Abstract:
Measurement of a precessional rate of a gas, such as an alkali gas, in a magnetic field is made by promoting a non-uniform precession of the gas in which substantially no net magnetic field affects the gas during a majority of the precession cycle. This allows sensitive gases that would be subject to spin-exchange collision de-phasing to be effectively used for extremely sensitive measurements in the presence of an environmental magnetic field such as the Earth&#39;s magnetic field.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation in part on U.S. patent application Ser. No. 13/198,940 filed Aug. 5, 2011 now U.S. Pat. No. 8,698,493 and hereby incorporated by reference. 
    
    
     This invention was made under HD057965 awarded by the National Institutes of Health and DE-FC02-03ER46093 awarded by the U.S. Department of Energy. The government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a magnetometer that may be sensitive to very small magnetic fluctuations in the presence of a much larger static field, and in particular to a magnetometer that measures precession of gas atoms in a way that the dephasing effects between gas atoms are reduced. 
     Atoms such as the alkali metals have a net spin which possesses a magnetic moment. Accordingly, if such atoms can be polarized and stimulated into precession, the frequency of precession can be used to precisely measure a magnetic field free from other influences. In this way, a precision magnetometer may be constructed. 
     The ability of alkali gas atoms to measure magnetic field is often limited by interactions between the alkali gas atoms (spin-exchange relaxation) themselves which cause de-phasing of the precessing alkali gas atoms. These interactions can be reduced, by eliminating any environmental magnetic field other than the field being measured (for example the Earth&#39;s magnetic field), for example by using nulling coils energized to produce a canceling countervailing magnetic field. Such magnetometers are termed “spin exchange relaxation-free (SERF) magnetometers. 
     Nulling the external magnetic field can be difficult and must be extremely precise to obtain the benefits of increased sensitivity of the alkali gas atoms. 
     SUMMARY OF THE INVENTION 
     The present invention provides a magnetometer with high magnetic field sensitivity comparable to a SERF magnetometer without the need to operate in a near zero magnetic field. This is accomplished by modulating the precession of the alkali atoms with a controllable time-dependent magnetic field to produce a time averaged stationary magnetic moment. This modulation pattern may provide a modulating field having extremely narrow pulses during which precession occurs and a long intervening duration during which precession is frozen. In this latter state, the atoms are inherently retained in a low magnetic field in which spin exchange collisions do not dephase the magnetic moments of the atoms. The modulation signal may be controlled by a feedback mechanism that largely eliminates requirements for precise pulse shaping or pulse amplitude. In this way, the problems attendant to nulling the external magnetic field with a static field are largely eliminated. 
     Specifically, the present invention provides a magnetometer having a chamber holding a gas exposable to an external magnetic field and directed along a z-axis. An electromagnet is positioned to apply a local magnetic field to the chamber and a signal source communicating with the electromagnet generates a field signal adapted to drive the electromagnet to produce a local magnetic field causing a non-uniform precession of a magnetic moment of the gas at an average frequency substantially equal to a frequency of uniform precession of the gas in the external magnetic field without an influence of the local magnetic field, while limiting a portion of each precession cycle during which substantial precession occurs. A monitor outputs a signal indicating the precession of the gas. 
     It is thus an object of at least one embodiment of the invention to provide a high sensitivity magnetometer that may operate in ambient magnetic field that might normally cause substantial spin exchange collision dephasing. 
     The signal may be adapted to limit the portion of each precession cycle during which substantial precession occurs to less than 50% of the precession cycle, or to less than 10% of the precession cycle. 
     It is thus an object of at least one embodiment of the invention to substantially minimize the effect of spin exchange collisions on dephasing of the precession of the atoms. 
     The magnetometer may further include a precession monitor providing a moment signal indicating orientation of a magnetic moment of the gas in the chamber and a feedback control system receiving the moment signal to control a shape of the field signal from the signal source to complete substantially 360 degrees of precession of the gas during the portion of each precession cycle during which substantial precession occurs. 
     It is thus an object of at least one embodiment of the invention to provide a feedback control mechanism eliminating the need for precise open-loop wave shaping and/or amplitude control. 
     The feedback control system may monitor a phase of the moment signal to control the duration of portions of the field signal during which substantial precession occurs. 
