Quantum sensors

Abstract

A substrate 101 comprising spin systems and a pair of mirrors or reflectors 100 and 102 to reflect excitation source photons through the substrate. The substrate may be diamond with nitrogen vacancy N

Description

Technical field The present invention relates to spin-based quantum sensors and systems for providing multipath illumination of excitation photons therein. The sensors of the present invention may be integrated into clothing or wearable devices and may be suitable for use in healthcare applications. Background Spin-based sensors (or quantum sensors / spin-based quantum sensors) exploit the fact that small changes in stimulus can alter the state of isolated quantum systems known as spin systems or sensing probes. Spin-based sensing probes comprise one or more degrees of freedom (e.g. electrons) with a spin state that is extremely sensitive to stimulus, and are capable of coherently transmitting changes in spin state into a detectable signal. A solid-state example of such a probe / spin system is the nitrogen-vacancy (NV) centre in diamond, a defect which involves a nitrogen atom and an adjacent vacant lattice site in a diamond lattice structure. In this defect, the negatively charged (NV-) centre forms a spin triplet in the ground state, the energy of which is susceptible to local magnetic and electric fields. The spin state of the NV- centre is highly sensitive to stimuli, and can be determined by a contrast in its fluorescence. As a result, the NV- centre has highly favourable and manipulatable properties of fluorescent signal production, which are sensitive to nanoscale changes in stimulus. Spin-based quantum sensors comprising NV centres have been used to detect changes in magnetic fields of a few nanotesla, as well as changes in electric fields and temperature. Thus, spin-based quantum sensors have been established as a powerful, versatile and low-cost means for sensitively detecting nanoscale stimuli (see e.g. WO2014051886A1, WO2016126436A1, US8885301B1, each of which are incorporated herein by reference). Solid-state optically-active spin-based quantum sensors, as tested in the lab, use a large amount of optical laser power (~1W) to sufficiently polarise and readout ensembles of spins (with a volume of X spins giving square root X increase in sensitivity but require X power per emitter). Firstly, by repeated absorption of e.g. a green photon (above band i.e. higher energy), and a resultant relaxation, each atom-like sensor builds up a spin polarisation. Following the quantum sensing event, where external stimuli change this spin polarisation, the sensor is again repeatedly excited by green photons. The spin converts this number of green photons into a number of red photons, the emitted fluorescence signal, at a ratio proportional to the final spin state. The red photons are then detected and counted to determine the spin state, and hence presence of external stimuli. This optical polarisation or excitation source typically and most effectively may be a green laser, for instance at 515 nm (semiconductor diode laser) or 532 nm (frequency-doubled Nd:YAG crystal), but can also be a light emitting diode. The interaction strength between individual point-like sensors and the micron-sized mode of the optical excitation source is very weak. This makes the absorbance per unit length low of the solid-state material containing the sensor, especially at lower densities of spin sensors where the spin sensors remain spatially separated, which have the best spin coherence properties as they are spaced sufficiently to not interact with each other. The sample illuminated is typically one to five hundred microns thick, and so only a small fractional amount of the laser beam is absorbed by the sample. Most laser radiation, therefore, leaves the solid-state volume and is scattered into the surrounding apparatus, whilst the optical excitation power used must be high in order to polarise an appreciable amount of the spin sensors in the material. Exposure to this radiation (at >0.5W, the highest hazard classification Class 4 laser) causes both heating (releasing hazardous fumes, gas or debris or igniting flammable material) and human hazards (permanent eye damage, skin burns), requiring adequate shielding to contain the radiation and eyewear protocols to prevent harm. Owing to these requirements, it has not been feasible to widely apply this technology operating at its highest sensitivity across a range of real-world applications relating to people and healthcare, where it would have important and tangible impact. For instance, in medicine, use in magnetic resonance imaging, magnetomyography, magnetocardiography or magnetoencephalography would require the sensor, and hence laser radiation, to be in close contact with the body of the patient, in some cases centimetres away from the eyes for applications in magnetoencephalography (brain scanning). In other contexts, it has been demonstrated that complete absorption of the optical radiation by solid-state spins can be achieved using a multi-pass illumination strategy, wherein the total optical path is increased from sub-mm to one metre through repeated total internal reflection inside the diamond sample. Clevenson et al. polished an edge facet of the diamond at 45° and carefully aligned the input laser to produce multi-path illumination (US Patent App. 10 / 197,515). The increased interaction with the laser field and the spins from the orders of magnitude increase in interaction length caused the green laser emission to be mostly absorbed by the material. Currently, this metre-length optical path technique, in combination with a 12 W laser beam, holds the sensitivity record for these spin-based quantum sensors, owing to the number of spins that can be optically excited compared to the standard optical excitation technique (Alsid, S. T. (2022). High-Sensitivity Nitrogen-Vacancy Center Magnetometry: from DC to GHz (Doctoral dissertation, Massachusetts Institute of Technology https: / / dspace.mit.edU / handle / 1721.1 / 150766). However, the multi-path technique requires the careful mechanical alignment of an external radiation beam with a polished edge facet of the diamond which is achieved in an optics lab and is not well suited to robust real-world applications. Misalignment could prove fatal with the initial reflection from the substrate significant. Additionally, the set path length is not well defined and limited by scattering loss at the rough diamond edges, which as opposed to the planar surface, cannot easily be polished. Thus, the art lacks a multi-pass optical excitation strategy