liquid xenon

In an underground laboratory in South Dakota, the LUX-ZEPLIN detector, filled with liquid xenon, waits for rare collisions. Usually, dark matter particles pass through it without a trace, but one day a xenon nucleus was displaced with an energy of 248 keV.

In a tank holding three tons of liquid xenon, buried deep underground, something stirred one day. Not the xenon itself, but one of its atomic nuclei: it received a kick with an energy of 248 thousand electronvolts and recoiled like a billiard ball struck by an invisible cue ball.

Inside a microscope that sees individual atoms, it's cold and silent: the temperature is around -268°C, and the interior is in perfect vacuum. To place individual iron atoms on such a stage, you usually need a direct 'corridor' from the evaporator to the sample. But in the coldest instruments, that corridor is blocked by thermal shields, making deposition a puzzle.

A laser beam strikes a thin target hidden inside layers of another material. Within billionths of a second, the material is compressed to a density of 25 grams per cubic centimeter, as if a piece of lead became more than twice as heavy without moving an inch.

No one has ever seen dark matter, but it should pass through detectors like wind through leaves. Sometimes, very rarely, a dark matter particle bumps into an atomic nucleus, and then the detector must catch that jolt. The trouble is that light dark matter particles can hardly move heavy nuclei: it's like trying to knock down a bowling pin by throwing a feather at it.

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.

Inside the Sun, reactions can produce light, almost undetectable particles called axions. One such reaction yields axions with an energy of 5.49 MeV, as if each one carried the energy of a tennis ball moving at 100 km/h. These particles pass freely through Earth, but occasionally they might collide with a xenon atom.

A sodium iodide crystal doped with thallium is like photographic film for invisible particles. When a gamma ray or a hypothetical dark matter particle hits it, it flashes with faint light. Two silicon photomultipliers pressed against the crystal's ends catch the flash from both sides.

The five-hundred-meter FAST radio telescope picked up a signal at the 21-centimeter wavelength — the telltale glow of cold hydrogen. The signal came from a cloud named Cloud-9. It hangs in space near the galaxy M94, but it contains not a single star.

In a two-dimensional crystal of tungsten diselenide (WSe₂), light creates pairs — an electron and a hole that stick together. Such pairs are called excitons. Among them are "dark" ones: they barely emit, but they set the tone at low temperatures.

Solar neutrinos are tiny particles born in the Sun's core by the trillions every second. They pass through Earth almost without noticing it, and for a long time they were considered absolutely stable. But perhaps they too can "age"—decaying imperceptibly, vanishing on their way to us.

At nine in the morning, the DarkSide-50 detector, 1400 meters beneath Gran Sasso, records a weak ionization pulse. By eleven at night, the frequency of such events drops noticeably. This is not an electronics glitch or a seasonal background variation.

Quantum technology urgently needs a universal 'translator' between the languages of microwaves and light. Superconducting qubits communicate on centimeter wavelengths, while photons in optical fiber talk at optical frequencies.

Dark matter leaves no trace in ordinary detectors: it passes through them like wind through gauze. To spot it, you need instruments that can feel a mosquito's punch in a storm. Quantum sensors with a threshold of a fraction of an electronvolt (a thousand times less than the energy of visible light) are just that.

A laser isn't just about light. If you make a particle oscillate like a pendulum and those oscillations sustain themselves, you get a phonon laser. Sounds complicated, but the idea is simple: instead of light waves, the device emits orderly waves of mechanical vibrations — phonons.

Dark matter detectors are essentially incredibly sensitive microphones, capable of hearing a whisper amid the roar of a rock concert. Their job is to catch the rare nudges from hypothetical dark matter particles, which are thought to occasionally collide with atomic nuclei of ordinary matter.

Every second, about a hundred invisible particles, muons, pass through your body. They are born high in the sky when cosmic rays collide with air atoms, raining down on Earth. These particles are so penetrating that they can pass through a multi-story building, but they slightly change direction when they encounter dense material, such as lead or uranium.

The story began with a puzzle: in the 1930s, Fritz Zwicky noticed that clusters of galaxies behave as if an enormous invisible mass is hidden within them. Half a century later, Vera Rubin confirmed that stars on the outskirts of galaxies rotate too fast to be held by visible matter alone.

In a dark room, a glass bead the size of a dust grain hangs on a laser beam. Beneath it are two mirrors: one aluminum, one silver. If axions (light, invisible particles that could make up the mysterious dark matter) whiz by, they exchange with the mirror atoms, and the bead shifts ever so slightly.

Dark matter has eluded direct observation so far, even though its gravity holds galaxies together. One hypothesis: it’s made of axions — ultra-light particles predicted by theory.

