ultracold atoms

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.

A quantum lab is like a kitchen where the chef waits a day for the stove to cool down. Superconducting qubits only work at temperatures of a few hundredths of a kelvin, and each 'cool, measure, warm up' cycle eats a whole day. A team of engineers built a fridge the size of a melon, weighing 3 kg, that does the job in just two hours.

A billiard ball hangs in midair. Tiny peas shoot at it from a blowgun. Each hit is barely perceptible, but if the ball is very light and barely trembles from heat, you can make out individual nudges. That's essentially the setup: a glass sphere the size of a bacterium levitates in a vacuum while a narrow beam of xenon atoms flies at it.

If you push water in a narrow channel too fast, a steep wave forms and breaks into foam. In the world of ultracold atoms, cooled to near absolute zero, quantum laws prevent such a wave from breaking. Physicists took a cloud of rubidium atoms stretched into a thin thread and sharply compressed it from one end with a movable wall of light.

In an ultracold cloud of atoms cooled to near absolute zero, regions with different spin orientations emerge, like patches with different magnetization. The boundaries between them are called domain walls. Usually they are stationary, but in this experiment the walls started moving on their own.

A radio receiver the size of a palm can measure signal power more accurately than expensive lab benches. Inside, there are no coils or antennas, just a glass head and a cloud of rubidium atoms cooled to near absolute zero.

In the far reaches of space, there exists a hypothetical object: an ultracold black hole. It is charged and resides in an expanding universe (de Sitter space). Its temperature hovers just above absolute zero — about one trillionth of a degree above it.

At temperatures billionths of a degree above absolute zero, a Bose–Einstein condensate emerges — a state where thousands of atoms march in a single quantum key.

Dark matter is the invisible scaffold that holds galaxies together. But how did it come to be? The standard answer: particles of dark matter once stopped colliding and "froze out" in the amount we see today. However, there's a trickier path — UFO, or ultra-fast freeze-out. Here, everything is like coffee that cools down and then heats up again as you carry it through a cold corridor.

Imagine a skyscraper assembled not from individual bricks but from entire studio apartments. That's exactly how the Au₆Te₁₂Se₈ crystal is built: its nodes are not atoms but superatomic clusters of gold and chalcogens that behave as single quantum objects.

The vacuum is not as empty as it seems. If an observer moves with acceleration, the vacuum around them heats up: this was predicted by William Unruh back in the 1970s. The effect is akin to black hole radiation, but in flat space.

At the center of many galaxies lurks an invisible halo of dark matter: it holds the stars at the edges, keeping them from flying apart. For a long time, it was described as a dilute gas of particles, but new work offers a different view: dark matter might gather into a quantum droplet.

The universe is like a layered cake, and our layer (stars, planets, and people) is just one of many. In a neighboring layer, which physicists call the hidden sector, processes are boiling away. This work describes one such process: a first-order phase transition, where matter abruptly changes state, like water turning into ice.

A pot of water always boils the same way: hit 100 degrees and bubbles start. Quantum fluids are different. When a cloud of atoms is cooled to near absolute zero, it can turn into a single matter wave — a Bose–Einstein condensate. But the transition itself, it turns out, can happen in three different ways.

An ordinary black hole is like a stove that heats up more as its mass shrinks. A tiny hole should glow at unimaginable temperatures and evaporate in an instant. But that scenario ignores quantum corrections — what happens when the very fabric of spacetime begins to tremble.

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.

Strike a bell, and it responds with its own note. Likewise, a black hole disturbed by the fall of a star or another black hole begins to 'ring,' emitting gravitational waves. But instead of sound, it's the trembling of space itself, and instead of a single note, a whole set of decaying frequencies. These are called quasinormal modes.

A pot of water has just been stirred: the whirlpools and eddies gradually calm down, and the liquid becomes smooth. A similar process, but in the quantum world, occurs in superfluid helium or in a cloud of ultracold atoms.

An orchestra where every musician plays their own part, and the conductor tries to catch the overall melody. That's roughly how physicists view electrons in cuprates — ceramic materials that conduct electricity without resistance at temperatures above liquid nitrogen.

A cold cloud of atoms is like a swarm of fireflies drawn to one another. The attraction between atoms tries to pack them into a single dense ball. But quantum jitter, the unavoidable motion of particles, keeps the ball from collapsing. When the attraction is strong, the cloud breaks into separate clumps.

