RU2789203C1 - Optical quantum magnetometer - Google Patents

Abstract

FIELD: magnetic field measuring techniques.

SUBSTANCE: invention relates to a technique for measuring a magnetic field, to quantum magnetometers with optical pumping and detection. An optical quantum magnetometer contains a controlled oscillator, an amplitude modulator, an electro-optical modulator connected in series in a closed ring of automatic frequency control, to the optical input of which linearly polarized laser radiation is supplied through a laser radiation input device, a quantum sensor in the form of a translucent container with an alkali metal, a signal processing unit in in the form of a device for measuring the polarization azimuth of laser radiation and a phase locked loop device. The magnetometer has increased sensitivity and accuracy of magnetic field measurement.

EFFECT: increasing the sensitivity and accuracy of measuring the magnetic field with small sensor sizes.

2 cl, 2 dwg

Description

The present invention relates to a technique for measuring a magnetic field, namely to the field of quantum magnetometers with optical pumping and detection, and can be used to study magnetic fields of biological origin.

One of the most urgent tasks of modern highly sensitive magnetometry is the study of magnetic fields of biological origin, primarily the fields generated by the human heart (magnetocardiography), muscles (magnetomyography) and brain (magnetoencephalography). This necessitates the creation of a quantum magnetometer, characterized by sensitivity at the level of tens or even units of fTl in a one Hz band, and speed (200-600) Hz with a volume of the sensitive element of a quantum magnetometer - a quantum sensor of units of cm ³ .

A radio-frequency atomic magnetometer is known (WO 2009079054, IPC G01R 33/26, publ. 06/25/2009), containing a cell in the form of a translucent container with alkali metal, equipped with a heater for the formation of alkali metal vapor, a magnetic field generator configured to apply a static magnetic field to the alkali metal vapor, the first source of linearly polarized light for optical pumping of the alkali metal vapor, the second linearly polarized light source configured to transmit light through the alkali metal vapor, the light polarization detector configured to determine the polarization angle of the light of the second linearly polarized source after it passing through the alkali metal vapor, and a processor configured to determine the component frequencies as the polarization angle changes.

The disadvantages of the known radio frequency atomic magnetometer is the use of a radio frequency magnetic field, which excludes the operation of several such sensors in a compact array, as well as the use of two laser beams for pumping and detection.

A radio frequency atomic magnetometer is known (US 20100289491, IPC G01R 33/44, publ. 11/18/2010) including a quantum sensor in the form of a translucent container with an alkali metal and an internal paraffin coating, equipped with a heater, a magnetic field generator configured to apply a substantially static magnetic field to an alkali metal vapor, a source of linearly polarized laser radiation configured to optically pump an alkali metal vapor, a laser radiation polarization detector configured to determine the polarization angle of the linearly polarized laser radiation after it has passed through the alkali metal vapor, and a processor configured to determination of component frequencies with a change in the angle of polarization. In this case, the laser radiation source is configured to irradiate the alkali metal vapor with laser radiation linearly polarized along the magnetic field. In a known magnetometer, the phasing of the magnetic moments is carried out by a radio frequency magnetic field, and the value of the measured magnetic field is determined by determining the frequency of the free precession of the magnetic moments with the optical pumping turned off; in this case, the second beam of linearly polarized laser radiation, detuned in frequency from the absorption line, measures the polarization azimuth of the laser radiation transmitted through the quantum sensor.

The disadvantage of the known radio frequency atomic magnetometer is the use of a radio frequency magnetic field, which excludes the operation of several such sensors in a compact array, as well as the use of two laser beams for pumping and detection.

An optical quantum magnetometer is known (WO2009073256, IPC G01R 33/26, publ. 06/11/2009), including a quantum sensor in the form of a translucent container with an alkali metal and an internal paraffin coating, equipped with a heater, the first source of linearly polarized laser radiation, made with the possibility of optical pumping alkali metal vapor, a second source of linearly polarized laser radiation, configured to transmit laser radiation through an alkali metal vapor, a detector for determining the angle of polarization of the laser radiation of the second source transmitted through an alkali metal vapor, an electro-optical modulator containing a Mach-Zehnder modulator for frequency modulation or the amplitude of the laser radiation of the first source in response to the polarization determined by the detector, and a processor configured to determine the magnetic field strength based on the frequency determined by the frequency counter. In this case, the polarization of laser radiation from the first source is not parallel to the polarization of laser radiation from the second source, the laser radiation of the first source and the second source comes from the same laser, the first and second sources contain polarizers, the laser radiation polarization detector includes a Rochon polarizer and a photodetector.

