Enhanced spectral profile in the study of Doppler-broadened Rydberg ensembles

www.nature.com/scientificreports OPEN Received: 12 April 2017 Accepted: 1 August 2017 Published: xx xx xxxx Enhanced spectral profile in the study of Doppler-broadened Rydberg ensembles Bo-Han Wu1, Ya-Wen Chuang1, Yi-Hsin Chen1, Jr-Chiun Yu1, Ming-Shien Chang2 & Ite A. Yu1 Combination of the electromagnetically-induced-transparency (EIT) effect and Rydberg-state atoms has attracted great attention recently due to its potential application in the photon-photon interaction or qubit operation. In this work, we studied the Rydberg-EIT spectra with room-temperature 87Rb atoms. Spectroscopic data under various experimental parameters all showed that the contrast of EIT transparency as a function of the probe field intensity is initially enhanced, reaches a maximum value and then decays gradually. The contrast of spectral profile at the optimum probe field intensity is enhanced by 2–4 times as compared with that at weakest intensity. Moreover, the signal-to-noise ratio of the spectrum can potentially be improved by 1 to 2 orders of magnitude. We provided a theoretical model to explain this behavior and clarified its underlying mechanism. Our work overcomes the obstacle of weak signal in the Rydberg-EIT spectrum caused by an apparent relaxation rate of the Rydberg polariton and weak coupling transition strength, and provides the useful knowledge for the RydbergEIT study. Rydberg atom has become a popular research topic in recent decades, especially in the context of quantum information science, thanks to its physical properties. The weak dipole transition between ground and highly-excited Rydberg states prolongs the lifetime of Rydberg atoms1, 2. The large polarizability of Rydberg atoms gives rise to strong long-range interactions. It would couple the nearby atoms strongly through the immense dipole-dipole interaction. The strong interaction between Rydberg atoms leads to a blockade effect, implying a double excitation for a distance smaller than the blockade radius is strongly suppressed3–6. With the unique features of above, Rydberg atom is a good candidate for the demonstration of novel quantum devices, such as single-photon transistors7, 8 as well as quantum phase gate9–11, single-photon sources12–14, and quantum simulator15. Electromagnetically-induced-transparency (EIT) spectrum provides the direct nondissipative optical detections of Rydberg energy levels, atom-atom interaction, and wall-atom interaction in a thin cell16–20. An additional microwave field can break the symmetry of Rydberg-EIT interference, making it a good way to precisely determine the electric field of the microwave21. Besides the EIT spectra, the quantum information carried by photons can be dynamically encoded in Rydberg polaritons, allowing for storage, control, and retrieval of quantum states22, 23. To perform the above mentioned studies with Rydberg-state atoms, it is necessary to lock laser frequencies to a two-photon transition frequency. The EIT spectrum provides a convenient way to stabilize the laser frequencies based on a high contrast EIT peak24. The EIT peak height increased with the probe field intensity has been experimentally observed and theoretically analyzed in a Λ-type open transition EIT system25, 26. Here we report that there exists an optimum probe intensity, which makes EIT peak height reach its maximum value, in a Ξ-type cycling Rydberg EIT transition. We carried out our study in a vapor cell which is filled with the admixture of 87Rb and 85Rb atoms at the room temperature of about 300 K. In this paper, we present the investigation of Rydberg-state EIT-type spectra based on 87Rb atoms. The EIT peak height, i.e. the difference between the probe transmission at the EIT peak and that in the absence of the EIT effect, has been enhanced by 2–4 times at the optimum probe intensity as compared with that at weakest probe intensity. Remarkably, the optimum intensity is influenced very little by the light polarization, the principal quantum number of Rydberg state n, and nS or nD Rydberg states. We will provide a theoretical model for the observed behavior of the EIT peak height as a function of the probe intensity. In addition, when one 1 Department of Physics and Frontier Research Center on Fundamental and Applied Sciences of Matters, National Tsing Hua University, Hsinchu, 30013, Taiwan. 2Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, 10617, Taiwan. Correspondence and requests for materials should be addressed to Y.-H.C. (email: yhchen920@ gmail.com) or I.A.Y. (email: ) SCIeNTIFIC Reports | 7: 9726 | DOI:10.1038/s41598-017-09953-0 1 www.nature.com/scientificreports/ Figure 1. (a) Rydberg-EIT transition scheme. |g〉 is the ground state |5S1/2, F = 2〉; |e〉 is the intermediate state |5P3/2, F′ = 3〉; and |r〉 is the Rydberg state |nS〉 or |nD〉 according to the experimental measurement. (b,c) Rydberg-state EIT-type spectra. The coupling field frequency was swept across the transitions of |5P3/2, F′ = 3〉 to |38D3/2, F″ = 2, 3〉 and |38D5/2, F″ = 2, 3, 4〉 in (b), and across the transition of that to |57S1/2, F = 2〉 state in (c), while the probe field frequency was fixed. In (b), we provide the definition of the normalized EIT peak height (NEPH), which is the difference between the transmissions of EIT peak and baseline. The intensities of the probe and coupling fields were 0.029 and 18 W/cm2, respectively. Both light fields had the same polarization σ+ in (b); and the probe field was σ− polarized and coupling field was σ+ polarized in (c). The values of NEPH are 0.014, 0.10, and 0.022 from left to right peaks in (b,c). applies a stronger probe intensity or power in the measurement, the signal level of the probe field is immediately enhanced. Thus, as for dominant noise being not caused by fluctuation of the probe power or intensity (but being caused by, for examples, stray light, electronic noise, detector’s dark current, etc.), the signal-to-noise ratio (S/N) can be significantly improved. On the other hand, the spectral linewidth increases only by 2 folds. Therefore, the comprehensive feature of EIT effect leads to a better way for locking the upper transition frequency through a high contrast of the Rydberg-state EIT-type spectrum, making it useful for the Rydberg-relevant researches. Results and Discussion Experimental Observation. Rydberg EIT has a cascade-type (Ξ-type) level structure, which consists of a ground state, an intermediate excited state, and a Rydberg state, as shown in Fig. 1(a). The probe field couples the ground state |5S1/2, F = 2〉 ≡ |g〉 and the intermediate state |5P3/2, F′ = 3〉 ≡ |e〉, while the coupling field drives the transition of |e〉 to Rydberg state |r〉. Further details of our setup can be found in the section of Setup and Measurement in Methods. For each EIT spectrum measurement, we kept the probe field frequency resonant to |g〉-|e〉 transition, while the frequency of the coupling field was swept across the |e〉-|r〉 transition. The p (...truncated)