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

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 during excitation with excitation light, a carrier substrate having a surface to which the sensor element is attached, and a detector, in particular a photodiode, for detecting the fluorescent light. Quantum sensors play a central role in many technical fields of application and can be used to determine various physical measured variables. Crystals doped with color centers are typically used as sensor elements for quantum sensors. In particular, a diamond crystal doped with color centers, typically nitrogen-vacancy (NV-) centers, can be used as a sensor element. NV centers in diamond exhibit a characteristic electronic structure that changes in response to variations in certain measured variables, such as an external magnetic field, temperature, pressure, or an electric field. 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, and the fluorescence light induced in the color centers as a result is detected and analyzed by a detector. A quantum sensor can be used to measure one or more measured variables. For example, DE 10 2014 219 550 A1 describes a combination sensor for measuring pressure and / or temperature and / or magnetic fields, comprising at least one sensitive component in the form of a deformable layer, in particular a deflectable membrane, having diamond structures that comprise NV centers. The manufacture of quantum sensors presents various challenges, for example regarding miniaturization, scalability, cost-efficiency, and assembly and interconnection technology. US 10,126,377 B2 describes a magnetometer having a magneto-optical defect center element, e.g., in the form of a diamond doped with NV centers, an excitation light source, and a collection device, all attached to a printed circuit board. The magnetometer features an optical waveguide arrangement that has an optical waveguide and at least one filter coating. The waveguide is designed to direct light emitted by the magneto-optical defect center element to the collecting device. To align one or more components attached to the printed circuit board, a two-point orientation system is used, in which two points are provided for each component that are aligned relative to a top plate, a bottom plate, and the printed circuit board. For alignment, alignment apertures may be provided in the top plate and bottom plate of the magnetometer, into which pins attached to the components engage. OBJECT OF THE INVENTION The object underlying the invention is to simplify the manufacture and assembly of a quantum sensor. SUBJECT MATTER OF THE INVENTION This object is achieved by a quantum sensor of the type mentioned at the outset, in which the carrier substrate has (at least) one waveguide for guiding the excitation light to the sensor element. The quantum sensor described here is based on the concept of (optical) waveguides on or within the carrier substrate. Such optical waveguides can be manufactured cost-effectively in large quantities using established (semiconductor) manufacturing techniques. The precision achievable using the semiconductor manufacturing technology allows the sensor element and other components attached to the carrier substrate to be positioned with a high degree of precision. By guiding the excitation light through the waveguide(s) between the components, active alignment of the components can be dispensed with. This eliminates the need for additional effort in positioning the individual components and applying adhesives compared to common / conventional sensor concepts, in which the excitation light is guided to the sensor element via free-space propagation, as described, for example, in US 10,126,377 B2. In addition, 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 can be miniaturized. The excitation light is guided to the sensor element via the waveguide. Other components attached to the carrier substrate, such as a reference detector or an additional sensor element (see below) can also be supplied with excitation light via the waveguide or via additional waveguides integrated into the carrier substrate. The configuration described here eliminates nearly all degrees of freedom in the positioning of the individual components relative to one another (angular degrees of freedom as well as 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. The sensor element and other components can be attached to the surface of the carrier substrate using a suitable mounting technique, such as surface-mount technology (SMT), for example, using a pick-and-place robot. In one embodiment, the waveguide is designed as a ridge waveguide that runs along the surface of the carrier, or the waveguide runs within the carrier substrate or is embedded in the carrier substrate. The waveguide can be produced, for example, by lithographic definition of etch masks followed by reactive ion etching or ion beam etching, by a laser structuring process, or by a nickel-induced etching process. The resulting waveguide(s) may run along the carrier substrate in the form of ridge waveguides that protrude above the surface of the carrier substrate or run within the carrier substrate or are embedded therein. Lattice structures can be inscribed into a waveguide in the form of a ridge waveguide to perform a filtering function. An embedded waveguide may be completely or partially covered by a cladding layer of the carrier substrate, wherein the cladding layer should have a lower refractive index than the active waveguide material. The carrier substrate may consist of a