Quantum interference between dissimilar particles

Nuclear physicists have found a new way to see details inside atomic nuclei ¹. They do so by tracking interactions between photons and gluons—the gluelike particles that hold together the building blocks of protons and neutrons. The method relies on harnessing a new type of quantum interference between two dissimilar particles. Tracking how these entangled particles emerge from the interactions lets scientists map out the arrangement of gluons.

This technique is similar to how positron emission tomography (PET) scans image the brain and other body parts, but it works at the scale of femtometres—quadrillionths of a metre. It will help scientists understand how gluons build up the structure of protons, neutrons, and the atoms that make up ordinary matter. The quantum interference measurement occurs between dissimilar particles that strike meters apart in the detector.

The researchers used the Relativistic Heavy Ion Collider (RHIC), a facility that accelerates and collides the nuclei of atoms such as gold. These speeding nuclei are surrounded by a cloud of polarized photons—particles of light. Through a series of quantum fluctuations, the photons surrounding one speeding ion can interact with the gluons in the other. By tracking the velocity and angles at which certain particles emerge from these interactions, the scientists can measure the photon polarization very precisely. This allows them to map out the distribution of gluons both along the polarization direction and perpendicular to it, resulting in a more precise gluon distribution than measured previously.

To make these measurements, the scientists tracked two pions—one with a positive charge, the other with a negative charge. Each is made up of the combined wavefunctions of particles emerging from a decay process that occurs inside each of the two nuclei passing at a “long” distance (for nuclei). Interference patterns between these particles’ wavefunctions indicated that the oppositely charged particles striking RHIC’s STAR detector are entangled, or in sync with one another.

This first-ever experimental observation of interference between dissimilar particles makes it possible to measure the photon polarization—and may open new avenues for harnessing quantum entanglement. Almost all such attempts to date, including in the field of quantum computing, have explored entanglement between identical particles.

More on quantum interference:

The dawn of the quantum network: quantum interference over 300 km of optical fiber