Method for generating entangled photons

Abstract

The invention relates to a method for generating entangled photon pairs by propagating a laser beam of wavelength [lambda] p through a crystal, preferably a crystal having a C3v or D3 or D3h symmetry class.

Description

Technical Field

[0001] The present invention relates to a method for generating entangled photon pairs according to the features of the preamble of claim 1, and an apparatus for generating entangled photon pairs according to the features of the preamble of claim 11. Background Technology

[0002] Entangled photon sources (EPS) are important quantum photonic devices. They are required for many methods in quantum technology, including quantum key distribution, quantum imaging, quantum spectroscopy, and quantum information technology, such as for boson sampling or quantum computer interconnects.

[0003] The most common method for generating entangled photons is through a spontaneous parametric down-conversion (SPDC) process, also known as parametric fluorescence, in which a high-energy (short-wavelength) "parent photon" is generated at an X-ray diffraction (χ²) wavelength. ( ² ) In a nonlinear crystal, the photon splits into two lower-energy (longer-wavelength) sub-photons. These two photons are correlated, meaning they share a common two-particle wavefunction. Although the process occurs locally, macroscopically only the photon pairs participating in the source constructive interference can be observed. If this process occurs in a homogeneous medium, it is called phase matching (PM) of a plane wave pair; if it occurs in a structured medium, it is generally called mode matching of a mode pair.

[0004] In order to generate entangled photon pairs from photon pairs, the crystal and optical system must be coordinated so that two independent and indistinguishable SPDC paths lead to the same mode pair. If the paths are indistinguishable, they will interfere and be able to emit entangled photons.

[0005] There are several known types of light sources that realize this concept in existing technologies. These can be broadly classified into two categories: co-path entangled photon sources (CP-EPS) and decoupled path entangled photon sources (DP-EPS).

[0006] DP-EPS has two independent channels in which photon pairs are generated. In a second step, these photon pairs are superimposed using a coherent mixer. For example, the Saniac ring light source belongs to this category, where paths are separated by propagation direction and superimposed using a beam splitter. Similarly, the resonator comb light source belongs to this category, where photon pairs are generated in different wavelength modes and superimposed within a single sideband using a mixer (modulator). DP-EPS structures are typically very complex and require a large number of high-quality photonic components. However, compared to CP-EPS, phase matching (PM), mode matching (MM), and conversion efficiency on each channel can be individually optimized. Therefore, DP-EPS generally offers superior performance.

[0007] CP-EPS abandons the independent conversion and superposition of two modes. Instead, CP-EPS uses two independent nonlinear channels along one mode or a spatially indistinguishable pair of modes (at least in the far field). The most common case here is polarization entanglement of plane waves along a common wave vector. Such a system—though conceptually simpler—must be able to achieve phase-matched PM (or mode-matched MM) for two independently propagating photons, and further, synchronize the SPDC efficiency along the two channels in the same system. This is typically only applicable to specific crystal classes and must be achieved along a specific propagation axis under very tightly controlled PM or MM conditions (which in turn require strictly controlled environmental conditions, such as temperature), with fixed polarization. Therefore, CP-EPS is generally less technically critical because, despite its conceptual ingenuity, it is technically difficult to control.

[0008] One approach to circumventing the phase-matching condition (PM condition) or mode-matching condition (MM condition) is to thin the crystal to a scale smaller than the wavelength of light (more precisely, smaller than the coherence length). It has been shown that, for example, in thin gallium phosphide crystals (but with different crystal symmetries than those described herein), a form of polarization-entangled photon pair can be generated. However, this does not allow for tunability of different maximally entangled polarization states; instead, the specific crystal symmetries described herein are required.

[0009] The following polarization entanglement light source has not been studied and is still unknown to date: it can generate different maximum entangled photon pairs in a manner that can be tuned based solely on special crystal symmetries and without the need for other optical components, or additionally generate photon pairs with tunable entanglement in the same system. Summary of the Invention

[0010] The object of the present invention is to provide an improved method for generating entangled photon pairs, and a correspondingly improved apparatus for generating entangled photon pairs.

[0011] According to the present invention, this objective is achieved by the method for generating entangled photon pairs as described in claim 1.

