Quantum sensor device

Abstract

The invention refers to a quantum sensor device for measuring a magnetic field using fluorescence dependence on the magnetic field, the quantum sensor device (18) comprising a substrate made of single-crystal diamond, and nitrogen atoms in the substrate, the nitrogen atoms being partially present in the form of negatively charged nitrogen-vacancy (NV-) centres, wherein a ratio of the concentration of NV- centres to the concentration of the nitrogen atoms in the substrate is at most 14%.

Description

[0001] QUANTUM SENSOR DEVICE

[0002] FIELD OF THE INVENTION

[0003] The invention relates to a quantum sensor device for measuring a magnetic field using fluorescence dependence on the magnetic field. The quantum sensor device comprises a substrate made of single crystal diamond, and nitrogen atoms in the substrate. The nitrogen atoms present in the substrate partially form negatively charged nitrogen-vacancy (NV-) centres.

[0004] The invention further relates to a method for manufacturing a quantum sensor device for measuring a magnetic field using fluorescence dependence on the magnetic field. The method comprises providing a substrate made of a single crystal diamond, implanting nitrogen atoms into the substrate, annealing the substrate to convert at least some of the implanted nitrogen atoms into negatively charged nitrogen-vacancy (NV-) centres.

[0005] BACKGROUND

[0006] Point defects in synthetic diamond material, particularly quantum spin defects and / or optically active defects, have been proposed for use in various sensing, detecting, and quantum processing applications including: magnetometers; spin resonance devices such as nuclear magnetic resonance (NMR) and electron spin resonance (ESR) devices; spin resonance imaging devices for magnetic resonance imaging (MRI); and quantum information processing devices such as for quantum computing.

[0007] Many point defects have been studied in synthetic diamond material including: silicon containing defects such as silicon-vacancy defects (Si-V), silicon di-vacancy defects (Si-V2), silicon-vacancy-hydrogen defects (Si-V:H), silicon di-vacancy hydrogen defects (S-V2:H); nickel containing defect; chromium containing defects; and nitrogen containing defects such as nitrogen-vacancy defects (N-V), di-nitrogen vacancy defects (N-V-N), and nitrogen-vacancy-hydrogen defects (N-V-H). These defects are typically found in a neutral charge state or in a negative charge state. These point defects extend over more than one crystal lattice point. The term point defect as used herein is intended to encompass such defects but not include larger cluster defects, such as those extending over ten or more lattice points, or extended defects such as dislocations which may extend over many lattice points. The negatively charged nitrogen-vacancy (NV-) defect in synthetic diamond material has attracted a lot of interest as a useful quantum spin defect because it has several desirable features including:

[0008] The electron spin states of the NV' centres can be coherently manipulated with high fidelity owing to a long coherence time (which may be quantified and compared using the transverse relaxation time or spin-spin relaxation time T2);

[0009] The electronic structure of the NV' centres allows the defect to be optically pumped into its electronic ground state allowing such defects to be placed into a specific electronic spin state even at non-cryogenic temperatures. This can negate the requirement for expensive and bulky cryogenic cooling apparatus for certain applications where miniaturization is desired. Furthermore, the defect can function as a source of photons which all have the same spin state.

[0010] The electronic structure of the NV' centres comprises emissive and non-emissive electron spin states which allows the electron spin state of the defect to be read out through photons. This is convenient for reading out information from synthetic diamond material used in sensing applications such as magnetometry, spin resonance spectroscopy and imaging. Furthermore, it is a key ingredient towards using the NV' defects as qubits for long-distance quantum communications and scalable quantum computation. Such results make the NV' defect a competitive candidate for solid-state quantum information processing (QIP).

[0011] The NV' defect in diamond consists of a substitutional nitrogen atom adjacent to a carbon vacancy. Its two unpaired electrons form a spin triplet in the electronic ground state (3A), the degenerate ms= ± 1 sublevels being separated from the ms= 0 level by 2.87 GHz at room temperature. The ms= 0 sublevel exhibits a high fluorescence rate when optically pumped. In contrast, when the defect is excited in the ms= ± 1 levels, it exhibits a higher probability to cross over to the non-radiative singlet state (1A) followed by a subsequent relaxation into ms= 0. As a result, the spin state can be optically read out, the ms= 0 state being “bright” and the ms= ± 1 states being dark. When an external magnetic field is applied, the degeneracy of the spin sublevels ms= ± 1 is broken via Zeeman splitting. This causes the resonance lines to split depending on the applied magnetic field magnitude and its direction. This dependency can be used for vector magnetometry as the resonant spin transitions can be probed by sweeping the microwave (MW) frequency resulting in characteristic dips in the optically detected magnetic resonance (ODMR) spectrum.

[0012] Ion implantation has been employed to create a layer of negatively charged NV' centres into {100} type Ila diamond. The ensemble NV' sensor was found to offer a higher magnetic sensitivity due to the amplified fluorescence signal from a plurality of sensing spins. Another option is vector reconstruction since the diamond lattice imposes four distinct tetrahedral NV' orientations. The magnetic field projections along each of these orientations can be measured as a single composite spectrum and a numerical algorithm used to reconstruct the full magnetic field vector. The magnitude (B) and orientation (0B, cpB) of the external magnetic field can be calculated by analysing the ODMR spectra based on an unconstrained least-square algorithm.

[0013] NV- centres are excellent magnetic field sensors and have been used for AC as well as DC magnetic field detection in various experiments. A significant parameter of each magnetometry setup is its sensitivity to external magnetic fields. A smaller sensitivity, in units TA / Hz, is favourable, measuring the ability to detect a smaller magnetic field in a shorter amount of time.

[0014] SUMMARY

[0015] An object of the invention is to provide a quantum sensor device having NV' centres which facilitates an improved measurement of a magnetic field.

[0016] According to a first aspect, there is provided a quantum sensor device for measuring a magnetic field using fluorescence dependence on the magnetic field. The quantum sensor device comprises a substrate made of single crystal diamond and nitrogen atoms in the substrate. The nitrogen atoms are partially present in the form of negatively charged nitrogen-vacancy (NV') centres. Optionally, a ratio of the concentration of NV' centres to the concentration of the nitrogen atoms in the substrate is between 0.01% and 1.4%.

[0017] According to a second aspect, there is provided a quantum sensor device for measuring a magnetic field using fluorescence dependence on the magnetic field. The quantum sensor device comprises a substrate made of single crystal diamond and nitrogen atoms in the substrate. The nitrogen atoms are partially present in the form of negatively charged nitrogen-vacancy (NV') centres. Optionally, the NV' centres have an optically detected magnetic resonance (ODMR) readout contrast of greater than 6% at a reciprocal initialisation time, 1 / treinit, of > 1 , optionally >1 1 / ps. The ODMR readout contrast is configured to be measured with an ODMR measurement using the quantum sensor device. The ODMR contrast refers to the maximum ratio between a fluorescence intensity after applying a resonant microwave pulse during a readout phase of the ODMR measurement and a fluorescence intensity without applying a resonant microwave pulse during the readout phase of the ODMR measurement. The fluorescence intensity is collected from an ensemble of NV' centres distributed over an area of at least 10 pm2, when initialized in the ms= 0 and ms= +1 or -1 state respectively. The treinit is the time required for re-polarizing the NV' centres.

[0018] According to a third aspect, there is provided a method for manufacturing a quantum sensor device for measuring a magnetic field using fluorescence dependence on the magnetic field, optionally using the quantum sensor device of any preceding claim. The method comprises the steps of (i) providing a substrate made of a single crystal diamond, (ii) implanting nitrogen atoms into the substrate, and (iii) annealing the substrate to convert at least some of the implanted nitrogen atoms into negatively charged nitrogen-vacancy (NV-) centres. Optionally, the step of annealing the substrate includes (a) providing a target temperature for the substrate, (b) heating the substrate by placing the substrate on a heater surface of a heater, (c) measuring the temperature of the substrate by a temperature sensor in direct contact with the substrate and / or the heater surface, and (d) controlling the heater based on the temperature measured by the temperature sensor such that the substrate has the target temperature.

