Methods for generating entangled photons

Abstract

A method is proposed for generating entangled photon pairs by propagating a laser beam with a wavelength <p through a crystal, preferably a crystal with a symmetry class C3v or D3 or D3h.

Description

The invention relates to a method for generating interdigitated photon pairs according to the features of the preamble of claim 1 and to an apparatus for generating interdigitated photon pairs according to the features of the preamble of claim 11.Entangled photon sources (engl. Excited photon sources EPS) are an important quantum photonic component. They are required for numerous methods in quantum technology. These include methods of quantum key distribution, quantum imaging, quantum spectroscopy or quantum information technology, e.g. for Boson sampling or the connection of quantum computers.The most widespread way of generating entangled photons makes use of the process of spontaneous parametric down conversion (SPDC), also called parametric fluorescence, in which a "mother photon" of high energy (short wavelength) breaks down into two daughter photons of lower energy (larger wavelength) in a 2)- nonlinear crystal. These two photons are correlated, i.e., they have a common two-particle wave function. Although the process takes place locally, macroscopically only those photon pairs can be observed in which all the involved sources are constructively superimposed. If the process occurs in a homogeneous medium, one speaks of phase matching of a plane wave pair (PM); in structured media, generalized mode matching of a mode pair.In order to generate an interleaved photon pair from the photon pair, the crystal and optical system must be matched to each other such that two independent and indistinguishable SPDC paths lead into the same mode pair. If the paths are indistinguishable, they interfere and interdigitated photons can be emitted.The current state of the art is familiar with a variety of source types which implement this concept. In this case, two categories can be roughly differentiated: common path EPS (CP-EPS) and distinct path EPS (DP-EPS).DP-EPS have two separate channels in which photon pairs are generated. These are superimposed in a second step by means of a coherent mixer. This class includes, for example. Sagnac loop sources; here, the paths are separated by the propagation direction and are superimposed via a beam splitter. This class also includes resonator comb sources; here, photon pairs are generated in different wavelength modes and superimposed in a sideband by means of a mixer (modulator). DP-EPS are generally very complex to construct and require a large number of photonic components of the highest quality. However, unlike CP-EPS, the phase matching PM and the mode matching MM and the conversion efficiency on each channel can be separately optimized. DP EPS are therefore often more efficient.CP-EPS do not require separate conversion along two modes with subsequent superposition. Instead, they use two independent channels of nonlinearity along a mode or mode pair that is spatially indistinguishable (at least in the far field). The most common case here is the polarization entanglement of plane waves along a common wave vector. However, such systems, although conceptually simpler, must allow phase matching PM (or mode matching MM) for both independently propagating photons and also synchronize SPDC efficiency along both channels in the same system. This generally only occurs for quite specific crystal classes along quite specific propagation axes, fixed polarizations under very sharply controlled PM or MM conditions, the sharply controlled ambient conditions (for example. Temperature) may then result. Thus, CP-EPS are generally not relevant from a technological standpoint because, although they are conceptually elegant, they are technically complicated to control.One way to undergo the phase matching (PM) conditions and the mode matching (MM) conditions, respectively, is to thin the crystals down to scales below the wavelength of the light (more precisely, below the coherence length). It could be shown that, for example, in thin gallium phosphide crystals, but with a crystal symmetry different from that described here, a form of polarization-entangled photon pairs can be produced. However, a tunable nature of different, maximally interlaced polarization states is not possible with this, but rather requires the specific crystal symmetry described here.Polarization-entangled sources which can generate different maximally entangled photon pairs in a tunable manner only on the basis of a specific crystal symmetry and without further optical components or, additionally, also in the same system, photon pairs with an adjustable degree of entanglement have not yet