Several studies have indicated that densely populated urban environments and the heavy dependence on traffic could increase the potential spread of COVID-19. This study investigated the association between changes in traffic volume and the spread of COVID-19 in South Korea. Methods: This study analyzed the daily national traffic and traffic trend for 3 months from January 1, 2020. Traffic data were measured using 6307 vehicle detection system (VDS). This study analyzed the difference in traffic levels between 2019 and 2020. Non-linear regression was performed to analyze the change in traffic trend in 2020. The relationship between traffic and confirmed COVID-19 cases was analyzed using single linear regression. Results: The mean daily nationwide level of traffic for the first 3 months of 2020 was 143 655 563 vehicles, which was 9.7% lower than the same period in 2019 (159 044 566 vehicles). All regions showed a decreasing trend in traffic in February, which shifted to an increasing trend from March. In Incheon there was a positive, but insignificant, linear relationship between increasing numbers of newly confirmed cases and increasing traffic (β = 43 146; p = 0.056). Conclusions: Numbers of newly confirmed COVID-19 patients have been decreasing since March, while the traffic has been increasing. The fact that traffic is increasing indicates greater contact between people, which in turn increases the risk of further COVID-19 spread. Therefore, the government will need to devise suitable policies, such as total social distancing.
We measured the absolute frequency of the optical clock transition 1 S 0 (F = 1/2) -3 P 0 (F = 1/2) of 171 Yb atoms confined in a one-dimensional optical lattice and it was determined to be 518 295 836 590 863.5(8.1) Hz. The frequency was measured against Terrestrial Time (TT; the SI second on the geoid) by using an optical frequency comb of which the frequency was phase-locked to an H-maser as a flywheel oscillator traceable to TT. The magic wavelength was also measured as 394 798.48(79) GHz. The results are in good agreement with two previous measurements of other institutes within the specified uncertainty of this work.
We report the observation of laser induced birefringence (LIB) in a wavelength-mismatched cascade system (J = 0 ↔ J = 1 ↔ J = 0 transitions) of inhomogeneously broadened ytterbium atoms with strong pump and probe fields. We investigate the transmission spectrum of two circular polarization (σ + p and σ − p ) components of strong probe field at fixed frequency, depending on the detuning of circularly polarized (σ − c ) coupling field from two-photon resonance. We find that σ proposed a simple scheme for laser cooling of alkalineearth and ytterbium atoms using a two-photon 1 S 0 ↔ 1 P 1 ↔ 1 S 0 cascade transition, which might be used as a second stage cooling after precooling with the 1 S 0 − 1 P 1 transition, and this two-photon laser cooling scheme has not been realized yet experimentally.In this paper, we present a Doppler-free two-photon atomic coherence spectroscopy of Yb atoms associated with the wavelength-mismatched cascade scheme considered in the two-photon cooling of Yb atoms [5]. We investigate the Doppler-free transmission spectrum of two circular polarization (σ + p and σ − p ) components of strong probe field at fixed detuning from one-photon resonance in a Yb hollow cathode lamp (HCL) depending on the detuning of circularly polarized (σ − c ) counterpropagating coupling field from two-photon resonance. We find that when the Rabi frequencies of the probe and coupling fields are similar to the decay rate of the J = 1 state, σ + p component exhibits a narrow electromagnetically induced absorption (EIA) spectrum, while σ − p component exhibits an electromagnetically induced transparency (EIT) spectrum due the multiphoton process in- * Electronic address: thyoon@kriss.re.kr duced via the strong probe field. The measured spectra show qualitative agreement with the calculated spectra from numerical solutions of semiclassical density matrix equations. A sensitive Doppler-free dispersive signal which is proportional to the birefringence experienced by two circular polarization components of the probe field is then subsequently obtained by detecting the Stokes parameter [6] of the transmitted probe field. This is the first measurement, as far as the authors know, of the electromagnetically induced birefringence originated from the EIA-type resonance of the strong probe field in the wavelength-mismatched three-level cascade system. Finally, we are able to stabilize the frequency of coupling laser at 1077 nm by use of the laser induced birefringence (LIB) signal with long-term stability below 1 MHz, which is stable enough to study the two-photon laser cooling of Yb atoms in a Yb magneto-optical trap (MOT) [7].Figure 1(a) and (b), respectively, show the three-level cascade scheme of Yb atoms having m-degenerate sublevels |2 and |3 relevant to the present study and the schematic diagram of the experimental set-up. In Fig. 1(a), the levels |1 , |2 , and |4 form a three-level cascade wavelength-mismatched configuration and the level |2 and |3 are coupled to the ground state |1 by an arbitrarily intense probe field...
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