[0012] According to the present invention, a method is proposed to make the wavelength λ p The laser beam propagates through the crystal, preferably having C 3v Or D3 or D 3h A method for generating entangled photon pairs using symmetric crystals.

[0013] Furthermore, this objective is achieved by the apparatus for generating entangled photon pairs as described in claim 9.

[0014] According to the present invention, an apparatus for generating entangled photon pairs is provided, the apparatus comprising a laser and a crystal for generating a laser beam, preferably having C 3v Or D3 or D 3h Symmetrical crystals.

[0015] This solution, which can be implemented in crystals with suitable crystal symmetry, and preferably only in crystals with suitable crystal symmetry, has the following advantages:

[0016] By means of the method and apparatus according to the invention, entangled photons can be generated with a very simple structure, a minimal number of components, and virtually no alignment operations required, and a light source for generating entangled photon pairs can be provided. Due to the small number of components, a stable and easily aligned entangled photon pair light source and a stable and easily aligned method for generating entangled photon pairs are advantageously obtained. Here, the setup of the light source requires minimal investment, and the entangled photon pair light source can provide constant, high-quality entangled photon pairs over a long period, unaffected by environmental factors (such as, for example, temperature variations).

[0017] Furthermore, the photon source according to the invention enables scalable fabrication, for example, by peeling off a thin film of a crystal and applying it onto a suitable substrate, without the need for other optical components. Moreover, the method and device according to the invention can be parallelized with high indistinguishability, thereby extending to light sources for more complex photonic quantum states.

[0018] It is also advantageous here that the method and apparatus according to the invention can be implemented in a variety of materials with partially self-controlled supply chains. Therefore, the availability of the method and apparatus according to the invention can be ensured in a better manner.

[0019] This invention describes a method and an apparatus by means of which entangled photons can be generated in optical nonlinear crystals, thin-layer crystals or crystalline monolayer materials without the assistance of other optical components, and without the need for other optical components other than a laser (which is required for the generation process by means of parametric fluorescence (SPDC) excitation of (entangled photons)).

[0020] It can be specified that crystals have C 3v Or D3 or D 3h Symmetry classes, or those possessing similar / comparable symmetry. Here, C... 3v This can be understood as follows: the crystal molecules of this point group possess an identity element, a C3 axis, and three symmetry planes containing that symmetry axis as symmetry elements, where C... n Let D represent the nth rotational symmetry axis. Here, D3 is understood as the crystal molecules of this point group having one C3 axis and three C2 axes perpendicular to it. 3h This can be understood as the crystal molecules of this point group having a D3 axis and a mirror plane perpendicular to it.

[0021] It can be specified that propagation occurs along the Z-axis. Here, propagation along the Z-axis is understood as the laser beam passing through the crystal along its Z-axis. The Z-axis is an axis in which the crystal has rotational symmetry, preferably n-fold rotational symmetry, about which it is oriented. Advantageously, to generate entangled photon pairs, only a (unique) laser for generating the laser beam and a crystal with the corresponding symmetry are required, wherein, for alignment, the propagation direction of the laser beam only needs to be oriented along the Z-axis of the corresponding symmetry.

[0022] Here, similarity symmetry is understood as the same symmetry described by other symbols, or other symmetries having triple rotational symmetry about a specific direction of propagation.

[0023] The following provides exemplary examples of crystals used in the methods and apparatus for generating entangled photon pairs.

[0024] C 3v 3R-MoS2 crystal (molybdenum disulfide), where 3R represents the rhombohedral 3R stacked structure in which adjacent layers slightly move but do not rotate. Barium β-borate (BaB2O4). Lithium niobate (LiNbO3).

[0025] D3: Quartz (SiO2).

[0026] D 3h: 2H-MoS2 crystal (molybdenum disulfide) with an odd number of layers D3h. 2H represents a hexagonal 2H stacked structure in which adjacent layers are rotated 180° and directly stacked on top of each other. Molybdenum diselenide (2H-MoSe2). Tungsten disulfide (2H-WS2).

