Radio frequency signal sensor

Abstract

A radio frequency signal sensor comprising a first module optimized for broadband radio frequency sensing, and a sub-module optimized for sensing at a pre-determined frequency. Any of the first module and the sub-module comprises a quantum host material comprising at least one spin defect, a magnetic field generator arranged to provide a non-uniform magnetic field across the quantum host material, an optical excitation source, and a detector arranged to detect resonance frequencies from the spin defect.

Description

[0001] RADIO FREQUENCY SIGNAL SENSOR

[0002] FIELD OF THE INVENTION

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

[0004] BACKGROUND

[0005] Point defects in quantum host materials, 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. Examples of quantum host materials include diamond and silicon carbide.

[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: (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);

[0009] (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.

[0013] Magaletti et al., Nature Communications Engineering 1 , 19 (2022) describes a similar system. Both the Chipaux and the Magaletti systems provide broadband resolution but with lower sensitivity, so is suitable when RF sensing where the frequency is unknown. The resolution is broadband over a range of frequencies because the application of a magnetic field across the diamond material is not uniform; the magnetic field will be stronger in some areas than others. This causes a difference in the resonance and allows signals to be detected over a range of frequencies. Typically this is provided by a magnet that has a field that is substantially non-uniform across the imaged diamond sample.

[0014] Wang et. al., Science Advances 8 (32) (2022) describes picotesla magnetometry of microwave fields using NV' centres in diamond. However, this provides narrower band resolution with higher sensitivity and so is suitable for RF sensing where the frequency is already known.

[0015] SUMMARY

[0016] The prior art RF sensors using NV' centres in diamond provide either broadband resolution with low sensitivity, or narrowband resolution with high sensitivity. An object of the invention is to provide a sensor system that provides both broadband resolution and high sensitivity.

[0017] According to a first aspect, there is provided a radio frequency signal sensor comprising a first module optimized for broadband radio frequency sensing, and a sub-module optimized for sensing at a pre-determined frequency. Any of the first module and the sub-module comprises a quantum host material comprising at least one spin defect, a magnetic field generator arranged to provide a non-uniform magnetic field across the quantum host material, an optical excitation source, and a detector arranged to detect resonance frequencies from the spin defect.

[0018] The signal to noise ratio (SNR) scales by sqrt(N), where N is the number of NV' defects in the diamond sampled. If an entire quantum host material used for sensing was dedicated to sensing only at 4 GHz, then we would apply a highly uniform magnetic field which would result in a detector that is sensitive but narrowband. By applying a magnetic field gradient across a quantum host diamond, we give the system broadband capabilities, but the sensitivity is smaller as the number of defects that are contributing to signal is reduced.

[0019] Once the first module has detected a signal at a given frequency, that frequency becomes the pre-determined frequency and the sub-module is then used to sense at that pre-determined frequency. As the sub-module is configured to sense at that predetermined frequency, the SNR is much improved and so better information can be obtained at the pre-determined frequency.

[0020] As an option, the sub-module is tunable to a desired frequency.

[0021] As an option, the radio frequency signal sensor further comprising a plurality of submodules. This allows multiple frequencies to be queried simultaneously.

[0022] Examples of first quantum host materials include diamond and silicon carbide.

[0023] As an alternative option, the first module comprises a Rydberg atom.

[0024] Alternatively, the first module is a passive module optimized for broadband radio frequency sensing.

[0025] The second magnetic field generator optionally comprises a solenoid. This has the advantage of being tunable and creating a uniform magnetic field.

[0026] However, it should be noted that in an alternative option, the sub-module need not comprise diamond material. There are non-diamond RF narrowband sensors that are highly sensitive that can serve equally well.

[0027] The radio frequency signal optionally further comprises a processor configured to control the first module and the sub-module.

[0028] According to a second aspect, there is provided a method of radio frequency signal analysis using the radio frequency signal sensor described above in the first aspect. The method comprises: obtaining a first signal from a radio frequency signal sensor first module optimized for broadband radio frequency sensing; and obtaining a second signal from a radio frequency signal sensor sub-module optimized for sensing at a pre-determined frequency.

[0029] As an option, the method further comprises tuning the pre-determined frequency of the radio frequency signal sensor sub-module.

[0030] As an option, the method further comprises tuning the pre-determined frequency of the radio frequency signal sensor sub-module in response to the first signal.

[0031] As an option, the method comprises tuning the predetermined frequency using a magnetic field generated by a solenoid.

[0032] BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which:

[0034] Figure 1 illustrates schematically an exemplary RF sensor;

[0035] Figure 2 illustrates schematically the sensor modules of the RF sensor illustrated in Figure 1; and

[0036] Figure 3 is a flow diagram showing exemplary steps of RF sensing using the RF sensor of Figure 1.

[0037] 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.

