DEVICE FOR GENERATING INDIVIDUAL PHOTONS

Abstract

A device for generating individual photons with energy E includes quantum emitters, having at least one determined transition with the energy E from an energy level N* to a lower energy level N1. The emitters are near a propagation path running from first to second regions. The device also includes at least one light source to output light, for propagation along the path. The light has the energy E for resonant excitation of the energy level N*. The emitters are arranged so that optionally exactly Z emitters are illuminated, forming an optical thickness τ>0 for the light along the path. The number Z lies in a range of ZO±10% and ZO is a number at which a maximum destructive interference in the second region occurs between a two-photon component of the light scattered on the ZO emitters and a two-photon component of the non-scattered light.

Claims

1-13. (canceled)

14. A device (1) for generating single photons (2) with an energy E, comprising a plurality of quantum emitters (3) which have at least one determined transition with the energy E from an energy level N* to a lower energy level N1, wherein the quantum emitters (3) are arranged in the region of a propagation path (4) extending from a first region (19) to a second region (20), the device (1) further comprising at least one light source (5) for emitting light (6) for propagation along the propagation path (4), wherein the light (6) has the energy E for resonantly exciting the energy level N*, wherein the quantum emitters (3) are arranged in such a way that only exactly Z quantum emitters (3) are illuminated, which form an optical thickness τ>0 for the light (6) along the propagation path (4), the number Z lying in a range Z0±10%, and wherein Z0 is that number at which, in the second region (20), a destructive interference between a two-photon component of the light (6) that is scattered at these Z0 quantum emitters (3) and a two-photon component of the light (6) that is not scattered is at a maximum.

15. The device (1) according to claim 14, wherein the light source is a laser source (5).

16. The device (1) according to claim 14, wherein the quantum emitters (3) comprise atoms and/or ions and/or molecules and/or quantum dots and/or defects in crystal structures.

17. The device (1) according to claim 14, wherein at least one optical element is provided for defining the propagation path.

18. The device (1) according to claim 14, wherein an optical waveguide (7) comprising a waveguide core (8) is provided for guiding the light (6), wherein the Z quantum emitters (3) are arranged in a near field, and wherein the near field comprises the electromagnetic field of the light (6) in the waveguide core (8) and outside the waveguide core (8).

19. The device (1) according to claim 18, wherein the waveguide is designed as an optical fiber (7).

20. The device (1) according to claim 18, wherein the waveguide core is arranged on a substrate.

21. The device (1) according to claim 14, wherein a photonic crystal fiber having a hollow core is provided for guiding the light (6).

22. The device (1) according to claim 14, wherein a crystal (11) is arranged in the propagation path (4), and wherein the Z quantum emitters (3) are formed by crystal defects in the crystal (11).

23. The device (1) according to claim 22, wherein the Z quantum emitters (3) are formed by dopant atoms, wherein the material of the crystal (11) comprises silicon, and wherein the dopant atoms comprise boron atoms or phosphorus atoms.

24. The device (1) according to claim 14, wherein the quantum emitters (3) are in gas form in the region of the propagation path (4).

25. The device (1) according to claim 22, wherein the quantum emitters (3) have a further transition with the energy E′ from the energy level N1 to a lower energy level NO, wherein at least one light source (17) is provided for emitting further light (18), and wherein the further light (18) has the energy E′, in order to resonantly excite the energy level N1 at quantum emitters (3) and then to excite the energy level N* with the light (6) of energy E at Z of these quantum emitters (3).

26. The device (1) according to claim 24, wherein the quantum emitters (3) comprise lithium atoms and/or sodium atoms and/or potassium atoms and/or rubidium atoms and/or caesium atoms.

27. The device (1) according to claim 14, wherein the device (1) comprises the at least one light source (5) for emitting light (6) of one light mode.

28. The device (1) according to claim 15, wherein the light source (5) comprises at least one laser diode.

29. The device (1) according to claim 20, wherein the waveguide core is at least in sections sunk into the substrate.

30. The device (1) according to claim 21, wherein the quantum emitters (3) are arranged in the hollow core.

31. The device (1) according to claim 22, wherein the Z quantum emitters (3) are formed by impurity atoms (12) and/or vacancies.

