WO2018174912A1

WO2018174912A1 - Bias magnetic array

Info

Publication number: WO2018174912A1
Application number: PCT/US2017/024174
Authority: WO
Inventors: Joseph W. Hahn; Andrew Raymond MANDEVILLE; Gregory Scott Bruce; Arul Manickam; Peter G. Kaup
Original assignee: Lockheed Martin Corporation
Priority date: 2017-03-24
Filing date: 2017-03-24
Publication date: 2018-09-27
Also published as: GB2575219B; GB2575219A; GB201915403D0

Abstract

A magnetometer includes a light source that provides excitation light and a magneto- optical defect center material with at least one defect center that transmits emitted light when excited by the excitation light. The magnetometer also includes a light sensor that receives the emitted light and a plurality of magnets that provide a bias magnetic field to the magneto-optical defect center material. The magnetometer further includes a ring magnet holder that has an outer ring with an outside surface and a plurality of holders extending from the ring. The plurality of holders hold the plurality of magnets in a same orientation with respect to one another. The magnetometer further includes a mount that has an inside surface. The outside surface of the outer ring slides along the inside surface of the mount.

Description

BIAS MAGNETIC ARRAY

CROSS REFERENCE TO RELATED APPLICATION

[0001] The present application is related to U.S. Patent Application No.

15/468,951 filed March 24, 2017, entitled BIAS MAGNETIC ARRAY, the content of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

[0002] The present disclosure relates, in general, to magnetometers using magneto-optical defect center materials. More particularly, the present disclosure relates to bias magnets within such a magnetometer.

BACKGROUND

[0003] The following description is provided to assist the understanding of the reader. None of the information provided or references cited is admitted to be prior art. Some magnetometers use magneto-optical defect center materials to determine a magnetic field. Such magnetometers can apply a magnetic bias to the magneto- optical defect center materials. There is a desire for improving and optimizing a method for applying the magnetic bias to the magneto-optical defect center materials while maintaining functionality of the magnetometer and/or adjustability of the magnetometer.

SUMMARY

[0004] Embodiments of a magnetometer may include a light source configured to provide excitation light and a magneto-optical defect center material with at least one defect center that transmits emitted light when excited by the excitation light. The magnetometers may also include a light sensor configured to receive the emitted light and a plurality of magnets that may be configured to provide a bias magnetic field to the magneto-optical defect center material. The

magnetometer may further include a ring magnet holder that has an outer ring with an outside surface and a plurality of holders extending from the ring. The plurality of holders may be configured to hold the plurality of magnets in a same orientation with respect to one another. The magnetometer may further include a mount that has an inside surface. The outside surface of the outer ring may slide along the inside surface of the mount.

[0005] Some device embodiments may include a plurality of magnets that can be configured to provide a bias magnetic field to a magneto-optical defect center material. The devices may also include a ring magnet holder that has an outer ring with an outside surface and a plurality of holders extending from the ring. The plurality of holders may be configured to hold a plurality of magnets in a same orientation with respect to one another. The devices may further include a mount that has an inside surface. The outside surface of the outer ring may slide along the inside surface of the mount.

[0006] The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the following drawings and the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Figure 1 illustrates one orientation of an NV center in a magneto- optical defect center material lattice in accordance with some illustrative embodiments.

[0008] Figure 2 is an energy level diagram showing energy levels of spin states for an NV center in accordance with some illustrative embodiments.

[0009] Figure 3 is a schematic diagram illustrating a NV center magnetic sensor system in accordance with some illustrative embodiments.

[0010] Figure 4 is a graph illustrating fluorescence as a function of an applied RF frequency of an NV center along a given direction for a zero magnetic field in accordance with some illustrative embodiments.

[0011] Figure 5 is a graph illustrating fluorescence as a function of an applied RF frequency for four different NV center orientations for a non-zero magnetic field in accordance with some illustrative embodiments. [0012] Figure 6 is a schematic diagram illustrating a magnetic field detection system in accordance with some illustrative embodiments.

[0013] Figure 7 is an illustration of an inside view of a magnetometer in accordance with some illustrative embodiments.

[0014] Figure 8 is an illustration of a magnet mount assembly in accordance with some illustrative embodiments.

[0015] Figures 9 and 10 are illustrations of parts of a disassembled magnet ring mount in accordance with some illustrative embodiments.

[0016] Figure 1 1 is an illustration of a magnet ring mount showing locations of magnets in accordance with some illustrative embodiments.

[0017] Figures 12 and 13 are illustrations of a bias magnet ring mount in accordance with some illustrative embodiments.

[0018] The foregoing and other features will become apparent from the following description and appended claims, taken in conjunction with the

accompanying drawings. Understanding that these drawings depict only several embodiments in accordance with the disclosure and are, therefore, not to be considered limiting of its scope, the disclosure will be described with additional specificity and detail through use of the accompanying drawings.