[0038] DETAILED DESCRIPTION

[0039] In order to detect a signal having broadband resolution and also high sensitivity, an RF sensor system is proposed that has multiple modules. A first module is a broadband RF sensor optimized for broadband RF frequencies detection. The first module is able to detect multiple signals in parallel but with a lower signal to noise ratio (SNR). Once the frequency values of potential frequencies of interest have been determined, a submodule, which is tuneable in frequency, is then be adjusted to sense at one of the given frequencies. Multiple sub-modules can be employed in parallel to allow higher-SNR detection over multiple frequencies. Each sub-module has a significantly higher SNR than the first module. Referring to Figure 1 , there is illustrated an RF sensor system 1 that has a first module 2 optimized for broadband RF frequencies detection, and a first and second sub-module 3, 4 that are optimised to sense at given frequencies. In the exemplary system of Figure 1 , the first module 2 and the sub-modules 3, 4 are controlled by a microprocessor 5. The skilled person will appreciate that the system 1 might have only one sub-module or more than two sub-modules, depending on the specific requirements of the sensor.

[0040] In the following description of the modules, diamond is used by way of example. However, it will be appreciated that other quantum host materials, such as silicon carbide, may be used, or other types of RF sensors such as Rydberg atoms.

[0041] Turning now to Figure 2, the first module 2 comprises a layer of single crystal diamond 6 that includes at least one NV' centre. Such diamond can be obtained using, for example, the processes described in WO 2017 / 112399. A first antenna 7 is also provided in proximity to the diamond 6, the first antenna being configured to respond to an RF signal. A first laser 8, for example a 532 nm laser, is provided for pumping the diamond 6. Note that an optical excitation sources other than a laser could be used. A first magnetic field generator 9 is also provided to cause resonance lines to split depending on the applied magnetic field magnitude and its direction, as described above. Finally, a first detector 10 is also provided to detect resonance frequencies from the NV' centre.

[0042] The magnetic field generator 9 provides a non-uniform magnetic field across the single crystal diamond layer 6, and so different NV- centres at different locations in the single crystal diamond layer 6 will respond to an RF source in different ways. This allows the first module 2 to analyse a range of frequencies, although it will have a poor signal to noise ratio. The detector 10 can detect which frequencies contain information and require further analysis.

[0043] A first sub-module 3 is also provided. In this exemplary embodiment, the first submodule 3 comprises a second layer of single crystal diamond 10 that includes at least one NV' centre. A second antenna 11 is also provided in proximity to the layer of single crystal diamond 10. A second laser 12 is provided for pumping the second layer of single crystal diamond 10. A second magnetic field generator 13 is also provided to cause resonance lines to split depending on the applied magnetic field magnitude and its direction, as described above. However, in this instance the second magnetic field generator 13 applies a uniform magnetic field across the entire second single crystal diamond layer 10, ensuring that while the second single crystal diamond layer 10 can only be used to analyse a specific frequency, it will have a much better signal to noise ratio than the first module described above. This ensures that while the first module 2 can analyse which frequencies may require analysis, the first sub-module 3 can analyse a signal at a given frequency as determined by the first module 2.

[0044] As an example, the second magnetic field generator 13 may be a solenoid that applies a uniform magnetic field across the second single crystal diamond layer. The solenoid can be operated at a frequency determine by the analysis of the first module. Alternatively, a Helmholtz coil may be used.

[0045] Finally, a second detector 14 is also provided to detect resonance frequencies from the NV' centre and analyse the signal at the predetermined frequency.

[0046] Of course, the RF sensor system 1 may comprise further sub-modules to allow the analysis of signals at other predetermined frequencies. In the example of Figures 1 and 2, a second sub-module 4 is provided. The second sub-module 3 comprises a third layer of single crystal diamond 15 that includes at least one NV' centre. A third antenna 16 is also provided in proximity to the third layer of single crystal diamond 15. A third laser 17 is provided for optically pumping the third layer of single crystal diamond 15. A third magnetic field generator 18 is also provided to cause resonance lines to split depending on the applied magnetic field magnitude and its direction, as described above. The third magnetic field generator 18 applies a uniform magnetic field across the entire third single crystal diamond later 15, ensuring that while the third single crystal diamond layer 15 can only be used to analyse a specific frequency, it will have a much better signal to noise ratio than the first module described above.

[0047] Again, a suitable exemplary third magnetic field generator 18 is a solenoid that applies a uniform magnetic field across the third single crystal diamond layer 15. The solenoid can be operated at a frequency determined by the analysis of the first module.

[0048] Finally, a third detector 19 is also provided to detect resonance frequencies from the NV' centre and analyse the signal at the predetermined frequency.

[0049] It will be appreciated that some components, such as the laser and the antenna, may be shared between the first module 2, the first sub-module 3 and the second sub-module 4. In the example described above, the sub-modules are diamond material containing at least one NV' centre. However, embodiments can be envisaged in which the first module is a diamond material comprising at least one NV' centre under an inhomogeneous magnetic field, but one or more of the sub-modules comprise non-diamond narrowband RF sensors. Examples of such sub-modules include a module that senses using Rydberg atoms, a network analyser or other narrowband electronic detection mechanism.