been studied and have not yet been known.The object of the present invention is to provide an improved method for generating interdigitated photon pairs and to provide a corresponding improved apparatus for generating interdigitated photon pairs.The object is achieved according to the invention by a method for generating interdigitated photon pairs according to the features of claim 1.According to the invention, a method for generating interdigitated photon pairs is proposed by propagating a laser beam having a wavelength λ p through a crystal, preferably a crystal having a symmetry class C 3v or D 3 or D 3h.The object is furthermore achieved by a device for generating interdigitated photon pairs according to the features of claim 9.According to the invention, a device for generating interdigitated photon pairs is proposed, comprising a laser for generating a laser beam and a crystal, preferably having a symmetry class C 3v or D 3 or D 3h.This approach, which can be converted into crystals with suitable crystal symmetry, preferably only into crystals with suitable crystal symmetry, has the following advantages:By means of the method according to the invention and the device according to the invention, interlaced photons can be generated with a very simple design with a minimum number of components and largely without the need for adjustment, and a source for generating interlaced photon pairs can be provided. The small number of components advantageously results in a stable and easily adjustable interdigitated photon pair source and a stable and easily adjustable method for generating interdigitated photon pairs. The source is set up without great effort and the interlaced photon pair source delivers interlaced photon pairs over a long period of time in a constantly high quality independently of environmental influences such as temperature changes.It is furthermore advantageous that scalable manufacturing capability of the photon source according to the invention is possible, for example by singulating crystal films of the crystal applied to suitable substrates, without further optical components being required. Furthermore, the method and the device according to the invention enable parallelizability with a high degree of indistinguishable for generalization to light sources for even more complex quantum states of light.It is also advantageous here that the method according to the invention and the device according to the invention can be implemented in a plurality of materials with partly souverous supply chains. The availability of the method according to the invention and of the device according to the invention can thus be ensured in an improved manner.The invention describes a method and apparatus by means of which entangled photons can be generated without the assistance of further optical components in a nonlinear optical crystal, thin film crystal or crystalline monolayer material, respectively, without the need for further optical components; with the exception of a laser required for parametric fluorescence (SPDC) excitation of the pair generation process.It can be provided that the crystal has the symmetry class C 3v or D 3 or D 3h or has comparable symmetry. In this context, C 3v is understood to mean that the molecules of the crystal of this group have, as symmetry elements, the identity, a C 3- axis and 3 symmetry planes which contain the symmetry axis with C n as an n-fold rotational symmetry axis. D 3 is understood here to mean that the molecules of the crystal of this group have a C 3- axis and 3 C 2- axes perpendicular thereto. In this context, D 3h is understood to mean that the molecules of the crystal of this group have a D 3- axis and a mirror plane perpendicular thereto.Provision can be made for the propagation to take place along the Z axis. Propagation along the Z axis is understood here to mean that the laser beam moves through the crystal along the Z axis of the crystal. The Z axis is the axis about which the crystal has a rotational symmetry, preferably an n-fold rotational symmetry. It is advantageous in that only one (single) laser is required for generating the laser beam and the crystal having the corresponding symmetry for generating the interdigitated photon pairs, wherein only one orientation of the propagation of the laser beam along the Z axis of the respective symmetry is necessary for the adjustment.Comparable balances are understood here to mean identical balances which are described in a different notation, or other balances which have triple rotational symmetry about a specific propagation direction.By way of example, the following lists crystals for the method and apparatus for generating interdigitated photon pairs. C 3v: 3 R-MoS 2 crystal (molybdenum disulfide, molybdenum disulfide) 3R