U.S. patent application number 16/196469 was filed with the patent office on 2019-04-11 for device and methodology for measuring minute changes in ambient magnetic field.
The applicant listed for this patent is BEN GURION UNIVERSITY OF THE NEGEV RESEARCH AND DEVELOPMENT AUTHORITY. Invention is credited to Andrei BEN AMAR BARANGA, David LEVRON, Eugene PAPERNO, Reuben SHUKER.
Application Number | 20190107588 16/196469 |
Document ID | / |
Family ID | 51299286 |
Filed Date | 2019-04-11 |
United States Patent
Application |
20190107588 |
Kind Code |
A1 |
BEN AMAR BARANGA; Andrei ;
et al. |
April 11, 2019 |
DEVICE AND METHODOLOGY FOR MEASURING MINUTE CHANGES IN AMBIENT
MAGNETIC FIELD
Abstract
An optical magnetometer comprising: a response frequency
measurement unit comprising a vapor cell, a pulsed-mode pump laser
and a probe laser; and a computing unit configured to compute a
magnetic field change based on a difference between at least two
temporally-distinct response frequency values received from the
frequency measurement unit. Optionally, the response frequency
measurement unit is magnetically non-shielded.
Inventors: |
BEN AMAR BARANGA; Andrei;
(Omer, IL) ; LEVRON; David; (Omer, IL) ;
PAPERNO; Eugene; (Beer Sheva, IL) ; SHUKER;
Reuben; (Omer, IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BEN GURION UNIVERSITY OF THE NEGEV RESEARCH AND DEVELOPMENT
AUTHORITY |
Beer Sheva |
|
IL |
|
|
Family ID: |
51299286 |
Appl. No.: |
16/196469 |
Filed: |
November 20, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14766250 |
Aug 6, 2015 |
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PCT/IL2014/050113 |
Feb 3, 2014 |
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16196469 |
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61814378 |
Apr 22, 2013 |
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61761752 |
Feb 7, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01R 33/032 20130101;
G01R 33/26 20130101 |
International
Class: |
G01R 33/26 20060101
G01R033/26; G01R 33/032 20060101 G01R033/032 |
Claims
1. An optical magnetometer comprising: a response frequency
measurement unit comprising a vapor cell, a pulsed-mode pump laser
and a probe laser; and a computing unit configured to compute a
magnetic field change based on a difference between at least two
temporally-distinct response frequency values received from the
response frequency measurement unit; wherein said response
frequency measurement unit is magnetically non-shielded.
2. The optical magnetometer according to claim 1, wherein said
vapor cell comprises alkaline atoms.
3. The optical magnetometer according to claim 2, wherein said
alkaline atoms are selected from the group consisting of Cesium
(Cs), Rubidium (Rb) and Potassium (K).
4. The optical magnetometer according to claim 2, wherein said
response frequency measurement unit further comprises an oven
configured to control a temperature in said vapor cell.
5. The optical magnetometer according to claim 1, wherein said pump
laser comprises a pulse diode laser configured to emit a pulsating
laser beam.
6. The optical magnetometer according to claim 5, wherein said pump
laser further comprises a diffraction grating configured to tune
the pulsating laser beam.
7. The optical magnetometer according to claim 6, wherein said
response frequency measurement unit further comprises an optical
delivery system and a polarimeter system.
8. The optical magnetometer according to claim 7, wherein said pump
laser further comprises a linear polarizer configured to filter the
pulsating laser beam.
9. The optical magnetometer according to claim 5, wherein said pump
laser further comprises a circular polarizer configured to
circularly polarize the pulsating laser beam.
10. The optical magnetometer according to claim 9, wherein said
circular polarizer is a .lamda./4 quarter wave plate.
11. The optical magnetometer according to claim 1, wherein said
probe laser comprises a single mode diode laser.
12. The optical magnetometer according to claim 11, wherein said
probe laser further comprises a linear polarizer.
13. The optical magnetometer according to claim 1, wherein said
response frequency measurement unit further comprises a polarized
splitter configured to split a beam transmitted by said probe
laser.