DETAILED DESCRIPTION

[0019] In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, and designed in a wide variety of different configurations, all of which are explicitly contemplated and make part of this disclosure. [0020] Magnetometers can use a magneto optical defect center material to determine a magnitude and direction of an external magnetic field. For example, the magneto optical defect center material can be a diamond with nitrogen vacancy (NV) centers. In other examples, any other suitable material with defect centers may be used. A bias magnetic field can be applied to the magneto optical defect center material. As explained in greater detail below, the bias magnetic field can be applied to distinguish an effect of a magnetic field on different axes of the defect center.

[0021] It can be advantageous in some embodiments to be able to adjust the bias magnetic field to achieve a desired output from the magneto optical defect center material, as discussed in greater detail below. In some illustrative embodiments, a magnet ring mount can hold a bias magnet ring in place around the magneto optical defect center material. The bias magnet ring can hold a plurality of magnets in place, and the magnets can apply the bias magnetic field to the magneto optical defect center material. The bias magnetic ring may be unsecured (e.g., via set screws) from the magnet ring mount and moved around the magneto optical defect center material to adjust the direction of the bias magnetic field that is applied to the magneto optical defect center material.

[0022] The NV center in a diamond comprises a substitutional nitrogen atom in a lattice site adjacent a carbon vacancy as shown in Figure 1. The NV center may have four orientations, each corresponding to a different crystallographic orientation of the diamond lattice. Although Figure 1 shows the NV center in a diamond, the same or similar principals can be applied to defect centers in any suitable magneto- optical defect center material.

[0023] The NV center may exist in a neutral charge state or a negative charge state. Conventionally, the neutral charge state uses the nomenclature NV°, while the negative charge state uses the nomenclature NV, which is adopted in this description.

[0024] The NV center has a number of electrons, including three unpaired electrons, each one from the vacancy to a respective of the three carbon atoms adjacent to the vacancy, and a pair of electrons between the nitrogen and the vacancy. The NV center, which is in the negatively charged state, also includes an extra electron. [0025] The NV center has rotational symmetry, and as shown in Figure 2, has a ground state, which is a spin triplet with A₂ symmetry with one spin state m_s = 0, and two further spin states m_s = +1, and m_s = -1. In the absence of an external magnetic field, the m_s = ±1 energy levels are offset from the m_s = 0 due to spin-spin interactions, and the m_s = ±1 energy levels are degenerate, i.e., they have the same energy. The m_s = 0 spin state energy level is split from the m_s = ±1 energy levels by an energy of 2.87 GHz for a zero external magnetic field.

[0026] Introducing an external magnetic field with a component along the NV axis lifts the degeneracy of the m_s = ±1 energy levels, splitting the energy levels m_s = ±1 by an amount 2gμBBz, where g is the g-factor, μ_Β is the Bohr magneton, and Bz is the component of the external magnetic field along the NV axis. This relationship is correct to a first order and inclusion of higher order corrections is a straightforward matter and will not affect the computational and logic steps in the systems and methods described below.

[0027] The NV center electronic structure further includes an excited triplet state E with corresponding m_s = 0 and m_s = ±1 spin states. The optical transitions between the ground state A₂ and the excited triplet E are predominantly spin conserving, meaning that the optical transitions are between initial and final states that have the same spin. For a direct transition between the excited triplet E and the ground state A₂, a photon of red light is emitted with a photon energy corresponding to the energy difference between the energy levels of the transitions.

[0028] There is, however, an alternative non-radiative decay route from the triplet E to the ground state A₂ via intermediate electron states, which are thought to be intermediate singlet states A, E with intermediate energy levels. Significantly, the transition rate from the m_s = ±1 spin states of the excited triplet E to the intermediate energy levels is significantly greater than the transition rate from the m_s = 0 spin state of the excited triplet E to the intermediate energy levels. The transition from the singlet states A, E to the ground state triplet A₂ predominantly decays to the m_s = 0 spin state over the m_s = ±1 spins states. These features of the decay from the excited triplet E state via the intermediate singlet states A, E to the ground state triplet A₂ allows that if optical excitation is provided to the system, the optical excitation will eventually pump the NV center into the m_s = 0 spin state of the ground state A₂. In this way, the population of the m_s = 0 spin state of the ground state A₂ may be "reset" to a maximum polarization determined by the decay rates from the triplet ¾ to the intermediate singlet states.

[0029] Another feature of the decay is that the fluorescence intensity due to optically stimulating the excited triplet ¾ state is less for the m_s = ±1 states than for the m_s = 0 spin state. This is so because the decay via the intermediate states does not result in a photon emitted in the fluorescence band, and because of the greater probability that the m_s = ±1 states of the excited triplet ¾ state will decay via the non- radiative decay path. The lower fluorescence intensity for the m_s = ±1 states than for the m_s = 0 spin state allows the fluorescence intensity to be used to determine the spin state. As the population of the m_s = ±1 states increases relative to the m_s = 0 spin, the overall fluorescence intensity will be reduced.

