Quantum sensor

Abstract

The invention relates to a quantum sensor (1), in particular for magnetic field measurement, comprising: a sensor element (4) in the form of a crystal doped with colour centres (5), in particular a diamond crystal doped with NV centres, wherein the colour centres (5) are designed to generate fluorescent light (7) during excitation with excitation light (6); a carrier substrate (2) having a surface (2a) to which the sensor element (4) is attached; and a detector (8), in particular a photodiode, for detecting the fluorescent light (7). In the quantum sensor (1), the carrier substrate (2) has a waveguide (3) for guiding the excitation light (6) to the sensor element (4).

Description

[0001] Quantum sensor

[0002] The present invention relates to a quantum sensor, in particular for magnetic field measurement, comprising: a sensor element in the form of a crystal doped with color centers, in particular a diamond crystal doped with NV centers, wherein the color centers are designed to generate fluorescent light upon excitation with excitation light, a carrier substrate with a surface to which the sensor element is attached, and a detector, in particular a photodiode, for detecting the fluorescent light.

[0003] Quantum sensors play a central role in many technical applications and can be used to determine various physical quantities. Quantum sensors typically use crystals doped with color centers as sensor elements. In particular, a diamond crystal doped with color centers, usually nitrogen vacancy (NV) centers, can be used as a sensor element. NV centers in diamond exhibit a characteristic electronic structure that changes when certain quantities, such as an external magnetic field, temperature, pressure, or electric field, change. The color centers are excited by irradiation with excitation light in the optical range and by irradiation with a microwave field of varying frequency in the microwave range. The fluorescent light induced in the color centers is detected and analyzed by a detector.

[0004] A quantum sensor can be used to measure one or more measurands. For example, DE 10 2014 219 550 A1 describes a combination sensor for measuring pressure and / or temperature and / or magnetic fields, which has at least one sensitive component in the form of a deformable layer, in particular a deflectable membrane, with diamond structures containing NV centers. The manufacture of quantum sensors presents various challenges, for example, with regard to miniaturization, scalability, cost efficiency, and assembly and connection technology.

[0005] US 10,126,377 B2 describes a magnetometer comprising a magneto-optical defect center element, e.g., in the form of a diamond doped with NV centers, an excitation light source, and a collector mounted on a circuit board. The magnetometer comprises an optical waveguide arrangement comprising an optical waveguide and at least one filter coating. The waveguide is configured to guide light emitted by the magneto-optical defect center element to the collector. To align one or more components mounted on the circuit board, a two-point alignment system is used. For each component, two points are provided that are aligned relative to a top plate, a bottom plate, and the circuit board.For alignment purposes, alignment holes may be provided in the top and bottom plates of the magnetometer, into which pins attached to the components engage.

[0006] Object of the invention

[0007] The invention is based on the object of simplifying the production and construction of a quantum sensor.

[0008] Subject of the invention

[0009] This task is solved by a quantum sensor of the type mentioned above, in which the carrier substrate has (at least) one waveguide for supplying the excitation light to the sensor element.

[0010] The quantum sensor described here is based on the concept of (optical) waveguides on or in the carrier substrate. Such optical waveguides can be manufactured cost-effectively in large quantities using established (semiconductor) manufacturing techniques. With the accuracy that can be achieved through semiconductor manufacturing technology, the sensor element and other components attached to the carrier substrate can be positioned very precisely. By guiding the excitation light in the waveguide(s) between the components, active alignment of the components is no longer necessary. This saves additional effort in positioning the individual components and adhesive bonds compared to common / conventional sensor concepts in which the excitation light is fed to the sensor element using free-beam propagation, as described, for example, in US Pat. No. 10,126,377 B2.Furthermore, the optical path can be integrated into the waveguide and does not need to be assembled from individual components. The integrated sensor concept described here is scalable and miniaturizable.

[0011] The excitation light is fed to the sensor element via the waveguide. Other components attached to the carrier substrate, such as a reference detector or another sensor element (see below), can also be supplied with excitation light via the waveguide or via additional waveguides integrated into the carrier substrate. The setup described here eliminates almost all positioning freedom between the individual components (angular degrees of freedom and vertical alignment can be mechanically fixed) and ensures that only the lateral alignment of the individual components relative to the carrier substrate is important for coupling efficiency. In this way, the sensor element in the form of the crystal and other components, such as a reference detector, another sensor element, and / or the excitation light source, can each be mounted independently of one another.For lateral positioning, passive alignment using positioning marks on the surface of the carrier substrate is sufficient. Attaching the sensor element and other components to the surface of the carrier substrate can be achieved using a suitable assembly technique, e.g., surface-mounted technology (SMT), e.g., using a pick-and-place robot.