14. The optical magnetometer according to claim 13, wherein said
response frequency measurement unit further comprises multiple
photodiodes, such that light of different polarizations is
transferred to different ones of the multiple photodiodes from said
polarized splitter, resulting in the at least two
temporally-distinct response frequency values.
15. A method for measuring change in a magnetic field, the method
comprising computing a difference between at least two
temporally-distinct response frequency values received from a
pulsed-mode atomic magnetometer, wherein said computing is
performed by a hardware computing unit; wherein said pulsed-mode
atomic magnetometer comprises a response frequency measurement unit
comprising a vapor cell, a pulsed-mode pump laser and a CW
(continuous wave) laser probe, and wherein said pulsed-mode atomic
magnetometer is magnetically non-shielded.
16. A method for measuring a change in a magnetic field, the method
comprising: providing a response frequency measurement unit
comprising a vapor cell, a pulsed-mode pump laser and a probe laser
pulsed-mode atomic magnetometer, wherein said response frequency
measurement unit is magnetically non-shielded; irradiating said
vapor cell with said pulsed-mode pump laser light while a magnetic
field changes; measuring at least two temporally-distinct responses
to the irradiation; and determining an amount of change to the
magnetic field based on a difference between the at least two
responses.
17. The method according to claim 16, wherein said irradiating is
at a repetition rate of 5 KHz or less.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 14/766,250, filed on Aug. 6, 2015, which is a
National Phase application of PCT Patent Application No.
PCT/IL2014/050113, filed Feb. 3, 2014, which claims the benefit of
priority of U.S. Provisional Patent Application Nos. 61/761,752,
filed Feb. 7, 2013 and 61/814,378, filed Apr. 22, 2013, all which
are incorporated herein by reference in their entirety.
FIELD OF THE INVENTION
[0002] The invention relates to the field of optical
magnetometers.
BACKGROUND
[0003] Optical magnetometers are used for the accurate measurement
of magnetic fields. These magnetometers utilize a phenomenon known
as Larmor precession, in which metal atom spins have a precession
at a frequency proportional to the ambient magnetic field. The
atoms are optically pumped by a pump laser beam, and their spin
rotation frequency is measured by a probe laser beam. Usually,
these magnetometers are used to measure absolute values of certain
magnetic fields in a controlled environment and thus, they are
often magnetically shielded to avoid the effect of external
interferences (e.g. the earth's magnetic field). They are also
commonly big and expensive, due to the laser components and
shielding they comprise.
[0004] The foregoing examples of the related art and limitations
related therewith are intended to be illustrative and not
exclusive. Other limitations of the related art will become
apparent to those of skill in the art upon a reading of the
specification and a study of the figures.
SUMMARY
[0005] The following embodiments and aspects thereof are described
and illustrated in conjunction with systems, tools and methods
which are meant to be exemplary and illustrative, not limiting in
scope.
[0006] On embodiment provides an optical magnetometer comprising: a
response frequency measurement unit comprising a vapor cell, a
pulsed-mode pump laser and a probe laser; and a computing unit
configured to compute a magnetic field change based on a difference
between at least two temporally-distinct response frequency values
received from the response frequency measurement unit.
[0007] In some embodiments, said response frequency measurement
unit is magnetically shielded.
[0008] In some embodiments, said response frequency measurement
unit is magnetically non-shielded.
[0009] In some embodiments, said vapor cell comprises alkaline
atoms.
[0010] In some embodiments, said alkaline atoms are selected from
the group consisting of Cesium (Cs), Rubidium (Rb) and Potassium
(K).
[0011] In some embodiments, said response frequency measurement
unit further comprises an oven configured to control a temperature
in said vapor cell.
[0012] In some embodiments, said pump laser comprises a pulse diode
laser configured to emit a pulsating laser beam.
[0013] In some embodiments, said pump laser further comprises a
diffraction grating configured to tune the pulsating laser
beam.
[0014] In some embodiments, said response frequency measurement
unit further comprises an optical delivery system and a polarimeter
system.
[0015] In some embodiments, said pump laser further comprises a
linear polarizer configured to filter the pulsating laser beam.
[0016] In some embodiments, said pump laser further comprises a
circular polarizer configured to circularly polarize the pulsating
laser beam.
[0017] In some embodiments, said circular polarizer is a .lamda./4
quarter wave plate.
