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Optics Express

Trap loss spectroscopy of ultra-cold Rydberg atoms involved in microwave fields

Yifei Cao, Hao Zhang, Linjie Zhang, Liantuan Xiao, Suotang Jia

Published Feb 6, 2025• 4 citations
Publisher

Abstract

Trap loss spectroscopy, based on fluorescence detection, provides a real-time measurement technique that is advantageous for ultra-cold atomic systems, to monitor the interaction between microwave fields and Rydberg atoms. By employing the polarization combinations of excitation lasers and microwave fields, we achieve trap loss spectroscopy of multi-path transitions in magnetic Rydberg hyperfine states and observe Zeeman broadening. The contributions of laser power, Zeeman effect, and collision effects to the broadening of the trap loss spectrum were comprehensively analyzed. Additionally, we have observed the broadening and splitting of the trap loss spectrum in response to variations in microwave field power and frequency. The transition from broadening to splitting of the spectral profile is observed with an increase in microwave field power. The full width at half maximum (FWHM) of trap loss spectroscopy exhibited a proportional relationship with the square root of microwave power, while the peak separation demonstrated a direct proportionality with higher microwave power. Our detailed exploration of trap loss spectroscopy under microwave fields may provide what we believe to be a novel approach for microwave field sensing.

DOI
10.1364/oe.551138

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Trap loss spectroscopy, based on fluorescence detection, provides a real-time measurement technique that is advantageous for ultra-cold atomic systems, to monitor the interaction between microwave fields and Rydberg atoms. By employing the polarization combinations of excitation lasers and microwave fields, we achieve trap loss spectroscopy of multi-path transitions in magnetic Rydberg hyperfine states and observe Zeeman broadening. The contributions of laser power, Zeeman effect, and collision effects to the broadening of the trap loss spectrum were comprehensively analyzed. Additionally, we have observed the broadening and splitting of the trap loss spectrum in response to variations in microwave field power and frequency. The transition from broadening to splitting of the spectral profile is observed with an increase in microwave field power. The full width at half maximum (FWHM) of trap loss spectroscopy exhibited a proportional relationship with the square root of microwave power, while the peak separation demonstrated a direct proportionality with higher microwave power. Our detailed exploration of trap loss spectroscopy under microwave fields may provide what we believe to be a novel approach for microwave field sensing.
We report on high-resolution microwave spectroscopy of cesium Rydberg $(n+2){D}_{5/2}\ensuremath{\rightarrow}n{F}_{J}$ transitions in a cold atomic gas. Atoms laser-cooled and trapped in a magnetic-optical trap are prepared in the DDD Rydberg state using a two-step laser excitation scheme. A microwave field transmitted into the chamber with a microwave horn drives the Rydberg transitions, which are probed via state selective field ionization. Varying duration and power of the microwave pulse, we observe Fourier sideband spectra as well as damped, on-resonant Rabi oscillations with pulse areas up to $\ensuremath{\gtrsim}3\ensuremath{\pi}$. Furthermore, we investigate the Zeeman effect of the clearly resolved nFJn{F}_{J}nFJ​ fine-structure levels in fields up to 120 mG, where the transition into nF7/2n{F}_{7/2}nF7/2​ displays a thee-peak Zeeman pattern, while nF5/2n{F}_{5/2}nF5/2​ shows a two-peak pattern. Our theoretical models explain all observed spectral characteristics, showing good agreement with the experiment. Our measurements provide a pathway for the study of high-angular-momentum Rydberg states, initialization and coherent manipulation of such states, Rydberg-atom macrodimers, and other Rydberg-atom interactions. Furthermore, the presented methods are suitable for calibration of microwave radiation as well as for nulling and calibration of dc magnetic fields in experimental chambers for cold atoms.