32. The device (1) according to claim 31, wherein the Z quantum emitters (3) are formed by dopant atoms.

33. The device (1) according to claim 25, wherein the at least one light source (17) is provided for emitting further light (18) of a further light mode.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0094] The invention will now be explained in greater detail on the basis of exemplary embodiments. The drawings are given by way of example and are intended to explain the concept of the invention but not to restrict it in any way or even exhaustively reflect it.

[0095] In the drawings:

[0096] FIG. 1 shows a schematic diagram of the basic mode of operation of the device according to the invention,

[0097] FIG. 2 shows a schematic diagram of an embodiment of the device according to the invention with an optical fibre, wherein quantum emitters are arranged in the latter,

[0098] FIG. 3 shows a schematic diagram of another embodiment of the device according to the invention, wherein the quantum emitters are impurity atoms in a crystal,

[0099] FIG. 4 shows a schematic diagram of another embodiment of the device according to the invention, wherein the quantum emitters are in gas form,

[0100] FIG. 5 shows a schematic diagram of another embodiment of the device according to the invention with an optical fibre, wherein the quantum emitters are arranged outside the fibre.

WAYS OF CARRYING OUT THE INVENTION

[0101] FIG. 1 shows a schematic illustration of the underlying principle of the present invention of a single-photon source or a device 1 for generating single photons or a single-photon stream 2 of a particular energy E. The device 1 uses the principle of photon-number-dependent scattering of light 6 of energy E by single quantum emitters 3, wherein the quantum emitters 3 can be implemented for example by atoms, ions, molecules, quantum dots or defects in crystal structures. A quantum emitter 3, which is coupled to the light 6 or to a light field with a well-defined optical mode, such as, for example, a laser beam from a laser source 5, interacts differently with the light 6 depending on whether one or two photons, or more specifically whether a single-photon component or a two-photon component 14 of the light 6, arrive at the quantum emitter 3. Here, light mode is to be understood to mean that the light 6 propagates along a particular propagation path 4 (indicated by the two solid black lines in

[0102] FIG. 1), which extends from a first region 19 to a second region 20. The quantum emitters 3 are arranged in the region of the propagation path 4. One or more optical elements may optionally be provided, in order to define the propagation path 4 more precisely.

[0103] Said components of the light 6 may be scattered at the respective quantum emitter 3 or also may not be scattered. Interference thus occurs between a non-scattered two-photon component and a scattered two-photon component of the outgoing light 6 for a selected observation direction or at a selected observation location (for reasons of clarity and presentation, only the two-photon component 14 of the incoming light 6 is shown in FIG. 1, but not the scattered and the non-scattered two-photon component). In the device 1, the observation direction is predefined by the propagation path 4, and the observation location corresponds to the second region 20.

[0104] A photon-number-dependent phase shift of the light 6 occurs, which in turn can lead to destructive interference of the scattered two-photon component with the non-scattered two-photon component at the observation location or in the second region 20. If the correct number Z0 of quantum emitters 3 is coupled to the light field mode, the point of perfect destructive interference can be reached, at which the losses for two jointly propagating photons are 100%. Remarkably, single photons still exhibit finite transmission at this working point. As a result, the light 6 transmitted through the ensemble of quantum emitters 3 has a perfect single-photon characteristic, apart from higher-order photon components (three-photon component, four-photon component, etc.) that statistically only play a role at extremely high intensities, and the single-photon stream 2 is obtained. This means that two photons never come through the ensemble at the same time.

[0105] In FIG. 1, total losses 13 are indicated by the wavy arrows pointing away from the propagation path 4, these losses being caused by the destructive interference between the scattered and non-scattered two-photon components and also comprising the attenuation of the single-photon component.

[0106] Said scattering of the two-photon component 14 of the light 6 is a resonant scattering, i.e. the quantum emitters 3 have a transition with the energy E from an energy level N* to a lower energy level N1, the energy level N* being excited by the light 6 or the two-photon component 14 of energy E thereof.

[0107] For the sake of good order, it should also be noted at this point that the energy E is to be understood not as an infinitely sharp value, but rather as a value that has a certain bandwidth, which is defined by the bandwidth of said transition or by the lifetime of the energy level N*. Accordingly, “light 6 of energy E (for resonantly exciting the energy level N*)” means not a perfectly sharp energy value E of the light 6, but rather that the light 6 has a bandwidth around the energy value E, it being necessary for the bandwidth to be of the same order of magnitude as or smaller than the bandwidth of the transition from the energy level N* to the energy level N1.