for spin-based sensors which will be robust and compatible with health-care applications, e.g. where 1mW or less excitation power exits the sensor system without requiring an initial low power excitation source such as a low-power LED which results in only low-quality sensing or one that requires careful alignment of a laser beam. In other works, it has been described that an optical path length can be extended through repeated reflections in an optical cavity, formed by two mirror surfaces. A so-called Fabry-Perot cavity is the means by which solid-state lasers are formed, where multiple passes of the radiation in a gain medium causes emission amplification. This extension of the optical path length has been realised in an external cavity to increase the interaction strength with the narrow-band spin-based sensor fluorescence useful for spin-photon entanglement (Riedel, Daniel, et al. "Deterministic enhancement of coherent photon generation from a nitrogenvacancy center in ultrapure diamond." Physical Review X 7.3 (2017): 031040, see also US12092939B2). This technique has thus far only been explored for long wavelength red photon fluorescence, and has not been applied to provide multi-pass optical excitation of spinbased sensors through a solid-state substrate. Summary of the invention In an aspect, the present invention provides a spin-based sensor system comprising: a) a substrate comprising spin systems; b) an excitation source of photons configured to excite the spin systems; and c) a pair of mirrors disposed on the planar surfaces of the substrate, wherein the pairs of mirrors is configured to reflect the excitation source photons through the substrate. In some embodiments, the pair of mirrors is configured to provide multi-path excitation of the spin systems in the substrate. In some embodiments, the mirrors are distributed Bragg reflectors. In some particularly advantageous embodiments, the substrate has a laminar normal that is (substantially) parallel or perpendicular to the orientation of a subset of the NV centres (e.g. 1 / 4 or more of the NV centres). In other aspects, the present invention provides an integrated solid-state device comprising a spin-based sensor system of the invention, or an item of clothing (e.g. headwear), wearable device or implantable device comprising a spin-based sensor system or integrated solid-state device of the invention. In another aspect, the present invention provides a method of sensing using the spin-based sensor system, integrated solid-state device or item of clothing, wearable device or implantable device of the invention, comprising: a) emitting photons from the optical excitation source; b) using the mirrors to reflect the photons through the substrate comprising NV- centres; c) exciting the NV- centres in the substrate with the emitted and reflected photons; d) detecting photons that are emitted from the excited NV- centres. Brief description of the drawings FIG1: (A) Optical path length increased by two dielectric Distributed Bragg Reflector mirrors first formed on bottom (100) of diamond substrate (101) and then formed on top (102). (B) Cascaded cavity formed by bonding a GaN laser (103: n-GaN substrate, 104: n-type electrical contact, 105: active layer, 106: p-GaN region, 107: p-type electrical contact, 108: top DBR). (C) Inclusion of a bottom side broadband metallic mirror (109) to reflect the spin-dependent fluorescence of the sensor towards a top silicon photodiode (110). FIG2: Broadband reflectivity in design band centred around 515 nm using dielectric Distributed Bragg Reflector (DBR) mirror. DBR created by depositing alternating layers of silica and silicon nitride on the diamond. Near unity reflectivity is seen with N = 20 pairs. FIG3: Finite Difference Time Domain (FDTD) electromagnetic simulation moving onto (centre point) and off resonance of the cavity structure by changing the frequency of the excitation source. At the resonant frequency, the energy is stored in the cavity (central x region). FIG4: Monitoring the electric (E) field at a point within the cavity for N=5 mirror pairs (light grey) and N=7 mirror pairs (dark grey). The increase in mirror pairs from N=5 to N=7 causes the excitation field to be stored for 300 ps as opposed to 100 ps. Detailed description of the invention Sensor systems The present invention provides spin-based sensor systems. The systems may be solid-state, and can be incorporated in solid-state devices. The systems and solid-state devices may be incorporated in items of clothing such as hats, gloves, bands or helmets, wearable devices and implantable devices such as implants or injectable devices. In an aspect, the present invention provides a spin-based sensor system comprising: a) a substrate comprising spin systems; and b) a pair of mirrors disposed on the planar surfaces of the substrate, wherein the pair of mirrors is configured to reflect photons between the mirrors and through the substrate, providing multi-path excitation of the spin systems in the substrate. In an aspect, the present invention provides a spin-based sensor system comprising: a) a substrate comprising spin systems; and b) one or more pairs of mirrors disposed on the planar surfaces of the substrate, wherein the pairs of mirrors are configured to reflect photons between the mirrors and through the substrate, providing multi-path excitation of the spin systems in the substrate. The basic elements of a spin-based sensor system, which may also be referred to as a quantum sensor or spin-based quantum sensor, and variations therein are known and practicable by the skilled person. Such systems generally comprise spin systems (e.g. NV- centres) which serve as a probe, a means for exciting the spin systems, thereby causing the spin systems to emit a signal, and means for detecting the signal produced by the spin systems. The probe function of the spin systems means that emitted signal will vary, e.g. be modulated, by the stimuli that the spinbased sensor system is being used to detect or measure. The stimuli may be as minor as nanoscale changes in single electrons, nuclei or a single photon. Such stimuli cause the emitted signal from the spin systems to vary, and that variance is detected by the means for detecting the signal produced by the spin systems, thereby providing information on the stimuli. Substrate comprising spin systems In some embodiments, it will generally be understood that the substrate is a solid-state substrate. In some embodiments, the substrate comprising the spin systems is generally understood as a solid-state source of spin systems. In preferred embodiments, the