Myoung-Sun Heo

Improved absolute frequency measurement of the 171Yb optical lattice clock at KRISS relative to the SI second

We measured the absolute frequency of the 1S0–3P0 transition of 171Yb atom confined in a one-dimensional optical lattice relative to the SI second. The determined frequency was 518 295 836 590 863.38(57) Hz. The uncertainty was reduced by a factor of 14 compared with our previously reported value in 2013 due to the significant improvements in decreasing the systematic uncertainties. This result is expected to contribute to the determination of a new recommended value for the secondary representations of the second.

Nonlinear, nonequilibrium and collective dynamics in a periodically modulated cold atom system

The physics of critical phenomena in a many-body system far from thermal equilibrium is an interesting and important issue to be addressed both experimentally and theoretically. The trapped cold atoms have been actively used as a clean and versatile simulator for classical and quantum-mechanical systems, deepening understanding of the many-body physics behind. Here we review the nonlinear and collective dynamics in a periodically modulated magneto-optically trapped cold atoms. By temporally modulating the intensity of the trapping lasers with the controlled phases, one can realize two kinds of nonlinear oscillators, the parametrically driven oscillator and the resonantly driven Duffing oscillator, which exhibit the dynamical bistable states. Cold atoms behave not only as the single-particle nonlinear oscillators, but also as the coupled oscillators by the light-induced inter-atomic interaction, which leads to the phase transitions far from equilibrium in a way similar to the phase transition in equilibrium. The parametrically driven cold atoms show the ideal mean-field symmetry-breaking transition, and the symmetry is broken with respect to time translation by the modulation period. Such a phase transition results from the cooperation and competition between the inter-particle interaction and the fluctuations, which lead to the nonlinear switching of atoms between the vibrational states, and the experimentally measured critical characteristics prove it as the ideal mean-field transition class. On the other hand, the resonantly driven cold atoms that possess the coexisting periodic attractors exhibit the kinetic phase transition analogous to the discontinuous gas-liquid phase transition in equilibrium, and interestingly the global interaction between atoms causes the shift of the phase-transition boundary.

Drift-compensated low-noise frequency synthesis based on a CryoCSO for the KRISS-F1(Cs)

This paper reports implementation and operation of a frequency synthesizer based on a cryocooled cryogenic Sapphire oscillator (cryoCSO) for the Cesium atomic fountain clock developing at KRISS, KRISS-Fl(Cs). With use of this highly stable local oscillator, the short-term stability of KRISS-F1(Cs) was greatly improved and reaches the quantum projection noise limit, resulting in the measured lowest Allan deviation of 2.6×10-14. In addition, the long-term drift (4.8×10-14/day) of the cryoCSO could be compensated and reach below 5×10-16/day as low as state-of-the-art active hydrogen masers.

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.

Operating atomic fountain clock using robust DBR laser system

We operate an atomic fountain clock KRISS-F1 using a laser system based on a DBR (Distributed Bragg Reflector) laser. We have found that there is no major difference in the fountain performance between laser systems with an ECDL (Extended-Cavity Diode Laser) and the DBR laser, even though spectral properties of the DBR is worse than that of ECDL. Moreover, quantum projection-noise limited stability is observed using DBR lasers. After replacing the ECDL to the DBR laser, laser system of the fountain clock became robust against acoustic and vibration noise mainly from mechanical shutters. Therefore, currently KRISS-F1 can be operated without failure of laser locking.

Increasement of effective number of detected atoms in KRISS-F1(Cs) fountain clock by optical pumping

We have performed optical pumping (OP) technique to increase the effective number of detected atoms in the KRISS-F1 (Cs) fountain clock using two laser frequencies. The maximum gain of detected signal is 3.5. We anticipate the short-term stability of KRISS-F1(Cs) will be improved with OP.

Continuous cold atomic beam in a Zerodur chamber for atom interferometry

We generated a continuous cold atomic beam using a two-dimensional magneto-optical trap for atom interferometry. The vacuum system is made of Zerodur glass with a large optical access for a compact system.

New clock laser system for the second Yb optical lattice clock at KRISS

We design and develop a new clock laser system for the second Yb optical lattice clock (OLC) at KRISS with the targeted uncertainty of 10−18 level. With a longer ULE cavity, the fused silica substrates, and the crystalline mirror coatings, the overall thermal limit is expected to be 4.3×10−17. The rapid evaluation of the systematic uncertainties of the second OLC will be possible with this new clock laser system.

Atom shutter using bender piezoactuator

A flag-type atom shutter based on a rotating lever that is driven by a bender piezoelectric actuator was developed to manipulate atomic beams. The shutter flag was displaced by ∼10 mm to open and close a 5-mm-diameter aperture with a shutter time of 13 ms that produced small mechanical vibrations. The short-term shutter time stability for each cycle was 0.03 ms and the long-term stability over an average of 20 000 cycles was 0.02 ms. The operational cycle number (lifetime) of the shutter reached 2.0 × 106 cycles after an intermittent operation over a period of eight months in an ultra-high vacuum chamber, and another shutter in an atmospheric environment swung for 2.6 × 107 cycles of continuous operation at 5 Hz for a period of 60 days without major problems. The shutter was shown to be compatible with the operation in an ultra-high vacuum at a low 10-7 Pa level after a gentle baking treatment.

Progress of an atomic gravimeter at KRISS

This paper introduces an atomic gravimeter under development at KRISS for precise measurement of gravitational acceleration. The gravimeter based on atom interferometer uses two counter-propagating Raman lasers of π/2-π-π/2 pulse sequence and laser cooled 87Rb atoms in free fall. Atomic interference fringes caused by Raman lasers were observed at various time intervals between the three Raman pulses. The frequency sweep rate α which compensates Doppler shift due to gravitational acceleration g was obtained from the interference fringes. In the presentation, the current status and recent results of the KRISS atomic gravimeter will be discussed.