single layer or multiple layers and is typically plate-shaped. One material used for the carrier substrate may be, for example, silicon. Materials such as SiN, SiCh, or LiNbOs can be used for the waveguide. In one embodiment, the detector is attached to the sensor element. By attaching the detector to the sensor element, it is possible to dispense with the use of a waveguide to guide the fluorescent light to the detector, as described, for example, in US 10,126,377 B2. The detector is typically connected over its entire surface to the sensor element in the form of the crystal and thus makes it possible to collect a large proportion of the fluorescent light generated at the color centers during excitation. In a further development, the detector is attached to a surface of the sensor element facing away from the carrier substrate or to a lateral surface of the sensor element. The excitation light is typically coupled into the crystal via a coupling surface on the underside of the crystal facing the carrier substrate or via a lateral surface. It may be advantageous if the detector is not directly struck by the excitation light, i.e., if it is positioned outside the beam path of the excitation light, since in this case the requirements for a filter element to filter out the excitation light are less stringent (see below). For example, if the excitation light is coupled into the crystal from the bottom, it may be advantageous for the detector to be attached to a side surface of the crystal, although this is not strictly necessary. The detector may, for example, be designed as a substantially plate-shaped photodiode whose detector surface is connected over its entire surface to a surface of the crystal. The detector may be connected to the carrier substrate, for example, via electrical wirebonds, in order to transmit information about the intensity of the detected fluorescent light. Electrical conductive traces may be provided in the carrier substrate to transmit this information. To evaluate the information regarding the detected fluorescent light, an evaluation device, for example, in the form of a chip or the like, may be attached to the carrier substrate. Alternatively, it is possible to transmit the information regarding the detected fluorescent light to an external evaluation device. In a further embodiment, a filter element for filtering the excitation light is attached between the detector and the sensor element. 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 introduce a filter—optionally only partially—between the sensor element and the detector to block the excitation light. The filter element can be designed, for example, as a filter layer that is attached, either as a coating or as a separate component, between the sensor element in the form of the crystal and the detector. In a further embodiment, a filter element for filtering infrared light is attached between the detector and the sensor element. When the color centers, in particular NV centers, are excited, infrared light may be generated in addition to the fluorescent light. If the detector is sensitive to infrared light, it is advantageous or necessary to block the infrared light by means of the filter element. The filter element can be designed, like the filter element described above for filtering the excitation light, e.g., as a filter layer in the form of a coating or as a separate component. In a further embodiment, the sensor element in the form of a crystal extends from a coupling surface for the excitation light toward the detector, wherein the crystal is preferably shaped as a truncated cone or a truncated pyramid, or has a parabolic lateral surface. A crystal that widens toward the detector is defined as one in which the surface of the crystal to which the detector is attached has a larger area than the surface of the crystal at which the excitation light is coupled into the crystal. For this purpose, the crystal may be designed in various ways. For example, it may be shaped as a truncated pyramid or a cone, or it may have a parabolic or otherwise curved lateral surface. The crystal in the form of a truncated pyramid may have three, four, or more lateral surfaces. Because of its shape, which widens toward the detector, the fluorescence yield of the detector can be increased. Furthermore, by shaping the crystal appropriately, the probability of total internal reflection along spatial directions in which no detector is located can be increased. In another embodiment, at least one metalens is attached between the waveguide and the crystal and / or between the crystal and the detector. Metalenses are optical components based on nanostructured surfaces or metamaterials and may be designed as coatings or as thin, plate-shaped components. A metalens between the waveguide and the crystal can be used to collimate the excitation light during coupling. A metalens between the crystal and the detector may be used as a converging lens to focus the excitation light onto the detector. 