The Quantum Observables for Collider Physics 2026 workshop, held at CERN from 20 to 24 April, was the third edition of a series that began at the Galileo Galilei Institute in Florence in 2023 – now brought to the heartland of high-energy physics. Around 100 invited specialists from across the world gathered to take stock of a programme that has grown, with surprising speed, from a mere theoretical curiosity into a substantial body of experimental results.
The study of quantum observables with colliders is not starting from scratch. Phenomena such as oscillations and CP violation in neutral meson systems, or quantum interference in decay amplitudes and entanglement between B mesons at the Υ(4S) resonance, have been under scrutiny for decades. The novelty lies in the systematic application of the conceptual vocabulary of quantum information science – entanglement witnesses, Bell inequalities, quantum state tomography, and magic and decoherence as observables – to the highest-energy collisions accessible today.
Catalogue of entanglement
The most sharply focused experimental news came from the Higgs sector, where both ATLAS and CMS reported evidence that the two Z bosons produced in Higgs decays are entangled. Tairan Xu (University of Michigan) presented the ATLAS result, with an observed significance of 4.7σ. Jeffrey Davis (Johns Hopkins University) reported the CMS analysis, which measured spin correlations and extracted helicity fractions within an effective-field-theory framework. Together, the two results mark the Higgs boson’s entry into the catalogue of entanglement sources at the LHC.
In the top sector, CMS presented a full tomographic characterisation of the top quark–antiquark quantum state, with Otto Hindrichs (University of Rochester) showing the reconstructed spin-density-matrix elements across the full kinematic range of Run 2. Fiona Jolly (DESY) presented complementary ATLAS measurements, probing quantum-information observables in top-quark pairs. What has changed since the first edition of the workshop is the ambition, with early proof-of-principle measurements giving way to precise reconstructions of the full quantum state.
What has changed since the first edition of the workshop is the ambition
One of the most striking results came from the STAR experiment at RHIC, presented by Zhoudunming Tu (BNL). The measurement reports spin correlations of Λ–Λ hyperon pairs in proton–proton collisions at a centre-of-mass energy of 200 GeV, with a clear angular dependence that existing predictions do not fully describe. Elsewhere, Dmitri Kharzeev (Stony Brook University) argued that partons within a proton approach maximal entanglement at high energies. Their entanglement entropy then becomes directly observable in final-state multiplicities, making structure functions themselves carriers of quantum information. Beatrix Hiesmayr (University of Vienna), whose earlier work on entanglement and Bell-inequality violations in neutral kaon systems helped lay the foundations of the field, provided perspectives from the meson sector that grounded the newer collider-based results.
Marcel Vos (IFIC Valencia) showed that, at a future lepton collider, the entanglement of the top spins in e+e– → ttg events is expected to fall with the gluon’s energy, vanishing near the kinematic endpoint. This would be a computable, measurable signature of decoherence from real gluon emission. Lian-Tao Wang (University of Chicago) offered a striking reinterpretation, according to which decoherence due to soft, collinear radiation in high-energy collisions can be understood as a renormalisation-group flow.
Two-faced
Several talks explored whether entanglement suppression and enhanced symmetry are two faces of the same phenomenon. Ian Low (Northwestern University) showed, in an abstract spin-system setting, that entanglement reaches a minimum precisely where the Hamiltonian acquires an enlarged symmetry. Carlos Wagner (University of Chicago), Kamila Kowalska (NCBJ Warsaw), and Spencer Chang (University of Oregon) pressed the question from the collider side, asking whether the causal arrow runs from symmetry to entanglement or vice versa. The discussion remained productively unresolved.
A dedicated session on Bell tests reflected a field that has matured in its self-understanding. After a period of a rapid proliferation of proposals, there is now broad agreement on what is and is not achievable at a collider: entanglement can be established, Bell-violating quantum states can be witnessed, but a loophole-free Bell test – of the kind achieved with photons in a standard laboratory setting – is structurally out of reach with existing techniques. The community has, by and large, made peace with this distinction, and is now focusing on what can actually be done.
The next generation of colliders promises a significant step forward
Many contributions pushed the programme’s conceptual boundaries outward. Vlatko Vedral (University of Oxford) argued that laboratory interference experiments could test quantum effects in the gravitational field, while Nikos Mavromatos (King’s College London) explored entangled squeezed states of gravitons – associated with black-hole environments – as a potential probe of quantum gravity. David Kaiser (MIT) recounted how foundational debates about Bell inequalities, pursued by an unlikely group of physicists in 1970s California, eventually catalysed the cosmic Bell experiments. The account served as a reminder that questions dismissed as purely philosophical have a habit of becoming precision science.
On a longer timescale, the next generation of colliders promises a significant step forward. Tao Han (University of Pittsburgh) made the case that FCC-ee and CEPC would deliver large statistical samples in cleaner conditions than the LHC, and that beam polarisation in lepton colliders would open access to quantum process tomography of fermion pairs. Paweł Horodecki (Gdańsk University of Technology) emphasised that, by controlling the initial quantum state, this technique could enable tests of subtle deformations of quantum mechanics – possible extensions of the theory that state tomography alone cannot reach. Yoshitaka Hatta (BNL) and Kun Cheng (University of Pittsburgh) pointed to a further frontier at Brookhaven’s Electron–Ion Collider, where polarised beams will allow the partonic structure of the proton to be probed through its entanglement content.
Phrases overheard in the corridors, in the panel sessions and at the well-attended poster session captured the mood: “That’s fascinating,” “I disagree with you,” and “Let’s talk over coffee.” As Yoav Afik (University of Chicago) observed, the field has grown from a handful of proposals into a genuine international programme. The questions being asked are real ones, and the answers, when they come, will sharpen our understanding of quantum mechanics at the highest accessible energies. The next edition of the workshop will be in Chicago.
