Atomic clocks are pendulums that tick at the frequency of light. Inside a trap, an aluminum ion hangs, and a laser probes it like a tuning fork. If the laser's frequency matches the atom's natural frequency, the atom responds. The clock's rate is checked against this response.
For a long time, accuracy calculations were done as if the atom had only two states: ground and excited. In reality, there are more levels, and an excited atom can decay not into one but into several different states. The authors of the work took this branching into account. It turned out that if you track exactly where the atom falls after decay and discard unsuccessful measurements, the clock becomes noticeably more accurate — a gain of up to 4.5 decibels. It's like a marksman not counting shots when his hand trembled.
This approach is especially useful for ion clocks, which are already hitting the limit set by the excited state's lifetime. The scientists proposed a specific measurement protocol for clocks based on an aluminum-27 ion and calculated new stability limits. Now the accuracy is limited not by our blindness to decays, but by the very nature of the atom.
A clock based on a single aluminum-27 ion would lose only one second in 33 billion years — twice the age of the Universe.