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 may form an end face of the waveguide that adjoins a lateral surface of the crystal, i.e., the waveguide may be directly attached to the crystal to couple the excitation light. If the waveguide is embedded in the carrier substrate, the carrier substrate may have a recess in the region of the crystal to lower the crystal and allow for lateral coupling of the excitation light. For the coupling of the excitation light, it may be advantageous to match the refractive index of the waveguide to that of the crystal. For this purpose, a suitable refractive index matching element may be provided between the waveguide and the crystal, or the refractive index of the waveguide may be specifically modified in the vicinity of the crystal through appropriate measures, such as doping. For the coupling of the excitation light, it may additionally be advantageous for the waveguide to diverge in the coupling region and for the width of the waveguide to increase, e.g., in a section that widens in a wedge shape. In a further development, the coupling device is designed as a grating coupler. In this embodiment, the waveguide is typically embedded in the substrate. The grating coupler is generally attached to the top surface of the waveguide to couple the excitation light from below into a coupling surface on the bottom surface of the crystal. In this case, the excitation light is coupled substantially perpendicular to the surface of the carrier substrate to which the crystal is attached. In a further embodiment, the quantum sensor comprises at least one additional component attached to the carrier substrate, as well as a splitting device for splitting the excitation light guided in the waveguide into a portion guided to the sensor element and into another portion to be guided to the additional component within the waveguide or in a separate waveguide. The splitting device may, for example, be designed in the form of a coupling device, such as a grating coupler (e.g., “apodized grating”) that 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 guided to the additional component. However, photonic elements such as directional couplers, multimode interference couplers, or Y-junctions may also serve as the splitting device. In the case that the waveguide is a ridge waveguide, the splitting device can transfer the excitation light via adiabatic coupling into a section of another waveguide that 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 the coupling length, the proportion of the excitation light that is coupled from the waveguide into the other waveguide can be specified. In a further development, the additional component serves as a reference detector for detecting excitation light. The reference detector can be designed as a photodiode, for example. The reference detector can be used to perform so-called “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. Typically, in this case, the splitting device, e.g., a suitably structured grating coupler, diverts the majority of the excitation light toward the crystal, while a small portion is transmitted to the reference diode. For balanced evaluation, it is typically advantageous for the measurement signal of the detector and the reference signal of the reference detector to be equal in magnitude at a particular value of the measured variable. In the case of a magnetic field sensor, this may be the case, for example, when no external magnetic field is present. To ensure that the measurement signal of the detector corresponds to the reference signal of the reference detector when no external magnetic field is present, an attenuator, such as a neutral-density filter, may be used to appropriately attenuate the excitation light guided to the detector or the reference detector. Alternatively, or in addition to the reference detector, a frequency stabilization arrangement may be used to reduce the intensity noise of the excitation light source in the form of an excitation laser source which 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 wirebond. 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 wirebond is described, for example, in the article “Hybrid externalcavity lasers (ECL) using photonic wirebonds as coupling elements," by Yilin Xu et aL, Scientific Reports (2021) 11:16426. In another embodiment, the additional component forms an additional sensor element in the form of an additional crystal doped with color centers, in particular an additional diamond crystal doped with NV centers. The color centers of the additional crystal are also designed to emit fluorescent light when excited by excitation light. In this case, the quantum sensor has an additional detector that is typically attached to the additional crystal. Typically, the splitting device in this case divides the excitation light substantially equally between the sensor element and the additional sensor element. Using the additional detector, it is possible to determine the gradient of a measured variable, such as a magnetic field gradient, i.e., a spatially dependent variation in the magnetic field between the positions of the two crystals. In this case, the quantum sensor having the two sensor elements is referred to as a magnetic field gradiometer. It is understood that the additional component does not necessarily have to be a reference detector or an additional sensor element. The additional component may be, for example, an additional 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 that is, for example, designed in the same way as the coupling device used to couple the excitation light into the sensor element. In a further development, the additional component is attached to the surface of the carrier substrate to which the sensor element is attached or is attached laterally to the carrier substrate. If the additional component is an additional crystal, it is typically attached to the surface of the carrier substrate. The reference detector may be attached to the surface of the carrier substrate or may be attached laterally to the carrier substrate. In the latter case, coupling may be achieved, for example, via an end