Abstract
A method is proposed for generating single photons with a predetermined wavelength f.sub.V, with the following steps: i) generating a single photon, preferably in a source and a resonator, wherein the single photon has a resonator wavelength f.sub.R and a resonator bandwidth f.sub.BR, ii) measuring the resonator wavelength f.sub.R, preferably in a wavelength standard, wherein the single photon is guided from the resonator to the wavelength standard via a beam guide, iii) comparing the resonator wavelength f.sub.R with the predetermined wavelength f.sub.V and generating a control signal on the basis of the comparison, preferably in a controller, iv) adjusting the resonator using the control signal in order to change the resonator wavelength f.sub.R toward or to the predetermined wavelength f.sub.V, v) repeating steps i to iv) until the resonator wavelength f.sub.R corresponds to the predetermined wavelength f.sub.V and then coupling out.
Claims
1. A method for generating single photons with a predetermined wavelength f.sub.V comprising: i) generating a single photon, wherein the single photon has a resonator wavelength f.sub.R and a resonator bandwidth f.sub.BR; ii) measuring the resonator wavelength f.sub.R, wherein the single photon is guided from a resonator to a wavelength standard via a beam guide; iii) comparing the resonator wavelength f.sub.R with the predetermined wavelength f.sub.V and generating a control signal on the basis of the comparison; iv) adjusting the resonator using the control signal in order to change the resonator wavelength f.sub.R toward or to the predetermined wavelength f.sub.V; and v) repeating steps i to iv) until the resonator wavelength f.sub.R corresponds to the predetermined wavelength f.sub.V and then coupling out a single photon with a predetermined wavelength f.sub.V into an output.
2. The method for generating single photons with a predetermined wavelength f.sub.V according to claim 1, wherein steps i) to iv) or step v) form a regulation in the case of or during the generation of single photons with the predetermined wavelength f.sub.V.
3. The method for generating single photons with a predetermined wavelength f.sub.V according to claim 1, wherein at least one of (i) the wavelength of the single photon corresponds to a control variable, (ii) the measured resonator wavelength f.sub.R corresponds to an actual value, or (iii) the predetermined wavelength f.sub.V corresponds to a set point.
4. The method for generating single photons with a predetermined wavelength f.sub.V according to claim 3, wherein in step iii) the controller compares the actual value and the set point and generates the control signal on the basis of the comparison.
5. The method for generating single photons with a predetermined wavelength f.sub.V according to claim 1 wherein the method comprises: a) measuring the resonator wavelength f.sub.R in the wavelength standard as measuring device, b) comparing the resonator wavelength f.sub.R with the predetermined wavelength f.sub.V and generating the control signal in a controller, and c) adjusting the resonator using the control signal as actuating means.
6. The method for generating single photons with a predetermined wavelength f.sub.V according to claim 1, wherein steps i) to iv) are repeated until the resonator wavelength f.sub.R of the single photon generated in step i) lies in a range of ±0.2 nm, around the predetermined wavelength f.sub.V.
7. The method for generating single photons with a predetermined wavelength f.sub.V according to claim 1, wherein the predetermined wavelength f.sub.V is a Fraunhofer line.
8. The method for generating single photons with a predetermined wavelength f.sub.V according to claim 1, wherein the single photon is generated in step i) by spontaneous emission or spontaneous parametric conversion.
9. The method for generating single photons with a predetermined wavelength f.sub.V according to claim 1, wherein in step i) source is excited by the resonator to emit the single photon with a resonator wavelength f.sub.R and a resonator bandwidth f.sub.BR, or the source generates a single photon with a source wavelength f.sub.Q and a source bandwidth f.sub.BQ and the resonator filters therefrom a single photon which has the resonator wavelength f.sub.R and the resonator bandwidth f.sub.BR, wherein the predetermined wavelength f.sub.V and the resonator wavelength f.sub.R are contained in the range of the source bandwidth f.sub.BQ.
10. The method for generating single photons with a predetermined wavelength f.sub.V according to claim 1, wherein in step iv) the resonator is regulated or is formed so that the resonator can be regulated at least one of chemically, thermally, electrically, mechanically or optically.
11. The method for generating single photons with a predetermined wavelength f.sub.V according to claim 1, wherein in step i) the resonator is coupled to the source at least one of photonically or mechanically.
12. The method for generating single photons with a predetermined wavelength f.sub.V according to claim 1, wherein the measuring in step ii) is effected through a dispersive element or through an absorber.
13. The method for generating single photons with a predetermined wavelength f.sub.V according to claim 1, wherein the beam guide is formed switchable in order to guide the single photon from a source and the resonator to a wave standard in step ii) and to guide the single photon from the source and the resonator to the output in step v), or the beam guide is formed as a passive optical element, wherein on average a particular proportion of single photons generated are either guided to the wavelength standard or guided to the output.
