Velocity-selective electromagnetically-induced-transparency measurements of potassium Rydberg states

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Velocity-selective electromagnetically-induced-transparency measurements of potassium Rydberg states

Wenchao Xu and Brian DeMarco

Phys. Rev. A 93, 011801(R) – Published 4 January 2016

Abstract

We demonstrate a velocity selection scheme that mitigates suppression of electromagnetically induced transparency (EIT) by Doppler shifts for coupling wavelengths larger than the probe wavelength. An optical pumping beam counterpropagating with the EIT probe beam transfers atoms between hyperfine states in a velocity-selective fashion. Measurement of the transmitted probe beam synchronous with chopping of the optical pumping beam enables a Doppler-free EIT signal to be detected. Transition frequencies between $5 P_{1 / 2}$ and $n S_{1 / 2}$ states for $n = 26$ , 27, and 28 in ${}^{39}K$ are obtained via EIT spectroscopy in a heated vapor cell with a probe beam stabilized to the $4 S_{1 / 2} \to 5 P_{1 / 2}$ transition. Using previous high-resolution measurements of the $4 S_{1 / 2} \to n S_{1 / 2}$ transitions, we make a determination of the absolute frequency of the $4 S_{1 / 2} \to 5 P_{1 / 2}$ transition. Our measurement is shifted by 560 MHz from the currently accepted value with a twofold improvement in uncertainty. These measurements will enable novel experiments with Rydberg-dressed ultracold Fermi gases composed of ${}^{40}K$ atoms.

Received 9 October 2015

DOI:https://doi.org/10.1103/PhysRevA.93.011801

Physics Subject Headings (PhySH)

Electromagnetically induced transparency

Atoms

Atomic, Molecular & Optical

Authors & Affiliations

Wenchao Xu and Brian DeMarco^*

Department of Physics, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA

^*bdemarco@uiuc.edu

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Issue

Vol. 93, Iss. 1 — January 2016

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Images

Figure 1
(a) Energy-level diagram of ${}^{39}K$ and corresponding transition wavelengths. The three-level ladder system used for EIT includes ground state $4 S_{1 / 2}$ , intermediate state $5 P_{1 / 2}$ , and Rydberg state $n S_{1 / 2}$ . The optical pumping beam transfers atoms between the $F = 1$ and $F = 2$ ground states in a velocity sensitive fashion, as shown in the schematic Gaussian velocity distributions $P (v)$ . The population imbalances at $v = 0$ and $v = c f_{h p} / f_{p}$ are modulated at the chopping frequency of the optical pumping beam. (b) Frequencies of the pump beam ${\tilde{f}}_{o p}$ , probe beam ${\tilde{f}}_{p}$ , and coupling beam ${\tilde{f}}_{c}$ in the atomic rest frame, where $β = v / c$ and $v$ is atomic velocity. The inset shows the frequencies in the atomic rest frame for velocity $v = c f_{h p} / f_{p}$ , in which case the frequency of EIT pump and probe beam are exchanged.
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Figure 2
Schematic of the EIT measurement apparatus. The coupling beam counterpropagates with the probe beam and copropagates with the optical pumping beam through a vapor cell. An acousto-optic modulator (AOM) shifts the frequency of optical pumping beam such that the frequency difference with the probe beam matches the ground-state hyperfine splitting. A 25 kHz square-wave generator and radio-frequency (rf) switch is used to modulate the signal generator that drives the AOM and chops the optical pumping beam. An optical chopping wheel modulates the coupling beam at 1.27 kHz. The transmitted power of the probe beam is measured using a photodetector (PD). The PD signal is doubly demodulated, first using a mixer followed by a low-pass (LP) filter, and then using a lock-in amplifier.
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Figure 3
EIT spectroscopy of the $28 S$ state. The signal from the lock-in amplifier, which is proportional to changes in the probe transmission, is shown as the frequency of the coupling laser is scanned. The maximum signal corresponds to approximately a 0.2% change in the transmission of the probe beam synchronous with the optical pumping and coupling beam. The abscissa is the frequency difference of the coupling laser relative to the $5 P_{1 / 2} \to n S$ transition predicted by previous measurements of the $4 S \to 5 P_{1 / 2}$ and $4 S \to n S$ transitions [20, 21]. Each point is an average of five measurements, and the error bars are the standard error of the mean. The dotted line is a fit to the sum of four Gaussian functions. The approximately 20 MHz FWHM of the peaks is consistent with broadening expected from the spread in velocities selected by the optical pumping beam and noise from the repeatability of the wavemeter.
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Figure 4
The measured frequency deviation for the $5 P_{1 / 2} \to n S_{1 / 2}$ transition with principal quantum numbers $n = 26$ , 27, and 28. The solid black (hollow red) points are for the transition to the $5 P, F^{'} = 1 (5 P, F^{'} = 2)$ state. The black solid and red dashed lines are the weighted average of each set of three points for the transitions to the $5 P, F^{'} = 1$ and $5 P, F^{'} = 2$ states, respectively. The error bars show the uncertainty from the fits to data such as those in Fig. 3. The 60 MHz uncertainty from the accuracy of the wavemeter is not included in the error bars.
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