New type of quantum entanglement observed, helping physicists “see” inside atoms

star-detector-heavy-ion-collider
The house-size STAR detector at the Relativistic Heavy Ion Collider (RHIC) acts like a giant 3D digital camera to track particles emerging from particle collisions at the center of the detector. Credit: Brookhaven National Laboratory.
January 10, 2023
Evrim Yazgin

Evrim Yazgin

Cosmos science journalist

Nuclear physicists have used a never before seen type of quantum entanglement to help them gain information about the inside of an atomic nucleus.

At the Relativistic Heavy Ion Collider (RHIC) – a particle collider at the US Department of Energy Brookhaven lab in the US – physicists were able to use the photons (particles of light) surrounding gold ions passing through the collider to observe the structure of atomic nuclei and entangled pairs of particles.

Called “spooky action at a distance” by Einstein, quantum entanglement links the physical states of particles, no matter how much they are separated. Until now, quantum entanglement has only ever been observed between particles of the same type – e.g. entangled pairs of electrons or photons.

In their experiment, the nuclear physicists observed photons interacting through a series of quantum fluctuations with gluons in gold ions, zipping through the RHIC. Imaginatively named, gluons are – wait for it – glue-like particles responsible for the strong force which holds quarks – which in turn make up protons and neutrons in atomic nuclei – together.

Read more: Quantum entanglement of many atoms observed for the first time

A new intermediate particle is produced by the interaction between the photons and gluons. This particle quickly decays into opposite charged particles called “pions” (denoted by the Greek letter π). The velocity and trajectory of the π+ and π-­­ can be used to get crucial information about the photon and work out the arrangement of gluons in the nucleus more accurately than ever before.

Schematic-showing-entangled-pions
Left: Scientists use the STAR detector to study gluon distributions by tracking pairs of positive (blue) and negative (magenta) pions (π). These π pairs come from the decay of a rho particle (purple, ρ0) — generated by interactions between photons surrounding one speeding gold ion and the gluons within another passing by very closely without colliding. The closer the angle (Φ) between either π and the rho’s trajectory is to 90 degrees, the clearer the view scientists get of the gluon distribution. Right/inset: The measured π+ and π- particles experience a new type of quantum entanglement. Here’s the evidence: When the nuclei pass one another, it’s as if two rho particles (purple) are generated, one in each nucleus (gold) at a distance of 20 femtometers. As each rho decays, the wavefunctions of the negative pions from each rho decay interfere and reinforce one another, while the wavefunctions of the positive pions from each decay do the same, resulting in one π+ and one π- wavefunction (a.k.a. particle) striking the detector. These reinforcing patterns would not be possible if the π+ and π- were not entangled. Credit: Brookhaven National Laboratory.

“This technique is similar to the way doctors use positron emission tomography (PET scans) to see what’s happening inside the brain and other body parts,” says former Brookhaven Lab physicist James Daniel Brandenburg, now an assistant professor at Ohio State University. “But in this case, we’re talking about mapping out features on the scale of femtometers – quadrillionths of a meter – the size of an individual proton.”

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“Now we can take a picture where we can really distinguish the density of gluons at a given angle and radius,” Brandenburg explains. “The images are so precise that we can even start to see the difference between where the protons and neutrons are laid out inside these big nuclei.”

A consequence of the interaction between the gluon and photons is what appears to be the discovery of an entirely new kind of quantum entanglement.

It seems that the resultant positive and negative pions are entangled. “This is the first-ever experimental observation of entanglement between dissimilar particles,” Brandenburg remarks.

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Daniel Brandenburg and Zhangbu Xu at the STAR detector of the Relativistic Heavy Ion Collider (RHIC). Credit: Brookhaven National Laboratory.

“We measure two outgoing particles and clearly their charges are different—they are different particles—but we see interference patterns that indicate these particles are entangled, or in sync with one another, even though they are distinguishable particles,” adds Brookhaven physicist Zhangbu Xu.

Read more: Quantum entanglement can be used to encrypt messages – making data more secure

The discovery has many potential applications beyond the important task of mapping out the ways in which the building blocks of matter come together to create atomic nuclei and, ultimately, everything we can see and touch.

Quantum entanglement is being researched to one day create significantly more powerful communication and computational tools than exist today.

The results of the experiment are published in the journal Science Advances.

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