Sensitive magnetometers based on dark states
Sensitive magnetometers based on dark states
l Luigi Moi
So-called Dark States provide another example of fundamental research which yields unexpected applications. Here is an all-optical magnetometer that can detect tiny fields even in an unshielded environment. As an obvious application a human magneto-cardiogram (MCG) has been recorded. And there are more promising applications.
m the first
observation of dark
by a star) in
of Na atoms, caused
by a spatially
magnetic field along
the laser beam.
Iexperiments have been carried out in alkali atoms
n recent years interest has been growing into the
fascinating properties of dark resonances. Most of the
due to their favourable level structure and the
commercially available diode lasers, resulting in numerous
applications. Teams from Siena and Sofia developed an
all-optical magnetometer based on such dark resonances.
The dark-state effect was discovered in 1976 by G.Alzetta,
A. Gozzini, L. Moi and G. Orriols [
], while they were
working on optical-pumping experiments with a
multimode dye laser. They wanted to make the observation
of multi-photon radio-frequency resonances in sodium
atoms easy, i.e., observable by the naked eye. To this end,
they introduced a magnetic field (MF) that was constant
in time but spatially inhomogeneous along the laser
beam, so as to produce Zeeman splitting of
groundstate hyperfine levels (Fig.1a, left). In this way, an applied
radio-frequency field was resonant only if it matched the
Zeeman splitting. As a result, a bright radio-frequency
resonance appeared (Fig.1a, right).
During these experiments, besides the bright resonances, a
new“dark”resonance was observed,which does not require
any radio-frequency field. In the introductory illustration
only the dark resonance is shown. The new dark state (DS)
effect appears in a position along the laser beam where the
frequency difference (ω10-ω20) between two optical
frequencies is equal to the frequency splitting ω12 between two
Zeeman sublevels (Fig.1b, left). This is the resonant condition
for the stimulated Raman transitions between the two
longliving ground levels. Theoretical studies described the dark
resonance as a coherent superposition of two long-living
ground levels prepared by the bi-chromatic laser field, and
‘atomic population trapping’was introduced,leading to the
term Coherent Population Trapping (CPT) [
A striking property of the DS is that its spectral width is
only 10 to 100 Hz (see Fig.1b, right). This is because it is
determined mainly by the long lifetime of the two ground
levels, so the much larger spectral width of the excited
level is not involved. Thus, the CPT resonances can be
extremely narrow features allowing performances both
at the level of basic physics experiments and of practical
applications. Recently a complete suppression of
fluorescence in narrow resonance has been realized exciting
all ground-state Zeeman sublevels of sodium [
Different approaches have been adopted to observe CPT
resonance. Most frequently Zeeman sublevels of the two
hyperfine ground levels (with ω12∼2÷9 GHz for
different alkali atoms) are coupled by two coherent optical
], commonly obtained by diode laser frequency
modulation around ω12 that requires relatively high
modulation power. To reduce this problem, we proposed [
to shift laser modulation to the kHz region by coupling
Zeeman sublevels of the same ground-state hyperfine
level, where ω12 is a few hundred kHz in earth and lab
MFs. In addition, the involvement of a single hyperfine
transition in the CPT preparation leads to observation
of a new narrow resonance of enhanced absorption [
The recently expanding interest in the topic is due
not only to the fascinating physics involving quantum
coherence but also to the fact that there are many
potential applications with relevance for fundamental
studies, such as slowing of light, STIRAP, velocity-selective
CPT and quantum information storage, as well as for
development of new techniques/devices like atomic
Here we describe the development of an all-optical
magnetometer based on the DS, which is promising for
realization of low-cost and easily-operated practical
devices for weak MF measurement.
Scheme for magnetic field (MF) measurements
In the developed approach (Fig. 2),
bi-chromatic/polychromatic coherent optical fields couple Zeeman sublevels
within a single hyperfine level, resulting in observation of
DS resonance with Γ<100Hz,in a magnetically unshielded
but partially compensated environment. The CPT
resonance observed as a function of the modulation frequency
makes absolute measurement of the MF value possible.