[0018] In some embodiments, said probe laser comprises a single
mode diode laser.
[0019] In some embodiments, said probe laser further comprises a
linear polarizer.
[0020] In some embodiments, said response frequency measurement
unit further comprises a polarized splitter configured to split a
beam transmitted by said probe laser.
[0021] In some embodiments, said response frequency measurement
unit further comprises multiple photodiodes, such that light of
different polarizations is transferred to different ones of the
multiple photodiodes from said polarized splitter, resulting in the
at least two temporally-distinct response frequency values.
[0022] Another embodiment provides a method for measuring change in
a magnetic field, the method comprising computing a difference
between at least two temporally-distinct response frequency values
received from a pulsed-mode atomic magnetometer.
[0023] In some embodiments, said computing is performed by a
hardware computing unit.
[0024] In some embodiments, said pulsed-mode atomic magnetometer
comprises a magnetically-shielded response frequency measurement
unit.
[0025] In some embodiments, said pulsed-mode atomic magnetometer
comprises a magnetically non-shielded response frequency
measurement unit.
[0026] In some embodiments, said pulsed-mode atomic magnetometer
comprises a response frequency measurement unit comprising a vapor
cell, a pulsed-mode pump laser and a CW (continuous wave) laser
probe.
[0027] In some embodiments, the method further comprises
irradiating said vapor cell with a pulsating laser beam from the
pulsed-mode pump laser.
[0028] In some embodiments, the method further comprises tuning the
pulsating laser beam using a diffraction grating.
[0029] In some embodiments, the method further comprises directing
the pulsating laser beam using an optical delivery system.
[0030] In some embodiments, the method further comprises filtering
the pulsating laser beam using a linear polarizer.
[0031] In some embodiments, the method further comprises circularly
polarizing the pulsating laser beam using a .lamda./4 quarter wave
plate.
[0032] In some embodiments, said probe laser comprises a single
mode diode laser.
[0033] In some embodiments, the method further comprises emitting a
continuous-wave laser beam from said single mode diode laser
through said vapor cell.
[0034] In some embodiments, the method further comprises filtering
the continuous-wave laser beam using a linear polarizer.
[0035] In some embodiments, the method further comprises splitting
the continuous-wave laser beam using a polarized splitter.
[0036] In some embodiments, the split probe laser beam impinges on
multiple photodiodes of said response frequency measurement
unit.
[0037] In some embodiments, the at least two temporally-distinct
response frequency values are obtained as a voltage difference
between said multiple photodiodes.
[0038] A further embodiment provides a method for measuring a
change in a magnetic field, the method comprising: irradiating a
vapor cell with pulsed laser light while a magnetic field changes;
measuring at least two temporally-distinct responses to the
irradiation; and determining an amount of change to the magnetic
field based on a difference between the at least two responses.
[0039] In some embodiments, said irradiating is at a repetition
rate of 5 KHz or less.
[0040] In some embodiments, the non-shielded optical magnetometer
is capable of measuring changes to the ambient magnetic field in
the order of a few pT (pico Tesla).
[0041] In addition to the exemplary aspects and embodiments
described above, further aspects and embodiments will become
apparent by reference to the figures and by study of the following
detailed description.
BRIEF DESCRIPTION OF THE FIGURES
[0042] Exemplary embodiments are illustrated in referenced figures.
Dimensions of components and features shown in the figures are
generally chosen for convenience and clarity of presentation and
are not necessarily shown to scale. The figures are listed
below.
[0043] FIG. 1 shows a block diagram of an exemplary atomic
magnetometer;
[0044] FIG. 2 shows a graph of a response to a short pulse of a
pump laser, in the course of a shielded experiment;
[0045] FIG. 3 shows a graph of the fit between a theoretical curve
and measured experiment results in the course of a shielded
experiment;
[0046] FIG. 4 shows a graph of a response to a short pulse of a
pump laser in the course of an unshielded experiment, under
influence of the earth magnetic field;
[0047] FIG. 5 shows a graph of a response to a short pulse of a
pump laser in the course of an unshielded experiment, under
influence of slight change in the earth magnetic field; and
[0048] FIG. 6 shows a graph of the fit between a theoretical curve
and measured experiment results in the course of an unshielded
experiment.