[0012] In one embodiment, the waveguide is formed as a ridge waveguide that runs along the surface of the carrier, or the waveguide runs within or is embedded in the carrier substrate. The waveguide can be created, for example, by lithographic definition of etching masks and subsequent reactive ion etching or ion beam etching, by a laser structuring process, or by a nickel-induced etching process. The resulting waveguide(s) can run on the carrier substrate in the form of ridge waveguides that protrude above the surface of the carrier substrate, or run within or be embedded in the carrier substrate. Grating structures can be inscribed into a waveguide in the form of a ridge waveguide to assume a filter function.An embedded waveguide can be fully or partially covered by a cover layer of the carrier substrate, whereby the cover layer should have a lower optical refractive index than the active waveguide material. The carrier substrate can be single- or multi-layered and is usually plate-shaped. The material(s) of the carrier substrate can be silicon, for example. Materials used for the waveguide include SiN, SiO?, or LiNbOa.

[0013] In one embodiment, the detector is attached to the sensor element. Attaching the detector to the sensor element eliminates the need for a waveguide to guide the fluorescent light to the detector, as described, for example, in US Pat. No. 10,126,377 B2. The detector is typically connected to the crystal sensor element in a planar manner, thus allowing the collection of a large portion of the fluorescent light generated upon excitation at the color centers.

[0014] In a further development, the detector is attached to a surface facing away from the carrier substrate or to a lateral surface of the sensor element. The excitation light is generally coupled into the crystal from a coupling surface facing the carrier substrate on the underside of the crystal or via a lateral surface. It can be advantageous if the detector is not directly hit by the excitation light, i.e. if it is arranged outside the beam path of the excitation light, since in this case the requirements for a filter element for filtering the excitation light are lower (see below). For example, if the excitation light is coupled into the crystal from the underside, it can be advantageous if the detector is attached to a lateral surface of the crystal, but this is not absolutely necessary.The detector can, for example, be designed in the form of a substantially plate-shaped photodiode whose detector surface is connected flatly to a surface of the crystal. The detector can be connected to the carrier substrate, e.g., via electrical wire bonds, in order to transmit information about the intensity of the detected fluorescent light. Electrical conductor tracks can be provided in the carrier substrate to transmit this information. To evaluate the information about the detected fluorescent light, an evaluation device, e.g., in the form of a chip or the like, can be attached to the carrier substrate. Alternatively, it is possible to transmit the information about the detected fluorescent light to an external evaluation device.

[0015] In a further development, a filter element is mounted between the detector and the sensor element to filter the excitation light. Typically, the detector is sensitive to both the wavelength of the excitation light and the wavelength of the fluorescent light. Therefore, it is typically necessary to insert a filter between the sensor element and the detector—possibly only partially—to block the excitation light. The filter element can, for example, be designed as a filter layer, which is mounted as a coating or in the form of a separate component between the sensor element in the form of the crystal and the detector.

[0016] In a further development, a filter element for filtering infrared light is mounted between the detector and the sensor element. When the color centers, particularly in the form of NV centers, are excited, infrared light can be generated in addition to fluorescent light. If the detector is sensitive to infrared light, it is advantageous or necessary to block the infrared light using the filter element. The filter element can be designed like the filter element described above for filtering the excitation light, e.g., in the form of a filter layer, a coating, or as a separate component.

[0017] In a further development, the sensor element in the form of a crystal widens from a coupling surface for the excitation light in the direction of the detector, wherein the crystal is preferably designed as a truncated cone or a truncated pyramid or has a parabolic lateral surface. A crystal that widens towards the detector is understood to mean that the surface of the crystal to which the detector is attached has a larger surface area than the surface of the crystal at which the excitation light is coupled into the crystal. The crystal can be designed in different ways for this purpose. For example, it can be truncated pyramid-shaped or conical, or have a parabolic or otherwise curved lateral surface. The crystal in the form of a truncated pyramid can have three, four or more side surfaces.The widening shape toward the detector can increase the detector's fluorescence yield. A suitable shape of the crystal can also increase the probability of total internal reflection along spatial directions where no detector is located.

