Quantum Gases cond-mat.quant-gas

In a dark room, a laser beam draws a bright spot on the wall. Light seems warm and uncontrollable, but for three decades physicists have used it as a conductor's baton, making atoms dance in unison.

A drop of ink in a glass of water quickly spreads, tinting the whole liquid. Similarly, electrons in a metal move freely, like molecules in a gas. But when a magnetic impurity, an atom with its own magnetic moment, enters this medium, something strange happens.

A cloud of 41K87Rb molecules held in an optical trap expands like fog at dawn. Each molecule is a weakly bound pair of potassium and rubidium atoms, teetering on the brink of dissociation. Cooling such a cloud to near absolute zero without shattering the molecules has long been a challenge.

Two actors on the same stage. One is a fermion, finicky and demanding, hard to make follow the rules of a quantum simulator. The other is a boson, easygoing and undemanding, ready to take any role. The director hopes the audience won't notice the substitution.

In the first moments after the Big Bang, the Universe underwent rapid expansion: inflation. But perhaps it was contracting before that, and then bounced. Both scenarios leave nearly identical imprints on the cosmic microwave background, and cosmologists have debated for decades which one is correct.

In 1867, James Clerk Maxwell imagined a demon that sorts gas molecules and thereby violates the second law of thermodynamics. The demon needs to know the speeds of the particles. In the quantum world, a similar task arises: keep entanglement from growing like an avalanche, but you cannot observe the entire system.

In the lab, time usually ticks on the wall: hands, digits, a frequency standard. But for the Universe, there are no external clocks. The Wheeler–DeWitt equation, the main law of quantum gravity, contains no time at all, and all fundamental theories lack an inbuilt arrow of time.

A stretched string between two quarks doesn't pop instantly like a rubber band. It first vibrates, develops knots, and only then breaks, if it breaks at all. This isn't just a metaphor; it's an accurate description of what physicists call confinement: quarks are permanently locked inside hadrons, and the only way to separate them is to snap the flux tube that binds them.

Physicists used a quantum simulator — a device that mimics the behavior of charged particles, to study their collisions at enormous energies.

According to Einstein's theory, Earth's rotation drags space-time itself along — like a spoon dragging thick honey. This effect is incredibly weak: about one revolution in billions of years. But it can be caught.

The Penrose tiling, assembled from two types of tiles, covers a plane without a repeating pattern — a mathematical marvel discovered by Roger Penrose that has found physical embodiment in the quantum world. Researchers modeled a chain of particles interacting according to a rule reminiscent of Fibonacci numbers and the golden ratio.

Everything tends toward equilibrium: tea cools, a pendulum swings to rest. The measure of this movement is entropy — growing disorder. But sometimes hot water freezes before warm. The Mpemba effect isn't just a curiosity: it shows up in the quantum world too. Scientists have devised a way to control this paradox.

Atoms, like strands in a braid, follow a strict rule: if one strand is 'charged', its neighbors must stay silent. This phenomenon, Rydberg blockade, allows stable quantum patterns to be woven. In such patterns, topological order emerges: a structure where the overall weaving pattern matters more than individual loops.

The law discovered by Wolfgang Pauli forbids identical particles from occupying the same spot. Hence, in cold matter, a Fermi sea is born: particles, like water, fill energy levels from bottom to top. The shape of the sea's surface — including 'islands' of emptiness — dictates the properties of the whole system.

Emptiness isn't always empty: it can hide energy, like supercooled water hides ice. Physicists call this false vacuum — a state that only needs a nudge to cascade into the true one.

In superfluid helium, a state where liquid flows without friction, quantized vortices spontaneously arise: microscopic funnels resembling whirlpools in ordinary Water, but governed by quantum mechanics.

A supersolid is ice that flows. Think of a pond with a frozen crust, but beneath it, a liquid depth without friction. Usually, creating that crust means changing the liquid's properties throughout its entire volume.

Time crystals pulse in time, like a perpetual metronome that never needs winding. The idea was proposed by Frank Wilczek almost ten years ago. Previously, to keep such a metronome from losing its beat, disorder was deliberately introduced into the system — a paradox, but chaos helped the rhythm.