face of the waveguide that extends to the lateral edge of the carrier substrate. The waveguide may have a varying refractive index in a section adjacent to the lateral edge in order to match the refractive index of the waveguide to the refractive index of the material at the detector surface of the detector, such as a photodiode. The quantum sensor typically has additional components to which no excitation light is guided. For example, the quantum sensor may have a microwave generator for generating a microwave field in the sensor element and, optionally, in the additional sensor element. The microwave field may be emitted, for example, via an omega resonator, a split-ring resonator, or a Helmholtz resonator, which is either integrated into the carrier substrate or attached or mounted as an additional component on the surface of the carrier substrate. In addition, the quantum sensor may 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 that 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 static magnetic field in the sensor element and, optionally, in the additional sensor element. 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 may be attached to the carrier substrate. However, it is also possible that the excitation light source is not attached to the carrier substrate but is appropriately aligned with it. In the simplest case, the coupling device forms an end face or end facet of the waveguide, and the excitation light is coupled into the waveguide via free-space propagation. If the waveguide is a ridge waveguide, the end facet for coupling the excitation light may be angled to achieve a 90° angle between a laser beam incident obliquely from above and the end facet of the waveguide. It is also possible to couple the excitation light from the excitation light source, e.g., in the form of a laser diode, into the waveguide via an optical waveguide in the form of a photonic wirebond. As described above in conjunction with the coupling device for the sensor element, the coupling device may be designed as a grating coupler if the waveguide is embedded in the carrier substrate. In this case, the excitation light may be directed onto the grating coupler via free-space propagation. However, it is also possible for the coupling device to have a prism or similar element for coupling the excitation light into the grating coupler, into which the excitation light is directed and then redirected toward the grating coupler. Refractive index matching is also possible in this case. For coupling the excitation light, direct contacting to the grating coupler via an optical fiber or glass fiber is also possible. The coupling of the excitation light into a waveguide embedded in the carrier substrate can also be achieved via an optical element attached, e.g., bonded or printed, to the embedded waveguide, or via a metalens. In this case, the excitation light is typically coupled into the optical element or metalens via an optical fiber. 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, optionally, into the additional crystal or into the reference detector, may be manufactured in various ways, for example by printing (in photoresist), which has the advantage of allowing for greater 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). Further advantages of the invention are apparent from the description and the drawing. Similarly, the features mentioned above and those listed below may be used individually or in any combination thereof. The embodiments shown and described are not to be understood as an exhaustive list, but rather serve as examples to illustrate the invention. In the drawings: Fig. 1a is 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 guided via a waveguide, Fig. 1b is a schematic representation of a quantum sensor similar to Fig. 1a, in which an additional diamond crystal is attached to the carrier substrate in place of the reference diode, Fig. 2a-c are 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 metalenses, Fig. 3a-c are representations of diamond crystals with three different geometries, Fig. 4a, b are representations of a quantum sensor similar to Fig. 1a, which has a ridge waveguide for guiding a first portion of the excitation light to the diamond crystal and a further ridge waveguide for guiding a second portion of the excitation light to the reference detector, and Fig. 5a-e are representations of examples of the coupling of excitation light into the waveguide of the carrier substrate. In the following description of the drawings, identical reference signs are used for identical or functionally equivalent components. It is understood that the components shown in the figures are not necessarily depicted to scale relative to one another. Fig. 1a shows a quantum sensor 1 that has 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 planar surface 2a of the carrier substrate 2, which forms the top surface 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 by way of an example in Fig. 1a, is attached to or mounted on the surface 2a of the carrier substrate 2. The NV centers 5 are designed to generate fluorescent light 7 when excited by 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 connected over its entire surface 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 via an electrical wirebond 9 and may be transmitted via electrical conductive traces embedded therein to an evaluation device which is attached to the carrier substrate 2 or, optionally, arranged elsewhere. 