# When should we change the definition of the second?

@article{Gill2011WhenSW, title={When should we change the definition of the second?}, author={Patrick Gill}, journal={Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences}, year={2011}, volume={369}, pages={4109 - 4130} }

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…

## 91 Citations

Demonstration of the nearly continuous operation of an 171Yb optical lattice clock for half a year

- PhysicsMetrologia
- 2020

Optical lattice clocks surpass primary Cs microwave clocks in frequency stability and accuracy, and are promising candidates for a redefinition of the second in the International System of Units…

Cryogenic optical lattice clocks with a relative frequency difference of $1\times 10^{-18}$

- Physics
- 2014

Time and frequency are the most accurately measurable quantities, providing foundations for science and modern technologies. The accuracy relies on the SI (Syst\'eme International) second that refers…

An optical lattice clock with neutral strontium

- Physics
- 2016

Strontium optical lattice clocks can measure the passage of time with extraordinary precision. Capitalising on this precision, we can anticipate the deployment of lattice clocks to search for new…

Towards a transportable aluminium ion quantum logic optical clock.

- PhysicsThe Review of scientific instruments
- 2019

A transportable optical clock based on a single trapped aluminum ion, which is interrogated via quantum logic spectroscopy, is developed, employing singly charged calcium as the logic ion for sympathetic cooling, state preparation, and readout.

Frequency ratio measurements at 18-digit accuracy using an optical clock network.

- PhysicsNature
- 2021

An advance in frequency ratio measurements lays the groundwork for future networks of mobile, airborne and remote optical clocks that will be used to test physical laws, perform relativistic geodesy and substantially improve international timekeeping.

A strontium optical lattice clock with 1 × 10−17 uncertainty and measurement of its absolute frequency

- PhysicsMetrologia
- 2020

We present a measurement of the absolute frequency of the 5 s2 1S0 to 5s5p 3P0 transition in 87Sr which is a secondary representation of the SI second. We describe the optical lattice clock apparatus…

Ultracold atoms and precise time standards

- PhysicsPhilosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences
- 2011

This work will discuss the current limitations in the performance of neutral atom atomic frequency standards and prospects for the future.

The $$^{229}$$Th isomer: prospects for a nuclear optical clock

- Physics
- 2020

The proposal for the development of a nuclear optical clock has triggered a multitude of experimental and theoretical studies. In particular the prediction of an unprecedented systematic frequency…

Frequency measurements of superradiance from the strontium clock transition

- Physics
- 2017

We present the first characterization of the spectral properties of superradiant light emitted from the ultra-narrow, 1 mHz linewidth optical clock transition in an ensemble of cold $^{87}$Sr atoms.…

## References

SHOWING 1-10 OF 104 REFERENCES

Frequency Ratio of Al+ and Hg+ Single-Ion Optical Clocks; Metrology at the 17th Decimal Place

- PhysicsScience
- 2008

Repeated measurements during the past year yield a preliminary constraint on the temporal variation of the fine-structure constant α of α ofbatchmode, a regime of operation for atomic clocks based on optical transitions, promising even higher performance.

Ultracold atoms and precise time standards

- PhysicsPhilosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences
- 2011

This work will discuss the current limitations in the performance of neutral atom atomic frequency standards and prospects for the future.

Frequency comparison of two high-accuracy Al+ optical clocks.

- PhysicsPhysical review letters
- 2010

An optical clock with a fractional frequency inaccuracy of 8.6x10{-18}, based on quantum logic spectroscopy of an Al+ ion, is constructed, consistent with the accuracy limit of the older clock.

Constraining the evolution of the fundamental constants with a solid-state optical frequency reference based on the 229Th nucleus.

- PhysicsPhysical review letters
- 2010

It is argued that the 229Th optical nuclear transition may be driven inside a host crystal with a high transition Q to allow for the construction of a solid-state optical frequency reference that surpasses the short-term stability of current optical clocks, as well as improved limits on the variability of fundamental constants.

Absolute frequency measurement of the In+ clock transition with a mode-locked laser.

- PhysicsOptics letters
- 2000

This work compared the (1)S(0)(3)P(0) transition with a methane-stabilized HeNe laser at 3.39 microm, which was calibrated against an atomic cesium fountain clock and found the frequency to be 1.267,402,452,899.92 (0.23) kHz.

Uncertainty evaluation of the atomic caesium fountain CSF1 of the PTB

- Physics
- 2001

A new primary frequency standard, the atomic caesium fountain CSF1, has been put into operation at the Physikalisch-Technische Bundesanstalt (PTB). 133Cs atoms are cooled in a magneto-optical trap…

Observation of the 1S0-->3P0 clock transition in 27Al+.

- PhysicsPhysical review letters
- 2007

We report, for the first time, laser spectroscopy of the 1S0-->3P0 clock transition in 27Al+. A single aluminum ion and a single beryllium ion are simultaneously confined in a linear Paul trap,…

Hertz-Level Measurement of the Optical Clock Frequency in a Single 88Sr+ Ion

- PhysicsScience
- 2004

Improvements required to obtain a cesium-limited frequency measurement are described and are expected to lead to a 88Sr+ optical clock with stability and reproducibility exceeding that of the primary cedium standard.

Limit on the present temporal variation of the fine structure constant.

- PhysicsPhysical review letters
- 2004

An optical transition frequency at 688 THz in 171Yb+ with a cesium atomic clock is measured and a value for the fractional variation of the frequency ratio f(Yb)/f(Cs) of (-1.2+/-4.4)x10(-15) yr(-1) (1sigma) is found, consistent with zero.

The LPTF preliminary accuracy evaluation of cesium fountain frequency standard

- Physics
- 1996

Most of the frequency shifts which limit the accuracy of cesium atomic frequency standards are considerably reduced when using ultra-cold atoms and long interrogation times in a Ramsey geometry.…