## 2 Setup and Measurement  
The experimental setup is shown in Figure 1. An external-cavity diode laser (ECDL) at 852 nm is used as a probe laser with a typical linewidth of \(\sim\)90 kHz. The optical power of a 1018-nm ECDL is amplified to \(\sim\)5 W by a fiber amplifier(FA), and then frequency-doubled by a single-passed periodically poled lithium niobate (PPLN) crystal to 509 nm. The two beams are overlapped in cesium (Cs) atomic vapor in a counter-propagating configuration. The spherical cesium cell has a radius of \(\sim\)1.2 cm to match the Rayleigh length of the focused beams (\(\sim\)40 \(\upmu\)m waist radius). We first performed the saturation absorption spectroscopy and determined the zero detuning of the 852-nm probe laser. The wavelength meter (HighFinesse WS-7, with wavelength deviation sensitivity of about 10 MHz) was calibrated using the Cs \(6S_{1/2}(F=4)\to 6P_{3/2}(F^{\prime}=5)\) hyperfine transition line. With the calibrated wavelength meter, we measured the wavelength of probe beam (when set to a certain detuning) and the coupling beam. The ladder-type Rydberg EIT signal is observed by scanning the frequency of 509-nm coupling laser while the 852-nm probe laser's frequency is fixed. The background-free EIT spectrum with high signal-to-noise ratio can be regarded as a low-noise detection system, which decreases the intensity noise and phase noise. Figure 2 shows the relevant energy level schematic and the transmitted intensity of a typical Rydberg EIT signal at 25 \({}^{\circ}\)C. The linewidth of the EIT signal is about 10 MHz, as measured by a RF modulation spectroscopy.  
Figure 1: Schematic diagram of the experimental setup.An ECDL at 852 nm is used as a probe laser, and an ECDL at 1018 nm is frequency-doubled and used as a 509-nm coupling laser. The frequency-doubling crystal is a PPLN. A SAS (saturation absorption spectroscopy) setup is used as a frequency reference for the hyperfine structure of the Cs D2 line, where OI is optical isolator; EOM, electro-optic modulator; \(\lambda/2\), half-wave plate; \(\lambda/4\), quarter-wave plate; PBS, polarization beam splitter cube; DM, dichroic mirror; FA, fiber amplifier; PD, photodiode (New Focus, model 2051); HR, high reflection mirror; SAS, saturation absorption spectroscopy.  
We study the relationship between the Rydberg EIT signal and the temperature as well as laser detuning in a ladder-type system. Figure 3a shows the dependence of the Rydberg EIT signal on probe detuning at a cell temperature of 40 \({}^{\circ}\)C, where each small peak in the figure represents an EIT signal. Zero detuning of the probe laser is identical with the Cs \(6S_{1/2}(F=4)\to 6P_{3/2}(F^{\prime}=5)\to 50S_{1/2}\) cascade resonance transition. We fix the 852-nm laser frequency at a chosen detuning and scan the 509-nm laser frequency in order to get EIT signal. When the detuning of the probe laser is \(\sim\)260 MHz, the EIT signal's strength is the largest, which is caused by the electric field generated by the collisions of Rydberg atoms causing the Rydberg level shift, a detailed explanation can be found in the discussion part. In Figure 3b, only the envelope curves are shown, for 40, 60, and 80 \({}^{\circ}\)C. Figure 3c shows that the 852-nm laser detuning for the maximum intensity of EIT signals changed with temperature (ground-state atomic density). It can be seen that probe laser detuning with the maximum EIT signal intensity increased nearly linear with increasing temperature. For example, at temperature of 90 \({}^{\circ}\)C, the probe laser detuning with the maximum EIT signal intensity is \(\sim\)510 MHz. Under high temperature conditions, the Rydberg EIT signal for the Cs \(6S_{1/2}(F=4)\to 6P_{3/2}(F^{\prime}=5)\to 50S_{1/2}\) cascade transition (zero detuning of the probe laser) is very weak. We observe that the linewidth of the EIT signal does not change significantly as the temperature increases, and is \(\sim\)10 MHz. There is no obvious linewidth broadening, which means that the electric field-induced shift is not a dissipative process.