[0018] In a further embodiment, at least one metal lens is mounted between the waveguide and the crystal and / or between the crystal and the detector. Metal lenses are optical components based on nanostructured surfaces or metamaterials and can be designed in the form of coatings or thin, plate-shaped components. A metal lens between the waveguide and the crystal can serve to collimate the excitation light during coupling. A metal lens between the crystal and the detector can serve as a converging lens to concentrate the excitation light onto the detector.

[0019] In a further embodiment, the quantum sensor has a coupling device for coupling excitation light into the sensor element. In the simplest case, the coupling device can form an end face of the waveguide which borders on a side surface of the crystal, i.e. the waveguide can be connected directly to the crystal in order to couple in the excitation light. If the waveguide is embedded in the carrier substrate, the carrier substrate can have a recess in the region of the crystal in order to lower the crystal and enable the excitation light to be coupled in from the side. For the coupling of the excitation light, it can be advantageous if the refractive index of the waveguide is matched to the refractive index of the crystal. For this purpose, a suitable refractive index matching element can be provided between the waveguide and the crystal, or the refractive index of the waveguide can be matched by suitable measures, e.g.The waveguide can be specifically modified near the crystal, e.g., by doping. For coupling the excitation light, it can also be advantageous if the waveguide runs divergently in the coupling region and the width of the waveguide increases, e.g., in a wedge-shaped widening section.

[0020] In a further development, the coupling device is designed in the form of a grating coupler. In this embodiment, the waveguide is typically embedded in the substrate. The grating coupler is usually attached to the top side of the waveguide to couple the excitation light from below into a coupling surface on the underside of the crystal. In this case, the excitation light is coupled essentially perpendicular to the surface of the carrier substrate to which the crystal is attached.

[0021] In a further embodiment, the quantum sensor comprises at least one further component attached to the carrier substrate, as well as a splitting device for splitting the excitation light guided in the waveguide into a portion supplied to the sensor element and a further portion supplied to the further component in the waveguide or in a further waveguide. The splitting device can, for example, be designed in the form of a coupling device, for example in the form of a grating coupler (e.g., "apodized grating"), which couples only a portion of the power of the excitation light from the waveguide into the crystal. The other portion of the excitation light remains in the waveguide and can be supplied to the further component. Photonic elements such as directional couplers, multimode interference couplers, or Y-junctions can also serve as the splitting device.

[0022] In the case of a ridge waveguide, the splitting device can transfer the excitation light through adiabatic coupling into a section of another waveguide, which is arranged at a short distance from the waveguide in which the excitation light is guided, over a predetermined coupling length. By selecting the distance and coupling length, the portion of the excitation light that is coupled from the waveguide into the other waveguide can be specified.

[0023] In a further development, the additional component forms a reference detector for detecting excitation light. The reference detector can be designed, for example, as a photodiode. The reference detector can be used to carry out what is known as "balanced detection" in an evaluation circuit, in which a difference between the measurement signal of the detector and the reference signal of the reference detector (in the form of voltage signals) is determined. In this way, the influence of the intensity noise of the excitation light on the measurement can be reduced and the signal-to-noise ratio can be increased. In this case, the splitting device, e.g. a suitably structured grating coupler, usually deflects the majority of the excitation light to the crystal, while a small portion is passed through to the reference diode.For balanced evaluation, it is typically advantageous if the detector's measurement signal and the reference detector's reference signal are equal for a specific value of the measured variable. In the case of a magnetic field sensor, for example, this can be the case when no external magnetic field is present. To ensure that the detector's measurement signal corresponds to the reference detector's reference signal when no external magnetic field is present, an attenuator such as a neutral density filter can be used to appropriately attenuate the excitation light applied to the detector or the reference detector.

