Timekeeping is all about counting steps. Regardless of whether it is the drop of a water clock, the tic-tooc of a mechanical clock or the oscillating crystal of a quartz clock. Every accurate clock counts the steps of something regular and periodic. Nothing is perfectly regular, so no watch keeps perfect time, but our timepieces get very, very accurate.
Time is traditionally measured using astronomy, e.g. B. on the basis of the rising and setting of the sun or the movement of the stars and the moon. There have been several popular methods, but in the 19th century time was measured by what was known as the mean sunny day. A day is not always exactly 24 hours. As the earth moves along its elliptical orbit, its speed around the sun changes slightly, making the day a little longer or shorter depending on the season. By finding the average (especially the mean) of the days over a year, astronomers can define a common standard.
As our mechanical watches got more accurate, it became clear that the mean sunny day was problematic. The rotation of the earth is not constant, but changes due to tectonic shifts. Due to its gravitational dance with the moon, the earth rotates more slowly over time. In 1956, for example, time was defined in terms of Earth's orbit rather than its rotation.
A series of atomic clocks that the U.S. military uses to keep time. Photo credit: US Naval Observatory
In 1967 we moved completely away from astronomy when the length of a second was redefined in the form of an atomic clock. The measured vibration is not based on an atom, but on the light emitted by an atom. Atoms produce light when an electron moves from a higher energy state to a lower one. Because the energy levels in an atom are quantized, light is emitted at an accurate frequency. In 1967 the length of one second was established by defining a specific emission of cesium-133 to be exactly 9,192,631,770 Hz. This is the standard we still use today.
Although the modern standard is officially accurate, it is not accurate. Two atomic clocks of the same design keep slightly different times. By statistically comparing atomic clocks, we know that they will be accurate to about one second in thirty million years. This is probably accurate enough for everyday use, but not accurate enough for some scientific purposes. If we had more accurate clocks, we could use them to study everything from geology to dark energy. So there is a constant attempt to develop a new, more precise standard.
The accuracy of the watches is getting better and better. Photo credit: Wikipedia
Most approaches aim at purely optical methods, but new work in nature uses atoms in quantum entanglement. One of the reasons modern atomic clocks are not perfect is because when light is emitted, the atoms retract, causing the frequency of the emitted light to shift slightly. If the atom could be kept perfectly stationary while emitting light, the frequency of the light would be accurate. However, quantum mechanics keeps the position of an atom a little fuzzy, which means that the frequency of the light emitted is also a little fuzzy. This effect is known as the standard quantum limit.
To address this problem, the team uses an effect known as quantum entanglement. By using lasers to press atoms together, they can force the atoms to interact at the quantum level so that if you measure one atom, you will measure all of them. Thus the states of these atoms are involved. When another laser is used to trigger an atom to emit light, a cascade occurs that synchronizes the atoms with each other. The light emitted is therefore much more accurate than the standard quantum limit.
The statistical analysis of this new clock shows that it can function with an accuracy of 100 milliseconds over the age of the universe. The clock is so precise that it can test whether universal physical constants change over time.
Our standard for timing isn't going to change anytime soon, but it's clear we can do better. At some point in the future we will use a more precise method. If we do that, a clock based on quantum entanglement could be the solution. In this case, our official clocks use quantum madness to overcome the quantum fuzziness.
Reference: Edwin Pedrozo-Peñafiel, et al. "Entanglement at an optical atomic clock junction." Nature 588 (2020): 414