     It is thus an object of at least one embodiment of the invention to provide a simple control strategy for adjusting the field signal largely indifferent to the exact field signal shape or amplitude. 
     Alternatively or in addition, the feedback control system may receive the moment signal to control a frequency of the signal from the signal source to produce an average signal value of substantially zero. 
     It is thus an object of at least one embodiment of the invention to provide a feedback mechanism conforming the non-uniform precession of the atoms to their natural precession in the absence of the local field. 
     The feedback control system may monitor an average value of the moment signal to control the frequency of the field signal. 
     It is thus an object of at least one embodiment of the invention to provide a simple technique for frequency control. 
     These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified perspective view of a magnetic resonator system described in the parent application to this continuation application invention and using a noble gas and showing orientation of a stimulating laser, magnetic field coils, and orthogonal sensing lasers about a gas chamber holding a noble gas and alkali gas; 
         FIG. 2  is a simplified perspective view of the precession of the magnetic moment of the atoms of the alkali gas stimulated into precession in the x-y plane at a changing precession rate plotted against time and showing the driving magnetic field signal for this nonuniform precession; 
         FIG. 3  is a vector diagram showing orientation of the magnetic moment of the alkali gas in the x-y plane as weighted by incremental dwell time at each angle, per the time plot of  FIG. 13 , illustrating a magnetic moment of the alkali gas having a time averaged upward vertical orientation; 
         FIG. 4  is a graph and vector diagram similar to those of  FIG. 14  and  FIG. 13 , showing a precession rate providing magnetic moment of the alkali gas having a time averaged downward vertical orientation; 
         FIGS. 5-8  are corresponding perspective, x-y plane and x-z plane depictions of the magnetic moments of the noble gas and alkali gas during transverse plane precession of the magnetic moments of the noble gas population as induced by switching between the modulation patterns producing upward and downward magnetic moment for the alkali gas; 
         FIG. 9  is a plot of the signal received by a z-axis sensing laser such as provides a measure of precession of the noble gas about the z-axis; 
         FIG. 10  is a functional block diagram of a control system for the present invention; 
         FIG. 11  is a representation of an alternative chamber of  FIG. 1  holding multiple species of atoms for providing a gyroscope output less sensitive to the external magnetic field; 
         FIG. 12  is a figure similar to  FIG. 1  showing a magnetometer of the present invention used, for example, with a single alkali gas species in contrast to the above described system using two gas species; 
         FIGS. 13-15  are figures similar to  FIGS. 2-4  for the embodiment of  FIG. 12 ; 
         FIG. 16  is a figure similar to  FIG. 10  for the embodiment of  FIG. 12 ; and 
         FIG. 17  is a figure similar to  FIG. 2  for the embodiment of  FIG. 12  showing phase sensitive feedback for control of pulse width. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Noble Gas Resonator 
     Referring now to  FIG. 1 , a magnetic resonator system  10 , described generally in co-pending application Ser. No. 11/198,940 filed Aug. 5, 2011 may include a chamber  12  holding an alkali gas  14  and noble gas  16 . In one embodiment, the alkali gas  14  may be rubidium (Rb) and the noble gas  16  may be a helium isotope (3-He). Each of the atoms of the alkali gas  14  and the noble gas  16  has magnetic moments  15  and  17 , respectively, represented by directional arrows in the figure. 
     The chamber  12  may have transparent walls allowing a laser beam  18  of a first Faraday rotational probe  21  to pass through the chamber  12  along a z-axis of a Cartesian coordinate system having its z-axis aligned with an external magnetic field  20  (B 0 ). This laser beam  18  may be emitted from a laser source  22  and received by a polarimeter  24  positioned on opposite sides of the chamber  12  along the z-axis from the laser source  22 . As will be understood in the art, this first Faraday rotational probe  21  provides a measure of a z-axis component of the magnetic moment  15  of the population of alkali gas  14 . 
     A set of magnetic coils  23  (for example a Helmholtz coil pair flanking the chamber and aligned along the z-axis) may provide an alternating or pulsed magnetic field (B 1 ) aligned along the z-axis. As will be discussed below, this field provides a means for controlling the time-averaged alkali spin precession in the presence of an external field B 0 . In particular, the field B 1  will be modulated to moderate the naturally fast precession rate of the alkali gas  14  in the external magnetic field B 0  to be aligned along the pump laser  32  direction. 
