The supercollider is now being used to explore quantum phenomena, including a “magic” form of quantum entanglement.

Ninety million times a year, when protons crash together at the Large Hadron Collider (LHC), they produce, in their wreckage, a top quark and an anti-top quark, the heaviest known elementary particles. In the trillionth of a trillionth of a second before the particles decay into lighter pieces, they fly apart. But they remain quantum mechanically entangled, meaning each particle’s state depends on the other’s. If the top quark is measured to spin in one direction, the anti-top quark must spin the opposite way.

Top quarks are special. Other types of quarks quickly group together to form composite particles (such as neutrons) before the LHC’s detectors can record their states. But top quarks decay before combining with other quarks. The particles they decay into contain a record of their spins — an observable fingerprint of their entanglement.

The ATLAS experiment at the LHC measured the correlations(opens a new tab) between top and anti-top quarks for the first time in 2023. A cascade of further entanglement measurements have followed.

Such efforts are new inside the walls of the LHC. Seventeen years after the machine switched on, particle physicists are realizing that they can use the collider to explore how information flows through quantum systems — a question at the foundations of quantum computing. The two possible spins of the quarks correspond to the 0 and 1 states of a qubit, a unit of quantum information. “It is treating the process of colliding things together and forming new particles as a quantum processor,” said Alan Barr(opens a new tab), a physicist at the University of Oxford who works on the ATLAS experiment. “You can investigate a whole different set of questions that colliders were not really designed to do in the first place but are very capable of addressing.”

...read more at quantamagazine.org