DETAILED DESCRIPTION
[0049] The terms "Non-Shielded" and/or "Unshielded" as used herein
refer to a magnetometer or any portion thereof (e.g., vapor cell)
being exposed to the effect of the ambient Earth magnetic field as
opposed to shielded magnetometers shielded by conductive or
magnetic materials and/or by compensating coils and magnets that
alter the effect of the ambient Earth magnetic field on the
device.
[0050] An atomic magnetometer is disclosed herein. The atomic
magnetometer includes a response frequency measurement unit having
a vapor cell, a pulsed-mode pump laser and a laser probe. The
atomic magnetometer further includes a computing unit configured to
compute a magnetic field change based on a difference between
response frequency values of at least two temporally-distinct
measurements, utilizing the Larmor frequency phenomenon associated
with atoms in the vapor cell. Optionally, the response frequency
measurement unit is magnetically non-shielded, but nonetheless
capable of measuring magnetic field changes, such as those
associated with a body of metal passing by the magnetometer, at pT
sensitivity.
[0051] Advantageously, the present atomic magnetometer, which
utilizes a relatively simple, cheap and small pulsed-mode pump
laser, may provide a solution to many of today's prominent
magnetometer needs.
[0052] A method for measuring a magnetic field change, in
accordance with some embodiments, may include computing a
difference between response frequency values of at least two
measurements, utilizing the Larmor frequency phenomenon associated
with the vapor cell. In the method, a vapor cell is irradiated with
pulsed laser light while the ambient magnetic field changes and at
least two temporally-distinct responses to the irradiation are
recorded. The amount of change to the magnetic field is then
computed based on a difference between each of the at least two
frequency responses.
[0053] The present atomic magnetometer may be better understood
with reference to the accompanying drawings. Reference is now made
to FIG. 1, which shows a block diagram of an optical magnetometer,
in accordance with an embodiment. In this example, the optical
magnetometer is of the atomic type. Alkaline atoms (such as Cs, Rb
and/or K) are contained in a vapor cell (100) which may be
temperature-controlled by an oven (128). A pulsating laser beam
(104) may be generated by a pulse diode laser (106), tuned by a
diffraction grating (108a), directed by an optical delivery system
(for example including one or more mirrors 108b, 108c, one or more
optical fibers, etc.), optionally filtered by a linear polarizer
(110) and circularly polarized by a circular polarizer being a
.lamda./4 quarter wave plate (112), which finally optically pumps
the atoms in vapor cell (100). In an ambient magnetic field, the
atom spins have a precession at a Larmor frequency, proportional to
the ambient magnetic field intensity.
[0054] A probe laser beam (114), optionally generated by a single
mode diode laser (116) and filtered by a linear polarizer (118),
may pass through the vapor cell (100). The probe laser is
optionally a CW (continuous wave) probe laser, or a pulsed probe
laser.
[0055] The probe beam linear polarization rotates due to the
ambient magnetic field. Time-dependent polarization may be measured
by a polarimeter or by a balanced polarimeter (102), which may
include a polarizing beam splitting cube (120) and multiple, for
example two, photodiodes (122a, 122b). The probe beam (114) may be
split by polarizing beam splitting cube (120) to fall on
photodiodes (122a, 122b). Due to the rotation of the probe beam
polarization, a different light intensity is transferred to and
impinges on each of the photodiodes (122a, 122b), namely--the
intensity transferred to photodiode 122a is maximal when the
intensity transferred to photodiode 122b is minimal, and vice
versa. The photodiodes (122a, 122b) may produce voltage which is
equivalent to the intensity of light projected onto them. The
voltage difference between them may then be amplified by a
differential amplifier (124) to yield the result, which is
processed by a computing unit (126) for further calculations.
Computing unit (126) may include hardware such as a microprocessor,
an integrated circuit and/or a discrete electronic circuit,
configured to compute a magnetic field change based on the voltage
difference between the photodiodes, namely--between at least two
temporally-distinct response frequency values.
[0056] The present optical magnetometer may accurately measure
small changes in the ambient magnetic field, at a pT sensitivity.
