Magnetometer

Abstract

A magnetometer is described. The magnetometer comprises a plurality of vacancy centres embedded in a body of material, a magnet arranged to apply a bias magnetic field to the plurality of vacancy centres along a first axis, a microwave source arranged to apply a microwave field to the plurality of vacancy centres, a probe laser source arranged to apply a probe laser to the plurality of vacancy centres along or parallel to the first axis, wherein the probe laser is polarised along a second axis orthogonal to the first axis, an excitation laser source arranged to apply an excitation laser to the plurality of vacancy centres along a third axis orthogonal to the first axis, and detection apparatus for measuring a change in a plane of polarisation of the probe laser.

Description

[0001] Magnetometer

[0002] Field

[0003] The present invention relates to a magnetometer and methods of operating a magnetometer.

[0004] Background

[0005] A magnetometer is a device which measures the strength of a magnetic field. Magnetometers employing different measuring schemes are known in the art. Atomic magnetometers detect magnetic field strength by measuring a change in the plane of polarisation of a laser; the polarised laser interacts with the spin states of atoms via the Faraday effect. Vacancy centre-based magnetometers, such as nitrogen vacancy centrebased magnetometers, detect a change in red fluorescence emitted by the vacancy centres when excited with an excitation laser and a microwave in order to determine the magnetic field strength.

[0006] A conventional nitrogen vacancy-based magnetometer is discussed in S. M. Graham et al.: “Fibre-coupled Diamond Magnetometry with an Unshielded i pT / VHz Sensitivity”, Phys. Rev. Appl. 19, 04402, 2023.

[0007] These conventional magnetometers have several drawbacks.

[0008] For example, atomic magnetometers must be operated inside a magnetically shielded environment (where the strength of the background magnetic field is in the order of a nanotesla) to achieve high sensitivity. Furthermore, the material containing the magnetically-sensitive atoms within the magnetometer must be heated significantly above room temperature, typically to between too°C and 200°C. The sensitivity of conventional vacancy centre-based magnetometers is limited by the proportion of total emitted fluorescence they are capable of collecting. These magnetometers are also limited by the power of the excitation laser. Increased excitation laser power enhances the sensitivity. However, the power required for such a laser makes the magnetometer hazardous to operate and creates other operational problems. Summary

[0009] According to a first aspect of the present invention, there is provided a magnetometer comprising a plurality of vacancy centres embedded in a body of material, a magnet arranged to apply a bias magnetic field to the plurality of vacancy centres along a first axis, a microwave source arranged to apply a microwave field to the plurality of vacancy centres, a probe laser source arranged to apply a probe laser to the plurality of vacancy centres along or parallel to the first axis, wherein the probe laser is polarised along a second axis orthogonal to the first axis, an excitation laser source arranged to apply an excitation laser to the plurality of vacancy centres along a third axis orthogonal to the first axis, and detection apparatus for measuring a change in a plane of polarisation of the probe laser.

[0010] The magnetometer according to the present invention has improved sensitivity (and, therefore, accuracy) compared to conventional magnetometers, specifically magnetometers using a fluorescence detection-based measuring scheme such as the vacancy centre-based magnetometers hereinbefore described.

[0011] The present magnetometer uses an optical plane of polarization-based measuring scheme, wherein collection of fluorescence is not required for measuring the magnetic field strength.

[0012] Vacancy centres are also herein referred to as “vacancy defects” or “defect centres”.

[0013] The magnetometer may be configured to determine the strength of a magnetic field according to the change in the plane of polarization of the probe laser.

[0014] The magnet may be a permanent magnet. The magnet may be an electromagnet. The magnet may be suitable for generating static magnetic fields. The magnetometer may be operable above myogenic temperatures.

[0015] As used herein, the term “ciyogenic temperatures” refers to any temperature below about 120 K. The magnetometer may be operable at ciyogenic temperatures. The magnetometer may be operable at temperatures between i mK and 500 K.

