IRONSIDE, Charles (University of Glasgow, Dept. of Electronics and Electrical EngineeringRankine Building,Oakfield Avenue, Glasgow G12 8LT, GB)
| CLAIMS 1. A frequency reference device having a population of atoms or ions providing a first frequency reference signal by virtue of at least one electronic transition, the device further including driving means for providing a driving signal to the population of atoms or ions to promote the electronic transition, and a detector for detecting a characteristic of the first frequency reference signal, wherein the driving means includes a negative differential resistance oscillator, the detected characteristic of the first frequency reference signal being used to control the driving signal. 2. A device according to claim 1 wherein the population of atoms or ions is held in a vapour cell. 3. A device according to claim 1 or claim 2 wherein the population of atoms or ions is selected from from atomic alkali metals and/or molecular gases. 4. A device according to any one of claims 1 to 3 wherein the driving means further includes a laser diode. 5. A device according to claim 4 wherein the output of the laser is coupled to the population of atoms or ions. 6. A device according to claim 4 or claim 5 wherein the frequency of the output of the laser is tuned to a frequency absorbed by the electronic transition of the population of the atoms/ions. 7. A device according to any one of claims 4 to 6 wherein the laser output is modulated by a modulation signal derived from the negative differential resistance oscillator. 8. A device according to claim 7 wherein the modulation of the laser output provides two optical fields, differing in frequency by an amount substantially corresponding to a characteristic of the electronic transition in order to provide coherent population trapping resonance. 9. A device according to any one of claims 1 to 8 wherein the negative differential resistance oscillator includes a resonant tunnelling diode. 10. A device according to claim 9 wherein, in operation, the resonant tunnelling diode is DC biased in order to tune the oscillator characteristics of the resonant tunnelling diode, the detected characteristic of the first frequency reference signal being used to control the DC bias of the resonant tunnelling diode. 11. A device according to any one of claims 1 to 10 wherein the detected characteristic of the first frequency reference signal is the amplitude of the CPT signal. 12. A method of producing a frequency reference by applying a driving signal to a population of atoms or ions to provide a first frequency reference signal by virtue of at least one electronic transition, and detecting a characteristic of the first frequency reference signal, wherein the driving signal is produced using a negative differential resistance oscillator, the detected characteristic of the first frequency reference signal being used to control the driving signal . 13. A magnetic field sensor including a frequency reference device according to any one of claims 1 to 11, the frequency of the first frequency reference signal providing a measure related to the magnetic field strength at the population of atoms or ions. 14. A method of sensing magnetic field strength by carrying out the method of claim 12, the frequency of the first frequency reference signal providing a measure related to the magnetic field strength at the population of atoms or ions . 15. An array of magnetic field sensors according to claim 13, the array in operation providing information relating to spatial and/or temporal variation of magnetic field. |
FREQUENCY REFERENCES
BACKGROUND TO THE INVENTION
Field of the invention
The present invention relates to frequency reference devices and to methods of providing frequency references for applications such as, for example, atomic clocks and magnetometers .
Related art
The term "atomic clock" is typically used to describe a device that provides a frequency standard based on microwave signals emitted by electrons changing between known energy levels of a known atom. For example, the SI system of units defines the second as the duration of 9,192,631,770 cycles of radiation corresponding to the transition between two energy levels of the ground state of the caesium-133 atom. Atomic clocks form the basis of the Global Positioning System (GPS) navigation system. Very precise, stable and accurate atomic clocks tend to be large, complex, expensive and power-hungry. Many atomic clocks use a microwave cavity in order to excite atoms held in a vapour cell. In order to provide resonance, the cavity is typically larger than one half of the wavelength of the microwave radiation used to excite the atomic resonance. This is of the order of several centimetres for hydrogen, caesium and rubidium, for example .
It would be advantageous to provide a miniaturized frequency standard or atomic clock, particularly with low power consumption. Miniature atomic clocks developed by the United States National Institute of Standards and Technology (NIST - http://tf.nist.gov) are passive vapour cell reference based on coherent population trapping (CPT) resonance .
