WO2018174907A1

WO2018174907A1 - Apparatus and method for resonance magneto-optical defect center material pulsed mode referencing

Info

Publication number: WO2018174907A1
Application number: PCT/US2017/024169
Authority: WO
Inventors: Gregory Scott Bruce; Arul Manickam; Peter G. Kaup
Original assignee: Lockheed Corp; Lockheed Martin Corp
Current assignee: Lockheed Martin Corp
Priority date: 2017-03-24
Filing date: 2017-03-24
Publication date: 2018-09-27
Anticipated expiration: 2019-09-24

Abstract

The present disclosure relates to apparatuses and methods for stimulating a magneto-optical defect material with defect centers in a magnetic detection system using a stimulation process to significantly increase magnetic sensitivity of the detection system. The system utilizes a modified Ramsey pulse sequence pair or a shifted magnetometry adapted cancellation (SMAC) pair to detect and measure the magnetic field acting on the system resulting in mitigation of low-frequency noise sources to provide improved sensor sensitivity. For a SMAC pair measurement, two different values of tau are used as well as two different values of the microwave pulse width.

Description

APPARATUS AND METHOD FOR RESONANCE MAGNETO-OPTICAL DEFECT CENTER MATERIAL PULSED MODE REFERENCING

CROSS REFERENCE TO RELATED APPLICATION

[0001] The present application is related to U. S. Patent Application No. 15/468,289 filed March 24, 2017, entitled "APPARATUS AND METHOD FOR RESONANCE MAGNETO-OPTICAL DEFECT CENTER MATERIAL PULSED MODE REFERENCING", the content of which is incorporated herein by reference in its entirety.

FIELD

[0002] The present disclosure relates to magnetic detection systems, and more generally, to measurement and signal processing methods for a magnetic detection system.

BACKGROUND

[0003] A number of industrial applications, as well as scientific areas such as physics and chemistry can benefit from magnetic detection and imaging with a device that has extraordinary sensitivity, ability to capture signals that fluctuate very rapidly (bandwidth) all with a substantive package that is extraordinarily small in size and efficient in power. Many advanced magnetic imaging systems can require operation in restricted conditions, for example, high vacuum and/or cryogenic temperatures, which can make them inapplicable for applications that require ambient or other conditions.

SUMMARY

[0004] According to some embodiments, a system for magnetic detection may include a magneto-optical defect center material comprising a plurality of defect centers, a radio frequency (RF) excitation source configured to provide RF excitation to the magneto-optical defect center material, an optical excitation source configured to provide optical excitation to the magneto- optical defect center material, an optical detector configured to receive an optical signal emitted by the magneto-optical defect center material, a bias magnet configured to separate RF resonance responses of the lattice oriented subsets of the magneto-optical defect center material, and a controller. The controller may be configured to control the optical excitation source and the RF excitation source to apply a first pulse sequence to the magneto-optical defect center material, the first pulse sequence comprising a first optical excitation pulse, a first pair of RF excitation pulses separated by a first time period, and a second optical excitation pulse to the magneto- optical defect center material. The controller may be configured to control the optical excitation source and the RF excitation source to further apply a second pulse sequence to the magneto- optical defect center material, the second pulse sequence comprising a third optical excitation pulse, a second pair of RF excitation pulses separated by a second time period, and a fourth optical excitation pulse to the magneto-optical defect center material. In some embodiments, a pulse width of the first pair of RF excitation pulses may be different than a pulse width of the second pair of RF excitation pulses, and the first time period may be different than the second time period. The controller may be further configured to receive a first light detection signal from the optical detector based on an optical signal emitted by the magneto-optical defect center material due to the first pulse sequence and may be configured to receive a second light detection signal from the optical detector based on an optical signal emitted by the magneto-optical defect center material due to the second pulse sequence. The controller may be further configured to compute a combined measurement based on a difference between a measured value of the first light detection signal and a measured value of the second light detection signal wherein the slope of the combined measurement is greater that the slope of the first light detection signal and the second light detection signal. The controller may be further configured to compute a combined measurement based on a difference between a measured value of the first light detection signal and a measured value of the second light detection signal wherein the slope of the combined measurement is greater than the slope of the measured value of the first and second light detection signals.

[0005] According to some embodiments, a method for magnetic detection using a magneto- optical defect center material comprising a plurality of defect centers may comprise applying a first pulse sequence to the magneto-optical defect center material, applying a second pulse sequence to the magneto-optical defect center material, receiving a first light detection signal using an optical detector, receiving a second light detection signal using the optical detector, and computing a combined measurement based on a difference between a measured value of the first light detection signal and a measured value of the second light detection signal. The first pulse sequence may comprise a first optical excitation pulse using an optical excitation source, a first pair of RF excitation pulses separated by a first time period using a radio frequency (RF) excitation source, and a second optical excitation pulse to the magneto-optical defect center material using the optical excitation source. The second pulse sequence may comprise a third optical excitation pulse using the optical excitation source, a second pair of RF excitation pulses separated by a second time period using the RF excitation source, and a fourth optical excitation pulse to the magneto-optical defect center material using the optical excitation source. In some embodiments, a pulse width of the first pair of RF excitation pulses is different than a pulse width of the second pair of RF excitation pulses. In some embodiments, the first time period is different than the second time period. Receiving the first light detection signal may be based on an optical signal emitted by the magneto-optical defect center material due to the first pulse sequence. The second light detection signal, may be based on an optical signal emitted by the magneto-optical defect center material due to the second pulse sequence.