     A second Faraday rotational probe  27  may include a laser source  26  directing a laser beam  30  along the y-axis through the chamber  12  to a corresponding receiving polarimeter  28  on the other side of the chamber  12 . This second Faraday rotational probe  27  provides a measure of the y-axis component of the magnetic moment  15  of the population of alkali gas  14 . 
     A “pump” laser  32  may direct a laser beam  33  along the x-axis through the chamber  12  after passing through a polarization modulator  34 . The pump laser  32  and polarization modulator  34  may “spin-polarize” the magnetic moment  15  of the alkali gas  14  to align in either of two directions along the x-axis (upward or downward as depicted) according to a modulation signal  36  received by the polarization modulator  34 . This polarization occurs by transfer of the angular momentum of the polarized photons of the laser beam  33  to the alkali gas  14  as will be generally understood in the art. 
     It will be understood that the various laser sources  22 ,  26 , and  32  in various combinations may be derived from one or more light sources. 
     Signals from the polarimeters  28  and  24  may be provided as electrical signal input to a control system  40  to be processed as will be described below. The control system  40  may in turn output the modulation signal  36  to the polarization modulator  34 . The control system  40  may also output the modulation signal  37  to the magnetic coils  23 . The control system  40  may be constructed of discrete components or functional blocks such as lock-in amplifiers, frequency counters and the like as will be described below or these elements may be implemented in software in an electronic computer  42  as depicted, or in dedicated hardware including an application-specific integrated circuit or digital signal processor, or as a combination of different elements in a hybrid configuration. In the case of implementation in a computer  42 , the computer  42  may execute a stored program  44  and may communicate with user input devices  46  such as a keyboard and/or mouse and may provide output for example through a graphic display screen  48  or other functionally similar device. Alternatively, or in addition, the control system  40  may provide a control output  50 , for example, providing a gyro output (e.g., angle or angular rate about the z-axis) or a magnetometer output (e.g. Gauss) use for control of an ancillary device such as aircraft or the like. 
     Referring also to  FIG. 2 , during operation of the magnetic resonator system  10 , the control system  40  will control the laser beam  33  and the applied magnetic field B 1  to drive the magnetic moment of the population of alkali gas  14  into precession substantially within the x-y plane. This precession is invoked by illuminating the alkali gas  14  with photons having alternate upward and downward angular momentum indicated by arrows  52 . The momentum of the photons is then transferred to the alkali gas  14  to align the magnetic moment  15  of the alkali gas  14  with the photon angular momentums. Ongoing precession of the alkali gas  14  is then controlled by varying the B 1  field by control signal  37 . The y-axis component of this precession of the magnetic moment  15  of the alkali gas  14  may be detected by the beam  30  of the second Faraday rotational probe  27   
     Control of the B 1  will be such that the precession  54  of the magnetic moment  15  of the alkali gas  14  in the x-y plane will not be at a uniform angular rate such as would be detected as a sinusoidal waveform by the second Faraday rotational probe  27 , but rather, as an irregular angular rate, progressing relatively slower in the upper half cycle such as will produce a compressed precession waveform  56 . The compressed precession waveform  56  represents the y-axis component of the magnetic moment  15  precessing at an irregular rate having a greater dwell time  29  when the magnetic moment of the alkali gas  14  is facing in an upward rather than the downward direction. 
     This compressed precession waveform  56  may be produced by modulating the B 1  field to a low relatively constant negative value  31  to substantially offset the B 0  field during the time  29  (greatly reducing the precession when the magnetic moment  15  is facing upward) for most of the period 1/ω of the normal precession of the alkali gas  14  in field B 0  The field B 1  may then be maximized during a short time remaining in 2π/ω by providing a positive pulse of amplitude  41  augmenting the field B 0  to promote rapid precession of the alkali gas  14  by 360 degrees back to the upward orientation. The field B 1  is controlled to have no direct current (that is areas  53  and  55  during times  29  and the remainder of 1/ω are equal and opposite) so that it has no average effect on the precession frequency of the alkali gas  14  or noble gas  16 . 