[0024] Alternatively or in addition to the reference detector, a frequency stabilization arrangement in the form of an excitation laser source can be used to reduce the intensity noise of the excitation light source. This frequency stabilization arrangement is also attached to the carrier substrate or, if necessary, integrated into the carrier substrate and connected to the excitation laser source via a photonic wire bond. A frequency stabilization arrangement in the form of an external photonic feedback circuit for stabilizing a reflective semiconductor amplifier coupled to the feedback circuit via a photonic wire bond is described, for example, in the article "Hybrid external-cavity lasers (ECL) using photonic wirebonds as coupling elements," Yilin Xu et al., Scientific Reports (2021) 11:16426.In a further embodiment, the further component forms a further sensor element in the form of a further crystal doped with color centers, in particular a further diamond crystal doped with NV centers. The color centers of the further crystal are also designed to generate fluorescent light when excited with excitation light. In this case, the quantum sensor has a further detector, which is typically attached to the further crystal. In this case, the splitting device generally splits the excitation light essentially equally between the sensor element and the further sensor element. With the help of the further detector, a gradient of a measurand can be determined, for example a magnetic field gradient, i.e. a location-dependent change in the magnetic field between the positions of the two crystals. In this case, the quantum sensor with the two sensor elements is referred to as a magnetic field gradiometer.

[0025] It is understood that the additional component does not necessarily have to be a reference detector or another sensor element. The additional component can be, for example, another splitting device, such as a grating coupler, or another type of coupling element. The excitation light can be coupled into the additional component using a coupling device, which is designed, for example, like the coupling device for coupling the excitation light into the sensor element.

[0026] In a further development, the further component is attached to the surface of the carrier substrate to which the sensor element is attached, or to the side of the carrier substrate. If the further component is another crystal, this is typically attached to the surface of the carrier substrate. The reference detector can be attached to the surface of the carrier substrate or to the side of the carrier substrate. In the latter case, coupling can occur, for example, via an end face of the waveguide, which extends to the lateral edge of the carrier substrate. The waveguide can have a varying refractive index in a section adjacent to the lateral edge in order to adapt the refractive index of the waveguide to the refractive index of the material on the detector surface of the detector, for example a photodiode.

[0027] The quantum sensor typically has additional components to which no excitation light is supplied. For example, the quantum sensor can have a microwave generator for generating a microwave field in the sensor element and possibly in the further sensor element. The microwave field can be radiated, for example, via an omega resonator, a split-ring resonator or a Helmholtz resonator, which is either integrated into the carrier substrate or is attached or mounted as a further component on the surface of the carrier substrate. In addition, the quantum sensor can have a magnetic field generator for generating a static magnetic field, which is designed, for example, in the form of current-carrying coils or permanent magnets, which can also be integrated into the carrier substrate or attached or mounted as separate components on the surface of the carrier substrate. The magnetic field generator generates a microwave field in the sensor element and possiblyin the further sensor element a static magnetic field.

[0028] In a further embodiment, the quantum sensor comprises an excitation light source for generating the excitation light and a coupling device for coupling the excitation light into the waveguide. The excitation light source can be attached to the carrier substrate. However, it is also possible for the excitation light source not to be attached to the carrier substrate, but to be suitably aligned with it. In the simplest case, the coupling device forms an end facet of the waveguide, and the excitation light is coupled into the waveguide in free-beam propagation. In the case of a ridge waveguide, the end facet for coupling the excitation light can be designed obliquely in order to achieve a 90° angle between a laser beam incident obliquely from above and the end facet of the waveguide. Coupling of the excitation light from the excitation light source, e.g.in the form of a laser diode, via an optical waveguide in the form of a photonic wire bond into the waveguide is possible. As described above in connection with the coupling device for the sensor element, the coupling device can be designed as a grating coupler if the waveguide is embedded in the carrier substrate. In this case, the excitation light can be radiated onto the grating coupler in free-beam propagation. However, it is also possible for the coupling device to have a prism or the like for coupling the excitation light into the grating coupler, into which the excitation light is radiated and deflected to the grating coupler. Refractive index matching is also possible in this case. For coupling the excitation light, direct contact with the grating coupler is also possible via an optical fiber or glass fiber.The excitation light can also be coupled into a waveguide embedded in the carrier substrate via an optic applied to the embedded waveguide, e.g., glued or printed onto it, or via a metal lens. In this case, the excitation light is typically coupled into the optic or metal lens via an optical fiber.