[0108] According to the invention, the arrangement of the quantum emitters 3 is such that only exactly Z quantum emitters 3 are illuminated, which form an optical thickness τ>0 for the light 6 along the propagation path 4, wherein the number Z lies in a range Z0±10%, and wherein Z0 is that number at which the destructive interference between the two-photon component of the light 6 that is scattered at these Z0 quantum emitters 3 and the two-photon component of the light 6 that is not scattered is at a maximum. In practice, the aforementioned range for Z around Z0 ensures a sufficient purity of the single-photon characteristic of the single-photon stream 2 for many applications.

[0109] In the exemplary embodiment of FIG. 2, a waveguide in the form of an optical fibre 7 is provided, into which the light 6 from a laser source 5 is coupled in a manner known per se, and which defines the propagation path 4 (indicated by the dashed line in FIG. 2). It should be noted that the propagation path need not be rectilinear, but rather—in a manner corresponding to the specific course of the optical fibre 7 —may also be curved practically at will. The optical fibre 7 comprises a fibre core 8 or a waveguide core for guiding the light 6.

[0110] Optical waveguides that have a waveguide core are known per se. The waveguide core or fibre core 8 constitutes a first optical medium having a refractive index n1 and is surrounded by at least one second optical medium having a refractive index n2 (not shown in FIG. 2 for reasons of clarity), or the waveguide core adjoins, by way of an optical interface, the at least one second optical medium, which forms a cladding for the waveguide core. In the exemplary embodiment shown, the optical interface is formed by the surface 10 of the fibre core 8. For the light 6, n1>n2 applies. The latter ensures the total reflection of the light 6 and thus the transporting of the light 6 in the waveguide core or fibre core 8.

[0111] When using optical fibres 7 in practice, particularly in data transfer applications, use is typically made of light 6 that has an energy corresponding to a vacuum wavelength λ.sub.0 of the light 6 of from approximately 100 nm to approximately 2000 nm, in particular from approximately 850 nm to 1550 nm.

[0112] The Z quantum emitters 3 are arranged in a near field, wherein the near field consists of the electromagnetic field of the light 6 in the waveguide core or fibre core 8 and outside the fibre core 8. This means that the Z quantum emitters 3 need not necessarily be arranged within the fibre core 8, as in the exemplary embodiment of FIG. 2, but rather can also be arranged outside the fibre core 8, provided that they are arranged close enough to the fibre core 8 to be able to couple sufficiently strongly to the near field. Such an arrangement outside the waveguide core is illustrated in FIG. 2 by the quantum emitter 3 shown in dotted line outside the fibre core 8, wherein this quantum emitter 3 is at a certain normal distance 9 from the surface 10 of the fibre core 8, the normal distance 9 being measured normal to the surface 10. To ensure sufficient coupling of the quantum emitters 3 to the near field, the normal distance 9 may lie for example in the range of up to 5*λ.sub.0 or up to 20*λ.sub.0, where λ.sub.0 is the vacuum wavelength of the light 6 of energy E. In this case, the light field need not drop to exactly zero outside this normal distance 9.

[0113] The light 6 guided in the optical fibre 7 propagates through the ensemble of Z quantum emitters 3, which are arranged in the fibre core 8 and which in particular may be impurity atoms or quantum dots. The quantum emitters 3 are arranged one behind the other, as viewed along the propagation path 4, and form an optical thickness T greater than zero, as a result of which the transmission of the light 6 is fundamentally attenuated. This results in the above-described resonant scattering of the two-photon component 14 of the light 6 at the quantum emitters 3 and in the destructive interference between the scattered two-photon component and the non-scattered two-photon component. A single-photon stream 2 accordingly exits from the optical fibre 7, which stream is of course significantly attenuated in comparison to the intensity of the incoming light 6, but still has a finite intensity or a relatively high rate of single photons per second (indicated in FIG. 2 by the different-sized arrows for the incoming light 6 and the single-photon stream 2).