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 adjoins 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 specifically, a laser diode. In the example shown, the excitation light source 11 is not attached to the carrier substrate 2 but is fixed relative to the carrier substrate 2 in an appropriate manner to ensure that the excitation light 6 strikes the first grating coupler 10a via free-space propagation and is coupled into waveguide 3. A second grating coupler 10b serves as a coupling device for coupling the excitation light 6, or, more specifically, 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 continues to propagate 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, which is guided to the diamond crystal 4, and a second portion, which is guided to a reference detector 12. The second grating coupler 10b is designed such that the first portion of the excitation light 6, which is coupled into the diamond crystal 4, is significantly larger than the second portion of the excitation light 6 that is not coupled into the diamond crystal 4. The second portion of the excitation light 6 is coupled out almost entirely via a third grating coupler 10c into the reference detector 12, which is designed as a photodiode. The reference detector 12 is attached to the surface 2a of the carrier substrate 2, just like the diamond crystal 4. An electronic signal proportional to the intensity of the portion of the excitation light 6 guided to the reference detector 12 is guided via an electrical wirebond 13 to the carrier substrate 2 and guided to the evaluation unit described above via electrical conductive traces running therein. In this case, the evaluation is performed using a “balanced detection” method, in which the electrical signal from the detector 8 and the electrical signal from the reference detector 12 are subtracted from one another in order to reduce the influence of the intensity noise of the excitation light 6 from the excitation light source 11 on the measurement. The quantum sensor 1 shown in Fig. 1a can be used to determine various physical measured variables, such as 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. The determination of the strength of the magnetic field based on the detected fluorescence radiation is known in principle and is described, for example, in US 10,126,377 B2. The underlying process is briefly summarized again below. To determine the magnetic field, the NV centers 5 in the diamond crystal 4 are excited by excitation light 6 having a wavelength between 515 nm and 570 nm. Depending on their initial state, the NV centers 5 are thereby excited into a state that either decays while emitting fluorescent light 7 at wavelengths between 600 nm and 900 nm, or is excited to a state that decays through several intermediate states while emitting light at a wavelength of 1042 nm. The ground state of the NV centers 5 can be manipulated using a resonantly incident microwave field. The resonance frequency depends on the external magnetic field and can be used to determine its magnitude. The drop in fluorescence light 7 is detected for resonant excitation with the microwave field. For this purpose, the fluorescence light 7, or rather, the voltage signal generated by the fluorescence light 7 in the detector 8, is monitored using the evaluation device. To generate the microwave field, the quantum sensor 1 has a microwave generator which may be designed in various ways. The microwave generator may be integrated into the carrier substrate 2 or mounted on the surface 2a of the carrier substrate 2 as an additional component. The quantum sensor 1 also has a magnetic field generator for generating a static magnetic field, which may, for example, be designed in the form of current-carrying coils or permanent magnets and may also be integrated into the carrier substrate or be attached to or mounted on the surface 2a of the carrier substrate 2 in the form of one or more additional components. In addition to or as an alternative to the reference detector 12, a frequency stabilization circuit may be used to stabilize the frequency of the excitation light source 11, which may, for example, be attached to the carrier substrate 2 and connected to the excitation light source 11 by photonic wirebonds. Fig. 1b shows a quantum sensor 1 that differs from the quantum sensor 1 shown in Fig. 1a in that, in place of the reference detector 12, an additional diamond crystal 4' is attached to the surface 2a of the carrier substrate 2. An additional detector 12' is attached to the additional diamond crystal 4', which is used to detect the fluorescent light 7' generated when the NV centers 5’ in the additional diamond crystal 4' are excited. In the example shown in Fig. 1b, 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 present at the location of the additional diamond crystal 4'. By comparing the magnetic fields determined using the diamond crystal 4 and the additional diamond crystal 4', a magnetic field gradient can be determined, which is why the quantum sensor 1 shown in Fig. 1b is also referred to as a magnetic field gradiometer. Fig. 2a-c show a detailed view of a diamond crystal 4 that has a rectangular geometry and a detector 8 that is attached thereto. As can be seen in Fig. 2a, a filter element 15 designed to filter or block the excitation light 6 is attached between the diamond crystal 4 and the detector 8. 