Context: The most accurate timekeeping devices are atomic clocks, which are accurate to seconds in many millions of years, and are used to calibrate other clocks and timekeeping instruments. Atomic clocks use the frequency of electronic transitions in certain atoms to measure the second. One of the most common atoms used is caesium, most modern atomic clocks probe caesium with microwaves to determine the frequency of these electron vibrations. Since 1967, the International System of Measurements bases its unit of time, the second, on the properties of caesium atoms. SI defines the second as 9,192,631,770 cycles of the radiation that corresponds to the transition between two electron spin energy levels of the ground state of the 133Cs atom.

Question: Which timekeeping devices are the most accurate?
Answer: atomic clocks

Question: What method do atomic clocks use to measure seconds?
Answer: the frequency of electronic transitions in certain atoms

Question: Which system bases its unit of time on the properties of caesium?
Answer: the International System of Measurements

Question: How long has the International System of Measurements based the second on caesium?
Answer: Since 1967

Question: In what year were caesium atoms discovered?
Answer: Unanswerable

Question: What are microwaves the most accurate at doing?
Answer: Unanswerable

Question: What are microwaves also used to calibrate?
Answer: Unanswerable

Question: What system bases its unit of time on microwave properties?
Answer: Unanswerable

Question: How many cycles of radiation are in microwaves?
Answer: Unanswerable

Question: Which timekeeping devices are the least accurate?
Answer: Unanswerable

Question: What method do atomic clocks use to measure cycles?
Answer: Unanswerable

Question: Which system bases its unit of time on the properties of vibrations?
Answer: Unanswerable

Question: How long has the International System of Measurements based the second on years?
Answer: Unanswerable

Question: How does caesium deine the second?
Answer: Unanswerable