substrate comprises diamond or is a diamond. In preferred embodiments, the substrate comprises diamond defects. In preferred embodiments, the substrate comprises NV- centres. In preferred embodiments, the substrate comprises diamond or is a diamond comprising NV- centres. In some embodiments, the spin systems may be referred to as sensing probes. In some embodiments, the spin systems may be referred to as spin-based sensing probes. In some embodiments, the spin systems comprise a point-like system with an associated electron spin triplet. In some embodiments, the spin systems comprise a solid-state defect. In some embodiments, the spin-based sensing probe is a near-surface defect. In some embodiments, the defect is no more than 5 pm, 1 pm, 500 nm, 400 nm, 300 nm, 200 nm, 100 nm, 50 nm, 5 nm or 1 nm from the surface of the probe, such as the surface of the diamond or nanodiamond. In some embodiments, the defect is a defect in diamond. In some embodiments, the diamond is a nanodiamond. In some embodiments, the spin systems comprise a NV- centre, e.g. one or more NV- centres. In some embodiments, the spin-based sensing probe comprises an electron comprised in a NV- centre. It will be understood that “NV- centre” herein refers to a negatively charged NV centre, which may also simply be referred to simply as a “NV centre” herein (and generally in the art of quantum information technology) for brevity. Excitation source In some embodiments, it will generally be understood that the system further comprises an (optical) excitation source of photons configured to excite the spin systems, optionally a laser or LED, such as a direct diode laser or vertical cavity surface emitting laser. In some embodiments, the excitation source comprises a laser. In some embodiments, the excitation source comprises an LED. In some embodiments, the excitation source is selected from a laser and LED. In some embodiments, the excitation source comprises a direct diode laser. In some embodiments, the excitation source comprises a vertical cavity surface emitting laser. In some embodiments the excitation source comprises an array or multiplicity of either laser or LED excitation sources. In some embodiments, it is generally understood that the excitation source of photons can be any source of photons capable of exciting the spin systems. Typically, the excitation source is a source of high energy photons, such as green / blue photons, such as photons having a wavelength of between 500 and 550 nm, such as a wavelength of 515 or 532 nm. In an embodiment, the excitation source photons have a higher energy than the fluorescence from the spin sensors. In some embodiments, the system comprises an excitation means and a signal detection means. In some embodiments, the excitation means is an optical excitation means. In some embodiments, the excitation means is configured to excite the spin systems. In some embodiments, the excitation means is configured to emit excitation light. In some embodiments, the spin systems are configured to emit a signal when excited by the excitation means. In some embodiments, the signal detection means is a fluorescence detection means. In some embodiments, the signal detection means is configured to detect a signal emitted by the spin systems. In some embodiments, the signal emitted by the spin systems is modulated by stimuli. In some embodiments, the signal is light, preferably fluorescent light, which is modulated by stimuli. In some embodiments, the intensity of the light is modulated by stimuli. In some embodiments, the phase of the light is modulated by stimuli. In some embodiments, the stimulus comprises a change in a magnetic field, electric field or temperature. In some embodiments, the change is from an absence to a presence of the stimulus. In some embodiments, the change is from a presence to an absence of the stimulus. In some embodiments, the stimulus comprises a nanoscale change. In some embodiments, the stimulus comprises the presence or state of single photons. In some embodiments, the stimulus comprises the presence or state of single stimulus spins. In some embodiments, the stimulus comprises the presence or state of sets of stimulus spins. Mirrors The present invention utilises a pair of mirrors. The number of pairs is not limited. Where configuration or function of the pair of mirrors is referred to, this is generally understood as the configuration or function of both mirrors within the pair of mirrors. A single pair of mirrors may be used, or more than one pair of mirrors may be used. A pair of mirrors is understood to comprise a first mirror and a second mirror. In some embodiments, the mirrors in a pair are generally understood to have the same optimum wavelength of reflectivity. In some embodiments, the mirrors in a pair are generally understood to have the same optimum wavelength of reflectivity but may have different reflectivity amplitudes. In some embodiments, the mirrors in a pair are substantially identical or identical. In some embodiments, “a pair of mirrors disposed on the planar surfaces of the substrate” comprises a first mirror disposed on a first planar surface of the substrate, and a second mirror disposed on a second planar surface of the substrate. In some embodiments, it will generally be understood that the pair of mirrors reflects photons of essentially the same wavelength as the photons emitted from the (optical) excitation source. In some embodiments, it will generally be understood that the pair of mirrors preferentially reflects photons of essentially the same wavelength as the photons emitted from the (optical) excitation source. In some embodiments, it will generally be understood that the pair of mirrors is configured to reflect photons of essentially the same wavelength as the photons emitted from the (optical) excitation source. In some embodiments, it will generally be understood that the pair of mirrors is configured to preferentially reflect photons of essentially the same wavelength as the photons emitted from the (optical) excitation source. In some embodiments, it will generally be understood that the pair of mirrors has an optimum wavelength of reflectivity for reflecting the photons that are emitted by the (optical) excitation source. In some embodiments, it will generally be understood that the pair of mirrors is configured to have an optimum wavelength of reflectivity at essentially the same wavelength as the photons emitted from the (optical) excitation source. In some embodiments, it is generally understood that the mirrors are used to increase the extinguishing of the excitation photons. Accordingly, in