Radio frequency sensor

Abstract

A radio frequency signal sensor comprising a material comprising at least one spin defect. A magnetic field generator is arranged to provide a magnetic field across the material. There is also provided an optical excitation source and a detector arranged to detect resonance frequencies from the spin defect. The magnetic field generator is located in a static position relative to the material and is configured to generate an inhomogeneous magnetic field comprising at least one inflection point, the inflection point having a non-zero second order derivative value and a gradient of at least 0.1 T / mm.

Description

[0001] RADIO FREQUENCY SENSOR

[0002] FIELD OF THE INVENTION

[0003] The invention relates to the field of radio frequency sensors, and in particular radio frequency sensors that utilise diamond.

[0004] BACKGROUND

[0005] 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. In addition, radio frequency (RF) sensors can also be developed using optically active defects in diamond.

[0006] 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. It will be noted that 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.

[0007] Similarly, point defects in SiC material have been studied that can be used in similar ways. Examples of such defects include silicon-vacancy defects (Si-V), divacancy centres (Si-V2) and nitrogen-vacancy centres (NV).

[0008] The 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:

[0009] (i) Its electron spin states can be coherently manipulated with high fidelity owing to an extremely long coherence time (which may be quantified and compared using the transverse relaxation time T2); (ii) Its electronic structure 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; and

[0010] (iii) Its electronic structure 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. 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.

[0012] It is known to use the NV' defect in diamond in RF sensing applications. For example, Chipaux et. al., Applied Physics Letters 107, 233502 (2015) describes a wide bandwidth RF spectrum analyser using NV' centres in diamond pumped by a 532 nm laser and the resultant photoluminescence is imaged. A microwave field in proximity to the NV' centres induces resonances that is detected through a decrease in the photoluminescence. A magnetic field gradient induces a Zeeman shift of the resonances and transforms the frequency information into spatial information, which allows for the simultaneous analysis of the microwave signal in the entire frequency bandwidth of the device. Magaletti et al., Nature Communications Engineering 1 , 19 (2022) describes a similar RF spectrum analyser.

[0013] The Chipaux and Magaletti disclosures use an inhomogeneous magnetic field that varies linearly across one of the three principal diamond axes (to extend bandwidth this is typically chosen as the axis where the diamond material has the greatest dimension). An external signal is converted to a magnetic field and applied across the diamond. As the inhomogeneous magnetic field varies across the width of the diamond, one can use the photoluminescence emitted by the NV' centres in particular areas of the diamond to determine the frequency and amplitude of an external signal. The frequency is determined by calibration measurements such that an RF frequency corresponds to a particular magnetic field. Typically, the inhomogeneity is provided by moving the magnetic field source relative to the diamond to allow for a large range of frequencies to be detected.

[0014] The resolution of a diamond-based RF sensor is determined by both the inhomogeneity of the magnetic bias field and the pixel resolution of the camera used for imaging the fluorescence emitted by the NV' centres. In addition, the sensitivity of the sensor is determined by the inhomogeneous magnetic field resolution. To maximize the bandwidth (the frequency range of the sensor), the magnetic field gradient should be as high as possible whilst still providing a high enough sensitivity to allow detection.

[0015] SUMMARY

[0016] Known RF sensors using spin centres in diamond or other materials require an inhomogeneous magnetic field. Typically an inhomogeneous magnetic field is created by a single solid state magnetic field source positioned such that a field impinging on the diamond is different at different points on the diamond. In general, this applied magnetic field is substantially linear. An object of the invention is to provide a highly inhomogeneous magnetic field across diamond for an RF sensor system.

[0017] According to a first aspect, there is provided a radio frequency signal sensor comprising: a material comprising at least one spin defect; a magnetic field generator arranged to provide a magnetic field across the material; an optical excitation source; a detector arranged to detect resonance frequencies from the spin defect; wherein the magnetic field generator is located in a static position relative to the material and is configured to generate an inhomogeneous magnetic field comprising at least one inflection point, the inflection point having a non-zero second order derivative value, and a gradient of at least 0.1 T / mm.