[0006] In some embodiments, an RF excitation frequency used for the first pair of RF excitation pulses and the second pair of RF excitation pulses in a system for magnetic detection may be associated with an axis of a defect center of the magneto-optical defect center material. In some embodiments, the controller may be further configured to compute a change in an external magnetic field acting on the magneto-optical defect center material based on the combined measurement. In some embodiments, a method for magnetic detection using a magneto-optical defect center material has the RF excitation frequency used for the first pair of RF excitation pulses and the second pair of RF excitation pulses is associated with an axis of a defect center of the magneto-optical defect center material. In some embodiments, a method for magnetic detection using a magneto-optical defect center material further comprises computing a change in an external magnetic field acting on the magneto-optical defect center material based on the combined measurement. In some embodiments, the second pair of RF excitation pulses of the first pulse sequence may be applied at a frequency detuned from a resonance frequency of the magneto-optical defect center material. The pulse width of the second pair of RF excitation pulses may be associated with a null at center frequency representing a lack of dimming in the fluorescence of the magneto-optical defect center material. The second time period may be associated with a null at a center frequency representing a lack of dimming in the fluorescence of the magneto-optical defect center material. The pulse width of the second pair of RF excitation pulses and the second time period may be associated with a null at a center frequency

representing a lack of dimming in the fluorescence of the magneto-optical defect center material. The RF excitation source may be a microwave antenna. In some embodiments, of a system for magnetic detection, the controller may be configured to apply the first pair of RF excitation pulses followed by the second pair of RF excitation pulses. In some embodiments, the pulse width of the first pair of RF excitation pulses and the first time period is associated with a high point at a center frequency representing dimming in the fluorescence of the magneto-optical defect center material. In some embodiments, a method for magnetic detection using a magneto- optical defect center material may have the first pair of RF excitation pulses applied followed by the second pair of RF excitation pulses. In some embodiments, the bias magnet is one of a permanent magnet, a magnet field generator, or a Halbach set of permanent magnets.

[0007] In some embodiments, computing the change in an external magnetic field acting on the magneto-optical defect center material based on the combined measurement comprise a plurality of pairs of RF excitation pulses. In some embodiments, once the magnetometry curves have been obtained for the pairs of RF excitation pulses at different frequencies, a SMAC measurement may be performed at a chosen frequency (e.g. at a frequency with a maximum slope for the curve) and the intensity of the SMAC measurement is monitored to provide an estimate of the magnetic field. In some embodiments, the maximum slope, positive and negative, may be determined from the curve obtained by the SMAC pairing and the corresponding frequencies. In some embodiments, the curve may be first smoothed and fit to a cubic spline. In some embodiments, only the corresponding frequencies may be stored for use in magnetic field measurements. In some implementations, the entire curve may be stored.

[0008] According to some embodiments, a magnetic detection system may comprise a defect center material responsive to an applied magnetic field, a radio frequency (RF) emitter operational to provide a first RF pulse sequence separated by at least one pause, a detector operational to measure the fluorescence of the defect center material in conjunction with the first RF pulse sequence and the second RF pulse sequence, thereby providing a first measurement curve and a second measurement curve affected by the applied magnetic field, respectfully, and a control circuit connected to the detector and operational to determine a difference between the first measurement curve and the second measurement curve to obtain greater sensitivity to variations in the applied magnetic field. The RF emitter may be operational to provide a second RF pulse sequence that is different from the first RF pulse sequence. The RF emitter may be operational to provide a second RF pulse sequence that is different from the first RF pulse sequence.

[0009] In some embodiments, the first RF pulse sequence and the second RF pulse sequence are applied at a frequencies detuned from a resonance frequency of the defect center material. In some embodiments, the first RF pulse sequence is applied followed by the second RF pulse sequence. The defect center material may be a nitrogen vacancy diamond. The defect center material may be Silicon Carbide (SiC).

[0010] According to some embodiments, a method for magnetic detection or a method for detecting a magnetic field, comprises emitting a first RF pulse sequence separated by at least one pause, using an RF emitter to a defect center material, emitting a second RF pulse sequence that is different from the first RF pulse sequence, using the RF emitter, to the defect center material, measure the fluorescence of the defect center material in conjunction with the first RF pulse sequence and the second RF pulse sequence, using a detector, providing a first

measurement curve and a second measurement curve of the measured fluorescence of the defect center material affected by the applied magnetic field, respectfully for the first RF pulse sequence and the second RF pulse sequence, and determining a difference between the first measurement curve and the second measurement curve to obtain greater sensitivity to variations in the applied magnetic field.

[0011] In some embodiments of a method for magnetic detection, determining the difference between the first measurement curve and the second measurement curve may be performed by a control circuit. In some embodiments, the first RF pulse sequence and the second RF pulse sequence may be applied at a frequency detuned from a resonance frequency of the defect center material. In some embodiments, the first RF pulse sequence may be emitted followed by the second RF pulse sequence. In some embodiments, the defect center material may be a nitrogen vacancy diamond. In some embodiments, the defect center material is Silicon Carbide (SiC). [0012] According to some embodiments, a system for magnetic detection may comprise, a magneto-optical defect center material comprising a plurality of defect centers, a means of providing RF excitation to the magneto-optical defect center material, a means of providing optical excitation to the magneto-optical defect center material, a means of receiving an optical signal emitted by the magneto-optical defect center material, and a means of controlling the provided RF excitation and provided optical excitation. The means of controlling the provided RF excitation and provided optical excitation may apply a first pulse sequence to the magneto- optical defect center material, the first pulse sequence comprising a first optical excitation pulse, a first pair of RF excitation pulses separated by a first time period, and a second optical excitation pulse to the magneto-optical defect center material, control the optical excitation source and the RF excitation source to apply a second pulse sequence to the magneto-optical defect center material, the second pulse sequence comprising a third optical excitation pulse, a second pair of RF excitation pulses separated by a second time period, and a fourth optical excitation pulse to the magneto-optical defect center material, receive a first light detection signal from the optical detector based on an optical signal emitted by the magneto-optical defect center material due to the first pulse sequence, receive a second light detection signal from the optical detector based on an optical signal emitted by the magneto-optical defect center material due to the second pulse sequence, and compute a combined measurement based on a difference between a measured value of the first light detection signal and a measured value of the second light detection signal. The pulse width of the first pair of RF excitation pulses may be different than the pulse width of the second pair of RF excitation pulses, and the first time period may be different than the second time period.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 illustrates a defect center in a diamond lattice in accordance with some illustrative embodiments.

[0014] FIG. 2 illustrates an energy level diagram showing energy levels of spin states for a defect center in accordance with some illustrative embodiments. [0015] FIG. 3 is a schematic diagram illustrating a defect center magnetic sensor system in accordance with some illustrative embodiments.

[0016] FIG. 4 is a graph illustrating the fluorescence as a function of an applied RF frequency of a defect center along a given direction for a zero magnetic field.

