Automatic,long-term frequency-stabilized lasers with sub-hertz linewidth and 10-16 frequency instability

We report two ultra-stable laser systems automatically frequency-stabilized to two high-finesse optical cavities.By employing analog-digital hybrid proportional integral derivative(PID)controllers,we keep the merits of wide servo bandwidth and servo accuracy by using analog circuits for the PID controller,and,at the same time,we realize automatic laser frequency locking by introducing digital logic into the PID controller.The lasers can be automatically frequency-stabilized to their reference cavities,and it can be relocked in 0.3 s when interruption happens,i.e.,blocking and unblocking the laser light.These automatic frequency-stabilized lasers are measured to have a frequency instability of 6×10^(-16)at 1 s averaging time and a most probable linewidth of 0.3 Hz.The laser systems were tested for continuous operation over 11 days.Such ultrastable laser systems in long-term robust operation will be beneficial to the applications of optical atomic clocks and precision measurement based on frequency-stabilized lasers.

Population aging and its economic consequences for China

Population aging in the People’s Republic of China(PRC)is mainly due to rapid fertility decline after 1970,a third of which was due to government policy,and the balance to social and economic development.Relaxing fertility policies now would probably not have a large effect on fertility,and rising educational attainment may reduce future fertility by 0.3 births per woman.The successes and failures of Euro-pean fertility policies can guide policy.In any case,the PRC’s population will age rapidly,with the old-age dependency ratio(OADR)more than tripling by 2065.This will cause severe fiscal problems for current old-age programs,particularly for pen-sions.The general economic impact will be less severe,as partially offset by rising education and increased capital per worker.Focusing on a particular age,like 65,is a mistake.Health,cognition,and vitality are more important than age itself and these will improve due to past increases in education and better living conditions for children.The retirement age should be raised.One good approach is building auto-matic adjustment mechanisms into old-age programs like pensions.For example,retirement age,taxes,and benefits can be linked to life expectancy and to the OADR as in some European countries.This would increase the labor supply of the elderly and avoid pension-related political problems in the future.