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 to filter the infrared light generated by the NV centers 5. As can also be seen in Fig. 2b, a first metalens 17a that is used to collimate the excitation light 6 coupled into the diamond crystal 4 is arranged between the second grating coupler 10b and the diamond crystal 4. A second metalens 17b that is used to focus the fluorescent light 7 onto the detector 8 is arranged between the diamond crystal 4 and the detector 8. The diamond crystal 4 shown in Fig. 2c differs from the diamond crystal 4 shown in 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 lateral surface 4b of the diamond crystal 4. This is advantageous for reducing the requirements imposed on the filter element 15 which is used to filter the excitation light 6. As mentioned earlier, the relative sizes of the components shown here do not necessarily reflect reality. In particular, the detector 8 may in reality be designed to be smaller than the diamond crystal 4. The second metalens 17b can thus be used to focus the fluorescent light 7 onto the detector. A smaller detector surface reduces the dark current and, consequently, the detector noise. The three diamond crystals 4 shown in Fig. 2a-c, each with the detector 8 attached thereto, may 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 metalenses 17a, b, and the detector 8 or the diamond crystal 4, respectively, are intended to make it easier to identify the respective components. The filter elements 15, 16 and the metalenses 17a, b are typically connected to the diamond crystal 4 and the detector 8 over their entire surface, for example, by bonding, or may be formed as coatings. Fig. 3a-c show examples of diamond crystals 4 having different geometries. Fig. 3a shows a diamond crystal 4 in the shape of a truncated pyramid which, in the example shown, has four lateral surfaces 4b. Alternatively, the diamond crystal 4 shown in Fig. 3a may also have the shape of a truncated cone. Fig. 3b shows a diamond crystal 4, which is shaped like a rectangular prism; Fig. 3c shows a diamond crystal 4 with a parabolic lateral surface 4b. The diamond crystals 4 shown in Fig. 3a and Fig. 3c widen in the direction of the detector 8, starting from a coupling surface 4c for the excitation light 6. Such a geometry of the diamond crystal 4 makes it possible to capture as much fluorescence light 7 as possible using the detector 8. Fig. 4a and 4b show a quantum sensor 1 that differs from the quantum sensor 1 shown in Fig. 1a primarily in that the waveguide 3 is not embedded in the carrier substrate 2 but is instead designed as a ridge waveguide that runs along the surface 2a of the carrier substrate 2. In Fig. 3a, a photonic wirebond connecting the excitation light source 11 to an end facet of the ridge waveguide 3 is used as the first coupling device 10a for coupling the excitation light 6 into the ridge waveguide 3. A section widening in a wedge-shape of the ridge waveguide 3 is used as a second coupling device 10b for laterally coupling excitation light 6 into the diamond crystal 4. In the quantum sensor 1 shown in Fig. 4a, b, the excitation light 6, which is guided in the waveguide 3, is split by a splitting device 18 into a first portion that is guided to the diamond crystal 4, while a second portion of the excitation light 6 is coupled via adiabatic coupling into another ridge waveguide 3'. In the example shown in Fig. 4a and 4b, the splitting device 18 consists of a section of the additional waveguide 3' that is arranged over a predetermined coupling length I at a small 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 proportion of the excitation light 6 that is coupled from the waveguide 3 into the additional waveguide 3' can be specified. The excitation light 6 coupled into the additional waveguide 3' is guided by the latter to the reference detector 12, which, in the example shown, is attached laterally on the carrier substrate 2. A third coupling device 10c for coupling the excitation light 6 into the reference detector 12 is an edge section of the additional ridge waveguide 3', whose refractive index is matched to the refractive index of the material of the detector surface of the reference detector 12, which is a photodiode, and which adjoins an end facet of the additional waveguide 3'. It is understood that there are other ways, besides those described above, to couple the excitation light 6 into the waveguide 3, the diamond crystal 4, and the reference detector 12. By way of example, several examples of coupling excitation light 6 into the waveguide 3 are described below with reference to Fig. 5a-e. In the example shown in Fig. 5a, the excitation light 6 is coupled into a ridge waveguide 3 via free-space propagation. The end facet of the ridge waveguide 3 used for coupling, which serves as the coupling device, is formed obliquely. The waveguides 3 shown in Fig. 5b to Fig. 5c are embedded in the carrier substrate 2. In the waveguide 3 shown in Fig. 5b, the excitation light 6 is coupled into the end facet of the waveguide 3, which is used as a coupling device, via free-space propagation. In the example shown in Fig. 5c, coupling occurs via a prism 19, into which the excitation light 6 is coupled via free-space propagation. In the example shown in Fig. 5c, a grating coupler may additionally be arranged between prism 19 and waveguide 3. In the example shown in Fig. 5d, the excitation light 6 is guided from the excitation light source (not shown) via an optical fiber 20 to the waveguide 3 and is coupled into the waveguide 3 via a grating coupler (not shown). As can be seen in Fig. 5e, an optical element 21, for example, in the form of a metalens, may be arranged between the optical fiber 20 and the waveguide to match the refractive index of the material of the optical fiber 20 to the refractive index of the material of the waveguide 3.

