A clock made from one atom

Every clock counts something that repeats. This one counts the light that a single aluminium ion absorbs, 1.121 quadrillion cycles every second.

0cycles of the clock laser since you opened this page

The instrument simulated here is the ²⁷Al⁺ quantum-logic clock at NIST. In 2025 it set the record for the lowest systematic uncertainty of any clock: $5.5\times10^{-19}$ in fractional frequency, or one second in about 58 billion years.

Three parts make any such clock work:

A flywheel. An ultrastable laser at 267.4 nm that ticks on its own, and slowly wanders.
A reference. One ²⁷Al⁺ ion, whose ${}^1S_0\leftrightarrow{}^3P_0$ resonance is set by nature and barely reacts to its surroundings.
A counter. A frequency comb that divides the optical cycles down to something electronics can count.

The trouble is that Al⁺ can't be laser cooled or read out directly: its strong transition lies at 167 nm, deep in the vacuum ultraviolet. So a second ion, ²⁵Mg⁺, sits beside it. The two share their motion, and quantum logic carries the answer from one to the other. That is what makes this a quantum clock.

Each step that follows runs live, with real parameters taken from the published experiments. Drag to orbit the trap, scroll to zoom. The scene is true to scale, from the titanium endcaps 4 mm away to the ions 3.9 µm apart.