page_title: Atomic clock
section_title: Evaluated accuracy
hierarchical_section_title: Atomic clock / Evaluated accuracy
caption_attribution_description: English: NIST physicists Steve Jefferts (foreground) and Tom Heavner with the NIST-F2 cesium fountain atomic clock, a new civilian time standard for the United States.
context_page_description: An atomic clock is a clock device that uses a hyperfine transition frequency in the microwave, or electron transition frequency in the optical or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element. Atomic clocks are the most accurate time and frequency standards known, and are used as primary standards for international time distribution services, to control the wave frequency of television broadcasts, and in global navigation satellite systems such as GPS.
  The principle of operation of an atomic clock is based on atomic physics; it measures the electromagnetic signal that electrons in atoms emit when they change energy levels. Early atomic clocks were based on masers at room temperature. Since 2004, more accurate atomic clocks first cool the atoms to near absolute zero temperature by slowing them with lasers and probing them in atomic fountains in a microwave-filled cavity. An example of this is the NIST-F1 atomic clock, one of the national primary time and frequency standards of the United States.
context_section_description: The evaluated accuracy uB reports of various primary frequency and time standards are published online by the International Bureau of Weights and Measures (BIPM). Several frequency and time standards groups as of 2015 reported uB values in the 2 × 10⁻¹⁶ to 3 × 10⁻¹⁶ range.
  In 2011, the NPL-CsF2 caesium fountain clock operated by the National Physical Laboratory (NPL), which serves as the United Kingdom primary frequency and time standard, was improved regarding the two largest sources of measurement uncertainties — distributed cavity phase and microwave lensing frequency shifts. In 2011 this resulted in an evaluated frequency uncertainty reduction from uB = 4.1 × 10⁻¹⁶ to uB = 2.3 × 10⁻¹⁶;— the lowest value for any primary national standard at the time. At this frequency uncertainty, the NPL-CsF2 is expected to neither gain nor lose a second in about 138 million (138 × 10⁶) years.
  The NIST-F2 caesium fountain clock operated by the National Institute of Standards and Technology (NIST), was officially launched in April 2014, to serve as a new U.S. civilian frequency and time standard, along with the NIST-F1 standard. The planned uB performance level of NIST-F2 is 1 × 10⁻¹⁶. "At this planned performance level the NIST-F2 clock will not lose a second in at least 300 million years." NIST-F2 was designed using lessons learned from NIST-F1. The NIST-F2 key advance compared to the NIST-F1 is that the vertical flight tube is now chilled inside a container of liquid nitrogen, at −193 °C (−315.4 °F). This cycled cooling dramatically lowers the background radiation and thus reduces some of the very small measurement errors that must be corrected in NIST-F1.
  The first in-house accuracy evaluation of NIST-F2 reported a uB of 1.1 × 10⁻¹⁶. However, a published scientific criticism of that NIST F-2 accuracy evaluation described problems in its treatment of distributed cavity phase shifts and the microwave lensing frequency shift, which is treated significantly differently than in the majority of accurate fountain clock evaluations. The next NIST-F2 submission to the BIPM in March, 2015 again reported a uB of 1.5 × 10⁻¹⁶, but did not address the standing criticism. There have been neither subsequent reports to the BIPM from NIST-F2 nor has an updated accuracy evaluation been published.
  At the request of the Italian standards organization, NIST fabricated many duplicate components for a second version of NIST-F2, known as IT-CsF2 to be operated by the Istituto Nazionale di Ricerca Metrologica (INRiM), NIST's counterpart in Turin, Italy. As of February 2016 the IT-CsF2 caesium fountain clock started reporting a uB of 1.7 × 10⁻¹⁶ in the BIPM reports of evaluation of primary frequency standards.
caption: NIST physicists Steve Jefferts (foreground) and Tom Heavner with the NIST-F2 caesium fountain atomic clock, a civilian time standard for the United States.
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