Coherent Optical Detection of Highly Excited Rydberg States Using Electromagnetically Induced Transparency

We demonstrate coherent optical detection of highly excited Rydberg states (up to $n=124$) using electromagnetically induced transparency (EIT), providing a direct nondestructive probe of Rydberg energy levels. We show that the EIT spectra allow direct optical detection of electric field transients in the gas phase, and we extend measurements of the fine structure splitting of the $nd$ series up to $n=96$. Coherent coupling of Rydberg states via EIT could also be used for cross-phase modulation and photon entanglement.

Figure 1

(a) Energy level diagram of the ${}^{85}\mathrm{Rb}$ ladder system. A probe beam at 780 nm measures the absorption on the $5s{}^{2}S_{1/2}\to 5p{}^{2}P_{3/2}$ transition, while an intense coupling beam at 480 nm dresses the $5p{}^{2}P_{3/2}\to nd{}^{2}D_{3/2,5/2}$ transition with $n=26–124$. The fine and hyperfine splitting of the $nd$ and $5p{}^{2}P_{3/2}$ states, respectively, give rise to six two-photon resonance lines. (b) A schematic of the experimental setup. The 480 and 780 nm beams counterpropagate through a room temperature Rb vapor cell. The transmission of the 780 nm light is detected on a photodiode. The vapor cell is placed between two electrodes inside a magnetic shield.

Figure 2

(a) Probe transmission as a function of probe laser detuning near the ${}^{85}\mathrm{Rb}$ $5s{}^{2}S_{1/2}(F=3)\to 5p{}^{2}P_{3/2}$ resonance with the coupling laser detuned nearly on resonance (red line) and far above resonance (blue line) with the $5p{}^{2}P_{3/2}(F=4)\to 44d{}^{2}D_{5/2}$ transition. (b) The change in the transmission $\Delta T$ is due to the coupling beam given by the difference between the two curves in (a). Six EIT resonances are observed.

Figure 3

Probe transmission as a function of detuning (thick red line) compared to Eq. (1) (thin black line) for (a) the $44d$ state with ${\Omega }_c=2\pi (3.5\mathrm{MHz})$ and ${\gamma }_3=2\pi (0.3\mathrm{MHz})$ and (b) $80d$ with ${\Omega }_c=2\pi (1.5\mathrm{MHz})$ and ${\gamma }_3=2\pi (0.3\mathrm{MHz})$.

Figure 4

The measured fine structure splitting (solid circles) of the $d$ state as a function of the principal quantum number $n$. The line is calculated using the quantum defects from Ref. 25.

Figure 5

The width (FWHM) of the EIT transient (rising edge) as the electric field is switched between $\pm 10\mathrm{V}/\mathrm{cm}$ as a function of the coupling laser power. The transient time decays exponentially to a fixed value at high power. Inset: The transient EIT response for a coupling laser power of 50 (dashed line) and 170 mW. Note that the EIT peak height is smaller at lower coupling laser power.

Figure 6

EIT spectra observed by scanning the coupling laser across the $5p{}^{2}P_{3/2}(F=4)\to 44d{}^{2}D$ resonance with the probe laser locked to the $5s{}^{2}S_{1/2}(F=3)\to 5p{}^{2}P_{3/2}(F=4)$ transition. A higher probe power is used, so the lines are power broadened and the enhanced absorption effect is lost. An rf field with frequency 90 MHz and amplitude (a) 0.0, (b) 0.18, (c) 0.32, and (d) $0.48\mathrm{V}/\mathrm{cm}$ is applied. The ${}^{3}D_{3/2}$ and ${}^{2}D_{5/2}$ lines, at $-139$ and 0 MHz in (a), are split into 2 and 3 $|m_J|$ components, labeled as $J$, $|m_J|$ in (d).