A superconductor is a material through which current flows without loss, like water through a perfectly smooth pipe. But to make the pipe perfect, you have to cool it to very low temperatures. Physicists have long been looking for ways to make such a pipe at warmer temperatures, and one clue is the quantum critical point.

Imagine trying to examine a snowflake without letting it melt or get damaged. That's roughly the challenge facing scientists who want to study individual proteins — the tiny molecular machines of life. In solution, a protein constantly jiggles and clumps with its neighbors, and in an electron microscope, the beam destroys it.

An ordinary aluminum pot on a stove doesn't emit molecules you can catch with light. But if you take aluminum and an aluminum salt, heat them together in a small oven, you get a stream of aluminum fluoride molecules, like steam from a kettle's spout, except instead of water droplets, tiny particles fly out.

Water in a cup cools down in minutes. Gas in a room spreads evenly in seconds. But if you trap atoms in a tube as thin as a laser beam and give them a good kick, they will take several seconds to "cool" toward equilibrium — an eternity by atomic standards.

In helium cooled to near absolute zero, the liquid stops rubbing against itself and begins to flow without any resistance.

Atoms settled along a chain of traps, like a line of coastal forts. Nearby, a quantum sea: a Bose-Einstein condensate (a substance with zero resistance, where particles move as a single wave).

Ordinary window glass may seem frozen forever, but inside, atoms never stay still. In amorphous germanium telluride (GeTe)—a material used in rewritable discs and non-volatile memory — physicists have for the first time directly tracked how atomic bonds stretch and bend in trillionths of a second.

Water in a pot, when heated, doesn't boil: it turns into ice. That's roughly how ultracold atoms behave in an optical lattice — a light-based crystalline grid that holds atoms in place. Scientists simulated this system on a computer to understand how order changes as temperature rises.

Ultracold molecules are a promising material for quantum computing, but assembling them into a neat array, like eggs in a carton, is no easy feat. A method has been proposed to load molecules one by one into optical tweezers or lattices directly from a gas. The molecules immediately end up in the calmest state of motion.

A tiny cloud of 300,000 ytterbium atoms, cooled to near absolute zero, was shuttled along a light track 34 centimeters long, like transporting a fragile cargo over a bumpy road without shaking it apart.

A layer of electrons trapped between two semiconductor plates lines up into a neat lattice at low temperatures, like beads on a string stretched between your fingers. As the temperature rises, order collapses: the crystal melts. But exactly how it melts has long been a puzzle.

In the macroscopic world we know, any motion inevitably decays: friction, a dissipative process that scatters energy, intervenes. But quantum reality has surprises in store.

Light from tiny perovskite crystals is like a choir performance, where voices sound sometimes solo, sometimes in a tight duet. Previously, such a duet could only be heard under extreme cooling, nearly to the chill of outer space. Now the choir sings at room temperature.

Light and matter in the tiny world obey laws that are hard to compute even on a supercomputer. Physicists build special "quantum simulators"—setups where cold atoms play the roles of particles and fields. But until now, such simulators could only depict the field in its simplest form: as if it were either present or absent.

Since Vera Rubin first convincingly showed that galaxies are surrounded by invisible matter, the quest to understand the nature of dark matter has remained one of physics' greatest challenges.

Cooling usually brings order: water freezes into ice. But in the quantum world, order takes unexpected forms. Experimenters cooled polaritons — particles that are both light and matter — to near absolute zero.

Ice cubes in a glass melt, and the water mixes on its own. Many quantum systems behave this way: particles exchange energy, and after a while, order disappears. But if you add a spoonful of honey to the glass, mixing slows down, and in intense cold, the water freezes into glass — without a crystal lattice, but also without flow.

A qubit, the heart of a quantum computer, is like a soap bubble: the slightest puff and it bursts. On Earth, gravity pulls bubbles down, creating vibrations, and the thermal hustle of atoms adds jitter. Entropy, the measure of this chaos, destroys quantum information.

Ultracold atoms can act as one. When a neutrino is born inside such a cloud, it usually slips away unnoticed.

Two black holes circle in a tight dance, and around them something remarkable is born: a gravitational molecule. A cloud of ultralight bosons, particles with masses below an electronvolt, envelops the pair like the electron cloud in an H2+ ion.