[0017] FIG. 5 is a graph illustrating the fluorescence as a function of an applied RF frequency for four different defect center orientations for a non-zero magnetic field.

[0018] FIG. 6 is a schematic illustrating a Ramsey sequence of optical excitation pulses and RF excitation pulses according to an operation of the system in some embodiments.

[0019] FIG. 7A is a free induction decay curve where a free precession time τ is varied using a Ramsey sequence in some embodiments.

[0020] FIG. 7B is a magnetometry curve where a RF detuning frequency Δ is varied using a Ramsey sequence in some embodiments.

[0021] FIG. 8 is a graphical diagram depicting a reference signal intensity relative to detune frequency and a measured signal intensity relative to detune frequency in accordance with some embodiments.

[0022] FIG. 9 is a plot showing a traditional magnetometry curve using a Ramsey sequence in accordance with some embodiments.

[0023] FIG. 10 is a plot showing an invented magnetometry curve using a Ramsey sequence in accordance with some embodiments.

[0024] FIG. 11 is a plot showing a combined magnetometry curve of a traditional and inverted curve in accordance with some embodiments.

[0025] FIG. 12 is a schematic diagram illustrating a system for magnetic field detection according to some embodiments. DETAILED DESCRIPTION

[0026] 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 indicates 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. 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.

[0027] Magneto-optical defect center materials such as diamonds with nitrogen vacancy (NV) centers can be used to detect magnetic fields. Atomic-sized nitrogen-vacancy (NV) centers in diamond lattices can have excellent sensitivity for magnetic field measurement and can enable fabrication of small magnetic sensors. The sensing capabilities of diamond NV (DNV) sensors may be maintained at room temperature and atmospheric pressure and these sensors can be even used in liquid environments.

[0028] Excitation light, such as green light, which enters a diamond structure with defect centers interacts with defect centers, and fluorescent light, which is red, is emitted from the diamond. The intensity of red light emitted can be used to determine the strength and direction of the magnetic field. The efficiency and accuracy of sensors using magneto-optical defect center materials such as diamonds with NV centers (generally, DNV sensors) is increased by transferring as much light as possible from the defect centers to the photo sensor that measures the amount of red light. Magneto-optical defect center materials include but are not be limited to diamonds, Silicon Carbide (SiC) and other materials with nitrogen, boron, or other chemical defect centers.

[0029] In some embodiments, the system utilizes a special Ramsey pulse sequence pair or a 'shifted magnetometry adapted cancellation' (SMAC) pair to detect and measure the magnetic field acting on the system. These parameters include the resonant Rabi frequency, the free precession time (tau), the RF pulse width, and the detuning frequency, all of which help improve the sensitivity of the measurement. For a SMAC pair measurement, two different values of tau are used as well as two different values of the pulse width for each measurement of the pair. This is in contrast to Ramsey excitation measurement where a single pulse sequence is repeated in which there may be repolarization of the system, double RF pulses separated by a gap for the free precession time, a start of the optical excitation and a readout during the optical excitation. In a SMAC excitation, there is a second set of RF pulses having a pulse width and tau values which may be different from the pulse width and tau of the first set. The first set of RF pulses is done with the first set of values, there is repolarization of the system, and then the second set of values is used to create an inverted curve. The SMAC pair estimate is a combination of the magnetometry curves of the two pulse sequences with different values. In some embodiments, the combination is the difference between the two curves. This creates a magnetometry curve with an improved slope and therefore improved performance.

[0030] In some embodiments, using the SMAC technique or SMAC pair measurements to perform a differential measurement technique, low-frequency noise such as vibrations, laser drift, low-frequency noise in the receiver circuits, and residual signals from previous

measurements (e.g., from previous measurements on other lattice vectors) get canceled out through the differential measurement technique. In some embodiments, this noise reduction may provide a sensitivity increase at lower frequencies where colored noise may be the strongest. In some embodiments, the low-frequency noise cancelation may be due to slowly varying noise in the time domain appearing almost identically in the two sequential sets of Ramsey measurements in the SMAC pair measurement. In some embodiments, inverting the second Ramsey set and subtracting the measurement from the first Ramsey set may largely cancel out any noise that is added post-inversion. Inverting the second Ramsey set and then subtracting its measurement off from the first may therefore largely cancel out any noise that is added post-inversion. In some embodiments, the low frequency noise cancelation may be understood by viewing the SMAC technique as a digital modulation technique, whereby, in the frequency domain, the magnetic signals of interest are modulated up to a carrier frequency of half the sampling rate (inverting every second set of Ramsey measurements is equivalent to multiplying the signal by e ¹⁷⁷⁷¹ where n is the sample (i.e., Ramsey pulse number). In some embodiments, this may shift the magnetic signals of interest to a higher frequency band that is separated from the low-frequency colored noise region. Then, a high-pass filter may be applied to the signal to remove the noise, and finally, the signal may be shifted back to baseband. In some embodiments, performing a differential measurement may be equivalent to a two-tap high-pass filter, followed by a 2x down- sampling. In some embodiments, higher-order filters may be used to provide more out-of-band noise rejection to leave more bandwidth for the signal of interest.

[0031] In some embodiments, when interrogating a single lattice vector via RF and laser excitation, the sidelobe responses from nearby lattice vectors will be present. The signals from these sidelobes may cause inter-lattice vector interference, resulting in corruption of the desired measurement. The SMAC technique may see lower sidelobe levels (and thus less inter-lattice vector interference) than those from regular Ramsey measurements. For regular Ramsey measurements, different lattice vectors have potentially different optimal pulse width & tau values, based on the RF polarization, laser polarization, and gradient of the bias magnetic field. Because of this discrepancy, applying the optimal pulse width and tau settings for one lattice vector may cause the nearby lattice vectors' responses to be lower than if they were interrogated at their respective optimal values. In some embodiments, for the SMAC technique, this reduction of the nearby lattice vector's responses can become even more pronounced. Not only are there different optimal pulsewidth and tau settings for the first Ramsey set, but there may be also potentially different optimal pulse width and tau settings for the second, inverted Ramsey set. This second Ramsey set discrepancy provides potential for even more reduction in neighboring lattice vectors' responses when using the optimal settings for the lattice vector of interest.