14. A device for generating single photons with a predetermined wavelength f.sub.V comprising: a source; a resonator; a beam guide; and a wavelength standard, wherein the source and the resonator generate single photons with a resonator wavelength f.sub.R and a resonator bandwidth f.sub.BR, the beam guide guides the single photon from the resonator to the wavelength standard or to an output, and the wavelength standard measures the wavelength of the single photon, and wherein the device has a controller, and the device has a control circuit for regulating the resonator wavelength f.sub.R to the predetermined wavelength f.sub.V, and wherein the control circuit is formed of the resonator as actuating means, the wavelength standard as measuring device and the controller for controlling the resonator.
15. The device for generating single photons with a predetermined wavelength f.sub.V according to claim 14, wherein the controller is formed as a continuous controller.
Description
[0105] FIG. 1 is a schematic representation of the method and the device for generating single photons with a predetermined wavelength f.sub.V;
[0106] FIG. 2a is a first embodiment example of an arrangement of the source and the resonator with the source in the resonator;
[0107] FIG. 2b is a second embodiment example of an arrangement of the source and the resonator with the source outside the resonator;
[0108] FIG. 3a is a first embodiment example of the wavelength standard with the gas cell;
[0109] FIG. 3b is a second embodiment example of the wavelength standard with the grating;
[0110] FIG. 4 is a wavelength intensity graph with the source bandwidth f.sub.BQ, the resonator bandwidth f RQ and the predetermined wavelength f.sub.V; and
[0111] FIG. 5 is a transmission spectrum of a gas in the wavelength standard.
[0112] FIG. 1 shows a schematic representation of the method and the device 1 for generating single photons 4 with a predetermined wavelength f.sub.V.
[0113] In the embodiment example of FIG. 1, a single photon 4 is generated in a source 2 and a resonator 3. The single photon 4 has a resonator wavelength f.sub.R and a resonator bandwidth f.sub.BR and is then coupled out of the resonator 3 and guided to a wavelength standard 6 via a beam guide 5. The resonator wavelength f.sub.R of the single photon 4 is measured in the wavelength standard 6, which generates an electrical signal 13 corresponding to the measured resonator wavelength f.sub.R. In the controller 7, the electrical signal 13 is compared with the predetermined wavelength f.sub.V. On the basis of the comparison of the resonator wavelength f.sub.R and the predetermined wavelength f.sub.V, a control signal 9 is generated, which is used to change the resonator 3 toward the predetermined wavelength f.sub.V or to the predetermined wavelength f.sub.V.
[0114] The generation, guiding and measurement of a single photon 4 as well as the generation of the control signal 9 and the adjustment of the resonator wavelength f.sub.R is repeated according to the invention until the resonator wavelength f.sub.R corresponds to the predetermined wavelength f.sub.V.
[0115] In the embodiment example of FIG. 1, the source 2 is arranged in the resonator 3. Through the photonic coupling of the resonator 3 to the source 2, the wavelength and the bandwidth of the single photon 4 are adjusted to the resonator wavelength f.sub.R and the resonator bandwidth f.sub.BR. The photonic coupling reduces the lifespan of the spontaneous emission in the source 2. The source 2 in the embodiment example of FIG. 1 is a two-dimensional hexagonal boron nitride structure with an impurity which is excited by a pulsed laser to generate single photons 4 with the resonator bandwidth f.sub.BR. The resonator 3 in the embodiment example of FIG. 1 is formed of two highly reflective resonator walls and a resonator material 14 between the resonator walls. The high reflectance of the resonator walls is generated by a multilayer system with different refractive indices. In the embodiment example of FIG. 1, the distance between the resonator walls can be changed using an electrical signal, for example a piezoelectric signal, as control signal 9, because the control signal 9 acts on the resonator material 14 and thereby brings about a change in length of the resonator material 14 and thus also the change in the distance between the resonator walls.
[0116] In the embodiment example of FIG. 1, the resonator wavelength f.sub.R of randomly selected single photons 4 is measured in the wavelength standard 6 through the passive beam guide 5. The passive beam guide 5 is formed as a beam splitter in this embodiment example. These measured single photons 4 are used to adjust the resonator 3 and the remaining single photons 4 generated are coupled out toward the output 8. The beam splitter is chosen such that for example on average only every 1000th single photon 4 is guided toward the wavelength standard 6 and the remaining single photons 4 are reflected toward the output 8. In this embodiment example, the regulation of the resonator 3 is always carried out when a single photon 4 is detected on the wavelength standard.
[0117] The embodiment example of FIG. 1 can also be formed as an active beam guide 5 in order to carried out the adjustment of the resonator wavelength f.sub.R to the predetermined wavelength f.sub.V first. After the resonator 3 has been adjusted to the predetermined wavelength f.sub.V, the active beam guide 5 can be repositioned in a targeted manner in order to guide the subsequently generated single photons 4 toward the output 8. The active beam guide 5 can be formed for example by a controllable mirror or an electro-optic modulator with a polarization element or an acousto-optic modulator or as a liquid crystal. In such embodiments, the adjustment can be effected in a targeted manner when a control and a regulation of the resonator 3 is carried out.
[0118] FIGS. 2a and 2b show different embodiment examples of the arrangement of the source 2 and the resonator 3.
