Accurate control of magnetic fields is crucial for cold-atom experiments, often necessitating custom-designed control systems due to limitations in commercially available power supplies. Here, we demonstrate precise and flexible control of a static magnetic field by employing a field-programmable gate array and a feedback loop. This setup enables us to maintain exceptionally stable current with a fractional stability of 1 ppm within 30 s. The error signal of the feedback loop exhibited a noise level of 10-5 A·Hz-1/2 for control bandwidths below 10 kHz. Utilizing this precise magnetic field control system, we investigate the second-order Zeeman shift in the context of cold-atom coherent population-trapping (CPT) clocks. Our analysis reveals the second-order Zeeman coefficient to be 574.21 Hz/G2, with an uncertainty of 1.36 Hz/G2. Consequently, the magnetic field stabilization system we developed allows us to achieve a second-order Zeeman shift below 10-14, surpassing the long-term stability of current cold-atom CPT clocks.

Studying Rydberg microwave frequency comb (MFC) spectroscopy helps increase the working bandwidth of the Rydberg receiver. This Letter demonstrates off-resonant Rydberg MFC spectroscopy in a meta-waveguide-coupled Rydberg atomic system. An off-resonant MFC field couples with the Rydberg atoms through a meta-waveguide. The system can receive the microwave field in the working band from 0.5 GHz to 13.5 GHz, and the MFC spectroscopy covers a span of 36 MHz at three different arbitrarily-chosen frequencies of 2 GHz, 3 GHz, and 5.8 GHz. The MFC spectrum that covers a wide range of 125 MHz is also verified. This work is significant for tunable wide-band instant microwave signal detection in the Rydberg atomic system, which is useful in microwave frequency metrology, communication, and radar.

Isotope shifts among different isotopes can be effectively addressed using narrow-linewidth lasers, facilitating laser isotope separation and achieving significant enrichment at a single stage. The separation of potassium isotopes, employing optical pumping and magnetic deflection, has proven to be efficient. To further improve the enrichment of 40K, we introduce 2D transverse cooling to minimize the divergence angle. Through this modification, we demonstrate enrichment of 40K, elevating it from 0.012% to 12%–20%. This represents an enrichment increase by three orders of magnitude, surpassing our previous result by one order. Our method is particularly well-suited for isotope enrichment of elements with extremely low abundance.

We developed a new single-layer atom chip with an additional U-shaped current-carrying structure. The new U-shaped microwire creates optimized magnetic field distribution, which increases the trapping volume of a magneto-optical trap (MOT) near the chip. Our approach allows one to localize more atoms, while a setup remains relatively simple (single-layer approach) and consumes low current (up to 10 A). The total number of trapped 87Rb atoms in our setup is 5 × 107.

We propose a scheme that utilizes weak-field-induced quantum beats to investigate the electronic coherences of atoms driven by a strong attosecond extreme ultraviolet (XUV) pulse. The technique involves using a strong XUV pump pulse to excite and ionize atoms and a time-delayed weak short pulse to probe the photoelectron signal. Our theoretical analysis demonstrates that the information regarding the bound states, initiated by the strong pump pulse, can be precisely reconstructed from the weak-field-induced quantum beat spectrum. To examine this scheme, we apply it to the attosecond XUV laser-induced ionization of hydrogen atoms by solving a three-dimensional time-dependent Schrödinger equation. This work provides an essential reference for reconstructing the ultrafast dynamics of bound states induced by strong XUV attosecond pulses.

To obtain cold atom samples with temperatures lower than 100 pK in the cold atom physics rack experiment of the Chinese Space Station, we propose to use the momentum filtering method for deep cooling of atoms. This paper introduces the experimental results of the momentum filtering method verified by our ground testing system. In the experiment, we designed a specific experimental sequence of standing-wave light pulses to control the temperature, atomic number, and size of the atomic cloud. The results show that the momentum filter can effectively and conveniently reduce the temperature of the atomic cloud and the energy of Bose–Einstein condensation, and can be flexibly combined with other cooling methods to enhance the cooling effect. This work provides a method for the atomic cooling scheme of the ultra-cold atomic system on the ground and on the space station, and shows a way of deep cooling atoms.