some typical embodiments, the mirrors will reflect photons having the same wavelength as the excitation photons, but are not necessarily required to reflect other photons. In some embodiments, it will generally be understood that the pair of mirrors has a wavelength bandwidth range configured to reflect the photons that are emitted by the (optical) excitation source such that the source can be modulated and that the modulated photons remain reflected in that bandwidth range. In some embodiments, the wavelength of the photons emitted from the (optical) excitation source is between 500 and 550 nm. In some embodiments, the pair of mirrors is configured to have an optimum wavelength of reflectivity that is between 500 and 550 nm. In some embodiments, the pair of mirrors is configured to have an optimum wavelength of reflectivity at about 515 nm and / or at about 532 nm. In some embodiments, the pair of mirrors reflects photons having one or more wavelengths between 500 and 550 nm, such as about 515 nm and / or 532 nm. In some typical embodiments, the mirrors will reflect photons having one or more wavelengths between 500 and 550 nm, but do not reflect other photons having any wavelength that is not between 500 and 550 nm. In some embodiments, the pair of mirrors is configured not to reflect photons that are emitted by the spin systems. In some embodiments, the photons emitted by the spin systems are typically red. In some embodiments, the mirrors are configured not to reflect red photons. In some embodiments, the mirrors do not (substantially) reflect photons of wavelength between 600-800 nm, such as about 637 nm. In some embodiments, the pair of mirrors (substantially) reflects photons having one or more wavelength between 500 and 550 nm, such as about 515 nm and / or 532 nm, and the pair of mirrors (substantially) does not reflect photons having one or more wavelength between 600-800 nm, such as about 637 nm. Mirror disposition In some embodiments, the spin-based sensor system comprises one or more pairs of mirrors disposed on the planar surfaces of the substrate. In some embodiments, it will generally be understood that “disposed on” the planar surfaces of the substrate means in contact (e.g. direct contact) with the substrate. In some embodiments, it will generally be understood that “in contact” means that photons passing through the substrate are reflected by the mirrors without exiting from the substrate. In preferred embodiments, it will generally be understood that the substrate and pair of mirrors form part of the same solid-state device, which may further comprise the other elements of the sensor systems of the present invention. In some embodiments, it may be described that the substrate comprises spin-systems within a solid-state cavity between the pair of mirrors. As generally used herein, a substrate is a laminar solid object, thereby comprising two “long” planar surfaces, e.g. a top surface and bottom surface, and two or more “short” edge surfaces. The planar surfaces of the substrate are therefore the flat (top and bottom) planes of the substrate. Accordingly, in some embodiments, the pair of mirrors is disposed on the top and bottom planes of the substrate. Herein, it will generally be understood that the mirrors within a given pair are disposed on different / opposite planar surfaces of the substrate. Thus, the spin-based sensor systems of the invention in some general embodiments will comprise a lamellar structure having the following layers from top to bottom: 1) a first mirror of the pair of mirrors, 2) the substrate comprising spin systems, 3) the second mirror of the pair of mirrors. In some embodiments, it will generally be understood that the mirrors are disposed on either side of the substrate. In some embodiments, it will generally be understood that the substrate is disposed between the mirrors. In some embodiments where there are more than one pair of mirrors, it will generally be understood that each pair comprises one mirror disposed on each of the two planar surfaces of the substrate. In some embodiments, a first mirror of the pair of mirrors is in (direct) contact with a first planar surface of the substrate and a second mirror of the pair of mirrors is in (direct) contact with a second (different) planar surface of the substrate. In some embodiments, the pair of mirrors is deposited (directly) onto the planar surfaces of the substrate, e.g. by vapour deposition as described herein. In some embodiments, the pair of mirrors is configured or configurable to reflect photons between the mirrors and through the substrate, e.g. as described above. In some embodiments, the photons are photons from an excitation source. In some embodiments, the photons have a wavelength suitable for exciting the spin systems. In some embodiments, this provides multi-path excitation of the spin systems in the substrate. In some embodiments, it is generally understood that multi-path excitation means that photons (e.g. from the excitation source) are able to take multiple paths through the substrate and thus have multiple opportunities to excite the spin systems. In some embodiments, “multi-path excitation” means that the excitation source photons are reflected back and forth between the mirrors in the pair until substantially all of the excitation source photons are absorbed by the spin systems. Multi-path excitation may also be referred to as multi-path illumination. In some embodiments, it will generally be understood that the multi-path excitation of the present invention reduces the amount of excitation power that exits the substrate and / or the sensor system. In some embodiments, 1 mW or less excitation power exits the sensor systems of the present invention. In some embodiments a first mirror of the pair of mirrors is grown to have a different (level of) reflectivity than the second mirror of the pair of mirrors such that the amount of excitation power that exits the substrate does so in a preferential direction. In some embodiments, the excitation source power that leaves the sensor system / the substrate is substantially reduced. In some embodiments, the excitation source power that leaves the sensor system / the substrate is substantially reduced by the pair of mirrors. In some embodiments, the excitation source power that leaves the sensor system / the substrate is substantially reduced by the multi-path illumination provided by the pair of mirrors, e.g. in that the excitation source power is instead absorbed by the spin systems due to the multi-path illumination. In some embodiments, only an insubstantial amount of residual