Time is one of the seven basic physical quantities in physics, and it is also the physical quantity with the highest measurement accuracy. As we all know, when measuring length with a ruler, the smaller the scale of the ruler, the more precise the measurement. The same is true of the measurement of time; the higher the frequency of the oscillation of the clock, the more accurate the measurement of time tends to be. At present, scientists in the laboratory to develop the world's most accurate clock, optical clock, the system uncertainty has reached 10-18 or 10-19 magnitude, equivalent to 10 billion years of exactly one second level. Calcium ion optical clock is a member of the big family of optical clock. Compared with other members, it has distinct characteristics: the energy level structure is relatively simple, the laser frequency required is in the visible spectrum, and it can be realized by semiconductor laser. Therefore, it is easy to achieve high integration, high reliability and high running rate of the portable optical clock.
The optical clock consists of three basic parts: physical system, oscillator and counter. A physical system consists of a system of atoms, ions, or molecules that are not subject to collision or interference from the external environment. At present, the optical clock mainly refers to two physical systems: the neutral atoms trapped in the optical lattice and the single ions trapped in the ion trap. The oscillator is an ultra-narrow linewidth ultra-stable laser. The counter is used to realize the connection and transmission of light wave and microwave. It is composed of a femtosecond optical comb and a microwave clock reference. According to the reference atomic system, there are various kinds of optical clocks, of which the calcium ion optical clock is an important member.
Calcium ion is an ideal reference system for building high precision optical clocks and realizing applications. The figure on the right lists the lowest energy levels associated with a calcium optical clock. The 42S1/2 state is the ground state of the ion. The 32D state is the lowest energy ion excited state, which is also metastable, with a lifetime of about 1 s, and the natural linewidth of the 42S1/2-32D transition is about 0.14 Hz. The long lifetime (narrow natural linewidth) of the 32D state makes the 42S1/2-32D transition an ideal reference transition for the optical clock. The 42S1/2-32D5/2 transition is chosen as the reference transition for the optical clock in the experiment.
The simple principle of the calcium optical clock: first, an ion trap device is used to trap individual calcium ions using alternating electric fields, and then laser cooling is performed to slow down the movement of calcium ions. Next, an ultra-stable narrow linewidth laser is used to detect the bell transition spectrum. Finally, the femtosecond comb is used to realize the conversion from visible frequency to microwave frequency, and finally the ultra-high stability frequency output can be used directly.
Partial energy level diagram of 40Ca+ showing the principal transitions used in cooling, repumping and probing
In the process of 40Ca+ optical clock construction, the stability of 40Ca+ optical frequency standard needs to be determined by using a single-photon camera with high sensitivity to observe the captive position of calcium ions and single Ca+ fluorescence (397 nm wavelength) detection. Meanwhile, realizing the comparison of two calcium ion optical frequency standards is needed in the research process. In this process, it is necessary to use highly sensitive single-photon camera to observe ion imaging and RF photon association technology, as well as to carry out detailed research on the dynamics of calcium ions in captivity, so as to realize the optimization of the captivity system.
In previous studies, EM-CCD was mainly used for imaging, mainly because EM-CCD has imaging sensitivity at the single-photon level. With the advent of qCMOS, it attracts the attention of many researchers in optical clock field. We conducted comparative experiments in the Innovation Academy for Precision Measurement Science and Technology, Chinese Academy of Sciences.
Exposure time: 500 ms
Scan mode : Ultraquiet scan
EM Gain: 300
Exposure time: 500 ms
The sensitivity of ORCA-Quest has reached the same level as EM-CCD, which can meet the application requirements of 40Ca+ optical clock. At the same time, ORCA-Quest has smaller pixels and higher frame speed. It is believed that the subsequent optimization of optical path system with more matching for the pixel size will achieve better results.
The International Bureau of Weights and Measures (BPIM) has updated the reference value of leap frequency for the international “second” definition. Innovation Academy for Precision Measurement Science and Technology, CAS Gao Kelin team has developed calcium ion optical clock system, which is listed as one of the standard time. It’s precision has reached to less than 1 second in 10.5 billion years, and the result is 350 times more accuracy than on the space station Rb optical clock to get the 3*10-18 level. It means that the time span of the sun's life cycle with this kind of optical clock error is only about one second.
The current calcium clock uncertainty and stability have entered the E-18, but not the extreme. Therefore, we should make great efforts to study the limit of calcium ion optical frequency standard. In addition, there is also a desire to do multi-ion, improve the stability of the ion optical frequency standard (in a short time into E-18). At the same time to achieve high precision optical clock practical application, we think ORCA-Quest ensures high frame speed and resolution while maintaining the advantages of high signal-to-noise ratio. Moreove, the readout noise is reduced to 0.27 electron rms. The realization of "Photon number resolving" will bring more possibilities of this research.
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