We experimentally demonstrate third-harmonic generation (THG) in gases ionized by a femtosecond laser pulse superimposed on its second-harmonic (SH). The mechanism of THG has been investigated, and it demonstrates that a third-order nonlinear process dominates at low pump intensity. Asymmetric third-harmonic (TH) spectra are observed at different time delays in two color fields, which are attributed to the process of the four-wave mixing (FWM) of the broad spectrum of pump pulses. A joint measurement on the terahertz (THz) and the TH is performed. It reveals that the optimized phase for the THG jumps from 0 to 0.5π as the pump intensity increases, which is different from the THz being a constant, and indicates that the THG arises from the nonlinearity of the third-order bound electrons to the tunnel-ionization current.

We report the measurement of the electromagnetically induced transparency (EIT) with Rydberg states in ultracold K40 Fermi gases, which is obtained through a two-photon process with the ladder scheme. Rydberg–EIT lines are obtained by measuring the atomic losses instead of the transmitted probe beam. Based on the laser frequency stabilization locking to the superstable cavity, we study the Rydberg–EIT line shapes for the 37s and 35d states. We experimentally demonstrate the significant change in the Rydberg–EIT spectrum by changing the principal quantum number of the Rydberg state (n=37/52 and l=0). Moreover, the transparency peak position shift is observed, which may be induced by the interaction of the Rydberg atoms. This work provides a platform to explore many interesting behaviors involving Rydberg states in ultracold Fermi gases.

We report two ultra-stable laser systems automatically frequency-stabilized to two high-finesse optical cavities. By employing analog-digital hybrid proportional integral derivative (PID) controllers, we keep the merits of wide servo bandwidth and servo accuracy by using analog circuits for the PID controller, and, at the same time, we realize automatic laser frequency locking by introducing digital logic into the PID controller. The lasers can be automatically frequency-stabilized to their reference cavities, and it can be relocked in 0.3 s when interruption happens, i.e., blocking and unblocking the laser light. These automatic frequency-stabilized lasers are measured to have a frequency instability of 6×10-16 at 1 s averaging time and a most probable linewidth of 0.3 Hz. The laser systems were tested for continuous operation over 11 days. Such ultra-stable laser systems in long-term robust operation will be beneficial to the applications of optical atomic clocks and precision measurement based on frequency-stabilized lasers.

We report the experimental realization of dark state atoms trapping in a nanofiber optical lattice. By applying the magic-wavelength trapping potentials of cesium atoms, the AC Stark shifts are strongly suppressed. The dark magneto-optical trap efficiently transfers the cold atoms from bright (6S1/2, F = 4) into dark state (6S1/2, F = 3) for hyperfine energy levels of cesium atoms. The observed transfer efficiency is as high as 98% via saturation measurement. The trapping lifetime of dark state atoms trapped by a nanofiber optical lattice is also investigated, which is the key element for realizing optical storage. This work contributes to the manipulation of atomic electric dipole spin waves and quantum information storage for fiber networks.

We present the frequency control of a 759 nm laser as a lattice laser for an ytterbium (Yb) optical clock. The frequency stability and accuracy are transferred from the Yb optical clock via an optical frequency comb. Although the comb is frequency-stabilized on a rubidium microwave clock, the frequency instability of the 759 nm laser is evaluated at the 10-15 level at 1 s averaging time. The frequency of the 759 nm laser is controlled with an uncertainty within 1 Hz by referencing to the Yb clock transition. Such a frequency-controlled 759 nm laser is suitable for Yb optical clocks as the lattice laser. The technique of laser frequency control can be applied to other lasers in optical clocks.

An ultranarrow bandwidth Faraday atomic filter is realized based on cold Rb87 atoms. The atomic filter operates at 780 nm on the 52S1/2, F=2 to 52P3/2, F′=3 transition with a bandwidth of 7.1(8) MHz, which is approaching the natural linewidth of the transition. The peak transmission achieves 2.6(3)% by the multi-pass probe method. This atomic filter based on cold atoms may find potential applications in self-stabilizing lasers in the future.

In this article, taking advantage of the special magnetic shieldings and the optimal coil design of a transportable Rb atomic fountain clock, the intensity distribution in space and the fluctuations with time of the quantization magnetic field in the Ramsey region were measured using the atomic magneton-sensitive transition method. In an approximately 310 mm long Ramsey region, a peak-to-peak magnetic field intensity of a 0.74 nT deviation in space and a 0.06 nT fluctuation with time were obtained. These results correspond to a second-order Zeeman frequency shift of approximately (2095.5±5.1)×10-17. This is an essential step in advancing the total frequency uncertainty of the fountain clock to the order of 10-17.

