Level 1 - Absolute Beginner

A clock tells us what time it is. Scientists make very exact clocks to measure time very carefully. These clocks are called atomic clocks.

Now scientists have made a new kind of clock called a nuclear clock. It uses a special material called thorium. Two teams of scientists made this clock at the same time but in different countries.

This new clock is very, very accurate. It will only be wrong by one second every three million years. That is much better than older clocks.

This clock can help us in many ways. It can make GPS navigation more exact. Scientists also hope to use it to find dark matter, which is something invisible in space that we do not fully understand yet.

clock

a device that measures and shows time

accurate

correct and without mistakes or errors

atomic clock

a very precise clock that uses the vibrations of atoms to measure time

nuclear clock

a new type of very precise clock that uses energy changes inside the centre of an atom

thorium

a radioactive element found in nature, used in the new nuclear clock

GPS

a satellite system that tells you your location anywhere on Earth

dark matter

an invisible substance in space that scientists cannot see directly but know exists through its effects

scientist

a person who studies the natural world through careful experiments and observation

Level 2 - Beginner

Scientists at two universities, one in China and one in Austria, have independently built the world's first nuclear clocks. Both teams used thorium-229, a radioactive isotope, embedded in crystals of calcium fluoride.

A nuclear clock works by probing energy transitions inside the nucleus of an atom, rather than the electrons orbiting it. These transitions are extremely stable, making the clock far more precise than today's best atomic clocks.

The two teams used special ultraviolet lasers with a wavelength near 148 nanometres to trigger and measure these tiny nuclear transitions. The results suggest the clocks will drift by only about one second every three million years.

Researchers say nuclear clocks could improve GPS accuracy, help detect dark matter, and allow precise tests of the fundamental constants of physics, which are the fixed numbers that describe how the universe works.

isotope

a version of a chemical element with a different number of neutrons in its nucleus

nucleus

the dense central part of an atom, made up of protons and neutrons

electron

a tiny particle with a negative charge that orbits the nucleus of an atom

transition

a change from one energy level to another inside an atom or nucleus

ultraviolet laser

a laser that produces light at wavelengths shorter than visible light, making it very energetic

nanometre

a unit of length equal to one billionth of a metre, used to measure wavelengths of light

fundamental constants

fixed numbers in physics, such as the speed of light, that describe how the universe works

drift

a slow, gradual error that builds up in a measuring instrument over time

Level 3 - Intermediate

Two independent research groups, one at Tsinghua University in Beijing and one at the Vienna Center for Quantum Science and Technology, published findings in June 2026 demonstrating the world's first operational nuclear clocks, both exploiting the uniquely low-energy nuclear transition of thorium-229.

Unlike conventional atomic clocks, which lock onto electronic transitions in the outer shells of atoms, a nuclear clock interrogates the energy levels of the atomic nucleus itself. The nucleus is far better shielded from environmental interference such as electromagnetic fields, making nuclear transitions exceptionally stable and reproducible.

Both teams drove the thorium-229 transition using vacuum-ultraviolet lasers tuned to approximately 148 nanometres, a challenging wavelength to generate and sustain. By embedding thorium ions in calcium fluoride crystals, the researchers stabilised the nuclear transition frequency and reduced sensitivity to collisions and heating.

The clocks are projected to drift by roughly one second per three million years. Beyond timekeeping, physicists are excited by the possibility of using nuclear clocks to search for ultralight dark matter and to detect tiny variations in the fine-structure constant, one of nature's dimensionless numbers whose stability underpins the entire standard model of particle physics.

operational

ready to use and working as intended

electromagnetic fields

regions of space where electric and magnetic forces act on charged particles

vacuum-ultraviolet

a range of ultraviolet light with very short wavelengths that is absorbed by air and requires a vacuum or special environment

fine-structure constant

a fundamental physical constant, approximately 1/137, that measures the strength of electromagnetic interactions

dimensionless number

a quantity with no physical units that describes a ratio or fundamental relationship

standard model

the theoretical framework in physics that describes the fundamental particles and forces in the universe

stabilised

made steady and prevented from changing or varying

reproducible

able to be repeated with the same result each time, an essential quality in scientific measurements

Level 4 - Advanced

June 2026 brought simultaneous publication of nuclear clock demonstrations from Tsinghua University and the Vienna Center for Quantum Science and Technology, both leveraging the M1 nuclear isomeric transition of thorium-229 at approximately 8.356 electronvolts -- a transition whose anomalously low energy places it within reach of laser excitation, a property unique among all known nuclides.

The thorium-229 nucleus, doped into a calcium fluoride crystal matrix, provides a host lattice that isolates the thorium ions from phonon noise and electromagnetic perturbations while maintaining optical transparency in the vacuum-ultraviolet regime. Both groups drove the transition using purpose-built 148-nanometre coherent sources, requiring frequency-quadrupled or high-harmonic-generation stages from near-infrared seed lasers, a technically formidable challenge that defined the barrier to entry for decades.

Projected fractional frequency instability falls below parts in 10^19, eclipsing the best optical lattice clocks -- themselves accurate to one second in roughly 15 billion years -- by another order of magnitude. This level of precision opens the door to geodetic measurements sensitive to centimetre-scale elevation differences via gravitational redshift, portable dark matter detectors capable of sensing oscillatory signatures from ultralight axion fields, and laboratory tests of whether alpha, the fine-structure constant, or the proton-to-electron mass ratio have drifted over geophysical timescales.

Beyond metrology, the thorium nuclear clock constitutes a first step toward controlling nuclear degrees of freedom with optical photons, a capability that could eventually enable nuclear quantum computing and laser-driven nuclear reactions. The independent simultaneous demonstrations eliminate the concern that the Tsinghua or Vienna results were artefacts, elevating the nuclear clock from a theoretical curiosity to an experimental certainty and formally opening a new chapter in precision physics.

isomeric transition

the decay of an excited nuclear isomer to a lower energy state, releasing energy in the process

electronvolt

a unit of energy used in atomic and particle physics; 1 eV is the energy gained by an electron moving through 1 volt of potential

phonon noise

random thermal vibrations in a crystal lattice that disturb quantum measurements

fractional frequency instability

a measure of how much a clock's frequency varies relative to its nominal value; lower is more precise

gravitational redshift

the effect by which clocks at lower gravitational potential tick more slowly, predicted by general relativity

axion

a hypothetical ultralight particle proposed as a dark matter candidate

fine-structure constant (alpha)

the dimensionless coupling constant characterising the strength of electromagnetic interaction, approximately 1/137.036