     Referring momentarily to  FIG. 3 , a diagram of the alkali magnetic moment  15  at various points in time as an angular vector having a length proportional to the incremental dwell time at each angle, it traces an oval outline  35  reflecting the increased time weighting of the magnetic moment in the upward direction. The centroid of this outline  35  may illustrate the time-averaged magnetic moment  15  as a stationary upward magnetic moment  15   a . The compressed precession waveform  56  which still retains the normal precession rate of the alkali gas  14  in the magnetic field B 0  boosts the length of the average magnetic moment  15  over that which might be provided by a sinusoidal B 1  field by a significant amount (for example 10 times) greatly increasing the effect on the noble gas  16 . In addition, effective neutralization of the B 0  field during time  29  comprising most of the precession cycle, substantially reduces dephasing of the precession due to spin-exchange between atoms of the alkali gas  14 . This stationary magnetic moment  15   a  represents moment experienced by the noble gas  16  during its precession during the irregular precession of the alkali gas  14 , the latter of which generally has precession rate as much is 1000 times higher the precession rate of the noble gas  16 . 
     Referring now to  FIG. 4 , it will be understood that the same waveform  56  with an inversion of signal  36  received by the polarization modulator  34  will produce a precession waveform  60  producing a time averaged magnetic moment  15   b  facing downward along the x-axis. Accordingly, by switching signal  36 , an effective upward or downward magnetic moment  15   a  or  15   b  of the alkali gas  14  may be generated within the transverse x-y plane. 
     Referring now to  FIG. 5 , the laser beam  33  from the pump laser  32  and the coils  23  initially may be modulated to produce the upward directed time averaged magnetic moment  15   a  causing the magnetic moments  17  of the population of noble gas to align therewith along the x-axis. Referring also to  FIG. 9 , at this time a z-component signal  62  from the first Faraday rotational probe  21  will show no z-axis component as indicated at waveform value  64 . 
     Referring now to  FIG. 6 , a short time later, the magnetic moments  17  of the noble gas will have precessed away from a vertical orientation along the x-axis caused by the influence of the external magnetic field B 0 . The divergence of the magnetic moments  15   a  and  17  causes a torque on the magnetic moment  15   a  pushing the magnetic moment  15   a  by an angle β out of the x-y plane in the direction of B 0 . This excursion of the magnetic moment  15   a  out of the x-y plane applies a slight additional z-axis magnetic field to the noble gas (adding to field B 0 ) causing the noble gas  16  to increase slightly in precession. A similar torque will be applied to the magnetic moment  17  which will be neglected at this time. 
     Referring now to  FIG. 7 , after an additional time, the magnetic moment  17  of the noble gas  16  will have precessed to be aligned with the y-axis so that the magnetic moments  15   a  and  17  are nearly perpendicular. In this state, the magnetic moment  17  produces its maximum torque on magnetic moment  15   a , which will afterwards begin to decrease as the magnetic moment  17  passes below the y-axis. Referring to  FIG. 9 , accordingly, at this time the z-component signal  62  is at a maximum waveform value  70  of zero slope. This point of zero slope may be used to change the polarization of the laser beam  33  to change the asymmetrical angular rotation of the alkali gas  14  to the pattern shown in  FIG. 4 , with the result of flipping the angle of the magnetic moment  15   a  to  15   b  so that it is facing vertically downward as depicted in  FIG. 7 . 
     As shown in  FIG. 9 , at this time the z-component signal  62  is at a negative waveform value  72  caused by a corresponding reversal of the torque on magnetic moment  15   b  from magnetic moment  17  still on the y-axis. This torque now causes the magnetic moment  15   a  to be deflected by an angle −β from the x-y plane but in a direction counter to that of B 0 . This negative deflection of the magnetic moment  15   b  produces a negative z-axis component that slows the precession of the magnetic moment  17  by an amount offsetting the previously described increase in precession, resulting in no net effect on the magnetic moment  17  by the magnetic moments  15   a  and  15   b  of the alkali gas  14 . 
     Referring now to  FIG. 8 , the magnetic moment  17  continues to precess until it is aligned with magnetic moment  15   b  directed downward along the x-axis. The torque between these magnetic moments  15   b  and  17  drops to zero. Referring to  FIG. 9 , z-component signal  62  returns to a zero value at waveform value  74 . 
