Patent Publication Number: US-10326071-B2

Title: Systems and methods for magnetic shielding

Description:
BACKGROUND 
     Field 
     The present systems and methods generally relate to magnetic shielding and particularly relate to providing an environment with low magnetic fields and/or low magnetic field gradients. 
     Refrigeration 
     According to the present state of the art, a superconducting material may generally only act as a superconductor if it is cooled below a critical temperature that is characteristic of the specific material in question. For this reason, those of skill in the art will appreciate that an electrical system that implements superconducting components may implicitly include a refrigeration system for cooling the superconducting materials in the system. Systems and methods for such refrigeration systems are well known in the art. A dilution refrigerator is an example of a refrigeration system that is commonly implemented for cooling a superconducting material to a temperature at which it may act as a superconductor. In common practice, the cooling process in a dilution refrigerator may use a mixture of at least two isotopes of helium (such as helium-3 and helium-4). Full details on the operation of typical dilution refrigerators may be found in F. Pobell,  Matter and Methods at Low Temperatures , Springer-Verlag Second Edition, 1996, pp. 120-156. However, those of skill in the art will appreciate that the present systems and devices are not limited to applications involving dilution refrigerators, but rather may be applied using any type of refrigeration system. 
     BRIEF SUMMARY 
     A magnetic shielding system may be summarized as including a shield structure having a longitudinal center axis; and a degaussing coil, wherein the degaussing coil is wrapped around at least a portion of the shield structure in a toroidal configuration such that the degaussing coil encloses at least a portion of the shield structure and at least a portion of the degaussing coil is parallel to the longitudinal center axis of the shield structure. The shield structure may be formed of a material having high magnetic permeability selected from the group consisting of Finemet® (available from Hitachi Metals, Ltd.), mu-metal, and cryoperm. The shield structure may be tubular and/or cylindrical in geometry. The shield structure may have a closed end, and the closed end may include a through-hole through which the degaussing coil is threaded. A cap may be position over the through-hole. In some embodiments, the magnetic shielding system may include a current source, wherein the degaussing coil is electrically coupled to the current source. 
     A system for compensating for a magnetic field gradient across an area may be summarized as including at least two magnetic field sensors respectively positioned at opposite ends of the area; at least two compensation coils respectively positioned proximate opposite ends of the area; and at least a first current source. The at least two compensation coils may be electrically coupled in series with one another and both coupled to the same current source. Alternatively, the system may include a second current source, wherein a first compensation coil is electrically coupled to the first current source and a second compensation coil is electrically coupled to the second current source. The at least two compensation coils may be respectively positioned proximate opposite ends along a width of the area, and the system may include at least two additional compensation coils that are respectively positioned proximate opposite ends along a length of the area. At least two magnetic field sensors may be respectively positioned at opposite ends along the width of the area, and the system may include at least two additional magnetic field sensors that are respectively positioned at opposite ends along the length of the area. 
     A superconducting chip may be summarized as including a plurality of superconducting devices; a superconducting plane positioned beneath the plurality of superconducting devices; a first current lead electrically coupled to the superconducting plane; and a second current lead electrically coupled to the superconducting plane. The superconducting chip may include a fluxon barrier carried on the superconducting plane, wherein the fluxon barrier is positioned adjacent to an edge of the superconducting plane and extends substantially parallel thereto, and wherein the fluxon barrier includes at least a first strip of superconducting material. The fluxon barrier may include at least a second strip of superconducting material that is stacked on top of the first strip of superconducting material. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
       In the drawings, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. For example, the shapes of various elements and angles are not drawn to scale, and some of these elements are arbitrarily enlarged and positioned to improve drawing legibility. Further, the particular shapes of the elements as drawn are not intended to convey any information regarding the actual shape of the particular elements, and have been solely selected for ease of recognition in the drawings. 
         FIG. 1  is a perspective view of mu-metal shield employing a prior art form of degaussing coil. 
         FIG. 2  is a perspective view of a mu-metal shield employing a toroidal degaussing coil in accordance with an embodiment of the present systems and methods. 
         FIG. 3  is a sectional view of a cylindrical magnetic shield employing a toroidal degaussing coil in accordance with an embodiment of the present systems and methods. 
         FIG. 4  is a top plan view of an embodiment of a system designed to actively compensate for magnetic field gradients within the environment of a device. 