[0029] The coupling device for coupling the excitation light into the waveguide as well as the coupling device for coupling the excitation light into the crystal and, if applicable, into the further crystal or into the reference detector can be manufactured in different ways, for example by printing (in photoresist), which has the advantage of higher tolerances in the positioning of the excitation light source relative to the waveguide, by drawing, by etching (advantage: faster process than printing, scalability), by reactive ion etching, by a nickel-induced etching process or by laser structuring (advantage: scalability).

[0030] Further advantages of the invention will become apparent from the description and the drawings. Likewise, the above-mentioned and further listed features can be used individually or in combination. The embodiments shown and described are not intended to be exhaustive, but rather serve as examples for describing the invention.

[0031] Shown are: Fig. 1a a schematic representation of a quantum sensor with a carrier substrate, on the surface of which a diamond crystal doped with NV centers and a reference detector are attached, to which excitation light is supplied via a waveguide,

[0032] Fig. 1 b is a schematic representation of a quantum sensor analogous to Fig. 1 a, in which another diamond crystal is attached to the carrier substrate instead of the reference diode,

[0033] Fig. 2a-c schematic representations of a diamond crystal with a detector and with one or more filter elements arranged between the diamond crystal and the detector, as well as with metal lenses,

[0034] Fig. 3a-c Representations of diamond crystals with three different geometries,

[0035] Fig. 4a, b a representation of a quantum sensor analogous to Fig. 1 a, which has a ridge waveguide for supplying a first portion of the excitation light to the diamond crystal and another ridge waveguide for supplying a second portion of the excitation light to the reference detector, and

[0036] Fig. 5a-e Representations of examples of coupling excitation light into the waveguide of the carrier substrate.

[0037] In the following description of the drawings, identical reference symbols are used for identical or functionally identical components.

[0038] It is understood that the components shown in the figures do not have to be shown in their actual size ratio to one another.

[0039] Fig. 1a shows a quantum sensor 1 having a plate-shaped carrier substrate 2 and a waveguide 3 embedded in the carrier substrate 2. The waveguide 3 runs within the carrier substrate 2 just below a flat surface 2a of the carrier substrate 2, which forms the top side of the carrier substrate 2. A sensor element in the form of a diamond crystal 4 doped with NV centers 5, one of which is shown as an example in Fig. 1a, is attached or mounted on the surface 2a of the carrier substrate 2. The NV centers 5 are designed to generate fluorescent light 7 upon excitation with excitation light 6. The fluorescent light 7 is detected by a detector 8, which in the example shown is attached to a surface 4a of the diamond crystal 4 facing away from the carrier substrate 2.In the example shown, the detector 8 is a photodiode whose detector surface is flatly connected to the surface 4a of the diamond crystal 4. An electrical signal dependent on the intensity of the fluorescent light 7 captured by the detector 8 is guided to the carrier substrate 2 by means of an electrical wire bond 9 and can be transmitted via electrical conductor tracks embedded therein to an evaluation device that is attached to the carrier substrate 2 or, if necessary, arranged elsewhere.

[0040] The waveguide 3 has a first coupling device in the form of a first grating coupler 10a, which serves to couple the excitation light 6 into the waveguide 3, which borders the surface 2a of the carrier substrate 2 in the region of the grating coupler 10a. In the example shown, the excitation light 6 is generated by an excitation light source 11 in the form of a laser, more precisely a laser diode. In the example shown, the excitation light source 11 is not attached to the carrier substrate 2, but is fixed in a suitable manner relative to the carrier substrate 2 to ensure that the excitation light 6 impinges on the first grating coupler 10a in free-beam propagation and is coupled into the waveguide 3.

[0041] A second grating coupler 10b serves as a coupling device for coupling the excitation light 6, more precisely a first portion of the excitation light 6, from the waveguide 3 into the diamond crystal 4, while a second portion of the excitation light 6 is not coupled into the diamond crystal 4 and propagates further in the waveguide 3. In the present example, the second grating coupler 10b serves as a splitting device for splitting the excitation light 6 into a first portion supplied to the diamond crystal 4 and a second portion supplied to a reference detector 12. The second grating coupler 10b is designed such that the first portion of the excitation light 6 coupled into the diamond crystal 4 is significantly larger than the second, uncoupled portion of the excitation light 6.The second portion of the excitation light 6 is coupled almost entirely at a third grating coupler 10c into the reference detector 12, which is designed as a photodiode. The reference detector 12, like the diamond crystal 4, is attached to the surface 2a of the carrier substrate 2.

