Optical atomic clocks with fractional uncertainties around and below 10−17 [1] and fractional instabilities at one second interrogation time of below 10−15 [2] have outperformed the best microwave clocks like the cesium atomic clock. In optical atomic clocks a highly stable laser is used to interrogate optical transitions in laser-cooled atoms, ions, or molecules. The increase in accuracy and stability of the optical atomic clocks results from the increased line quality factor Q= ν/Δν given by the five orders of magnitude higher frequency of optical transitions as compared to the transition relevant in the microwave domain. To utilize this potential it is furthermore necessary to realize a suitable narrow linewidth of the interrogated line. The linewidth of any optical transition is given by the different broadening mechanisms of the respective quantum transition relevant for particular interrogation process as well as by the coherence properties of the interrogating laser.
Optical atomic clocks with fractional uncertainties around and below 10−17 [1] and fractional instabilities at one second interrogation time of below 10−15 [2] have outperformed the best microwave clocks like the cesium atomic clock. In optical atomic clocks a highly stable laser is used to interrogate optical transitions in laser-cooled atoms, ions, or molecules. The increase in accuracy and stability of the optical atomic clocks results from the increased line quality factor Q= ν/Δν given by the five orders of magnitude higher frequency of optical transitions as compared to the transition relevant in the microwave domain. To utilize this potential it is furthermore necessary to realize a suitable narrow linewidth of the interrogated line. The linewidth of any optical transition is given by the different broadening mechanisms of the respective quantum transition relevant for particular interrogation process as well as by the coherence properties of the interrogating laser.
8half a century ago and have provided the definition of the second since 1967. The international atomic time (TAI) is currently kept by an ensemble of Cs clocks with a fractional uncertainty of Δν/ν0 ≈ 10 –15 , and some Cs fountain clocks 9,10 operate at even smaller uncertainties. Although Cs atomic clocks operate at the microwave transition of ν0 ≈ 9.2 GHz, using optical transition frequencies of several hundred terahertz should improve the fractional uncertainty Δν/ν0 by 4–5 orders of magnitude, as most measurement uncertainties Δν are independent of ν0 (except the Doppler shift Δ νD = (v/c)ν0, where v is the atomic velocity and c is the speed of light). The invention of the laser and the subsequent improvement of laser frequency stabilities 11 has allowed the realization of optical clocks to become a reality 2,3,12
Abstract The atomic clocks, whether operating at optical or microwave region, can be divided into two categories according to their working mode, namely the passive clocks and active clocks. The passive clocks, whose standard frequency is locked to an ultra-narrow atomic spectral line, such as laser cooled Cs beam or lattice trapped Sr atoms, depend on the spontaneous emission line. On the contrary, the active clocks, in which the atoms are used as the gain medium, are based on the stimulated emission radiation, their spectrum can be directly used as the frequency standard. Up to now, the active hydrogen maser has been the most stable microwave atomic clocks. Also, the Sr superradiant active atomic clock is prospects for a millihertz-linewidth laser. Moreover, the optical clocks are expected to surpass the performance of microwave clocks both in stability and uncertainty, since their higher working frequency. The active optical clock has the potential to improve the stability of the best clocks by 2 orders of magnitude. In this work, we introduce the development of active optical clocks, and their types is classified according to the energy-level structure of atoms for stimulated radiation.
Optical lattice clocks with uncertainty and instability in the 1017-range and below have so far been demonstrated exclusively using fermions. Here, we demonstrate a bosonic optical lattice clock with 3 1018 instability and 2:0 1017 accuracy, both values improving on previous work by a factor 30. This was enabled by probing the clock transition with an ultra-long interrogation time of 4 s, using the long coherence time provided by a cryogenic silicon resonator, by careful stabilization of relevant operating parameters, and by operating at low atom density. This work demonstrates that bosonic clocks, in combination with highly coherent interrogation lasers, are suitable for highaccuracy applications with particular requirements, such as high reliability, transportability, operation in space, or suitability for particular fundamental physics topics. As an example, we determine the 88Sr { 87Sr isotope shift with 12 mHz uncertainty.
Frequency and its inverse, time, are the most accurately measured quantities. Historically, improvements in the accuracy of clocks have enabled advances in navigation, communication, and science. Since 1967, the definition of the International System (SI) second has been based on the frequency of a microwave transition in cesium, and present day cesium atomic clocks have a fractional uncertainty near 10-16. Recently, a new type of atomic clock based on optical transitions has been developed, with a current fractional uncertainty near 10-18 (approximately one second divided by the age of the universe), and they are rapidly improving. This talk presents a brief summary of the development of optical atomic clocks, with a focus on the Al+ quantum-logic clock developed at NIST. We discuss the current state-of-the-art in optical clock performance, and describe new applications in sensing and fundamental physics. Future directions in optical atomic clock research are also considered.
An optical clock or frequency standard based on the interrogation of an atomic transition in the optical domain, operates at ~1014Hz is capable of providing orders of magnitude better accuracy than the present primary standard of frequency or time operating in the microwave range. Optical transitions of an ions, confined within a radio frequency ion trap and laser cooled to ~mK temperature, provides better long term stability since they are free from Coulomb and intra-atomic interactions. Optical frequency standards have been realized with several ions, e.g.,199Hg+,171Yb+,115In+,88Sr+,40Ca+,27Al+and among those171Y+has multiple ultra-narrow optical transitions suitable for serving as frequency standards [1-2].
In the mid-20th century, atoms were realized to be good candidates for time or frequency standards since transition energies or frequencies between two energy levels of some unperturbed atoms are more stable than those defined by the rotation of the earth or artefact-based clocks such as quartz crystal clocks. This motivated extensive research on realization and improvement of such kind of quantum-based time or frequency standards. The ultra-stable laser, referred to as a local oscillator in a clock, still drifts over time due to environmentally induced length fluctuations of the reference cavity. With the development of ion traps, ultra-narrow linewidth lasers and optical frequency combs, optical clocks based on single trapped ions as a new type of clocks have been realized in many laboratories. Ions can only stay in the trap within a certain region, which is called stability region. There is a variety of potential sources of systematic shifts associated with the clock transition in ion optical clocks.