excitation source power leaving the spin-based sensor system. In some embodiments, the residual excitation source power that leaves the spin-based sensor system is below the tolerable amount for contact with or close proximity to the human or animal body, e.g. as little as 10 cm, 9 cm, 8 cm, 7 cm, 6 cm, 5 cm, 4 cm, 3 cm, 2 cm, 1 cm, or 0 cm from the human or animal body. In some embodiments, the excitation source power that leaves the spin-based sensor system is no more than about 1 mW, about 0.9 mW, about 0.8 mW, about 0.7 mW, about 0.6 mW, about 0.5 mW, about 0.4 mW, about 0.3 mW, about 0.2 mW, about 0.1 mW or about 0.0 mW. In some embodiments, the excitation source power that leaves the spin-based sensor system is no more than 1 mW, 0.9 mW, 0.8 mW, 0.7 mW, 0.6 mW, 0.5 mW, 0.4 mW, 0.3 mW, 0.2 mW, 0.1 mW or 0.0 mW. In some embodiments, the mirrors are disposed on opposite surfaces of the substrate. In some embodiments, a first mirror of the pair is deposited on the same side of the substrate that the excitation photons enter from the excitation source, and a second mirror of the pair is deposited on the opposite side of the substrate from the side of the substrate that the excitation photons enter. In some embodiments, the mirrors are distributed Bragg reflectors (DBRs). In some embodiments, the distributed Bragg reflectors comprise a plurality of silicon dioxide and silicon nitride layers. In some embodiments, the distributed Bragg reflectors comprise a plurality of silicon dioxide and silicon nitride layers deposited on two opposite surfaces of the substrate. It will generally be understood, particularly in embodiments of the present invention comprising DBRs, that the DBRs have a wavelength of reflectivity, which is influenced by the thickness of each layer of the DBRs, and an amplitude of reflectivity (or level of reflectivity) which is influenced by the number of layer pairs of the DBRs. It will generally be understood that, in some embodiments of the present invention comprising DBRs, each DBR in the pair of DBRs may comprise one or more layer units, preferably a plurality of layer units. Each layer unit comprises the minimum number of constituent layers to form a working single DBR layer. For example, a layer unit can comprise a layer of silicon dioxide and a layer of silicon nitride. In some embodiments, one or more of the pair of the DBRs (e.g. the first and / or second DBR in the pair) comprises or consists of a number of layer units that is independently selected from 1,2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29 or 30. In some embodiments, the number of layer units is between 1 and 100, more preferably between 5 and 20. In some embodiments wherein the substrate comprises diamond, the mirrors are in (direct) contact with the diamond. In some such embodiments, the diamond is a planar diamond and the mirrors are in (direct) contact with the planar surfaces of the diamond. Further system elements In some embodiments, the system further comprises an excitation source. In highly preferred embodiments, the excitation source is an in-situ excitation source with the substrate of the present invention. In highly preferred embodiments, the excitation source is provided as a solid-state with the substrate of the present invention. In some embodiments, the system further comprises a detector. In some embodiments, the system further comprises a vector magnetic field source, e.g. which is used to align the spin systems to maximise or improve their sensitivity to stimuli. In general, in substrates (e.g. diamonds) that comprise NV centres, the NV centres have an orientation that is not parallel to (e.g. at around 55 degrees from) the plane of growth, which is the original laminar normal of the diamond as grown. In some embodiments of the present invention, the substrate (e.g. diamond) is polished at an angle such that the orientation of a subset of the NV centres (e.g. 1 / 4 or more) is (substantially) parallel or perpendicular to the laminar normal of the substrate. Accordingly, in some embodiments, the substrate has a laminar normal that is (substantially) parallel or perpendicular to the orientation of a subset of the NV centres (e.g. 1 / 4 or more of the NV centres). In some embodiments, the subset of spin systems can be magnetically aligned by a vector magnetic field source. In some embodiments, the substrate is polished at an angle such that a subset of the spin systems is aligned to the optical excitation source and can be magnetically aligned by a vector magnetic field source. In some preferred embodiments of this type, it is understood that the system further comprises a vector magnetic field source. In some embodiments, the substrate is polished at an angle before the mirror deposition such that a subset of the spin systems is aligned to the optical excitation source. In some embodiments, the system further comprises one or more or all of a broadband reflector, a microwave driving field source, a vector magnetic field source, a communications module, a control module and / or a power source. In some embodiments, the system further comprises a broadband reflector. In some embodiments, the system further comprises a microwave driving field source. In some embodiments, the system further comprises a vector magnetic field source. In some embodiments, the system further comprises a communications module. In some embodiments, the system further comprises a control module. In some embodiments, the system further comprises a power source. Other forms of the sensor system In an aspect, the present invention provides an integrated solid-state device comprising a spinbased sensor system according to the invention. In embodiments, the solid-state device comprises the elements of the sensor system as a single physical unit. In embodiments, there is no substantial empty space, air or vacuum between the elements of the sensor system. The integrated solid-state device comprises all essential elements of the sensor system and may further comprise any optional elements. In some embodiments, the present invention provides an integrated solid-state device comprising: a) a substrate comprising spin systems; and b) a pair one or more pairs of mirrors disposed on the planar surfaces of the substrate, wherein the pair of mirrors is configured to reflect photons between the mirrors and through the substrate, providing multi-path excitation of the spin systems in the substrate. In some embodiments, the solid-state device further comprises a broadband reflector. In some embodiments, the solid-state device further comprises a microwave driving field source. In some