[0027] It can be specified that the laser beam is reflected back using a suitable device. Therefore, the laser beam can propagate through the crystal multiple times. Thus, entangled photons are generated each time the laser beam passes through the crystal. As a suitable device, one or more reflective elements, such as mirrors or prisms, arranged behind the crystal can be used. Here, the entangled photons generated on the first pass can also be reflected, or extracted / coupled out through an element that only reflects the laser beam.

[0028] It can be specified that the crystal is positioned on the substrate in a manner that allows it to be suspended or embedded in the optical system, so that the wavelength is λ. p The laser beam, preferably linearly polarized, elliptically polarized, or circularly polarized, propagates through the crystal. It can be specified that C... 3v Or D3 or D 3h A symmetric or equivalently symmetric nonlinear crystal is thus positioned on a substrate in a manner that allows it to be suspended or embedded in an optical system, such that the wavelength is λ. p Linearly, elliptically, or circularly polarized laser beams propagate through the crystal. The advantage of arranging the crystal on the matrix is ​​that it provides additional stability relative to the effects of the mechanical environment and allows for the possibility of using very thin crystals to generate entangled photon pairs. Additionally, besides stability, the matrix can fulfill other functions, such as filtering the laser beam after the generation of entangled photon pairs. In the case of a suspended crystal, high transmittance of entangled photons and the laser beam is achieved, and a very cost-effective design is realized. In the case of a crystal embedded in an optical system, it is advantageous to have a compact structure while achieving additional functionality through the resulting optical system. For example, in such a design, the crystal can be arranged on a filter that filters the laser beam after the generation of entangled photons.

[0029] It can also be specified that the laser is designed to generate wavelengths of λ. p The laser beam. Here, the wavelength λ can be specified. p It is designed to operate in the range of 300 nm to 900 nm. With this laser beam, entangled photon pairs in the wavelength range of around 800 nm can be generated to achieve high transmittance as a free beam and high detection efficiency of the detector, and entangled photon pairs in the wavelength range of around 1350 nm and 1550 nm can be generated to achieve good transmission rate in glass optical fibers.

[0030] It can be specified that the crystal has a thickness greater than 10 nm, preferably 100 nm, and preferably greater than 200 nm. The maximum thickness of the crystal can be in the range of 4 cm to 5 cm. It can be specified that the crystal is designed as a single-layer material or has only a few layers. The thickness of such crystals is in the range of a few nanometers. The thickness of the crystal (preferably a thin-film crystal in the case of a small thickness) needs to be fixed in advance by a suitable device. Here, the thickness can be achieved by additive manufacturing (such as crystal growth), subtractive manufacturing (such as removal, polishing or etching), or by deformation / conversion (by peeling and transfer). The advantage obtained by the method and device according to the invention is that crystals with very small thicknesses can be used. As a result, less material is consumed per light source, and a very compact light source for generating entangled photons can be obtained. When using thin-layer crystals, i.e., crystals in the nanometer range, low requirements for size (<1 cubic micrometer), mass and energy consumption are advantageous.

[0031] It can be stipulated that the crystal does not exhibit fluorescence transitions within the relevant wavelength range, that is, within the wavelength range of the generated photons. Importantly, this significantly improves the quality of entangled photons, or entangled states, because there are no interfering radiation photons generated by fluorescence. For example, crystals such as 3R-MoS2, lithium niobate (LiNbO3), or quartz (SiO2) can be used for this purpose.

[0032] It can be stipulated that entangled photon pairs are generated by superimposing two nonlinear paths in a tensor, where these two paths are indistinguishable due to the choice of crystal symmetry. Thus, the two paths interfere with each other, thereby emitting entangled photons.

