These days it's all about optical lattice clocks; they're redefining the way we measure time. Not that like that matters to the average person.
The negative time thing, on the other hand...I'll never understand it, but it sure sounds like something you'd want to understand, so I feel obligated to include it.
Entanglement-enhanced optical lattice clock achieves unprecedented precision
Nov 2025, phys.org
Optical lattice clocks are devices that measure the passing of time via the frequency of light that is absorbed or emitted by laser-cooled atoms trapped in a repeating pattern of light interference known as optical lattice. ... 30,000 strontium atoms trapped in a 2D laser light grid (i.e., an optical lattice); they spin-squeezed two groups of atoms in the lattice, entangling them in a way that boosts the clock's precision.
via JILA National Institute of Standards and Technology and University of Colorado: Y. A. Yang et al, Clock Precision beyond the Standard Quantum Limit at 10−18 Level, Physical Review Letters (2025). DOI: 10.1103/6v93-whwq. https://dx.doi.org/10.1103/6v93-whwq
On arXiv: DOI: 10.48550/arxiv.2505.04538
Image credit: found on the watchmaker website Esslinger
Optical clock sets new accuracy record, bringing us closer to a new definition of the second
Dec 2025, phys.org
The official definition of the second is set to be updated for the first time in decades. The change will be based on new optical clocks, which are far more precise than today's standards.Now, researchers at VTT MIKES have demonstrated a strontium single-ion optical clock with an exceptionally low systematic uncertainty of 7.9×10⁻¹⁹, among the lowest ever reported. Over 10 months, the clock's frequency was measured against International Atomic Time (TAI) with an impressive 84% uptime. The record-setting total uncertainty of this measurement was just 9.8×10⁻¹⁷, limited by the cesium clocks that realize the current definition of the second and calibrate TAI. The study is published in Physical Review Applied.
via Technical Research Centre of Finland: T. Lindvall et al, 88Sr+ optical clock with 7.9 ×10-19 systematic uncertainty and measurement of its absolute frequency with 9.8 ×10-17 uncertainty, Physical Review Applied (2025). DOI: 10.1103/czlf-bfvp
Strontium optical clock accurate to within 1 second over 30 billion years
Mar 2026, phys.org
Stability and uncertainty are both surpassing the 10⁻¹⁹ level, meaning the clock would lose or gain less than one second over roughly 30 billion years. Only a few leading institutions, such as the National Institute of Standards and Technology in the United States and Germany's Physikalisch-Technische Bundesanstalt, had approached this level of precision.
via University of Science and Technology of China: Zhi-Peng Jia et al, Improved systematic evaluation of a strontium optical clock with uncertainty below 1X10-18, Metrologia (2026). DOI: 10.1088/1681-7575/ae449e
Ytterbium atomic clock could open a new window on fundamental physics
Apr 2026, phys.org
We trapped ytterbium atoms in a three-dimensional optical lattice using a 'magic wavelength,' which eliminates the frequency shifts caused by the trapping light.
via Kyoto University: Taiki Ishiyama et al, Orders-of-magnitude improvement in precision spectroscopy of an inner-shell orbital clock transition in neutral ytterbium, Nature Photonics (2026). DOI: 10.1038/s41566-026-01857-8
Bringing quantum time into the lab—a single clock can run young and old at once
Apr 2026, phys.org
Rather than just cooling the atoms, they show that one can instead manipulate the vacuum itself, creating so-called squeezed states in which the position and velocity of the clock exhibit subtle quantum behavior.The result is a new manifestation of relativistic time in the quantum regime, where superpositions and entanglement of time arise: a single clock can measure how it ticks both faster and slower simultaneously, and entangle with the squeezed motion. The team now aims to demonstrate the effects in the laboratory.
via Stevens Institute of Technology, Colorado State University and National Institute of Standards and Technology: Quantum signatures of proper time in optical ion clocks, Physical Review Letters (2026). doi.org/10.1103/qhj9-pc2b
Physicists have measured 'negative time' in the lab
May 2026, phys.org
Our experiment used photons and the against-the-odds journey they must undertake to pass straight through a cloud of rubidium atoms, which means they have a "resonance" with the photons, meaning the energy of the photon can be transferred temporarily to the atoms as an atomic excitation. This allows the photon to "dwell" in the atomic cloud for a time before being released.If the photon does make it straight through, a strange thing happens. Based on the average time when the photon enters the cloud, one can calculate the expected average time it would arrive at the far side of the cloud, assuming it travels at the speed of light (as photons usually do). What one finds is that the photon actually arrives far earlier than that. In fact, it arrives so early it appears to have spent a negative amount of time inside the cloud - to exit, on average, before it enters.This effect has been known for decades and was observed in a 1993 experiment. But physicists had mostly decided not to take this negative time seriously.We fired a weak laser beam - unrelated to the single photon pulse - through the cloud of atoms, and measured small changes in the phase of the beam's light to probe whether the atoms were excited. Any single run of the experiment gives only a very rough indication of whether the photon dwelt in the atoms, but averaging millions of runs yields an accurate dwell time. Amazingly, the result of this weak measurement of dwell time, when the photon goes straight through the cloud, exactly equals the negative time suggested by the photons' average arrival time. Prior to our work, no-one suspected that these two times, measured in entirely different ways, would be equal.
via University of Toronto: Daniela Angulo et al, Experimental Observation of Negative Weak Values for the Time Atoms Spend in the Excited State as a Photon Is Transmitted, Physical Review Letters (2026). DOI: 10.1103/gjfq-k9dv