[0019] According to a fourth aspect, there is provided a quantum device for imaging and / or measuring a magnetic field of a sample. The device comprises a sample holder for supporting the sample in a sample area, the quantum sensor device as describe herein, an excitation light source, an optical sensor, imaging optics, a microwave source, and / or a controller. The quantum sensor device is configured to be arranged adjacent to the sample area. The excitation light source is configured to generate excitation light configured to be directed to the quantum sensor device for energetically exciting the NV' centres. The optical sensor includes a plurality of pixels and / or is configured to generate electronic signals indicative of an intensity of the light emitted by the NV' centres. The imaging optics are configured to project the NV' centres onto the optical sensor. The microwave source has a microwave antenna for emitting microwave radiation. The controller is in data-communication with the microwave source and the optical sensor. The sample holder is configured to support the sample such that the NV' centres are subjected to the microwave radiation. Optionally, the controller is configured to (i) control the microwave radiation for generating microwave radiation, (ii) record - for each pixel - the electronic signals received from the optical sensor for the microwave radiations generated, (iii) determine - for each pixel - changes in the electronic signals, and / or (iv) generate an image based on the changes for each pixel, the changes for each pixel indicating (local) variations in magnetic field to which the quantum sensor device is subjected.

[0020] According to a fifth aspect, there is provided the use of the quantum sensor device as described herein for measuring and / or imaging a magnetic field.

[0021] The single-crystal diamond may be grown, e.g. is not a natural diamond. For example, the single-crystal diamond is grown by Chemical Vapor Deposition (CVD) and / or Physical Vapor Deposition (PVD). The substrate prior to the step of implanting nitrogen atoms into the substrate may include a total concentration of NV centres of less than 0.1 ppb (parts per billion), 0.05 ppb, 0.03 ppb, or 0.01 ppb. The NV centres include neutral and charged NV' centres. The single-crystal diamond may be commercially available.

[0022] The quantum device can be provided for measuring a magnetic field that is present in the sample and / or generated by the sample. For example, the sample interacts with an external magnetic field and the quantum device is provided for measuring and / or analysing the interaction of the sample with the magnetic field. An example for this type of measurement are commonly known NMR measurements using the quantum device for measuring the NMR response.

[0023] The quantum device can include magnetometers, spin resonance devices such as nuclear magnetic resonance (NMR) and electron spin resonance (ESR) devices, spin resonance imaging devices for magnetic resonance imaging (MRI), and / or quantum information processing devices such as for quantum computing.

[0024] The quantum device can also be provided for non-destructively probing the sample, for example the presence (or absence) of, e.g., metallic structures in optically non- transmissible material (such as deep metallization layers in semiconductor circuitry). Thus, the quantum device may allow probing samples that cannot be analysed using optical imaging techniques. Further, non-ionising radiation is used reducing the risk of the destruction of the sample as present with X-ray imaging techniques.

[0025] The NV- centres may be provided only in a sensor area of the substrate. The quantum device optionally allows to project the sensor area onto the optical sensor. For example, the quantum device may be configured to project the NV' centres onto the optical sensor, optionally all NV' centres of the substrate. The sensor area may cover the entire sample or a substantial portion thereof. In this way, sample probing is a fast process, for example compared to known scanning techniques. This may allow to probe the sample in-line and / or a high throughput in fingerprinting samples. Optionally, the quantum device allows widefield imaging of the sensor area.

[0026] The sample holder may be any device that is configured to removably support the sample and / or to which the sample can be removably attached, e.g. using fastening means. Commonly known sample holders may be used. The sample holder defines a sample area in which the sample is located when the sample is probed. The sample area may have a circular, quadratic or rectangular cross-section. The size of the sample area can include a maximal length / diameter of 100 pm, 200 pm, 300 pm, 400 pm, 500 pm, 600 pm, 700 pm, 800 pm, 900 pm or 1000 pm and / or a minimal length / diameter of 10pm. The size of the sample area may be, for example, 1000 pm2, 2000 pm2, 3000 pm2, 4000 pm2, 5000 pm2, 6000 pm2, 7000 pm2, 8000 pm2, 9000 pm2or 10000 pm2.

[0027] Further, the sample holder is configured to support the sample in such a position that the sample is close to or in contact with the sensor area of the quantum sensor device. In this way, the NV' centres of the quantum sensor device can be arranged close to or directly adjacent to the sample. Optionally, the sample holder is configured to support the sample as close as possible to the sensor area.

[0028] The quantum sensor device may be attached to the sample holder. For example, the sample is placed on the quantum sensor device that is in turn supported by the sample holder.

[0029] The quantum sensor device is optically transparent such that the excitation light can reach the NV' centres without or only minimal loss of intensity. The quantum sensor device may include a sample side and an optics side which opposes the sample side. The quantum sensor device and the sample holder are configured to be arranged such that the sample is in contact with or directly adjacent to the sample side of the quantum sensor device.

[0030] The light generated by the excitation light may impinge on the optics side and travel through the quantum sensor device to reach the sensor area. The light emitted by the NV- centres may exit the quantum sensor device at the optics side. It is also possible that the excitation light enters the quantum sensor device at the sample side and exits the quantum sensor device at the optics side. The optics side may face the imaging optics and / or the sample side may face the sample.

[0031] The quantum sensor device may have a cuboid or cylinder shape. For example, the quantum sensor device may have the shape of a plate or disc. In this case, the sample side and / or the optics side may form a flat, non-curved surface. Alternatively, the quantum sensor device may form a lens wherein optionally the sample side is also a flat surface and the optics side is curved and / or shaped to optically refract the impinging and / or exiting light.

[0032] The sensor area and / or the quantum sensor device may have maximal length / diameter of 100 pm, 200 pm, 300 pm, 400 pm, 500 pm, 600 pm, 700 pm, 800 pm, 900 pm or 1000 pm and / or a minimal length / diameter of 10pm. The size of the sample area may be, for example, 1000 pm2, 2000 pm2, 3000 pm2, 4000 pm2, 5000 pm2, 6000 pm2, 7000 pm2, 8000 pm2, 9000 pm2or 10000 pm2.

[0033] As commonly known, the NV' centres can be energetically excited by the excitation light and emit fluorescent light. The NV' centres may be provided in the single-crystal diamond by doping the material and / or treating the single-crystal diamond (e.g. using a laser) as known in the prior art. For example, the step of implanting nitrogen atoms into the substrate may include subjecting the substrate to a beam of nitrogen atoms and / or ions. Optionally,14 / 15N atoms at an energy of 1 - 60 keV are used. The beam may have an off-centre inclination of 5° to 10°, optionally 7°, with regard to a normal to the sample side of the substrate. The beam may have a fluence of 1011to 1014cm'2.

[0034] The implantation of nitrogen atoms and / or ions may provide nitrogen atoms within the lattice of the diamond substrate. The subsequent annealing step converts some of the implanted nitrogen atoms into the NV' centres. The annealing step may include heating the substrate in low-pressure or vacuum conditions, e.g. using a furnace and / or an annealing oven.

[0035] Further, the quantum sensor device may be grown by epitaxy. This process can be controlled in such a way that the NV' centres are provided in the desired concentration and / or location.

[0036] The NV' centres may be arranged in a predefined volume or layer within the substrate. For example, the NV' centres are arranged in a layer at or close to the sample side of the substrate. The layer in which the NV' centres are arranged form the sensor area. It is possible that the NV' centres are arranged in patterns.

[0037] The sensor area may extend along the entire sample side or may extend over a section of the sample side. The sensor area may cover an area of between 0.01 mm2and 20 mm2, optionally 0.2 mm2to 2mm2, further optionally between 0.5 mm2to 1 mm2. The sensor area may have a thickness between 0.01 pm to 1 nm, optionally between 0.1 pm to 100 pm, further optionally between 1 pm and 50 pm. The thickness and / or the size of the sensor area may depend on the size and / or shape of the substrate and / or the imaging optics.

[0038] Optionally, the sensor area may have a longitudinal extension and / or a thickness such that the entire sensor area can be projected onto the optical sensor, e.g. using wide- field imaging. For example, the thickness of the sensor area is sufficiently small such that a sharp image of the NV' centres can be projected onto the optical sensor. A sensor area having a large thickness would result in a blurred image of the NV' centres on the optical sensor. Further, if the sample area has a large thickness, the distance between an individual NV' centres and the sample would substantially vary which may introduce undesired differences in the interaction of the interacting microwave radiation with the NV' centres.

[0039] The quantum sensor device may be permanently or removably attached to the sample holder. Thus, upon attaching a sample to the sample holder, the sample holder may support both the quantum sensor device and the sample. For example, the sample holder is configured such that the sample can be put or lie on the sample side of the quantum sensor device. The excitation light source may comprise one or more lasers, one or more LEDs (Light Emitting Diodes) and / or one or more light sources with a broadband emission spectrum. The light source may generate light of a wavelength or narrow wavelength range (for example 10 nm, 20 nm, 50 nm or 100 nm). Optionally the wavelength or wavelength range generated by the excitation light source includes one or more wavelengths absorbed by NV' centres (e.g. that excite the NV' centres). The excitation light source may also include a broadband light source which generates light having a wavelengths over a wavelength range, for example a white light source. The wavelength(s) generated by the excitation light source may be chosen and / or selected such that the excitation light is absorbed by the NV' centres for exciting the NV' centres. In other words, the energy of the light generated by the excitation light source may correspond to the energy required for exciting NV' centres, for example for increasing the energy state of an electron of the NV' centres.