[0016] The magnetometer may be operable at room temperature. As used herein, the term “room temperature” refers to temperatures between about 15°C and 3O°C.

[0017] The plurality of vacancy centres may consist of between 0.1 ppb and too ppm vacancy centres.

[0018] The body of material may be diamond. The diamond may be single-ciystal diamond.

[0019] The vacancy centres may be negatively charged nitrogen vacancy centres. The vacancy centres may be negatively charged silicon vacancy centres, germanium vacancy centres, tin vacancy centres, or lead vacancy centres.

[0020] The first axis may be the axis of symmetry of the vacancy centres. The axis of symmetry is defined by the crystallographic orientation of the vacancy centres in the material. For example, nitrogen vacancy centres exist in one of four crystallographic orientations.

[0021] Therefore, for nitrogen vacancy centres, the first axis may be along one of the four axes of symmetry of the nitrogen vacancy centres.

[0022] The magnetometer may further comprise an optical cavity. Use of the optical cavity may help the magnetometer to detect a magnetic field having a strength of at least 1 x io18T.

[0023] The optical cavity may be formed of two mirrors and the body of material may be provided between the two mirrors.

[0024] The optical cavity may be formed of a mirror coating provided on at least a portion of the surface of the body of material.

[0025] The mirror coating may completely cover the surface of the body of material. The mirror coating may be provided on portions of the surface which the probe laser intersects. The detection apparatus may comprise a polarising beam splitter and a balanced photodetector. The balanced photodetector may comprise first and second photodiodes. An imbalance between output signals of the first and second photodiodes may be indicative of a change in the plane of polarisation of the probe laser.

[0026] The magnetometer may be for use in continuous wave magnetometry or pulsed magnetometry.

[0027] According to a second aspect of the present invention, there is provided a method of operating the magnetometer according to the first aspect of the present invention. The method comprises determining the strength of a magnetic field according to a change in the plane of polarisation of the probe laser.

[0028] The method may follow a continuous wave scheme or a pulsed scheme.

[0029] The pulsed scheme may be a Ramsey scheme. The pulsed scheme may be a TT pulse scheme.

[0030] According to a third aspect of the present invention, there is provided a method of operating a magnetometer. The method comprises providing a body of material comprising a plurality of vacancy centres, applying a bias magnetic field to the vacancy centres, applying a microwave field and an excitation laser simultaneously to the vacancy centres, applying a polarised probe laser to the plurality of vacancy centres, applying an unknown magnetic field to the plurality of vacancy centres, and determining the strength of the unknown magnetic field according to a change in the plane of polarization of the probe laser.

[0031] According to a fourth aspect of the present invention, there is provided a method of operating a magnetometer. The method comprises providing a body of material comprising a plurality of vacancy centres, applying a bias magnetic field to the vacancy centres, applying an excitation laser pulse to the vacancy centres, applying a first 71 / 2 microwave pulse to the vacancy centres, applying an unknown magnetic field to the vacancy centres, applying a second 71 / 2 microwave pulse to the vacancy centres, applying a polarised probe laser to the plurality of vacancy centres, and determining the strength of the unknown magnetic field according to a change in the plane of polarisation of the probe laser. The method may comprise repeating an application sequence at least once. The application sequence consists of the application of the excitation laser pulse, the first TT / 2 microwave pulse, the unknown magnetic field, the second rm / 2 microwave pulse, and the polarised probe laser. The application sequence may be repeated between 1 and 100,000 times.

[0032] After each application sequence, the strength of the unknown magnetic field may be determined. An average value of the magnetic field strength may be determined from the values determined for each iteration of the application sequence.

[0033] According to a fifth aspect of the present invention, there is provided a method of operating a magnetometer. The method comprises providing a body of material comprising a plurality of vacancy centres, applying a bias magnetic field to the vacancy centres, applying an excitation laser pulse to the vacancy centres, applying a TT microwave pulse to the vacancy centres, applying an unknown magnetic field to the vacancy centres, applying a polarised probe laser to the plurality of vacancy centres, and determining the strength of the unknown magnetic field according to a change in the plane of polarisation of the probe laser.