[0030] Figure 3 is a schematic diagram illustrating a conventional magneto- optical defect center material magnetic sensor system 300 that uses fluorescence intensity to distinguish the m_s = ±1 states, and to measure the magnetic field based on the energy difference between the m_s = +1 state and the m_s = -1 state. The system 300 includes an optical excitation source 310, which directs optical excitation to a magneto-optical defect center material 320 with defect centers. The system further includes an RF excitation source 330, which provides RF radiation to the magneto- optical defect center material320. Light from the magneto-optical defect center material may go through an optical filter 350 and to an optical detector 340.

[0031] The RF excitation source 330 may be a microwave coil, for example. The RF excitation source 330, when emitting RF radiation with a photon energy resonant with the transition energy between ground m_s = 0 spin state and the m_s = +1 spin state, excites a transition between those spin states. For such a resonance, the spin state cycles between ground m_s = 0 spin state and the m_s = +1 spin state, reducing the population in the m_s = 0 spin state and reducing the overall fluorescence at resonances. Similarly, resonance occurs between the m_s = 0 spin state and the m_s = -1 spin state of the ground state when the photon energy of the RF radiation emitted by the RF excitation source is the difference in energies of the m_s = 0 spin state and the m_s = -1 spin state, or between the m_s = 0 spin state and the m_s = +1 spin state, there is a decrease in the fluorescence intensity. [0032] The optical excitation source 310 may be a laser or a light emitting diode, for example, which emits light in the green, for example. The optical excitation source 310 induces fluorescence in the red, which corresponds to an electronic transition from the excited state to the ground state. Light from the magneto-optical defect center material 320 is directed through the optical filter 350 to filter out light in the excitation band (in the green, for example), and to pass light in the red fluorescence band, which in turn is detected by the detector 340. The optical excitation light source 310, in addition to exciting fluorescence in the diamond material 320, also serves to reset the population of the m_s = 0 spin state of the ground state A₂ to a maximum polarization, or other desired polarization.

[0033] For continuous wave excitation, the optical excitation source 310 continuously pumps the defect centers, and the RF excitation source 330 transmits a radio frequency field that sweeps across a frequency range that includes the zero splitting (when the m_s = ±1 spin states have the same energy) photon energy of 2.87 GHz. The light from optical excitation source 310 passes in front of RF excitation source 330. The fluorescence for an RF sweep corresponding to a magneto-optical defect center material 320 with defect centers aligned along a single direction is shown in Figure 4 for different magnetic field components Bz along the defect axis, where the energy splitting between the m_s = -1 spin state and the m_s = +1 spin state increases with Bz. Thus, the component Bz may be determined. Optical excitation schemes other than continuous wave excitation are contemplated, such as excitation schemes involving pulsed optical excitation, and pulsed RF excitation. Examples of pulsed excitation schemes include Ramsey pulse, and spin echo pulse sequence.

[0034] In general, the magneto-optical defect center material 320 may have defect centers aligned along directions of four different orientation classes. Figure 5 illustrates fluorescence as a function of RF frequency for the case where the magneto- optical defect center material 320 has defect centers aligned along directions of four different orientation classes. In this case, the component Bz along each of the different orientations may be determined. These results, along with the known orientation of crystallographic planes of a magneto-optical defect center material lattice, allow not only the magnitude of the extemal magnetic field to be determined, but also the direction of the magnetic field. [0035] Figure 3 illustrates a defect center magnetic sensor system 300 with magneto-optical defect center material 320 with a plurality of defect centers, in general. The magnetic sensor system 300 may employ any suitable magneto-optical defect center material with a plurality of magneto-optical defect centers. The electronic spin state energies of the magneto-optical defect centers shift with magnetic field, and the optical response, such as fluorescence, for the different spin states is not the same for all of the different spin states. In this way, the magnetic field may be determined based on optical excitation, and possibly RF excitation.

[0036] Figure 6 is a schematic diagram of a system 600 for a magnetic field detection system according to some embodiments. The system 600 includes an optical excitation source 610, which directs optical excitation to a magneto-optical defect center material 620 with defect centers. An RF excitation source 630 provides RF radiation to the magneto-optical defect center material 620. A magnetic field generator 670 generates a magnetic field, which is detected at the magneto-optical defect center material 620.

[0037] The magnetic field generator 670 may generate magnetic fields with orthogonal polarizations, for example. In this regard, the magnetic field generator 670 may include two or more magnetic field generators, such as two or more Helmholtz coils. The two or more magnetic field generators may be configured to provide a magnetic field having a predetermined direction, each of which provide a relatively uniform magnetic field at the magneto-optical defect center material620. The predetermined directions may be orthogonal to one another. In addition, the two or more magnetic field generators of the magnetic field generator 670 may be disposed at the same position, or may be separated from each other. In the case that the two or more magnetic field generators are separated from each other, the two or more magnetic field generators may be arranged in an array, such as a one- dimensional or two-dimensional array, for example.