[0032] The defect center in a diamond comprises a substitutional nitrogen atom in a lattice site adjacent a carbon vacancy as shown in FIG. 1. The NV center may have four orientations, each corresponding to a different crystallographic orientation of the diamond lattice.

[0033] 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.

[0034] 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. [0035] The NV center has rotational symmetry, and as shown in FIG. 2, has a ground state, which is a spin triplet with ³A₂ 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.

[0036] 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 2gμ_BBz, 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.

[0037] The NV center electronic structure further includes an excited triplet state ³E with corresponding m_s = 0 and m_s = ±1 spin states. The optical transitions between the ground state ³A₂ and the excited triplet ³E 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 ³E and the ground state ³A₂, a photon of red light is emitted with a photon energy corresponding to the energy difference between the energy levels of the transitions.

[0038] There is, however, an alternative non-radiative decay route from the triplet ³E to the ground state ³A₂ 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 ³E to the intermediate energy levels is significantly greater than the transition rate from the m_s = 0 spin state of the excited triplet ³E to the intermediate energy levels. The transition from the singlet states A, E to the ground state triplet ³A₂ 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 ³E state via the intermediate singlet states A, E to the ground state triplet ³A₂ 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 ³A₂. 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 ³E to the intermediate singlet states.

[0039] Another feature of the decay is that the fluorescence intensity due to optically stimulating the excited triplet ³E 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 ³E 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.

[0040] FIG. 3 is a schematic diagram illustrating a magneto-optical defect center magnetic sensor system 300 that uses fluorescence intensity to distinguish the ms = ±1 states, and to measure the magnetic field based on the energy difference between the ms = +1 state and the ms = -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 (e.g, NV diamond material). The system further includes an RF excitation source 330, which provides RF radiation to the magneto-optical defect center material 320. Light from the NV diamond may be directed through an optical filter 350 to an optical detector 340.

[0041] In some implementations, the RF excitation source 330 may be a microwave coil. The RF excitation source 330, when emitting RF radiation with a photon energy resonant with the transition energy between ground ms = 0 spin state and the ms = +1 spin state, excites a transition between those spin states. For such a resonance, the spin state cycles between ground ms = 0 spin state and the ms = +1 spin state, reducing the population in the ms = 0 spin state and reducing the overall fluorescence at resonances. Similarly, resonance occurs between the ms = 0 spin state and the ms = -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 ms = 0 spin state and the ms = -1 spin state, or between the ms = 0 spin state and the ms = +1 spin state, there is a decrease in the fluorescence intensity. [0042] In some implementations, the optical excitation source 310 may be a laser or a light emitting diode which emits light in the green. In some implementations, the optical excitation source 310 induces fluorescence in the red, which corresponds to an electronic transition from the excited state to the ground state. In some implementations, 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 ms = 0 spin state of the ground state 3 A2 to a maximum polarization, or other desired polarization.

[0043] For continuous wave excitation, the optical excitation source 310 continuously pumps the magneto-optical defect centers, and the RF excitation source 330 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 fluorescence for an RF sweep corresponding to a magneto-optical defect center material 320 with magneto-optical defect centers aligned along a single direction is shown in FIG. 4 for different magnetic field components Bz along the defect center 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 sequence (described in more detail below), and spin echo pulse sequence.

[0044] For continuous wave excitation, the optical excitation source 310 may continuously pump the defect centers, and the RF excitation source 330 may sweep across a frequency range that includes the zero splitting (when the ms = ±1 spin states have the same energy) photon energy of 2.87 GHz. The fluorescence for an RF sweep corresponding to a magneto-optical defect center material 320 with defect centers aligned along a single direction for different magnetic field components Bz along the defect center axis, where the energy splitting between the ms = -1 spin state and the ms = +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 sequence, and spin echo pulse sequence. The excitation scheme utilized during the measurement collection process (i.e., the applied optical excitation and the applied RF excitation) may be any appropriate excitation scheme. For example, the excitation scheme may utilize continuous wave (CW) magnetometry, pulsed magnetometry, and variations on CW and pulsed magnetometry (e.g., pulsed RF excitation with CW optical excitation). In cases where Ramsey pulse RF sequences are used, pulse parameters π and τ may be determined as described in, for example, U.S. Patent Application No. 15/003,590, which is incorporated by reference herein in its entirety.

[0045] In general, the magneto-optical defect center material 320 has defect centers aligned along directions of four different orientation classes. FIG. 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 external magnetic field to be determined, but also the direction of the magnetic field.

[0046] In general, the magnetic sensor system may employ a variety of different magneto- optical defect center material, with a variety of magneto-optical defect centers. Magneto-optical defect center materials include but are not be limited to diamonds, Silicon Carbide (SiC) and other materials with nitrogen, boron, or other 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, in a corresponding way to that described above with NV diamond material.

[0047] Ramsey pulse sequence is a pulsed RF laser scheme that is believed to measure the free precession of the magnetic moment in the magneto-optical defect material 320 with defect centers, and is a technique that quantum mechanically prepares and samples the electron spin state. FIG. 6 is an example of a schematic diagram illustrating the Ramsey pulse sequence using a SMAC pair for the two pulse sequences. Several pulse sequences are shown. As shown in FIG. 6, a Ramsey pulse sequence includes optical excitation pulses (e.g., from a laser) and RF excitation pulses over a five-step period. In a first step, a first optical excitation pulse is applied to the system to optically pump electrons into the ground state (i.e., ms = 0 spin state). This is followed by a first RF excitation pulse (in the form of, for example, a pulse width / 2 (pwi/2) microwave (MW)). The first RF excitation pulse may set the system into superposition of the ms = 0 and ms = +1 spin states (or, alternatively, the ms = 0 and ms = -1 spin states, depending on the choice of resonance location). During a period 2, the spins are allowed to freely precess (and dephase) over a time period referred to as tau (τι). During this free precession time period, the system measures the local magnetic field and serves as a coherent integration. Next, a second RF excitation pulse (in the form of, for example, a MW pwi/2 pulse) is applied to project the system back to the ms = 0 and ms = +1 basis. Finally, a second optical pulse is applied to optically sample the system and a measurement basis is obtained by detecting the fluorescence intensity.