[0040] The excitation light source may be configured to continuously emit the excitation light during the entire measurement. This is often called continuous wave (CW) measurement. Alternatively, the excitation light source may be configured to emit pulses of the excitation light. For example, the excitation light source emits a first pulse of the excitation light prior to the emission of the microwave radiation and a second pulse of the excitation light after the emission or during the emission of the microwave radiation. The optical sensor can be triggered in such a way that it only records the fluorescent light during the emission of the second excitation light pulse and / or after the emission of the second excitation light pulse. The wavelength of the excitation light may be 532 nm.

[0041] The excited NV' centres emit fluorescent light because the energy state eventually returns to a lower energy state after being excited by the excitation light (an example is described in the background section above). Usually, the energy gap between the excited state and the lower energy state is lower than the energy of the excitation light such that the fluorescent light as a higher wavelength compared to the wavelength of the excitation light. For example, the excitation light of NV' centres may be green and the fluorescent light may be red.

[0042] The excitation light source, the optical sensor, and the imaging optics may form a wide- field microscope for exciting and / or imaging the NV' centres. The beam path of the wide-field microscope may not have an aperture (also known as a pinhole), which is used to select a layer along the z-axis or optical axis of the optical path, as is common in confocal microscopy. In this way, the setup of the present invention can be simplified and / or the light yield increased.

[0043] The NV- centres may be exclusively located in the sensor area and the remainder of the quantum sensor device may be essentially free of NV' centres. This enables the use of wide-field microscopy because there is no superposition of signals from the sensor area with signals from the remainder of the quantum sensor. The arrangement of the NV' centres in a layer (and the thickness thereof) can be controlled by the energy of the beam of the nitrogen atoms in the implanting step.

[0044] The magnification of the wide-field microscope can be 1x, 10x, 20x, 50x, 100x, 200x or 400x. The magnification can be adjusted, for example, by the magnification of the imaging optics. The imaging can be adjusted by varying the parameters magnification, number of pixels, and size of the optical sensor.

[0045] The optical sensor may be a device for recording two-dimensional images from light by electrical or electronical means. Semiconductor-based optical sensors that can record light up to the mid-infrared range can be used. Examples of optical sensors in the visible and near infrared range are CCD sensors. The optical sensor can convert an incident light into electrical or electronic signals, with the electrical and electronic signals indicating the intensity of the incident light and / or the wavelength of the incident light, optionally with spatial resolution. For example, the optical sensor has a plurality of pixels, each of which generates electrical or electronic signals. The electrical and electronic signals indicate the intensity of the incident light and / or the wavelength of the incident light. The plurality of electrical and electronic signals generated by the pixels could be converted into a two-dimensional image. However, as outlined in the following, the electronic signals are processed before the image is generated as outlined in the following.

[0046] The image sensor may have a number of pixels of 50x50 pixels; 250x250 pixels; 1000x1000 pixels; 2000x2000 pixels, 4000x4000 pixels or 8192x8192 pixels. The image sensor may be non-square and have one of the above values for the number of pixels along one direction. The size of the image sensor is optionally 1x1 pm2, 2.5x2.5 pm2, 5x5 pm2, 10x10 pm2, 25x25 pm2or 50x50 pm2. As mentioned, the image sensor can also be non-square and then have an edge length with the values mentioned. The imaging optics may include one or more optical elements that are configured to provide optical paths for the excitation light and / or the fluorescent light emitted by the NV- centres. For example, the one or more optical elements include an objective, lenses, a beam splitter, an optical filter, a mirror, and / or a dichroic mirror.

[0047] An optical path for exciting the NV' centres may start from the excitation light source over a dichroic mirror, through an objective to the NV' centres. An optical path for the fluorescent light may start from the NV' centres through the objective and the dichroic mirror onto the optical sensor. Thus, there might be a common optical path through the objective. The dichroic mirror may be used to couple in the excitation light in the common optical path or couple out the fluorescent light from the common optical path. Those optical setups are known in the prior art.

[0048] The imaging optics project the NV' centres onto the optical sensor. The fluorescent light emitted by a group of NV' centres may therefore be projected onto a single pixel. In other words, the electronic signals generated by each pixel of the optical sensor may correspond to the intensity and / or wavelength of a corresponding group of NV' centres. Stated differently, the imaging optics projects a sub-volume of the sensor area onto a respective pixel. The fluorescent light emitted by the NV' centres in this sub-volume is recorded by the respective pixel.

[0049] The microwave source may include a generator for generating microwave electromagnetic energy and a microwave antenna that is electrically coupled to the generator. The microwave antenna is configured to emit the microwave electromagnetic energy that is generated by the generator. Thus, the microwave radiation is emitted at the microwave antenna. The microwave radiation may have a wavelength between 1 mm to 1 m or a frequency of between 300 GHz - 300 MHz

[0050] The generator of the microwave source may be configured to generate the microwave electromagnetic energy in a wavelength range having an energy range that corresponds to a resonance state of the NV' centres. In other words, the energy of the microwave radiation may be set to such a wavelength that corresponds with an energy gap of the NV' centres. Thus, the microwave radiation can be used to excite the NV' centres and / or depopulate excited energy states of the NV' centres. For example, the microwave radiation may be used to initiate a population transfer from on energy level to another energy level of the NV' centres.

[0051] The microwave antenna may be a monopolar antenna or dipole antenna. The microwave antenna may include a ring structure, have the shape of a loop, and / or include one or more elongated structures (e.g. a rod) for emitting the microwave radiation. For example, the microwave antenna at least partially extends around the quantum sensor device. The microwave antenna may be shaped and / or located such that the electric field generated by the microwave radiation is approximately homogeneous over the sensor area and / or the sample area.

[0052] In some examples, the microwave radiation is used to manipulate the NV' centres in such a way that it affects the fluorescence response of the NV' centres upon excitation by the excitation light source. For example, the microwave radiation is provided for increasing the intensity of the fluorescent light emitted by the NV' centres. This may be done by exciting the NV' centres to an energy level at which the NV' centres can absorb the excitation light.

[0053] Alternatively, the microwave radiation is provided for decreasing the intensity of the fluorescent light emitted by the NV' centres. This may be done by depopulating energy states that are excited by the excitation light. Thus, the microwave radiation that is emitted by the microwave antenna and directly interacting with the NV' centres influences the intensity of the fluorescent light emitted by the NV' centres. For example, the microwave radiation may have a frequency that is in resonance with the NV' centres or is off resonance. Thus, the application of the microwave radiation (directly from the antenna) varies the fluorescence response of the NV' centres.

[0054] The presence of the sample and / or an external magnetic field changes the variation of the fluorescence response of the NV' centres as induced by the microwave radiation. Thus, the quantum device is based on determining deviations in the fluorescence response which is caused by the sample and / or an external magnetic field.

[0055] The quantum device may include a magnetic source which optionally includes a permanent magnet and / or one or more coils that are configured to generate a magnetic field. The magnetic source may also be configured to vary a configuration of the magnetic field. The configuration of the magnetic field may refer to the orientation and / or strength of the magnetic field. The magnetic field of a first configuration may differ in the orientation and / or strength from the magnetic field of a second configuration. For example, the magnetic field of the first configuration may have to same orientation as the magnetic field of the second configuration and differs in the strength of the respective magnetic field. The configuration of the magnetic field (e.g. the strength of the magnetic field) may be tuned such that the gradient of change in the electronic signals (e.g. a change in the absorption and / or emission of the light by the NV' centres) is maximal.

[0056] The magnetic field generated by the magnetic source may be constant for the time over which the electronic signals, the microwave radiation, and / or the excitation light are generated. In other words, the magnetic field generated by the magnetic source may be constant for the time of one measurement. The magnetic field generated by the magnetic source is at least present in the sample area and / or the sensor area. Due to the proximity of the sample area and the sensor area, the magnetic field generated by the magnetic source may also be present in both the sensor area and the sample area. The magnetic field generated by the magnetic source may be configured to be constant and / or homogeneous throughout the sample area and / or the sensor area. The magnetic source may be provided for induce Zeeman splitting as described in the background section above.

[0057] The controller can be implemented by a computer and / or can implemented the data- processing steps of the methods described herein. The controller can read out the electrical or electronic signals generated by the optical sensor, convert them into a digital image and / or calculate an image of the physical quantity from the digital image and / or the electronic signals. The controller may include a processor and a memory which stores programs, algorithms, and / or software that are executed by the processor.