It does so by launching the pump laser beam (104) towards an
advantageously unshielded vapor cell (100) at a repetition rate of,
for example, up to about 5 KHz, constantly measuring the probe beam
(114) rotation, and calculating the frequency offset between
measurements.
[0057] The present invention may be a system, a method, and/or a
computer program product. The computer program product may include
a computer readable storage medium (or media) having computer
readable program instructions thereon for causing a processor to
carry out aspects of the present invention.
[0058] The computer readable storage medium can be a tangible
device that can retain and store instructions for use by an
instruction execution device. The computer readable storage medium
may be, for example, but is not limited to, an electronic storage
device, a magnetic storage device, an optical storage device, an
electromagnetic storage device, a semiconductor storage device, or
any suitable combination of the foregoing. A non-exhaustive list of
more specific examples of the computer readable storage medium
includes the following: a portable computer diskette, a hard disk,
a random access memory (RAM), a read-only memory (ROM), an erasable
programmable read-only memory (EPROM or Flash memory), a static
random access memory (SRAM), a portable compact disc read-only
memory (CD-ROM), a digital versatile disk (DVD), a memory stick, a
floppy disk, a mechanically encoded device such as punch-cards or
raised structures in a groove having instructions recorded thereon,
and any suitable combination of the foregoing. A computer readable
storage medium, as used herein, is not to be construed as being
transitory signals per se, such as radio waves or other freely
propagating electromagnetic waves, electromagnetic waves
propagating through a waveguide or other transmission media (e.g.,
light pulses passing through a fiber-optic cable), or electrical
signals transmitted through a wire.
[0059] Computer readable program instructions described herein can
be downloaded to respective computing/processing devices from a
computer readable storage medium or to an external computer or
external storage device via a network, for example, the Internet, a
local area network, a wide area network and/or a wireless network.
The network may comprise copper transmission cables, optical
transmission fibers, wireless transmission, routers, firewalls,
switches, gateway computers and/or edge servers. A network adapter
card or network interface in each computing/processing device
receives computer readable program instructions from the network
and forwards the computer readable program instructions for storage
in a computer readable storage medium within the respective
computing/processing device.
[0060] Computer readable program instructions for carrying out
operations of the present invention may be assembler instructions,
instruction-set-architecture (ISA) instructions, machine
instructions, machine dependent instructions, microcode, firmware
instructions, state-setting data, or either source code or object
code written in any combination of one or more programming
languages, including an object oriented programming language such
as Java, Smalltalk, C++ or the like, and conventional procedural
programming languages, such as the "C" programming language or
similar programming languages. The computer readable program
instructions may execute entirely on the user's computer, partly on
the user's computer, as a stand-alone software package, partly on
the user's computer and partly on a remote computer or entirely on
the remote computer or server. In the latter scenario, the remote
computer may be connected to the user's computer through any type
of network, including a local area network (LAN) or a wide area
network (WAN), or the connection may be made to an external
computer (for example, through the Internet using an Internet
Service Provider). In some embodiments, electronic circuitry
including, for example, programmable logic circuitry,
field-programmable gate arrays (FPGA), or programmable logic arrays
(PLA) may execute the computer readable program instructions by
utilizing state information of the computer readable program
instructions to personalize the electronic circuitry, in order to
perform aspects of the present invention.
[0061] Aspects of the present invention are described herein with
reference to flowchart illustrations and/or block diagrams of
methods, apparatus (systems), and computer program products
according to embodiments of the invention. It will be understood
that each block of the flowchart illustrations and/or block
diagrams, and combinations of blocks in the flowchart illustrations
and/or block diagrams, can be implemented by computer readable
program instructions.
[0062] These computer readable program instructions may be provided
to a processor of a general purpose computer, special purpose
computer, or other programmable data processing apparatus to
produce a machine, such that the instructions, which execute via
the processor of the computer or other programmable data processing
apparatus, create means for implementing the functions/acts
specified in the flowchart and/or block diagram block or blocks.
These computer readable program instructions may also be stored in
a computer readable storage medium that can direct a computer, a
programmable data processing apparatus, and/or other devices to
function in a particular manner, such that the computer readable
storage medium having instructions stored therein comprises an
article of manufacture including instructions which implement
aspects of the function/act specified in the flowchart and/or block
diagram block or blocks.