         FIG. 5  is a top plan view of an embodiment of a system implementing curved compensation coils designed to actively compensate for magnetic field gradients within the environment of a device. 
         FIG. 6  is a perspective view showing an embodiment of a superconducting chip (e.g., a superconducting quantum processor chip) including a superconducting plane used to remove trapped fluxons from the chip. 
         FIG. 7  is a side elevational view of an embodiment of a superconducting processor chip and a pyramidal superconducting shield in accordance with the present systems and methods. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, some specific details are included to provide a thorough understanding of various disclosed embodiments. One skilled in the relevant art, however, will recognize that embodiments may be practiced without one or more of these specific details, or with other methods, components, materials, etc. In other instances, well-known structures associated with electronics systems, such as power supplies, signal generators, and control systems including microprocessors and drive circuitry have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments of the present systems and methods. 
     Unless the context requires otherwise, throughout the specification and claims which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense, that is as “including, but not limited to.” 
     Reference throughout this specification to “one embodiment,” or “an embodiment,” or “another embodiment” means that a particular referent feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment,” or “in an embodiment,” or “another embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
     It should be noted that, as used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. Thus, for example, reference to a shielding system including “a magnetic shield” includes a single magnetic shield, or two or more magnetic shields. It should also be noted that the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise. 
     The headings provided herein are for convenience only and do not interpret the scope or meaning of the embodiments. 
     The various embodiments described herein provide systems and methods for achieving low magnetic fields and/or low magnetic field gradients over a particular area or volume. The present systems and methods include both passive and active techniques for reducing magnetic fields and/or magnetic field gradients in an environment. 
     An established passive technique for reducing magnetic fields within an environment is to enclose the environment with a shield formed of a material having high magnetic permeability. Such shields are typically cylindrical in geometry with at least one open end providing access to the enclosed volume. Exemplary materials that are appropriate for this purpose include Finemet®, Mumetal® and Cryoperm®. Both Mumetal® and Cryoperm® are nickel-alloys of high magnetic permeability (e.g., with a maximum permeability typically in the range of 100,000 to 500,000) and are known in the art. Thus, throughout this specification, any reference to a “Mumetal®” shield may be considered to also include embodiments that implement Cryoperm® shields, or shields made of any other material of high magnetic permeability. For the purposes of the present systems and methods, the term “high magnetic permeability” is used to describe a material with a maximum magnetic permeability on the order of 100,000 or more. Cylindrical Mumetal® shields having a specially-designed longitudinal mating seam are described in PCT Patent Publication 2009-099972. 
     The performance of a magnetic shield, such as a passive Mumetal® shield, may be significantly enhanced by degaussing. Degaussing is a process by which a residual magnetism within a material is reduced, or “wiped.” The material forming a magnetic shield (e.g., a Mumetal® shield) typically exhibits some residual magnetism which can be reduced by, for example, wrapping a coil of conductive wire around the shield and applying a periodic waveform of gradually decreasing amplitude through the coil. Procedures for degaussing magnetic shields are well known in the art; however, the present systems and methods describe improvements to established degaussing techniques. 
     A typical degaussing coil is carried on or near the surface of a magnetic shield and coiled around or about the longitudinal center axis of the shield in a solenoid-like fashion.  FIG. 1  is a perspective view of a shield  101  employing a prior art form of degaussing coil  102 . In accordance with the prior art, degaussing coil  102  resembles a solenoid that is carried on the outer surface of shield  101  and coiled around or about (e.g., substantially perpendicular to) the longitudinal center axis  103 . That is, a cross-section of the magnetic shield taken perpendicular to the central longitudinal axis is within a volume defined by a projected perimeter of the degaussing coil. This approach can undesirably result in a non-uniform distribution of the degaussing field over the shield volume, particularly at an end (e.g., the base) of the cylindrical shield. The present systems and methods provide an improved geometry of degaussing coils of particular benefit in applications involving a cylindrical magnetic shield. This improved geometry implements degaussing coils in a toroidal arrangement with respect to the shield body. 