[0048] Continuing with FIG. 6, to create a SMAC pair, a second Ramsey pulse sequence includes a third optical excitation pulse applied to the system to optically pump electrons into the ground state (i.e., ms = 0 spin state). This is followed by a third RF excitation pulse (in the form of, for example, a second MW pulse width / 2 (pw₂/2) ). The third RF excitation pulse may again set the system into superposition of the ms = 0 and ms = +1 spin states (or, alternatively, the ms = 0 and ms = -1 spin states, depending on the choice of resonance location). The spins are allowed to freely precess (and dephase) over a time period referred to as tau₂ (τ₂). During this free precession time period, the system measures the local magnetic field and serves as a coherent integration. Next, a fourth RF excitation pulse (in the form of, for example, a MW pw₂/2 pulse) is applied to project the system back to the ms = 0 and ms = +1 basis. Finally, a fourth optical pulse is applied to optically sample the system and a measurement basis is obtained by detecting the fluorescence intensity of the system. FIG. 6 depicts the pulse sequences continuing with another sequence with pwi.

[0049] In some embodiments, a reference signal may be determined by using a reference signal acquisition prior to the RF pulse excitation sequence and measured signal acquisition. A contrast measurement between the measured signal and the reference signal for a given pulsed sequence is then computed as a difference between a processed read-out fluorescence level from the measured signal acquisition and a processed reference fluorescence measurement from the reference signal. The processing of the measured signal and/or the reference signal may involve computation of the mean fluorescence over each of the given intervals. The reference signal acts to compensate for potential fluctuations in the optical excitation power level (and other aspects), which can cause a proportional fluctuation in the measurement and readout fluorescence measurements. Thus, in some implementations the magnetometer includes a full repolarization between measurements with a reference fluorescence intensity (e.g., the reference signal) captured prior to RF excitation (e.g., RF pulse excitation sequence) and the subsequent magnetic b field measurement data. This approach may reduce sensor bandwidth and increase

measurement noise by requiring two intensity estimates per magnetic b field measurement. For a magneto-optical defect material with defect centers magnetometer, this can means that it needs full repolarization of the ensemble defect center states between measurements. In some instances, the bandwidth considerations provide a high laser power density trade space in sensor design, which can impact available integration time and achievable sensitivity.

[0050] In some embodiments, the magnetometer system may omit a reference signal acquisition prior to RF pulse excitation sequence and measured signal acquisition. The system processes the post RF sequence read-out measurement from the measured signal directly to obtain magnetometry measurements. The processing of the measured signal may involve computation of the mean fluorescence over each of the given intervals. In some implementations, a fixed "system rail" photo measurement is obtained and used as a nominal reference to compensate for any overall system shifts in intensity offset. In some implementations, an optional ground reference signal may be obtained during the RF pulse excitation sequence to be used as an offset reference. Some embodiments provide faster acquisition times, reduced or eliminated noise from the reference signal, and increased potential detune Vpp contrast.

[0051] In some embodiments, an approximation of the readout from a Ramsey pulse sequence when the pulse width is much less than the free precession interval may be defined as equation (1) below:

1— m * a ^■.n. )) * (τ + 0)) (1)

[0052] In equation (1) above, τ represents the free precession time, Γ₂ ^* represents spin dephasing due to inhomogeneities present in the system 600, ω_τβ3 represents the resonant Rabi frequency, ω_β^ represents the effective Rabi frequency, a_n represents the hyperfine splitting of the NV diamond material 620 (~2.14 MHz), Δ represents the MW detuning, and Θ represents the phase offset.

[0053] When taking a measurement based on a Ramsey pulse sequence, the parameters that may be controlled are the duration of the MW π/2 pulses, the frequency of the MW pulse (which is referenced as the frequency amount detuned from the resonance location, Δ), and the free precession time τ. FIGS. 7 A and 7B show the effects on the variance of certain parameters of the Ramsey pulse sequence. For example, as shown in FIG. 7A, if all parameters are kept constant except for the free precession time τ, an interference pattern, known as the free induction decay (FID), is obtained. The FID curve is due to the constructive/destructive interference of the three sinusoids that correspond to the hyperfine splitting. The decay of the signal is due to

inhomogeneous dephasing and the rate of this decay is characterized by Γ₂ ^* (characteristic decay time). In addition, as shown in FIG. 7B, if all parameters are kept constant except for the microwave detuning Δ, a magnetometry curve is obtained. In this case, the x-axis may be converted to units of magnetic field through the conversion 1 nT = 28 Hz in order to calibrate for magnetometry.

[0054] Figure 8 is a graphical diagram of an intensity of a measured signal 810 from an optical detector 340 relative to an intensity of a reference signal 820 from the optical detector 340 over a range of detune frequencies. When using a reference signal 820, the reference signal 820 will contain signal information from a prior RF pulse for a finite period of time. This prior signal information in the reference signal 820 reduces available detune Vpp and slope for a detune point for positive slope 830 and a detune point for negative slope 840. Thus, to remove the prior signal information, the system would need to wait until the prior signal information is eliminated from the reference signal or operate without the reference signal.

[0055] In some embodiments, there may be implementation of a reference signal and in some embodiments, omitting of the reference signal may be possible through the use of the SMAC pair due to the increased performance. Eliminating the need for a reference signal reduces the number of pulse measurements that need to be taken and increases the bandwidth of gathering magnetic field data (i.e., an increase in sample rate). [0056] FIG. 9 depicts a plot of a magnetometry curve using a Ramsey sequence in accordance with some embodiments. The plot depicts intensity decreasing as you go up the y- axis, so curves seen in the plot going up represent a dimming in intensity. In some embodiments, the intensity is the measured fluorescence intensity obtained from a magneto-optical defect material with defect centers. In some embodiments, the x-axis represents an RF excitation frequency of a microwave source used in the Ramsey sequence. The magnetometry curve is due to the constructive/destructive interference of the three sinusoids that correspond to the hyperfine splitting in addition to side lobes caused by the Ramsey pulse. In some embodiments, this curve is a representative depiction of the first pulse sequence as depicted in FIG. 6. In some embodiments, the curve shows an upward curve at the center frequency, representing dimming.

