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SPIE Quantum Technologies

A read-out enhancement for microwave electric field sensing with Rydberg atoms

Harald Kübler, James P. Shaffer

Published May 21, 2018• 33 citations
Publisher

Abstract

We have shown that Rydberg atoms can be used for high-sensitivity, absolute sensing of radio frequency (RF) electric fields. We achieved a sensitivity of 3 μVcm-1Hz-1/2 for two read-out strategies. Results using a Mach-Zehnder interferometer2 and frequency modulated spectroscopy both achieve similar photon shot noise limited sensitivity. Fundamental limits to the sensitivity of the Rydberg atom-based RF electric field sensing have been addressed. Depending on the spectral resolution of the read-out, either the RF induced transmission line frequency splitting, the Autler-Townes regime, or a change in the on-resonant absorption, the amplitude regime, can be used to determine the RF electric field. Here, we present theoretical results of a 3-photon read-out scheme which enables the Autler-Townes regime of Rydberg atom-based RF electrometry to be extended to lower RF electric field strengths. We show that the residual Doppler shifts can, in principle, be reduced to ~11.8 kHz, on the order of the Rydberg atom natural lindewidths, using the approach.

DOI
10.1117/12.2309386

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We have shown that Rydberg atoms can be used for high-sensitivity, absolute sensing of radio frequency (RF) electric fields. We achieved a sensitivity of 3 μVcm-1Hz-1/2 for two read-out strategies. Results using a Mach-Zehnder interferometer2 and frequency modulated spectroscopy both achieve similar photon shot noise limited sensitivity. Fundamental limits to the sensitivity of the Rydberg atom-based RF electric field sensing have been addressed. Depending on the spectral resolution of the read-out, either the RF induced transmission line frequency splitting, the Autler-Townes regime, or a change in the on-resonant absorption, the amplitude regime, can be used to determine the RF electric field. Here, we present theoretical results of a 3-photon read-out scheme which enables the Autler-Townes regime of Rydberg atom-based RF electrometry to be extended to lower RF electric field strengths. We show that the residual Doppler shifts can, in principle, be reduced to ~11.8 kHz, on the order of the Rydberg atom natural lindewidths, using the approach.
We have shown that Rydberg states can be used for high-sensitivity, absolute sensing of microwave (MW) electric fields. We achieved a sensitivity of 3 μVcm-1Hz-1/2 for two read-out strategies. Depending on the spectral resolution of the read-out, either the MW induced transmission line frequency splitting, the Autler-Townes regime, or a change in the on-resonant absorption, the amplitude regime, can be used to determine the MW electric field. Results using a Mach-Zehnder interferometer and frequency modulated spectroscopy both achieve similar photon shot noise limited sensitivity. In addition, we have also explored amplitude modulation and the displacement of a probe laser beam due to index of refraction changes in a prism shaped vapor cell. These latter methods were not able to achieve photon shot noise limited performance. Fundamental limits to the sensitivity of the Rydberg atom-based MW electric field sensing have been addressed, but it is important to clarify the differences between noise in different parts or subsystems of the sensor. Shot noise in the probe laser usually dominates the projection noise of the atoms participating in the measurement of the MW electric field because of the desire to operate at low effective Rydberg atom densities in order to avoid collisional dephasing and ionization.
We describe experiments relevant to Rydberg atom-based sensing. In Rydberg atom-based sensing, atoms contained in a vapor cell are used as a probe of an incident radio frequency electric field. We present a novel approach based on a collinear three-photon read-out and detection scheme, which extends the sensing range of the self-calibrated Autler-Townes sensing mode to lower electric field strengths. We demonstrate proof of concept and present concrete results from first experiments. We show that we are able to measure electric field strengths below 1 mV/cm in the self-calibrated Autler-Townes regime for a radial dipole moment of 2206 ea0for a transition at 10.7 GHz. We measure an Autler-Townes splitting of 503(6) kHz, which is equivalent to a radio frequency electric field strength of micro-V/cm.
