiation .sub.in (e.g. that have been generated by the signal source 29, and encoded and transmitted together as a telecommunications signal) that are desired to be separated for decoding, are directed through the input 28 of the de-multiplexer 21 to be incident upon the input mirror 24 of the cavity 22. At the same time, the control sources 32, 33 generate respective time-dependent control fields R.sub.1(t), R.sub.2(t) to be incident upon the respective electro-optic modulators .sub.1, .sub.2 such that they overlap both spatially and contemporaneously on the electro-optic modulators .sub.1 .sub.2 with the two co-propagating modes of electromagnetic radiation .sub.in.

[0164] The incidence of the time-dependent control fields R.sub.1(t), R.sub.2(t) on the electro-optic modulators .sub.1, .sub.2 of the cavity 22 causes the cavity 22 to accept one of the co-propagating modes of electromagnetic radiation .sub.in onto the stationary mode {circumflex over (B)} of the cavity 22. The other one of the co-propagating modes of electromagnetic radiation .sub.in is thus output by the Mach-Zehnder interferometer through the output mirror 25 where it is received by the output 30 of the de-multiplexer 21 as a single mode of electromagnetic radiation .sub.1.

[0165] The accepted mode of electromagnetic radiation that is mapped onto the stationary mode {circumflex over (B)} of the cavity 22 by the action of the electro-optic modulators .sub.1, .sub.2 in the Mach-Zehnder interferometer is retained in the cavity 22 by the cavity mirrors 26, 27. However, owing to leaky lower mirror 27, the accepted mode leaks out through the mirror 27 where it is received by the other output 34 of the de-multiplexer 21 as a single mode of electromagnetic radiation .sub.1. As the two different output modes .sub.1, .sub.2 are output through different outputs 30, 34, owing to the action of the electro-optic modulators .sub.1, .sub.2 driven by the control fields R.sub.1(t), R.sub.2(t), the two co-propagating modes of electromagnetic radiation .sub.in input into the de-multiplexer 21 have thus been separated spatially.

[0166] The spatially separated output modes of electromagnetic radiation modes .sub.1, .sub.2 can then be detected and decoded as appropriate.

[0167] A further embodiment of a de-multiplexer that is implemented using a quantum memory device will now be described. FIG. 5 is a diagram showing schematically a quantum memory de-multiplexer 41 according to an embodiment of the present invention.

[0168] The quantum memory de-multiplexer 41 includes a vapour cell 42 containing an atomic ensemble 44, e.g. of rubidium or caesium atoms. The quantum memory de-multiplexer 41 also includes an input signal 46 (for inputting two co-propagating modes of electromagnetic radiation .sub.in) and a control laser 48 that acts as a control field source of pulsed near-infrared radiation .sub.j to be incident upon the atomic ensemble 44 within the vapour cell 42.

[0169] FIG. 6 shows the vapour cell 42 shown in FIG. 5 in more detail. The vapour cell 42 is formed as a hollow core 54 that contains the atomic ensemble 44 within a photonic crystal fibre 56 having a lattice 58 surrounding the hollow core 54. The photonic crystal fibre 56 containing the hollow core 54 is spliced into an optical fibre 60. The optical fibre 60 is connected at either end to the input signal 46 and the control laser 48 to enable them to direct their radiation .sub.in, .sub.j so to be incident upon the atomic ensemble 44 with the vapour cell 42.

[0170] Operation of the quantum memory de-multiplexer 41 shown in FIGS. 5 and 6 will now be described with reference to FIGS. 7 and 8. FIG. 7 shows a specific example of an atomic level scheme used in the quantum memory de-multiplexer 41 shown in FIGS. 5 and 6, when the vapour cell 42 contains an atomic ensemble 44 of rubidium atoms. FIG. 8 shows schematically two co-propagating modes of electromagnetic radiation being separated by the quantum memory de-multiplexer 41 shown in FIGS. 5 and 6.

[0171] The optical fibre 60 is prepared with the crystal fibre 56 spliced into it and containing an atomic ensemble 44 of rubidium atoms in the hollow core 54 of the crystal fibre 56 that forms the vapour cell 42. The optical fibre 60 is connected between the input signal 46 (which generates the two co-propagating modes of electromagnetic radiation .sub.in at a wavelength of 1528 nm (corresponding to a frequency of 196 THz)) and the control laser 48 having a wavelength of 780 nm (corresponding to a frequency of 384 THz).

[0172] The two co-propagating modes of electromagnetic radiation .sub.in generated by the input signal 46 is thus arranged to stimulate the 1528 nm 5P-4D transition of the atomic valence electrons in the rubidium atomic ensemble 44, and the electromagnetic radiation .sub.j of the control field generated by the control laser 48 is arranged to stimulate the 780 nm 5S-5P transition of the atomic valence electrons in the rubidium atomic ensemble 44. However, the electromagnetic radiation .sub.in, .sub.j generated by the input signal 46 and the control laser 48 respectively is arranged to be detuned from the 5S-5P and 5P-4D transitions respectively by up to 1 THz, but such that the combined application of the electromagnetic radiation .sub.in, .sub.j of one of the two co-propagating modes and the control field is resonant with the two-photon 5S-4D transition {circumflex over (B)}.