[0042] An electronic signal proportional to the intensity of the portion of the excitation light 6 supplied to the reference detector 12 is routed to the carrier substrate 2 via an electrical wire bond 13 and fed to the evaluation device described above via electrical conductor tracks running there. In this case, the evaluation is carried out in the manner of a "balanced detection," in which the electrical signal of the detector 8 and the electrical signal of the reference detector 12 are subtracted from one another in order to reduce the influence of the intensity noise of the excitation light 6 of the excitation light source 11 on the measurement.

[0043] Using the quantum sensor 1 shown in Fig. 1a, various measured variables can be determined, for example, the strength of a magnetic field, the temperature, etc. In the present example, the evaluation device is designed to determine the strength of an (external) magnetic field in the diamond crystal 4. Determining the strength of the magnetic field based on the detected fluorescence radiation is generally known and described, for example, in US Pat. No. 10,126,377 B2. The underlying process is briefly summarized below.

[0044] To determine the magnetic field, the NV centers 5 in the diamond crystal 4 are excited with excitation light 6 having a wavelength between 515 nm and 570 nm. Depending on the initial state, the NV centers 5 are thereby excited into a state that either decays by emitting fluorescent light 7 with wavelengths between 600 nm and 900 nm, or into a state that decays via several intermediate states by emitting light with a wavelength of 1042 nm. The ground state of the NV centers 5 can be manipulated using a resonantly radiated microwave field. The resonance frequency depends on the external magnetic field and can be used to determine its magnitude. The dip in the fluorescent light 7 is detected for resonant excitation with the microwave field. For this purpose, the fluorescent light 7 orthe voltage signal generated by the fluorescent light 7 in the detector 8 is monitored by means of the evaluation device.

[0045] To generate the microwave field, the quantum sensor 1 has a microwave generator, which can be designed in different ways. The microwave generator can be integrated into the carrier substrate 2 or mounted in the form of a further component on the surface 2a of the carrier substrate 2. The quantum sensor 1 also has a magnetic field generator for generating a static magnetic field, which can be designed, for example, in the form of current-carrying coils or permanent magnets and can also be integrated into the carrier substrate 2 or attached or mounted in the form of one or more further components on the surface 2a of the carrier substrate 2. In addition to or as an alternative to the reference detector 12, a frequency stabilization circuit can be used to stabilize the frequency of the excitation light source 11, which, for example,can be attached to the carrier substrate 2 and is connected to the excitation light source 11 via photonic wire bonds.

[0046] Fig. 1 b shows a quantum sensor 1 which differs from the quantum sensor 1 shown in Fig. 1 a in that, at the location of the reference detector 12, another diamond crystal 4' is attached to the surface 2a of the carrier substrate 2. Another detector 12' is attached to the additional diamond crystal 4', which serves to detect fluorescent light 7' generated upon excitation of the NV centers 5' in the additional diamond crystal 4'. In the example shown in Fig. 1 b, a portion of the excitation light 6 corresponding to approximately half of the excitation light 6 generated by the excitation light source 11 is supplied to the additional diamond crystal 4' by the third coupling device 10c. The additional diamond crystal 4' is used to determine the (external) magnetic field prevailing at the location of the additional diamond crystal 4'.

[0047] By comparing the magnetic fields determined by means of the diamond crystal 4 and the further diamond crystal 4', a magnetic field gradient can be determined, which is why the quantum sensor 1 shown in Fig. 1 b is also referred to as a magnetic field gradiometer.

[0048] Fig. 2a-c show a diamond crystal 4 having a cuboidal geometry and a detector 8 attached to it in a detailed view. As can be seen in Fig. 2a, a filter element 15 is mounted between the diamond crystal 4 and the detector 8, which filter element is designed to filter or block the excitation light 6. Fig. 2b shows a diamond crystal 4 in which, in addition to the filter element 15 for filtering the excitation light 6, a filter element 16 for filtering infrared light, in particular in the wavelength range around approximately 1042 nm, is arranged between the detector 8 and the diamond crystal 4 in order to filter the infrared light generated by the NV centers 5.

