When should we change the definition of the second
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Options for redefinition are considered, the performance of various trapped ion and optical lattice systems are contrasted, and potential limiting environmental factors, such as magnetic, electric and light fields, collisions and gravity are pointed to.Abstract:
The microwave caesium (Cs) atomic clock has formed an enduring basis for the second in the International System of Units (SI) over the last few decades. The advent of laser cooling has underpinned the development of cold Cs fountain clocks, which now achieve frequency uncertainties of approximately 5×10(-16). Since 2000, optical atomic clock research has quickened considerably, and now challenges Cs fountain clock performance. This has been suitably shown by recent results for the aluminium Al(+) quantum logic clock, where a fractional frequency inaccuracy below 10(-17) has been reported. A number of optical clock systems now achieve or exceed the performance of the Cs fountain primary standards used to realize the SI second, raising the issues of whether, how and when to redefine it. Optical clocks comprise frequency-stabilized lasers probing very weak absorptions either in a single cold ion confined in an electromagnetic trap or in an ensemble of cold atoms trapped within an optical lattice. In both cases, different species are under consideration as possible redefinition candidates. In this paper, I consider options for redefinition, contrast the performance of various trapped ion and optical lattice systems, and point to potential limiting environmental factors, such as magnetic, electric and light fields, collisions and gravity, together with the challenge of making remote comparisons of optical frequencies between standards laboratories worldwide.read more
Citations
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Journal ArticleDOI
Optical two-way time and frequency transfer over free space
Fabrizio R. Giorgetta,William C. Swann,Laura C. Sinclair,Esther Baumann,Ian Coddington,Nathan R. Newbury +5 more
TL;DR: Using two-way exchange between coherent frequency combs, each phase-locked to the local optical oscillator, optical time-frequency transfer is demonstrated in free space across a 2-km-long link, with a timing deviation of 1 fs, a residual instability below 10−18 at 1,000 s and systematic offsets below 4 × 10−19 as discussed by the authors.
Proceedings Article
RF Spectroscopy in an Atomic Fountain
TL;DR: In this article, an atomic fountain was created with a sample of laser cooled Na atoms using the Ramsey resonance technique, which has an observed linwidth of 2.0 Hz at ν = 1.772,626,129.
Journal ArticleDOI
Optical clock networks
TL;DR: This Review covers optical clock networks that are established to synchronize remote optical clocks that may allow a future redefinition of the unit of time based on an optical reference transition.
Journal ArticleDOI
The CIPM list of recommended frequency standard values: guidelines and procedures
TL;DR: A list of standard reference frequency values (LoF) of quantum transitions from the microwave to the optical regime has been recommended by the International Committee for Weights and Measures (Comité international des poids et mesures, CIPM) for use in basic research, technology, and for the metrology of time, frequency and length as mentioned in this paper.
Journal ArticleDOI
Towards a redefinition of the second based on optical atomic clocks
TL;DR: In this article, the status of the optical clocks based on either single ions in radio-frequency traps or on neutral atoms stored in an optical lattice is described, with special emphasis on the current work at the Physikalisch-Technische Bundesanstalt (PTB, Braunschweig, Germany).
References
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