The disadvantages of the known optical quantum magnetometer are the use of two laser beams for pumping and detection, as well as the use of linearly polarized laser radiation modulated in amplitude or frequency for optical pumping of atoms, which leads to the creation of an alignment of the magnetic moments of the atoms, and not their orientation, and, as a result , to insufficient detection efficiency and to deterioration of the magnetic field measurement accuracy. The use of a translucent container with an internal paraffin coating limits the operating temperature range of the sensor from above to ~65°C, which leads to a significant decrease in sensitivity at small sensor sizes.

Known optical quantum magnetometer (US 8421455, IPC G01V 3/00, G01R 33/02, publ. 04/16/2013), coinciding with the present solution for the largest number of essential features and taken as a prototype. Optical quantum magnetometer prototype includes a quantum sensor in the form of a glass container equipped with a heater with an internal anti-relaxation coating and with an alkali metal placed in it, a laser generating linearly polarized radiation connected to the first input of the Mach-Zehnder modulator and to a device for blocking the dichroic vapor line of an alkali metal connected to the input of the laser control unit, a controlled generator, the input of which is connected to the output of the signal processing unit, and the output is connected to the second input of the Mach-Zehnder modulator, the outputs of the quantum sensor through multimode fibers, photodiodes and a differential amplifier are connected to the input of the signal processing unit.

The disadvantages of the known optical quantum magnetometer prototype is the use for optical pumping of atoms of linearly polarized laser radiation modulated in amplitude or frequency, which leads to the creation of an alignment of the magnetic moments of the atoms, and not their orientation, and, as a result, to insufficient detection efficiency and to deterioration magnetic field measurement accuracy; as well as tuning the laser frequency to the frequency of the optical transition in atoms from the same hyperfine level at which the alignment of magnetic moments is carried out, which does not allow the most efficient pumping and detection of magnetic resonance. The use of a translucent container with an internal paraffin coating limits the operating temperature range of the container to ~65°C from above, which leads to a significant decrease in sensitivity at small sensor sizes.

The objective of this technical solution is to develop an optical quantum magnetometer, which would provide increased sensitivity and accuracy of magnetic field measurements with small sensor sizes.

The problem is solved by the fact that the optical quantum magnetometer includes a quantum sensor in the form of a translucent container with an alkali metal and a buffer gas (nitrogen) equipped with a heater, an input device for linearly polarized laser radiation (LI), a controlled generator, an amplitude modulator, a signal processing unit, the output of which connected to the input of the controlled generator. What is new is that the magnetometer contains an electro-optical modulator, by means of which the LR polarization ellipticity is modulated from the left circular (circular) polarization to the right circular polarization with a frequency close to the magnetic resonance frequency (Larmor frequency). The magnetometer also contains a signal generator, the signal processing unit is made in the form of a device for measuring the polarization azimuth of laser radiation and a phase locked loop device, the optical input of the electro-optical modulator is connected to the device for inputting laser radiation, and the optical output is connected to a quantum sensor, a device for measuring the azimuth of polarization of laser radiation connected in series with the first input of the phase locked loop device, while the second input of the phase locked loop device is connected to the output of the controlled oscillator and the input of the amplitude modulator, the output of which is connected to the modulating input of the electro-optical modulator, and the output of the signal generator is connected to the control input of the phase locked loop device and to modulating input of the amplitude modulator.