A detector filled with superfluid helium-4 is like a perfectly still lake. The liquid doesn't slosh or bubble: it's frozen in a single quantum state. An invisible guest, a dark matter particle, flies through it. It doesn't shine or leave a track. But sometimes it nudges a helium nucleus.

A neutron star is the dead core of a massive star, crushed to the size of a city. A teaspoon of its material weighs as much as a mountain. Dark matter particles fly through such a dense medium almost unimpeded, but occasionally they do hit neutrons and get stuck, like bullets bogging down in deep sand.

A tiny glass bead hangs in a laser beam like a speck of dust in sunlight. Around it is a rarefied gas: atoms of krypton, xenon, and heavy molecules of sulfur hexafluoride. They collide with the bead, and each hit transfers a tiny impulse, like a pea thrown at a taut sheet makes it tremble. Impulse is a measure of motion: mass times velocity.

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 silicon wafer as thin as half a human hair catches a passing particle the way a palm catches a raindrop. But if that drop is a proton accelerated to enormous energy, the palm can get burned. Inside the detector, an avalanche of charge is born, and sometimes it's enough to burn a microscopic hole. This is called single-event burnout.

In a dark room, an invisible lamp flashes. Its light is unlike anything familiar: it has no steady brightness, only rare but powerful bursts. This is bright squeezed vacuum — a state of light with no ordinary wave but strong quantum fluctuations. It is with such light that physicists bombarded xenon atoms.

Deep beneath the Apennine Mountains, in a tank holding three tons of liquid xenon, something gives a barely perceptible shudder every few days. It's a solar neutrino, a ghostly particle born in the Sun's thermonuclear furnace, that has traveled 150 million kilometers through empty space and solid rock to finally collide with a xenon nucleus.

A metal needle descends onto a piece of superconductor. All of this happens inside a refrigerator that cools the sample to 30 thousandths of a degree above absolute zero — a hundred times colder than outer space. A weak current is passed through the needle, and by how electrons travel through the tiny contact point, physicists read what is happening inside the material.

An ordinary antenna picks up radio waves, but there are fields it barely feels. We're talking about slow electric fields: those that change once a second or less. For them, physicists built a sensor from a beam of rubidium atoms.

Underground, in a former gold mine, sits a tank of liquid xenon. If a dark matter particle flies through it, the xenon flashes. But it also flashes from ordinary particles, like neutrinos or leftover radioactivity. The task is to tell which flash comes from dark matter and which from noise.

The camera that hunts for tumors works like a highly sensitive camera: it catches the faint glow from a substance that accumulates in diseased cells. In conventional machines, the light is born in a crystal — a solid piece that resembles glass. But a crystal doesn't always convey exactly where the light flashed, so the image comes out slightly blurry.

In the kitchen, two flashlight beams pass through each other without noticing. Light barely interacts with light. But if you accelerate particles to enormous energies, two photons can collide and give birth to something new.

A pot of boiling water sits on the stove: the higher the heat, the hotter the water and the more steam escapes. Hot gas in galaxy clusters behaves much the same way. But in small galaxy groups, this "steam" leaks out faster than you'd expect from the temperature alone.

In a dark tank holding thirty tons of water, the liquid glows when a ghostly particle streaks through. Neutrinos barely interact with matter, but occasionally they bump into atoms, and a flash is born. To catch it, a special substance, a liquid scintillator, is added to the water to amplify the light.

In a dark mine under the Italian Alps, a tank holds three tons of liquid xenon. When a particle hits it, it knocks faint flashes of light and electron clouds out of the atoms. Following these tracks, physicists hope to see an ultra-rare decay: a xenon-136 nucleus turning into barium-136, emitting two electrons and no neutrinos.

Deep in a Canadian mine, there's a barrel holding 3269 kilograms of liquid argon — a clear fluid that looks like very cold water. This barrel is the DEAP-3600 detector, and its job is to catch dark matter particles, which are thought to permeate the universe but barely interact with ordinary matter.

In deep cold, at a temperature just two degrees above absolute zero, a copper cavity the size of a saucepan listens to the silence of the universe.

When treating epilepsy, doctors adjust the drug dose almost blindly. Too little, and seizures continue; too much, and side effects appear. Usually, a patient's blood is sent to a lab, and the result comes back a day or two later. Now, the same test can be done on a tiny silicon chip. The sensor works like a highly sensitive nose.

A dark matter detector is like a room where you're trying to hear a whisper while static electricity crackles nearby. This crackle is produced by radon atoms — a radioactive gas that seeps out from materials and air.

The universe is held together by an invisible scaffolding of dark matter. Part of it could be dark photons, ghostly copies of light that pass right through matter. The Dandelion experiment turned a spherical mirror into a super-sensitive ear: it gathers dark photons and translates them into the language of radio waves.