atomic clock

Miners took canaries underground not for their songs. The bird would fall off its perch before the miner noticed carbon monoxide. In ultrahigh vacuum, the canary's role is played by a cloud of one hundred thousand lithium or rubidium atoms.

The two states of a strontium atom are like twins running down the same track, but one is slightly higher than the other. Usually, the light used to manipulate atoms pushes one twin harder and the other more weakly. Here, they chose a wavelength of 813 nanometers—'magic'—where both get the same kick.

A quantum particle moves in a circle, passing through several stations. At each station, something happens to it — for example, its properties change slightly. After completing a full circle, the particle returns, but its state is no longer exactly what it was at the start.

The quantum world does not forgive forgetfulness. Throw a stone into a pond — the ripples spread and vanish, the information about the impact dissipates without a trace. In quantum mechanics, everything is different: the evolution of a system is reversible, and the past is never completely erased.

Moiré patterns are familiar to everyone. Two combs with different spacing, placed on top of each other, show dark fringes. Something similar has emerged in time. Physicists took a cloud of Rydberg atoms, particles with unusually large electron orbits, and illuminated it with two lasers.

An orchestra of billions of cesium atoms struck a single note, and it sounded for 17 hours without fading. That’s no exaggeration: Chinese physicists created a continuous time crystal with a record-breaking lifespan. A time crystal is a system that breaks time symmetry, spontaneously pulsating in an unchanging rhythm.

Two synchronized swimmers dive into the sea. One hits calm water, the other a whirlpool. Their movements, perfectly coordinated at the start, diverge. Something similar happens to two quantum particles when one moves in flat space and the other in curved space.

A pendulum that swings on its own, without any winding — in the quantum world, such 'pendulums' are called time crystals. In them, rhythmic motion arises spontaneously and does not decay. But until now, controlling this rhythm was nearly impossible: any intervention threatened to destroy the fragile order.

A pendulum that swings almost half a quadrillion times per second. This isn't mechanics: it's the pulse of light trapped by ytterbium atoms. Instead of gears, there's an optical lattice — a standing wave that holds atoms at its nodes like peas in a pod.

The world's most accurate pendulum isn't a steel rod: it's a tiny atomic nucleus. The isotope thorium-229 has a special excited state: to nudge the nucleus into it, you need surprisingly little energy — about what an ultraviolet laser delivers.

A pendulum that almost nothing can knock off balance (not shaking, not heat, not magnetic fields) because it is hidden in the very heart of the atom. That is the kind of pendulum physicists used when they first switched on a nuclear clock. Previously, the best clocks, optical ones, relied on electron transitions.

The most precise modern clocks count time by the oscillations of electrons in atoms. But their rhythm can be thrown off, like a mechanical watch's spring gets jolted by shaking. Physicists found a more reliable pendulum: the atomic nucleus.

Time crystals are quantum phases of matter that spontaneously oscillate, like a tuning fork with an infinitely decaying echo. If an ordinary spatial crystal repeats an atomic pattern in space, breaking translational symmetry, a time crystal exhibits periodicity in time, breaking time translation symmetry.

A time crystal resembles an eternal clock: its atoms spontaneously perform rhythmic movements. In ordinary clocks, a pendulum is driven by a spring or battery, but here the motion arises without an energy supply — the system simply returns to its initial state at regular intervals. This idea was proposed by Frank Wilczek in 2012.

In a benzene molecule, electrons buzz around the nuclei like a swarm of bees around a hive. But this swarm doesn't just hum: it pulses: every 856 attoseconds, the electron cloud completes a full oscillation cycle. The hive itself, the carbon nuclei, rocks more slowly, with a period of 36 femtoseconds.

In giant accelerators, particles zip through detectors in billionths of a second, and telling one from another requires a stopwatch of incredible precision. The new µRWELL-PICOSEC detector acts as such a stopwatch: it captures the moment a particle passes with a precision of 48 picoseconds: that's 48 trillionths of a second.

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.

A typical quantum bit is destroyed by the slightest disturbance — heat, magnetic fields. Physicists made a pair of atoms work together so that disorder wouldn't erase the record. Information was encoded not in each atom individually, but in the difference of their states — like the tension of a rope between two buoys.

A film of a shattered cup played backward looks absurd: shards don't spontaneously reassemble. This intuitive sense of irreversibility is tied to the increase of entropy — a measure of disorder in a system. The second law of thermodynamics states that the entropy of an isolated system never decreases, setting time's direction from past to future.

Two phones with identical batteries: one charges in a basement with a weak signal, and the process goes slower. The battery percentage here plays the role of a clock hand. Physicists applied the same logic to the quantum world. Their clock is a microscopic battery that stores energy. The charging rate depends on a hidden quantum 'mode'—like a switch.

A regular wall clock runs smoothly, but quantum clocks are a different story. They tick at random moments, and each tick can have a different duration. On average, if you watch such a clock for a long time, it will show the correct time. But at any given moment, its hand might jump forward by an unexpected amount.

A magnetic compass in a test tube sounds like a magic trick, but it's pure physics. In a solution of fumarate (a salt of malic acid), carbon-13 nuclei behave like tiny spinning tops.

Time is like a highway: from afar it looks flawlessly paved, but zoom in and every pothole appears. These bumps are the quantum fluctuations of the vacuum, making it impossible to measure the journey exactly. Physicists built a clock model whose operation relies on spontaneous particle creation from the void.

Conventional atomic clocks are essentially optical counters: they rely on spectroscopy — a method for measuring the frequency of radiation with high precision as electrons jump between energy levels.

We’re used to time flowing only forward. But quantum laws let it become like a coin showing heads and tails at once. Take a particle with its own spin — its intrinsic rotation. In a magnetic field, it spins like a top. Send this particle through a device where two opposing magnetic flows fight each other — and it will start moving into the future and the past at the same time.

On an ordinary optical table, amid lasers and mirrors, a cloud of a hundred strontium-87 atoms begins to live by the laws of curved spacetime. Half a second later, its quantum interference fades: gravitational time dilation destroys the superposition.