[0057] FIG. 10 depicts a plot of an inverted magnetometry curve using a Ramsey sequence in accordance with some embodiments. The plot depicts intensity decreasing as you go up the y- axis so curves seen in the plot going up represent a dimming in intensity. In some embodiments, the intensity is the measured fluorescence intensity obtained from a magneto-optical defect material with defect centers. In some embodiments, the x-axis represents an RF excitation frequency of a microwave source used in the Ramsey sequence. The magnetometry curve is due to the constructive/destructive interference of the three sinusoids that correspond to the hyperfine splitting in addition to side lobes caused by the Ramsey pulse. In some embodiments, this curve is a representative depiction of the second pulse sequence as depicted in FIG. 6. The values of pulse width and τ2 of the second pulse sequence are chosen such that a null is seen at the center frequency, representing a lack of dimming.

[0058] Still referring to FIG. 10 and expanding on a null seen at the center frequency representing a lack of dimming in the fluorescence. In some embodiments using a spin state of the defect center electrons, the null can be thought of in terms of a representation on a Bloch sphere where the zero reference of the spin state and the minus one spin state of the defect center electrons on a sphere are the North Pole and South Pole. In the first sequence, represented in FIG. 9, the first RF pulse may move the state from the baseline zero spin state to the equator of the Bloch sphere. The precession time after the first RF pulse may move the state around the equator of the Bloch sphere representation with time. If the chosen precession time (i.e., τι) allows for the state to go around the circumference all or most of the way before application of the second RF pulse, the second RF pulse may create maximum dimming in the fluorescence. However, if in the sequence, represented in FIG. 10, the first RF pulse was longer and for an amount of time that moved the state from the baseline zero spin state all the way to the South Pole of the Block sphere, then the precession time (i.e., τ2) allows for the state to simply go around the South Pole which is not doing anything, and the second RF pulse to create minimum dimming or take advantage of a null point in the dimming of the fluorescence.

[0059] Therefore, in some embodiments, the curve shows a downward curve at the center frequency, representing a lack of dimming. In some embodiments, the inverted curve is created because the pulse width and τ₂ value are chosen such that the time given to the precession is enough to take advantage of a null point at the chosen frequency.

[0060] FIG. 11 depicts a plot showing a combined magnetometry curve of a traditional and inverted curve in accordance with some embodiments, where the curves from FIG. 9 and FIG. 10 are combined. The curves are combined by combining the intensities at each frequency value, such as for example, by taking the difference between intensities at each frequency value. In some embodiments, the intensity is the measured fluorescence intensity obtained from a magneto-optical defect material with defect centers. In some embodiments, the x-axis represents an RF excitation frequency of a microwave source used in the Ramsey sequence. In some embodiments, the plot combines the curves as depicted in FIG. 10 and FIG. 11. In some embodiments, the combined plot is obtained by taking the difference between the traditional curve and the inverted curve. The plot depicts intensity decreasing as you go up the y-axis so curves seen in the plot going up represent a dimming in intensity. The magnetometry curve is due to the constructive/destructive interference of the three sinusoids that correspond to the hyperfine splitting in addition to side lobes caused by the Ramsey pulse.

[0061] In some implementations such as depicted in FIGS. 9-11, when performing a magnetic field measurement using a magnetometer system, once the magnetometry curves have been obtained, a SMAC measurement is performed at a chosen frequency (e.g. at a frequency with a maximum slope for the curve) and the intensity of the SMAC measurement is monitored to provide an estimate of the magnetic field. In some embodiments, the maximum slope, positive and negative, is determined from the curve obtained by the SMAC pairing and the corresponding frequencies. In some implementations, the curve is first smoothed and fit to a cubic line. In some implementations, only the corresponding frequencies are stored for use in magnetic field measurements. In some implementations, the entire curve is stored. Various implementations may use different numbers of measurement points to plot out the curve. For example, to obtain a width of curve comprising 12.5 MHZ, 500 different frequencies separated by 25 KHz may be measured. Other widths of the curve with differing granularity of the separation of measurement points are possible. In some implementations, a plurality of measurements are done at each measurement point.

[0062] FIG. 12 is a schematic diagram of a system 1200 for a magnetic field detection system according to some embodiments. The system 1200 includes an optical excitation source 1210, which directs optical excitation through a waveplate assembly 1225 to a diamond with nitrogen vacancy (NV) centers or another magneto-optical defect center material with magneto- optical defect centers 1220. An RF excitation source 1230 provides RF radiation to the magneto- optical defect center material 1220. A magnetic field generator 1270 generates a magnetic field, which is detected at the magneto-optical defect center material 1220. Specifically, the magnetic field generator 1270 may be used to apply a bias magnetic field that sufficiently separates the intensity responses corresponding to electron spin resonances for each of the lattice vectors. The controller 1280 may then control the optical excitation source 1210 to provide optical excitation to the magneto-optical defect center material 1220 and the RF excitation source 1230 to provide RF excitation to the magneto-optical defect center material with magneto-optical defect centers 1220. The resulting fluorescence intensity responses for each of the lattice vectors may be collected over time to determine the components of the external magnetic field Bz aligned along directions of the lattice vectors corresponding to magneto-optical defect center material crystallographic axes which may then be used to calculate the estimated vector magnetic field acting on the system 1200. In some embodiments, the magnetic field generator 1270 may be a permanent magnet positioned relative to the magneto-optical defect center material 1220, which generates a known, uniform magnetic field (e.g., a bias or control magnetic field) to produce a desired fluorescence intensity response from the magneto-optical defect center material 1220.

[0063] In some implementations, the magnetic field generator 1270 may generate magnetic fields with orthogonal polarizations. The magnetic field generator 1270 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 material 1220. The predetermined directions may be orthogonal to one another. In addition, the two or more magnetic field generators of the magnetic field generator 1270 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.

