Laser frequency stabilization in the 10 -14 range via optimized modulation transfer spectroscopy on the 87Rb D2 line.

We present a high-performance laser frequency stabilization method using modulation transfer spectroscopy (MTS) on the rubidium 87D2 transition line. A substantial improvement of the laser frequency stability was achieved by searching for the optimal diameter and intensity settings of the probe and pump beam. The frequency instability measured from the beat frequency of two locked external cavity diode lasers (ECDLs) reached a short-term stability of 4.5×10-14/τ and did not exceed 2 × 10-12 until 105 s, which is the best performance reported thus far with a D2 transition. The long-term stability is limited by the offset fluctuations of the baseline induced by the residual amplitude modulation (RAM), which can be further improved by reducing the current temperature variation of about 0.2 K by means of temperature stabilization or through a further reduction of the RAM.

Photon-pair generation from a chip-scale Cs atomic vapor cell.

The realization of a narrowband photonic quantum source based on an atomic device is considered essential in the practical development of photonic quantum information science and technology. In this study, we present the first step toward the development of a photon-pair source based on a microfabricated Cs atomic vapor cell. Time-correlated photon pairs from the millimeter-scale Cs vapor cell are emitted via the spontaneous four-wave mixing process of the cascade-type 6S1/2-6P3/2-8S1/2 transition of 133Cs. The maximum normalized cross-correlation value between the signal and idler photons is measured as 622(8) under a weak pump power of 10 µ;W. Our photon source violates the Cauchy-Schwartz inequality by a factor of >105. We believe that our approach has very important applications in the context of realizing practical scalable quantum networks based on atom-photon interactions.

Moving-frame imaging of transiting cold atoms for precise long-range transport.

Transporting cold atoms between interconnected vacuum chambers is an important technique for increasing the versatility of cold atom setups, particularly for those that couple atoms to photonic devices. In this report, we introduce a method where we are able to image the atoms at all points during transport via moving optical dipole trap. Cooled 87Rb atoms are transported ∼50 cm into an auxiliary vacuum chamber while being monitored with a moving-frame imaging system for which in-situ characterization of the atom transport is demonstrated. Precise positioning of the atoms near photonic devices is also tested across several tapered fibers showing an axial positioning resolution of ∼450 µm.

Maximized atom number for a grating magneto-optical trap via machine-learning assisted parameter optimization.

We present a parameter set for obtaining the maximum number of atoms in a grating magneto-optical trap (gMOT) by employing a machine learning algorithm. In the multi-dimensional parameter space, which imposes a challenge for global optimization, the atom number is efficiently modeled via Bayesian optimization with the evaluation of the trap performance given by a Monte-Carlo simulation. Modeling gMOTs for six representative atomic species - 7Li, 23Na, 87Rb, 88Sr, 133Cs, 174Yb - allows us to discover that the optimal grating reflectivity is consistently higher than a simple estimation based on balanced optical molasses. Our algorithm also yields the optimal diffraction angle which is independent of the beam waist. The validity of the optimal parameter set for the case of 87Rb is experimentally verified using a set of grating chips with different reflectivities and diffraction angles.

Chip-Scale Ultra-Low Field Atomic Magnetometer Based on Coherent Population Trapping.

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/Hz1/2 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.

Optical phase-locking of two extended-cavity diode lasers by serrodyne modulation.

We report the optical phase-locking of two extended-cavity diode lasers with a frequency difference of 6.9 GHz by serrodyne modulation. The bandwidth of the phase-locking loop is extended up to 9.5 MHz. The residual phase noise of the two phase-locked lasers reaches -130 dBrad2/Hz in the offset frequency range of 1.5 kHz to 9 kHz and below -120 dBrad2/Hz in the range of 150 Hz to 350 kHz, respectively. It is expected that the sensitivity limit of atom interferometers will be enhanced when the phase-locked lasers are used.

Mie resonance-enhanced pumping and detection efficiency for shallow nitrogen-vacancy centers in diamond.

We investigate Mie resonances of a diamond nano-resonator as a means to enhance the pumping and detection efficiency of shallow nitrogen-vacancy color centers. We show it is possible to tune a couple of high-order modes of a single resonator to each absorption and emission spectrum of the color center, and thereby the resonator plays a dual role of pump field concentration and emission field guiding. Furthermore superposition of the resonator field and the uncoupled near field results in even stronger pump intensity in the shallow top layer of the resonator. We also examine possible coupling between adjacent resonators when they form a periodic array. This approach allows us to achieve lower excitation power and higher signal intensity at local sites defined by resonators providing a way to enhance wide-field metrology in the sampled region of shallow color centers.

Generation of 578-nm yellow light over 10 mW by second harmonic generation of an 1156-nm external-cavity diode laser.

578-nm yellow light with an output power of more than 10 mW was obtained using a waveguide periodically-poled-lithium-niobate crystal as a nonlinear medium for second harmonic generation, which is the highest output power at this wavelength using second harmonic generation of a solid state laser source without an enhancement ring cavity, to our knowledge. To achieve this result we made a high power 1156-nm external-cavity diode laser with the maximum output power of more than 250 mW. This system is expected to be an excellent alternative to the system using the sum-frequency generation with the advantage of simplicity and cost-effectiveness, and will be used as a clock laser of the ytterbium optical lattice clock with robust and reliable operation.