[0119] In FIG. 2a, as in the embodiment example of FIG. 1, the source 2 is arranged in the resonator 3, whereby the source 2 is excited only to emit single photons 4 with a resonator wavelength f.sub.R and a resonator bandwidth f.sub.BR through the arrangement in the resonator 3. The advantage of an arrangement of the source 2 in the resonator 3 is that a spontaneous emission of the single photons 4 in the source 2 with the resonator wavelength f.sub.R and with the resonator bandwidth f.sub.BR is increased by the resonator 3. In the process, the linewidth of the emission falls to resonator wavelength f.sub.R through the coupling of the source 2 to the resonator 3. Further, through the coupling the resonator 3 reduces the lifespan of the excited state and thus increases the emission rate of the single photons 4 with the resonator wavelength f.sub.R. In contrast to an arrangement of the source 2 outside the resonator 3, here the single photons 4 are not filtered out of the source bandwidth f.sub.BQ, but rather the source 2 is excited directly to emit the single photons 4 with the resonator bandwidth f.sub.BR.
[0120] In FIG. 2b, the source 2 is arranged in front of the resonator 3. The source 2 has a source bandwidth f.sub.BQ predefined by the source 3. In the source, single photons 4 with the source bandwidth f.sub.BQ are generated and then coupled into the resonator 3. In the resonator 3, single photons 4 with the resonator bandwidth f.sub.RQ are filtered out by the resonator geometry and only these single photons are coupled out of the resonator 3. The advantage of an arrangement of the source 2 outside the resonator 3 is the simple arrangement and formation of the source 2 and the resonator 3.
[0121] FIGS. 3a and 3b show different embodiment examples of the wavelength standard 6.
[0122] FIG. 3a shows a first embodiment example of the wavelength standard 6 with gas cell 11 and a single-photon detector 12. A single photon 4 is conducted through the gas cell 11 and can be detected behind the gas cell 11 by the single-photon detector 12, which generates an electrical signal 13 in the case of a detection and relays this to the controller 7. The transmission spectrum of the gas cell 11 is represented in FIG. 5. In this embodiment example, the gas in the gas cell 11 absorbs the single photon 4 if the single photon 4 has the predetermined wavelength f.sub.V. Thus, when the predetermined wavelength f.sub.V is reached, single photons 4 are no longer detected by the single-photon detector 12. The controller 7 is formed as a PI controller in this embodiment example and regulates the resonator 3 such that the electrical signal 13 of the single-photon detector 12 is minimized Since the transmission spectrum of the gas in the gas cell 11 has a minimum both in the case of the predetermined wavelength f.sub.V and at the left and right edge, in this embodiment example the transmission spectrum of the gas is measured first in order to determine the starting position of the resonator. This can be effected by actively readjusting the resonator over a broad wavelength range.
[0123] FIG. 3b shows a second embodiment example of the wavelength standard 6 with a grating 10. In this embodiment example, corresponding to their resonator wavelength f.sub.R single photons 4 are reflected at different angles and reflected toward the single-photon detector 12. In the case of the detection of a single photon 4, the single-photon detector 12 generates the electrical signal 13 and relays it to the controller 7. The predetermined wavelength f.sub.V can be adjusted using the position of the single-photon detector 12 and the angle of incidence of the single photon 4. The resolving power of the second embodiment example can be improved through the arrangement of several gratings one behind another.
[0124] FIG. 4 shows a schematic representation of the spectrum of the source with the source bandwidth f.sub.BQ, the resonator bandwidth f.sub.BR, the resonator wavelength f.sub.R, the predetermined wavelength f.sub.V, to which the resonator 3 is to be adjusted, and the direction of the regulation X. A source 2, arranged outside the resonator 3, has the source bandwidth f.sub.BQ. The resonator 3 filters single photons 4 with the resonator bandwidth f.sub.BR out of the source bandwidth f.sub.BQ. In the case of a source 2 inside the resonator 3, the source 2 has the theoretical source bandwidth f.sub.BQ, wherein the source 2 is, however, excited by the resonator 3 only to generate single photons 4 with a resonator bandwidth f.sub.BR. The regulation X indicates the direction and the difference in wavelength from the source bandwidth f.sub.BQ to the predetermined wavelength f.sub.V. It is possible to achieve the adjustment of the resonator 3 through one step or else through several small steps in order to approach the predetermined wavelength f.sub.V.
[0125] FIG. 5 shows a schematic representation of the transmission spectrum of a gas in a gas cell 11 of an embodiment example of the wavelength standard 6, wherein 0 corresponds to the predetermined wavelength f.sub.V.
LIST OF REFERENCE NUMBERS
[0126] 1 device for generating single photons with a predetermined wavelength f.sub.V [0127] 2 source [0128] 3 resonator [0129] 4 single photon [0130] 5 beam guide [0131] 6 wavelength standard [0132] 7 controller [0133] 8 output [0134] 9 control signal [0135] 10 grating [0136] 11 gas cell [0137] 12 detector [0138] 13 electrical signal [0139] 14 resonator material [0140] f.sub.R resonator wavelength [0141] f.sub.BR resonator bandwidth [0142] f.sub.Q source wavelength [0143] f.sub.BQ source bandwidth [0144] f.sub.V predetermined wavelength [0145] X regulation
