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In this paper, we demonstrate a frequency modulation technique to accurately measure the probe Stark shift of optical clocks. The effective coupling strength of the probe laser to the clock transition is precisely controlled by this frequency modulation technique, which is different from the traditional method by controlling the effective coupling strength via changing the probe light power. In an 87Sr optical lattice clock, the probe Stark shifts are measured as a function of the square of the bare state coupling strength. The probe Stark shift coefficient of 87Sr is experimentally determined as −6.0(5)×10−7/Hz using this frequency modulation technique. With the help of frequency modulation, the probe light power can be as large as possible without decreasing the measurement precision and the achieved measurement lever arm is improved by more than one order of magnitude compared with previous results.

We report a transportable one-dimensional optical lattice clock based on 87Sr at the National Time Service Center. The transportable apparatus consists of a compact vacuum system and compact optical subsystems. The vacuum system with a size of 90 cm× 20 cm× 42 cm and the beam distributors are assembled on a double-layer optical breadboard. The modularized optical subsystems are integrated on independent optical breadboards. By using a 230 ms clock laser pulse, spin-polarized spectroscopy with a linewidth of 4.8 Hz is obtained which is close to the 3.9 Hz Fourier-limit linewidth. The time interleaved self-comparison frequency instability is determined to be 6.3 × 10–17 at an averaging time of 2000 s.

The frequency-drift-induced self-comparison measurement error was experimentally demonstrated by measuring the frequency difference between two interleaved clock loops with the same systemic parameters in the 87Sr optical lattice clock at the National Time Service Center of China. Combining the experimental and simulated results, this error was precisely determined by the total clock laser frequency drift during the time interval between two adjacent operations of interleaved clock loops.

The work presented in this paper is based on an optical frequency comb injection-locking, active filtering and amplification method to build a narrow linewidth laser for second cooling of the strontium atoms in an optical lattice clock. An optical femtosecond laser comb with mode spacing of 250 MHz as master laser is applied to directly inject into an external cavity semiconductor laser at 689 nm called slave laser, a single mode is selected and amplified from the optical femtosecond laser comb.

We show that single-layer graphene presents negative irradiance-dependent nonlinear refraction. We discriminate between a low-irradiance regime, where the conventional Kerr effect applies and a high-irradiance regime where the nonlinear refraction becomes irradiance-dependent.

We report experiments on the observation of the 1S0-3P0 transition spectrum of 88Sr in the Lamb-Dicke regime. After going through a two-stage magneto-optical trap (MOT), 88Sr cold atoms with number of about 1 \times 105 and a longitudinal temperature of 8.4 μK are loaded into a one-dimensional (1D) optical lattice, which is realized with a semiconductor laser at 813.4 nm. Using the magnetic field-inducing 3P1 state mixing into 3P0 state, the spectroscopy of the 1S0-3P0 transition with a linewidth of 180 Hz is detected.

We describe a dual-entropy-source quantum random number generator based on atomic spontaneous emission. Using the intensity fluctuations of a coherent beam to modulate the detuning frequency of the driving field, the atomic ensemble exhibits larger fluctuations of the spontaneous emission noise. The quantum-to-classical-noise ratio of the measured noise is increased to 10.14 dB. After post-processing, a string of random numbers at rates up to 5.12 Mb/s is extracted, which passes all the NIST tests. The presented scheme shows the feasibility to combine multiple independent randomness sources to produce secure quantum random bits.

We report the 87Sr optical lattice clock developed at the National Time Service Center. We achieved a closed-loop operation of the optical lattice clock based on 87Sr atoms. The linewidth of the spin-polarized clock peak is 3.9 Hz with a clock laser pulse length of 300 ms, which corresponds to a Fourier-limited linewidth of 3 Hz. The fitting of the in-loop error signal data shows that the instability is approximately , affected primarily by the white noise. The fractional frequency difference averages down to for an averaging time of 3000 s.

We report on frequency measurement of the intercombination (5s2)1S0–(5s5p)3P1 transition of the four natural isotopes of strontium, including 88Sr (82.58%), 87Sr (7.0%), 86Sr (9.86%), and 84Sr (0.56%). A narrow-linewidth laser that is locked to an ultra-low expansion (ULE) optical cavity with a finesse of 12000 is evaluated at a linewidth of 200 Hz with a fractional frequency drift of 2.8×10−13 at an integration time of 1 s. The fluorescence collector and detector are specially designed, based on a thermal atomic beam. Using a double-pass acousto-optic modulator (AOM) combined with a fiber and laser power stabilization configuration to detune the laser frequency enables high signal-to-noise ratios and precision saturated spectra to be obtained for the six transition lines, which allows us to determine the transition frequency precisely. The optical frequency is measured using an optical frequency synthesizer referenced to an H maser. Both the statistical values and the final values, including the corrections and uncertainties, are derived for a comparison with the values given in other works.