The error is less than one second in 39.6 billion years: the world's most accurate atomic clock sets a new record
2024-08-19
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How long is one second? How should the precision of a second be defined in science? On September 22, 2023, the Mozi Salon was honored to invite Jun Ye, a professor of physics at the University of Colorado and winner of the Mozi Quantum Award, to give a speech entitledThe speech introduced the research background of atomic clocks and the research team's efforts to continuously improve the accuracy of time measurement in an easy-to-understand way.Today, can the upper limit of atomic clock accuracy be further improved? The latest achievement of Ye Jun's team has once again broken the record of atomic clock accuracy, achieved a leap in time measurement accuracy, and set a new benchmark for the next generation of atomic clocks. In January 2014, the team successfully developed the best optical clock system in China ()'s optical clock team from USTC was invited by Physics to introduce this achievement.
The advent of the atomic clock in the 1950s marked a major breakthrough in our ability to measure time with ultra-high precision.After 70 years of development, it is currently the most accurate timekeeping device. If it were to start timing at the beginning of the Big Bang, its error would not exceed one second until now.These precise atomic clocks have a wide range of applications in fundamental physics, metrology, navigation, and more. Further improvements could lead to a host of new applications and new tests of fundamental physics.However, further improvements face many challenges, chief among which are environmental noise, such as magnetic field fluctuations and temperature changes, and the complex interactions of atoms that govern the operation of the clock but are difficult to control.JILA) and a research team at the University of Colorado Boulder have broken the record for atomic clock accuracy.Figure 1:Strontium atoms in1S0and3P0The transitions between the states serve as a reference for the optical lattice clock. When a light signal resonates with the transitions, its frequency can be defined very precisely.
The research team used an optical lattice clock (OLC) based on neutral strontium atoms to measure the frequency of strontium atomic transitions, with a systematic uncertainty of 8.1×10-19, more than tripling the previous record (earlier work by the team). This achievement marks a leap forward in the precision of time measurement and sets a new benchmark for the next generation of atomic clocks.The earliest atomic clocks used the frequency of a microwave signal as their "pendulum" to keep time. Today, the best time-measuring technology is based on light emitted by certain atomic transitions, called clock transitions. The high frequencies of these transitions (typically a few hundred terahertz) and the narrow linewidths (typically 1-100 millihertz) mean that optical atomic clocks can measure time more accurately than microwave-based atomic clocks, which keep time at lower frequencies. Thanks to the tireless efforts of researchers over the past few decades, optical clocks now outperform microwave clocks by more than two orders of magnitude. Improving their performance further means reducing the size of the systematic errors.To achieve this goal, the JILA and CU Boulder team re-evaluated the coefficients of certain atomic parameters that are critical to the operation of optical atomic clocks. In particular, the researchers studied the least magnetically sensitive clock transition in strontium atoms (3P0and1S0The transitions between states were precisely calibrated (see Figure 1) and the second-order Zeeman coefficient was determined.The Zeeman coefficient describes the effect of a magnetic field on the energy levels of an electron, and therefore the frequency change of the light emitted during the associated transition. Typically, magnetically insensitive clock transitions are chosen to minimize the dominant first-order Zeeman frequency shift, which reduces the clock's sensitivity to fluctuations in the ambient magnetic field. But weaker second-order effects still exist. The team's calibration of this coefficient reduced the uncertainty introduced by the second-order Zeeman frequency shift to 1×10-19, double the previous calibration.Figure 2:In September 2023, at the 2023 International Conference on Emerging Quantum Technologies in Hefei, Jun Ye, as the winner of the "Mozi Quantum Award", gave a wonderful live award-winning report.
The researchers also addressed a second factor affecting the clock's uncertainty: dynamic blackbody radiation correction. Blackbody radiation can affect the energy levels of atoms through its electric field, which is an inevitable consequence of operating the clock in a room temperature environment. The dynamic component of this effect refers to the differential frequency shifts between the atomic energy levels. In previous generations of strontium optical lattice clocks, the accuracy was particularly limited by3P0The magnitude of this frequency shift is related to a transition in the energy spectrum of blackbody radiation, namely3P0and higher energy levels3D1The transition between3D1By making these measurements, the team reduced the uncertainty in the blackbody radiation shift from the previous 1.5×10-18Reduced to 7.3×10-19Combining the reduction in the uncertainty of the blackbody radiation frequency shift with other environmental controls, such as stabilizing the temperature, the researchers determined that the total uncertainty of all system effects on the clock transition energy level is less than 1×10-18。To control and measure the atoms in the optical lattice clock, the researchers also used an optical lattice with a "magic wavelength." In an optical lattice well, the energy levels of atoms can drift due to the electric field of a laser beam. However, in a potential well manipulated at the magic wavelength, the potential well is the same regardless of the electronic state of the atom. This means that the relative energy level frequency shifts caused by the laser beam between the clock's transition states are minimized, which helps make the line width of the transition as narrow as possible. The researchers also incorporated a cooling procedure that allowed them to confine the atoms using a shallow lattice. The energy level frequency shifts caused by the laser beam are larger when the atoms are more tightly confined, so the shallow potential minimizes this frequency shift.These methods allow their device to surpass the accuracy of all previous optical lattice clocks, measuring time to within a fraction of a second in 39.6 billion years. The implications of this improvement are far-reaching. For example, combined with the Colorado team's advances, a new generation of instruments could set a new benchmark for the definition of the second. Future efforts may focus on improving these techniques, further reducing uncertainties through cryogenic operation, for example. This ultra-high precision measurement technology could be used to study fundamental questions at the forefront of physics, such as the quantum nature of gravity, which could be revealed through gravitational wave observations, and the nature of dark matter.
Mozi was a famous thinker and scientist in ancient my country. His thoughts and achievements embody the early scientific buds in my country. The establishment of the Mozi Salon aims to inherit and carry forward the scientific tradition, advocate and promote the scientific spirit, improve citizens' scientific literacy, and build a social atmosphere that respects science.
Mozi Salon is aimed at the general public who love science, have a spirit of exploration and curiosity. Through face-to-face public activities and various new media platforms, we hope to let everyone understand the world's most cutting-edge scientific progress and most advanced scientific ideas, explore the secrets of science, and feel the beauty of science.
The Mozi Salon is hosted by the Shanghai Research Institute of the University of Science and Technology of China and the Nanqi Quantum Technology Exchange Center of Pudong New Area, and is supported by the USTC New Alumni Foundation, the University of Science and Technology of China Education Foundation, the Pudong New Area Science and Technology Association, the China Association for Science and Technology, and the Pudong New Area Science and Technology and Economic Committee.