Laser radiation is tuned to a frequency close to the frequency of optical transitions from the lower hyperfine sublevel of the ground state of an alkali metal atom. In this case, as a result of optical pumping by circularly polarized time-modulated LR, the orientation of the magnetic moments of atoms is created. Magnetic resonance detection is performed by a linearly polarized LI, and it mainly detects resonance at a level from which it is detuned in frequency by an amount on the order of hyperfine splitting of the ground state; due to this, an increased accuracy of measuring the magnetic field is achieved in comparison with the prototype. The use of a translucent container with a buffer gas allows, in comparison with the prototype, to expand the operating temperature range of the sensor to 110-120°C and above, which makes it possible to reduce the size of the container without a significant decrease in sensitivity.

A translucent container with a heating element can be placed in a thermostat with windows for input and output of laser radiation. The present invention is illustrated by the drawing, where:

in fig. 1 is a schematic representation of a real optical quantum magnetometer (B ₀ is the magnetic field of the object under study, S(t) is the control signal, f _q is the frequency of the signal at the output of the controlled oscillator, f ₁ is the Larmor frequency, f _OUT is the frequency of the output signal;

in fig. Figure 2 shows the waveforms in a real optical quantum magnetometer (21 - signal at the output of a controlled generator, 22 - signal at the output of a signal generator, 23 - signal at the output of an amplitude modulator, 24 - signal at the output of a device for measuring the polarization azimuth of laser radiation, processed by a phase-locked loop frequencies).

This optical quantum magnetometer (see Fig. 1) includes a controlled oscillator (UG) 1, an amplitude modulator (AM) 2, an electro-optical modulator (EOM) 3, connected in series in a closed loop of automatic frequency control, to the optical input of which through the laser radiation input device (SWLI) 4 laser radiation (LI) is supplied to the optical input of the EOM 3 characterized by linear polarization, a quantum sensor (QD) 5, which is a translucent container (SC) 6 with an alkali metal and a buffer gas, placed in a thermostat 7 with a heating element 8 and windows 9 for input and output of laser radiation, a signal processing unit in the form of a device (UAP) 10 for measuring the azimuth of the polarization of laser radiation and a device (UFACH) 11 phase locked loop, the output of the UPA 10 is connected to the first input of the UPACH 11, the output of which is connected to the input HS 1, while the second input of UPACH 11 is connected to the output of HS 1, as well as the signal generator (GS) 12, the output of which connected to the modulating input AM 2 and to the control input UPACH 11, while the output of the HS 1 is the output of the optical quantum magnetometer. The measured value of the magnetic field is obtained from the frequency of the output signal f _OUT according to the formula B ₀ =f _OUT /y, where y is the gyromagnetic ratio.

This optical quantum magnetometer works as follows.

основного состояния

щелочного металла (цезия, рубидия или калия) в КД 5. УГ 1 формирует периодический синусоидальный сигнал с частотой f_g (см. фиг. 2, кривая 21). ГС 12 формирует управляющий сигнал S(t), представляющий собой последовательность импульсов длительностью Ti, повторяющихся через промежуток времени Т₂ (см. фиг. 2, кривая 22). В течение промежутка времени T₁ поляризация ЛИ варьируется от левой циркулярной поляризации до правой циркулярной поляризации по синусоидальному закону с частотой f_g, близкой к ларморовской частоте f_L. При этом два раза за период поляризация луча становится чисто круговой (σ^±), и два раза за период - чисто линейной (п); в остальные моменты эллиптичность принимает промежуточные значения. Оптическая накачка и одновременно возбуждение магнитного резонанса осуществляется σ^± компонентой луча. В течение промежутка времени Т₂ поляризация ЛИ становится чисто линейной (п), что позволяет использовать луч ЛИ в качестве пробного для детектирования сигнала прецессии магнитных моментов. При этом, поскольку частота ЛИ настраивается близко к частоте оптического перехода с уровня

