Both the Al-based quantum clock and theHg-based opticalatomic clock track time by the ion vibration at an optical frequency using aUV laser, that is 100,000 times higher than the microwave frequencies used inNIST-F1 and other similar time standards around the world. Quantum clocks like this are able to befar more precise than microwave standards.
ANIST 2010 quantum logic clock based on a single aluminum ion
The NIST team are not able to measure clock ticks per second because the definition of a second is based on the standard NIST-F1, which cannot measure a machine more precise than itself. However, the aluminum ion clock's measured frequency to the current standard is1121015393207857.4(7) Hz.[2] NIST have attributed the clock's accuracy to the fact that it is insensitive to background magnetic and electric fields, and unaffected by temperature.[3]
In March 2008, physicists atNIST described an experimental quantum logic clock based on individualions ofberyllium andaluminum. This clock was compared to NIST'smercury ion clock. These were the most accurate clocks that had been constructed, with neither clock gaining nor losing time at a rate that would exceed a second in over a billion years.[4]
In February 2010, NIST physicists described a second, enhanced version of the quantum logic clock based on individualions ofmagnesium andaluminium. Considered the world's most precise clock in 2010 with a fractional frequency inaccuracy of8.6 × 10−18, it offers more than twice the precision of the original.[5][6]In terms ofstandard deviation, the quantum logic clock deviates one second every 3.68 billion (3.68 × 109) years, while the then current international standard NIST-F1Caesium fountain atomic clock uncertainty was about 3.1 × 10−16 expected to neither gain nor lose a second in more than 100 million (100 × 106) years.[7][8] In July 2019, NIST scientists demonstrated such a clock with total uncertainty of9.4 × 10−19 (deviates one second every 33.7 billion years), which is the first demonstration of a clock with uncertainty below10−18.[9][10][11]
"Two clocks are depicted as moving in Minkowski space. ClockB is moving in a localized momentum wave packet with average momentum pB, while clockA is moving in a superposition of localized momentum wave packets with average momentum pA and p0A. ClockA experiences a quantum contribution to the time dilation it observes relative to clockB due to its nonclassical state of motion."[12]
In a 2020 paper scientists illustrated that and how quantum clocks could experience a possibly experimentally testablesuperposition of proper times via time dilation of the theory of relativity by which time passes slower for one object in relation to another object when the former moves at a higher velocity. In "quantum time dilation" one of the two clocks moves in a superposition of two localized momentumwave packets,[further explanation needed] resulting in a change to the classical time dilation.[13][14][12]
The accuracy of quantum-logic clocks was briefly superseded byoptical lattice clocks based onstrontium-87 andytterbium-171 until 2019.[9][10][11] An experimental optical lattice clock was described in a 2014 Nature paper.[15]In 2015JILA evaluated the absolute frequency uncertainty of their lateststrontium-87 429 THz (429228004229873.0 Hz[16]) optical lattice clock at2.1 × 10−18, which corresponds to a measurablegravitational time dilation for an elevation change of 2 cm (0.79 in) on planet Earth that according to JILA/NIST FellowJun Ye is "getting really close to being useful forrelativistic geodesy".[17][18][19]At this frequency uncertainty, this JILA optical lattice optical clock is expected to neither gain nor lose a second in more than 15 billion (1.5 × 1010) years.[20]
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