represents a rhombohedral 3R stack order in which the adjacent layers shift slightly with respect to one another without rotation. Beta Barium Borate (BaB 2 O 4). Lithium niobate (LiNbO 3). D 3: Quartz (SiO 2). D 3h: 2 H-MoS 2 crystal (molybdenum disulfide, molybdenum disulfide) with an odd number of layers D3h. 2H represents a hexagonal 2H stack arrangement in which the adjacent layers are rotated by 180° and lie directly one above the other. Molybdenum diselenide (2H-MoSe 2), tungsten disulfide (2H-WS 2).It can be provided that the laser beam is reflected back by a suitable device. The laser beam can thus be propagated repeatedly through the crystal. This produces entangled photons each time the laser beam passes through the crystal. As a suitable device, one or more reflecting members such as mirrors or prisms may be used which are arranged behind the crystal. In this case, the entangled photons generated during the first pass can also be reflected or can already be decoupled by a component which reflects only the laser beam.It can be provided that the crystal is positioned on a substrate, cantilevered or embedded in an optical system, so that the laser beam, preferably a linearly polarized or elliptically polarized or circularly polarized laser beam, with wavelength λ p propagates through the crystal. It may be provided that the non-linear crystal of symmetry class C 3v or D 3 or D 3h or equivalent symmetry is positioned on a substrate, cantilevered or embedded in an optical system, such that a linearly polarized or elliptically polarized or circularly polarized laser beam having wavelength λ p propagates through the crystal. The advantage of arranging the crystal on a substrate lies in the additional stability of the arrangement with respect to mechanical environmental influences and the possibility of providing very thin crystals for generating interdigitated photon pairs. In addition to stabilization, the substrate can also perform further functions, such as, for example, filtering the laser beam after the generation of the interdigitated photon pairs. In the case of a self-supporting crystal, a high transmission of the entangled photons and of the laser beam results and a very cost-effective configuration. An advantage of embedding the crystal in an optical system is the compact construction with simultaneously additional function due to the optical system thereby produced. For example, in such an embodiment, the crystal may be placed on a filter that filters out the laser beam after the generation of entangled photons.Furthermore, it can be provided that the laser is designed to generate a laser beam having a wavelength λ p. In this case, it can be provided that the wavelength λ p is formed in the range from 300 nm to 900 nm. Such a laser beam can be used to generate interlaced photon pairs in the wavelength range around 800 nm for high transmission as free beam and high detection efficiency of the detectors, and also around 1350 nm and 1550 nm for good transmission into glass fibers.It can be provided that the crystal has a thickness of greater than 10 nm, preferably 100 nm, preferably greater than 200 nm. The maximum thickness of the crystal may be in the range of 4-5 cm. It can be provided that the crystal is formed as monolayer material or with only a few layers. The thickness of such crystals is in the range of a few nm. The thickness of the crystal, preferably the thin film crystal at a small thickness, is to be fixed in advance by a suitable device. The thickness can be additive (by crystal growth methods), subtractive (by removal, polishing or etching) or else transformative (by exfoliation and transfer). Advantageously, the method and the device according to the invention result in that crystals with very small thickness can be used. This results in a lower material outlay per source and a very compact source for generating entangled photons. When using a thin-film crystal, i.e. a crystal in the nm range, the low demands on size (<1 cubic micrometer), mass and energy requirement are advantageous.It can be provided that the crystal does not have any fluorescent transitions in the relevant wavelength range, i.e. in the wavelength range of the photons generated. It is essential here that the quality of the entangled photons, i.e. of the entangled state, can be significantly improved as a result, since no photons generated by fluorescence are present as interfering radiation. For example, the following crystals can be used for this: 3R-MoS 2, lithium niobate (LiNbO 3), or quartz (SiO 2).It can be provided that the generation of interdigitated photon pairs arises from the superposition of two nonlinear paths in the tensor, which are indistinguishable by the selection of crystal symmetry. This causes the two paths to