Optical clock

Optical clocks are a subset of atomic clocks and are based on transitions that emit electromagnetic radiation in the range of 200 nm to 1000 nm. Like other atomic clocks, they are based on the measurement of the resonant frequency of atoms. However, the first atomic clocks mostly operated at microwave frequencies, and therefore were much slower than optical-frequency clocks could be. Optical light oscillates at frequencies near 500 THz, more than 50,000 times faster than the cesium microwave atomic clock. Because of their speed, optical clocks have been demonstrated to keep time with less error than microwave caesium clocks for the definition of the second. John L. Hall and Theodor W. Hansch shared the 2005 Nobel Prize in Physics for their contributions to optical clock development. Optical clocks have been based on a number of atoms, including magnesium, aluminum, potassium, calcium, rubidium, strontium, indium, ytterbium, mercury, and radium. These atoms emit electromagnetic radiation when stimulated by coherent, intense light, for example the 728 nm transition in singly-ionized calcium or the 467 nm and 436 nm transitions in singly-ionized ytterbium. State-of-art optical clocks, which can measure atomic clock transition frequencies to better than one part in 1018, represent the most precise measurements in the world. The precision of a clock is the smallest unit of time it can measure, and comes from counting oscillations of visible light, which oscillates at approximately 700 quadrillion times a second. These oscillations divide a second into 700 quadrillion intervals, with each of those intervals being roughly 10−18 seconds. By counting oscillations of laser light, one can measure time to within one such interval. The laser light is stabilized by the atomic transition; the trapped atom or atoms are excited when the laser light is resonant with the transition frequency. Oscillations of light in the optical range are counted using a frequency comb.

Overview The development of femtosecond frequency combs and optical lattices has led to a new generation of atomic clocks. These clocks are based on atomic transitions that are resonant with visible light instead of microwave radiation. The major obstacle in operating an optical clock was the difficulty of directly measuring optical frequencies. Before the demonstration of the frequency comb in 2000, terahertz techniques were needed to bridge the gap between radio and optical frequencies, and the systems for doing so were cumbersome and complicated. This problem has been solved with the development of self-referenced mode-locked lasers, commonly referred to as femtosecond frequency combs. The frequency comb has dramatically increased accessibility and numerous optical clock systems are in development.

Operation The local oscillator is referenced to the clock transition. In optical clocks, the oscillator is laser light stabilized to the atomic clock transition.

Configurations Optical clocks using neutral or ionized atoms are operated in a variety of experimental systems. For example, millions of neutral strontium atoms are trapped in an optical lattice, which is composed of many shallow atom trap sites. Ion clocks such as the co-trapped aluminum and mercury ion clock confine single or a few ionized atoms within a deep, well-isolated ion trap. In atomic species with atomic transitions that cannot be read out with conventional lasers, a second atom with an accessible transition is co-trapped and coupled to the internal state of the clock ion, and the clock state is transferred to this co-trapped atom. This technique is known as quantum logic spectroscopy. Clocks using neutral and ionized atoms form the bases for state-of-the-art optical clocks. These systems are carefully characterized to account for shifts in the resonant frequency of the atomic transition due to external electromagnetic perturbations. Lasers and magneto-optical traps are used to cool the atoms for improved precision.

Optical Clock Transitions

Optical clocks are based on narrow electronic transition in the optical domain. These transitions include electric quadrupole, electric octupole, and spin-flipping transitions. The transition is used as a frequency reference for the clock laser. The wavelength required for the clock laser to reach this transition must be carefully considered when designing an optical clock. Otherwise very promising atomic species are not widely pursued because of inaccessible clock transitions, such as Th-229 and highly charged ions. In both of these cases, the clock transition is far into the ultraviolet. Laser technology at these wavelengths is not robust, and they must be operated in vacuum because air otherwise strongly absorbs light in this frequency range. Another desirable characteristic is an electronic structure amenable to laser cooling. If the atom or ion cannot be reliably laser cooled, it must be co-trapped with another, easily coolable, atomic species that can provide sympathetic cooling. Other desired features include properties that reduce the effect of perturbations from external electric and magnetic fields, such as a large mass, and a reliable, long-term term source that can be sealed in vacuum for years. The rare-earth element ytterbium (Yb) is valued not so much for its mechanical properties but for its complement of internal energy levels. "A particular transition in Yb atoms, at a wavelength of 578 nm, currently provides one of the world's most accurate optical atomic frequency standards," said Marianna Safronova. The estimated uncertainty achieved corresponds to about one second over the lifetime of the universe so far, 15 billion years, according to scientists at the Joint Quantum Institute (JQI) and the University of Delaware in December 2012.

History

2000s

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