13 By Sam Fan 樊潤璋 Clocks Almost Perfect Time 如何精確計時？ years for a quartz clock [3], there was still room for improvement for the timekeeping accuracy. As a result, scientists developed more advanced timekeeping technologies. Cesium-beam clocks are just one member of the atomic clock family: Others include rubidium-beam standards, hydrogen masers, compact chip-scale clocks, and the newest optical lattice clocks. Cesium-beam designs remain the most widely used standard worldwide. In fact, the Hong Kong Observatory has been relying on cesium-beam atomic clocks to provide official time service since 1980, with an accuracy kept within 0.01 microsecond (10-8 seconds) per day [5]. At its core, the clock does not “tick” atoms but counts the cycles of a microwave signal precisely locked to an atomic reference. The cesium atoms act as a built-in tuning fork; cesium only resonate to the frequency at 9,192,631,770 Hz within the microwave band to change between two very slightly different energy states. This happens when cesium atoms pass through a microwave cavity [6]. If the microwave frequency is above or below 9,192,631,770 Hz, fewer atoms undergo a change in energy levels. The irregularity can be detected, and the oscillator will be steered back onto the exact cesium resonance frequency, ensuring the microwave oscillator stays locked to atomic standard. Once the oscillator is held exactly at the resonance frequency, every single cycle becomes one “tick” of the clock. By simply tallying 9,192,631,770 of these ticks, the device measures one second [6, 7]. Since this shift in energy levels is a fundamental property of cesium atoms, every clock built anywhere with cesium atoms can reproduce the same resonance frequency of 9,192,631,770 Hz signal, ensuring that time is uniform around the world. Importance of Time Synchronization Every GPS satellite carries multiple atomic clocks and broadcasts signals with an accurate timestamp of when the signal is emitted, so that receivers on the ground can determine their distance to the satellite by multiplying the signal’s travel time and the speed of light [8]. Without the cesium or rubidium standards, minute timing drifts would quickly lead to substantial positional errors in just a few minutes due to the cumulative time error of the onboard clock. Summary In just a few decades, we’ve gone from pendulums and quartz crystals to atomic clocks and optical lattices that keep time to within a quintillionth of a second (10-18 seconds), so precise that they can sense minuscule shifts in Earth’s gravity or hunt for hints of dark matter in the cosmos [9]. Although the principles behind atomic precision may seem complicated, the technologies they enable are integral to our daily lives. Every time we check our phones, play a game, look up our location, or simply glance at the clock, we tap into nature’s steady rhythms.
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