[0063] The computer readable program instructions may also be
loaded onto a computer, other programmable data processing
apparatus, or other device to cause a series of operational steps
to be performed on the computer, other programmable apparatus or
other device to produce a computer implemented process, such that
the instructions which execute on the computer, other programmable
apparatus, or other device implement the functions/acts specified
in the flowchart and/or block diagram block or blocks.
[0064] The flowchart and block diagrams in the Figures illustrate
the architecture, functionality, and operation of possible
implementations of systems, methods, and computer program products
according to various embodiments of the present invention. In this
regard, each block in the flowchart or block diagrams may represent
a module, segment, or portion of instructions, which comprises one
or more executable instructions for implementing the specified
logical function(s). In some alternative implementations, the
functions noted in the block may occur out of the order noted in
the figures. For example, two blocks shown in succession may, in
fact, be executed substantially concurrently, or the blocks may
sometimes be executed in the reverse order, depending upon the
functionality involved. It will also be noted that each block of
the block diagrams and/or flowchart illustration, and combinations
of blocks in the block diagrams and/or flowchart illustration, can
be implemented by special purpose hardware-based systems that
perform the specified functions or acts or carry out combinations
of special purpose hardware and computer instructions.
[0065] In the description and claims of the application, each of
the words "comprise" "include" and "have", and forms thereof, are
not necessarily limited to members in a list with which the words
may be associated. In addition, where there are inconsistencies
between this application and any document incorporated by
reference, it is hereby intended that the present application
controls.
Experimental Results 1
[0066] In an experiment conducted, a 70 nT (nano-Tesla) magnetic
field was applied to a shielded vapor cell which included 99% Rb
and 1% K. The pump beam wavelength was tuned to 795 nm (Rb D1 line)
and the probe beam was tuned to 770 nm (K D1 line). The magnetic
field was slightly changed, and the system then measured the
magnetic field change.
[0067] Referring now to FIG. 2, the system response to a short
pulse (200 ns) of the pump laser is shown. The measured frequency
of the decaying oscillations is 162.4 Hz. Knowing the Potassium
atoms Larmor frequency, which is a 233,333 Hz/Gauss (or 2.333
Hz/nT), one concludes a magnetic field of 69.57 nT.
[0068] To extract the frequency shift between measurements, the sum
of absolute values of difference of the spectra, taken at all
sampling points, was calculated.
[0069] Five additional measurements were taken, with a magnetic
field change of 4=16.2 pT (equivalent to 0.038 Hz frequency shift),
5.DELTA., 10.DELTA., 50.DELTA., and 100.DELTA.. In addition, a
simulation of these five measurements was performed and compared
with the real measured results. FIG. 3 shows the simulation and
measurements results. The fit between the theoretical curve and
measured result starts to be adequate at about 0.08 Hz, which is
equivalent to -34 pT. Thus, this experimental system can measure
magnetic field changes at a sensitivity of -34 pT.
Experimental Results 2
[0070] An additional experiment was performed, this time without
shielding, using a Cs cell. The pump was tuned to 895 nm and the
probe was tuned to near 852 nm. Referring to FIG. 4, the response
of the system to a short pulse of pump light is depicted. The
spectrum represents decaying oscillations at approximately 148 kHz.
The ambient earth field at the laboratory is approximately 0.395
Gauss (39500 nT). The magnetic field that corresponds to 1 Hz is
266 pT. The fit to the experimental spectrum is shown in FIG.
5.
[0071] The operation was repeated with small changes in the
magnetic field. The response of the system looks similar to the one
depicted in FIG. 4, with a small frequency change. A measure of the
frequency change was determined as the sum of the absolute values
of the difference between the fits to the spectra, taken at all
sampling points.
[0072] Referring to FIG. 6, the results of several measurements are
depicted. The difference of the frequency between measurements is
calculated by comparing the fits to the spectra using the sum of
absolute differences between the fits at all sampling points. As
shown, frequency differences of 0.1 Hz may be detected. This
corresponds to approximately 27 pT. Further improvement of the
signal-to-noise ratio may result in better accuracy, namely <5
pT.
* * * * *