synchrotron light source

An assembly line spits out parts one by one per cycle. If the cycle is short and defects are few, the line runs fast. In quantum optics, a single-photon source plays the role of such a line: it must emit exactly one quantum of light per cycle, and the more cycles per second, the more photons go into the fiber.

When a laser beam hits a gas, it tears electrons from atoms and accelerates them to nearly the speed of light. As these electrons fly through the plasma, they emit X-rays, like a glowing trail from a fast particle. In the new work, physicists made a single laser pulse create not one but two such trails in succession.

A shooter aims at a target, pulls the trigger, and hears two shots: one loud, one quiet. That's how the new setup works, except instead of bullets, there's an electron bunch, and instead of shots, there are two flashes of light. One flash is X-ray, the other is terahertz. They fly to the same point with perfect timing.

A synchrotron beam is like a jet of water from a hose: wide and awkward for fine work. To examine a tiny sample, you need a fine needle, not a hose. In the new setup, the needle is a glass capillary — a tube through which light glides, reflecting off the walls like a voice echoing down a long corridor.

To probe deeper into the workings of the universe, physicists accelerate electrons and smash them together. An ordinary electron is like a spinning top that wobbles randomly. For delicate experiments, you need particles whose spins all point in the same direction: this is called polarization.

A stone falls into a pond, and ripples spread across the water. Likewise, light from distant galaxies spreads in all directions — calmly and steadily. But at the center of some galaxies, it's not a stone but a veritable cannon: it fires particles with monstrous force. Astronomers call such objects HSP BL Lac.

When matter is heated to millions of degrees, it turns into plasma — a mix of free electrons and ions, like a boiling soup of charged particles. You can't measure the temperature of such a 'soup' with an ordinary thermometer, but you can shine X-rays through it.

A conventional laser is like an orchestra where musicians must warm up before playing in unison. You first need to pump enough atoms with energy so they 'flip' and are ready to emit light: this is called population inversion. Only then does the beam flash. A polariton laser plays by different rules: its 'musicians'—a mix of light and matter — join in almost without preparation.

Modern X-ray light sources are like an orchestra without a conductor: electrons in the beam fly every which way, and the radiation comes out blurry. The new idea is to make electrons line up in neat rows with a spacing of a few nanometers. Then the light they emit becomes coherent, like a laser beam.

Two clocks in different rooms need to tick in unison. Usually, this is done by sending bright pulses or pairs of photons that arrive one right after the other. But the signal can be so weak that photons barely make it through. In this work, clocks were synchronized with light a billion times dimmer than usual.

A charged particle spiraling in a magnetic field doesn't stay silent: it emits light. This light is called synchrotron radiation, and it shows up everywhere — from distant galaxies to laboratory accelerators.

A thin crystal of hexagonal boron nitride is a relative of graphite, but transparent and insulating. Occasionally, single defects appear in its lattice: a boron or nitrogen atom is missing, leaving a trap for electrons. Such a trap can absorb light and re-emit it, like a tiny flashlight.

A laser pulse enters a tiny cavity made of a transparent crystal. Inside, light bounces between mirrors, and occasionally one photon splits into a pair: one goes one way, the other goes another. One becomes the signal, and the other acts as a 'bell' announcing that a single photon is ready.

You can't catch X-rays from stars with an ordinary mirror: they pass through glass like light through a window pane. To reflect them, you need a mirror almost parallel to the beam: then the X-rays skim along the surface like a flat stone skipping on water, only slightly changing direction.

In a particle accelerator, electrons race around a ring at nearly the speed of light. When such an electron collides with a photon, a particle of light, the photon bounces off like a ball hitting a moving truck and flies away with much greater energy.

An ordinary camera flash lasts a thousandth of a second. The new setup at the ELI ALPS laboratory in Hungary produces flashes a billion times shorter. Each flash consists of a few hundred oscillations of the light wave, packed into a time interval in which light travels a distance smaller than the thickness of a human hair.

A light pulse can be sliced into short time bins, like a train into carriages. If two photons are born together and their carriages are tightly linked, the arrival of one in a specific bin guarantees that the other arrives in the paired bin, this connection is called time-bin entanglement.

The laser clicks ten million times per second, and each click kicks exactly one photon out of a polystyrene film. Not a beam, not a burst, just a single particle of light, neatly packaged in space and time. The source is a molecule you can order from a chemical supply catalog: C60 fullerene, sixty carbon atoms folded into a soccer ball.