[0058] The data-communication of the controller with the excitation light source, the optical sensor, the microwave source, and the magnetic source may be provided by a wired connection or by a wireless connection. The controller may receive the electronic signals from the optical sensor and / or can control the excitation light source, the microwave light source, and / or the magnetic source. For example, the controller may control the wavelength and / or the intensity of the excitation light, the parameter of the microwave radiation, and / or the configuration of the magnetic field. Here readout contrast is the difference in fluorescence intensity of the NV' centre quantum state, determining the information gained per measurement. Treinit is the time it takes to repolarize the NV' centres. The NV' centres may be simultaneously readout (i.e. during the repolarization). Thus, Treinit may be the time it takes to repolarize and readout the NV' centres. A measurement time T is the interaction time of the NV' centre with the measurement quantity and can be set to 100 ps, which is a typical value for NMR experiments. Generally, this time is limited by the coherence properties of the NV' centres. Exemplary details on how the readout contrast can be defined and / or measured is described in Barry et al., Rev. Mod. Phys. 92, 015004 (2020) which is incorporated by reference in its entirety.

[0059] The inventors have surprisingly found that the quantum sensor device is more stable against photoionization and / or has an increased readout contrast compared to the prior art if the concentration of NV' centres to the concentration of the nitrogen atoms in the substrate is less than or at most 2%, 1.5%, 1.4%, 1.3%, 1.2%, 1.1% 1%, 0.9%, 0.8%, 0.7%, 0.6%, or 0.5 %. In some examples, the ratio of the concentration of NV' centres to the concentration of the nitrogen atoms in the substrate is about 1%, is at least 0.001%, is between 0.01% and 1.4%, is between 0.05% and 1.3%, is between 0.1% and 1.2%, is between 0.2% and 1.1%, is less than 1 %, and / or at most 1%. In words, the improved characteristics of the quantum sensor device can be provided if the following relationship is fulfilled: wherein [NV'] is the concentration of NV' centres, [Ntot] is the concentration of nitrogen atoms in the substrate, and P can be a percentage of 2%, 1.5%, 1.4%, 1.3%, 1.2%, 1.1% 1%, 0.9%, 0.8%, 0.7%, 0.6%, or 0.5 %. [Ntot] is the total nitrogen concentration which includes the neutral nitrogen [N°], the positively charged nitrogen [N+], and the negatively charged nitrogen [N']. In other words, total nitrogen concentration [Ntot] includes all forms of nitrogen-related defects and / or every defect type that contains nitrogen and nitrogen interstitials. The highest margin by far of the total nitrogen concentration [Ntot] are neutral nitrogen [N°] which are often called Ns° or P1 centres. The concentration of neutral nitrogen-vacancy (NV°) centres may be less than 0.05 ppm, 0.03 ppm, 0.02 ppm, or 0.02 ppm. The above ratio of NV' centres to the concentration of the nitrogen atoms in the substrate can be provided by the method described herein. This means that the percentage P is the conversion rate of converting the amount or concentration of nitrogen atoms in the substrate into NV' centres. For example, if [Ntot] is approximately 25ppm, then the [NV'] is approximately less than 0.25 ppm after completion of the annealing process. Further, the percentage P is determined after the annealing step, i.e. refers to the completed quantum sensor device.

[0060] This conversion rate and the resulting concentration of NV' centres can be provided by precisely controlling the temperature and / or pressure during the annealing step. In particular, it has been found that the temperature can be precisely controlled by measuring the temperature of the heater surface and / or the substrate itself. This is in contrast of the commonly known techniques where the temperature is indirectly measured. For example, the temperature of components surrounding the substrate (e.g. using a temperature sensor attached to the housing of the vacuum chamber) are measured. It is assumed that such indirect measurements do not track fast deviations from the target temperature such that the temperature of the substrate overshoots or falls below the target temperature. Further, the temperature of the substrate is often measured using optical pyrometers in the prior art, i.e. the colour change of the diamond. The optical pyrometer works well for coloured diamonds. However, for ultrapure diamond as used herein, the colour change is very low which can lead to massive overshoots or undershoots of the temperature of the substrate.

[0061] It is assumed that substantial deviations from the target temperature adversely affect the conversion process resulting in less-than-ideal characteristics of the NV' centres. This is overcome by the method described herein which provides NV' centres having increased stability to photoionization and / or improved readout contrast.

[0062] The step of annealing the substrate may be executed in a furnace or annealing oven which includes a heater. The heater may include a heater surface onto which the substrate can be placed. Commonly known heaters can be used. For example, the heater may provide temperatures ranging from 200°C to 1000°C. The heater may be in data communication with a heater controller which controls the temperature that the heater generates, for example by controlling the power supply to the heater. The heater may be Tectra BORALECTRIC HTR-1003. The heater controller may be a PI D controller that allows precise control of the temperature of the heater. In other words, the heater controller controls the heater such that temperature of the heater corresponds to the target temperature. The target temperature may be manually set using the heater controller. Alternatively, the heater controller is in data-communication with a further input device, such as a computer, with which to target temperature can be set.

[0063] The target temperature may be gradually and / or stepwise changed over time. In other words, the target temperature may include a curve of the target temperature over time.

[0064] A temperature sensor, such as a thermocouple, may be arranged in or on the heater. For example, the temperature sensor may be arranged in or on the heater surface. The temperature sensor may be flush with the heater surface. For example, the heater surface includes a cavity or recess in which the temperature sensor is arranged. The heater may be in direct contact with the heater surface for measuring the temperature of the heater surface by thermal conduction between the heater surface and the temperature sensor.

[0065] If the substrate is placed on the heater surface, the temperature sensor is configured to measure directly to temperature of the substrate. It is also possible that the substrate is placed on the heater surface offset of the temperature sensor. For example, the heater includes one or more resistive tracks which are placed on or below the heater surface for providing heat to the heater surface. The temperature sensor may be offset to the one or more heater tracks. In this case, the temperature sensor is configured to measure the temperature of the heater surface which is considered to correspond to the temperature of the substrate because the substrate is in direct contact with the heater surface and / or the volume of the substrate is usually small such that the substrate essentially has the temperature of the heater surface (because temperature diffusion inside the substrate is considered to be fast such that no temperature variations inside the substrate are present).

[0066] The heater and / or the temperature sensor are arranged inside a low-pressure chamber and / or vacuum chamber. The low-pressure chamber may be part of annealing oven. The heater controller may be arranged outside the low-pressure chamber. The temperature sensor may be in data-communication with the heater controller. The heater controller is configured to control the heater in such a way that the temperature at the heater surface corresponds to the target temperature. This may be done by controlling the power supplied to the heater.

[0067] In an optional embodiment of the quantum sensor device and / or the manufacturing method, the concentration of nitrogen atoms is more than 18 ppm (parts per million), 19 ppm, 20 ppm, 21 ppm, 22 ppm, 23 ppm, 24 ppm, or 25 ppm and / or less than 100 ppm.

[0068] The concentration of nitrogen atoms may refer to a portion or section of the substrate in which the nitrogen atoms are actually provided. Thus, the concentration of nitrogen atoms may refer to the sensor area. Outside the sensor area, the concentration of nitrogen atoms may be equal to concentration of nitrogen atoms prior to the step of implanting nitrogen atoms.

[0069] The concentration of nitrogen atoms may be adjusted by varying the parameters of the step of implanting nitrogen ions into the substrate. For example, the time which the beam of nitrogen atoms is directed to the substrate, the fluence of the beam, the energy of the beam, and / or further parameters may be varied for setting the concentration of nitrogen atoms.

[0070] In the prior art, a lower concentration of nitrogen atoms is often used. However, the inventors found out that a higher concentration of nitrogen atoms in connection with a lower conversion rate into NV' centres (compared to the prior art) increases the stability against photoionization and / or the readout contrast. One potential explanation for this effect is that the charge stability of the NV' centres are linked to the concentration of NV' centres versus the overall substitutional neutral nitrogen (so called: Ns° or P1 centres) at an P1-to-NV' conversion rate of < 1 %. To this end, the total nitrogen [Ntot] density in the diamond may be bigger than 10 ppm.

[0071] Optionally, the step of implanting the diamond material with nitrogen comprises implanting nitrogen with an implantation dose selected from any of at least 105N / cm2, 106N / cm2, 107N / cm2, 108N / cm2, 109N / cm2, 101° N / cm2, or 1011N / cm2and no more than 1014N / cm2or 1013N / cm2

[0072] The step of annealing the substrate may be executed at a pressure of less than 10'5mbar, optionally less than 50'6mbar, further optionally less than 10'6mbar. In an optional embodiment of the quantum sensor device and / or the manufacturing method, the substrate includes a layer extending along a side surface of the substrate. Optionally, the nitrogen atoms and the NV' centres are only present in the layer.