Significant progress has been made in atom-based measurements of length, time, gravity and electromagnetic fields in recently years. Rydberg atom-based microwave electric field measurement, using electromagnetically induced transparency (EIT) in room temperature alkali-metal vapors, has been extensively investigated and aroused the broad interest. This approach may establish a new standard for the measurements of microwave (MW) and radio frequency (RF) electric fields.In this review, we describe the work on a new method of measuring electric fields based on quantum interference by using either cesium or rubidium atoms contained in a dielectric vapor cell. Rydberg atoms with principal quantum number n >>1 have large direct current (DC) polarizabilities and microwave transition dipole moments, thereby making them extremely sensitive to external electric fields. Using the Rydberg three-level EIT to detect the level splitting and shift that is induced by the external field, we can realize a rapid and robust self-calibration method of measuring the electric field in a frequency range from 0.01 GHz to 1000 GHz. For the MW electric field (frequency range > 1 GHz), the MW field causes the Rydberg states to split, known as an Autler-Townes splitting (A-T) effect when the applied microwave can resonate with adjacent Rydberg states. The MW coupled A-T splitting is proportional to the applied electric field strength, from which the field strength is measured. Using the EIT window, a high sensitivity of 3 μV·cm-1·Hz-1/2 and small electric field of 1 μV/cm are expected to be achieved with a modest setup, and the limitations of the sensitivity are also addressed in the review. For the RF field at frequency mj EIT lines, and avoided crossings formed with the fine-structure levels of equal mj and different J's, which is used to calibrate and measure the RF field amplitude. On the other hand, the dependence of the EIT-line strength on the RF field polarization provides a fast and robust polarization measurement of RF fields based on matching experimental data with a theoretical simulation. The measurements of minimum strengths and sensitivity of RF fields based on Rydberg atoms are one order magnitude below the values obtained by traditional antenna methods. The atom-based field measurement paves the way for determining fields through calibration-free, invariable atomic properties and miniaturization. We also propose its various potential applications in the future.
We present a Rydberg atom-based microwave electric field sensor that achieves extended dynamic range and enhanced sensitivity across a broad bandwidth. By characterizing the Autler-Townes (AT) splitting induced by a single-tone microwave field, we demonstrate a spectroscopic method that simultaneously extracts both the microwave frequency and electric field strength directly from the splitting pattern. We implement dual-tone heterodyne detection, achieving a minimum detectable field strength on the order of uV/cm and a sensitivity in the sub-uV/cm/Hz^1/2 regime, while extending the operational bandwidth up to 3 GHz. Through systematic characterization of frequency and power dependencies, we identify optimal operating conditions to minimize power broadening in the resonant AT regime and maximize sensitivity in the far-off-resonance AC Stark regime. The resulting platform combines high sensitivity, broad bandwidth, and a dynamic range of approximately 90 dB, establishing Rydberg atoms as practical sensors for precision electric field metrology.
Rydberg atom-based radio frequency electromagnetic field sensors are drawing wide-spread interest because of their unique properties, such as small size, dielectric construction, and self-calibration. These photonic sensors use lasers to prepare atoms and read out the atomic response to a radio frequency electromagnetic field based on electromagnetically induced transparency, or related phenomena. Much of the theoretical work has focused on the Autler-Townes splitting induced by the radio frequency wave. The amplitude regime, where the change in transmission observed on resonance is measured to determine electric field strength, has received less attention. In this paper, we deliver analytic expressions that are useful for calculating the absorption coefficient in the amplitude regime. Our main goal is to describe the analytic expressions for the absorption coefficient and demonstrate their validity over a large range of the interesting parameter space. The effect of the thermal motion of the atoms is explicitly addressed. The analytic formulas for the absorption coefficient for different types of Doppler broadening are compared to estimate the sensitivity under conditions where it is limited by the laser shot noise. Residual Doppler shifts are shown to limit sensitivity. The expressions, approximations and descriptions presented in the paper are important for understanding the absorption of Rydberg atom-based sensors in the amplitude regime. This provides insight into the physics of multi-level interference phenomena.
Atom-based radio-frequency (RF) electric field probes have the potential to improve electric field measurements for a broad range of frequencies (from a few GHz to 100s of GHz) and field strengths (mV/m to kV/m). For these probes to become a common measurement method, their range must be extended to high frequency (>100 GHz) and low field strength regimes. We present SI-traceable electric field measurements of RF fields above 100 GHz, using Autler-Townes splitting of Rydberg electromagnetically-induced transparency in a rubidium (Rb) vapor. We also demonstrate several techniques, including RF detuning from resonance and enhanced absorption, for increasing the probe sensitivity.