interfere with each other, resulting in emission of entangled photons.It can be provided that the selection of the interlaced state takes place via the rotation of the polarization of the laser field, preferably of the laser beam. This can be done, for example, by rotating the laser, the crystal or by using a suitable wave plate. It can be provided that for selecting the interdigitated state, the lasers and / or the crystal are designed to be rotatable, preferably rotatable about the propagation direction of the laser beam. Alone or in addition, it can be provided that, for the selection of the interdigitated state between the laser and the crystal, an optical element for rotating the polarization of the laser beam is arranged, preferably a fiber and / or a wave plate and / or an electro-optical modulator and / or a liquid crystal. This advantageously results in a very flexible source which can generate interleaved photon pairs in different interleaved states and at the same time with a very simple possibility of selecting the interleaved state of the photon pairs. The advantage of the wave plates is the cost-saving and simple construction. The advantage of the electro-optical modulator and the liquid crystals lies in the rapid and precise switching of the polarization of the laser beam. It is also possible to combine a plurality of components with one another. In the case of a linear polarization along the axes of symmetry of the crystal (often called armchair and zigzag axes), a so-called φ + / - state arises. The φ + / - state mentioned by way of example is a so-called fully or maximally interlaced bell state. It should also be noted that by rotating the crystal or the laser and thereby the laser beam, it is also possible to generate another, completely interlaced bell state ψ + / -. Advantageously, such a configuration results in a source which is simple to adjust and very flexible and can be used for a broad range of functions.It can be provided that the selection of the degree of entanglement is effected continuously via the polarization, preferably the ellipsoidity of the input polarization, preferably of the laser beam. It can be provided that, for the selection of the degree of entanglement, one or more optical elements are arranged in front of the crystal, preferably a fiber and / or a wave plate and / or an electro-optical modulator and / or a liquid crystal. If elliptically polarized light is irradiated instead of linearly polarized light, the entanglement can be gradually reduced. In the case of circular polarization, perfectly independent photon pairs are produced. This advantageously results in a simple source which can be adjusted and flexible and which can be used for many applications in which the state of entanglement and the degree of entanglement are adjustable by simple selection of the polarization of the pump laser.The method and the device thus permit the generation of various maximum interleaved states by changing the linear excitation polarization direction of the laser and the control of the degree of interleaving by changing the ellipticity of the polarization of the excitation laser without requiring further changes at the source. In this case, a particular symmetry of the nonlinear response of the crystal is utilized, namely that the crystal couples along the propagation direction of the laser beam light wave polarization components at the fundamental and converted length via nonlinear coefficients with the same amplitude for different polarization directions. This is the case, for example, for symmetry classes with 3-fold rotational symmetry such as C 3v or D 3 or D 3h or comparable symmetry.It can be provided that the crystal has a 3-fold rotational symmetry, preferably a 3-fold rotational symmetry about the propagation axis of the laser beam. This is given, for example, in the case of crystals having a C 3v or D 3 or D 3h rotational symmetry during propagation along the z-axis. The 3-fold rotational symmetry can also be achieved by selecting a suitable propagation direction in a crystal with a different symmetry class. At the same time, the crystal requires a χ(2)- nonlinearity and a sufficiently small fluorescence. Thus, it can be provided that the selection of the interlaced state takes place via the rotation of the polarization of the laser field, preferably of the laser beam, and the selection of the degree of interlace continuously takes place via the polarization, preferably the ellipsoid of the input polarization. It can be provided that the optical elements between laser and crystal are used both for selecting the interlaced state and for selecting the degree of interlace.It can be provided that the entanglement takes place in the polarization wavelength phase space. Such entangled photons