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Whenever you glance at the time on your mobile device, conduct an internet transaction, or utilize a navigation tool, you are relying on the accuracy of atomic timepieces.
An atomic timepiece maintains time by depending on the “ticks” of atoms as they oscillate naturally at unwavering frequencies. Modern atomic clocks function by monitoring cesium atoms, which tick over 10 billion times each second. Each of these ticks is meticulously monitored using lasers that oscillate in harmony, at microwave frequencies.
Researchers are crafting next-generation atomic clocks that depend on even quicker-ticking atoms like ytterbium, which can be monitored with lasers at elevated optical frequencies. If they can maintain stability, optical atomic clocks could measure even smaller time intervals, potentially up to 100 trillion ticks per second.
Now, physicists at MIT have discovered a method to enhance the stability of optical atomic clocks by reducing “quantum noise” — a fundamental limitation in measurement caused by quantum mechanical effects, which distorts the pure oscillations of atoms. Additionally, the team found that an influence of a clock’s laser on the atoms, previously thought insignificant, can be leveraged to further stabilize the laser.
The researchers devised a technique to harness a laser-induced “global phase” in ytterbium atoms and amplified this effect using a quantum-amplification method. This new strategy doubles the precision of an optical atomic clock, allowing it to distinguish twice the number of ticks per second compared to the identical setup without the novel technique. Furthermore, they expect that the precision of this method should progressively increase with the quantity of atoms in an atomic clock.
The researchers outline the method, referred to as global phase spectroscopy, in a study released today in the journal Nature. They envision that this clock-stabilizing technique could eventually lead to portable optical atomic clocks that can be transported to different locations for measuring a variety of phenomena.
“With these clocks, researchers are aiming to detect dark matter and dark energy, and test whether there are indeed just four fundamental forces, and even to investigate whether these clocks can predict earthquakes,” states study author Vladan Vuletić, the Lester Wolfe Professor of Physics at MIT. “We believe our methodology can facilitate the transportability and deployability of these clocks wherever needed.”
The paper’s co-authors include Leon Zaporski, Qi Liu, Gustavo Velez, Matthew Radzihovsky, Zeyang Li, Simone Colombo, and Edwin Pedrozo-Peñafiel, all of whom are affiliated with the MIT-Harvard Center for Ultracold Atoms and the MIT Research Laboratory of Electronics.
Precision Ticking
In 2020, Vuletić and his colleagues demonstrated that atomic clocks could achieve greater precision by quantumly entangling the atoms of the clock. Quantum entanglement is a phenomenon in which particles behave in a highly correlated, collective manner. When atoms are quantumly entangled, they distribute any noise, or uncertainty in measuring the atoms’ oscillations, in a way that reveals a clearer, more discernible “tick.”
In their prior research, the team achieved quantum entanglement among hundreds of ytterbium atoms that they first cooled and confined in a cavity formed by two curved mirrors. They directed a laser into the cavity, causing it to bounce thousands of times between the mirrors, interacting with the atoms, and leading the ensemble to entangle. They demonstrated that quantum entanglement could enhance the precision of existing atomic clocks by essentially diminishing the noise, or uncertainty, between the tick rates of the laser and the atoms.
At that time, however, they were constrained by the ticking instability of the clock’s laser. In 2022, the same team developed a method to further amplify the difference in tick rates between the laser and atoms through “time reversal” — a technique that relies on entangling and de-entangling the atoms to enhance the signal acquired in between.
Nonetheless, in that research, the team was still utilizing traditional microwaves, which oscillate at significantly lower frequencies than the optical frequency standards that ytterbium atoms can provide. It was akin to meticulously cleaning a painting only to photograph it with a low-resolution camera.
“When atoms tick 100 trillion times per second, that’s 10,000 times faster than the frequency of microwaves,” Vuletić notes. “At that time, we didn’t know how to apply these methods to higher-frequency optical clocks that are much more challenging to stabilize.”
About Phase
In their latest study, the team discovered a way to apply their previously developed time reversal approach to optical atomic clocks. They then introduced a laser that oscillates near the optical frequency of the entangled atoms.
“The laser ultimately inherits the ticking of the atoms,” states first author Zaporski. “However, for this inheritance to be sustained over time, the laser must be remarkably stable.”
The researchers realized they could enhance the stability of an optical atomic clock by leveraging a phenomenon that had previously been deemed inconsequential to its operation. They found that when light is directed through entangled atoms, this interaction can cause the atoms to elevate in energy and then return to their original energy state while still retaining memory of their journey.
“One might think we’ve achieved nothing,” says Vuletić. “You acquire this global phase of the atoms, which is typically considered trivial. But this global phase holds information about the laser frequency.”
In essence, they recognized that the laser was inducing a measurable alteration in the atoms, despite returning them to the original energy state, and that the extent of this change is dependent on the frequency of the laser.
“Ultimately, we are investigating the difference between the laser frequency and the atomic transition frequency,” clarifies co-author Liu. “When that difference is minimal, it becomes obscured by quantum noise. Our method amplifies this difference above the level of quantum noise.”
In their experiments, the team applied this novel approach and discovered that through entanglement, they could double the precision of their optical atomic clock.
“We observed that we can now resolve nearly twice as small a difference in the optical frequency or clock ticking frequency, without encountering the quantum noise threshold,” Zaporski explains. “Although it’s generally a challenging task to operate atomic clocks, the technical advantages of our method will simplify this, and we believe this can facilitate stable, portable atomic clocks.”
This research received support, in part, from the U.S. Office of Naval Research, the National Science Foundation, the U.S. Defense Advanced Research Projects Agency, the U.S. Department of Energy, the U.S. Office of Science, the National Quantum Information Science Research Centers, and the Quantum Systems Accelerator.
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