[0033] It can be specified that the entangled state is selected by polarization rotation of the laser field (preferably the laser beam). This can be achieved, for example, by rotating the laser, the crystal, or by using a suitable waveplate. It can be specified that, for selecting the entangled state, the laser and / or crystal are designed to be rotatable, preferably rotatable about the propagation direction of the laser beam. It can be specified, individually or additionally, that, for selecting the entangled state, optical elements, preferably optical fibers and / or waveplates and / or electro-optic modulators and / or liquid crystals, are arranged between the laser and the crystal to rotate the polarization of the laser beam. Advantageously, this results in a very flexible light source that can generate entangled photon pairs with different entanglement states, while simultaneously enabling a very simple scheme for selecting the entangled state of photon pairs. The advantage of waveplates is their cost-effectiveness and simple structure. The advantage of electro-optic modulators and liquid crystals is their ability to quickly and accurately switch the polarization of the laser beam. Multiple elements can also be combined with each other. When linearly polarized along the crystal's axis of symmetry (often called the armchair axis and sawtooth axis), so-called... + / - State. Examples mentioned + / -The state is a so-called fully entangled Bell state or a maximally entangled Bell state. It should also be noted that another fully entangled Bell state can be generated by rotating the crystal or rotating the laser and thus rotating the laser beam. + / - Advantageously, this type of design allows for easy alignment and highly flexible light sources that can be applied to a wide range of functions.

[0034] It can be specified that the degree of entanglement is selected steplessly by—preferably—the polarization of the laser beam, and preferably the ellipticity of the incident polarization. It can be specified that, for selecting the degree of entanglement, one or more optical elements, preferably optical fibers and / or waveplates and / or electro-optic modulators and / or liquid crystals, are arranged before the crystal. If elliptically polarized light is incident instead of linearly polarized light, the entanglement can be gradually reduced. In the case of circular polarization, perfect, that is, completely independent photon pairs are produced. Advantageously, this results in an easily aligned and flexible light source that can be used in a variety of applications where the tunability of the entangled state and the degree of entanglement is achieved by simply selecting the polarization of the pump laser.

[0035] Therefore, this method and apparatus allow for the generation of different maximally entangled states by changing the linear excitation polarization direction of the laser, and allow for control of the degree of entanglement by changing the ellipticity of the polarization of the excitation laser without requiring other modifications to the light source. This utilizes the special symmetry of the crystal's nonlinear response, i.e., the crystal couples the fundamental wavelength and the polarization components of the light wave at the converted / variable wavelength along the propagation direction of the laser beam via a nonlinear coefficient, where the amplitudes of different polarization directions are the same. For example, for a crystal with triple rotational symmetry (such as C... 3v Or D3 or D 3h (or similar symmetry classes, i.e., such cases exist.)

[0036] It can be specified that the crystal possesses triple rotational symmetry, preferably triple rotational symmetry about the propagation axis of the laser beam. For example, for a crystal with C... 3v Or D3 or D 3h Rotational symmetry exists in crystals when propagating along the z-axis. Triple rotational symmetry can also be achieved by selecting a suitable propagation direction in crystals with other symmetry classes. Simultaneously, the crystal requires x-axis... ( ² ) Nonlinearity and sufficiently low fluorescence. Therefore, it can be specified that the entangled state is selected by the polarization rotation of the laser field, preferably the laser beam, and the degree of entanglement is selected steplessly by polarization (preferably the incident polarization ellipticity). It can be specified that the optical elements between the laser and the crystal are used not only to select the entangled state but also to select the degree of entanglement.

[0037] It can be specified that entanglement occurs in the polarization-wavelength phase space. Such entangled photons are also called polarization-entangled photons or photon pairs. Here, each photon pair consists of a signal photon and an idler photon. This means that the resulting photons with wavelength λ... s and λ i The photon pairs are polarization-entangled. Here, λ s λ represents the wavelength of the signal photon in the photon pair. i λ represents the wavelength of the idle photon in the photon pair. Here, (λ) s + λ i ) (-1) = λ p (-1) , where λ p It is equal to the wavelength of the laser beam. Due to multiple λ... s and λ i The pairings can all satisfy this formula, thus producing broadband entangled light. This can be limited to a narrower bandwidth using appropriate filters or resonators.

[0038] It can be stipulated that the generated photon pairs propagate together along a direction preset by the laser. This means that the generated photons propagate along the direction of the laser beam. Thus, a compact scheme for generating entangled photon pairs can be provided, because the entangled photons propagate in the direction of the laser beam after generation.