CPT is a nonlinear phenomenon in atoms in which coherences (electromagnetic multipole moments) between atomic energy levels are excited by pairs of optical fields. In one example of CPT, a coherence between two components of the hyperfine-split ground state of an atom is generated through the simultaneous coupling of both levels to a common excited state with the optical fields. When the difference of the frequencies of the optical fields is near the atomic hyperfine splitting frequency, atoms in one specific superposition of the two ground-state sub-levels do not interact with the optical field at all. This superposition state is commonly referred to as a "dark state" or "CPT state". Atoms in the orthogonal superposition interact strongly with the optical field. Therefore, if an atom starts off in some arbitrary state, it will absorb photons. Through the optical pumping process, atoms accumulate in the dark state and stop absorbing photons from the light field, and the absorption of the atomic sample decreases. A resonance therefore occurs. CPT is explained at http: //tf .nist.gov/ofm/smallclock/CPT.htm (accessed 22 July 2008) and characterization of CPT for atomic frequency references is discussed in Knappe et al ("Characterization of coherent population-trapping resonances as atomic frequency references" Knappe, S.; Wynands, R.; Kitching, J. ; Robinson, H. G.; Hollberg, L., Vol. 18, No. 11 [November 2001] J. Opt. Soc. Am. B pages 1545-1553), incorporated herein by reference in its entirety.
CPT resonance occurs at a narrow frequency range. It is characterized, as explained above, by a reduction in absorption of light at that narrow frequency range.
Kitching et al 2000 ("A microwave frequency reference based on VCSEL-driven dark line resonance in Cs vapor" J. Kitching, S. Knappe, N. Vukicevic, L. Hollberg, R. Wynands, and W. Weidemann, IEEE Trans. Instrum. Meas . , 49, 1313-1317, 2000) disclose a microwave frequency reference based on CPT resonance in caesium vapour. The Cs vapour was driven by light from a vertical cavity surface emitting laser (VCSEL) tuned to the Cs D 2 line (852 nm) . The vapour cell contains Cs and Ne buffer gas (the buffer gas reducing the transit time and Doppler broadening) . The laser injection current was modulated near 4.6 GHz with 12.6 mW of RF power using an external oscillator. The absorption of the light in the vapour cell was detected with a large area Si photodiode. The external oscillator was locked to the narrow CPT resonance in order to create a stable frequency reference. The content of Kitching et al 2000 is incorporated herein by reference in its entirety.
Knappe et al 2005 ("A chip-scale atomic clock based on 87 Rb with improved frequency stability" S. Knappe, P. D. D. Schwindt, V. Shah, L. Hollberg, J. Kitching, L. Liew and J. Moreland, Opt. Exp. 13, 1249, 2005) disclose the application of CPT resonance based on the Di lines of 87 Rb. A vapour cell was manufactured using etched Si wafers bonded to glass windows. The interior volume of the cell was 1 mm 3 . Two phase-stable circularly polarized light fields were produced by modulating the current of a VCSEL at half the frequency of the ground-state hyperfine splitting, creating two first- order sidebands on the optical carrier separated by the full atomic hyperfine splitting of 6.8 GHz. The laser wavelength of 795 nm was then tuned so that these two sidebands were resonant with the transitions from the two ground states to the excited 5Pi /2 state. The resonance was detected by measuring the transmitted optical power by use of phase- sensitive detection with a lock-in amplifier. A feedback loop stabilized the modulation frequency of 3.4 GHz onto the centre of the CPT resonance. The resultant frequency reference demonstrated fractional frequency instability (short term instability) of 4 x 10 '11 Zx 1 ' 2 for integration times between 1 s and 10 s. Residual long term drift was -5 x 10 ~9 /day. The content of Knappe et al 2005 is incorporated herein by reference in its entirety.
SUMMARY OF THE INVENTION
The present inventors consider that it would be advantageous to improve the frequency reference devices described above by further simplifying their design and/or further miniaturising their design.
Accordingly, in a first preferred aspect, the present invention provides a frequency reference device having a population of atoms or ions providing a first frequency reference signal by virtue of at least one electronic transition, the device further including driving means for providing a driving signal to the population of atoms or ions to promote the electronic transition, and a detector for detecting a characteristic of the first frequency reference signal, wherein the driving means includes a negative differential resistance oscillator, the detected characteristic of the first frequency reference signal being used to control the driving signal. In a second preferred aspect, the present invention provides a method of producing a frequency reference by applying a driving signal to a population of atoms or ions to provide a first frequency reference signal by virtue of at least one electronic transition, and detecting a characteristic of the first frequency reference signal, wherein the driving signal is produced using a negative differential resistance oscillator, the detected characteristic of the first frequency reference signal being used to control the driving signal.