     It will be appreciated that the zero crossings  64  and  74  of waveform  62  may alternatively be used for synchronization of the modulation. 
     It will also be appreciated that the amount of deflection of the magnetic moment  15   a  and  15   b  out of the x-y plane is symmetrical not only in its peak value but also in its decline to have no net effect on the time average value of the precession of the magnetic moment  17 . 
     Referring to  FIG. 9 , it will be understood that the periodicity of z-component signal  62  over one complete cycle represents the inverse of the precession frequency of the noble gas  16  without influence by the alkali gas  14  and can therefore be used to accurately measure the precession of the population of the noble gas  16  without additional sensing structure. 
     It should be noted that the magnetic moment  17  of the noble gas will also be affected by the torque caused by magnetic moments  15   a  and  15   b  of the alkali gas but again generally this deflection along the z-axis will be positive during a first-half cycle of the precession of the magnetic moment  15  and negative during a second-half cycle of that precession to be fully offset over one cycle. 
     Referring now to  FIG. 10 , the control system  40  may implement a number of functional blocks either through discrete components or software or a combination of the same as described above. In one embodiment, precession waveform  56  from the polarimeter  28  representing the y-axis component of the precession of the alkali gas  14  may be received at a phase comparator  80  of a phase locked loop type lock-in amplifier  82 . The phase comparator  80  may also receive an output of a voltage controlled oscillator  84  divided by a divider  86 , and may operate to lock the phase and frequency of the voltage controlled oscillator  84  with the phase of the precession waveform  56  representing the precession of the magnetic moment  15  of the alkali gas  14 . 
     The undivided high-frequency output of the voltage controlled oscillator may then be used to drive a synthesizer  87  synchronized to the precession waveform  56  providing a desired waveform implementing the modulation signal for driving the coils providing B 1 . The synthesized modulation signal  37  for coils  23  may be back-calculated from the desired precession waveforms  56  or  60 , as will be understood by those of ordinary skill in the prior art, to maintain the time averaged alkali spin along the x-axis at substantially the frequency of the freely precessing alkali gas  14  in field B 0 . Generally the amplifier  82  thus adjusts the phase and frequency of the synthesized modulation signal  37  for the coils  23  to match the natural precession frequency of the alkali gas  14   
     As noted, the synthesized modulation signal  36  may be selected to generate either the upward magnetic moment  15   a  or the downward magnetic moment  15   b  and this synthesized waveform may be selected by an input signal  93  to the synthesizer  87 . This input signal  93  may be generated from the z-component signal  62  from polarimeter  24  of the first Faraday rotational probe  21  (shown in  FIG. 9 ) by detecting the zero slope waveform value  70  of the positive peak of the z-component signal  62 , for example, using a differentiator  92  and zero crossing detector  94  triggering a toggle or flip-flop  96 . The flip-flop  96  provides a binary output producing the input signal  93  to switch the magnetic moment  15  of the alkali gas  14  appropriately using the polarization modulator  34 . 
     In an alternative embodiment, the precession of the noble gas  16  may be measured directly using the Faraday rotational probe  27  which may be used to control the polarization modulator  34 . 
     A frequency counter  90  may be used to produce a count signal  100  over a period of time, which may be scaled or otherwise processed by scaler  102  to provide for a display on display screen  48  indicating the precession frequency of the noble gas  16  or to provide the control output  50  for use as a gyroscope or magnetometer. 
     Referring now to  FIG. 11 , it will be appreciated that these principles and techniques described above may be extended to a chamber  12  holding a first and second isotope of noble gas  16   a  and  16   b  having magnetic moments  17   a  and  17   b  respectively with different gyromagnetic constants. The use of these two different isotopes permits the production of a control output  50  for a gyroscope that is largely indifferent to the value of the external magnetic field B 0  using the equations (1) (2) as discussed above. In such a system, the waveforms needed for each species of the isotope may be combined by multiplication and the sign of the product used to provide the signal to the polarization modulator  34 . Frequency demultiplexing techniques may be used to extract the individual signals from the waveforms from the Faraday rotational probes  21  and  27 . The control output  50  will then reveal the rotation of a coordinate system fixed with respect to the reference frame used to determine the precession of the noble gas  16 , e.g. the reference frame of the Faraday rotational probe  21 . 