We demonstrate a new approach for the metrology of microwave fields based on the trap-loss spectroscopy of cold Rydberg atoms in a magneto-optical trap. Compared to state-of-the-art sensors using room-temperature vapors, cold atoms allow longer interaction times, better isolation from the environment, and a reduced Doppler effect. Our approach is particularly simple, as the detection relies on fluorescence measurements only. Moreover, our signal is well described by a two-level model across a broad measurement range, allowing one, in principle, to reconstruct the amplitude and the frequency of the microwave field simultaneously without the need for an external reference field. We report on a scale-factor linearity at the percent level and no noticeable drifts over two hours, paving the way for new applications of cold Rydberg atoms in metrology, such as calibrating black-body shifts in state-of-the-art optical clocks, monitoring the Earth's cryosphere from space, measuring the cosmic microwave background, or searching for dark matter.
We present a method for the simultaneous measurement of the amplitude and the ellipticity of microwave fields using trap-loss spectroscopy based on Rydberg excitation of a cold ensemble of 87Rb atoms. The measurement consists of probing the energy spectrum of the 61S1/261S_{1/2}61S1/2​ to 61P1/261P_{1/2}61P1/2​ transition in the presence of microwave, which depends on the electric field amplitude ∣E∣|E|∣E∣ and the third Stokes parameter ∣S3∣|S_3|∣S3​∣. We use a millimeter-size magneto-optical trap to perform strongly-localized measurements, which mitigates the effect of microwave field inhomogeneities. This allows us to work with large Rabi frequencies and thus resolve small deviations from linear or circular states of polarization. Taking into account the effect of non-resonant couplings to neighboring Rydberg states, we measure ∣E∣|E|∣E∣ and ∣S3∣|S_3|∣S3​∣ with an accuracy on the order of 1 percent in the Autler-Townes regime where the measurements can be traced to the international system of units. By studying the visibility of the spectral features as a function of the polarization of the excitation laser beams, we extract some information on the angle of arrival of the microwave field onto the atoms, with good agreement between theory and experiment.
While most experimental developments have focused on room-temperature vapors so far, using cold atoms has the advantage of strongly reducing the frequency broadening associated with Doppler effect, while increasing the interaction time between the atoms and the probing beams, offering prospects to make the most of the intrinsic narrow linewidth of Rydberg atoms and perform long-term, stable measurements. Scientists researchers at ONERA have previously used cold atoms to perform absolute measurements of the gravity field onboard moving carriers, showing that this technology is now compatible with practical applications out of the lab. We have recently developed at ONERA a novel approach to sense electromagnetic fields with cold
We demonstrate the single-photon Rydberg excitation of cesium atoms in a magneto-optical trap (MOT). We excite atoms directly from 6S1/2ground state to nP3/2(n = 70-100) Rydberg state with a narrow-linewidth 319-nm ultra-violet laser. The detection of Rydberg states is performed by monitoring the reduction of fluorescence signal of the MOT as partial population on 6S1/2(F = 4) ground state are transferred to Rydberg state. We clearly observe Autler-Townes doublet in the trap-loss spectra due to the cooling lights. Utilizing the large electric polarizibility of Rydberg atoms, we observe Stark splitting in the Autler-Townes doublet induced by background DC electric fields. We investigate the dependence of Stark shift on electric fields by theoretical analysis, and then infer the DC electric field from the measured Stark splitting. We find that there is a 44.8(4) mV/cm DC electric field in the vicinity of the cold ensemble. It indicates that high-lying Rydberg atoms can be used as sensors for DC electric fields.
Electric field measurement techniques based on Rydberg atoms have proven to have unique advantages in high sensitivity and have drawn wide attention in microwave electric field metrology and communication. The Rydberg states employed for microwave sensing are usually excited by two- or three-step laser irradiation. It leads to the Rydberg atoms being populated by some specific magnetic quantum number states determined by the laser polarization configurations, whose further coupling to another Rydberg state is also constrained by the microwave polarization. Based on a three-laser optical excitation, we demonstrate this effect on the microwave spectra of the 85Rb transitions 41F7/2 → 42D5/2 and 41F7/2 → 41G7/2 at various combining polarization configurations. It shows a strong polarization dependence on the microwave spectrum and then affects the sensitivity of Rydberg atom-based microwave field sensing. Advisable polarization schemes for higher sensitivity are suggested by the two types of transitions.