[0173] Therefore, when the two co-propagating modes of electromagnetic radiation .sub.in and a first pulse of the control field .sub.1 are incident contemporaneously on the atomic ensemble 44 (stimulating the 5P-4D and 5S-5P transitions of the rubidium atomic ensemble 44 respectively), the control field .sub.1 and one of the two co-propagating modes of electromagnetic radiation .sub.in create a coherent excitation of the two-photon 5S-4D transition {circumflex over (B)}. This has the effect of storing the one of the two co-propagating modes of electromagnetic radiation .sub.2 and allowing the other of the two co-propagating modes of electromagnetic radiation .sub.2 to pass through the atomic ensemble 44 in the vapour cell 42 such that it is output as a single mode of electromagnetic radiation .sub.1 at time t.sub.0 (as shown in FIG. 8).

[0174] To retrieve the stored mode of electromagnetic radiation .sub.2 from the atomic ensemble 44, a further pulse of the control field .sub.0 is generated by the control laser 48 to be incident upon the atomic ensemble 44. This control pulse Co stimulates the 5S-5P transition, causing the emission of the stored mode of electromagnetic radiation .sub.2 from the atomic ensemble 44 at a time t.sub.1 (as shown in FIG. 8).

[0175] The temporally separated output modes of electromagnetic radiation .sub.1, .sub.2 can then be detected and decoded as appropriate.

[0176] FIG. 9 shows schematically a quantum memory de-multiplexer according to another embodiment of the present invention.

[0177] In the de-multiplexer 61 shown in FIG. 9, the atomic ensemble 62 is placed inside a cavity 64. The two co-propagating modes of electromagnetic radiation .sub.in to be separated are input through a mirror 66 of the cavity, with the temporally separated output modes .sub.out being output through the same mirror 66.

[0178] In operation, the quantum memory de-multiplexer 61 shown in FIG. 9 operates in the same manner as the embodiment shown in FIGS. 5-8, except that the cavity 64 helps to increase the coupling of the control field and the one of the two co-propagating modes of electromagnetic radiation to the respective transitions of the atomic valence electrons of the atomic ensemble.

[0179] FIG. 10 shows schematically a de-multiplexer 71 used in combination with a mode converter 72 according to another embodiment of the present invention.

[0180] The de-multiplexer 71 (which may be any of the quantum memory de-multiplexers according to the embodiments shown in FIGS. 5-10) is used as described above, to separate two co-propagating modes of electromagnetic radiation .sub.in into two temporally separate output modes of electromagnetic radiation .sub.1, .sub.2. For example, one of the output modes .sub.1 may correspond to the mode HG.sub.0 shown in FIG. 2 and the other of the output modes .sub.2 may correspond to the mode HG.sub.1 shown in FIG. 2.

[0181] The mode converter 72 is configured in the same way as the de-multiplexer 71, except that only a single mode of electromagnetic radiation .sub.2 is input to be accepted into the atomic ensemble. Thus the mode converter 72 is positioned relative to the de-multiplexer 71 so that one of the output modes of electromagnetic radiation .sub.2 is received by and input into the mode converter 72. The control field .sub.j is shaped and used, with the mode of electromagnetic radiation .sub.2 input into the mode converter 72 to create a coherent excitation of the atomic ensemble of the mode converter 72 such that the mode of electromagnetic radiation .sub.2 is stored in the atomic ensemble of the mode converter 72. In the same manner as for the de-multiplexer, the control field .sub.j is subsequently shaped and caused to be incident upon the atomic ensemble of the mode converter 72 such that the mode of electromagnetic radiation .sub.2 is output as a different mode of electromagnetic radiation, e.g. corresponding to the mode HG.sub.0 shown in FIG. 2.

[0182] Therefore the combination of the de-multiplexer 71 and the mode converter 72 can be used to separate two co-propagating modes of electromagnetic radiation .sub.in into two temporally separate output modes of electromagnetic radiation which are described by the same functional (e.g. Hermite-Gauss) mode.

[0183] It can be seen from the above that, at least in preferred embodiments, the invention provides a de-multiplexing device that is able to separate temporal field orthogonal modes of electromagnetic radiation temporally and/or spatially, owing to the mode selective time-dependent control field acting on the volume in a way that is free from parametric classical non-linear optics. Thus the de-multiplexer allows such temporal field orthogonal modes to be used for encoding data for telecommunications, for example. This helps to open up more space for the encoding of data, thus enabling such modes to be able to pack more data into the existing telecommunication bands, for example, and may be compatible with dense wavelength division multiplexing (DWDM).