[0064] The system 1200 includes, in some implementations, a waveplate assembly 1225. In some implementations, the waveplate assembly 1225 is configured to adjust the polarization of the light (e.g., light from a laser) as it the light is passed through the waveplate assembly 1225. In some implementations, the waveplate assembly 1225 is configured to mount a waveplate to allow for rotation of the waveplate with the ability to stop the plate into a position at a specific rotation. In some implementations, the waveplate assembly 1225 is configured to allow for rotation of the waveplate with the ability to lock the plate in to a position at a specific rotation. Stopping the waveplate at a specific rotation allows the configuration of the waveplate assembly 1225 to tune the polarization of the light passing through the waveplate. In some

implementations, the waveplate assembly 1225 is configured to adjust the polarization of the light such that the orientation of a given lattice of a magneto-optical defect center material allows the contrast of a dimming Lorentzian to be deepest and narrowest such that the slope of each side of the Lorentzian is steepest. In some implementations, the waveplate assembly 1225 is configured such that the polarization of the light is lined up with the orientation of a given lattice of a magneto-optical defect center material such that it allows extraction of maximum sensitivity of the lattice (i.e., maximum sensitivity of a vector in free space). In some implementations, the waveplate assembly 1225 is configured such that four determined positions of the waveplate maximize the sensitivity across all the different lattices of a magneto-optical defect center material. In some implementations, the waveplate assembly 1225 is configured where the position of the waveplate is such that similar sensitivities are achieved to the four Lorentzians corresponding to lattices of a magneto-optical defect center material. Different waveplates may be used in different implementations, including but not limited to half-wave plates and quarter- wave plates.

[0065] The system 1200 may be arranged to include one or more optical detection systems, comprising an optical detector 1240, optical excitation source 1210, and magneto-optical defect center material 1220. Furthermore, the magnetic field generator 1270 may have a relatively high power as compared to the optical detection systems. In this way, the optical detection systems may be deployed in an environment that requires a relatively lower power for the optical detection systems, while the magnetic field generator 1270 may be deployed in an environment that has a relatively high power available for the magnetic field generator 1270 so as to apply a relatively strong magnetic field.

[0066] In some implementations, the system 1200 further includes a controller 1280 arranged to receive a light detection signal from the optical detector 1240 and to control the optical excitation source 1210, the RF excitation source 1230, and a second magnetic field generator (not shown). 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 1200. The second magnetic field generator may be controlled by the controller 1280 via an amplifier.

[0067] In some implementations, the RF excitation source 1230 is a microwave coil, for example. The RF excitation source 1230 is controlled to emit RF radiation with a photon energy resonant with the transition energy between the ground ms = 0 spin state and the ms = ±1 spin states as discussed above with respect to FIG. 3.

[0068] The optical excitation source 1210 may be a laser or a light emitting diode, for example, which emits light in the green, for example. The optical excitation source 1210 induces fluorescence in the red from the Magneto-optical defect center material 1220, where the fluorescence corresponds to an electronic transition from the excited state to the ground state. Light from the Magneto-optical defect center material 1220 is directed through the optical filter 1250 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 1240. The optical excitation light source 1210, in addition to exciting fluorescence in the Magneto-optical defect center material 1220, also serves to reset the population of the ms = 0 spin state of the ground state 3 A2 to a maximum polarization, or other desired polarization.

[0069] In some implementations, the controller 1280 is arranged to receive a light detection signal from the optical detector 1240 and to control the optical excitation source 1210, the waveplate assembly 1225, and the RF excitation source 1230, and the second magnetic field generator. The controller may include a processor 1282 and a memory 1284, in order to control the operation of the optical excitation source 1210, the waveplate assembly 1225, the RF excitation source 1230, and the second magnetic field generator. The memory 1284, which may include a non-transitory computer readable medium, may store instructions to allow the operation of the optical excitation source 1210, the RF excitation source 1230, and the second magnetic field generator to be controlled. That is, the controller 1280 may be programmed to provide control. In some implementations, the controller 1280 is configured to control an angle of the rotation of a half-wave plate.

[0070] In an illustrative embodiment, 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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] 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 system for magnetic detection, comprising:

a magneto-optical defect center material comprising a plurality of defect centers and lattice oriented subsets;

a radio frequency (RF) excitation source configured to provide RF excitation to the magneto-optical defect center material;

an optical excitation source configured to provide optical excitation to the magneto- optical defect center material;

an optical detector configured to receive an optical signal emitted by the magneto- optical defect center material;

a bias magnet configured to separate RF resonance responses of the lattice oriented subsets of the magneto-optical defect center material;

and

a controller configured to:

control the optical excitation source and the RF excitation source to apply a first pulse sequence to the magneto-optical defect center material, the first pulse sequence comprising a first optical excitation pulse, a first pair of RF excitation pulses separated by a first time period, and a second optical excitation pulse to the magneto-optical defect center material;

control the optical excitation source and the RF excitation source to apply a second pulse sequence to the magneto-optical defect center material, the second pulse sequence comprising a third optical excitation pulse, a second pair of RF excitation pulses separated by a second time period, and a fourth optical excitation pulse to the magneto-optical defect center material, wherein a pulse width of the first pair of RF excitation pulses is different than a pulse width of the second pair of RF excitation pulses, and wherein first time period is different than the second time period;

receive a first light detection signal from the optical detector based on an optical signal emitted by the magneto-optical defect center material due to the first pulse sequence;

receive a second light detection signal from the optical detector based on an optical signal emitted by the magneto-optical defect center material due to the second pulse sequence; and compute a combined measurement based on a difference between a measured value of the first light detection signal and a measured value of the second light detection signal wherein the slope of the combined measurement is greater than the slope of the measured value of the first and second light detection signals.

2. The system of claim 1, wherein an RF excitation frequency used for the first pair of RF excitation pulses and the second pair of RF excitation pulses is associated with an axis of a defect center of the magneto-optical defect center material.

3. The system of claim 1, wherein the controller is further configured to compute a change in an external magnetic field acting on the magneto-optical defect center material based on the combined measurement.

4. The system of claim 1, wherein the second pair of RF excitation pulses of the first pulse sequence are applied at a frequency detuned from a resonance frequency of the magneto- optical defect center material.

5. The system of claim 1, wherein the pulse width of the second pair of RF excitation pulses is associated with a fluorescence null point of the magneto-optical defect center material.

6. The system of claim 1, wherein the second time period is associated with a fluorescence null point of the magneto-optical defect center material.