[0114] FIG. 5 shows a further exemplary embodiment with an optical fibre 7, wherein in this case the quantum emitters 3 are arranged not in the fibre core 8, but rather outside the fibre core 8. Specifically, the fibre core 8 has a region with a narrowed cross-section or with a very small lateral dimension, the quantum emitters 3 being arranged along this region. This arrangement can take place in a manner known per se, for example by means of an optical trap, cf. for example R. Mitsch et al., Physical Review A 89, 063829 (2014). By way of example, the quantum emitters 3 may be Cs atoms, wherein the transition between the energy levels N* and N1 is defined by the Cs-D.sub.2 line with a vacuum wavelength λ≈852 nm.

[0115] The above-described resonant scattering of the two-photon component 14 of the light 6 at the quantum emitters 3 and the destructive interference between the scattered two-photon component and the non-scattered two-photon component in the second region 20 thus also occur in the exemplary embodiment of FIG. 5, so that a single-photon stream 2 exits from the optical fibre 7.

[0116] FIG. 3 shows a schematic diagram of another embodiment of the device 1 according to the invention, wherein the quantum emitters 3 are implemented by crystal defects in the form of impurity atoms 12 in a crystal 11. By way of example, the crystal 11 may be a semiconductor crystal, in particular a silicon crystal, wherein dopant atoms, for example boron atoms or phosphorus atoms, are provided as impurity atoms 12. The crystal 11 is transparent to the light 6 used and barely attenuates the light 6.

[0117] Also in this exemplary embodiment, one or more optical elements (not shown) may be provided in order to define the propagation path 4 (indicated by the curved solid lines in FIG. 3). By way of example, at least one lens (not shown) may be provided for focusing the light 6 onto a limited spatial region of the crystal 11, resulting in the propagation path 4 shown in FIG. 3. The Z quantum emitters 3 illuminated by the light 6 are arranged in the region of this propagation path 4. The above-described resonant scattering of the two-photon component 14 of the light 6 at the quantum emitters 3 and the destructive interference between the scattered two-photon component and the non-scattered two-photon component in the second region 20 again occurs. A single-photon stream 2 thus exits from the crystal 11, which stream is of course significantly attenuated in comparison to the power of the incoming light 6, but still has a finite power or a relatively high rate of single photons per second (indicated in FIG. 3 by the different-sized arrows for the incoming light 6 and the single-photon stream 2).

[0118] FIG. 4 shows a schematic diagram of another embodiment of the device 1 according to the invention, wherein the quantum emitters 3 are in gas form. In particular, the quantum emitters 3 may in this case be formed by atoms; by way of example, the gas from quantum emitters 3 may be vaporized alkali metal atoms. In principle, the gas from quantum emitters 3 may be part of another gas, in particular a transparent or barely absorbent gas. In the exemplary embodiment shown, a gas 15 is illustrated which consists only of atomic quantum emitters 3.

[0119] In the exemplary embodiment shown, the gas 15 is arranged in a container 16 that is transparent to the light 6, so that the light 6 can pass largely unhindered through the walls of the container 16. This means that an attenuation of the light 6 when passing through the container is largely caused only by the gas 15.

[0120] Also in this exemplary embodiment, one or more optical elements (not shown) may be provided, in order to define the propagation path 4 (indicated by the curved solid lines in FIG. 4). By way of example, at least one lens (not shown) may be provided for focusing the light 6 onto a limited spatial region within the container 16, resulting in the propagation path 4 shown in FIG. 4. The Z quantum emitters 3 illuminated by the light 6 are arranged in the region of this propagation path 4.

[0121] It should be noted that, instead of the container 16, a photonic crystal fibre that has a hollow core may also be provided for example, the gas 15 of the quantum emitters 3 being arranged in the hollow core. The photonic crystal fibre or the course of its hollow core at the same time defines the propagation path 4.

[0122] The arrangement of these Z quantum emitters 3 in the gas 15 is in any case not static, but rather is a momentary arrangement since the quantum emitters 3 move. The velocity distribution of the quantum emitters 3 can be described at least approximately by the Maxwell-Boltzmann distribution. Due to the Doppler effect, the energy E of the transition between the energy levels N* to N1 is shifted as a function of the velocity of the respective quantum emitter 3 (so-called “Doppler shift”). Depending on the temperature, and according to the velocity distribution, there are more or fewer atoms having a velocity that is so great that the resulting Doppler shift is larger than the bandwidth of the transition, so that a resonant excitation of the energy level N* by means of the light 6 of energy E is not possible for these atoms.