основного состояния

щелочного металла, а атомы концентрируются в состоянии

m_F=F, то пробное ЛИ преимущественно детектирует резонанс на уровне, от которого оно отстроено по частоте на величину порядка сверхтонкого расщепления основного состояния атома щелочного металла (для цезия это 9.192 ГГц); тем самым реализуются условия для неразрушающего квантового измерения. Таким образом, достигаются почти оптимальные условия для детектирования. Сигналы с УГ 1 и с ГС 12 подают на входы AM 2, на выходе которого образуется сигнал, представляющий собой синусоидальный сигнал с частотой f_g за время Т₁ и нулевой сигнал за время Т₂ (см. фиг. 2, кривая 23). Этот сигнал подают на вход ЭОМ 3, который осуществляет модуляцию поляризации ЛИ. ЭОМ 3 должен быть настроен таким образом, чтобы при нулевом значении напряжения на входе он пропускал линейно поляризованное ЛИ без изменения поляризации. Линейно поляризованное ЛИ подают через УВЛИ 4 на оптический вход ЭОМ 3. При проходе через ЭОМ 3 в течение промежутка времени T₁ поляризация ЛИ изменяется от левой циркулярной поляризации до правой циркулярной поляризации по синусоидальному закону с частотой f_g, что регулируется амплитудой сигнала с УГ 1, а в течение промежутка времени Т₂ ЛИ поляризовано линейно. Такое ЛИ проходит через СК 6 КД 5, содержащий атомы щелочного металла (цезия или рубидия, или калия) в газообразном состоянии, а также буферный газ, препятствующий релаксации магнитных моментов атомов на стенках СК 6, помещенный в теплоизолирующий корпус термостата 7, включающий в себя нагревательный элемент 8 и датчик температуры (на чертеже не показан). После прохождения СК 6 в течение промежутка времени Т₂ азимут поляризации ЛИ измеряется УАП 10, в простейшем случае представляющим собой разделитель линейных поляризаций и два фотоприемника. Сигнал с УАП 10 в течение промежутка времени Т₂, являющийся сигналом свободной прецессии магнитных моментов в магнитном поле В₀, представляющий собой затухающий синусоидальный сигнал на ларморовской частоте f_L (см. фиг. 2, кривая 24), подается на первый вход УФАЧ 11, на второй вход которого подается сигнал с УГ 1 с частотой f_g. На управляющий вход устройства УФАЧ 11 подается сигнал s(t), при этом устройство УФАЧ 11 работает таким образом, что сравнение сигналов с частотами f_g и f_L происходит в течение промежутка времени Т₂. Устройство УФАЧ 11 в течение промежутка времени Т₂ формирует сигнал U, который подается на вход УГ 1, обеспечивая подстройку частоты f_g к ларморовской частоте f_L. Выходным сигналом устройства является частота fouT=f_g. Измеряемая величина магнитного полярассчитывается, исходя из частоты выходного сигнала W по формуле Bo=four/y, где у - гиромагнитное отношение.The pump/detection LI supplied from the UVLI 4 is tuned to a frequency close to the frequency of the optical transition from the level

ground state

alkali metal (cesium, rubidium or potassium) in CD 5. UG 1 generates a periodic sinusoidal signal with a frequency f _g (see Fig. 2, curve 21). HS 12 generates a control signal S(t), which is a sequence of pulses with a duration Ti, repeated after a period of time T ₂ (see Fig. 2, curve 22). During the time interval T ₁ the LI polarization varies from the left circular polarization to the right circular polarization according to a sinusoidal law with a frequency f _g close to the Larmor frequency f _L . In this case, twice per period, the polarization of the beam becomes purely circular (σ ^± ), and twice per period, purely linear (n); at other moments, the ellipticity takes intermediate values. Optical pumping and simultaneous excitation of magnetic resonance are carried out by the σ ^± component of the beam. During the time interval T _2, the LR polarization becomes purely linear (n), which makes it possible to use the LR beam as a test beam for detecting the precession signal of magnetic moments. In this case, since the LR frequency is tuned close to the frequency of the optical transition from the level