     It will be appreciated that the present invention may be used, for example, with a magnetic shield  11  (shown in  FIG. 1 ) to moderate the influence of external magnetic fields that may have variability, when a gyroscope is being constructed. In addition the invention may be used with nulling coils to provide a field B 2  generally aligned with the z-axis to null or control the B 0  field. The laser detectors shown may be replaced by other magnetic detectors including for example pickup coils. It will be understood that the gas mixtures described may include other gaseous elements and the invention may also use noble gases with quadrupole interactions. In addition it is contemplated that the invention may work in with hybrid spin-exchange optical pumping in which there are two species of alkali atoms and one interacts with the laser and the other works as a spin-bath to exchange angular momentum between the first alkali and the noble gas. 
     Alkali Gas Magnetometer 
     Referring now to  FIG. 12 , a magnetic resonator system  10 , per another embodiment of the present invention, may include a chamber  12  holding as little as one species of alkali gas  14 . In one embodiment, the alkali gas  14  may be rubidium (Rb). As discussed above, each of the atoms of the alkali gas  14  has a magnetic moment  17  represented by directional arrows in the figure. 
     A set of magnetic coils  23  (for example a Helmholtz coil pair flanking the chamber and aligned along the z-axis) may provide an alternating or pulsed magnetic field (B 1 ) aligned along the z-axis. As will be discussed below, this field provides a means for controlling the time-averaged alkali spin precession in the presence of an external field B 0 . In particular, the field B 1  will be modulated to promote non-uniform precession to the alkali gas  14  in the external magnetic field B 0 . 
     A Faraday rotational probe  27  may include a laser source  26  directing a laser beam  30  along the y-axis through the chamber  12  to a corresponding receiving polarimeter  28  on the other side of the chamber  12 . This Faraday rotational probe  27  provides a measure of the y-axis component of the magnetic moment  15  of the population of alkali gas  14 . 
     A “pump” laser  32  may direct a laser beam  33  along the x-axis through the chamber  12  after passing through a polarization filter  34 . The pump laser  32  and polarization filter  34  may “spin-polarize” the magnetic moment  15  of the alkali gas  14  to align along the x-axis (upward as depicted). This polarization occurs by transfer of the angular momentum of the polarized photons of the laser beam  33  to the alkali gas  14   
     It will be understood that the various laser sources  26 , and  32  in various combinations may be derived from one or more light sources. 
     Signals from the polarimeter  28  may be provided as an electrical signal input to a control system  40  to be processed as will be described below. The control system  40  may in turn output the modulation signal  37  to the magnetic coils  23 . The control system  40  may be constructed of discrete components or functional blocks such as lock-in amplifiers, frequency counters and the like as will be described below or these elements may be implemented in software in an electronic computer  42  as depicted, or in dedicated hardware including an application-specific integrated circuit or digital signal processor, or as a combination of different elements in a hybrid configuration. 
     In the case of implementation in a computer  42 , the computer  42  may execute a stored program  44  and may communicate with user input devices  46  such as a keyboard and/or mouse and may provide output for example through a graphic display screen  48  or other functionally similar device. 
     Referring also to  FIG. 13 , during operation of the magnetic resonator system  10 , the control system  40  will control the applied magnetic field B 1  to drive the magnetic moment of the population of alkali gas  14  into precession substantially within the x-y plane. This precession is invoked by illuminating the alkali gas  14  with photons having upward angular momentum indicated by arrow  52 . The momentum of the photons is then transferred to the alkali gas  14  to align the magnetic moment  15  of the alkali gas  14  with the photons&#39; angular momenta. Ongoing precession of the alkali gas  14  is then controlled by varying the B 1  field by control signal  37 . The y-axis component of this precession of the magnetic moment  15  of the alkali gas  14  may be detected by the beam  30  of the Faraday rotational probe  27   
     The waveform of the B 1  field will be such that the precession  54  of the magnetic moment  15  of the alkali gas  14  in the x-y plane will not be at a uniform angular rate such as would be detected as a sinusoidal waveform by the Faraday rotational probe  27 , but rather, as an irregular or non-uniform angular rate, progressing relatively slower in the upper half cycle such as will produce a compressed precession waveform  56 . The compressed precession waveform  56  represents the y-axis component of an a precession having an angular rate with a greater dwell time  29  of the magnetic moment  15  of the alkali gas  14  during precession when the magnetic moment of the alkali gas  14  is facing in an upward rather than the downward direction. 