[0039] It can be specified that, at the output end, residual laser radiation and any parasitic radiation that may arise are filtered out using a suitable device. It can be specified that a filter is arranged after the crystal, designed to filter out residual laser radiation and any parasitic radiation that may arise. Parasitic radiation can be understood as, for example, interference radiation caused by fluorescence. With a suitable filter, this broadband interference radiation can be suppressed at least within the wavelength range unused by entangled photons, thereby reducing measurement errors.

[0040] It can be specified that the crystal possesses second-order and / or third-order nonlinearity. This enables various nonlinear optical effects, such as second harmonic generation (SHG), sumfrequency generation (SFG), difference frequency generation (DFG), parametric processes (e.g., spontaneous parametric down-conversion, SPDC), and four-wave mixing (FWM). It can be specified that such a crystal is used for nonlinear parametric processes, preferably second-order or third-order nonlinear processes.

[0041] In the case of a second-order nonlinear process, it can be defined that the second-order nonlinear process is a process of sum frequency generation, second harmonic difference frequency mixing generation, spontaneous parametric generation, optical rectification and / or electro-optic modulation.

[0042] In the case of a third-order nonlinear process, it can be defined that the third-order nonlinear process is a process of self-phase modulation, supercontinuum generation, cross-phase modulation, four-wave mixing and / or spontaneous four-wave mixing.

[0043] It can be specified that the device is designed to perform one of the aforementioned methods. Attached Figure Description

[0044] Other embodiments of the invention are shown in the figures and described below. The figures exemplarily illustrate one possible design of the invention. This design is intended to explain possible implementations of the invention and should not be construed as limiting it. Wherein:

[0045] Figure 1 A schematic diagram of sum-frequency generation (SFG), second-harmonic generation (SHG), and spontaneous parametric down-conversion (SPDC) is shown.

[0046] Figure 2 The image shows a VSI (vertical scanning interferometry) image of a 3R stacked MoS2 crystal sample and a schematic diagram of the crystal structure.

[0047] Figure 3 A schematic diagram and imaging analysis of the characteristics of second-harmonic generation (SHG) are shown;

[0048] Figure 4 The coincidence count histogram and spectrum of the resulting photon pairs are shown;

[0049] Figure 5 Measurements of parametric fluorescence (SPDC) and frequency doubling generation (SHG) under laser beams with different polarization states are shown.

[0050] Figure 6 A schematic diagram of an embodiment of the device according to the present invention is shown. Detailed Implementation

[0051] Figure 1The left side shows a schematic diagram of sum-frequency generation (SFG) and second-harmonic generation (SHG), and the right side shows a schematic diagram of spontaneous parametric down-conversion (SPDC), where ω s ω is the frequency of the signal photon. i ω is the frequency of the idle photon. p ω is the frequency of the pump photons of the laser beam. SF ω is the frequency of the photons generated by sum-frequency generation. SHG Let P be the frequency of the photons generated by frequency doubling, P be the electric field, and χ be the polarizability of the crystal.

[0052] like Figure 1 As shown on the left, sum-frequency generation (SFG) is a nonlinear optical effect in which a frequency is generated through a path of length L and a polarizability of χ. (2) Two different frequencies (referred to here as ω) propagating in a nonlinear medium. s The frequency of the signal photon is represented by ω. i The sum of the frequencies ω of the laser beam that produces the signal photon and the idle photon (representing the frequency of the idle photon) is ω. SF In this example, under second-harmonic generation (SHG), the signal photon and the idler photon have the same frequency, that is, ω. s = ω i = ω, and when passing through a length of L and a polarizability of χ (2) When propagating in a nonlinear medium, the frequency doubles. = 2ω photons. The law of conservation of energy applies here, and for high intensity, it applies evenly. Where P is polarization, E is the electric field, and χ is the polarization field. (2) It is the second-order polarizability.

[0053] like Figure 1 As shown on the right, parametric down-conversion (SPDC) is a nonlinear optical effect in which the frequency is ω. p The pump photon passes through a length of L and a polarization of χ (2) The propagation of the nonlinear medium, and simultaneously generating frequencies of ω s The signal photon and frequency ω i Photon pairs consisting of idle photons, that is, ωp = ω s + ω i Or, correspondingly, the wavelength applicable is The law of conservation of energy applies here.

