A research team at the National Institute of Standards and Technology (NIST) has recently built the world's most accurate clock. Powered by a single aluminum ion at its core, the new optical atomic clock enables extremely precise time measurements, with a fractional frequency uncertainty as low as 5.5 × 10⁻¹⁹—meaning it takes one second faster or slower than the age of the universe. At the same time, the clock’s fractional frequency stability reaches 3.5 × 10⁻¹⁶/√τ seconds, which is 2.6 times higher than other current ion clocks.
(L-R) Mason Marshall, David Hume, Willa Arthur-Dvorak, and Daniel Rodriguez-Castillo stand in front of the National Institute of Standards and Technology's aluminum ion atomic clock. After recent improvements, the atomic clock will not only pave the way for redefining the second, but also enable new explorations in physics.
Optical atomic clocks are judged on accuracy (how close they are to "true" time) and stability (consistency of measurements). The achievement of this record is due to the team's 20 years of continuous research and development and optimization of the laser, ion trap and vacuum cavity of the aluminum ion atomic clock. "It's exciting to be involved in the development of the most accurate clocks," said NIST researcher Mason Marshall, first author of the paper.
The clock is based on quantum logic spectroscopy measurements of a single ²⁷Al⁺ ion, with a ²⁵Mg⁺ ion trapped in it to assist with “homomorphic cooling” and reading the state of the aluminum ion. The "beating" of aluminum is extremely stable and has minimal impact on temperature and magnetic field. It is very suitable for time measurement, but laser control is difficult. Magnesium ions are easier to control, so they are used to aid cooling and allow researchers to indirectly read the signal from aluminum ions.
Important innovations by the research team include extending the Rabi detection time to 1 second, which is achieved by stably transmitting laser light from an ultra-cold silicon cavity remotely from the JILA laboratory to the NIST team laboratory (3.6 kilometers away). This technology reduces clock instability by about one-third compared to previous aluminum-ion clocks.
National Institute of Standards and Technology (NIST) physicist David Hume holds a newly modified ion trap for an aluminum ion clock. By modifying the trap, aluminum ions and their magnesium ion partner particles can continue to tick away undisturbed.
In addition, the team also made a new design of the ion trap to reduce redundant micro-movements (such tiny unexpected movements will affect timing accuracy); they selected thicker diamond wafers and adjusted the metal coating of the electrodes to correct the electric field imbalance. The vacuum chamber has also been upgraded to titanium alloy, and the amount of background hydrogen has been reduced by 150 times, which greatly extends the "holding" time of ions in the device and reduces the collision error of hydrogen molecules.
The team also performed sensitivity measurements of the direction of the AC magnetic field in the radio frequency trap, eliminating the uncertainty caused by the orientation of the magnetic field.
A new improved ion trap for the National Institute of Standards and Technology (NIST) aluminum ion clock. Inset shows a CCD image of an aluminum-magnesium ion pair. The circle marks the location of the aluminum ion - this ion is dark to the camera because it can only be read out via quantum logic spectroscopy via magnesium ions.
Various innovations allow the clock to achieve 19 decimal places of accuracy in about 36 hours, instead of three weeks in the past. NIST graduate student Willa Arthur-Dworschack said: "With this platform, we will explore new structures of multi-ion clocks and even entangled ions to further improve measurement capabilities."
This breakthrough is expected to redefine the length of the "second" with higher precision and open up new prospects in the fields of earth science and basic physics, including basic scientific questions such as verifying whether the constants of nature are truly constant.