Claims

1. A quantum sensor (1), in particular for measuring magnetic fields, comprising:a sensor element (4) in the form of a crystal doped with color centers (5), in particulara diamond crystal doped with NV centers, wherein the color centers (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; anda detector (8), in particular a photodiode, for detecting the fluorescent light (7), characterized in thatthe carrier substrate (2) has a waveguide (3) for guiding the excitation light (6) to the sensor element (4).

2. The quantum sensor according to claim 1, in which the waveguide (3) is designed as a ridge waveguide that runs along the surface (2a) of the carrier substrate (2), or in which the waveguide (3) runs within the carrier substrate (2).

3. The quantum sensor according to claim 1 or 2, in which the detector (8) is attached to the sensor element (4).

4. The quantum sensor according to claim 3, in which 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. The quantum sensor according to claim 3 or 4, in which a filter element (15) for filtering the excitation light (6) is arranged between the detector (8) and the sensor element (4).

6. The quantum sensor according to any of claims 3 to 5, in which a filter element (16) for filtering infrared light is attached between the detector (8) and the sensor element (4).

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

8. The quantum sensor according to any of the preceding claims, in which at least one optical element, preferably a metalens (17a, 17b), is attached between the waveguide (3) and the crystal and / or between the crystal and the detector (8).

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

10. The quantum sensor according to claim 9, wherein the coupling device (10b) is designed as a grating coupler.

11. The quantum sensor according to any of the preceding claims, further comprising: at least one additional component (12, 4') 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 guided to the sensor element (4) and into another portion guided to an additional component (12, 4') in the waveguide (3) or to an additional waveguide (3').

12. The quantum sensor according to claim 11, in which the additional component forms a reference detector (12) for detecting excitation light (6).

13. The quantum sensor according to claim 11 or 12, in which the additional component forms an additional sensor element (4') in the form of an additional crystal doped with color centers, in particular an additional diamond crystal doped with NV centers (5').

14. The quantum sensor according to any of claims 11 to 13, in which the additional component (12, 14) is attached to the surface (2a) of the carrier substrate (2) or laterally to the carrier substrate (2).

15. The quantum sensor according to any 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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