CERN ALPHA Collaboration Cools Antihydrogen Atoms With Laser Light

Researchers at the ALPHA collaboration at CERN have for the first time succeeded in cooling down antihydrogen atoms using laser light. Antihydrogen is the simplest form of atomic antimatter, and the technique known as laser cooling was first demonstrated four decades ago on normal matter. Laser cooling is commonly used in many research fields.

The first application of laser cooling to antihydrogen opens the door to more precise measurements of the internal structure of antihydrogen and how it behaves under the influence of gravity. Comparing measurements with those of normal hydrogen atoms may reveal differences between matter and antimatter atoms. Those differences, if present, could help researchers determine why the universe is made up of matter only, an imbalance known as matter-antimatter asymmetry.

Researchers on the project say the ability to laser-cool antihydrogen atoms is a game-changer for spectroscopic and gravitational measurements. The new capability could lead to new perspectives in antimatter research, including the creation of antimatter molecules and the development of anti-atom interferometry. Jeffrey Hangst, an ALPHA spokesperson, says that laser cooling of antimatter was science fiction about a decade ago.

The team creates their antihydrogen atoms but taking antiprotons from the CERN Antiproton Decelerator and binding it with positrons originating from a sodium-22 source. The antihydrogen atoms produced are confined inside a magnetic trap preventing them from coming into contact with matter. Researchers say that measurement of antihydrogen behavior inside the Earth's gravitational field is limited by the kinetic energy or, equivalently, the temperature of the antiatoms.

Laser cooling helps to control the temperature of the antiatoms. The antiatoms absorb laser photons, causing them to reach a higher energy state. They then emit photons and spontaneously decay back into their initial state. The interaction depends on the velocity of the atoms, and since photons impart momentum, repeating the absorption-emission cycle leads to cooling of the atoms to a low temperature.