[0050] The first module 2 is a low-SNR broadband sensor that can detect over a range of frequencies (typically of the order of several GHz), owing to the application of an inhomogeneous magnetic field applied across the longest dimension of a diamond. The inhomogeneous magnetic field can be varied by altering the magnetic position to provide both a large instantaneous bandwidth and an overall large frequency bandwidth for the system.

[0051] The diamond would contain NV' centres which emit red fluorescence under green illumination. The signal from the NV' centres for both the first module and the submodules can be detected either through the emitted NV' fluorescence or by directly measuring the charges generated. In other words, the signal can be detected either through optically detected magnetic resonance (ODMR) or photoelectric detection of magnetic resonance (PDMR). Detection through ODMR involves the use of a camera with a sufficient resolution to detect the NV' fluorescence. An example of such a camera is a lock-in camera, which enhances the sensitivity and makes the module more sensitive. Detecting the signal through PDMR requires depositing wires on the diamond surface and connecting these wires to a CMOS detector. Any external RF signal is converted to a magnetic field that will be applied to the diamond and the charges created will be detected using the wires and directed to the CMOS detector. The use of PDMR gives rise to a more compact set-up.

[0052] The purpose of the first module 2 is to detect unknown frequencies over a large range. The sub-modules 3, 4 are designed so that the magnetic field produced across the diamond is highly uniform, as a solenoid or other magnetic field generator that generates a uniform magnetic field is used to generate the magnetic field. The field for the submodules 3, 4 is tuneable such that each sub-module 3, 4 can be independently adjusted to detect at a single frequency detected by the first module 2. Each sub-module 3, 4 is optimized such that the entire signal across the diamond can be dedicated to a single frequency, vastly improving the SNR for the unknown signal.

[0053] Turning now to Figure 3, a flow diagram showing exemplary steps is illustrated. The following numbering corresponds to that of Figure 3:

[0054] 51. A radio frequency signal sensor first module is used to analyse a signal over a wide range of frequencies using an inhomogeneous magnetic field as described above. Ideally the inhomogeneous magnetic field varies substantially linearly across the material, but inhomogeneous magnetic fields that vary non-linearly can also be used.

[0055] 52. Using the information obtained by the first module, a frequency is determined that requires further analysis.

[0056] 53. A sub-module with a tuneable homogeneous magnetic field is used to analyse the signal at the determined frequency, giving a much better SNR than that obtained by the first module. As described above, further sub-modules can be provided that allow multiple individual frequencies to be analysed at the same time. Alternatively or additionally, the solenoid can be dynamically tuned to ‘sweep’ frequencies of interest.

[0057] Either or both of the first module and the sub-module comprises a quantum host material with at least one spin defect, a magnetic field generator arranged to provide a non- uniform magnetic field across the quantum host material, an optical excitation source, and a detector arranged to detect resonance frequencies from the spin defect.

[0058] 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 radio frequency signal sensor comprising: a first module optimized for broadband radio frequency sensing; a sub-module optimized for sensing at a pre-determined frequency; and wherein any of the first module and the sub-module comprises: a quantum host material comprising at least one spin defect; a magnetic field generator arranged to provide a non-uniform magnetic field across the quantum host material; an optical excitation source; and a detector arranged to detect resonance frequencies from the spin defect.

2. The radio frequency signal sensor according to claim 1 , wherein the sub-module is tunable to a desired frequency.

3. The radio frequency signal sensor according to claim 1 or claim 2, further comprising a plurality of sub-modules.

4. The radio frequency signal sensor according to any one of claims 1 to 3, wherein the quantum host material is selected from any of diamond and silicon carbide.

5. The radio frequency signal sensor according to any one of claims 1 to 4, wherein the first module comprises a Rydberg atom.

6. The radio frequency signal sensor according to any one of claims 1 to 4, wherein the first module comprises a passive module optimized for broadband radio frequency sensing.

7. The radio frequency signal sensor according to claim 9, wherein the magnetic field generator comprises a solenoid.

8. The radio frequency signal sensor according to any one of claims 1 to 7, further comprising a processor configured to control the first module and the sub-module.

9. A method of radio frequency signal analysis using the radio frequency signal sensor according to any one of claims 1 to 8, the method comprising:obtaining a first signal from the radio frequency signal sensor first module optimized for broadband radio frequency sensing; and obtaining a second signal from the radio frequency signal sensor sub-module optimized for sensing at a pre-determined frequency.

10. The method according to claim 9, further comprising tuning the pre-determined frequency of the radio frequency signal sensor sub-module.

11. The method according to claim 9, further comprising tuning the pre-determined frequency of the radio frequency signal sensor sub-module in response to the first signal.

12. The method according to any one of claims 10 or 11 , further comprising tuning the predetermined frequency using a magnetic field generated by a solenoid.

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