Quantum entanglement is one of the most astonishing properties of quantum mechanics. If two particles are entangled, the state of one particle cannot be described independently from the other. This is a unique property of the quantum world and forms a crucial difference between classical and quantum theories of physics. It is so important, the 2022 Nobel Prize in Physics was awarded to Alain Aspect, John F. Clauser and Anton Zeilinger "for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science". The large mass of the top quark, which is greater than any other particle, remains one of the most enduring mysteries of the Standard Model. Why this is so remains unexplained, however, the top quark has many unique properties to exploit as a result. The top quark is so heavy that it is extremely unstable and decays before it has time to hadronise, transferring all of its quantum numbers to its decay particles. Physicists can detect these decay particles and thus reconstruct the quantum state of a top quark, a feat that is impossible with any other quark. Most importantly, they can measure its spin and use it to show that entanglement can be studied in top-quark-pair production at the LHC. Entanglement has indeed been measured in the past, but not quite like this. Most previous entanglement measurements involved low non-relativistic energies, typically utilising photons or electrons. The LHC collides protons with an incredibly high centre-of-mass energy. The data used in ATLAS’ new measurement were obtained from collisions at 13 TeV collected between 2015 and 2018. This means researchers are delving into an energy scale over 12 orders of magnitude (a thousand billion times) higher than typical laboratory experiments. This is the first-ever observation of quantum entanglement between a pair of quarks and the highest-energy measurement of entanglement. Figure 1: The observed (black dots) value of D for different ranges of top-pair masses (mtt), compared to various theoretical predictions (coloured dots). The value of the entanglement limit (D= -1/3) at parton level is converted to particle level (black dashed line). (Image: ATLAS Collaboration/CERN) In a new result from the ATLAS Collaboration, physicists studied the effects of entanglement in top quarks. They looked at top-quark pairs at their “production threshold”, i.e. when the invariant mass of the pair is at its minimum (approximately twice the mass of the top quark) and the top quarks are expected to be maximally entangled. The degree of entanglement (D) is related to the angular distribution of the particles the top quarks decay into – giving physicists a direct way to study this quantum effect. Researchers measured the angular separation of the top-quarks’ decay products (in the rest frames of the top quarks), correcting for detector effects that may impact its measured value. As a result, they calculated D at the particle level to be –0.547 ± 0.021. The result is significantly lower than the minimum value indicative of a non-entangled state, and more than passes the five standard deviation threshold required for an observation of the entanglement of top-quark pairs (see Figure 1). This is the first-ever observation of entanglement between a pair of quarks and the highest-energy measurement of entanglement. Apart from the fundamental interest of testing quantum entanglement in a new environment, this measurement paves the way to use the LHC as a laboratory to study quantum information and other foundational problems in quantum mechanics. About the banner image: Artistic visualisation of top-quark entanglement. The line between the particles emphasises the non-separability of the top-quark pair, which is produced by LHC collisions and recorded by ATLAS. (Image: Daniel Dominguez/CERN) Learn more Observation of quantum entanglement in top quark pair production using pp collisions of 13 TeV with the ATLAS detector (ATLAS-CONF-2023-069) TOP2023 presentation by Yoav Afik: Spin and entanglement ATLAS Afik, Y., de Nova, J.R.M., Entanglement and quantum tomography with top quarks at the LHC (Eur. Phys. J. Plus 136, 907 (2021)) Highest-energy observation of quantum entanglement, CERN Courier, 29 September 2023