[0034] The body of material may be provided in an optical cavity prior to application of the bias magnetic field. Brief Description of Drawings

[0035] Certain embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings in which:

[0036] Figure 1 is a schematic block diagram of a magnetometer; Figure 2 is an energy level diagram of a nitrogen vacancy centre;

[0037] Figure 3 schematically illustrates components within a magnetometer;

[0038] Figure 4 schematically illustrates a body of material within an optical cavity;

[0039] Figure 5 is a process flow diagram of a first method of operating a magnetometer;

[0040] Figure 6 is a process flow diagram of a second method of operating a magnetometer; and

[0041] Figure 7 is a process flow diagram of a third method of operating a magnetometer.

[0042] Detailed Description of Certain Embodiments

[0043] In the following, like parts are denoted by like references.

[0044] The present application is concerned with a magnetometer which uses an optical plane of polarization-based measuring scheme for detecting the strength of a magnetic field. The present magnetometer uses the spin states of vacancy centres, preferably nitrogen vacancy centres, to detect the magnetic field strength.

[0045] Referring to Figure 1, a magnetometer 1 is shown.

[0046] The magnetometer 1 comprises a body of material 2 (or “host material”) into which are embedded (or “positioned”) a plurality of vacancy centres 3. As will be hereinafter explained, the vacancy centres 3 interact with an applied probe laser to change the plane of polarization of that probe laser via the Faraday effect. This change in the plane of polarization is used by the magnetometer 1 to detect the strength of an unknown magnetic field. Preferably, the body of material 2 is diamond, for example single crystal diamond, and the vacancy centres 3 are negatively charged nitrogen vacancy centres (“NV centres”). However, any suitable host material 2 and vacancy centres 3 may be used. For example, negatively charged silicon vacancy centres, germanium vacancy centres, tin vacancy centres, or lead vacancy centres may be used as the vacancy centres 3 in a diamond host material 2. The body of material 2 includes between 0.1 ppb and too ppm vacancy centres 3. Using a body of material 2 with a high density of vacancy centres 3, compared to typical magnetometers, may help to reduce signal noise when the magnetometer 1 is operated at around room temperature.

[0047] Referring also to Figure 2, an example energy level diagram is shown. The energy level diagram is for NV centres.

[0048] The magnetometer 1 further includes a magnet 4. The magnet 4 is arranged to apply a bias magnetic field to the plurality of vacancy centres 3 along a first axis. As for a typical magnetometer, application of the bias magnetic field to the vacancy centres 3 removes the degeneracy between the ms= ±1 spin state of the vacancy centres 3 via the Zeeman effect. As will be hereinafter explained, the magnetically sensitive nature of either the ms=+1 spin state or the ms= -1 spin state can be exploited in measuring an unknown magnetic field. Without being bound by theoiy, the unknown magnetic field can modulate the spin population of the ms= +1 spin state or the ms= -1 spin state. For the example methods of operation hereinafter described, the ms= +1 is utilised.

[0049] Any suitable magnet 4 may be used. The magnet 4 is preferably suitable for generating static magnetic field so that the bias magnetic field is a static field. When the vacancy centres 3 are NV centres, the bias magnetic field may be in the order of a few microTesla to 1 T.

[0050] The magnetometer 1 further comprises a probe laser source 5, a microwave source 6, and an excitation laser source 7.

[0051] The microwave source 6 is configured to generate a microwave field and is arranged to apply that microwave field to the plurality of vacancy centres 3. Application of a microwave field to the vacancy centres 3 causes at least some of the vacancy centres 3 in the ms= o spin state to be excited into one of the higher energy level spin states ms= +1 or ms= -1. The ms= o, ms= +1, and ms= -1 spin states herein referred to exist in the ground state manifold3A2, as shown in Figure 2.

