We report a chip-scale atomic magnetometer based on coherent population trapping, which can operate near zero magnetic field. By exploiting the asymmetric population among magnetic sublevels in the hyperfine ground state of cesium, we observe that the resonance signal acquires sensitivity to magnetic field in spite of degeneracy. A dispersive signal for magnetic field discrimination is obtained near-zero-field as well as for finite fields (tens of micro-tesla) in a chip-scale device of 0.94 cm³ volume. This shows that it can be readily used in low magnetic field environments, which have been inaccessible so far in miniaturized atomic magnetometers based on coherent population trapping. The measured noise floor of 300 pT/Hz^1/2 at the zero-field condition is comparable to that of the conventional finite-field measurement obtained under the same conditions. This work suggests a way to implement integrated atomic magnetometers with a wide operating range.

We present an evaluation of the frequency shifts in an optically pumped caesium-beam frequency standard caused by the neighbouring transitions. We analyse the shifts caused by the Rabi and Ramsey pulling effects and find a well-known result for the Rabi pulling but a new result for the Ramsey pulling. The revised Ramsey shift is found to be proportional to not the imaginary but the real part of the ground-state atomic coherence. The value of the Ramsey shift is proportional to the first power of the Rabi frequency defined by a small orthogonal component of the microwave field and the inverse value of the Zeeman splitting. We compare the shifts caused by the neighbouring transitions and show that the relative contributions of the Rabi and Ramsey pulling effects crucially depend on the quality of optical pumping. We estimate that the Rabi shift for the KRISS-1 caesium-beam frequency standard is at a relative level of 10⁻¹⁴ while the Ramsey shift is at a relative level of 10⁻¹².

We introduce a compact atomic clock based on laser-cooled atoms trapped inside a loop-gap microwave cavity. The cavity occupies a volume eight times smaller than conventional cylindrical cavities while maintaining a high quality factor in the presence of the ten apertures required for optical manipulation of cold atoms. The measured linewidth of the Ramsey spectrum, which is limited by the free fall distance of the atomic cloud in the cavity, was 19.6 Hz. The corresponding frequency instability was σ_y(τ) = 2.5 × 10⁻¹² τ^−1/2, which could be further improved by reducing the local oscillator noise. This work paves a way toward high-performance portable atomic clocks.

We introduce a miniaturized magnetic shield integrated into the physics package of a chip-scale atomic clock. The primary shield, which is formed by a stack of precision-machined high-permeability plates, compactly sits inside the package as it is designed to be the internal supporting frame. Further improved by a Kovar lid, the best frequency sensitivity to a longitudinal external field is 1.1(3)×10⁻⁹/G, which is consistent with the measured shielding factor, while the transverse sensitivity is well below 10⁻¹⁰/G. The sealed unit on a chip carrier alone allows stable clock operation with a frequency instability of 4.8(2)×10⁻¹² at 1000 s.

To realize the chip-scale atomic clocks (CSACs) for atomic reference with miniaturization and low power consumption, there are technological challenges in the physics package for compact and stable frequency standards. Here, the precise thermal management and the exact magnetic-field control of the physics package are required for accurate clocks. Also, high thermal resistance for the physics package helps to considerably reduce power consumption of the CSAC. Thus, in this study, a compact physics package with embedded magnetic shielding has been developed by using flexible hybrid approach for a chip-scale atomic clock. The vertical-cavity surface-emitting laser (VCSEL) and the MEMS hotplate on a flexible circuit board (FPCB) are vertically stacked and bound with a QWP and a MEMS vapor cell by aluminum foil. The aluminum bound structure is suspended by the polyimide core of the FPCB for high thermal resistance. Also, the mu-metal fixture is applied for magnetic shield and FPCB suspension, while the MEMS hotplate with a resistance heater and an amorphous Si based thermistor is adopted for thermal management of the physics package. Then, the planar integrated FPCB is folded and stacked vertically at an intersection with the mu-metal fixture. As a result, the CSAC incorporating the proposed physics package reveals the CPT linewidth of 5.6 kHz and low power consumption of about 200mW. The measured frequency short-term stability is about 5.2×10-12 τ-1/2 at 1000-second integration. The observed results demonstrate promising performances of the flexible hybrid physics package for its use in miniaturized atomic clocks.

An ensemble time, which is indicated by TA (temps atomique, in French), was generated every minute by combining four active hydrogen masers with the reduced Kalman filter (Kred) algorithm. The TA frequency was averaged over 12 h and then the mean frequency was steered to the primary and secondary frequency standards developed by KRISS with a micro-phase stepper (MPS). The steered and physically realized TA frequency is called KRISS ensemble time (KET). We investigated the frequency-state estimate outputted from the Kred algorithm and the control signal applied to the MPS, according to the abnormal temperature change of the room where the hydrogen masers were placed. When the frequency of the TA was corrected by the KRISS-F1, the Allan deviation of KET was 1.72 × 10⁻¹⁵ at the average time τ = 1 d, and the variation of [UTC–KET] was stable within 2 ns for 75 d.