       FIG. 2  shows a perspective view of a shield  201  employing a toroidal degaussing coil  202  in accordance with an embodiment of the present systems and methods. Shield  201  may be formed of any material having a magnetic permeability, such as Mumetal® or Cryoperm®. Toroidal degaussing coil  202  is coiled lengthwise around the surface of shield  201  with the majority of the length of the coil being substantially parallel to the longitudinal center axis  203  of the shield  201 . That is, only a portion of the cross-section of the magnetic shield taken perpendicular to the central longitudinal axis is within a volume defined by a projected perimeter of the degaussing coil, while another portion is not within the volume. This toroidal arrangement provides improved degaussing field uniformity over the volume of the shield  201  compared to the solenoidal arrangement of the prior art, especially at an end (e.g., the base) of the cylindrical shield  201 . Furthermore, in the toroidal arrangement of  FIG. 2  the curved shield body  201  is better aligned with the degaussing field lines produced by the coil  202 , the result being that the degaussing fields are better concentrated within the shield  201 . Those of skill in the art will appreciate that a toroidal degaussing coil may be implemented in conjunction with other shapes and configurations of shield structures and are not limited to use with cylindrical shields or, in particular, cylindrical shields employing longitudinal mating seams as illustrated. 
     Toroidal degaussing coil  202  is shown in  FIG. 2  as being wrapped around only a small portion of the total body of shield  201 . In most applications, toroidal degaussing coil  202  is only required to enclose a small portion of the total shield body  201 , and for this reason the toroidal coil geometry of  FIG. 2  is lower in mass, easier to produce, and generally less expensive than the solenoidal coil geometry employed in the prior art (e.g.,  FIG. 1 ). However, those of skill in the art will appreciate that, in alternative embodiments, the number of turns in coil  202  (and/or the spacing between turns) may be increased or decreased such that coil  202  encloses a greater or lesser portion of the total body of shield  201 . In order for degaussing coil  202  to employ a toroidal geometry, it may be necessary to insert a through-hole in the base of shield  201 . 
       FIG. 3  is a sectional view of a cylindrical magnetic shield  301  employing a toroidal degaussing coil  302  in accordance with an embodiment of the present systems and methods. To facilitate the toroidal geometry of coil  302 , shield  301  includes a through-hole  303  in its base. Coil  302  is wrapped substantially lengthwise around the body of shield  301  and passes through through-hole  303 . Shield  301  also includes a cap  304  formed of a material having high magnetic permeability, such as Mumetal®, Cyroperm®, or Finemet® which covers through-hole  303  to reduce degradation of the shielding characteristics of shield  301  caused by through-hole  303 . Cap  304  substantially covers through-hole  303  to reduce (or, preferably, eliminate) the passage of magnetic fields therethrough. Though not shown in the Figure, degaussing coil  302  is communicably coupled to a current source for controlling the degaussing field(s). In some embodiments, the current source may be part of a feedback system that further includes a magnetic field sensor positioned within the volume enclosed by shield  301  (e.g., at position  305 ). As previously described, the degaussing signal applied by the current source may be periodic (e.g., sinusoidal). The amplitude of the degaussing signal may produce a field that initially exceeds the saturation field for the shield  301  and gradually decreases over time. Some embodiments of shielding systems may employ at least two shields, with a first “inner” shield nested within a second “outer” shield. In such embodiments, each shield may employ a respective toroidal degaussing coil, but the coils may be electrically coupled in series with one another. It may be advantageous for the coil on the outer shield to mediate the coupling between the current source and the coil on the inner shield such that the outer shield is effectively degaussed before the inner shield. In some embodiments, at least one shield may be used to shield a superconducting device, such as a superconducting quantum processor chip, and the shield may be operated at a cryogenic temperature. In such embodiments, it may be advantageous to complete the degaussing process at a cold temperature to avoid changes in the shield magnetization with temperature. For example, it may be advantageous to complete the degaussing process at the coldest possible temperature that is above the critical temperature of the superconducting device being shielded, and then to proceed with cooling the superconducting device after the shield has been degaussed. 
     In accordance with the present systems and methods, in some embodiments the effective permeability of a magnetic shield may be increased by applying a signal (e.g., a high frequency signal) through the degaussing coils after the degaussing process has been completed. This effect can be achieved because the actual permeability of the degaussed shield may be non-linear in the operating region and the applied signal may effectuate an average permeability that is higher than the permeability realized when no such signal is applied. 
