Quarks Entangled

This post is about the recent observation of entanglement of quarks at the Large Hadron Collider, published in Nature. This is the detection of entanglement at the most fundamental level in the universe ever probed by human beings (quarks are, as far as we know, the smallest constituents of matter), and at the highest energies.

Entanglement

Entanglement is the existence of correlations (between two or more physical systems) which cannot be explained using classical physics, and requires quantum physics for its appropriate characterization. The term entanglement was coined by Schrodinger, who identified it as the essential feature of quantum mechanics. Specifically, if two systems are entangled, neither of them cannot be completely described without reference to the other.

Entanglement plays a critical role in practically all of physics. There is hardly a field where it is not being studied currently - black hole physics, condensed matter physics, optics, nanomechanics, information theory, quantum fluids, even in biology. Entanglement is also the workhorse of quantum information processing. To give a specific example, it is the critical ingredient behind quantum teleportation (a topic I just taught this semester) and quantum cryptography (for which commercial devices have existed for a while). The Nobel prize for physics in 2022 was awarded for pioneering studies of entanglement to Aspect, Clauser and Zeilinger.

Between systems with many degrees of freedom, entanglement is difficult to measure experimentally and quantify theoretically. A major effort in ongoing physics research is involved in addressing such complicated questions. Perhaps the simplest entangled system consists of two particles, each of which has only two states available to it. For example, a spin 1/2 particle, such as an electron, can be either have its spin up or down. Or a photon could have right or left circular polarization. These (2 states per particle and thus 4 states for 2 particles) restricted number of degrees of freedom allowed for the earliest detection of entanglement, following the pathbreaking proposal of John Bell.

Typically, entanglement is easier to observe at low temperatures, where thermal fluctuations do not interfere with quantum processes. For photons, which are practically immune to thermal effects and electrons, which can be cooled using refrigeration, entanglement can now be detected and even manipulated in many laboratories worldwide.

The Top quark

The top quark was discovered in 1995, the year I joined graduate school. I remember the excitement associated with it, especially since at that time I used to do laps around the school track with professor Paul Tipton, whose was directly involved in the experiments at Fermi Lab, and who later moved to Yale. The top quark is the heaviest fundamental particle, about as heavy as a gold atom.

Like the electron, the top quark is a spin 1/2 particle. Top-antitop quark pairs are produced via proton-proton collisions in particle accelerators. These quarks are both very short-lived, but their entanglement properties can be deduced from their much more stable decay products. Nonetheless, even this takes sophisticated data processing, making the achievement of detecting entanglement all the more impressive.

Summary

From a larger perspective, this experiment opens up the use of particle colliders for studying fundamental problems in quantum information. One of the novelties of these systems is that unlike most table top experiments, the (massive) particles are usually moving at relativistic speeds. The effects of relativity on quantum information can thus be studied using these platforms. Conversely, since quantum information science is now fairly well developed, maybe it can shed some new light on particle physics (this kind of back-and-forth is typical in science).