7. The system of claim 1, wherein the pulse width of the second pair of RF excitation pulses and the second time period is associated with a fluorescence null point of the magneto- optical defect center material.

8. The system of claim 1, wherein the RF excitation source is a microwave antenna.

9. The system of claim 1, wherein the controller is configured to apply the first pair of RF excitation pulses followed by the second pair of RF excitation pulses.

10. The system of claim 7, wherein the pulse width of the first pair of RF excitation pulses and the first time period is associated with a fluorescence low point of the magneto- optical defect center material.

11. The system of claim 1, wherein the bias magnet is one of a permanent magnet, a magnet field generator, or a Halbach set of permanent magnets.

12. A method for magnetic detection using a magneto-optical defect center material comprising a plurality of defect centers, the method comprising:

applying a first pulse sequence to the magneto-optical defect center material, the first pulse sequence comprising a first optical excitation pulse, using an optical excitation source, a first pair of RF excitation pulses separated by a first time period using a radio frequency (RF) excitation source, and a second optical excitation pulse to the magneto-optical defect center material using the optical excitation source;

applying a second pulse sequence to the magneto-optical defect center material, the second pulse sequence comprising a third optical excitation pulse, using the optical excitation source, a second pair of RF excitation pulses separated by a second time period using the RF excitation source, and a fourth optical excitation pulse to the magneto-optical defect center material, using the optical excitation source, wherein a pulse width of the first pair of RF excitation pulses is different than a pulse width of the second pair of RF excitation pulses, and wherein first time period is different than the second time period;

receiving a first light detection signal, using an optical detector, based on an optical signal emitted by the magneto-optical defect center material due to the first pulse sequence;

receive a second light detection signal, using the optical detector, based on an optical signal emitted by the magneto-optical defect center material due to the second pulse sequence; and

computing a combined measurement based on a difference between a measured value of the first light detection signal and a measured value of the second light detection signal.

13. The method of claim 12, wherein an RF excitation frequency used for the first pair of RF excitation pulses and the second pair of RF excitation pulses is associated with an axis of a defect center of the magneto-optical defect center material.

14. The method of claim 12, further comprising computing a change in an external magnetic field acting on the magneto-optical defect center material based on the combined measurement.

15. The method of claim 12, wherein the second pair of RF excitation pulses of the first pulse sequence are applied at a frequency detuned from a resonance frequency of the magneto- optical defect center material.

16. The method of claim 12, wherein the pulse width of the second pair of RF excitation pulses is associated with a fluorescence null point of the magneto-optical defect center material.

17. The method of claim 12, wherein the second time period is associated with a fluorescence null point of the magneto-optical defect center material.

18. The method of claim 12, wherein the pulse width of the second pair of RF excitation pulses and the second time period is associated with a fluorescence null point of the magneto- optical defect center material.

19. The method of claim 12, wherein the first pair of RF excitation pulses is applied followed by the second pair of RF excitation pulses.

20. The method of claim 18, wherein the pulse width of the first pair of RF excitation pulses and the first time period is associated with a fluorescence low point of the magneto- optical defect center material.

21. A magnetic detection system comprising:

a defect center material responsive to an applied magnetic field;

a radio frequency (RF) emitter operational to provide a first RF pulse sequence separated by at least one pause, said RF emitter operational to provide a second RF pulse sequence that is different from the first RF pulse sequence;

a detector operational to measure a fluorescence of the defect center material in conjunction with the first RF pulse sequence and the second RF pulse sequence, thereby providing a first measurement curve and a second measurement curve affected by the applied magnetic field, respectfully; and

a control circuit connected to the detector and operational to determine a difference between the first measurement curve and the second measurement curve to obtain greater sensitivity to variations in the applied magnetic field.

22. The system of claim 21, wherein the first RF pulse sequence and the second RF pulse sequence are applied at a frequencies detuned from a resonance frequency of the defect center material.

23. The system of claim 21, wherein the first RF pulse sequence is applied followed by the second RF pulse sequence.

24. The system of claim 21, wherein the defect center material is a nitrogen vacancy diamond.

25. The system of claim 21, wherein the defect center material is Silicon Carbide (SiC).

26. A method for detecting a magnetic field comprising:

emitting a first RF pulse sequence separated by at least one pause, using an RF emitter to a defect center material;

emitting a second RF pulse sequence that is different from the first RF pulse sequence, using the RF emitter, to the defect center material; measure a fluorescence of the defect center material in conjunction with the first RF pulse sequence and the second RF pulse sequence, using a detector;

providing a first measurement curve and a second measurement curve of the measured fluorescence of the defect center material affected by the applied magnetic field, respectfully for the first RF pulse sequence and the second RF pulse sequence; and

determine a difference between the first measurement curve and the second measurement curve to obtain greater sensitivity to variations in the applied magnetic field.

27. The method of claim 26 wherein determining the difference between the first measurement curve and the second measurement curve is performed by a control circuit.

28. The method of claim 26, wherein the first RF pulse sequence and the second RF pulse sequence are applied at a frequency detuned from a resonance frequency of the defect center material.

29. The method of claim 26, wherein the first RF pulse sequence is emitted followed by the second RF pulse sequence.

30. The method of claim 26, wherein the defect center material is a nitrogen vacancy diamond.

31. The method of claim 26, wherein the defect center material is Silicon Carbide (SiC).

32. A system for magnetic detection, comprising:

a magneto-optical defect center material comprising a plurality of defect centers; a means of providing RF excitation to the magneto-optical defect center material; a means of providing optical excitation to the magneto-optical defect center material; a means of receiving an optical signal emitted by the magneto-optical defect center material;

a means of providing a bias magnetic offset to distinguish response curves of different lattice orientation subsets of the magneto-optical defect center material; and

a means of controlling the provided RF excitation and provided optical excitation to: apply a first pulse sequence to the magneto-optical defect center material, the first pulse sequence comprising a first optical excitation pulse, a first pair of RF excitation pulses separated by a first time period, and a second optical excitation pulse to the magneto-optical defect center material;

receive a second light detection signal from the optical detector based on an optical signal emitted by the magneto-optical defect center material due to the second pulse sequence; and

compute a combined measurement based on a difference between the measured value of the first light detection signal and a measured value of the second light detection signal.