     The challenge of providing a low-magnetic field environment has received a lot of attention. However, much of this attention is focused on achieving a specific field level (e.g., on the order of nanoTeslas) at a specific measurement point. Comparatively little attention is devoted to providing a substantially uniform low-field environment over an appreciable area, which requires that not only field levels but also field gradients be controlled. The various embodiments described herein provide systems and methods for actively compensating for and thereby reducing magnetic field gradients. 
       FIG. 4  is a top plan view of an embodiment of a system  400  designed to actively compensate for magnetic field gradients within the environment of a device  401 . In some embodiments, device  401  may include a superconducting device, such as a superconducting quantum processor chip. In the illustrated embodiment, device  401  is a chip that includes a plurality of on-chip magnetometers  410   a - 410   d , each of which provides a measure of the magnetic field(s) impingent thereon. In some embodiments, on-chip magnetometers  410   a - 410   d  may include the magnetometers described in PCT Patent Application Serial No. PCT/US 2009/060026, now published as PCT Publication 2010/042735, or variations thereof. Each of magnetometers  410   a - 410   d  provides a measure of the magnetic field(s) in the vicinity of chip  401 . A discrepancy between the readings of any or all of magnetometers  410   a - 410   d  can indicate a non-uniformity in the magnetic field(s) around chip  401 , which may manifest itself in the form of at least one magnetic field gradient. System  400  includes a set of four gradient compensation coils  420   a - 420   d  that are designed to reduce magnetic field gradients over the area of chip  401 . Each of gradient compensation coils  420   a - 420   d  is formed of conductive wire (e.g., in some embodiments, superconductive wire) and is connected to an electrical current source. In some embodiments, each of gradient compensation coils  420   a - 420   d  is connected to a respective independently controlled current source. In other embodiments, at least two of gradient compensation coils  420   a - 420   d  are connected in series with one another and both connected to the same current source. For example, gradient compensation coils  420   a  and  420   c  may be coupled in series with one another and both controlled by a first current source and gradient compensation coils  420   b  and  420   d  may be coupled in series with one another and both controlled by a second current source. 
     As an exemplary operation, take the scenario where magnetometers  410   a ,  410   b , and  410   d  all measure approximately the same magnetic field but magnetometer  410   c  measures a higher magnetic field than the others. This implies a magnetic field gradient over chip  401  in the direction (from lower field to higher field) indicated by arrow  450 . This field gradient may be compensated by using coils  420   a - 420   d  to generate a compensation field gradient  455  that is approximately opposite to gradient  450 . In some embodiments, coils  420   a - 420   d  may be used strictly for gradient compensation to produce a uniform field over chip  401  with little added attention to the magnitude of the uniform field. In other embodiments, coils  420   a - 420   d  may be simultaneously used to provide both gradient compensation and field compensation, such that a uniform field of a desired level is achieved over the area of chip  401 . In the latter case, it may be advantageous to retain individual control of the currents passed through each of coils  420   a - 420   d  (i.e., the coils  420   a - 420   d  may preferably be electrically isolated from one another). 
     In the illustrated embodiment, chip  401  and coils  420   a - 420   d  are all carried on the same base  430 . In some embodiments, base  430  may include a printed circuit board (“PCB”) and coils  420   a - 420   d  may be realized by conductive (e.g., in some embodiments, superconductive) traces carried on the PCB  430 . In other embodiments, coils  420   a - 430   d  may be formed by wound wiring that is carried by PCB  430 . 
     The coiling direction of coils  420   a - 420   d  may depend on, among other things, the electrical configuration of the coils. In applications employing at least two serially connected coils, their relative coiling directions are important in determining the sort of compensation that can be provided. For example, two serially-connected coils may be better equipped to provide gradient compensation if they are respectively coiled in such a way that, for any given applied signal, one coil produces a positive field on chip  401  and the other coil produces a negative field on chip  401  (e.g., their respective fields are in opposite direction or in anti-parallel arrangement). In the illustrated embodiment, if coils  420   a  and  420   c  are electrically coupled in series with one another, then a signal applied in the direction shown by the arrows will enable coil  420   a  to induce a compensation field in the direction into the page at the chip  401  and coil  420   c  to induce a compensation field in the direction out of the page at the chip  401 , thereby forming a compensation gradient across chip  401 . However, relative coiling direction is less significant in embodiments where each of coils  420   a - 420   d  is independently controlled and electrically isolated from the others. 