embodiments, the solid-state device further comprises a vector magnetic field source. In some embodiments, the solid-state device further comprises a communications module. In some embodiments, the solid-state device further comprises a control module. In some embodiments, the solid-state device further comprises a power source. In an aspect, the present invention provides an item of clothing (e.g. headwear), wearable device or implantable device comprising a spin-based sensor system or integrated solid-state device according to the invention. Method of sensing In an aspect, the present invention provides method of sensing using the spin-based sensor system, integrated solid-state device or item of clothing, wearable device or implantable device of the invention. In an embodiment, the method of sensing comprises: a) emitting photons from the (optical) excitation source; b) using the mirrors to reflect the photons through the substrate; c) exciting the spin systems in the substrate with the emitted and reflected photons; d) detecting photons that are emitted from the excited spin systems. In other embodiments, the spin systems are other defects in solid-state materials. In some embodiments, the spin systems are defects in silicon carbide or hexagonal boron nitride. In some embodiments, the spin systems are defects in diamond (e.g. NV centres), silicon carbide or hexagonal boron nitride. In a preferred embodiment, the spin systems are NV- centres. In a related aspect, the present invention provides a data processing apparatus, device or system comprising or a processor configured to perform the method according to the present invention. In a related aspect, the present invention provides a computer program comprising instructions which, when executed by a computer, carry out the method according to the present invention. In a related aspect, the present invention provides a computer-readable medium comprising instructions which, when executed by a computer, cause the computer to carry out the method according to the present invention. Herein, “a” or “an” encompasses the singular and also the plural, unless context or explicit limitation dictates otherwise. Herein, an object “comprising” an element indicates that the object has that element but is not limited thereto, and may further comprise additional, unrecited elements. Herein, the use of “comprising” should be recognised as encompassing a further, preferred embodiment using the wording “consisting”, in which only the recited elements are present. The scope of the present invention is determined with reference to the accompanying claims, and the relevant jurisdictional law under which the scope of protection is being assessed. Examples Herein, devices are provided that extend the concept of an optical cavity to increase the optical path length to a solid-state example, and exploit this integration to realise a fully-integrated quantum sensor safe for use on and in proximity to humans, such as in the application of brain scanning (optically-pumped magnetometer magnetoencephalography or OPM-MEG). Solid-state diamond cavity A means is presented to increase the optical path length by coating the diamond top and bottom surface with a mirror realised using alternate layers of silicon dioxide and silicon nitride designed to be highly reflective at a target laser wavelength, known as a Distributed Bragg Reflector or DBR. One advantage of the integrated approach is its stability as opposed to external cavity designs. The resonant frequency of the cavity may be modulated by tuning the temperature, which modulates the refractive index of the material constituents. The layer stack can be generated through chemical vapour deposition of alternate layers of these two materials. Stoichiometric silicon nitride emits in the red when illuminated with green laser light owing to the presence of local emitters. Nitrogen-rich variants have a low number of emitters, which improves the signal to noise operation of the device when used in combination with red fluorescing spin systems, although these compositions have a lower refractive index contrast resulting in a larger number of layer pairs stack (20 pairs for reflectivity R>0.99). The wavelength-dependent reflectance of some of these mirrors are shown in Fig 2. By fabricating one of these mirrors on both the top and bottom side of the diamond an optical cavity is realised wherein light remains trapped for multiple reflections. The cavity finesse, equivalent to the number of reflections in the cavity, is parameterised by its quality factor (Q). The quality factor of the cavity (Q) extends the duration that light stays in the cavity and therefore must be kept low enough to permit the use of rapid rise-time pulse sequences (laser on I off) for high sensitivity magnetometry, whilst still allowing enough reflections to effectively absorb all radiation in the diamond cavity medium. Consider a simplified ray optics model where the laser reflects one million times off each mirror interface inside the 100 pm of diamond (refractive index n = 2.43), then the minimum on / off time possible is (106 x 100 pm) x 2.43 / c = 800 ns. Phrased conversely, a rise time of 1 ns limits the optical path length to 12 cm. From the diamond in Clevenson et al. (Clevenson, Hannah, et al. "Broadband magnetometry and temperature sensing with a light-trapping diamond waveguide." Nature Physics 11.5 (2015): 393-397), with an absorption constant a = 0.45 cm'1, using the Beer-Lambert law, a target 0.99 of the power will be absorbed at logw(1 / 0.01) / 0.45 = 4.4 cm path length. For the present invention, an aim is to enable radiation to decay to safe levels (i.e. Class 1 <1 mW). Given an injection of 1 W of optical power per module, the extinction needed is 0.999, giving a path length of 6.7 cm by the same expression, or 8.9 cm for 10 W per module. Therefore, a target path length of 12 cm both allow the module to be classified as safely below Class 1, yet allow 1 ns pulses. The parameters that can be used to control the absorption are the NV density in the diamond, the diamond thickness, and the number of DBR mirror pairs at each interface. Through photonic simulation, the green laser absorption by the NV centre is modelled by adding a small complex refractive index term e = 1.905e-6j to the diamond dielectric model, equivalent to the measured absorption constant a = 0.45 cm'1 at 532 nm in optimal ensemble densities for quantum sensing (US Patent App. 10 / 197,515, incorporated by reference herein). Increased NV densities beyond this level lead to each NV centre affecting the coherence of surrounding NV