We have observed a Bose-Einstein condensate in a dilute gas of ${}^4\mathrm{He}$ in the ${}_{}{}^3{}_{}{}^{}2S_1^{}{}_{}{}^{}$ metastable state. We find a critical temperature of $(4.7\pm 0.5)\mu \mathrm{K}$ and a typical number of atoms at the threshold of $8\times {10}^6$. The maximum number of atoms in our condensate is about $5\times {10}^5$. An approximate value for the scattering length $a=(16\pm 8)\mathrm{nm}$ is measured. The mean elastic collision rate at threshold is then estimated to be about $2\times {10}^4{\mathrm{s}}^{-1\mathrm{}}$, indicating that we are deeply in the hydrodynamic regime. The typical decay time of the condensate is $2\mathrm{s}$, which places an upper bound on the rate constants for two-body and three-body inelastic collisions.

With the help of Stokes operator Ŝ ₂ squeezed state (also called polarization squeezed state (PSS)) of 795 nm light, rubidium-87 ( ⁸⁷ Rb) atomic magnetometer based on Faraday rotation (FR) has been implemented and characterized. The PSS of Stokes operator Ŝ ₂ of 795 nm light has been prepared by means of coherently combining the polarization coherent state (PCS) of a linearly p-polarized bright 795 nm light beam and a linearly s-polarized squeezed vacuum state (SVS) generated by a 397.5 nm ultraviolet laser pumped sub-threshold optical parametric oscillator (OPO) with a PPKTP bulk crystal inside the OPO cavity. PSS with a squeezing level of −3.7 ± 0.3 dB has been achieved around the analysis frequency of 10 kHz. At different transitions of D ₁ line, various frequency detuning, and reasonable atomic vapor cell’s temperature, FR has been measured and compared. To decrease absorption (scattering) losses and the back-action from atomic spin noise to the probe beam’s polarization noise for maintaining the quantum properties of PSS of Stokes operator Ŝ ₂ of 795 nm light, we had to run our magnetometer with ⁸⁷ Rb vapor cell’s temperature below 60 °C, at which the PSS was almost destroyed. The sensitivities of magnetic field measurement were characterized via measuring signal-to-noise ratio of the alternating current calibrated magnetic field signal with a balanced polarimeter. Under the conditions of the atomic number density of ∼5.8 × 10 ¹⁰ cm ⁻³ ( T ∼ 40 °C) and the probe beam with a detuning of ∼−400 MHz relative to the 5S _1/2 ( F _g = 2) − 5P _1/2 ( F _e = 1) transition of ⁸⁷ Rb D ₁ line, a typical sensitivity of ∼19.5 pT (Hz ^1/2 ) ⁻¹ has been achieved employing PSS of Stokes operator Ŝ ₂ as the probe, compared with a sensitivity of ∼28.3pT (Hz ^1/2 ) ⁻¹ using PCS as the probe. We preliminarily demonstrated that the quantum-enhanced sensitivity in a Faraday-rotation-based ⁸⁷ Rb atomic magnetometer with the help of PSS of 795 nm light.

With the help of light-emitting diodes at the center wavelength of 470 nm, we demonstrate that light-induced atom desorption (LIAD) can be used for flexibly controlling the loading of magneto-optical traps (MOT) of cesium atoms. Under an ultralow background pressure in a quartz cell without any wall coating, we show that low intensity blue light can be used to control the loading rates (from 200 to 4000 atoms/s) and the number of cesium atoms (on the order of ${10}^4$ in our experiment) in a MOT without the use of dispensers or any secondary atom source. A theoretical model based on an atom loading rate equation is built which can simulate the magneto-optical trap loading process with LIAD. The theoretical results are in agreement with experimental data. Some important parameters of the vacuum system are determined accordingly. The decay time of the vacuum pressure is about 70 ms, which is much shorter than the usual vacuum system for experiments on atomic ensembles. It is very difficult to measure such short times by the traditional fluorescence detection of cold atoms due to the slow loading process. The change of desorption rate (on the order of ${10}^{13}\text{atoms}/\text{s}$) for the different desorption intensities is also determined based on the background pressure caused by noncesium atoms in the cell. According to the experimental data and theoretical calculations, we obtain a partial pressure of about $1.4\times {10}^{-10}\text{Torr}$ for the untrapped cesium atoms and about $6.4\times {10}^{-10}\text{Torr}$ for the noncesium atoms at the moment the desorption light is turned off. The system is almost an ideal vacuum system for transferring atoms into another region for further experiments.

Based on the cesium $6S_{1/2}$-$6P_{3/2}$-$8S_{1/2}$ ladder-type atomic system, double-resonance optical pumping (DROP) spectra including electromagnetically induced transparency (EIT) effects have been investigated with a room-temperature cesium vapor cell. For both cases of the probe and the coupling laser beams passing through the cesium vapor cell with the counter-propagation (CTP) and co-propagation (CP) configurations, the DROP spectra measured in the experiment display explicitly different linewidths. Thanks to the EIT effect, the linewidth of the DROP spectrum is explicitly narrower for the CTP configuration than for the CP configuration. Experimental results agree with the theoretical analysis considering Doppler averaging. Furthermore, when the coupling laser has moderate power, the DROP spectrum for the CTP configuration clearly shows two components: the narrow part due to the EIT effect and the broad part caused by optical pumping (but these two different components are never seen in the CP configuration). Also, the effect of the intensity of the coupling and probe lasers on the DROP spectra is investigated.

We report on efficient generation of second harmonic laser and single-mode vacuum squeezed light of 795 nm with periodically poled KTiOPO₄ (PPKTP) crystals. We achieved 111 mW of ultra-violet (UV) light at 397.5 nm from 191 mW of fundamental light with a PPKTP crystal in a doubling cavity, corresponding to a conversion efficiency of 58.1%. Using the UV light to pump an optical parametric oscillator with a PPKTP crystal, we realized −5.6 dB of a maximum squeezing. We analyzed the pump power dependence of the squeezing level and concluded that the UV light induced losses limit the improvement of the squeezing level. The generated squeezed light has huge potential application in quantum memory and ultra-precise measurement.

We present an investigation of electromagnetically induced transparency (EIT) without Doppler background due to a locked probe laser. The EIT is theoretically studied based on a multilevel ladder-type cesium atomic system 6$S_{1/2}$–6$P_{3/2}$–8$S_{1/2}$ in a room-temperature vapor cell. The experimental results agree with the theoretical calculations. Compared with the traditional EIT spectra with a Doppler profile limiting the spectral resolution for keeping the coupling laser locked and scanning the probe laser, these EIT spectra with the probe laser locked and the coupling laser scanned have a flat background, which seem be of great benefit for applications such as the measurement of hyperfine intervals between excited states, the study of highly excited Rydberg states, laser-frequency stabilization, etc.