[0052] The microwave source 6 may take the form of a wire (not shown), such as a copper wire, which is in contact with or in close proximity to the body of material 2. The frequency of the microwave field is dependent on the strength of the bias magnetic field, but is typically in the range of between 2 GHz and 4 GHz.

[0053] The excitation laser source 7 is arranged to apply an excitation laser to the vacancy centres 3. When the excitation laser is applied to the vacancy centres 3, at least some of the vacancy centres 3 are excited into the excited state manifold3E (Figure 2). Each excited vacancy centre 3 eventually returns (“de-excites”) to the ground state manifold and predominantly to the ms= o spin state. In this way, the excitation laser spin polarises the plurality of vacancy centres 3. The excited vacancy centres 3 may de-excite via one or more intermediate states, such as, for NV centres, the two intermediate singlet states 'A, and 'E (Figure 2).

[0054] In an example mode of operation in which the ms= +1 spin state population is used to measure the unknown magnetic field, an excitation laser (such as a green excitation laser) is used to excite at least some vacancy centres 3 from the ground state manifold3A2into the excited state manifold3E.

[0055] When NV centres are used as the vacancy centres 3 embedded in a diamond host material 2, the excitation laser has a wavelength of between 500 nm and 640 nm, preferably 532 nm.

[0056] The probe laser source 5 is configured to generate the probe laser hereinbefore described along or parallel to the first axis. The probe laser is polarised along a second axis orthogonal to the first axis. The excitation laser is applied to the plurality of vacancy centres 3 along a third axis orthogonal to the first axis.

[0057] The probe laser has a frequency not resonant with the energy level transitions of the vacancy centres 3— either between the ms= o and ms= ±1 spin states or between the ground state manifold,3A2, and the excited state manifold,3E. For example, where NV centres are used as the vacancy centres 3, the probe laser has a frequency between 800 nm and 1600 nm, for example 1064 nm.

[0058] Figure 3 shows an example orientation of the excitation laser and probe laser each applied to the body of material 2. In this example, the first axis is along the z axis and the third axis is along the x axis. The first axis may be the axis of symmetry of the vacancy centres 3. The axis of symmetry is defined by the crystallographic orientation of the vacancy centres 3 in the body of material 2. Therefore, the first, second, and third axes may be defined by the type of host material 2 and vacancy centres 3 employed in the magnetometer 1. The magnetometer 1 further comprises detection apparatus 8. The detection apparatus

[0059] 8 is arranged to receive the probe laser after the probe laser has been applied to the vacancy centres 3 (“rotated probe laser”).

[0060] The detection apparatus 8 is for measuring a change in the plane of polarisation of the probe laser.

[0061] In one example, the detection apparatus 8 comprises a polarising beam splitter (“PBS”)

[0062] 9 and a balanced photodetector 10. The balanced photodetector 10 includes first and second photodiodes 11, Hi, n2. The PBS 9 is configured to split the rotated probe laser into first and second components, each component having a different plane of polarization.

[0063] The first photodiode Hi is arranged to receive the first component of the rotated probe laser and the second photodiode n2is arranged to receive the second component of the rotated probe laser. Each photodiode 11 is configured to generate an electrical signal (e.g. current signal) indicative of the received component of the rotated probe laser.

[0064] When no magnetic field is applied, the photodiodes 11 are configured such that their output signals balance and the balanced photodetector 10 outputs either a constant electrical signal or no signal. When a magnetic field is applied, the balance between the output signals of the photodiodes 11 is perturbed because the plane of polarisation of the probe laser is rotated. Thus, the balanced photodetector 10 is configured to output an electrical signal indicative of the imbalance between the output signals of the first and second photodiodes lOi, io2. In this way, the output signal of the photodetector 10 is indicative of a change in the plane of polarisation of the probe laser.