[0123] To illustrate an example, consider, as quantum emitters, gaseous caesium atoms and the Cs-D.sub.2 transition with an energy of approximately 1.45 eV and a vacuum wavelength of approximately 852 nm and a frequency of approximately 352 THz. The lifetime of approximately 30.5 ns relevant to the transition results in a bandwidth of the transition of approximately 2*10.sup.−8 eV, which corresponds to a frequency of approximately 5 MHz. The maximum of the Maxwell-Boltzmann distribution yields v.sub.m ≈193.7 m/s as the most probable velocity v.sub.m of a Cs atom at a temperature of T=300 K. The resulting Doppler shift is approximately 227 MHz or 9.4*10.sup.−7 eV and is therefore significantly larger than the bandwidth of the transition.

[0124] According to the Maxwell-Boltzmann distribution, however, there is also a fraction of atoms that are so slow that the Doppler shift does not lead out of the bandwidth of the transition. For these atoms, a resonant excitation by means of the light 6 of energy E is still possible. In the exemplary embodiment of FIG. 4, it can be ensured, by suitably selecting the propagation path 4, that statistically exactly Z quantum emitters 3 of the gas 15 are illuminated, these Z quantum emitters 3 being slow enough for resonant excitation of the energy level N*.

[0125] It should be noted that, in the above-mentioned variant with the photonic crystal fibre, the propagation path 4 is predefined by the course of the hollow core, but the latter represents a very precisely defined volume. The number of quantum emitters 3 arranged in the hollow core can therefore be set very precisely, as a result of which it is likewise possible to ensure that statistically exactly Z quantum emitters 3 of the gas 15 are illuminated, these Z quantum emitters 3 being slow enough for resonant excitation of the energy level N*.

[0126] With the device 1 of the exemplary embodiment of FIG. 4, a two-stage method is implemented, in order to be able to control with particular ease the function of the device 1 according to the invention by using the described resonant scattering process. First, from all the quantum emitters 3 of the gas 15, those that are slow enough for resonant scattering with the light 6 are pre-selected; of these, in turn, Z quantum emitters 3 are then illuminated with the light 6. To this end, the quantum emitters 3 have a further transition with the energy E′ from the energy level N1 to a lower energy level NO, and a further laser source 17 is provided for emitting further light 18, preferably of a further light mode, the further light 18 having the energy E′. In the exemplary embodiment shown, the further light 18 impinges on the quantum emitters 3 in the same and/or parallel direction or approximately in the same and/or parallel direction as the light 6; however, an arrangement running in opposite directions would also be conceivable for example. The design is as follows: If the quantum emitters 3 are slow enough that the energy level N1—starting from the energy level NO—can be resonantly excited by means of the further light 18 of energy E′, then these quantum emitters 3 are also slow enough that subsequently the energy level N*—now starting from the energy level N1—can be resonantly excited by means of the light 6 of energy E. Only the arrangement of these Z quantum emitters 3 then plays a role for the resonant scattering process, with the destructive interference occurring between the scattered two-photon component and the non-scattered two-photon component of the light 6. Exiting from the container 16, therefore, is a single-photon stream 2 which is of course significantly attenuated in comparison to the intensity of the incoming light 6 but still has a finite intensity and a relatively high rate of single photons per second (indicated in FIG. 4 by the different-sized arrows for the incoming light 6 and the single-photon stream 2).

[0127] Vapours from alkali metal atoms, in particular from sodium, rubidium or caesium atoms, are suitable for implementing this exemplary embodiment. For caesium atoms, for example, the following atomic energy levels (in ascending order of energy) N0, N1, N* can be selected (transitions upon excitation: from N0 to N1 and from N1 to N*): N*=5D, N1=6P and N0=6S.

LIST OF REFERENCE SIGNS

[0128] 1 device [0129] 2 single-photon stream [0130] 3 quantum emitter, in particular atom, ion, molecule or [0131] quantum dot [0132] 4 propagation path [0133] 5 laser source [0134] 6 light [0135] 7 optical fibre [0136] 8 fibre core [0137] 9 normal distance [0138] 10 surface of the fibre core [0139] 11 crystal [0140] 12 impurity atoms [0141] 13 total losses [0142] 14 two-photon component of the incoming light [0143] 15 atomic gas [0144] 16 container for the gas [0145] 17 further laser source [0146] 18 further light [0147] 19 first region [0148] 20 second region