Constant earth and lab MFs shift the resonance center at
ωmod = B/k without causing its broadening. The first step
of the measurement is to find this point and to determine
. FiG. 1: (a) Formation of bright optical pumping resonance. (left) due to fluorescence decay, a light field of frequency ω10, resonant with the 1-0 transition, produces optical pumping
(accumulation) of atoms in the ground level 2, non-interacting with the light. consequently, the population in level 1 decreases. (right) as the splitting (ω12) between levels 1 and 2
is determined by the applied magnetic field, the radio-frequency field (with ωrF ) will couple the ground levels if ωrF = ω12, repopulating level 1 and producing enhanced absorption
and a bright resonance (enhanced fluorescence). (b) dark-state resonance formation: (left) basic scheme for coherent population trapping (cpt ) - only two optical fields are used,
i.e., no radio-frequency field is involved in the cpt preparation; (right) when the two optical fields (ω 10 and ω20) are components of a modulated laser field separated by modulation
frequency ωmod (= ω10 − ω20), the dark resonance is centered at the modulation frequency determined by the ground level splitting ω12 = ω10 − ω20.
seNsitiVe ma GNetometers
t “his mcG measurement is
passive (no radiation imposed
to subject), completely
noninvasive and contact-free.
B. At the second step, the modulation frequency ωmod is
fixed and the small magnetic field to be measured (in the
interval B∆ = kΓ) is switched on. Thus, the resonance pro
file enables precise measurement of small MF fluctuations
in time, superimposed on a
large magnetic background.
In this way, the requirements
for earth and lab MF
shielding are strongly relaxed,
which is a significant
advantage because the shielding
” is about an order of
tude more expensive than the
magnetometer.Note that other techniques performing the
most precise measurement of MF fluctuations around zero
value require expensive MF shielding.
First human magneto-cardiogram (MCG)
in unshielded environment
In 2007, an encouraging result was obtained [
] with an
experimental apparatus based on CPT and aimed at
detecting a MCG in a magnetically compensated but unshielded
surrounding. The system was suitable to compensate for
some dc components of the environmental MF and the
first order MF inhomogeneities, thus reducing the
resonance line width to few tens of Hz. Environmental noise
was cancelled by making use of a differential measurement.
Synchronous data acquisition with a reference signal
(electro-cardiogram/pulse-meter) improved the signal-to-noise
ratio by off-line averaging. The setup has the significant
advantages of working at room temperature with a
smallsize magnetic sensor,and the possibility of fast adjustments
of the dc bias MF, which makes the sensor suitable for
detecting a biomagnetic signal of any orientation and in
any position on the patient’s chest.
This all-optical technique does not require coils in the
proximity of the sensor cell to produce direct
radiofrequency magnetic excitation. The sensor consists of
a glass cell containing alkali vapor, and the laser light is
brought to and from the cell by optical fibers.
The potential of the magnetometer for
magnetocardiographic applications in unshielded environments is
illustrated by Fig. 3. Multiple MCG traces are averaged,
in order to improve the signal-to-noise. The signals are
filtered with a single low-pass filter and the
deterministic noises (e.g., originating from the main supply and its
harmonics) are subtracted off-line.
The simplicity of the method, its low cost, and the low
cost of maintenance may be crucial factors for its
dissemination in clinical applications. Note that this MCG
measurement is passive (no radiation imposed to
subject), completely non-invasive and contact-free.
Other promising applications
A dual-channel self-oscillating approach has been
developed, based on a wide frequency modulation of the pump
laser and registration of the nonlinear magneto-optical
rotation of the probe laser [
]. The self-oscillating
operation makes the magnetometer insensitive to the slow drifts
of the MF (of several nano-Tesla), thus making possible
long time records up to the duration of the working days.
The sensor was used for the nuclear magnetic resonance
(NMR) registration, detecting both the dc magnetization
and the nuclear spins precession of remotely polarized
. FiG. 2: principle scheme of an all-optical ds magnetometer. the modulated light (with components separated by ωmod) is effectively produced by direct modulation of a diode
laser current. the measured magnetic field B splits levels 1 and 2 of alkali atoms (Fig.1) at frequency ω12 related to the mF by B = kω12, where k is determined by atomic constants.
in unshielded and partially-compensated environments, the light produces a dark resonance, centered at ω12 = 91228Hz (denoted by asterisk), which is the modulation frequency
(ωmod) of the laser. the dark-resonance spectral width is Γ ∼ 90Hz; its first derivative is shown by curve (a). the resonance corresponds to a mF variation ΔB = k Γ. if the modulation
frequency is kept constant (ωmod = ω12 = constant), the obtained linear dependence cd (within the resonance profile) allows measuring mF fluctuations less than ΔB, based on the
registration of the transmitted light intensity with time – shown in the curve on the right (b). in unshielded and partially-compensated environments the value ΔB is less than B by
several orders of magnitude.
hydrogen nuclei in water samples.The optical
magnetometer reaches a sensitivity of 100fT.Hz-1/2 worsened to 2pT.
Hz-1/2 due to MF gradients in an unshielded environment.