[0038] The system 600 may be arranged to include one or more optical detection systems 605, where each of the optical detection systems 605 includes the optical detector 640, optical excitation source 610, and Magneto-optical defect center material620. Furthermore, the magnetic field generator 670 may have a relatively high power as compared to the optical detection systems 605. In this way, the optical systems 605 may be deployed in an environment that requires a relatively lower power for the optical systems 605, while the magnetic field generator 670 may be deployed in an environment that has a relatively high power available for the magnetic field generator 670 so as to apply a relatively strong magnetic field.

[0039] The system 600 further includes a controller 680 arranged to receive a light detection signal from the optical detector 640 and to control the optical excitation source 610, the RF excitation source 630, and a second magnetic field generator (not shown in Figure 6). The controller may be a single controller, or multiple controllers. For a controller including multiple controllers, each of the controllers may perform different functions, such as controlling different components of the system 600. The second magnetic field generator may be controlled by the controller 680 via an amplifier 660, for example.

[0040] The RF excitation source 630 may be a microwave coil, for example behind the light of the optical excitation source 610. The RF excitation source 630 is controlled to emit RF radiation with a photon energy resonant with the transition energy between the ground m_s = 0 spin state and the m_s = ±1 spin states as discussed above with respect to Figure 3.

[0041] The optical excitation source 610 may be a laser or a light emitting diode, for example, which emits light in the green, for example. The optical excitation source 610 induces fluorescence in the red from the Magneto-optical defect center material620, where the fluorescence corresponds to an electronic transition from the excited state to the ground state. Light from the Magneto-optical defect center material 620 is directed through the optical filter 650 to filter out light in the excitation band (in the green, for example), and to pass light in the red fluorescence band, which in turn is detected by the optical detector 640. The optical excitation light source 610, in addition to exciting fluorescence in the Magneto-optical defect center material620, also serves to reset the population of the m_s = 0 spin state of the ground state A₂ to a maximum polarization, or other desired polarization.

[0042] The controller 680 is arranged to receive a light detection signal from the optical detector 640 and to control the optical excitation source 610, the RF excitation source 630, and the second magnetic field generator (not shown in Figure 6). The controller may include a processor 682 and a memory 684, in order to control the operation of the optical excitation source 610, the RF excitation source 630, and the second magnetic field generator. The memory 684, which may include a nontransitory computer readable medium, may store instructions to allow the operation of the optical excitation source 610, the RF excitation source 630, and the second magnetic field generator to be controlled. That is, the controller 680 may be programmed to provide control.

[0043] Figure 7 is an illustration of an inside view of a magnetometer in accordance with some illustrative embodiments. An illustrative magnetometer 700 includes a housing 705, light sources 710 and 715, a magneto-optical defect center material 720, a light detector 725, a magnet ring mount 730, and a bias magnet ring 735. In alternative embodiments, additional, fewer, and/or different elements may be used. For example, although two light sources 710 and 715 are shown in the embodiment of Figure 7, any suitable number of light sources can be used, such as one, three, four, etc. light sources.

[0044] As noted above, a bias magnetic field may be applied to the magneto- optical defect center material 720. In the embodiment illustrated in Figure 7, permanent magnets are mounted to the bias magnet ring 735, which is secured within the magnet ring mount 730. The magnet ring mount 730 is mounted or fixed within the housing 705 such that the magnet ring mount 730 does not move within the housing 705. Similarly, the light sources 710 and 715 are mounted within the housing 705 such that the light sources 710 and 715 do not move within the housing 705.

[0045] As shown in Figure 7, the magneto-optical defect center material 720 is mounted within the magnetic ring mount 730, but the light sources 710 are mounted outside of the magnetic ring mount 730. The light sources 710 transmit light to the magneto-optical defect center material 720 which excites the defect centers, and light emitted from the defect centers is detected by the light detector 725. In the embodiments shown in Figure 7, the light sources 710 and 715 transmit the light such that the magnet ring mount 730 and the bias magnet ring 735 do not interfere with the transmission of the light from the light sources 710 and 715 to the magneto-optical defect material 720. [0046] Figure 8 is an illustration of a magnet mount assembly in accordance with some illustrative embodiments. An illustrative magnet mount assembly 800 includes the magnet ring mount 730 and the bias magnet ring 735. In alternative embodiments, additional, fewer, and/or different elements may be used.

[0047] As shown in Figure 8, the magnet ring mount 730 includes a first portion 805 and a second portion 810 held together with fasteners 820. The bias magnet ring 735 can be fixed within the magnet ring mount 730. The bias magnet ring 735 can hold magnets such that a uniform or substantially uniform magnetic field is applied to a central portion of the magnet mount assembly 800. For example, the uniform magnetic field can be applied to the magneto-optical defect center material 720.