Preferred and/or optional features of the invention are set out below. These are applicable singly or in any combination to any aspect of the invention, unless the context demands otherwise.
Preferably, the population of atoms or ions is maintained in a closed container. For example, the population of atoms or ions may be held in a vapour cell. The vapour cell may have environmental control means, such as temperature control means, magnetic field control means, electric field control means, etc.
The population of atoms or ions may be selected from atomic alkali metals and/or molecular gases. For example, the population of atoms or ions may include at least one of Cs, Rb or NH3. One or more buffer gases may be provided in addition.
Preferably, the driving means further includes a laser, typically a laser diode. The driving signal is thus preferably includes at least one optical field. The output of the laser is preferably coupled to the population of atoms or ions via a light guide such as an optical fibre. Alternatively, the vapour cell may be formed in direct optical contact with the laser output. Preferably the frequency of the output of the laser is tuned to a frequency absorbed by the electronic transition of the population of the atoms/ions.
Preferably, the laser output is modulated by a modulation signal derived from the negative differential resistance oscillator. Most preferably, this modulation includes rapid on-off switching of the laser output. The frequency of the modulation of the laser is determined by the oscillator. Preferably the device provides a frequency reference signal. This is typically a microwave frequency reference signal derived from the oscillator.
Modulation of the laser as explained above can lead to the production of two optical fields, differing in frequency by an amount substantially corresponding to a characteristic of the electronic transition (e.g. the atomic hyperfine splitting frequency) in order to provide coherent population trapping (CPT) resonance. Preferably the CPT resonance utilizes electronic states that are substantially- insensitive to magnetic fields, except in magnetometry applications as set out in more detail below.
Preferably the negative differential resistance oscillator includes a resonant tunnelling diode (RTD) .
It is known that a laser diode (LD) may be controlled using a negative differential resistance oscillator circuit. For example, a resonant tunnelling diode (RTD) may be allowed to produce a high frequency oscillating output to control a laser diode in an RTD-LD circuit. See, for example, Figueiredo et al ("Self-oscillation and period adding from resonant tunnelling diode-laser diode circuit" J. M. L. Figueiredo, B. Romeira, T.J. Slight, L. Wang, E. Wasige, CN. Ironside, Electronics Letters, Volume 44, Issue 14, July 3 2008, pages 876-877); Slight et al ("Integration of a resonant tunneling diode and an optical communications laser" T.J. Slight, CN. Ironside, CR. Stanley, M. Hopkinson, CD. Farmer, Photonics Technology Letters, IEEE Volume 18, Issue 14, July 2006, pages 1518-1520); and Slight and Ironside ("Investigation into the Integration of a Resonant Tunnelling Diode and an Optical Communications Laser: Model and Experiment", T.J. Slight, CN. Ironside, IEEE J. Quant. Elec. 43, 7, 580-587, 2007), the content of each of which is hereby incorporated by reference in its entirety.
The use of resonant tunnelling diodes in optoelectronic Communications systems is disclosed in WO 00/72383 and in WO 02/088834, in the context of modulation of light of wavelength 1550 nm by modulation of the absorbance characteristics of a waveguide associated with an RTD. However, these documents do not disclose the modulation of the laser itself using the output from the resonant tunnelling diode.
Preferably, the RTD and the laser diode are integrated in a monolithic device. However, alternatively, the RTD and the laser diode may be formed as a hybrid driving means.
The vapour cell (or other container holding the population of atoms or ions) may be formed in direct optical contact with the laser diode. The detector may be formed in direct optical contact with the vapour cell.
Preferably, in operation, the resonant tunnelling diode is DC biased towards the negative differential resistance region of the I-V characteristics of the combination of the resonant tunnelling diode and laser diode. The oscillator characteristics (including the oscillation frequency) of the resonant tunnelling diode typically depend, at least in part, on this DC bias. Thus, it is preferred that the DC bias of the resonant tunnelling diode is varied in order to tune the oscillator characteristics of the resonant tunnelling diode. This amounts to "fine-tuning" of the resonant tunnelling diode. Preferably, "gross tuning" of the resonant tunelling diode is determined by the external components of the circuit. Typically, the values of the inductance and capacitance determine the approximate value of the oscillation frequency and this is then fine tuned by adjusting the DC supply voltage.