are also called polarization entangled photons or photon pairs. Each photon pair has a signal photon and an idle photon. This means that photon pairs with wavelengths λ s and λ i are generated, which are polarization-constrained. Here, λ s stands for the wavelength of a signal photon of a photon pair and λ i stands for the wavelength of an idle photon of this photon pair. Here, (λ s+ λ i)(-1)= λ p(-1), where λ p is the same as the wavelength of the laser beam. Since this equation can be fulfilled for a plurality of pairings λ s and λ i broadband interlaced light arises. This can be limited to low bandwidths by suitable filters or resonators.It can be provided that the photon pairs that are produced co-propagate along the direction that is predetermined by the laser. This means that the generated photons propagate along the direction of the laser beam. This makes it possible to provide a compact generation of interdigitated photon pairs, since the propagation direction of the interdigitated photons occurs along the direction of the laser beam after the generation.It can be provided that the remaining laser radiation and any parasitic radiation at the output is filtered by a suitable device. It can be provided that a filter is arranged after the crystal, which filter is designed to filter the remaining laser radiation and any parasitic radiation. Parasitic radiation can be understood to mean interfering radiation, for example by fluorescence. Such broad interference radiation can be reduced by suitable filters at least in the unused wavelengths of the entangled photons in order to reduce erroneous measurements.It can be provided that the crystal has a nonlinearity of second and / or third order. This enables various nonlinear optical effects such as second harmonic generation (SHG), sum frequency generation (SFG), difference frequency generation (DFG), parametric process such as parametric fluorescence (spontaneous parametric down conversion SPDC), and four-wave mixing (FWM). It can be provided that the crystal is used for non-linear parametric processes, preferably non-linear processes of second order or non-linear processes of third order.In the case of a non-linear process of second order, it can be provided that this is a process of sum frequency generation, generation of the second harmonic difference frequency mixture, spontaneous parametric generation, rectification and / or electro-optical modulation.In the case of a non-linear process of third order, it can be provided that this is a process of self-phase modulation, supercontinuum generation, cross-phase modulation, four-wave mixing and / or spontaneous four-wave mixing.It can be provided that the device is designed to carry out one of the aforementioned methods.Further embodiments of the invention are illustrated in the figures and described below. The figures show, by way of example, a possible configuration of the invention. This embodiment serves to explain a possible implementation of the invention and should not be understood as limiting. The following are shown: FIG. 1 : Schematic illustration of sum-frequency generation (SFG), second-harmonic generation (SHG) and parametric fluorescence (SPDC); FIG. 2: VSI image (vertical scanning interferometry) of a sample of a 3R-stacked MoS 2 crystal and schematic illustration of the crystal structure; FIG. 3 : Schematic representation and mapping of the characterization of the second-harmonic generation (SHG); FIG. 4 : coincidence histogram and spectrum of the photon pairs generated; FIG. 5 : Measurements of parametric fluorescence (SPDC) and frequency doubling (SHG) with differently polarized laser beams; FIG. 6 : Schematic illustration of an exemplary embodiment of the device according to the invention.FIG. 1 shows on the left a schematic illustration of the sum-frequency generation (SFG) and the frequency doubling (second-harmonic generation SHG) and on the right of the parametric fluorescence (spontaneous parametric down conversion, SPDC) with ω s the frequency of the signal photon, ω i the frequency of the idler photon, ω p the frequency of the pump photon of the laser beam, ω SF the frequency of the generated photon by sum frequency generation and ω SHG the frequency of the generated photon by frequency doubling, P is the electric field and χ is the electric susceptibility of the crystal.As shown on the left in FIG. 1, sum-frequency generation (SFG) is a nonlinear optical effect in which two laser beams having different frequencies, here with ω s of the frequency of the signal photon and ω i of the frequency of the idle photon, which propagate through a nonlinear medium having the length L having the susceptibility χ (2) generate photons having the sum of the frequency ω SF of the two signal and idle photons. In the