centre, and the average spin coherence and hence sensitivity dropping significantly. Increasing the thickness of the diamond substrate results in diffractive losses in the volume. Decreasing the thickness of the diamond substrate results in damage to the quality of the NV centre. By monitoring the total light transmitted through the bottom mirror pair, the total light reflected above the top mirror pair, and the optical source power, it is modelled that the loss, i.e. the absorbed power, increases as the number of mirror pairs increases. In Fig 3, the electric (E) field across the simulation is monitored and the total E field stored in the device increases significantly on resonance owing to the multiple passes between the DBR mirrors. Off resonance, the E field rapidly leaves the simulation. The density of the optical mode and hence the effect of its confinement is reduced for a larger length cavity, requiring a larger quality factor (Q) for an equivalent effect. In Fig 4, by monitoring a single point in the time domain, the electromagnetic field is captured and hence absorbed on a longer time scale for a greater number of mirror pairs (N = 7 pairs compared to N =5 pairs results in 350 ps to 250 ps increase in time storage). The number of mirror pairs increases the optical path length in the diamond cavity and hence the NV centre absorption. In tandem, as expected, the spectral resonance narrows (Q greater) as the number of mirror pairs increases. Cascaded cavity with integrated laser Following the above, a two-part solid-state cascaded cavity is designed. The cascaded cavity includes a first cavity section to realise a laser that emits an emission line between 510 and 540 nm for the excitation of NV centres, followed by a second cavity section to realise the absorption of this emission by the spin-based sensors as already described. A breakthrough of the present invention is in including the emission source in-situ with the absorption in one solid-state structure, by which any ex-situ optics are eliminated. This increases the efficiency of the device, reduces its footprint and weight, and prevents the chance of misalignment of the high-power source away from the absorption source, for human applications. The first part of the device consists of a Vertical Cavity Surface-emitting Laser (VCSEL) by growing a pair of dielectric mirrors in contact and on opposing sides of a lamellar GaN substrates. As a standalone structure, this is already well demonstrated. GaN is chosen to realise a direct diode. Direct diode lasers are highly efficient, often surpassing q = 0.6 wall plug efficiency. Watt-class lasers at the target emission wavelength have been realised in GaN through arrays of 16 by 16 VCSELs, at 100 pm pitch ("Watt-class blue vertical-cavity surfaceemitting laser arrays." Applied Physics Express 12.9 (2019): 091004. https: / / iopscience.iop.org / article / 10.7567 / 1882-0786 / ab3aa6). In some embodiments of the present invention, the laser may be a single laser, this laser may be single mode or multimode or may be an array of single or multimode lasers. Dielectric DBRs are often used to create lasers in GaN owing to the stronger refractive index contrast compared to doped layers of GaN, and high controllability and reproducibility compared to the formation of porous GaN layers. Owing to its best-in-class thermal conductivity, diamond is regularly suggested as a heat absorber layer for GaN light sources. However, the lattice mismatch between GaN and diamond is large (11.8 % (Liu T, Kong Y, Wu L, et al. 3-inch GaN-on-Diamond HEMTs with device-first transfer technology. IEEE Electron Device Lett. 2017; 38(10):1417-1420,)) precluding direct growth. Instead of direct epitaxial growth, a dielectric mirror spacer layer and wafer bonding is used in the example to absorb this lattice-mismatch. The addition of a broadband reflector on the bottom facet of the diamond following the DBR directs generated spin signal upwards to a photodiode detector on the opposite top surface. This broadband reflector can be realised using a metallic layer such as silver or gold. The in-situ photodiode detects the signal from the spins. The spin sensor fluorescence (600 nm to 800 nm) is not significantly affected by the DBR designed for the excitation signal. Incident excitation is given a second pass to the DBR by the presence of the broadband reflector, further increasing the absorbance. Brain scanner application A soft fabric hat is proposed containing a set of cascaded cavity modules comprising sensor systems of the present invention, for the sensing of neural activity in the brain. Electrical activity present in neurons is recorded by the magnetic field sensor. This technique is known as optically-pumped magnetometer magnetoencephalography or OPM MEG. MEG equipments typically require bulky and costly apparatus such as Superconducting Quantum Interference Devices or Atomic Vapour Cells. The equipment cost, large space requirement, and running cost of these machines means that few are operational despite their superior performance compared to other technology such as EEG using electrodes to directly detect electrical signals — either low sensitivity on the surface of the head or very invasively implanted in the head (such as Neuralink™) or lower sensitivity fNIRS which detects blood flow linked to brain activity via a laser and corresponding laser absorbance detector. The device proposed herein is an analogue to the atomic vapour cell Optically Pumped Magnetometer yet, as opposed to the cell containing the vapour, it has the solid-state robustness of fNIRs. The comparison is drawn between the vacuum tube and the transistor. Integration with microelectronics allows in-situ vector magnetic fields and microwave driving fields to be produced at low power. A comfortable wearable is achieved using small millimetre-sized sensor modules placed closed to the human head and flexible materials including electronics and cabling for the rest of the hat. This design is opposed to vapour cell OPM-MEG which requires millimetre sized tubes (Quspin) in a hard plastic hat (Cerca Magnetics Ltd). The hat has a battery such as a lithium battery to power the sensors via a common flexible power line to each sensor. Either each module can have its own wireless connectivity through a microcontroller operating over, for example, Wi-Fi ™ / Bluetooth standards, or sets of modules can be connected with a communication bus realised by flexible electronics to a central communications module. A communication band is chosen to not interfere with the microwave operation of the spin-based sensors (such as 5 GHz).

