Time Atom

A clock based on just a single atom -- the simplest clock yet -- has now been devised, researchers say.

This new device to measure time could help lead to a radically new way to define mass as well, scientists added.
In addition, this achievement suggests that researchers might one day build even more exotic clocks -- ones based on antimatter, or ones based on no particles at all.
Fundamentally, all clocks measure time by relying on parts that repeat behavior in regular patterns. For instance, a year is defined by how long it takes for Earth to complete an orbit around the sun.
The most accurate clocks that currently exist are atomic clocks. These depend on how atoms switch between two distinct energy levels. Essentially, these clocks rely on at least two particles -- the nucleus of an atom, and an electron leaping back and forth between different levels of energy.
Defining Time
However, could clocks get simpler still?
"We were interested in what the simplest clocks are to explore the question of what time is," said researcher Holger Muller, a physicist at the University of California at Berkeley. "If you say that, say, you can't measure time with less than two particles, does that mean that anything below two particles doesn't experience time at all?"
The researchers theorized it was possible to create a clock made up of just one particle. To understand, one starts with Einstein's famous equation E=mc2, which showed that matter can be converted to energy and vice versa. One consequence of this, called de Broglie's matter-wave hypothesis, suggests that matter can also behave like waves. As such, a particle of matter can in principle behave like a wave that oscillates in a regular manner, thus acting like a clock.
"We've shown that one single particle really can measure time," Muller told LiveScience.
The problem with making a clock from a particle of matter is that the frequency at which it oscillates "should be so high that one should never be able to measure it," Muller said. To get over this hurdle, the scientists relied on a phenomenon known as time dilation, another consequence of Einstein's theory of relativity. This suggests that as objects move away from and back to a location, they experience less elapsed time than objects that stayed at that location the entire time.
Splitting Atoms
The researchers recreated this phenomenon using lasers on cesium atoms. "We essentially split an atom into two halves, and had one stay where it is and the other go forward and come back," Muller said. "A tiny, tiny bit less time elapsed for the half that moved, so it oscillated less."
The fact that one half of the atom oscillated less than the other meant that when these halves are reunited, they did not recombine perfectly, but interference occurred that the scientists could measure. By knowing the size of this discrepancy and the extent to which the researchers disturbed the atom, the researchers could deduce the original frequency at which the atom oscillated.
The moving half of the atom took about a third of a second less than the other half to make its round trip. Each half of the atom made about 10 to the 25th oscillations -- a 1 with 25 zeroes behind it, equal to 10 trillion trillion -- but the moving half made about 100,000 fewer oscillations than the still half.
"We have demonstrated that you can make a clock from a single massive particle," said researcher Justin Brown, a physicist at the University of California at Berkeley.
At present, this new clock can tell time about as precisely as the first atomic clocks developed about 60 years ago and about a billion times less precisely than the best current atomic clocks, known as optical clocks. Although it remains uncertain whether this new clock will ever match the performance of optical clocks, the researchers say it could help solve a problem today regarding one of the world's most important units of measurement -- the kilogram.

Since 1889, the kilogram has been defined as the mass of a specific golf-ball-size cylinder of platinum and iridium, which is housed in a vault outside Paris. The problem with defining the kilogram on this object -- known formally as the International Prototype Kilogram and more familiarly called Le Grande K -- is that contaminants settling on its surface can make it gain weight while cleaning it could make it lose weight, potentially wreaking havoc on one of the main ways science describes everything in the universe.
As such, researchers have in recent years sought to base the kilogram not arbitrarily on an artifact, but on more fundamental constants. The new clock that Müller and his colleagues developed links time to the mass of an atom. As such, not only could atoms be used to measure time, but also time could be used to help define mass.
For instance, as new standard weights, scientists can manufacture incredibly pure crystals of silicon dubbed Avogadro spheres, which are created so precisely that the number of atoms inside is known to high accuracy.
"Our clock and the current best Avogadro spheres would make one of the best realizations of the newly defined kilogram," Muller said. "Knowing the ticking rate of our clock is equivalent to knowing the mass of the particle, and once the mass of one atom is known, the masses of others can be related to it."
There are other strategies that exist on which to base the kilogram -- for instance, by using what is known as a watt balance that uses magnetic force to levitate objects, defining their masses by how much they levitate in response to the magnetic field.
"It's good to have multiple ways of measuring mass -- it provides a cross-check for consistency," Muller said.
Future of Measuring Time
In the future, Muller suggested it might be possible to create even simpler clocks — ones that are based on no particles at all. Quantum theory suggests that what may seem like vacuum is actually filled with "virtual particles" that regularly pop in and out of existence, generating measurable forces.
"It'd be fascinating to see if we can make a clock based off zero particles -- you don't even need one particle, just the hypothetical possibility of a particle to measure time," Muller said.
Another interesting possibility is developing a version of this clock that is based on antimatter instead of normal matter. When antimatter is brought into contact with its normal matter, it annihilates its counterpart. One of the greatest mysteries in the universe is why the visible matter in the universe is nearly all normal matter and not antimatter.
"You can have an antimatter clock run for a year as the Earth moves closer to the sun and then farther away, since the Earth's orbit around the sun is not perfectly circular, but slightly elliptical. This means the strength of the gravitational field it experiences would change over time," Muller said. "It would be interesting to compare a clock of normal matter with a clock of antimatter, to see if they behave the same way in relation to gravity as expected. Such a test of the laws of physics would be fascinating if it was found that matter and antimatter behaved differently."
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