     The size and shape of coils  420   a - 420   d  relative to chip  401  may depend on, among things, the range of field/gradient compensation expected from the coils. In some embodiments, it may be advantageous for coils  420   a - 420   d  to be smaller than chip  401  and/or quite close to chip  401  to enable very fine adjustments to the fields and/or gradients. In other embodiments, it may be advantageous for coils  420   a - 420   d  to be larger (e.g., longer and/or wider) than chip  401  and/or spaced quite far away from chip  401  so that the chip effectively “sees” a substantially uniform field from each of the coils. 
     Realistically, a compensation field produced by any of coils  420   a - 420   d  may not be perfectly uniform over chip  401  due to the geometry of the coil itself. For example, coils  420   a - 420   d  may produce compensation fields and/or gradients that have non-uniformities near the ends of the coils. In some embodiments, this effect can be made less severe by implementing coils that are curved in shape.  FIG. 5  is a top plan view of an embodiment of a system  500  implementing curved compensation coils  520   a - 520   d  designed to actively compensate for magnetic field gradients within the environment of a device  501 . In some embodiments, the curvature in each of coils  520   a - 520   d  can help to produce more uniform compensation fields and/or gradients compared to the right-angled coil geometry illustrated in  FIG. 4 . 
     Despite the implementation of passive magnetic shielding and/or active compensation systems, it can be extremely difficult to reduce magnetic fields below a certain point. In applications where extremely low fields (e.g., on the order of nanoTeslas or less) are desired, such as in a system employing a superconducting quantum processor chip, it can be advantageous to introduce a mechanism for dealing with unwanted magnetic fields that cannot be completely shielded or compensated. In the case of a superconducting quantum processor chip, unwanted magnetic fields may manifest themselves as unwanted magnetic flux (i.e., “fluxons”) trapped within at least some of the superconducting devices on the chip. In U.S. Pat. No. 7,687,938, an on-chip superconducting plane is used to passively shield on-chip devices from unwanted magnetic fields originating from either on or off the chip. In accordance with the present systems and methods, a similar superconducting plane (or, in some embodiments, the same plane) may be used to actively remove trapped fluxons from the chip by an application of the Lorentz force. 
       FIG. 6  is a perspective view showing an embodiment of a superconducting chip (e.g., a superconducting quantum processor chip)  600  including a superconducting plane  610  used to remove trapped fluxons from the chip  600 . In some embodiments, superconducting plane  610  may also serve as a superconducting ground plane. In accordance with the present systems and methods, fluxons that are trapped in the devices of chip  600  (e.g., fluxons that are trapped in superconducting plane  610 ) may be expelled from the system by applying a current through plane  610 . To this end, current leads  621  and  622  are electrically coupled to superconducting plane  610 . Lorentz&#39;s Law dictates that a current travelling from lead  622  to lead  621  will experience a Lorentz force causing the current to move in the direction indicated by the arrow  650 . Thus, fluxons that are trapped in plane  610  may be expelled through one side (e.g., through the right side in  FIG. 6 ) of the chip  600 . 
     In operation, it is necessary to cool chip  600  below the critical temperature of plane  610  such that plane  610  becomes superconducting. It is the transition to the superconducting regime that can cause plane  610  to trap unwanted fluxons. In order to expel the fluxons by the Lorentz force, the current applied through leads  621  and  622  must exceed the critical current of the plane  610  such that the plane temporarily ceases to superconduct while the current is applied. The magnitude of this critical current is dependent, at least in part, on the temperature of plane  610 . It can be desirable to limit the magnitude of the current that may be applied to the superconducting plane  610  in order to accommodate constraints of other aspects of the system. For a conservative current range (e.g., &lt;1.0 A) the temperature range for which Lorentz force may be induced in the superconducting plane  610  may be fairly narrow. In applications where the temperature of the system cannot be precisely controlled, a preferred way to implement the “Lorentz force expulsion” described herein is to apply a current through the plane  610  during cool down of the chip  600 . In this way, the plane  610  will pass through the desired temperature range while the fluxon-expulsion current is being applied. 