[0018] The use of a magnetic field with an inflection point allows high gradients to be achieved in the magnetic field that approximate a linear magnetic field, which allows a wider range of frequencies to be queried.

[0019] As an option, the portion of the magnetic field that includes the inflection point acts on a largest dimension of the material.

[0020] As an option, the largest dimension of the material is selected from any of at least 0.5 mm, at least 1 mm, at least 2 mm, at least 3 mm and at least 4 mm.

[0021] The magnetic field generator is optionally configured to provide a magnetic field gradient over a largest dimension of the material selected from any of at least 0.2 T / mm, and at least 0.5 T / mm.

[0022] The magnetic field generator is optionally configured to provide a magnetic field gradient over the largest dimension of the material of no more than 1.0 T / mm.

[0023] The detector is optionally configured to detect radio frequency signals over a bandwidth selected from any of at least 1 GHz, at least 5 GHz, at least 10 GZ, at least 20 GHz and at least 50 GHz.

[0024] An optional example of a magnetic field generator is a neodymium magnet.

[0025] As an option, the magnetic field generator comprises magnet having opposing teeth with opposite polarity.

[0026] The material is optionally selected from any of single crystal diamond and single crystal silicon carbide.

[0027] Where the material is single crystal diamond, the spin defect is optionally selected from any of a silicon-vacancy defects (Si-V), a silicon di-vacancy defect (Si-V2), a silicon- vacancy-hydrogen defect (Si-V:H), a silicon di-vacancy hydrogen defects (S-V2:H), a nickel containing defect, a chromium containing defects, a nitrogen-vacancy defect (N- V), a di-nitrogen vacancy defect (N-V-N), and a nitrogen-vacancy-hydrogen defect (N-V- H).

[0028] Where the material is single crystal silicon carbide, the spin defect is optionally selected from any of a silicon-vacancy defects (Si-V), a silicon divacancy centre (Si-V2), and a nitrogen-vacancy centre (NV).

[0029] According to a second aspect, there is provided a method of sensing radio frequency signals, the method comprising: providing a material comprising at least one spin defect; providing an inhomogeneous magnetic field comprising at least one inflection point, the inflection point having a non-zero second order derivative value and a gradient of at least 0.1 T / mm, the magnetic field being static relative to the material, and wherein at least a portion of the magnetic field acts on a largest dimension of the material; pumping the material using an optical excitation source; and detecting resonance frequencies from the spin defect using a detector.

[0030] As an option, the portion of the magnetic field that includes the inflection point acts on the largest dimension of the material.

[0031] The largest dimension of the material is optionally selected from any of at least 0.5 mm, at least 1 mm, at least 2 mm, at least 3 mm and at least 4 mm.

[0032] The method optionally comprises providing a magnetic field gradient over a largest dimension of the material selected from any of at least 0.2 T / mm, and at least 0.5 T / mm.

[0033] The method optionally comprises providing a magnetic field gradient over the largest dimension of the material of no more than 1 .0 T / mm.

[0034] As an option, the method comprising detecting radio frequency signals over a bandwidth selected from any of at least 1 GHz, at least 5 GHz, at least 10 GZ, at least 20 GHz and at least 50 GHz.

[0035] The material is optionally selected from any of single crystal diamond and single crystal silicon carbide. Where the material is single crystal diamond, the spin defect is optionally selected from any of a silicon-vacancy defects (Si-V), a silicon di-vacancy defect (Si-V2), a silicon- vacancy-hydrogen defect (Si-V:H), a silicon di-vacancy hydrogen defects (S-V2:H), a nickel containing defect, a chromium containing defects, a nitrogen-vacancy defect (N- V), a di-nitrogen vacancy defect (N-V-N), and a nitrogen-vacancy-hydrogen defect (N-V- H).

[0036] Where the material is single crystal silicon carbide, the spin defect is optionally selected from any of a silicon-vacancy defects (Si-V), a silicon divacancy centre (Si-V2), and a nitrogen-vacancy centre (NV).

[0037] As an option, the method comprising detecting resonance frequencies by correlating a known result of a radio frequency interaction with the material.