[0065] The balanced photodetector 10 can be calibrated by applying known magnetic fields. Once calibrated, the output signal of the balanced photodetector 10 can be used to determine the strength of an unknown magnetic field. As hereinbefore stated, the unknown magnetic field can modulate the spin population of the ms= +1 spin state. Thus, the output signal of the balanced photodetector to is proportional to the difference in the number of vacancy centres 3 in the ms= +1 spin state before and after the unknown magnetic field is applied.

[0066] Referring also to Figure 4, optionally, the magnetometer includes an optical cavity 12.

[0067] In one example, the optical cavity 12 is formed of two mirrors 13, as shown in Figure 4. The body of material 12 is provided between the mirrors 13. In another example, the optical cavity 12 takes the form of a mirror coating (not shown) on the surface of the body of material 2. The mirror coating may be on all surfaces of the body of material 2. The mirror coating may be on the surfaces of the body of the material 2 which the probe laser intersects only (in other words, the entrance and exit surfaces of the body of material 2).

[0068] The mirror coating may be formed of multiple layers of different materials. The mirror coating may be formed of tantalum pentoxide (Ta2O5), aluminium oxide (A12O3), or hafnium oxide (Hf02). The optical cavity 12 may help to enhance the Faraday effect. This is done by confining the probe laser within the cavity 12 for the duration of the spin coherence time of the vacancy centres 3, T*2, which in turn increases the interaction time between the probe laser and the vacancy centres 3. The finesse of the optical cavity is dictated by the spin coherence time and can be adjusted as required. In one example, the optical cavity 12 has a finesse of at least too, preferably 5,000. In this way, use of the optical cavity 12 may help the magnetometer to detect a magnetic field having a strength of at least 1 x io18T.

[0069] The magnetometer 1 according to the present application is operable at or above cryogenic temperatures. Advantageously, the magnetometer 1 is operable at room temperature.

[0070] Method of operation

[0071] The present application is further concerned with a method of operating a magnetometer, such as a nitrogen vacancy centre magnetometer. The method involves measuring a change in a plane of polarisation of a probe laser due to the interaction of the probe laser with the vacancy centres via the Faraday effect.

[0072] The magnetometer may be operated according to a continuous wave scheme or a pulsed scheme.

[0073] By way of illustration, the following example methods of operation are described with reference to Figures i to 4. Referring also to Figure 5, a first example method will now be described. The first example method is a continuous wave scheme method employed by the magnetometer 1.

[0074] First, the vacancy centres 3 are applied with the bias magnetic field (step S1.1). The bias magnetic field is applied throughout the first method. Preferably, the bias magnetic field is static.

[0075] Then, the vacancy centres 3 embedded within the body of material 2 are applied with the microwave field and the excitation laser simultaneously (step Si.2).

[0076] The frequency of the microwave field is selected such that at least some of the vacancy centres 3 in the ms= o spin state are excited into the ms= +1 spin state. The excitation laser spin polarises at least some of the vacancy centres to the ms= o spin state as hereinbefore described.

[0077] The microwave field and excitation laser are applied continuously throughout the remining steps of the first method.

[0078] The vacancy centres 3 are then applied with the probe laser (step S1.3). Interaction via the Faraday effect with at least some of the vacancy centres 3, as hereinbefore described, results in the rotation of the plane of polarisation of the probe laser by an angle, 0F.

[0079] During step S1.3, the balanced photodetector 10 is outputting either a constant signal or no signal. Whilst the probe laser is applied, the unknown magnetic field (in other words, the magnetic field to be measured) is applied to the vacancy centres 3 (Step S1.4).

[0080] The magnitude of the rotation angle 0F is proportional to the number of vacancy centres 3 in the ms= +1 state. In the presence of the unknown magnetic field, the number of vacancy centres 3 in the ms= +1 state is reduced since the resonance frequency changes (in other words, the energy difference between the ms= o and the ms= +1 states in the ground state manifold,3A2). This change is reflected in a corresponding change in the plane of polarisation of the probe laser.