To facilitate averaging of bias-MF-dependent signals and
to improve the signal/noise ratio in unshielded
environment, an automatic system has been developed [
active compensation of time-dependent earth and urban
MFs using the dual channel scalar magnetometer. The
system made possible to stabilize the bias field within a
bandwidth of several tens of Hz, counteracting MF
fluctuation with attenuation levels exceeding 40dB.
Developed optical magnetometers have been successfully
tested for monitoring of earth MF variations, showing
that the approach is quite robust against urban
electromagnetic noise [
A lot of effort has been put by US scientists for the
development of practical chip-scale atomic clock and
magnetometer, based on the DS. More recently a special
care has been taken to avoid electrical connections to the
sensor, i.e. making the sensor head largely non-magnetic
and its performance free of internal MFs [
The Siena group, lead by V. Biancalana, is presently
involved in a project for the magnetic characterization of
magneto-nano-particle hydrogels. Further campaigns
are foreseen in MCG as well as in ultra-low-field NMR,
including collaboration with researchers involved in
developing unconventional techniques for nuclear
prepolarization. The Sofia team is involved in studies of
miniaturized optical magnetometer based on DS and in
the improvement of its signal/noise.
The DS magnetometer is a versatile device offering the
possibility to build up non-magnetic sensors: no internal
MFs perturb the field to be measured. This is an
important issue when multichannel sensors are
developed.Absence of coils minimizes the size and optical fibers may
easily address each single detection point. It can be used
also for remote control or work in hostile environment. n
b FiG. 3: mcG signal
(83 averages) of an 8
year old boy recorded
in an unshielded
that signals of
order 10-12 tesla are
detected. this result
is kindly provided
by V. Biancalana
About the Authors
Luigi Moi studied Physics at the
University of Pisa, and holds the Chair of
General Physics at the University of
Siena since 1990. He received the Panizza
Award of Italian Physical Society for the
discovery of the CPT effect.
Stefka Cartaleva studied at Moscow
State University, where she received a
Ph.D degree in 1973. Since 1991 she is
Associate Professor at the Institute of
Electronics of the Bulgarian Academy
of Sciences in Sofia.
 G. Alzetta , A. Gozzini , L. Moi , G. Orriols, Nuovo Cimento 36 , 5 ( 1976 ).
 E. Arimondo , Prog.Opt. 35 , 257 ( 1996 ).
 G. Alzetta , S. Gozzini , A. Luchesini , S. Cartaleva , T. Karaulanov , C. Marinelli , L. Moi, Phys. Rev. A 69 , 063815 ( 2004 ).
 M. Stahler , S. Knappe , C. Affolderbach , W. Kemp , R. Wynands , Europhys.Lett. 54 , 323 ( 2001 ).
 V. Biancalana , Y. Dancheva , E. Mariotti , L. Moi , S. Cartaleva , C. Andreeva , European Patent No EP1570282, Priority 03 /12/ 2002 .
 C. Andreeva , G. Bevilacqua , V. Biancalana , S. Cartaleva , Y. Dancheva , T. Karaulanov , C. Marinelli , E. Mariotti , L. , Appl. Phys. B 76 , 667 ( 2003 ),.
[7. Y. Dancheva , G.Alzetta, S. Cartaleva , M. Taslakov , Ch.Andreeva, Opt. Commun. 178 ( 2000 ) 103; F. Renzoni , S. Cartaleva , G. Alzetta , E. Arimondo, Phys.Rev. A 63 , 065401 ( 2001 ).
 J. Belfi , G. Bevilacqua , V. Biancalana , S. Cartaleva , Y. Dancheva , L. Moi, JOSA B 24 , 2357 ( 2007 ).
 J. Belfi , G. Bevilacqua , V. Biancalana , S. Cartaleva , Y. Dancheva , K. Khanbekyan , L. Moi, JOSA B 26 , 910 ( 2009 ).
 J. Belfi , G. Bevilacqua , V. Biancalana , R. Cecchi , Y. Dancheva , and L. Moi , Rev. Sci. Instrum . 81 , 065103 ( 2010 ).
 S. Cartaleva , S. Gateva , E. Alipieva , A. Yanev , G. Todorov , D. Slavov , C. Andreeva , T. Karaulanov , E. Taskova , L. Petrov , V. Sarova , N. Petrov , Annual report 2004 , p. 109 : www.ie-bas.dir .bg/Presentation.htm
 P. D. D. Schwindt , S. Knappe , V. Shah , L. Hollberg , J. Kitching , L.-A. Liew , et al., Appl. Phys. Lett . 85 , 6409 ( 2004 ) ; J. Preusser , S. Knappe , J. Kitching , V. Gerginov , 2009 IEEE International Frequency Control Symposium Joint with the 22nd European Frequency and Time Forum , art . no. 5168385 , p. 1180 .