second-harmonic generation (SHG), in this example, the frequencies of the signal photons and the idle photons are the same, i.e. ω s= ω i= ω applies, and when propagated through a nonlinear medium having the length L with the susceptibility χ (2) generate photons having the double frequency ω SHG= 2 ω. The energy conservation and high intensities are applicable here with P of the polarization, E the electric field and χ (2) the susceptibility of the second order.As shown on the right in FIG. 1, the parametric fluorescence (spontaneous parametric down conversion, SPDC) is a nonlinear optical effect in which the pump photons with the frequency ω p propagate through a nonlinear medium with the length L with the susceptibility χ (2) and in the process each generate a photon pair consisting of a signal photon with the frequency ω s and an idler photon with the frequency ω i i.e. ω p= ω s+ ω i, or correspondingly referred to the wavelengths.FIG. 2 shows a 3R-stacked (stacked) MoS 2 crystal (molybdenum disulfide, molybdenum disulfide). MoS2(3R phase) is a semiconductor with a tunable band gap of 0.80 eV for bulk and 1.81 eV for single layer samples. The crystal is naturally present and also has good thermodynamic stability. Molybdenum disulfide in the 3R phase is used in the fields of photonics, optoelectronics and catalysis. The layers are interconnected by van der Waals interactions and can be layered into thin 2D layers. MoS2belongs to the Group VI transition metal dichalcogenides (TMDC). 3R represents a rhombohedral 3R stack arrangement in which the adjacent layers are easily displaced relative to one another without rotation. The following also applies to this crystalFIG. 2 shows a sample on the left and the crystalline structure on the right with the symmetry of the 3R-stacked (stacked) MoS 2 crystal (molybdenum disulfide, molybdenum disulfide) for generating photon pairs by means of parametric fluorescence (spontaneous parametric down conversion, SPDC). This material is one of the thinnest sources of interdigitated photon pairs and allows tunable generation of maximum interdigitated states with constant efficiency. With such a 3R-stacked MoS 2 crystal, entangled photons can be generated even with a few crystal layers.FIG. 3 shows on the left a schematic illustration for frequency doubling (SHG) for the case in which the incident polarization is parallel to the detected polarization. The intensity of the frequency doubling and the incident polarization are parallel to the detected polarization. Figure 3 shows on the right a frequency doubling (SHG) map for the sample of Figure 2.FIG. 4 shows at the top a coincidence histogram and at the bottom a spectrum of the generated photon pairs which were generated with a 3R-stacked (stacked) MoS 2 crystal (molybdenum disulfide) from FIG. 2. The crystal has a thickness of 278 nm and was pumped with a laser beam (CW continuous beam) having 788 nm wavelength. In addition, a long-pass filter with an edge from 1500 nm was used. The ratio of the coincidences to random (accidental) photons is 5.5. Spectral measurement was carried out using a single mode fiber (SMF-28 fiber) 1 km long. The linear relationship between the power of the pump laser and the photon pairs generated by measuring the coincidences is shown at the top in FIG. 4.FIG. 5 shows the measured parametric fluorescence (SPDC) in the 3R-stacked MoS 2 crystal (molybdenum disulfide) of FIG. 2 with differently polarized laser beams, upper left with parallel incident polarization to the measured polarization and upper right with perpendicular incident polarization to the measured polarization. FIG. 5 shows a measurement without analyzer at the bottom right, it being evident from this that the same efficiency is given in this material for all polarizations. Figure 5 at the bottom left shows a second-harmonic generation (SHG) measurement for parallel and perpendicular incident polarization to the measured polarization.Using the 3R-stacked MoS 2 crystal (molybdenum disulfide) from FIG. 2, a fidelity of 84% and concurrence C=0.82 could be measured for a ψ + state. In this case, with an incident polarization, H is the horizontally linearly polarized photon and V is the vertically linearly polarized photon.Using the 3R-stacked MoS 2 crystal (molybdenum disulfide) from FIG. 2, a fidelity of 96% and concurrence C=0.95 could be measured for a Φ - state. In this case, with an incident polarization, H is the horizontally linearly polarized photon and V is the vertically linearly polarized photon.It is thus shown that 3R-stacked MoS 2 offers unique properties: parametric fluorescence (engl. Broadband Spontaneous Parametric Down-Conversion, SPDC); Parametric Fluorescence (Engl. Spontaneous parametric down-conversion, SPDC) efficiency independent of