[0081] Thus, during step Si.4, the balanced photodetector 10 outputs a signal (if no signal was output in step S1.3) or the output signal changes (if a constant signal was output in step S1.3). This output signal is indicative of the change in the plane of polarisation of the probe laser.

[0082] The change in the plane of polarisation of the probe laser is measured (Step S1.5) using the detection apparatus 8 as hereinbefore described. The change in the plane of polarisation is used to measure the magnetic field strength of the unknown magnetic field (Step Si.6). This may be done by comparison of the output signal of step S1.5 with one or more signals obtained during calibration of the detection apparatus 8.

[0083] Referring also to Figure 6, a second example method will now be described. The second example method is a pulsed scheme method. This method uses a Ramsey scheme. First, the bias magnetic field is applied (step S2.1) in the same way as the first example method.

[0084] Then, a pulse of the excitation laser is applied to the vacancy centres (step S2.2), followed by a 71 / 2 pulse of the microwave field (step S2.3). Steps S2.2 and 2.3 in combination result in the preparation of at least some of the vacancy centres in the ( | ms= o) + |ms= +i)) / 2 spin superposition state. The duration of each pulse in steps S2.2 and S2.3 are equal to the spin coherence time, T*2, of the vacancy centres 3.

[0085] Next, the unknown magnetic field is applied to the vacancy centres 3 (step S2.4) and the spin superposition state interacts with the unknown magnetic field for T*2 seconds. Whilst the unknown magnetic field is being applied, a further TT / 2 micro wave pulse is applied (step S2.5) for T*2 seconds. This latest pulse, depending on the strength of the unknown magnetic field, determines the number of vacancy centres in the ms= +1 spin state.

[0086] Immediately after step S2.5, the probe laser is applied to the vacancy centres 3 (step S2.6). In the same way as for the first example method, a change in the plane of polarisation of the probe laser is measured (Step S2.7), and this change is used to determine the magnetic field strength of the unknown magnetic field (Step S2.8).

[0087] This pulse sequence (steps S2.2 to S2.5) along with the application of the probe laser (step S2.6) may be repeated between 1 and 100,000 times. After each iteration, the strength of the magnetic field is determined in the way hereinbefore described. An average of the magnetic field strength values can then be calculated to provide a more accurate value for the strength of the unknown magnetic field.

[0088] Referring now to Figure 6, a JT pulse scheme can be used in place of the Ramsey scheme. Steps S3.1 and S3.2 of the method which utilises the TT pulse scheme (“third example method”) are the same as steps S2.1 and S2.2 of the second example method hereinbefore described. However, the first and second n / 2 microwave pulses (steps S2.3 and S2.5) are replaced by a single TT microwave pulse (step S3.3 of Figure 6). In other words, the TT microwave pulse is applied, followed by application of the unknown magnetic field (step S3.4) . Steps S3.5 to S3.7 of the third example method are the same as steps S2.6 to S2.8 of the second example method.

[0089] Modifications

[0090] It will be appreciated that various modifications may be made to the embodiments hereinbefore described. Such modifications may involve equivalent and other features which are already known. Features of one embodiment may be replaced or supplemented by features of another embodiment.

[0091] Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel features or any novel combination of features disclosed herein either explicitly or implicitly or any generalization thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicant hereby gives notice that new claims may be formulated to such features and / or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Claims

Claims1. A magnetometer comprising : a plurality of vacancy centres embedded in a body of material; a magnet arranged to apply a bias magnetic field to the plurality of vacancy centres along a first axis; a microwave source arranged to apply a microwave field to the plurality of vacancy centres; a probe laser source arranged to apply a probe laser to the plurality of vacancy centres along or parallel to the first axis, wherein the probe laser is polarised along a second axis orthogonal to the first axis; an excitation laser source arranged to apply an excitation laser to the plurality of vacancy centres along a third axis orthogonal to the first axis; and detection apparatus for measuring a change in a plane of polarisation of the probe laser.

2. The magnetometer of claim 1, wherein the magnetometer is configured to determine the strength of a magnetic field according to the change in the plane of polarization of the probe laser.

