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Conference on Precision Electromagnetic Measurements

Measurement of microwave magnetic field in free space using the Rabi frequency

Moto Kinoshita, Masanori Ishii

Published Jul 1, 2016• 4 citations
Publisher

Abstract

Quantum-mechanics-based measurement of microwave magnetic field strength has been suggested and has attracted considerable research interest. Such measurements exploit the fact that the Rabi frequency, which indicates the degree of combination between atoms and microwaves, is proportional to the magnetic field strength of the microwaves interacting with the atoms. This new method is expected to facilitate realization of a quantum-based electromagnetic sensor that minimally perturbs the microwaves, being both nonmetal and wireless. This paper demonstrates measurement of the microwave magnetic field strength at a 9.2-GHz frequency using the Rabi frequency of cesium-133 atoms. The microwaves are radiated from a horn antenna.

DOI
10.1109/cpem.2016.7540672

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Quantum-mechanics-based measurement of microwave magnetic field strength has been suggested and has attracted considerable research interest. Such measurements exploit the fact that the Rabi frequency, which indicates the degree of combination between atoms and microwaves, is proportional to the magnetic field strength of the microwaves interacting with the atoms. This new method is expected to facilitate realization of a quantum-based electromagnetic sensor that minimally perturbs the microwaves, being both nonmetal and wireless. This paper demonstrates measurement of the microwave magnetic field strength at a 9.2-GHz frequency using the Rabi frequency of cesium-133 atoms. The microwaves are radiated from a horn antenna.
Detection of the microwave (MW) field with high accuracy is very important in the physical science and engineering fields. Herein, an atomic Rabi resonance-based MW magnetic field sensor with a high-dynamic-range is reported, where α and β Rabi resonances are used to measure MW fields. In MW measurement experiments, the sensor successfully measured a magnetic field of about 10 nT at 9.2 GHz using the α Rabi resonance line on the cesium clock transition and continuously detected the MW magnetic field in the X-band over a high dynamic power range of >60 dB from the β Rabi resonance. Finally, the MW power frequency shift and power broadening are investigated to support more sensitive field measurements. The proposed MW detection method can be extended to cover a higher dynamic range and a wider frequency band by applying stronger excitations and exploring non-clock atomic transitions, respectively. In addition to MW magnetic field sensing, other potential application of the proposed method can be explored, including SI-traceable MW calibration and atomic communication.
We present an initial proof-of-concept experiment to measure microwave power based on quantum-mechanical principles. Ground-state cesium atoms exposed to microwaves at 9.192 631 770 GHz oscillate between two hyperfine states at a rate that is proportional to the rf magnetic field strength. This provides a quantum-based method of measuring rf field strength that depends only on the fundamental parameters in the proportionality constant. A small fountain apparatus was used to prepare laser-cooled cesium atoms in a single hyperfine state, which were then launched through a cylindrical cavity operating in the TE011 mode. After passing through the cavity, the fraction of atoms in the two hyperfine states was measured. Rabi oscillations between the two states were observed as a function of microwave field strength. The scaling with field strength and with time in the cavity agreed with theory to within 0.4%. The field strength in the cavity was used, together with measured S parameters, to determine the microwave power incident at a reference plane outside the fountain apparatus. The difference between the quantum-based microwave power measurement and a traditional microwave power measurement was less than 5% of the measured power.
In this work, we demonstrate a microwave magnetic field imaging technique based on Rabi resonance with a cesium atom vapor cell. Rabi resonance signals are generated when atoms interact with a phase-modulated microwave (MW) field and are detected by a photodiode and camera. A low noise, high quantum efficiency camera is used to capture a series of frames for different phase modulation frequencies. Rabi frequencies of each spatial point in the field can be measured by scanning the frames. Thus, the strength of the MW magnetic field distribution is obtained by combing the Rabi frequencies, measured with camera pixels in the range of the probe laser beam. The simple architecture of this imaging setup holds great potential for the construction of compact/miniature MW field sensors for material testing, field imaging of MW components, and biomedical imaging.
In this work, the theoretical and measurement results have been reported of microwave E-field sensing using an atomic spectroscopy instead of conventional methods (using antennae) under practice till date. This method is based on interaction of microwave with the Rb atoms excited to the Rydberg states by using two laser beams. This significantly new technique may get accepted internationally as a new primary standard for accurate and precise measurements of various Radio-Frequency parameters such as E-field, power, radiation pressure etc as it provides traceability to physical constants and SI units. The atomic model is also studied theoretically and Optical Bloch equations of the same are reported. It has been shown that microwave E-field strength is measured by studying the frequency difference between the peaks in laser transmission curve. The ability of atoms to sense E-field is shown by varying the power and distance of the antenna from atomic cell. In addition, results are shown for weak RF field detection by detuning the RF field by MHz order.
We experimentally investigated continuously frequency-tunable microwave (MW) magnetic field detection based on Rabi resonance with a single cesium-rubidium hybrid vapor cell. The multispecies atomic systems, with their tunable abilities in transition frequencies, enabled this atomic sensing head to cover a broader detectable MW field scope compared to the use of a single alkali atom. Here, we demonstrated the simultaneous observation of atomic Rabi resonance signals with 85Rb, 87Rb, and 133Cs in the same vapor cell. Using an experimentally feasible static magnetic field (DC field) below 500 G, we realized a MW magnetic field strength detection with a bandwidth of 4.8 GHz around 8.1 GHz. The use of a hybrid cell system cell also enabled the detection of an identical MW field for different atomic species with the help of the DC field, allowing us to perform a perfect comparison for different applications that require the same electromagnetic environment. The results may be useful for the realization and application of many atomic detectors based on different physical principles.
In this letter, an atom-based approach for measuring the microwave (MW) cavity response (including cavity frequency and Q-factor) is presented, which utilizes a MW magnetic field detection technique based on atomic Rabi resonances. We first identify the Rabi resonances on seven π transitions in Cs atoms and demonstrate their uses in continuously frequency-tunable field detectors. With the atom-based field detectors, we then indicate the possibility of reconstructing the MW cavity response by measuring the MW frequency-dependent Rabi frequency (i.e., MW field strength) inside the cavity. To demonstrate this approach, we measured the response curves of a 9.2-GHz cavity and a cavity resonating at 8.3 GHz and 9.7 GHz using π transitions and σ transitions, respectively. We compared the results measured by our approach with those measured by Vector Networker Analyzer and obtained good agreement. From such atom-based, SI-traceable measurements, the MW cavity response can be linked directly to the Rabi frequency, which could be referred to an atomic clock.