[0054] Figure 2 A 3R stacked MoS2 crystal (molybdenum disulfide) is shown. MoS2 (3R phase) is a bandgap-tunable semiconductor, with a bandgap of 0.80 eV for the bulk sample and 1.81 eV for the monolayer sample. This crystal is naturally occurring and also exhibits good thermodynamic stability. 3R phase molybdenum disulfide has applications in photonics, optoelectronics, and catalysis. The layers are interconnected by van der Waals forces and can be exfoliated into thin two-dimensional layers. MoS2 belongs to group VI transition metal dichalcogenides (TMDCs). Here, 3R represents the rhombohedral 3R stacked structure, in which adjacent layers slightly move relative to each other but do not rotate. Also applicable to this crystal are... .

[0055] Figure 2 The sample is shown on the left, and a crystal structure with 3R stacked MoS2 (molybdenum disulfide) symmetry is shown on the right for generating photon pairs via parametric down-conversion (SPDC). This material is one of the thinnest light sources for entangled photon pairs and can generate maximally entangled states tunably with constant efficiency. With this 3R stacked MoS2 crystal, entangled photons can be generated using very few crystal layers.

[0056] Figure 3 The left side shows a schematic diagram of frequency harmonic generation (SHG) when the incident polarization is parallel to the detection polarization. In this case, it is applicable to... ,in, The intensity of the harmonic generation when the incident polarization and the detection polarization are parallel. Figure 3 Shown on the right Figure 2 Imaging analysis of frequency halving generation (SHG) of the sample.

[0057] Figure 4 The above shows the use Figure 2The coincidence count histogram of photon pairs generated by a 3R stacked MoS2 crystal (molybdenum disulfide) is shown below, along with its spectrum. The crystal has a thickness of 278 nm and is pumped using a 788 nm laser beam (continuous wave (CW) beam). Additionally, a long-pass filter with a cutoff wavelength of 1500 nm is used. The coincidence count to random photon / accidental coincidence ratio is 5.5. Spectroscopic measurements were performed using a 1 km long single-mode fiber (SMF-28 fiber). Figure 4 The upper middle section shows the linear relationship between pump laser power and the generated photon pairs obtained by measuring coincidence counts.

[0058] Figure 5 The measured values ​​of laser beams under different polarization states are shown. Figure 2 Parametric fluorescence (SPDC) in 3R stacked MoS2 crystals (molybdenum disulfide). The upper left shows the case where the incident polarization is parallel to the measured polarization, and the upper right shows the case where the incident polarization is perpendicular to the measured polarization. Figure 5 The measurement results without the use of an analyzer are shown in the lower right corner, which show that all polarizations have the same efficiency in this material. Figure 5 The lower left shows the measurements of second-harmonic generation (SHG) under two conditions: incident polarization parallel to and perpendicular to the measurement polarization.

[0059] use Figure 2 The 3R stacked MoS2 crystal (molybdenum disulfide) in the middle, for + The state can be measured with a fidelity of 84% and a concurrency C = 0.82. At this point, with incident polarization... The following situations apply: Where H represents a horizontally linearly polarized photon and V represents a vertically linearly polarized photon.

[0060] use Figure 2 The 3R stacked MoS2 crystal (molybdenum disulfide) in the middle, for - The state can be measured with a fidelity of 96% and a concurrency C = 0.95. At this point, with incident polarization... The following situations apply: Where H represents a horizontally linearly polarized photon and V represents a vertically linearly polarized photon.

[0061] This indicates that 3R stacked MoS2 has the following unique characteristics:

[0062] Spontaneous parametric down-conversion (SPDC) has a broadband spectrum;

[0063] The efficiency of parametric fluorescence (SPDC) is independent of incident polarization.

[0064] The inherent, high-fidelity ability to generate Bell states.

