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Could clock made from a single atom finally answer the question: What is time?

A researcher from the University of California-Berkeley has made a clock from a single atom. Holger Müller, an associate professor of physics at Berkeley, has found a way to tell time by counting the oscillations of a matter wave. A matter wave’s frequency is 10 billion times higher than that of visible light.

“A rock is a clock, so to speak,” Müller said in a statement.

Müller and his Berkeley colleagues refer to their method of telling time using only the matter wave of a cesium atom as a Compton clock because it is based on the the so-called Compton frequency of a matter wave.

“When I was very young and reading science books, I always wondered why there was so little explanation of what time is,” said Müller. “Since then, I’ve often asked myself, ‘What is the simplest thing that can measure time, the simplest system that feels the passage of time?’ Now we have an upper limit: one single massive particle is enough.”

Müller’s Compton clock is still 100 million times less precise than today’s best atomic clocks (which use aluminum ions), but improvement in the technique could better its precision to that of atomic clocks, according to researchers.

“This is a beautiful experiment and cleverly designed, but it is going to be controversial and hotly debated,” said John Close, a quantum physicist at The Australian National University, in a statement. “The question is, ‘Is the Compton frequency of atoms a clock or not a clock?’ Holger’s point is now made. It is a clock. I’ve made one, it works.”

Müller and his research team welcome debate, since his experiment deals with a basic concept of quantum mechanics that has puzzled scientists for nearly a century.

“We are talking about some really fundamental ideas,” Close said. “The discussion will create a deeper understanding of quantum physics.”

Müller and his team can also use time to measure mass. The reference mass today is kept under lock and key in a vault in France. Müller’s matter wave technique gives researchers a new way to build their own kilogram reference.

Louis de Broglie famously discussed the idea that matter can be seen as a wave. De Broglie combined Albert Einstein’s idea that mass and energy are equivalent with Ernst Planck’s idea that every energy is associated with a frequency.

Prior to Müller’s discovery, using matter as a clock seemed nearly impossible because the frequency of the wave might be unobservable. Müller, however, discovered a way to use matter waves to confirm Einstein’s gravitational redshift. He and his team constructed an atom interferometer that treats atoms as waves and measures their interference.

“At that time, I thought that this very, very specialized application of matter waves as clocks was it,” Müller said. “When you make a grandfather clock, there is a pendulum and a clockwork that counts the pendulum oscillations. So you need something that swings and a clockwork to make a clock. There was no way to make a clockwork for matter waves, because their oscillation frequency is 10 billion times higher than even the oscillations of visible light.”

In 2012, however, he realized that he might be able to combine two well-known techniques to design such a clockwork and clearly demonstrate that the Compton frequency of a single particle is useful as a reference for a clock.

A cesium atom that moves away and then returns is younger than one that stays in place. Given this, a moving cesium matter wave will have oscillated fewer times. Müller concluded that the difference frequency might be measurable.

In the lab, Müller demonstrated that he could measure the difference frequency if he allowed the matter waves of the fixed and moving cesium atoms to interfere in an atom interferometer.

“Our clock is accurate to within 7 parts per billion,” Müller said. “That’s like measuring one second out of eight years, about as good as the very first cesium atomic clock about 60 years ago. Maybe we can develop it further and one day define the second as so many oscillations of the Compton frequency for a certain particle.”

Müller’s idea could assist the international General Conference on Weights and Measures’ plan to replace the standard kilogram with a more fundamental measure.

“Our clock and the current best Avogadro spheres would make one of the best realizations of the newly defined kilogram,” Müller 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 still some unanswered questions.

“I don’t think that anyone will ever have a final answer, but we know a bit more about its properties,” Müller said in response to the question: What is time? “Time is physical as soon as there is one massive particle, but it definitely is something that doesn’t require more than one massive particle for its existence. We know that a massless particle, like a photon, is not sufficient.”

Someday, Müller wants to measure time with even smaller particles, such as electrons or even positrons.

Müller’s finding is described in detail in the January 11 issue of the journal Science.