We take H++CO as a prototype to analyze the effect of ion or proton collision on molecular orientation modulated by a two-color shaped pulse combined with the time-delayed terahertz (THz) pulse. Through examining the effect of ion collision on the molecular orientation, we found that when the impact parameter and collisional velocity have weak inverse influences on the maximal orientation degree, the appropriate two-color and THz field intensity employed can improve the molecular orientation degree. The carrier envelope phase and frequency of the THz laser pulse as well as the temperature also have certain influence on the collision-induced molecular orientation.

The gravimeter based on atom interferometry has potential wide applications on building gravity networks and geophysics as well as gravity assisted navigation. Here, we demonstrate experimentally a portable atomic gravimeter operating in the noisy urban environment. Despite the influence of noisy external vibrations, our portable atomic gravimeter reaches a sensitivity as good as 65 μGal/Hz and a resolution of 1.1 μGal after 4000 s integration, being comparable to state-of-the-art atomic gravimeters. Our achievement paves the way for bringing the portable atomic gravimeter to field applications.

The nitrogen vacancy (NV) center in diamond has been well applied in quantum sensing of electromagnetic field and temperature, where the sensitivity can be enhanced by the number of NV centers. Here, we used electron beam irradiation to increase the generation rate of NV centers by nearly 22 times. We systematically studied the optical and electronic properties of the NV center as a function of an electron irradiation dose, where the detection sensitivity of magnetic fields was improved. With such samples with dense NV centers, a sub-pico-Tesla sensitivity in magnetic fields detection can be achieved with optimal controls and detections.

A cavity-stabilized 578 nm laser is used to probe the clock transition of ytterbium atoms trapped in optical lattice sites. We obtain a Fourier-limited 4.2-Hz-linewidth Rabi spectrum and a Ramsey spectrum with fringe linewidth of 3.3 Hz. Based on one of the spectra, the 578 nm laser light is frequency-stabilized to the center of the transition to achieve a closed-loop operation of an optical clock. Based on interleaved measurement, the frequency instability of a single optical clock is demonstrated to be 5.4 × 10-16/√τ.

We demonstrate two ultra-stable laser systems at 1064 nm by independently stabilizing two 10-cm-long Fabry–Pérot cavities. The reference cavities are on a cubic spacer, which is rigidly mounted for both low sensitivity to environmental vibration and ability for transportation. By comparing against an independent ultra-stable laser at 578 nm via an optical frequency comb, the 1064 nm lasers are measured to have frequency instabilities of 6 × 10?16 at 1 s averaging time.

Nonsequential double ionization (NSDI) of noble gas atoms in counter-rotating two-color circularly polarized (CRTC) laser fields is investigated. A scaling law is concluded by qualitatively and quantitatively comparing the momentum distributions of two electrons from NSDI in CRTC laser fields for different atoms with different parameters. The scaling law indicates that the momentum distributions from an atom driven by CRTC laser frequency ω1, ω2, and laser intensity I are the same as that from another atom irradiated by CRTC laser frequency kω1, kω2, and laser intensity k3I. This study can provide an avenue in the research of two-color laser field ionization.

Coulomb potential may induce a significant angular offset to the two-dimensional photoelectron momentum distributions for atoms subject to strong elliptically polarized laser fields. In the attoclock experiment, this offset usually cannot be easily disentangled from the contribution of tunneling delay and poses a main obstacle to the precise measurement of tunneling delay. Based on semiclassical calculations, here, we propose a method to extract the equivalent temporal offset induced solely by Coulomb potential (TOCP) in an attoclock experiment. Our calculations indicate that, at constant laser intensity, the TOCP shows distinctive wavelength dependence laws for different model atoms, and the ratio of the target atom’s TOCP to that of H becomes insensitive to wavelength and linearly proportional to (2Ip) 3/2, where Ip is the ionization potential of the target atom. This wavelength and Ip dependence of TOCP can be further applied to extract the Coulomb potential influence. Our work paves the way for an accurate measurement of the tunneling delay in the tunneling ionization of atoms subject to intense elliptically polarized laser fields.