[0073] The layer may be an example of the sensor area described above. Thus, more generally, the substrate includes a sensor area. Optionally, the nitrogen atoms and the NV' centres are only present in the sensor area.

[0074] In an optional embodiment of the quantum sensor device and / or the manufacturing method, the NV' centres have an optically detected magnetic resonance (ODMR) readout contrast of greater than 6% at a reciprocal initialisation time, 1 / treinit, of > 1 , optionally >1 1 / ps. Optionally, the ODMR readout contrast is configured to be measured with an ODMR measurement using the quantum sensor device, wherein the ODMR contrast refers to the maximum ratio between a fluorescence intensity after applying a resonant microwave pulse during a readout phase of the ODMR measurement and a fluorescence intensity without applying a resonant microwave pulse during the readout phase of the ODMR measurement, The fluorescence intensity is collected from an ensemble of NV- centres distributed over an area of at least 10 pm2, when initialized in the ms= 0 and ms= +1 or -1 state respectively. treinit is the time required for re-polarizing the NV- centres.

[0075] In this way, the quantum sensor device has a higher sensibility compared to the prior art to the increased readout contrast. The readout contrast can be measured as discussed in Edmonds et al., Mater. Quantum. Technol. 1 , 025001 (2021)), the content of which is incorporated here in its entirety. Compared to known methods, the quantum sensor device and the method for manufacturing method described herein provide a less effective conversion of the implanted nitrogen into NV' centres. However, this can be balanced by increasing the concentration of nitrogen atoms, e.g. Ntot. One explanation for the increased contrast is that the increasing the total nitrogen linearly reduces the spin-spin relaxation time T2 with which the sensitivity scales at ^T2. However, the sensitivity scales linear with the readout contrast. Thus, the improvement of the readout contrast becomes dominant with the approach described herein.

[0076] The readout contrast is measured in optically detected magnetic resonance (ODMR) measurements. The readout contrast may also be determined using different types of measurement as known in the prior art. However, the readout contrast may differ if it is measured using a different type of measurement.

[0077] As described in Edmonds et al., the readout contrast is a physical parameter of the quantum sensor device. In other words, the readout contrast is determined by the inherent properties of the quantum sensor device, such as concentration of nitrogen atoms and / or the concentration of NV' centres. Thus, the readout contrast can be used to characterise a quantum sensor device and / or to distinguish one quantum sensor device from another quantum sensor device.

[0078] The ODMR measurement includes applying a microwave pulse (e.g. a TT / 2 pulse) in the readout phase that shifts the spin state of the NV' centres by 90° in the Bloch sphere (an exemplary description of ODMR measurements can be found in Barry et al., Rev. Mod. Phys. 92, 015004 (2020) which is incorporated herein in its entirety). The readout phase may be that phase of the ODMR measurement in which the fluorescent light emitted by the NV' centres is detected. To determine the readout contrast, the maximum intensity of the fluorescence light as detected by the optical sensor is determined and compared to an identical measurement which only differs in that the microwave pulse during the readout phase is not applied.

[0079] The maximum intensity of the fluorescent light is determined by collecting the light from a plurality of NV' centres (e.g. an ensemble of NV' centres) which are distributed over an area of at least 10 pm2. Of course, the maximum intensity of the light depends on the number of NV' centres contributing to the measurement. The number of NV' centres can be determined by the concentration of the NV' centres multiplied by the area and / or thickness of the sensor area.

[0080] As discussed and ready herein, treinit is the time required for re-polarizing the NV' centres and, therefore, an intrinsic property of the NV' centres. The unit of 1 / treinit may

[0081] The readout contrast may be higher than 6%, 6.5%, 7%, 7.5 %, or 8% for 1 / treinit, of > 1 1 / ps, optionally for treinit of >0.65 1 / ps. The readout contrast may be greater than 4% at a reciprocal initialisation time treinit of >1.05 1 / ps. In an optional embodiment of the quantum sensor device and / or the manufacturing method, the NV' centres have a spin-lattice relaxation time Ti between 500 ps and 1000 ps, between 600 ps and 900 ps, or between 700 ps and 800 ps.

[0082] The spin-lattice relaxation time Ti can be measured by extending the time between optical initialisation and readout.

[0083] In an optional embodiment of the quantum sensor device and / or the manufacturing method, the NV' centres have spin-spin relaxation time T2 between 0.01 ps and 10 ps, optionally between 0.1 ps and 7 ps, further optionally between 0.2 ps and 6ps, in one example between 0.4 ps to 4 ps.

[0084] The spin-spin relaxation time T2 can be measured with a Hahn echo sequence.

[0085] In an optional embodiment of the quantum sensor device and / or the manufacturing method, the step of controlling the heater includes controlling the temperature of the substrate and / or the heater surface to be within ± 5°C, ± 3°C, ± 2°C, ± 1.5°C, ± 1°C, ± 0.5°C or ± 0.1 °C with regard to the target temperature.

[0086] This precise control of the temperature of the substrate and / or the heater surface can be facilitated by the provision of the temperature sensor in direct contact with the substrate and / or heater surface as described above. It has been found that the precise control of the temperature of the substrate improves the conversion of the nitrogen atoms into NV' centres for achieving the advantageous characteristics as described herein.

[0087] In an optional embodiment of the quantum sensor device and / or the manufacturing method, the target temperature is between 380°C and 420 °C, optionally between 390°C and 410 °C, further between 395°C and 405 °C, or at 400°C for a first time span and then between 780°C and 820 °C, optionally between 790°C and 810 °C, further between 895°C and 805 °C, or at 800°C for a second time span.

[0088] It has been found that this temperature profile provides the best conversion of the nitrogen atoms into NV' centres. Thus, the step of annealing includes two temperature windows over which the temperature is held constant. Optionally, the first time span and / or the second time span may be 6 hours ± 10%, ± 5%, ± 2.5%, ± 1 %, ± 0.5% or ± 0.1%.

[0089] In an optional embodiment of the quantum sensor device and / or the manufacturing method, changes or increases in target temperature have a (maximal) rate between 3°C / min and 4°C / min, optionally between 2.5°C / min and 3.5°C / min, further optionally 3.0°C / min.

[0090] In this way, the temperature of the substrate can be held at the target temperature because of the relatively slow change in the target temperature. In other words, any change in the temperature provided by the heater happens at such a pace such that the temperature of the substrate can follow. In other words, changing a target temperature is chosen in that there is no substantial difference between the temperature of the heater and the temperature of the substrate. This helps of the improve the conversion rate of the nitrogen atoms into NV' centres.

[0091] The heater may also be powered when the temperature of the substrate is reduced. For example, the heater is not switched off after the second time span. Rather, the temperature is reduced in a controlled manner, e.g. by appropriately powering the heater. The rate of decrease of the temperature may be the same as the rate of increase of the temperature. Alternatively, it may be possible that the rate of decrease of the temperature is higher than the rate of increase of the temperature, for example between 3°C / min and 10°C / min.

[0092] An example of the invention may also be summarized as follows:

[0093] The readout contrast C and the repolarisation time treinit have an effect on the magnetometer’s sensitivity. As described herein, the readout contrast refers to the difference in fluorescence intensity of the NV centres quantum state, determining the information gained per measurement, treinit is the time it takes to readout and repolarize the NV centres. The measurement time T is the interaction time of the NV centre with the measurement quantity and is set to 100 ps, which is a typical value for NMR experiments. Generally, this time is limited by the coherence properties of the NV centres. The contrast has the largest impact on the NV centres sensitivity so improving the readout contrast close to the theoretical limit for epi-illumination of 14% is important. Since the qubit (e.g. the NV' centre) is not interacting with the target field (e.g. a magnetic field to be determined) during the treinit, the total sensitivity decreases the longer it takes to repolarize the NV centre. Shorter repolarization times can be achieved by using higher laser powers (up to the saturation power ~ 1 mW / pm2) and is limited by the photoionization of the NV' centre which typically reduces the readout contrast by shifting the NV7NV0ratio (NV° is approximately equal to the number of the nitrogen atoms prior to the conversion step (e.g. using annealing)). In addition to shorter treinit, higher possible laser powers (or equivalently higher resistance to photoionization) increase the total fluorescence intensity of the NV centres further increasing the sensitivity. Therefore, ensuring that the NV centres are exceptionally stable against photoionization (measured as readout contrast vs. the reciprocal initialisation time treinit) is important.