[0048] The magnet mount assembly 800 includes through-holes 815. The through-holes 815 can be sufficiently large to allow light from the light sources 710 and 715 to pass into a center portion of the magnet mount assembly 800 (e.g., to apply light to the magneto-optical defect center material 720). As noted above, the magnetometer 700 may include any suitable number of light sources 710 and 715. Similarly, the magnet mount assembly 800 may include any suitable number of through-holes 815. In some illustrative embodiments, the magnet mount assembly 800 incudes the same number of through-holes 815 as the magnetometer 700 includes light sources 710 and 715. In alternative embodiments, the magnet mount assembly 800 includes a different number of through-holes 815 as the magnetometer 700 includes light sources 710 and 715. For example, two or more light sources 710 and 715 may pass light through the same through-hole 815. In another example, one or more through-holes 815 may not have light passing therethrough.

[0049] The magnet mount assembly 800 as shown in Figure 8 includes six fasteners 820. The fasteners 820 can be used to secure the first portion 805 to the second portion 810. In some illustrative embodiments, the fasteners 820 can be used to secure the magnet mount assembly 800 to the housing 705 of the magnetometer 700. The fasteners 820 can be any suitable device for securing the first portion 805 to the second portion 810. In the embodiment shown in Figure 8, the fasteners 820 are screws. Other examples of fasteners 820 may include bolts, studs, nuts, clips, etc. In alternative embodiments, any suitable means of securing the first portion 805 and the second portion 810 to one another, such as glue, welds, epoxy, etc. Although Figure 8 shows six fasteners 820 being used, any other suitable number can be used. For example, the magnet mount assembly 800 may have one, two, three, five, ten, etc. fasteners 820.

[0050] As shown in Figure 8, the inside surface of the magnet ring mount 730 is circular or semi-spherical and the outside surface is an octagonal prism. In such an embodiment, a center of the circular shape or semi-spherical shape of the inside surface is on a central axis of the octagonal prism of the outside surface. Any other suitable shapes may be used. For example, the inside surface of the magnet ring mount 730 may be elliptical. In another example, the outside surface of the magnet ring mount 730 may have more or fewer sides than eight.

[0051] In some illustrative embodiments, the inner diameter (e.g., the inner spherical diameter) of the magnet ring mount 730 is 2.75 inches. In such an embodiment, the tolerance may be +0.002 inches and -0.000 inches. In alternative embodiments, the inner diameter of the magnet ring mount 730 is greater than or less than 2.75 inches, and any suitable tolerance may be used.

[0052] As shown in Figure 8, the bias magnet ring 735 can include an outside ring that is circular. In some illustrative embodiments, the outside circumference of the bias magnet ring 735 is the same or slightly less than the inside diameter of the magnet ring mount 730. In such an embodiment, when not secured, the bias magnet ring 735 can move freely within the magnet ring mount 730. As discussed in greater detail below, the bias magnet ring 735 can be secured in place inside of the magnet ring mount 730 using, for example, set screws.

[0053] The magnet ring mount 730 and the bias magnet ring 735 may be made of any suitable material. In some illustrative embodiments, the magnet ring mount 730 and the bias magnet ring 735 are non-ferrous and/or non-magnetic. For example, the magnet ring mount 730 and the bias magnet ring 735 may be made of plastic (e.g., Black Noryl ® PPO™, polystyrene, polyphenylene ether, etc.), titanium (e.g., Grade 5, Ti 6A1-4V, etc.), aluminum (e.g., 6061-T6 per ASTM B209, may have a chemical conversion coating per military standard MIL-DTL-5541 , etc.), etc. The fasteners 820, the set screws, and any other component of the system may be made of the same or similar materials.

[0054] Figures 9 and 10 are illustrations of parts of a disassembled magnet ring mount in accordance with some illustrative embodiments. Figure 9 is an illustration of the first portion 805 of the magnet ring mount 730, and Figure 10 is an illustration of the second portion 810 of the magnet ring mount 730. The first portion 805 includes fastener holes 905, and the second portion 810 includes fastener holes 1005. In some illustrative embodiments, the fastener holes 905 align with

corresponding fastener holes 1005 to accept the fasteners 820. The first portion 805 includes a hole larger than the fastener holes 905 above the fastener holes 905 to accept a head of the fasteners 820 (e.g., the head of a screw). For example, the fastener holes 905 and the fastener holes 1005 may be 0.1 inches in diameter and may be suitable to accept fasteners 820 that are #2-56 screws. In some illustrative embodiments, the fasteners 820 screw into threaded holes in the housing 705 or a surface secured to the housing 705 (e.g., a circuit board). In alternative embodiments, any other suitable securing mechanism or arrangement may be used.

[0055] The first portion 805 of the magnet ring mount 730 includes a height 950, a length 951 , and a width 952. In some illustrative embodiments, the width 952 can be as wide as the length 951 is long. In some illustrative embodiments, the height 950 is 0.475 inches, and the width 952 and the length 951 are 2.875 inches each. In alternative embodiments, any other suitable dimensions may be used.