     As fluxons are expelled through the right side of plane  610 , it is possible for roughly the same number of fluxons to enter from the left side of plane  610 . In order to prevent such fluxons from entering the left side of plane  610 , a fluxon barrier  640  may be constructed. In some embodiments, such a barrier  640  may comprise a stack of superconducting strips  641 ,  642  on one side (e.g., on the left side in the illustrated embodiment) of the plane  610 . Since there is no current passing through the strips  641 ,  642 , the strips  641 ,  642  may remain superconducting while the current flowing through plane  610  causes plane  610  to go normal and induce the Lorentz force. With barrier  640  remaining in a superconducting state, fluxons are naturally prevented from penetrating barrier  640 . In various embodiments, barrier  640  may include any number of strips of superconducting material. In some embodiments, barrier  640  may be formed of a material that has a higher critical temperature than plane  610 . 
     In some embodiments, it may be advantageous for superconducting plane  610  to be formed of a material that has a higher critical temperature than the rest of chip  600 . In some embodiments, it may be advantageous to connect current leads  621  and  622  to diagonally opposite corners of plane  610 . If chip  600  includes a superconducting ground plane and/or a superconducting shielding plane, such a plane may be adapted for the removal of fluxons as described herein by adding current leads  621  and  622  and barrier  640 . However, in some embodiments, a plane  610  may be added to the existing structure of chip  600  for use in the removal of trapped fluxons even if chip  600  already includes a superconducting shield/ground plane. In some embodiments, plane  610  may be physically separate from chip  600 . 
     In accordance with the present systems and methods, trapped fluxons may be moved by using an applied current to induce a Lorentz force. In addition to this, trapped fluxons may also be moved by introducing a preferential flux gradient and/or a temperature gradient. In some embodiments, a conical or pyramidal superconducting shield may be constructed on or adjacent to a superconducting chip (e.g., a superconducting quantum processor chip) as shown in  FIG. 7 .  FIG. 7  is a side elevational view of an embodiment of a superconducting processor chip  701  and a pyramidal superconducting shield  710 . Shield  710  is thermalized by a crossbeam  720  (which is elsewhere thermally coupled to a cold source—not shown) that makes physical contact with its apex. As the system is cooled, it is the apex of shield  710  that becomes superconducting first and the superconductivity then spreads down the sides and to the base of shield  710 . This transition pattern creates a preferential gradient for magnetic flux towards the outer edges of the base of shield  710 . Thus, the cooling of shield  710  naturally expels flux away from the center of chip  701  and out towards its perimeter (and beyond if the base of shield  710  is made wider than chip  701 ). In some embodiments, shield  710  may be pyramidal in shape; in other embodiments, shield  710  may be conical in shape. Furthermore, while shield  710  is illustrated as a single structure that is in close proximity to chip  701 , in alternative embodiments shield  710  may be in physical contact with chip  701  and/or shield  710  may be formed directly on chip  701 . For example, shield  710  may be formed by a lithographic process using a stack of metal layers to form a pyramidal shape, where each metal layer is shorter in length than the one beneath it and longer than the one above it. 
     Certain aspects of the present systems and methods may be realized at room temperature, and certain aspects may be realized at a superconducting temperature. Thus, throughout this specification and the appended claims, the term “superconducting” when used to describe a physical structure such as a “superconducting plane” is used to indicate a material that is capable of behaving as a superconductor at an appropriate temperature. A superconducting material may not necessarily be acting as a superconductor at all times in all embodiments of the present systems and methods. 
     The above description of illustrated embodiments, including what is described in the Abstract, is not intended to be exhaustive or to limit the embodiments to the precise forms disclosed. Although specific embodiments of and examples are described herein for illustrative purposes, various equivalent modifications can be made without departing from the spirit and scope of the disclosure, as will be recognized by those skilled in the relevant art. The teachings provided herein of the various embodiments can be applied to other systems, methods and apparatus, not necessarily the exemplary systems, methods and apparatus generally described above. 
     The various embodiments described above can be combined to provide further embodiments. All of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, including but not limited to U.S. patent application Ser. No. 13/050,742, filed Mar. 17, 2011, and entitled “Systems and Methods For Magnetic Shielding,” U.S. Provisional Patent Application Ser. No. 61/316,744, filed Mar. 23, 2010, and entitled “Systems and Methods For Magnetic Shielding,” PCT Patent Publication 2009-099972, PCT Patent Application Serial No. PCT/US2009/060026, now published as PCT Patent Publication 2010/042735, and U.S. Pat. No. 7,687,938 are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary, to employ systems, circuits and concepts of the various patents, applications and publications to provide yet further embodiments. 
     These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.