3. The magnetometer of claims 1 or 2, wherein the magnetometer is operable above myogenic temperatures.

4. The magnetometer of any preceding claim, wherein the magnetometer is operable at temperatures between 1 mK and 500 K.

5. The magnetometer of any preceding claim, wherein the magnetometer is operable at room temperature.

6. The magnetometer of any preceding claim, wherein the plurality of vacancy centres consists of between 0.1 ppb and too ppm vacancy centres.

7. The magnetometer of any preceding claim, wherein the body of material is diamond.

8. The magnetometer of claim 7, wherein the vacancy centres are negatively charged nitrogen vacancy centres.

9. The magnetometer of claim 7, wherein the vacancy centres are negatively charged silicon vacancy centres, germanium vacancy centres, tin vacancy centres, or lead vacancy centres.

10. The magnetometer of any preceding claim, the magnetometer further comprising: an optical cavity.

11. The magnetometer of claim 10, wherein the optical cavity is formed of two mirrors and the body of material is provided between the two mirrors.

12. The magnetometer of claim 10, wherein the optical cavity is formed of a mirror coating provided on at least a portion of the surface of the body of material.

13. The magnetometer of any preceding claim, wherein the detection apparatus comprises: a polarising beam splitter; and a balanced photodetector.

14. The magnetometer of claim 13, wherein the balanced photodetector comprises first and second photodiodes, and wherein an imbalance between output signals of the first and second photodiodes is indicative of a change in the plane of polarisation of the probe laser.

15. The magnetometer of any preceding claim, wherein the magnetometer is for use in continuous wave magnetometry or pulsed magnetometry.

16. A method of operating the magnetometer according to any preceding claim, the method comprising determining the strength of a magnetic field according to a change in the plane of polarisation of the probe laser.

17. The method of claim 16, wherein the method follows a continuous wave scheme or a pulsed scheme.

18. A method of operating a magnetometer, the method comprising: providing a body of material comprising a plurality of vacancy centres; applying a bias magnetic field to the vacancy centres; applying a microwave field and an excitation laser simultaneously to the vacancy centres; applying a polarised probe laser to the plurality of vacancy centres; applying an unknown magnetic field to the plurality of vacancy centres; and determining the strength of the unknown magnetic field according to a change in the plane of polarization of the probe laser.

19. A method of operating a magnetometer, the method comprising: providing a body of material comprising a plurality of vacancy centres; applying a bias magnetic field to the vacancy centres; applying an excitation laser pulse to the vacancy centres; applying a first 71 / 2 microwave pulse to the vacancy centres; applying an unknown magnetic field to the vacancy centres; applying a second n / 2 microwave pulse to the vacancy centres; applying a polarised probe laser to the plurality of vacancy centres; and determining the strength of the unknown magnetic field according to a change in the plane of polarisation of the probe laser.

20. The method of claim 19, wherein an application sequence consists of the application of the excitation laser pulse, the first n / 2 microwave pulse, the unknown magnetic field, the second 71 / 2 microwave pulse, and the polarised probe laser, wherein the method comprises repeating the application sequence at least once.

21. The method of claim 20, wherein the application sequence is repeated between 1 and 100,000 times.

22. The method of claims 20 or 21, wherein, after each application sequence, the strength of the unknown magnetic field is determined.

23. A method of operating a magnetometer, the method comprising: providing a body of material comprising a plurality of vacancy centres; applying a bias magnetic field to the vacancy centres;applying an excitation laser pulse to the vacancy centres; applying a TT microwave pulse to the vacancy centres; applying an unknown magnetic field to the vacancy centres; applying a polarised probe laser to the plurality of vacancy centres; and determining the strength of the unknown magnetic field according to a change in the plane of polarisation of the probe laser.

24. The method of any one of claims 18 to 23, wherein the body of material is provided in an optical cavity prior to application of the bias magnetic field.

Images