Atoms have been caught in a ring of light. This is not a metaphor: laser beams form a ring-shaped trap in which atoms move in circles. The light not only holds them but also couples the atom's rotation to its internal 'compass'—the spin. As a result, an atom running around the ring senses which way it's going, and this changes its internal state. This coupling is not just elegant.

An ultracold atomic cloud, resembling fog, hints that gravity might not work as Newton said. On the outskirts of galaxies stars move faster than expected, and MOND theory tweaks the law of gravitation over large distances, removing the need for dark matter.

Our Universe may have arisen from a highly unstable state that physicists call the false vacuum. Imagine a glass of water cooled below its freezing point: the liquid remains liquid only until the first jolt, which instantly triggers crystallization into ice.

Two particles cling to each other like climbers on the same rope and fly toward a wall. When they hit, will they bounce off together, or will the rope snap? What does it depend on? Physicists considered the simplest case: everything happens on a single line, the particles attract only when they occupy the same point, and the wall is perfectly rigid.

Inside a tiny mirrored cavity, smaller than the thickness of a hair, light is trapped and bounces off the walls many times. If you place a material that can absorb and emit light inside, the light particles (photons) start to 'dress' in excitations of the material — this is how polariton modes are born.

Ordinary light is a stream of independent particles: photons fly through each other without noticing their neighbors. But in superconducting circuits cooled to near absolute zero, the situation changes. There, photons travel along narrow metal tracks, and if tiny elements called Josephson junctions are embedded in these tracks, light begins to 'feel' itself.

In a cold cloud of sodium and rubidium atoms, physicists have for the first time assembled molecules that rotate like spinning tops. Usually, in ultracold experiments, atoms pair up without rotation, like two people holding hands and standing still. Here, the pairs got an extra kick: they spin around a common axis, and this rotation changes their behavior.

Toss a handful of peas into a pot of semolina porridge — the peas quickly sink, pushing the grains aside. But if you make the porridge very thick and sticky, the peas get stuck. That's roughly how light lithium atoms behaved in a cloud of heavy potassium atoms at temperatures just a few millionths of a degree above absolute zero.

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.

A quantum computer is like an orchestra where each musician plays their own part. With few instruments, the conductor can manage each one separately. But when there are thousands of musicians, individual commands become overwhelming.

Physicists built a chain of Rydberg atoms — atoms with electrons in enormous orbits, comparable to planetary paths. These giant atoms feel their neighbors intensely, turning the chain into a quantum analog of a string.

A vortex in a superfluid gas is like a whirlpool in a sink: the fluid swirls around an empty core. If you spin such a vortex three times harder than usual, it won't hold: it breaks into three separate whirlpools, much like a thick stream of water splits into droplets.

Fourteen rubidium atoms lined up like beads on a string. Each atom is a tiny magnet that can point up or down. Together they follow strict rules, similar to a train schedule: everything is predictable, everything repeats. But introduce a barely noticeable change, and the schedule goes off the rails.

A rubber band is stretched, and suddenly snaps. In the atomic world, a similar picture emerges: the string binding particles can break. In a new study, physicists took a chain of Rydberg atoms and tracked how such a string breaks. Under weak tension, the string breaks completely, and the system splits into several independent pieces, as if a rubber band shattered into segments.

A black hole is a place from which even light cannot escape. But quantum theory says it still glows faintly. This glow is called Hawking radiation. In space, it's impossible to spot: it's too faint. So physicists build models of black holes from ordinary materials.

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.

In an ultracold cloud, potassium atoms behave like waves that can add up and amplify each other — much like sound waves in a musical instrument create a loud chord. Physicists have learned to control this process by tuning the strength of atomic interactions with a magnetic field.

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.

Ordinary tiles form a repeating pattern. But atoms can do more: under certain conditions, they themselves lay out a complex mosaic where the pattern never repeats, like on antique tile panels.

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.

Physicists took 114 rubidium atoms, cooled them to near absolute zero (as in experiments with liquid helium), and neatly arranged them in a honeycomb pattern. They then smoothly adjusted the interaction strength, as if slowly turning up the heat under a pot.

A tiny cloud of atoms, cooled to millionths of a degree above absolute zero, floats in a ring-shaped trap. It looks like a glowing necklace, but physicists have made it live on the surface of an invisible bagel. Using laser beams and microwave pulses, three internal states of the atoms were linked into a cycle.