[0056] The second portion 810 of the magnet ring mount 730 includes a height 1050, a length 1051, and a width 1052. In some illustrative embodiments, the width 1052 can be as wide as the length 1051 is long. In the embodiments shown in Figures 9 and 10, the height 950 is the same as the height 1050, the length 951 is the same as the length 1051 , and the width 952 is the same as the width 1052. In some such embodiments, the height 1050 is 0.475 inches, and the width 1052 and the length 1051 are 2.875 inches each. In such an embodiment, the inside surface 910 and the inside surface 1010 are matching but opposite portions of a sphere. That is, the circle at which the inside surface 910 and the inside surface 1010 meet is a circumference of a sphere, and the inside surface 910 and the inside surface 1010 are along the sphere. In alternative embodiments, any other suitable dimensions may be used. [0057] Figure 1 1 is an illustration of a magnet ring mount showing locations of magnets in accordance with some illustrative embodiments. Figure 1 1 includes the magnet ring mount 730 and magnets 1105. In Figure 11 , six sets of three magnets 1105 are shown. Each magnet 1105 in a set are arranged in the same direction (e.g., the poles of each magnet 1 105 are pointed in the same direction). In alternative embodiments, additional, fewer, and/or different elements may be used. For example, in alternative embodiments, each set of magnets 1105 may include greater than or fewer than three magnets 1 105. Similarly, the total number of magnets 1105 may be greater than or fewer than eighteen.

[0058] Figure 1 1 shows an illustrative arrangement of the magnets 1105 within the magnet ring mount 730 without the bias magnet ring 735. Although the bias magnet ring 735 is not shown, the bias magnet ring 735 may hold the magnets 1105 in the same position relative to one another. But, the bias magnet ring 735 may move within the magnet ring mount 730 while maintaining the magnets 1 105 in the same position relative to one another. Accordingly, the magnets 1105 may be rotated around the center portion of the bias magnet ring 735 and/or the magnet ring mount 730 (e.g., around the magneto-optical defect center material 720). For reference, a detailed discussion of diamond axes crystal alignment and magnet orientation is provided in U. S. Patent Application No. 15/003,718 (now U. S. Patent No. 9,541 ,610) and U. S. Patent Application No. 15/003,704, both filed on January 21, 2016, and both of which are incorporated herein by reference in their entireties.

[0059] Figures 12 and 13 are illustrations of a bias magnet ring mount in accordance with some illustrative embodiments. The bias magnet ring mount 735 includes magnet holders 1205 with magnet holes 1210 and securing tabs 1215 with set screw holes 1220. In alternative embodiments, additional, fewer, and/or different elements may be used.

[0060] As shown in Figures 12 and 13, the bias magnet ring mount 735 has an outer ring, and the magnet holders 1205 and the securing tabs 1215 are fixed to the outer ring. In some illustrative embodiments, the outside diameter 1250 of the outer ring and the bias magnet ring mount 735 is 2.745 inches. The height 1251 of the magnet holders 1205 can be 0.290 inches. In some illustrative embodiments, the outside surface of the outer ring is spherically shaped to fit within and slide along the inner surface 910 and the inner surface 1010.

[0061] As noted above, the magnet holders 1205 have magnet holes 1210. The magnet holes 1210 may hold the magnets 1105 in the orientation to one another shown in Figure 11. The securing tabs 1215 may each include one or more set screw holes 1220. The set screw holes 1220 may be configured to receive a set screw. For example, the set screw holes 1220 may be threaded. In some illustrative

embodiments, set screws may be threaded into the set screw holes 1220 and be pressed against the inner surface 910 and/or the inner surface 1010 to secure the bias magnet ring mount 735 within the magnet ring mount 730. In some illustrative embodiments, the set screws 1220 may be #2-56 screws. In alternative embodiments, any other suitable set screws may be used.

[0062] In the embodiment shown in Figure 13, two of the securing tabs 1215 each include one set screw hole 1220 and six through-holes 1305. Each of the six through-holes 1305 can be used to drill or otherwise form the magnet holes 1210. For example, each of the through-holes 1305 may be aligned along a same central axis as a corresponding magnet hole 1210. For example, the inside diameter of the magnet holes 1210 can be 0.070 inches. The inside diameter of the through-holes 1305 can be the same or larger than the inside diameter of the magnet holes 1210. Following the example, the inside diameter of the through-holes 1305 may be 0.070 inches (or larger). In alternative embodiments, any other suitable inside diameters may be used.

[0063] Thus, the magnet mount assembly 800 can be used to adjust the magnetic bias applied to the magneto-optical defect center material 720 by moving the magnets 1105 about the magneto-optical defect center material 720. Similarly, once a desired position is selected, the bias magnet ring mount 735 may be secured within the magnet ring mount 730.