[0094] The invention at least partly relates to a protocol for the production of near-surface nitrogen vacancy (NV) centres ensembles in diamonds with high stability to photoionization. Therefore, these NV centres exhibit large readout contrast even at high laser powers.

[0095] The method for manufacturing a quantum sensor device includes a nitrogen implementation step and an annealing step. The nitrogen implantation step can include providing an electronic grade diamond (1.1 %13C natural abundance or12C enriched, e.g. as manufactured by Element Six) which is implanted using14 / 15N at an energy of 1 - 60 keV with an off-centre inclination of 7° and a fluence of 1011- 1014cm-2. The implanted sample was then vacuum annealed in a furnace for 32 hours using a Tectra BORALECTRIC® sample heater. The diamond was cleaned with equal parts of boiling sulphuric, nitric and perchloric acid (ratio 1 :1 :1) before and after implantation and the annealing step.

[0096] The annealing of implanted diamond for conversion to NV centres includes annealing the diamond after implantation in vacuum to form NV centres using gradual ramp rates (~3 °C / min) and precisely controlled temperatures (400 °C for 6h and 800 °C for 6h) and ultra-high vacuum (> 10'6mbar). The annealing process is thought to be important for forming NV centres and maintaining their readout contrast C.

[0097] The implanted NV centre ensembles (prepared as outlined herein) can out-perform the N-doped CVD reference at all intensities. Thus, allowing the usage of higher laser powers resulting in shorter treinit without sacrificing readout contrast / sensitivity. The properties of our near-surface NV ensemble in diamond are summarised in the following for an example for quantum sensor device having a15N implantation energy of 2.5 keV and fluence 2x1012cm-2. The NV resonance for the transition from ms= 0 to -1 is used to determine the magnetic field strength. The contrast of the Rabi oscillations is typically about 7% for the dry state but decreases to 4% in the case of liquid samples on the diamond. An alternative to detecting NMR signals at the nanoscale with correlation spectroscopy is dynamic decoupling sequences, where the time between TT- pulses is swept. In this experiment, two dips are observed, corresponding to the19F and the1H. The spin-lattice relaxation time Ti was measured by extending the time between optical initialisation and readout. Fitting an exponential gives a range of 700 ps to 800 ps for pure diamond. The spin-spin relaxation time T2 is measured with a Hahn echo sequence. The pulse sequence consists of a sequence rotate in the plane due to the magnetic noise. The TT-pulse, which lies between the two TT / 2-pulses, cancels the phase accumulation if the magnetic noise in the two free precession times is identical. As the magnetic noise B fluctuates over time, the spin echo will decay, which is called a Hahn echo or NV-T2. The dips are due to coupling with13C-containing in the diamond. The data is fitted to a stretched exponential function, resulting in a range of 4-6 ps.

[0098] BRIEF DESCRIPTION OF THE DRAWINGS

[0099] The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which:

[0100] Fig. 1 shows a schematic view of a quantum device;

[0101] Fig. 2 shows a schematic view of an annealing oven used for preparing a quantum sensor device of the quantum device of Fig. 1 ;

[0102] Fig. 3 shows a block diagram for manufacturing the quantum sensor device using the annealing oven of Fig. 2

[0103] Fig. 4 shows schematic diagram of the pressure and temperature curves used with the method shown in Fig. 3; Fig. 5 is a graph showing experimental data of the readout contrast C against the reciprocal treinit;

[0104] Fig. 6 shows other exemplary properties of the NV' centres, namely A) ESR line shape and resonance frequency; B) Typical Rabi contrast in dry state and in ethanol; C) XY8-12 depression of the1H peak on the left and the19F peak of drop-shaped phosphonic acid on the surface on the right; D) Spin-lattice relaxation Ti; and E) Spin-spin relaxation T2.

[0105] The figures are not drawn to scale. Throughout the description, similar parts have been assigned the same reference numerals, and a detailed description is omitted for brevity.

[0106] DETAILED DESCRIPTION

[0107] The inventors have determined that taking diamond with a very low concentration of nitrogen as a starting point, and implanting the diamond with nitrogen before irradiating and annealing to form NV' centres, can lead to a quantum device that uses near- surface NV' centres in diamond with high stability to photoionization.

[0108] Fig. 1 shows an embodiment of a quantum device 10 for imaging a sample 12 and / or measuring a magnetic field inside the sample 12. The magnetic field inside the sample 12 may be the response of the sample 12 to an external magnetic field and / or to external stimulation. For example, the magnetic field inside the sample 12 that can be measured using the quantum device 10 may include NMR signals.

[0109] The quantum device 10 comprises a sample holder 16, a quantum sensor device 18, a microwave source 20, a magnetic source 22, a controller 24, imaging optics 28, an excitation light source 30, and / or an optical sensor 32. The sample holder 16 is configured to support the sample 12. Further, in this example, the sample holder 16 further supports the quantum sensor device 18 such that the sample 12 is in direct contact with the quantum sensor device 18.

[0110] The sample 12 may be an integrated circuit, a printed circuit board, an electric device and / or material interface. These types of samples 12 may be imaged using the quantum device 10. For example, metallic structures may be imaged by detecting the magnetic field emitted or manipulated by the metallic structures. Alternatively, the sample 12 includes particles interacting with an external magnetic field. The variation in the external magnetic field caused by the particles in the sample 12 may be detected using the quantum device 10.

[0111] The imaging optics 28, the excitation light source 30, and / or the optical sensor 32 may provide a wide-field microscope 14 which may additionally comprise a beam splitter 26.

[0112] The quantum sensor device 18 includes a substrate made of single-crystal diamond which contains a plurality of negatively charged nitrogen-vacancy (NV-) centres. The NV' centres can be projected onto the optical sensor 32 by means of the imaging optics 28 which may include an objective. In other words, the wide-field microscope 14 allows the NV' centres to be projected onto the optical sensor 32 by means of wide-field imaging. The optical sensor 32 converts spatially resolved incident light into electrical or electronic signals which may be used when imaging the sample 12. For example, the optical sensor 32 comprises a plurality of pixels, each pixel generating an electrical or electronic signal indicative of an intensity and / or a wavelength of the light impinging on the respective pixel. The plurality of electrical or electronic signals for each pixel can thus be used to image the sample 12. This can be done by the controller 24, which is in data communication with the optical sensor 32, for example to trigger the electrical or electronic signals of the optical sensor 32 and / or to drive the optical sensor 32. Alternatively, the electrical or electronic signals of all pixels may be analysed to determine the magnetic field inside the sample 12, e.g. an averaged magnetic field of the sample 12.

[0113] The controller 24 includes, for example, a processor and a memory. The memory may store one or more programs and / or algorithms that are executed by the processor. The controller 24 may also be in data-communication with the excitation light source 30 to control the excitation light source 30. The excitation light source 30 may include one or more lasers or other devices for generating light. The light generated by the excitation light source 30 may be in the visible wavelength range, the infrared wavelength range, or the ultraviolet wavelength range. The excitation light source 30 may be configured to selectively generate light in a selected wavelength range, for example in a wavelength range that is absorbed by the NV' centres.

[0114] The light generated by the excitation light source 30 is coupled into the wide-field microscope 14 via the beam splitter 26 and directed to the objective of the imaging optics 28. The objective focuses the incident light onto the NV' centres in the quantum sensor device 18. The light emitted by the NV' centres is projected by the imaging optics 28 onto the optical sensor 32. The light beams in the beam paths described here do not have to be parallel, as shown in Fig. 1.

[0115] The excitation light source 30 may continuously emit excitation light which may result in so-called CW (continuous wave) measurements. Alternatively, the excitation light generated by the excitation light source 30 may be pulsed. For example, a first pulse of excitation light is emitted prior to the generation of the microwave radiation and a second pulse of excitation light is emitted after or during the generation of the microwave radiation. In this case, the emitted fluorescent light may only be imaged during the excitation of the second pulse.

[0116] The beam splitter 26 may be a semi-transparent mirror and / or a dichroic mirror. The choice of the type of beam splitter 26 depends on the light generated by the excitation light source 30 and which light is emitted and / or absorbed by the NV' centres.

[0117] The quantum sensor device 18 can be supported by the sample holder 16. For example, the sample holder 16 has a hole in which the quantum sensor device 18 is embedded. A surface of the sample holder 16 may form a plane with a sample side of the quantum sensor device 18. The sample 12 can be placed on the quantum sensor device 18 and / or the sample holder 16. The area where the sample 12 can be placed on the sample holder 16 and / or supported by the sample holder 16 may be regarded as a sample area.