     This compressed precession waveform  56  may be produced by modulating the B 1  field signal  59  to a low relatively constant negative value  31  to substantially offset the B 0  field during the time  29  (greatly reducing the precession when the magnetic moment  15  is facing upward) for most of the period 2π/ω of the normal precession of the alkali gas  14  in field B 0  The B 1  field signal  59  may then be maximized during a short time remaining in 2π/ω by providing a positive pulse  57  of amplitude  41  augmenting the field B 0  to promote rapid precession of the alkali gas  14  by 360 degrees back to the upward orientation. The field B 1  signal  59  is controlled to have no direct current (that is areas  53  and  55  during times  29  and the remainder of 2π/ω are equal and opposite) so that it has no net effect on the precession frequency of the alkali gas  14 . 
     Referring momentarily to  FIG. 14 , a diagram of the alkali magnetic moment  15  at various points in time as an angular vector having a length proportional to the incremental dwell time at each angle, it traces an oval outline  35  reflecting the increased time weighting of the magnetic moment in the upward direction. The centroid of this outline  35  may illustrate the time-averaged magnetic moment  15  as a stationary upward magnetic moment  15   a . The effective neutralization of the B 0  field during time  29 , comprising most of the precession cycle, substantially reduces dephasing of the precession due to spin-exchange between atoms of the alkali gas  14 . 
     Referring now to  FIG. 15 , it will be understood that the same waveform  56  with an inversion of signal  36  received by the polarization modulator  34  will produce a precession waveform  60  producing a time averaged magnetic moment  15   b  facing downward along the x-axis. Accordingly, by replacing the polarization filter  34  with a polarization modulator, an effective upward or downward magnetic moment  15   a  or  15   b  of the alkali gas  14  may be generated within the transverse x-y plane. This switching of polarity may be useful for the measurement of AC magnetic fields. 
     Referring now to  FIG. 16 , the control system  40  may implement a number of functional blocks either through discrete components or software or a combination of the same as described above. In one embodiment, precession waveform  56  from the polarimeter  28  representing the y-axis component of the precession of the alkali gas  14  may be received at the control system  40  The control system  40  also synthesizes the driving signal  37  providing field signal  59  to the magnetic coils  23 . 
     Referring now to  FIGS. 16 and 17 , the control system  40  monitoring the precession waveform  56  from the polarimeter  28  may control the frequency of the pulse  57  to ensure proper alignment of the magnetic moment  15  along the x-axis. One way to accomplish this is to monitor the zero crossing of the signal  56  with respect to the width or beginning of the pulses  57 . When the pulse width  57  is correct, the zero crossing of signal  56  will will have a period  61  such that it will bisect the pulse  57  in time (for an integer multiple of 360 degrees of precession during the pulse  57 ). When the area of the pulse  57  is too great (as shown in  FIG. 17 ) the magnetic moment  15  will overshoot the x-axis during the precessing cycle (greater than an integer number of revolutions), advancing the phase of the succeeding waveform at the next pulse  57  by a phase error Δφ. This phase error Δφ may be readily detected by the control system  40  to decrease the pulse width  57  by synthesis techniques well known in the art and to be discussed below. Conversely, it will be understood that if the pulse width  57  is too narrow, the magnetic moment  15  will undershoot the x-axis during the precessing cycle (less than an integer number of revolutions) causing a detectable delay in the zero crossing of the waveform  56  (not shown) effecting the opposite correction. 
     The control system also monitors the time-average of the detected waveform  56  from the Faraday probe. The repetition period of the waveform  57  of B 1  is adjusted to hold the time-averaged value of waveform  56  to be zero. Then the frequency is precisely equal to the mean Larmor precession frequency in the field B 0 . This frequency may be used to deduce the strength of the external magnetic field. 