[0038] BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which:

[0040] Figure 1 is a graph showing a linear magnetic field with respect to frequencies that can be detected;

[0041] Figure 2 is a graph showing a non-linear magnetic field having at least one inflection point with respect to frequencies that can be detected;

[0042] Figure 3 is a prior art drawing taken from Wood et al, Phys. Rev. A 105, 012824 (2022), showing an example of a magnet geometries that can generate an inhomogeneous magnet field;

[0043] Figure 4 illustrates schematically in a block diagram an exemplary radio frequency signal sensor; and

[0044] Figure 5 is a flow diagram showing exemplary steps in detecting radio frequency signals.

[0045] 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. DETAILED DESCRIPTION

[0046] In the following description of the modules, single diamond is used by way of example of a quantum host material for spin defects. However, it will be appreciated that other quantum host materials, such as silicon carbide, may be used. Furthermore, the spin defect referred to by way of example is the NV' centre in diamond, but as described above there are many other possible spin centres that can be used.

[0047] With a standard linear magnetic field across a material of a fixed size and with a given optical detector resolution, the gradient of the magnetic field determines the resolution of the frequencies that can be measured. A lower gradient leads to a higher frequency resolution.

[0048] However, for a fixed diamond size and fixed optical detector resolution, the difference in the magnitude of the field determines the frequency bandwidth. A higher difference in the magnitude of the magnetic field leads to the ability to detect frequencies a larger frequency bandwidth.

[0049] This is illustrated in Figure 1 , in which a linear field allows a high resolution of frequency but a lower bandwidth.

[0050] In order to achieve a higher bandwidth, as described above, possible solutions include applying a moving magnetic field over the material, or using a large magnetic field at a greater distance from the material. Each of these solutions means that the detector will be bulkier and heavier.

[0051] The inventors have discovered that a structured magnet that provides an inhomogeneous magnetic field with at least one inflection point can provide large magnetic field gradients. This means that the magnetic field generator does not need to be moveable, and can be lighter and closer to the material, thereby allowing lighter and smaller devices to be produced.

[0052] This is illustrated in Figure 2, in which the magnetic field takes the form of a sine wave. This shape of field provides much higher gradients than are possible using a static linear magnetic field, meaning that a higher bandwidth of frequencies can be detected.

[0053] The term ‘inflection point’ is used herein to refer to a change in the sign of the gradient of the magnetic field. The inflection point has a non-zero value for the second order i derivative. The inflection point in Figure 2 is at frequency f2. The gradient of the sine wave between frequencies fi and fa is such that linearity of the magnetic field can be approximated over a relatively large linear dimension, and this can act on a material with a largest linear dimension of at least 0.5 mm.

[0054] Examples of fields that have inflection points include sin wave forms or sawtooth wave forms. In the case of a saw-tooth waves, imperfections in the magnetic field will provide non-zero values for the second order derivative of the inflection point.

[0055] A magnet geometry that generates a highly inhomogeneous magnetic field with at least one inflection point is therefore proposed across a single crystal diamond axis. As described above, in order to achieve a large bandwidth when analysing an RF signal, previous works have moved the magnet to achieve a bandwidth over tens of GHz. By optimizing the magnet geometry to generate a highly inhomogeneous magnetic field, one can remove the need to modify the magnet position allowing the creation of smaller and simpler devices.

[0056] Turning to Figure 3, there is shown an exemplary magnet geometry that can generate an inhomogeneous magnet field with an inflection point that provides regions with a high gradient. In this example, a teeth-like magnet structure is proposed.

[0057] Turning now to Figure 4, an exemplary radio frequency signal sensor 1 is illustrated. The radio frequency signal sensor 1 is provided with a quantum host material 2 that comprises at least one spin defect. In practice, for a useful device a plurality of spin defects is provided. A magnetic field generator 3 is provided to provide a magnetic field across the material. The magnetic field generator 3 is located in a static position relative to the material and is configured to generate an inhomogeneous magnetic field over a largest dimension of the material. An optical excitation source 4 is provided that can be used to excite the spin defect. A detector 5 is provided that is arranged to detect resonant frequencies from the spin defect.