[0065] Figure 6 A schematic diagram of an apparatus 1 for generating entangled photon pairs according to the present invention is shown. The apparatus includes a laser 2 for generating a laser beam 4 and a crystal 3 having triple rotational symmetry about a propagation axis, for example, C0. 3v Or D3 or D 3h Symmetrical type. Laser beam 4 propagates through crystal 3, generating entangled photon pairs consisting of signal photon 5 and idle photon 6, respectively. (As in...) Figure 6 As shown in the diagram. Signal photon 5 and idle photon 6 also propagate along the direction of laser beam 4 after crystal 3. For clarity, laser beam 4, as well as signal photon 5 and idle photon 6, are shown offset from each other.

[0066] Optionally, a filter 8 may be arranged after the crystal 3 to filter out the laser beam and any other possible radiation (such as fluorescence).

[0067] Alternatively, a waveplate 7 can be arranged between the laser 2 and the crystal 3 for polarization rotation or alteration of the laser beam. This allows for adjustment of the entanglement state and degree of entanglement of the resulting photon pairs. Alternatively or additionally, the laser 2 and / or crystal 3 can also be rotatably arranged within the device 1 for this purpose.

[0068] List of reference numerals in the attached diagram:

[0069] 1. Device for generating entangled photon pairs

[0070] 2 lasers

[0071] 3 crystals

[0072] 4 laser beams

[0073] 5 signal photons

[0074] 6 Idle Photons

[0075] 7-wave plate

[0076] 8 filters.

Claims

1. A method for generating entangled photon pairs, wherein, Entangled photon pairs pass through a wavelength of λ p The laser beam propagates through a crystal, preferably a crystal with C0... 3v Or D3 or D 3h Symmetrical crystals.

2. The method according to claim 1, characterized in that, The crystal is positioned on the substrate in a suspended or embedded manner within an optical system, such that the wavelength is λ. p The laser beam, preferably linearly polarized, elliptically polarized, or circularly polarized, propagates through the crystal.

3. The method according to claim 1 or 2, characterized in that, The propagation occurs along the Z-axis.

4. The method according to any one of claims 1 to 3, characterized in that, Entangled photon pairs are generated by superimposing two nonlinear paths in a tensor, the two nonlinear paths being indistinguishable through the selection of crystal symmetry.

5. The method according to any one of claims 1 to 4, characterized in that, Entangled states are selected by polarization rotation of the laser field and the preferred laser beam.

6. The method according to any one of claims 1 to 5, characterized in that, The degree of entanglement is selected by polarization, preferably by the incident polarization ellipticity, preferably by steplessly selecting the degree of entanglement, and preferably by the polarization of the laser beam.

7. The method according to claims 5 and 6, characterized in that, The crystal has triple rotational symmetry, preferably triple rotational symmetry about the laser beam propagation axis.

8. The method according to any one of claims 1 to 7, characterized in that, Entanglement occurs in the polarization-wavelength phase space.

9. The method according to any one of claims 1 to 8, characterized in that, The resulting photon pairs propagate together in a direction predetermined by the laser.

10. The method according to any one of claims 1 to 9, characterized in that, At the output end, residual laser radiation and any possible parasitic radiation are filtered out.

11. An apparatus for generating entangled photon pairs, the apparatus comprising a laser and a crystal for generating a laser beam, preferably a crystal arranged in the laser beam, and preferably having a C 3v Or D3 or D 3h Symmetrical crystals.

12. The device according to claim 11, characterized in that, Crystals have C 3v Or D3 or D 3h Symmetry classes or similar symmetries.

13. The device according to claim 11 or 12, characterized in that, The crystal exhibits second-order and / or third-order nonlinearity.

14. The device according to any one of claims 11 to 13, characterized in that, The crystal does not exhibit fluorescence transitions within the relevant wavelength range, and / or a filter for laser radiation and / or parasitic radiation is arranged after the crystal.

15. The device according to any one of claims 11 to 14, characterized in that, In order to select entangled states, the laser and / or crystal are designed to rotate, or an optical element, preferably a waveplate, is arranged between the laser and the crystal for polarization rotation.

16. The device according to any one of claims 11 to 15, characterized in that, To select the degree of entanglement, one or more optical elements, preferably waveplates, are arranged in front of the crystal.

17. The device according to claims 15 and 16, characterized in that, The crystal has triple rotational symmetry, preferably triple rotational symmetry about the laser beam propagation axis.

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