[0064] As noted above with respect to Figures 4 and 5, each of the dips (e.g., Lorentzians) in the graphs may correspond to one or more axes of the defect centers within the magneto-optical defect center material 720. The bias magnetic field applied to the magneto-optical defect center material 720 may adjust the order and orientation of the Lorentzian dips in the graphs. Accordingly, there are forty-eight unique orientations of the Lorentzians such that each Lorentzian is distinguishable from the others (e.g., as in the graph of Figure 5). Thus, there are forty-eight unique positions of the magnets 1105 around the magneto-optical defect center material 720 corresponding to each of the forty-eight orientations of the Lorentzians.

[0065] In some illustrative embodiments, the magnet ring mount 730 is movable within the bias magnet ring 735 and the housing 705 such that twelve of the forty-eight positions of the magnets 1105 are accessible. That is, the magnet ring mount 730 cannot be positioned into all of the forty-eight positions because the magnet ring mount 730 would interfere with the housing 705, which may span across the top and bottom of the magnet ring mount 730. In some instances, only a portion of the twelve positions may position the bias magnet ring 735 within the magnet ring mount 730 such that the bias magnet ring 735 does not interfere with the light that passes through the through-holes 815. In some illustrative embodiments, the bias magnet ring 735 is positioned such that the Lorentzians are distinguishable from one another and such that the light is not interfered with as it passes through the through- hole 815 to the magneto-optical defect center material 720.

[0066] In some illustrative embodiments, any of the operations described herein can be implemented at least in part as computer-readable instructions stored on a computer-readable memory. Upon execution of the computer-readable instructions by a processor, the computer-readable instructions can cause a node to perform the operations.

[0067] The herein described subject matter sometimes illustrates different components contained within, or connected with, different other components. It is to be understood that such depicted architectures are merely exemplary, and that in fact many other architectures can be implemented which achieve the same functionality. In a conceptual sense, any arrangement of components to achieve the same functionality is effectively "associated" such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality can be seen as "associated with" each other such that the desired functionality is achieved, irrespective of architectures or intermedial components. Likewise, any two components so associated can also be viewed as being "operably connected," or "operably coupled," to each other to achieve the desired functionality, and any two components capable of being so associated can also be viewed as being "operably couplable," to each other to achieve the desired functionality. Specific examples of operably couplable include but are not limited to physically mateable and/or physically interacting components and/or wirelessly interactable and/or wirelessly interacting components and/or logically interacting and/or logically interactable components.

[0068] With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity.

[0069] It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as "open" terms (e.g., the term "including" should be interpreted as "including but not limited to," the term "having" should be interpreted as "having at least," the term "includes" should be interpreted as "includes but is not limited to," etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases "at least one" and "one or more" to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles "a" or "an" limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases "one or more" or "at least one" and indefinite articles such as "a" or "an" (e.g., "a" and/or "an" should typically be interpreted to mean "at least one" or "one or more"); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of "two recitations," without other modifiers, typically means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to "at least one of A, B, and C, etc." is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., "a system having at least one of A, B, and C" would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances where a convention analogous to "at least one of A, B, or C, etc." is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., "a system having at least one of A, B, or C" would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase "A or B" will be understood to include the possibilities of "A" or "B" or "A and B." Further, unless otherwise noted, the use of the words "approximate," "about," "around,"

"substantially," etc., mean plus or minus ten percent.

[0070] The foregoing description of illustrative embodiments has been presented for purposes of illustration and of description. It is not intended to be exhaustive or limiting with respect to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the disclosed embodiments. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.

Claims

WHAT IS CLAIMED IS:

1. A magnetometer comprising:

a light source configured to provide excitation light;

a magneto-optical defect center material with at least one defect center that transmits emitted light when excited by the excitation light;

a light sensor configured to receive the emitted light;

a plurality of magnets configured to provide a bias magnetic field to the magneto-optical defect center material;

a ring magnet holder comprising:

an outer ring with an outside surface, and

a plurality of holders extending from the ring, wherein the plurality of holders are configured to hold the plurality of magnets in a same orientation with respect to one another; and

a mount comprising an inside surface, wherein the outside surface of the outer ring slides along the inside surface of the mount.

2. The magnetometer of claim 1, further comprising a processor configured to:

receive an indication of a frequency of the excitation light;

receive an indication of a frequency of the emitted light; and

determine a magnitude of an external magnetic field based at least in part on a comparison between the frequency of the excitation light and the frequency of the emitted light.

3. The magnetometer of claim 2, wherein the processor is further configured to determine a direction of the external magnetic field based at least in part on a comparison between the frequency of the excitation light and the frequency of the emitted light.

4. The magnetometer of claim 1 , wherein the ring magnet holder further comprises a fixation member configured to secure the ring magnet holder in a location within the mount.