[0058] A microprocessor 6 may also be provided for controlling the operation of the radio frequency signal sensor 1 , and various in / out communication devices 7 may also be provided to allow the radio frequency signal sensor 1 to communicate with other devices. The microprocessor 6 may also detect resonance frequencies by correlating a known result of a radio frequency interaction with the material. In this example, the magnetic field generator 3 comprises a magnet having opposing teeth with opposite polarity, as shown in Figure 3. This applies an inhomogeneous magnetic field over the largest dimension of the quantum host material 2. An example of such a magnet is a neodymium magnet. For a useful device, the largest dimension of the quantum host material is at least 2 mm and the magnetic field generator 3 is configured to provide a magnetic field gradient over the largest dimension of the quantum host material of at least 0.1 T / mm. To avoid SNR issues the magnetic field gradient over the largest dimension of the quantum host material 2 is expected to be no more than 1.0 T / mm.

[0059] The design using the magnetic field generator 3 shown in Figure 3 maintains a periodic gradient across a dimension of 2.4 mm and offers a maximum field gradient of 1.45 T / mm. This magnetic field gradient is orders of magnitude larger than sphere magnet used in the Magaletti paper described above, in which the gradient achieved is 0.032 T / mm, which allowed for a maximum bandwidth of 4 GHz. Variation of the magnet position in the Magaletti paper allowed a tuneable range over 25 GHz. The design described herein allows the same and greater tuneable ranges without having to move the position of the magnetic field generator 3. This provides several advantages; a first advantage is that there is no need to ‘sweep’ the magnetic field over the quantum host material to achieve the tuneable range, making readout quicker. A second advantage is that smaller devices can be produced as nothing is required to power and move a magnetic field generator 3. A third advantage is that there are fewer parts that may require servicing or replacement. The invention therefore provides an improved radio frequency signal sensor 1 over the prior art.

[0060] As the field strength and periodicity are determined by the dimensions of the teeth-like structure a modification of its dimensions can allow a gradient to optimize for high bandwidth RF sensing.

[0061] Turning now to Figure 5, there is shown a flow diagram that illustrates exemplary steps. The following numbering corresponds to that of Figure 5:

[0062] 51 . A quantum host material 2 is provided that comprises at least one spin defect.

[0063] 52. A highly inhomogeneous magnetic field having at least one inflection point is provided. The inflection point has a non-zero second order derivative value, and a gradient of at least 0.1 T / mm. This field is static relative to the material 2. At least a portion of the magnetic field acts on a largest dimension of the material. It may be that the magnetic field across the material has more than one inflection point, providing several regions with a high gradient. In this case, readings can be taken using several different regions and integrated.

[0064] 53. The quantum host material 2 is pumped using the optical excitation source 4.

[0065] 54. Resonance frequencies are detected from the spin defect using a detector 5. This may be achieved by, for example, correlating a known result of a radio frequency interaction with the material.

[0066] 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.

[0067] 1 U

Claims

CLAIMS:1 . A radio frequency signal sensor comprising: a material comprising at least one spin defect; a magnetic field generator arranged to provide a magnetic field across the material; an optical excitation source; a detector arranged to detect resonance frequencies from the spin defect; wherein the magnetic field generator is located in a static position relative to the material and is configured to generate an inhomogeneous magnetic field comprising at least one inflection point, the inflection point having a non-zero second order derivative value and a gradient of at least 0.1 T / mm.

2. The radio frequency signal sensor according to claim 1 , wherein at least a portion of the magnetic field including the inflection point acts on a largest dimension of the material.

3. The radio frequency signal sensor according to claim 1 or claim 2, wherein the largest dimension of the material is selected from any of at least 0.5 mm, at least 1 mm, at least 2 mm, at least 3 mm and at least 4 mm.

4. The radio frequency signal sensor according to any one of claims 1 to 3, wherein the magnetic field generator is configured to provide a magnetic field gradient over a largest dimension of the material selected from any of at least 0.2 T / mm, and at least 0.5 T / mm.