Claims

1. A spin-based sensor system comprising:a) a substrate comprising spin systems;b) an excitation source of photons configured to excite the spin systems; andc) a pair of mirrors disposed on the planar surfaces of the substrate,wherein the pairs of mirrors is configured to reflect the excitation source photons through the substrate.

2. The spin-based sensor system of claim 1, wherein the pair of mirrors is configured to provide multi-path excitation of the spin systems in the substrate.

3. The spin-based sensor system of claim 1 or 2, wherein the excitation source of photons is a laser or LED, such as a direct diode laser or vertical cavity surface emitting laser.

4. The spin-based sensor system of any preceding claim, wherein the pair of mirrors is configured to reflect photons of essentially the same wavelength as the photons emitted from the optical excitation source.

5. The spin-based sensor system of any preceding claim, wherein the pairs of mirrors are configured to have an optimum reflectivity at essentially the same wavelength as the photons emitted from the optical excitation source.

6. The spin-based sensor system of any preceding claim, wherein the wavelength of the excitation source photons is between 500 and 550 nm.

7. The spin-based sensor system of any preceding claim, wherein the excitation source power that leaves the substrate is no more than about 1 mW.

8. The spin-based sensor system of any preceding claim, wherein the mirrors within the pair of mirrors are disposed on opposite surfaces of the substrate.

9. The spin-based sensor system of any preceding claim, wherein the mirrors are distributed Bragg reflectors.

10. The spin-based sensor system of any preceding claim, wherein the distributed Bragg reflectors are formed of layers of silicon dioxide and silicon nitride layers deposited on opposite surfaces of the substrate.

11. The spin-based sensor system of any preceding claim, wherein the system further comprises a detector.

12. The spin-based sensor system of any preceding claim, wherein the substrate has a laminar normal that is (substantially) parallel or perpendicular to the orientation of a subset of the NV centres (e.g. 1 / 4 or more of the NV centres).

13. The spin-based sensor system of any preceding claim, further comprising one or more or all of a broadband reflector, a microwave driving field source, a vector magnetic field source, a communications module, a control module and / or a power source.

14. An integrated solid-state device comprising a spin-based sensor system according to any preceding claim.

15. An item of clothing (e.g. headwear), wearable device or implantable device comprising a spin-based sensor system or integrated solid-state device according to any preceding claim.

16. A method of sensing using the spin-based sensor system, integrated solid-state device or item of clothing, wearable device or implantable device of any preceding claim, comprising: a) emitting photons from the optical excitation source;b) using the mirrors to reflect the photons through the substrate comprising NV centres;c) exciting the NV centres in the substrate with the emitted and reflected photons;d) detecting photons that are emitted from the excited NV centres.

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