[0049] As can also be seen in Fig. 2b, a first metal lens 17a is arranged between the second grating coupler 10b and the diamond crystal 4, which serves to collimate the excitation light 6 coupled into the diamond crystal 4. A second metal lens 17b is arranged between the diamond crystal 4 and the detector 8, which serves to focus the fluorescent light 7 onto the detector 8. The diamond crystal 4 shown in Fig. 2c differs from the diamond crystal 4 of Fig. 2b in that the detector 8 is not attached to the side 4a of the diamond crystal 4 facing away from the carrier substrate 2, but rather to a side surface 4b of the diamond crystal 4. This is advantageous for reducing the requirements placed on the filter element 15, which serves to filter the excitation light 6. As already mentioned, the relative sizes of components shown here do not necessarily correspond to reality.In particular, the detector 8 can be designed smaller in reality than the diamond crystal 4. The second metal lens 17b can thus focus the fluorescent light 7 onto the detector. With a smaller detector area, the dark current and thus the detector noise can be reduced.

[0050] The three diamond crystals 4 shown in Fig. 2a-c with the detector 8 attached thereto can each be attached to the surface 2a of the carrier substrate 2 of the quantum sensors 1 shown in Fig. 1a,b. The distances shown in Fig. 2a-c between the filter elements 15, 16, the metal lenses 17a,b, and the detector 8 or the diamond crystal 4 serve to facilitate the identification of the respective components. The filter elements 15, 16 and the metal lenses 17a,b are typically bonded to the diamond crystal 4 and the detector 8 over a large area, for example, by adhesive bonding, or can be formed as coatings.

[0051] Fig. 3a-c show examples of diamond crystals 4 with different geometries. Fig. 3a shows a diamond crystal 4 in the shape of a truncated pyramid, which in the example shown has four side surfaces 4b. Alternatively, the diamond crystal 4 of Fig. 3a can also have the shape of a truncated cone. Fig. 3b shows a diamond crystal 4 in the shape of a cuboid, and Fig. 3c shows a diamond crystal 4 with a parabolic-shaped lateral surface 4b. The diamond crystals 4 shown in Fig. 3a and Fig. 3c expand from a coupling surface 4c for the excitation light 6 in the direction of the detector 8. Such a geometry of the diamond crystal 4 makes it possible to capture as much fluorescent light 7 as possible with the help of the detector 8.

[0052] Fig. 4a, b show a quantum sensor 1 that differs from the quantum sensor 1 shown in Fig. 1a in that the waveguide 3 is not embedded in the carrier substrate 2, but is designed as a ridge waveguide that runs along the surface 2a of the carrier substrate 2. In Fig. 3a, a photonic wire bond serves as the first coupling device 10a for coupling the excitation light 6 into the ridge waveguide 3, which connects the excitation light source 11 to an end facet of the ridge waveguide 3. A wedge-shaped widening section of the ridge waveguide 3 serves as the second coupling device 10b for laterally coupling excitation light 6 into the diamond crystal 4.

[0053] In the quantum sensor 1 shown in Fig. 4a, b, the excitation light 6 guided in the waveguide s is split by means of a splitting device 18 into a first portion, which is fed to the diamond crystal 4, while a second portion of the excitation light 6 is coupled into a further ridge waveguide 3' via adiabatic coupling. In the example shown in Fig. 4a, b, the splitting device 18 consists of a section of the further waveguide 3', which is arranged over a predetermined coupling length I at a short distance d from the waveguide 3, in which the excitation light 6 is guided. By selecting the distance d and the coupling length I, the portion of the excitation light 6 that is coupled from the waveguide 3 into the further waveguide 3' can be predetermined.

[0054] The excitation light 6 coupled into the further waveguide 3' is guided by the latter to the reference detector 12, which in the example shown is mounted laterally on the carrier substrate 2. A peripheral portion of the further ridge waveguide 3' serves as the third coupling device 10c for coupling the excitation light 6 into the reference detector 12. The peripheral portion of the further ridge waveguide 3' has a refractive index matched to the refractive index of the material of the detector surface of the reference detector 12 in the form of a photodiode, and is adjacent to an end facet of the further waveguide 3'. It is understood that there are other options for coupling the excitation light 6 into the waveguide 3, the diamond crystal 4, and the reference detector 12 besides those described above.

[0055] Several examples for coupling excitation light 6 into the waveguide 3 are described below using Fig. 5a-e. In the example shown in Fig. 5a, the excitation light 6 is coupled into a ridge waveguide 3 using free-beam propagation. The end facet of the ridge waveguide 3 used for coupling, which serves as the coupling device, is inclined.