ground state

alkali metal, and the atoms are concentrated in the state

m _F =F, then the probe LR mainly detects the resonance at the level from which it is detuned in frequency by a value on the order of the hyperfine splitting of the ground state of the alkali metal atom (for cesium, this is 9.192 GHz); thus, the conditions for a nondestructive quantum measurement are realized. In this way, almost optimal conditions for detection are achieved. Signals from UG 1 and from GS 12 are fed to the inputs of AM 2, at the output of which a signal is formed, which is a sinusoidal signal with a frequency f _g for time T ₁ and a zero signal for time T ₂ (see Fig. 2, curve 23). This signal is fed to the input of EOM 3, which modulates the polarization of the LI. EOM 3 must be configured in such a way that, at zero input voltage, it passes a linearly polarized LI without changing the polarization. A linearly polarized LI is fed through the UVLI 4 to the optical input of the EOM 3. When passing through the EOM 3 for a period of time T ₁ , the polarization of the LI changes from the left circular polarization to the right circular polarization according to a sinusoidal law with a frequency f _g , which is controlled by the amplitude of the signal with CG 1 , and during the time interval T ₂ LI is linearly polarized. Such LR passes through SC 6 KD 5, containing atoms of an alkali metal (cesium or rubidium, or potassium) in a gaseous state, as well as a buffer gas that prevents the relaxation of the magnetic moments of atoms on the walls of SC 6, placed in a heat-insulating thermostat housing 7, including heating element 8 and a temperature sensor (not shown). After passing through the SC 6 for a period of time T _2, the azimuth of the LI polarization is measured by the UAP 10, which in the simplest case is a linear polarization separator and two photodetectors. The signal from the UAP 10 for a period of time T ₂ , which is a signal of the free precession of magnetic moments in a magnetic field B ₀ , which is a damped sinusoidal signal at the Larmor frequency f _L (see Fig. 2, curve 24), is fed to the first input of the UFACH 11 , the second input of which is fed with a signal from the HS 1 with a frequency f _g . A signal s(t) is applied to the control input of the UPACH device 11, while the UPACH device 11 operates in such a way that the comparison of signals with frequencies f _g and f _L takes place over a period of time T ₂ . The UPACH device 11 during a period of time T ₂ generates a signal U, which is fed to the input of the HS 1, providing adjustment of the frequency f _g to the Larmor frequency f _L . The output signal of the device is the frequency fouT=f _g . The measured value of the magnetic field is calculated based on the frequency of the output signal W according to the formula Bo=four/y, where y is the gyromagnetic ratio.

основного состояния

щелочного металла и детектировании магнитного резонанса в состоянии

m_F=F.According to the preliminary experimental results given in [Petrenko MV, Pazgalev AS, Vershovskii A.K. Single-beam all-optical non-zero field magnetometric sensor for magnetoencephalography applications // Phys. Rev. Applied. 2021 Vol. 15, #6. P. 064072], in comparison with the prototype, the proposed quantum magnetometer makes it possible to realize a higher magnetic field measurement accuracy, limited by fundamental quantum noise, since it uses time-modulated circular polarization of laser radiation to orient the magnetic moments, which makes it possible to increase the amplitude of the magnetic signal by almost an order of magnitude. resonance. An additional increase in accuracy is realized by pumping at the optical transition frequency from the level

ground state

alkali metal and magnetic resonance detection in the state

m _F =F.

Since the present magnetometer must be able to work as part of large arrays of magnetometers pumped by a single laser pump source, the laser itself is not included in the magnetometer. The present quantum magnetometer is characterized by simplicity, compactness and economy, since it uses a single laser optical channel and a compact container with alkali metal and buffer gas.

Claims (2)

1. An optical quantum magnetometer, which includes a quantum sensor in the form of a translucent container with an alkali metal, equipped with a heater, a laser radiation input device, a controlled generator, an amplitude modulator, a signal processing unit, the output of which is connected to the input of a controlled generator, while the laser radiation input device is optically is connected to a quantum sensor, characterized in that it contains an electro-optical modulator and a signal generator, the signal processing unit is made in the form of a device for measuring the polarization azimuth of laser radiation and a phase-locked loop device, the optical input of the electro-optical modulator is connected to the input device for laser radiation, and the optical output with a quantum sensor, a device for measuring the polarization azimuth of the laser radiation is connected in series with the first input of the phase locked loop device, while the second input of the phase locked loop device is connected to the output of the control oscillator and the input of an amplitude modulator, the output of which is connected to the modulating input of the electro-optical modulator, and the output of the signal generator is connected to the control input of the phase locked loop device and to the modulating input of the amplitude modulator. 2. Magnetometer according to claim. 1, characterized in that the translucent container with a heating element is placed in a thermostat with windows for input and output of laser radiation.

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