5. The magnetometer of claim 4, wherein the fixation member comprises a set screw.

6. The magnetometer of claim 1 , wherein the mount comprises a through- hole configured to allow the excitation light to pass through the through-hole of the mount.

7. The magnetometer of claim 1, wherein the inside surface of the mount has a shape that is semi-spherical.

8. The magnetometer of claim 7, wherein the outside surface of the mount has a shape that is semi-spherical.

9. The magnetometer of claim 1, wherein the mount comprises a first portion and a second portion that are secured together with a plurality of fasteners.

10. The magnetometer of claim 9, wherein the first portion comprises half of the inside surface.

11. The magnetometer of claim 1 , wherein the plurality of magnets are permanent magnets.

12. The magnetometer of claim 1, wherein the plurality of holders each comprise at least one magnet hole, wherein each of the at least one magnet hole is configured to hold one of the plurality of magnets.

13. The magnetometer of claim 12, wherein the ring magnet holder further comprises at least one mounting tab, and wherein the at least one mounting tab comprises a fixation member configured to secure the ring magnet holder in a location within the mount.

14. The magnetometer of claim 13, wherein the mounting tab further comprises at least one through-hole, wherein the at least one through-hole comprises a central axis that is coaxial to a central axis of one of the at least one magnet hole.

15. The magnetometer of claim 1, wherein the bias magnetic field is substantially uniform through the magneto-optical defect center material.

16. A device comprising:

a plurality of magnets configured to provide a bias magnetic field to a magneto-optical defect center material;

a ring magnet holder comprising:

an outer ring with an outside surface, and

a plurality of holders extending from the ring, wherein the plurality of holders are configured to hold a plurality of magnets in a same orientation with respect to one another; and

17. The device of claim 16, wherein the ring magnet holder further comprises a fixation member configured to secure the ring magnet holder in a location within the mount.

18. The device of claim 17, wherein the fixation member comprises a set screw.

19. The device of claim 16, wherein the mount comprises a through-hole configured to allow the excitation light to pass through the through-hole of the mount.

20. The device of claim 16, wherein the inside surface of the mount has a shape that is semi-spherical.

21. The device of claim 20, wherein the outside surface of the mount has a shape that is semi-spherical.

22. The device of claim 16, wherein the mount comprises a first portion and a second portion that are secured together with a plurality of fasteners.

23. The device of claim 22, wherein the first portion comprises half of the inside surface.

24. The device of claim 16, wherein the plurality of magnets are permanent magnets.

25. The device of claim 16, wherein the plurality of holders each comprise at least one magnet hole, wherein each of the at least one magnet hole is configured to hold one of the plurality of magnets.

26. The device of claim 25, wherein the ring magnet holder further comprises at least one mounting tab, and wherein the at least one mounting tab comprises a fixation member configured to secure the ring magnet holder in a location within the mount.

27. The device of claim 26, wherein the mounting tab further comprises at least one through-hole, wherein the at least one through-hole comprises a central axis that is coaxial to a central axis of one of the at least one magnet hole.

28. A device for generating substantially uniform magnetic flux across a magneto-optical material, the device comprising:

a magneto-optical material that is capable of fluorescing upon the application of certain light and that provides different fluorescence depending upon applied magnetic fields;

a biasing magnet assembly comprising:

an outer ring with an outside surface, and a plurality of holders extending from the ring, wherein the plurality of holders are configured to hold the plurality of magnets in a same orientation with respect to one another; and

a mount comprising an inside surface, wherein the outside surface of the outer ring slides along the inside surface of the mount,

wherein the biasing magnet assembly is adjustable to provide a uniform magnetic field to the magneto-optical material.

29. The device of claim 28, wherein the biasing magnet assembly further comprises a fixation member configured to secure the ring magnet holder in a location within the mount.

30. The device of claim 29, wherein the fixation member comprises a set screw.

31. The device of claim 28, wherein the mount comprises a through-hole configured to allow the excitation light to pass through the through-hole of the mount.

32. The device of claim 28, wherein the inside surface of the mount has a shape that is semi-spherical.

33. The device of claim 32, wherein the outside surface of the mount has a shape that is semi-spherical.

34. The device of claim 28, wherein the mount comprises a first portion and a second portion that are secured together with a plurality of fasteners.

35. The device of claim 34, wherein the first portion comprises half of the inside surface.

36. The device of claim 28, wherein the plurality of magnets are permanent magnets.

37. The device of claim 28, wherein the plurality of holders each comprise at least one magnet hole, wherein each of the at least one magnet hole is configured to hold one of the plurality of magnets.

38. The device of claim 37, wherein the ring magnet holder further comprises at least one mounting tab, and wherein the at least one mounting tab comprises a fixation member configured to secure the ring magnet holder in a location within the mount.

39. The device of claim 38, wherein the mounting tab further comprises at least one through-hole, and wherein the at least one through-hole comprises a central axis that is coaxial to a central axis of one of the at least one magnet hole.