[0056] The waveguides 3 shown in Fig. 5b to Fig. 5e are embedded in the carrier substrate 2. In the waveguide 3 shown in Fig. 5b, the excitation light 6 is coupled in free-jet propagation into the end facet of the waveguide 3, which serves as a coupling device. In the example shown in Fig. 5c, the coupling takes place via a prism 19, into which the excitation light 6 is coupled in free-jet propagation. In the example shown in Fig. 5c, a grating coupler can additionally be arranged between the prism 19 and the waveguide 3. In the example shown in Fig. 5d, the excitation light 6 is guided from the excitation light source (not shown) via a glass fiber 20 to the waveguide 3 and coupled into the waveguide 3 via a grating coupler (not shown). As shown in Fig.As can be seen in Figure 5e, an optic 21, for example in the form of a metal lens, can be arranged between the glass fiber 20 and the waveguide in order to adapt the refractive index of the material of the glass fiber 20 to the refractive index of the material of the waveguide 3.

Claims

Patent claims 1. Quantum sensor (1), in particular for magnetic field measurement, comprising: a sensor element (4) in the form of a crystal doped with color centers (5), in particular a diamond crystal doped with NV centers, wherein the color centers (5) are designed to generate fluorescent light (7) when excited with excitation light (6), a carrier substrate (2) with a surface (2a) to which the sensor element (4) is attached, and a detector (8), in particular a photodiode, for detecting the fluorescent light (7), characterized in that the carrier substrate (2) has a waveguide (3) for supplying the excitation light (6) to the sensor element (4).

2. Quantum sensor according to claim 1, wherein the waveguide (3) is designed as a ridge waveguide which is arranged on the surface (2a) of the carrier substrate (2), or in which the waveguide (3) runs in the carrier substrate (2).

3. Quantum sensor according to claim 1 or 2, wherein the detector (8) is attached to the sensor element (4).

4. Quantum sensor according to claim 3, wherein the detector (8) is attached to a surface (4a) of the sensor element (4) facing away from the carrier substrate (2) or to a lateral surface (4b) of the sensor element (4).

5. Quantum sensor according to claim 3 or 4, wherein a filter element (15) for filtering the excitation light (6) is mounted between the detector (8) and the sensor element (4).

6. Quantum sensor according to one of claims 3 to 5, wherein a filter element (16) for filtering infrared light is mounted between the detector (8) and the sensor element (4).

7. Quantum sensor according to one of claims 3 to 6, wherein the sensor element (4) in the form of the crystal widens from a coupling surface (4c) for the excitation light (6) in the direction of the detector (8), wherein the crystal is preferably designed as a truncated cone or a truncated pyramid or has a parabolic lateral surface (4b).

8. Quantum sensor according to one of the preceding claims, in which at least one optic, preferably a metal lens (17a, 17b), is mounted between the waveguide (3) and the crystal and / or between the crystal and the detector (8).

9. Quantum sensor according to one of the preceding claims, further comprising: a coupling device (10b) for coupling excitation light (6) into the sensor element (4).

10. Quantum sensor according to claim 9, wherein the coupling device (10b) is designed in the form of a grating coupler.

11. Quantum sensor according to one of the preceding claims, further comprising: at least one further component (12, 4') which is attached to the carrier substrate (2), and a splitting device (10b, 18) for splitting the excitation light (6) guided in the waveguide (3) into a portion supplied to the sensor element (4) and into a further portion supplied to the further component (12, 4') in the waveguide (3) or in a further waveguide (3').

12. Quantum sensor according to claim 11, wherein the further component forms a reference detector (12) for detecting excitation light (6).

13. Quantum sensor according to claim 11 or 12, wherein the further component forms a further sensor element (4') in the form of a further crystal doped with color centers, in particular a further diamond crystal doped with NV centers (5').

14. Quantum sensor according to one of claims 11 to 13, wherein the further component (12, 14) is attached to the surface (2a) of the carrier substrate (2) or laterally to the carrier substrate (2).

15. Quantum sensor according to one of the preceding claims, further comprising: an excitation light source (11) for generating the excitation light (6) and a coupling device (10a) for coupling the excitation light (6) into the waveguide (3).

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