New type of entanglement lets scientists “see” inside nuclei – Zoo House News
Nuclear physicists have found a new way to use the Relativistic Heavy Ion Collider (RHIC) — a particle accelerator at the US Department of Energy’s (DOE) Brookhaven National Laboratory — to see the shape and detail inside atomic nuclei. The method relies on light particles surrounding gold ions as they race around the collider and a new type of quantum entanglement never seen before.
Through a series of quantum fluctuations, the light particles (also known as photons) interact with gluons — glue-like particles that hold quarks together inside the protons and neutrons of nuclei. These interactions create an intermediate particle that rapidly decays into two differently charged “pions” (π). By measuring the velocities and angles at which these π+ and π- particles hit RHIC’s STAR detector, scientists can go back to get crucial information about the photon – and use that to study the array of the gluons within the nucleus with greater precision than ever before.
“This technique is similar to how doctors use positron emission tomography (PET scans) to see what’s happening in the brain and other parts of the body,” said former Brookhaven Lab physicist James Daniel Brandenburg, a STAR collaboration member who joined Ohio State University as an assistant professor in January 2023. “But in this case, we’re talking about mapping features on the scale of femtometers — billionths of a meter — the size of a single proton.”
Even more amazing, the STAR physicists say, is the observation of an entirely new type of quantum interference that makes their measurements possible.
“We’re measuring two outgoing particles and their charges are clearly different – they’re different particles – but we’re seeing interference patterns that suggest these particles are entangled or synchronized with each other, even though they’re distinguishable particles,” the Brookhaven physicist said and STAR staff member Zhangbu Xu.
This discovery could have applications far beyond the lofty goal of mapping the building blocks of matter.
For example, many scientists, including those who won the 2022 Nobel Prize in Physics, are attempting to exploit entanglement — a type of “consciousness” and interaction of physically separate particles. One goal is to create much more powerful communication tools and computers than exist today. But most other observations of entanglement to date, including a recent demonstration of the interference of lasers with different wavelengths, have been between photons or identical electrons.
“This is the first-ever experimental observation of entanglement between dissimilar particles,” said Brandenburg.
The work is described in an article just published in Science Advances.
Gluons in the light
RHIC acts as the DOE Office of Science user facility, where physicists can study the innermost building blocks of nuclear matter — the quarks and gluons that make up protons and neutrons. They do this by smashing the nuclei of heavy atoms like gold, which are traveling in opposite directions around the collider at nearly the speed of light. The intensity of these collisions between nuclei (also called ions) can “melt” the boundaries between individual protons and neutrons, allowing scientists to study the quarks and gluons as they existed in the very early Universe – before protons and neutrons formed.
But nuclear physicists also want to know how quarks and gluons behave in today’s atomic nuclei – to better understand what force holds these building blocks together.
A recent discovery using “clouds” of photons surrounding RHIC’s fast ions suggests a way to use these light particles to peer inside the nuclei. When two gold ions pass very close to each other without colliding, the photons surrounding one ion can probe the internal structure of the other.
“In this previous work, we showed that these photons are polarized, with their electric field radiating outward from the center of the ion. And now we’re using this tool, polarized light, to effectively image the nuclei at high energy,” Xu said.
The observed quantum interference between π+ and π- in the newly analyzed data makes it possible to measure the direction of polarization of the photons very precisely. This in turn allows physicists to view the gluon distribution both along the photon’s direction of travel and perpendicular to it.
This two-dimensional mapping turns out to be very important.
“All previous measurements, in which we did not know the direction of polarization, have measured the density of gluons on average – as a function of the distance from the center of the nucleus,” says Brandenburg. “It’s a one-dimensional image.”
These measurements all revealed that the core looked too large compared to what was predicted by theoretical models and measurements of the charge distribution in the core.
“With this 2D imaging technique, we were able to solve the 20-year mystery of why this happens,” said Brandenburg.
The new measurements show that the momentum and energy of the photons themselves become intertwined with those of the gluons. If you only measure along the direction of the photon (or don’t know that direction), the image will be distorted by these photon effects. But measuring in the transverse direction avoids the photon blur.
“Now we can take an image where we can really distinguish the density of gluons at a given angle and radius,” Brandenburg said. “The images are so precise that we can even tell the difference where the protons and the neutrons are in these large nuclei.”
The new images are qualitatively consistent with theoretical predictions of gluon distribution, as well as measurements of electric charge distribution within the nuclei, say the scientists.
To understand how physicists make these 2D measurements, let’s go back to the particle created by the photon-gluon interaction. It is called rho and decays very quickly – in less than four quintillionths of a second – into π+ and π-. The sum of the momenta of these two pions gives physicists the momentum of the parent Rho particle – and information that includes the gluon distribution and the photon blur effect.
To extract only the gluon distribution, the scientists measure the angle between the path of either π+ or π- and the trajectory of the rho. The closer this angle is to 90 degrees, the less blur you will get from the photon probe. By tracking pions that originate from Rho particles moving at a range of angles and energies, scientists can map the gluon distribution across the nucleus.
Now for the quantum oddity that makes the measurements possible – proving that the π+ and π- particles hitting the STAR detector result from interference patterns produced by the entanglement of these two distinct, oppositely charged particles.
Remember that all the particles we are talking about exist not only as physical objects but also as waves. Like ripples on the surface of a pond radiating outward when they hit a rock, the mathematical “wave functions” that describe the crests and troughs of particle waves can interfere to reinforce or cancel each other out.
When the photons surrounding two nearly missing ions interact with gluons in the nuclei, it is as if these interactions actually create two rho particles, one in each nucleus. As each rho decays into a π+ and π-, the negative pion wavefunction from one rho decay interferes with the negative pion wavefunction from the other. When the amplified wave function hits the STAR detector, the detector sees a π-. The same thing happens with the wave functions of the two positively charged pions, and the detector sees a π+.
“The interference occurs between two wavefunctions of the identical particles, but without the entanglement between the two dissimilar particles – π+ and π- – this interference would not happen,” said Wangmei Zha, a STAR collaborator at the University of Science and Technology Technology of China and one of the original proponents of this statement. “This is the madness of quantum mechanics!”
Could the Rhos just get tangled up? The scientists say no. The rho particle wave functions originate at a distance 20 times the distance they could travel in their short lifetime, so they cannot interact with each other before decaying to π+ and π-. But the π+ and π- wave functions of each rho decay retain the quantum information of their parent particles; their peaks and troughs are in phase, “aware of each other” even though they strike the detector meters apart.
“If π+ and π- were not entangled, the two π+ (or π-) wavefunctions would have random phase with no discernible interference effect,” said Chi Yang, a STAR collaborator from Shandong University in China who also helped with the analysis of this result. “We would not see any orientation related to photon polarization — or be able to make these precision measurements.”
Future measurements at the RHIC with heavier particles and different lifetimes – and at an Electron-Ion Collider (EIC) built in Brookhaven – will investigate more detailed distributions of gluons within nuclei and test other possible quantum interference scenarios.
This work was funded by the DOE Office of Science, the US National Science Foundation, and a number of international agencies listed in the published paper. The STAR team leveraged computing resources from the RHIC and ATLAS Computing Facility/Scientific Data and Computing Center at Brookhaven Lab, the National Energy Research Scientific Computing Center (NERSC) – a user facility of the DOE Office of Science at Lawrence Berkeley National Laboratory – and the Open- Science Grid Consortium.