In operation, it is preferred that the detected characteristic of the first frequency reference signal (or one of such characteristics) is the amplitude of the CPT signal. Preferably this amplitude signal is used to control the DC bias of the resonant tunnelling diode. The effect of this is that the frequency of modulation of the laser diode can become stably locked to the CPT signal. In effect, this amounts to feedback control of the oscillator using the CPT signal.
The frequency reference device may be used in a GPS system, for example. Alternatively it may be used (either singly or as one of a group of similar devices) in sensing applications such as temperature sensing or magnetic field sensing. The application of the device and method to sensing of magnetic fields is set out in more detail below. Balabas et al ("Magnetometry with millimeter-scale antirelaxation-coated alkali-metal vapor cells" M. V. Balabas, D. Budker, J. Kitching, P. D. D. Schwindt, and J. E. Stalnaker, J. Opt. Soc. Am. B, Vol. 23, Issue 6, June 2006, pages 1001- 1006) disclose the observation and investigation of dynamic nonlinear magneto-optical-rotation signals with frequency- modulated (FM NMOR) and amplitude-modulated laser light with a spherical glass cell of 3 mm diameter containing Cs metal with inner walls coated with paraffin. The laser frequency was tuned to the Cs Di line (894.6 nm) . The FM NMOR effect harnesses the rotation of polarized light due to the application of a magnetic field. This effect is sensitive to very small magnetic fields. The content of the Balabas et al document is incorporated herein by reference in its entirety.
BeIfi et al ("All optical sensor for automated magnetometry based on coherent population trapping" J. Belfi, G. Bevilacqua, V. Biancalana, Y. Dancheva, and L. Moi, J. Opt. Soc. Am. B, Vol. 24, Issue 7, pages 1482-1489) use coherent population trapping in a Cs vapour cell in order to measure small magnetic fields, such as biomagnetic fields. The exact frequency at which the CPT absorption resonant decrement occurs is a function of the magnetic field in the Cs vapour cell. The content of the Belfi et al document is incorporated herein by reference in its entirety. Schwindt et al 2004 ("Chip-scale atomic magnetometer" P. D. D. Schwindt, S. Knappe, V. Shah, L. Hollberg, J. Kitching, L.A. Liew and J. Moreland, Applied Physics Letters, Vol. 85. No. 26, December 2004, pages 6409-6411) disclose a magnetometer operating using CPT resonance based on the 5S 1Z2 ground-state hyperfine splitting between two magnetically sensitive Zeeman states in 87 Rb. The device is operated by tuning a laser diode to the Dl line of 87 Rb at 795 ran. The content of the Schwindt et al 2004 document is incorporated herein by reference in its entirety.
Accordingly, in a third preferred aspect, the present invention provides a magnetic field sensor (magnetometer) including a frequency reference device according to the first aspect, the frequency of the first frequency reference signal providing a measure related to the magnetic field strength at the population of atoms or ions.
In a fourth preferred aspect, the present invention provides a method of sensing magnetic field strength by carrying out the method of the second aspect, the frequency of the first frequency reference signal providing a measure related to the magnetic field strength at the population of atoms or ions . In a fifth preferred aspect, the present invention provides an array of magnetic field sensors according to the third aspect, in order to provide information relating to spatial and/or temporal variation of magnetic field.
Further preferred and/or optional features will now be set out. These have particular relevance to the third, fourth and/or fifth aspects, but may also be applied singly or in any combination, with any other aspect of the invention.
Preferably, the frequency of the first frequency reference signal varies with varying magnetic field strength. This is considered to be due to the hyperfine splitting becoming larger with increasing magnetic field strength.
Preferably, the measurement of magnetic field strength is related to the human or animal body. In particular, it is preferred that the invention is embodied in one of magnetoencephalography or magnetocardiography .
Thus, in a further preferred aspect of the invention, there is provided a method of diagnosis of the human or animal body, including the step of carrying out the method of the fourth aspect, or using a device according to the third or fifth aspect, in order to provide a magnetoencephalography measurement, a magnetocardiography measurement, a magnetoencephalogram or a magnetocardiogram. BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 shows a schematic diagram illustrating the atomic energy levels of a Rb atom relevant to the CPT effect. Fig. 2 shows a schematic diagram illustrating hyperfine splitting in a modulated later spectrum of Rb. Fig. 3 shows a schematic graph illustrating the CPT effect on optical transmission through a Rb vapour cell. Fig. 4 shows the schematic layout of a preferred embodiment of the invention.