incident polarization; intrinsic true bell state generation.FIG. 6 shows a schematic illustration of the apparatus 1 according to the invention for generating interdigitated photon pairs. This comprises a laser 2 for generating a laser beam 4 and a crystal 3 having a 3-fold rotational symmetry about the propagation axis, for example of the symmetry class C 3v or D 3 or D 3h. The laser beam 4 propagates through the crystal 3 and generates interlaced photon pairs, each consisting of a signal photon 5 and an idle photon 6. As shown in FIG. 6, the signal and idler photons 5 and 6 propagate downstream of the crystal 3 in the direction of the laser beam 4.Optionally, a filter 8 can be arranged behind the crystal 3 in order to filter the laser beam and also possibly further radiation such as, for example, fluorescence.Further, a wave plate 7 may optionally be arranged between the laser 2 and the crystal 3 in order to rotate or change the polarization of the laser beam. Thus, both the entanglement state and the entanglement degree of the generated photon pairs can be adjusted. Alternatively or additionally, the laser 2 and / or the crystal 3 can also be rotatably arranged in the device 1 for this purpose.List of reference numbers:1 Device for generating interdigitated photon pairs 2 Laser 3 Crystal 4 Laser beam 5 Signal photons 6 Idle photons 7 Wave plates 8 Filters

Claims

Method for generating interdigitated photon pairs by propagating a laser beam having a wavelength λ p through a crystal, preferably a crystal having a symmetry class C 3v or D 3 or D 3h.Method according to claim 1, characterized in that the crystal is positioned on a substrate in a cantilevered or embedded manner in an optical system, so that the laser beam, preferably a linearly or elliptically or circularly polarized laser beam, with wavelength λ p propagates through the crystal.Method according to Claim 1 or 2, characterized in that the propagation takes place along the Z axis.Method according to one of Claims 1 to 3, characterized in that the generation of interlaced photon pairs is produced by the superposition of two nonlinear paths in the tensor, which are indistinguishable by the selection of the crystal symmetry.Method according to one of Claims 1 to 4, characterized in that the selection of the interlaced state takes place via the rotation of the polarization of the laser field, preferably of the laser beam.Method according to one of Claims 1 to 5, characterized in that the selection of the degree of entanglement is effected, preferably continuously, via the polarization, preferably the ellipsoid of the input polarization, preferably via the polarization of the laser beam.Method according to claims 5 and 6, characterized in that the crystal has a 3-fold rotational symmetry, preferably a 3-fold rotational symmetry about the propagation axis of the laser beam.Method according to one of Claims 1 to 7, characterized in that the entanglement takes place in the polarization wavelength phase space.Method according to one of Claims 1 to 8, characterized in that the photon pairs which are produced co-propagate along the direction which is predefined by the laser.Method according to one of Claims 1 to 9, characterized in that the remaining laser radiation and any radiation which may arise parasitically are filtered at the output.Device for generating interdigitated photon pairs, comprising a laser for generating a laser beam and a crystal, preferably arranged in the laser beam, preferably with a symmetry class C 3v or D 3 or D 3h.Device according to claim 11, characterised in that the crystal has the symmetry class C 3v or D 3 or D 3h or comparable symmetry.Device according to claim 11 or 12, characterised in that the crystal has a non-linearity of second and / or third order.Device according to one of Claims 11 to 13, characterized in that the crystal has no fluorescent transitions in the relevant wavelength range, and / or in that a filter for the laser radiation and / or for parasitic radiation is arranged behind the crystal.Device according to one of Claims 11 to 14, characterized in that, in order to select the interdigitated state, the lasers and / or the crystal are designed to be rotatable, or in that an optical element for rotating the polarization, preferably a waveplate, is arranged between the laser and the crystal.Device according to one of Claims 11 to 15, characterized in that, in order to select the degree of entanglement, one or more optical elements are arranged upstream of the crystal, preferably waveplates.Device according to claims 15 and 16, characterised in that the crystal has a 3-fold rotational symmetry, preferably a 3-fold rotational symmetry about the propagation axis of the laser beam.

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