5. The radio frequency signal sensor according to any one of claims 1 to 4, wherein the magnetic field generator is configured to provide a magnetic field gradient over the largest dimension of the material of no more than 1 .0 T / mm.

6. The radio frequency signal detector according to any one of claims 1 to 5, wherein the detector is configured to detect radio frequency signals over a bandwidth selected from any of at least 1 GHz, at least 5 GHz, at least 10 GZ, at least 20 GHz and at least 50 GHz.

7. The radio frequency signal detector according to any one of claims 1 to 6, wherein the magnetic field generator comprises a neodymium magnet.

8. The radio frequency signal detector according to any one of claims 1 to 7, wherein the magnetic field generator comprises magnet having opposing teeth with opposite polarity.

9. The radio frequency signal detector according to any one of claims 1 to 8, wherein the material is selected from any of single crystal diamond and single crystal silicon carbide.

10. The radio frequency signal detector according to any one of claims 1 to 9, wherein the material is single crystal diamond and the spin defect is selected from any of a si I iconvacancy defects (Si-V), a silicon di-vacancy defect (Si-V2), a silicon-vacancy-hydrogen defect (Si-V:H), a silicon di-vacancy hydrogen defects (S-V2:H), a nickel containing defect, a chromium containing defects, a nitrogen-vacancy defect (N-V), a di-nitrogen vacancy defect (N-V-N), and a nitrogen-vacancy-hydrogen defect (N-V-H).11 . The radio frequency signal detector according to any one of claims 1 to 9, wherein the material is single crystal silicon carbide, and the spin defect is selected from any of a silicon-vacancy defects (Si-V), a silicon divacancy centre (Si-V2), and a nitrogenvacancy centre (NV).

12. A method of sensing radio frequency signals, the method comprising: providing a material comprising at least one spin defect; providing an inhomogeneous magnetic field comprising at least one inflection point, the inflection point having a non-zero second order derivative value and a gradient of at least 0.1 T / mm, the magnetic field being static relative to the material, and wherein at least a portion of the magnetic field acts on a largest dimension of the material; pumping the material using an optical excitation source; and detecting resonance frequencies from the spin defect using a detector.

13. The method according to claim 12, wherein the portion of the magnetic field acting on the largest dimension of the material included the inflection point.

14. The method according to claim 12or claim 13, wherein the largest dimension of the material is selected from any of at least 0.5 mm, at least 1 mm, at least 2 mm, at least 3 mm and at least 4 mm.

15. The method according to claim 13 or claim 14, further comprising providing a magnetic field gradient over a largest dimension of the material selected from any of at 0.2 T / mm, and at least 0.5 T / mm.

16. The method according to any one of claims 13 to 15, further comprising providing a magnetic field gradient over the largest dimension of the material of no more than 1 .0 T / mm.

17. The method according to any one of claims 13 to 16, further comprising detecting radio frequency signals over a bandwidth selected from any of at least 1 GHz, at least 5 GHz, at least 10 GZ, at least 20 GHz and at least 50 GHz.

18. The method according to any one of claims 13 to 17, wherein the material is selected from any of single crystal diamond and single crystal silicon carbide.

19. The method according to any one of claims 13 to 17, wherein the material is single crystal diamond and the spin defect is selected from any of a silicon-vacancy defects (Si-V), a silicon di-vacancy defect (Si-V2), a silicon-vacancy-hydrogen defect (Si- V:H), a silicon di-vacancy hydrogen defects (S-V2:H), a nickel containing defect, a chromium containing defects, a nitrogen-vacancy defect (N-V), a di-nitrogen vacancy defect (N-V-N), and a nitrogen-vacancy-hydrogen defect (N-V-H).

20. The method according to any one of claims 13 to 17, wherein the material is single crystal silicon carbide, and the spin defect is selected from any of a silicon-vacancy defects (Si-V), a silicon divacancy centre (Si- 2), and a nitrogen-vacancy centre (NV).21 . The method according to any one of claims 13 to 20, further comprising detecting resonance frequencies by correlating a known result of a radio frequency interaction with the material.

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