[0118] The quantum sensor device 18 may have a cuboid shape. The NV' centres may be arranged in a layer that is located on the side of the quantum sensor device 18 that contacts the sample 12. The layer of NV' centres may have a thickness sufficiently low so that the NV' centres can be imaged or projected onto the optical sensor 32. The quantum sensor device 18 may be entirely made from diamond. The area over which the NV' centres are provided within the quantum sensor device 18 may be considered a sensor area. The sensor area may be in direct contact with the sample 12.

[0119] The sample 12 is arranged close to or in contact with the NV' centres in the quantum sensor device 18. In one example, the NV' centres of the quantum sensor device 18 are in the near-field of the microwave radiation that is interacting with the sample 12. The near-field can be defined as a fraction of the wavelength. As the wavelength of the microwave radiation is between 1 mm and 1 m, the distance between the sample 12 and the NV' centres in the quantum sensor device 18 may be between 1 pm and 1 mm, optionally between 10 pm and 100 pm.

[0120] The microwave source 20 may include a generator (not shown in the figures) and a microwave antenna. The microwave antenna may be arranged on the side of the sample holder 16 that faces the imaging optics 28. In other words, the microwave antenna and the imaging optics 28 are arranged on the same side of the sample holder 16 and / or the sample area which is the area where the sample 12 can be placed on the sample holder 16. In the embodiment of Fig. 1 , the microwave antenna 20 is arranged in the space between the objective of the imaging optics 28 and the sample holder 16.

[0121] The microwave antenna may have a ring shape or a loop shape and / or may at least partially surround the quantum sensor device 18 such that microwave radiation emitted from the antenna penetrates the quantum sensor device 18, optionally from all sides, and, optionally, is reflected back from the sample 12 to the quantum sensor device 18. In this way, the NV' centres of the quantum sensor device 18 are subjected to the microwave radiation reflected by the sample as well as the microwave radiation directly emitted by the antenna of the microwave source 20.

[0122] The microwave source 20 can be controlled by the controller 24 and / or is in data communication with the controller 24. The microwave radiation generated by the microwave source 20 may be used to decrease and / or increase the absorption and / or emission of light by the NV' centres of the quantum sensor device 18. In an example for imaging the sample 12, the microwave radiation directly emitted by the antenna of the microwave source 20 may induce a population transfer in the excited states of the NV' centres because the energy of the microwave radiation corresponds to a gap in between two energy levels of the NV' centres. This may lead to a reduction of the emitted fluorescent light. The dips in the intensity of the fluorescent light or local extrema may correspond to the different orientations of the NV centres relative to the microwave radiation which results in different energy levels.

[0123] The microwave radiation reflected from the sample 12 and interacting with the NV' centres of the quantum sensor device 18 may increase this population transfer, for example because the microwave radiation locally reflected an interface in the material (e.g. a conductive layer in a dielectric material) locally increases the intensity of the microwave radiation at the NV' centres. This may result in a local decrease in the intensity of the fluorescent light. This change in the intensity of the fluorescent light may be spatially resolved and / or mirrors the structure within the sample 12 that reflects the microwave radiation. This is due to the fact that the NV' centres of the quantum sensor device 18 are in the near-field of the microwave radiation that is reflected by the sample 12. In this case, sub-wavelength resolution is possible compared to the far- field. Thus, it is possible to image structures within the sample 12 that reflect the microwave radiation.

[0124] The microwave radiation may be reflected by electrical conductors within the dielectric material of the sample 12. More generally, the microwave radiation may be reflected at interfaces between two different types of material, for example at the metal-dielectric interface. In this way, it is possible to image electrical conductors within an optically non-transparent material, such as semiconductor material. The differences in the intensity of the fluorescent light emitted by the NV' centres can be displayed in image.

[0125] The scale of fluorescence image is set to a baseline at which no or little microwave radiation is reflected back. This may correspond to the dip or decrease in the microwave radiation.

[0126] Alternatively, the microwave source 20 can be used to emit microwave pulses for Ramsey measurements, CW-ODMR, pulsed ODMR, Hahn echo measurements, and / or dynamical decoupling for AC magnetometry.

[0127] The magnetic source 22 may include a permanent magnet that generates a constant and / or homogeneous magnetic field within the sensor area. The permanent magnet may induce a Zeeman split in the energy levels of the NV' centres.

[0128] In an alternative embodiment, the magnetic source 22 may alternatively or additionally include a generator (not shown in figures) and a coil for generating the magnetic field. Again, the generated magnetic field may be constant and / or homogeneous, for example within the sample 12 and / or the sensor area. The controller 24 may be configured to vary the orientation and / or the strength of the magnetic field between measurements. This means that, for a particular measurement, the magnetic field is constant and may be changed for the next measurement. A measurement may be considered as imaging the fluorescent light emitted by the NV' centres. The magnetic source 22 may generate the magnetic field for generating a magnetic response by the sample 12 that is measured using the quantum sensor device 18.

[0129] Fig. 2 shows an exemplary embodiment of an annealing oven 40 which can be used to anneal a substrate made of single-crystal diamond that is implanted with nitrogen atoms. The annealing converts some of the nitrogen atoms into the NV' centres.

[0130] The annealing oven 40 includes a vacuum chamber 42, a heater 44, a temperature sensor 46, a heater controller 48, and / or a vacuum pump 50. The vacuum chamber 42 can be closed in a gas tight manner such that negative pressure or vacuum can be generated inside the vacuum chamber 42. The vacuum pump 50 is in gas communication with the vacuum chamber 42 for pumping gas out of the vacuum chamber 42. The vacuum chamber 42 and / or the vacuum pump 50 are configured to generate a pressure of less than 10'5mbar, optionally less than 50'6mbar, further optionally less than 10'6mbar, inside the vacuum chamber 42.

[0131] The heater 44 is arranged inside the vacuum chamber 42 and includes a heater surface 44a. The heater surface 44a is configured and / or arranged such that the quantum sensor device 18 can be placed on the heater surface 44a. For example, the heater surface 44a is a horizontal surface of the heater 44. The heater 44 is configured to heat this heater surface 44a to a temperature of up to 1000°C. The heater 44 may be a resistance heater and / or includes resistive tracks which run below the heater surface 44a. In this way, the quantum sensor device 18 can be heated to a temperature which corresponds to the temperature of the heater surface 44a.

[0132] The temperature sensor 46 is located inside a cavity of the heater 44, optionally of the heater surface 44a. The temperature sensor 46 is in direct contact with the heater surface 44a for measuring the temperature thereof. The temperature sensor 46 can be in thermal conduction with the heater surface 44a. This may provide an accurate measurement of the temperature of the heater surface 44a. The heater 44 may include a thermocouple and / or is in data-communication with the heater controller 48.

[0133] The heater controller 48 is also in data communication with the heater 44 for the temperature control thereof. The heater controller 48 may include a PID-controller for controlling the heater 44 in such a way that the temperature of the heater surface 44a corresponds to a target temperature. The heater controller 48 may read the temperature of the heater surface 44a using the temperature sensor 46 and adjusts the power supply to the heater 44 for setting the temperature at the heater surface 44a. For example, the heater controller 48 is configured to control the temperature of the heater surface 44a to be within ± 5°C, ± 3°C, ± 2°C, ± 1.5°C, ± 1°C, ± 0.5°C or ± 0.1 °C with regard to the target temperature.

[0134] The heater controller 48 may include an interface for inputting the target temperature and / or may be in data communication with a controller or computer for setting the target temperature. The target temperature may vary over time as described in connection with Figs. 3 and 4.

[0135] Fig. 3 shows some steps of an exemplary method for manufacturing the quantum sensor 18. In step S1 , the substrate made of single-crystal diamond is provided. The substrate may be essentially free of nitrogen atoms. For example, a total NV concentration of the substrate used in step S1 is less than 0.1 ppb (parts per billion), 0.05 ppb, 0.03 ppb, or 0.01 ppb. The single-crystal diamond may be commercially available, for example electronic grade diamond (1.1%13C natural abundance or12C enriched).

[0136] In step S2, nitrogen atoms are implanted into the substrate. Commonly known techniques for implanting nitrogen atoms into a diamond substrate can be used. In one example, a beam of14 / 15N atoms at an energy of 1 - 60 keV with an off-centre inclination of 7° and a fluence of 1011- 1014cm-2can be used. The diamond can be cleaned with equal parts of boiling sulphuric, nitric and perchloric acid (ratio 1:1:1) before and after the implantation step. The depth of implantation may be to a depth selected from any of no more than 1 pm, 500 nm, 100 nm, 50 nm, 30 nm, 10 nm, and 5 nm. Thus, a layer including the NV' centres can be formed. For example, the layer covers the entire sample surface of the diamond substrate. The concentration of the nitrogen atoms after implantation in the layer may be 20 ppm.