Fig. 5 shows a schematic cross sectional view of an RTD-LD integrated chip for use in preferred embodiments of the invention. Fig. 6 illustrates an example of the RTD-LD circuit. Fig. 7 shows a tuning curve of the RTD-LD oscillator frequency as a function of the applied DC bias.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS, AND FURTHER PREFERRED AND/OR OPTIONAL FEATURES
Fig. 1 shows a schematic diagram illustrating the atomic energy levels of a Rb atom relevant to the CPT effect. As is well known to the skilled person, hyperfine splitting of the ground state corresponds to a microwave frequency of 6.834682610904324 GHz for rubidium. Fig. 2 shows a schematic diagram illustrating hyperfine splitting in a modulated later spectrum of Rb.
Fig. 3 shows a schematic graph illustrating the CPT effect on optical transmission through a Rb vapour cell. The graph shows the variation in optical transmission with modulation frequency of the light being passed through the vapour cell. From a simplified perspective, it would perhaps be expected that there would be a maximum in absorption (and thus a minimum in optical transmission) at the modulation frequency corresponding to the hyperfine splitting of the ground state. However, what is observed instead is a maximum in optical transmission at this modulation frequency, corresponding to the CPT effect described above.
In what follows, there is provided technical detail on how the preferred embodiments of the invention are put into effect. The RTD-LD circuit is described in detail, including how it interacts with the vapour cell. The vapour cell is also described. Also described is an example of how the detected amplitude from the vapour cell is used to control the DC bias to the RTD.
In one embodiment of the invention, there is used an integrated RTD-LD circuit, a "holey fibre" (a hollow core photonic crystal fibre) containing Rb vapour in the hollow core, a microcontroller and associated detectors. The system layout is described first.
The system employs a signal from the coherent population trapping (CPT) effect in an atomic vapour as a feedback signal to control the frequency of modulation of a RTD-LD circuit that acts as an optoelectronic voltage controlled oscillator (OVCO) .
Fig. 4 shows the schematic layout of the frequency reference device 10. An RTD-LD circuit 12 provides light with a wavelength of approximately 790 nm modulated at approximately 3.4 GHz and a radio frequency output "RF out" that is used as the frequency standard provided by the device 10. The RTD-LD circuit is in effect a voltage controlled oscillator (VCO) . An optical fibre 14 collects the light from the RTD-LD 12. The light from the RTD-LD 12 is split into two paths. The first path leads into and through a holey fibre vapour cell 16. The output beam from the vapour cell 16 is directed to a first optical detector 18. The vapour cell 16 is heated by heating means (not shown) and contains Rb vapour. The second path acts as a reference beam and is sent directly to a second optical detector 20. Analogue signals from both optical detectors 18, 20 are low passed filtered (not shown) so that the signals from both detectors do not have frequency components high enough to contain information about the microwave modulation. Both signals are directed to a microcontroller 22. The microcontroller 22 divides the signal from the first optical detector 18 (derived from the vapour cell 16) by the signal from the second optical detector 20 (reference signal) to obtain a value that is proportional to the transmission of light through the vapour cell 16.
The microcontroller 22 runs a programme that adjusts the DC bias to the RTD-LD and thereby the modulation frequency of the optical and electrical output from the RTD-LD circuit 12 to maximise the transmission through the vapour cell 16. The electrical output RF out of the RTD-LD circuit 12 is thereby locked to the hyperfine splitting of the Rb atoms in the atomic vapour cell 16 and is the frequency reference.
Next the RTD-LD device is described in more detail.
In this embodiment we describe an integrated RTD-LD with an edge emitting laser. As the skilled person will appreciate, the RTD-LD could also be separate RTD and laser diode (LD) chips. The laser could be a vertical-cavity surface- emitting laser (VCSEL) . The RTD-LD act as an optoelectronic voltage controlled oscillator (OVCO) . The frequency of oscillation is coarsely tuned by external components consisting of inductors or capacitors and in use in the embodiments of the present invention is finely tuned by altering the DC bias of the RTD-LD circuit. Fig. 5 shows a schematic cross sectional view of an RTD-LD integrated chip for use in preferred embodiments of the invention.
