A global comparison of optical atomic clocks has paved the way for redefining the SI unit of time,  second by 2030.

Current Definition of a Second

• Currently the second has been defined as the time taken for 9,192,631,770 cycles of microwave radiation from the caesium-133 atom during its hyperfine energy transition.
  • Caesium (Cs) atomic clocks have served as the global standard, with extremely high accuracy losing just one second every 300 million years.

Why Redefine the Second?

• With growing applications in GPS navigation, radio astronomy, and climate monitoring, the demand for even more precise timekeeping has increased.
• Optical atomic clocks, which count vibrations at frequencies nearly 10,000 times higher than caesium clocks, offer precision to 18 decimal places, losing less than a second in 15 billion years.

Old vs New Definition of the ‘Second’

About the Global Optical Clock Experiment

• Between February and April 2022, researchers from six national metrology institutes across Europe and Japan ran the largest-ever intercontinental comparison of optical atomic clocks.
• Clocks Used: 10 optical clocks based on atoms like strontium-87, ytterbium-171, indium-115, and their charged ion states.
• Linking Methods: Clocks were connected via optical fibres and GPS-based techniques like Integer Precise Point Positioning (IPPP).
• Time Span: 45 days of continuous operation, with backup clocks used during downtime.
• Outcome: 38 frequency ratios were recorded, several of them never measured before, achieving uncertainty levels as low as 4.4 × 10⁻¹⁸.
• Error Identification: Certain discrepancies were found in Italian Yb clocks and others, needing further refinement before 2030.

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