     It will be understood that the level  31  of the waveform  59  may be independently adjusted within this control strategy to decrease the width of the pulses  57  and thus increase the time during which the atoms  14  are subject to substantially zero total field (B 1  plus B 0 ) as desired. Generally, the duty cycle of the pulses  57  (that is the ratio of their width to the period of the precessing cycle measured pulse to pulse) will be much less than 50 percent of the cycle and more typically much less than 10 percent of the cycle and ideally will be minimized within the practical constraints of the apparatus. 
     It will be understood then, that the field signal  59  will in this way be synchronized to the precession waveform  56  while maintaining the time averaged alkali spin along the x-axis at substantially the frequency of the freely precessing alkali gas  14  in field B 0 . Generally, the control system  40  thus adjusts the phase and frequency of the synthesized modulation signal  37  as well as the area  53  relative to area  55  for the coils  23  to match the natural precession frequency of the alkali gas  14 . 
     A frequency counter  90  may be used to produce a count signal  100  over a period of time, which may be scaled or otherwise processed by scaler  102  to provide for a display on display screen  48  indicating the precession frequency of the alkali gas  14  indicating magnetic field strength of B 0 . 
     It will be appreciated that the control system  40  as described above may be replaced with a simple manual adjustment system in cases where the external magnetic field is largely static. In such a system, the signal from the Faraday probe  27  would be observed and the signal  59  adjusted to provide the desired phasing and average value. The frequency of the signal from the Faraday probe  27  would then be used to deduce magnetic field strength. Another way to implement a magnetometer applied field B 1  as described above and to provide a feedback magnetic field B 2  that works to maintain a constant value of B 0 +B 2  to maintain resonance. 
     It will be appreciated that the present invention may be used in all of these embodiments, for example, with a magnetic shield  11  (a fragment shown in  FIGS. 1 and 12 ) to moderate the influence of external magnetic fields that may have variability. In addition the invention may be used with nulling coils to provide a field B 2  generally aligned with the z-axis to null or control the B 0  field. The laser detectors shown may be replaced by other magnetic detectors including for example pickup coils. It will be understood that the gas described may include other gaseous elements including for example other alkali gases or metastable helium or the like. A hybrid spin-exchange optical pumping mixture may be used using for polarizing the gases, for example, using two species of alkali atoms. 
     An embodiment of the magnetometer that is appropriate to measure AC magnetic fields at a frequency f can be realized by applying a static field B 0 =f 0 /γ, the modulating field B 1  at frequency f 1 , and additionally modulating the pump laser polarization at the frequency f=f 0 −f 1 . This produces a rotating polarization of the spins at frequency f. A second Faraday probe added along the z direction then detects a DC rotation for AC fields that are orthogonal to the rotating spin polarization. 
     Generally, the term “magnetic field” as used herein should be understood to refer to both or either of the classical magnetic field and a quantum mechanical term that looks like a magnetic field, as context would require. The terms “alkali” and “alkali gas” as used herein should be understood to refer to “alkali-metal atom” or “alkali-metal gas” or “alkali-metal magnetic moment” as context would require per the understanding of those of ordinary skill in the art. 
     Certain terminology is used herein for purposes of reference only, and thus is not intended to be limiting. For example, terms such as “upper”, “lower”, “above”, and “below” refer to directions in the drawings to which reference is made. Terms such as “front”, “back”, “rear”, “bottom” and “side”, describe the orientation of portions of the component within a consistent but arbitrary frame of reference which is made clear by reference to the text and the associated drawings describing the component under discussion. Such terminology may include the words specifically mentioned above, derivatives thereof, and words of similar import. Similarly, the terms “first”, “second” and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context. 
     When introducing elements or features of the present disclosure and the exemplary embodiments, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of such elements or features. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements or features other than those specifically noted. It is further to be understood that the method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
     References to “a controller” and “a processor” or “the microprocessor” can be understood to include one or more microprocessors that can communicate in a stand-alone and/or a distributed environment(s), and can thus be configured to communicate via wired or wireless communications with other processors, where such one or more processor can be configured to operate on one or more processor-controlled devices that can be similar or different devices. Furthermore, references to memory, unless otherwise specified, can include one or more processor-readable and accessible memory elements and/or components that can be internal to the processor-controlled device, external to the processor-controlled device, and can be accessed via a wired or wireless network. 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. All of the publications described herein, including patents and non-patent publications, are hereby incorporated herein by reference in their entireties.