A typical atomic clock does this by first using a system of lasers to corral a gas of ultracooled atoms into a trap formed by a laser. This is why today’s atomic clocks are designed to measure a gas composed of thousands of the same type of atom, in order to get an estimate of their average oscillations. “When you increase the number of atoms, the average given by all these atoms goes toward something that gives the correct value,” says Colombo. This limitation is what physicists refer to as the Standard Quantum Limit. But at that scale, an atom is so small that it behaves according to the mysterious rules of quantum mechanics: When measured, it behaves like a flipped coin that only when averaged over many flips gives the correct probabilities. To keep perfect time, clocks would ideally track the oscillations of a single atom. Furthermore, one cesium atom will oscillate at exactly the same frequency as another cesium atom. Today, vibrations in atoms are the most stable periodic events that scientists can observe. Since humans began tracking the passage of time, they have done so using periodic phenomena, such as the motion of the sun across the sky. The paper’s other co-authors from MIT are Simone Colombo, Chi Shu, Albert Adiyatullin, Zeyang Li, Enrique Mendez, Boris Braverman, Akio Kawasaki, Saisuke Akamatsu, Yanhong Xiao, and Vladan Vuletic, the Lester Wolfe Professor of Physics. If state-of-the-art atomic clocks were adapted to measure entangled atoms the way the MIT team’s setup does, their timing would improve such that, over the entire age of the universe, the clocks would be less than 100 milliseconds off. “Entanglement-enhanced optical atomic clocks will have the potential to reach a better precision in one second than current state-of-the-art optical clocks,” says lead author Edwin Pedrozo-Peñafiel, a postdoc in MIT’s Research Laboratory of Electronics. The new setup can achieve the same precision four times faster than clocks without entanglement. The atoms are correlated in a way that is impossible according to the laws of classical physics, and that allows the scientists to measure the atoms’ vibrations more accurately. The researchers report today in the journal Nature that they have built an atomic clock that measures not a cloud of randomly oscillating atoms, as state-of-the-art designs measure now, but instead atoms that have been quantumly entangled. Now a new kind of atomic clock designed by MIT physicists may enable scientists to explore such questions and possibly reveal new physics. With better atomic clocks, scientists could also start to answer some mind-bending questions, such as what effect gravity might have on the passage of time and whether time itself changes as the universe ages. If atomic clocks could more accurately measure atomic vibrations, they would be sensitive enough to detect phenomena such as dark matter and gravitational waves. The best atomic clocks in the world keep time with such precision that, if they had been running since the beginning of the universe, they would only be off by about half a second today. These exquisite instruments use lasers to measure the vibrations of atoms, which oscillate at a constant frequency, like many microscopic pendulums swinging in sync. Atomic clocks are the most precise timekeepers in the world.
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