When should we change the definition of the second?

Area of Science:

• Metrology
• Atomic Physics
• Quantum Information Science

Background:

• The cesium (Cs) atomic clock has been the standard for the SI second for decades, with cold Cs fountain clocks reaching frequency uncertainties of ~5x10⁻¹⁶.
• Optical atomic clock research has accelerated since 2000, surpassing Cs fountain clock performance.

Purpose of the Study:

• To evaluate optical atomic clock systems as potential replacements for the current SI second standard.
• To compare the performance of different optical clock technologies, including trapped ion and optical lattice systems.
• To identify environmental factors and challenges that may impact the redefinition of the SI second.

Main Methods:

• Utilizing frequency-stabilized lasers to probe atomic or ionic transitions.
• Employing laser cooling techniques to prepare cold atoms or ions.
• Confining single cold ions in electromagnetic traps or ensembles of cold atoms in optical lattices.

Main Results:

• Optical clocks, such as the aluminum Al(+) quantum logic clock, have demonstrated fractional frequency inaccuracies below 10⁻¹⁷.
• Several optical clock systems now meet or exceed the performance of Cs fountain primary standards.
• Potential limiting factors for optical clocks include magnetic, electric, and light fields, collisions, and gravity.

Conclusions:

• Optical atomic clocks are viable candidates for redefining the SI second due to their superior performance.
• Further research is needed to address environmental sensitivities and inter-laboratory comparisons for optical clock networks.
• The transition to an optical standard for the second requires careful consideration of various technical and logistical challenges.