[0137] In step S3, the substrate is annealed to convert at least some of the implanted nitrogen atoms into negatively charged nitrogen-vacancy centres. To this end, the implanted substrate is placed on the heater surface 44a of the heater 44. Thereafter, the vacuum chamber 42 of the annealing oven 40 may be closed and a vacuum generated using the vacuum pump 50. The pressure over the entire heating process may be less than 50'7mbar. As shown in Fig. 4, the heater 44 is then heated up to a temperature of 400°C. The rate of change in the target temperature can be 3°C / min. This can ensure that entire substrate is at the same temperature, i.e. that there are no temperature differences inside the substrate. The temperature of 400°C is kept for 6 hours. Thereafter, the temperature is raised to 800°C for another 6 hours. As a next step, the heater 44 is cooled down to room temperature. The rate of the temperature change for reaching 800°C as well as cooling down to room temperature may be 3°C / min. The substrate is cleaned with equal parts of boiling sulphuric, nitric and perchloric acid (ratio 1 :1 :1) before and after the annealing step.

[0138] The method described in connection with Figs. 3 and 4 is an exemplary method for manufacturing the quantum sensor device 18 having the characteristics as outlined in Figs. 5 and 6.

[0139] The quantum sensor device 18 is configured to provide information based on an excitation response from the NV' centres. The NV' centres provide a fluorescence intensity readout contrast of greater than or at least 6% at a reciprocal initialisation time, 1 / treinit, of >1 (see Fig. 5). The unit of 1 / treinit may be 1 / ps. treinit is the time required for re-polarizing the NV' centres. The fluorescence intensity readout contrast may be greater than 8% at a reciprocal initialisation time treinit of >0.65, and / or may be greater than 4% at a reciprocal initialisation time treinit of >1.05.

[0140] It can be seen from Fig. 5 that the implanted NV' centres show a better contrast at all intensities as the NV' centres prepared using known methods (for example the one described in Edmonds et al.). This means that higher power lasers can be used resulting in shorter treinit without sacrificing readout contrast / sensitivity.

[0141] The readout contrast is an ODMR readout contrast which is measured with an ODMR measurement using the quantum sensor device 18. The ODMR contrast refers to the maximum ratio between a fluorescence intensity after applying a resonant microwave pulse during a readout phase of the ODMR measurement and a fluorescence intensity without applying a resonant microwave pulse during the readout phase of the ODMR measurement. The fluorescence intensity is collected from an ensemble of NV' centres distributed over an area of at least 10 pm2, when initialized in the ms= 0 and ms= +1 or -1 state, respectively. One example of the measurement method is also described in Edmonds et al.. Other parameters of the quantum sensor device 18 prepared according to Figs. 3 and 4 are summarised in Fig. 6 as an example for a diamond with a15N implantation energy of 2.5 keV and fluence 2x1012cm-2. The NV' resonance for the transition from ms= 0 to -1 was used to determine the magnetic field strength. The contrast of the Rabi oscillations is typically about 7% for the dry state but reduces to 4% in the case of liquid samples on the diamond (see Fig. 6B).

[0142] An alternative to detecting NMR signals at the nanoscale with correlation spectroscopy is dynamic decoupling sequences, where the time between TT-pulses is swept. In this experiment, two dips were observed, corresponding to the19F and the1H (see Fig. 6C).

[0143] The spin-lattice relaxation time Ti was measured by extending the time between optical initialisation and readout. Fitting an exponential gives a range of 700 ps to 800 ps for pure diamond, as shown in Fig. 6D.

[0144] The spin-spin relaxation time T2, was measured with a Hahn echo sequence. The pulse sequence consists of a sequence of TT / 2-t-TT-t-TT / 2 pulses, where t is swept. After the TT / 2 pulse, the Bloch vector starts to rotate in the plane due to the magnetic noise. The 77-pulse, which lies between the two 77 / 2-pulses, cancels the phase accumulation if the magnetic noise in the two free precession times is identical. As the magnetic noise B fluctuates over time, the spin echo will decay, which is called a Hahn echo or NV-T2. The dips are due to coupling with13C-containing in the diamond. The data is fitted to a stretched exponential function, resulting in a range of 4-6 ps.

[0145] While this invention has been particularly shown and described with reference to embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims.

Claims

CLAIMS:1 . A quantum sensor device for measuring a magnetic field using fluorescence dependence on the magnetic field, the quantum sensor device (18) comprising a substrate made of single-crystal diamond, and nitrogen atoms in the substrate, the nitrogen atoms being partially present in the form of negatively charged nitrogen-vacancy (NV-) centres, wherein a ratio of the concentration of NV' centres to the concentration of the nitrogen atoms in the substrate is at most 1 .4 %.

2. The quantum sensor device of claim 1 , wherein the concentration of nitrogen atoms is at least 20 ppm and / or at most 100 ppm.

3. The quantum sensor device of claim 1 or 2, wherein the substrate includes a layer extending along a side surface of the substrate, wherein the nitrogen atoms and the NV' centres are only present in the layer.

4. The quantum sensor device of any preceding claim, wherein the NV' centres have an optically detected magnetic resonance (ODMR) readout contrast of at least 6% at a reciprocal initialisation time, 1 / treinit, of > 1 1 / ps, wherein the ODMR readout contrast is configured to be measured with an ODMR measurement using the quantum sensor device (18), wherein the ODMR contrast refers to the maximum ratio between a fluorescence intensity after applying a resonant microwave pulse during a readout phase of the ODMR measurement and a fluorescence intensity without applying a resonant microwave pulse during the readout phase of the ODMR measurement, wherein the fluorescence intensity is collected from an ensemble of NV' centres distributed over an area of at least 10 pm2, when initialized in the ms= 0 and ms= +1 or -1 state respectively, and wherein treinit is the time required for re-polarizing the NV' centres.

5. The quantum sensor device of any preceding claim, wherein the NV' centres have a spin-lattice relaxation time Ti between 500 ps and 1000 ps.

336. The quantum sensor device of any preceding claim, wherein the NV' centres have spin-spin relaxation time T2 between 0.4 ps and 4 ps.

7. A quantum sensor device for measuring a magnetic field using fluorescence dependence on the magnetic field, the quantum sensor device (18) comprising a substrate made of single-crystal diamond, and nitrogen atoms in the substrate, the nitrogen atoms being partially present in the form of negatively charged nitrogen-vacancy (NV') centres, wherein the NV' centres have an optically detected magnetic resonance (ODMR) readout contrast of at least 6% at a reciprocal initialisation time, 1 / init, of > 1 1 / ps, wherein the ODMR readout contrast is configured to be measured with an ODMR measurement using the quantum sensor device (18), wherein the ODMR contrast refers to the maximum ratio between a fluorescence intensity after applying a resonant microwave pulse during a readout phase of the ODMR measurement and a fluorescence intensity without applying a resonant microwave pulse during the readout phase of the ODMR measurement, wherein the fluorescence intensity is collected from an ensemble of NV' centres distributed over an area of at least 10 pm2, when initialized in the ms= 0 and ms= +1 or -1 state respectively, and wherein treinit is the time required for re-polarizing the NV' centres.

8. A method for manufacturing a quantum sensor device (18) for measuring a magnetic field using fluorescence dependence on the magnetic field, optionally using the quantum sensor device (18) of any preceding claim, the method comprising the steps of providing a substrate made of a single crystal diamond, implanting nitrogen atoms into the substrate, and annealing the substrate to convert at least some of the implanted nitrogen atoms into negatively charged nitrogen-vacancy (NV') centres, wherein the step of annealing the substrate includes providing a target temperature for the substrate, heating the substrate by placing the substrate on a heater surface (44a) of a heater (44), measuring the temperature of the substrate by a temperature sensor in direct contact with the substrate and / or the heater surface (44a), and34controlling the heater (44) based on the temperature measured by the temperature sensor such that the substrate has the target temperature.

9. The method of claim 8, wherein the step of controlling the heater (44) includes controlling the temperature of the substrate and / or the heater surface (44a) to be within± 5°C, ± 3°C, ± 2°C, ± 1 ,5°C, ± 1 °C, ± 0.5°C or ± 0.1 °C with regard to the target temperature.

10. The method of claim 8 or 9, wherein the target temperature is between 390°C and 410 °C for 6 hours and then between 790°C to 810 °C for 6 hours.

11. The method of any one of the claims 8 to 10, wherein changes in target temperature have a rate of 2.5°C / min to 3.5°C / min.

Images