The RTD-LD chip consists of semiconductor epitaxial layers grown by molecular beam epitaxy or metal organic vapour phase epitaxy. The RTD part of the wafer consists of AlAs/GaAs/AlGas layers with thickness in the range 2-6 nm. The GaAs layer acts as a quantum well that has a resonant electronic state and maximum current through the RTD is obtained when the electron has energy equal to the resonant state energy. This means that the current-voltage curve of the RTD has a maximum. When the voltage is increased beyond the maximum then the current declines giving the current- voltage characteristic of the device a negative slope or negative differential resistance (NDR) region. It is this NDR region that causes the device to oscillate when it is place in an external, resonant circuit.
Table 1 shows typical layer thickness and dopant levels in the different layers of the device of Fig. 5. Table 1
As the skilled person will understand, this is merely one example of a suitable wafer design. The aim here is to form a RTD integrated with a single mode semi-conductor layer section designed to emit light at the Rb absorption close to 790 nm. A major determining factor for the emission wavelength is the y variable in Table 1.
An example of the RTD-LD circuit is illustrated in Fig. 6.
The properties of the components of the external circuit, i.e. resistance R, capacitance C and inductance L, coarsely determine the frequency of the oscillation. In addition the applied voltage (the DC bias voltage applied to the RTD-LD) selects the exact part of the current -voltage curve and thereby the exact value of the NDR region in which the RTD- LD operates. This has the effect of fine tuning the frequency of oscillation. The current from the RTD part of the RTD-LD device is used to drive the laser diode. Since the current to the LD is modulated the light output from the LD is also modulated at the same frequency.
The laser diode part of the RTD-LD is required to emit a wavelength appropriate for the CPT effect in Rb (about 790 nm) . For a Cs vapour cell the wavelength required would be about 830 nm. The wavelength of the light generated by the LD is determined by the semiconductor used in the active region of the LD and, for example, the alloy semiconductor AlGaAs may be used. In that case, the Al fraction has a strong effect on the wavelength emitted.
As the skilled person will understand, alternative RTD devices may be used, for example based on a 2 nm layer of AlAs, a 6 nm layer of InGaAs and a 2 nm layer of AlAs made by epitaxial growth on an InP substrate. Particularly for an integrated RTD-LD, the materials used for the RTD may be varied in order to suit the laser material.
Next, the atomic vapour cell is described in more detail.
In the preferred embodiments of the invention, a Rb-filled coated hollow core photonic crystal fibre is used as the atomic vapour cell. For example, a product such as that described in Light et al (P. S. Light, F. Benabid, F. Couny, M. Marie and A. N. Luiten "Electromagnetically induced transparency inRb-filled coated hollow-core photonic crystal fiber" May 15, 2007, Vol. 32, No. 10, Optics Letters, pp. 1323-1325) may be used. The content of the Light et al document is hereby incorporated by reference in its entirety.
Next, the microcontroller and servo-loop are described in more detail.
Referring once more to Fig. 4, the signals from the optical detectors 18, 20 are processed by microcontroller 22. The first task of the microcontroller 22 is to divide the signal from the first optical detector 18 (corresponding to the signal transmitted through the vapour cell 16) by the signal from the second optical detector 20 (reference signal) . The result is a signal only proportional to the transmission through the vapour cell. This transmission signal is used in a feedback (or servo) loop to control the DC bias of the RTD-LD and thereby fine tune the modulation of the light emitted by the RTD-LD. The person skilled in the art will understand how to produce a control programme for the microcontroller that adjusts the DC bias so that the transmission signal is maximised. Thus this feedback loop uses the CPT effect to ensure that the modulation frequency of the RTD-LD is adjusted to the hyperfine splitting of the Rb atoms in the vapour cell. The radio frequency output from the RTD-LD is thus locked to the atomic transition. Fig. 7 shows a tuning curve of the RTD-LD oscillator frequency as a function of the applied DC bias. The centre frequency is set by the value of the L and C components in the external circuit.
The preferred embodiments above are described by way of example. On reading this disclosure, modifications of these embodiments, further embodiments and modifications thereof will be apparent to the skilled person. In particular, it is contemplated that the preferred embodiments described above may be utilised as highly sensitive magnetometers in view of the effect of an applied magnetic field on the hyperfine splitting energy and thus on the modulation frequency corresponding to CPT. Highly sensitive magnetoencephalography measurements and/or magnetocardiography measurements may then be carried out on the human or animal body in order to perform diagnosis.