arXiv:2607.08816

High Energy Photon Polarimetry at Lepton Colliders: Quantum Information from Converted Photons

Carlos Henrique de Lima, Navin McGinnis, David McKeen

Published Jul 9, 2026• 0 citations

Abstract

We study high-energy photons produced at a lepton collider that convert into an $e^+e^-$ pair in the detector, as a tool for measuring quantum information observables. We consider single- and double-conversion processes in $e^+e^- \to γγ$ and $e^+e^- \to e^+e^-γ$. Single conversions enable an in situ extraction of the spin-analyzing power, while double conversions probe polarization correlations. Focusing on the Belle-II detector, we show that, depending on the reconstruction resolution of the opening angle of the conversion $e^+e^-$ pair, quantum correlations of the diphoton system can be probed. In particular, measurements of violations of the Bell inequality, quantum discord, concurrence, nonstabilizerness, and steerability with spatially separated GeV-scale photons can be made at high significance.

DOI: 10.48550/arxiv.2607.08816

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arXiv:2607.08816

High Energy Photon Polarimetry at Lepton Colliders: Quantum Information from Converted Photons

Carlos Henrique de Lima, Navin McGinnis, David McKeen

Published Jul 9, 2026• 0 citations

arXiv HTML PDF

Abstract

We study high-energy photons produced at a lepton collider that convert into an

e^+e^-

pair in the detector, as a tool for measuring quantum information observables. We consider single- and double-conversion processes in

e^+e^- \to γγ

and

e^+e^- \to e^+e^-γ

. Single conversions enable an in situ extraction of the spin-analyzing power, while double conversions probe polarization correlations. Focusing on the Belle-II detector, we show that, depending on the reconstruction resolution of the opening angle of the conversion

e^+e^-

pair, quantum correlations of the diphoton system can be probed. In particular, measurements of violations of the Bell inequality, quantum discord, concurrence, nonstabilizerness, and steerability with spatially separated GeV-scale photons can be made at high significance.

DOI: 10.48550/arxiv.2607.08816

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Quantum properties of heavy-fermion pairs at a lepton collider with polarised beams

Mohammad Mahdi Altakach, Priyanka Lamba, Fabio Maltoni B +1 more

A bstract We investigate the quantum properties of heavy-fermion pairs, such as $t\overline{t}$ t t ¯ or τ + τ − , produced in lepton-lepton collisions with polarised beams. Focusing on spin correlations, entanglement, Bell-inequality violation, and quantum-information-theoretic measures such as purity and magic, we analyse how beam polarisation shapes the structure of the spin-density matrix. We derive analytic expressions for a wide range of helicity configurations, including both Standard Model contributions and generic new physics effects parametrised by scalar, vector, and tensor four-fermion operators within an effective field theory framework. We show that beam polarisation unlocks a substantially richer set of spin configurations and significantly enhances sensitivity to non-standard interactions. As a phenomenological application, we study $t\overline{t}$ t t ¯ production at a future linear collider and demonstrate that quantum observables provide a comprehensive and complementary probe of top-quark interactions and stronger constraints on the scale of new physics.A bstract We investigate the quantum properties of heavy-fermion pairs, such as $$ t\overline{t} $$ t t ¯ or τ + τ − , produced in lepton-lepton collisions with polarised beams. Focusing on spin correlations, entanglement, Bell-inequality violation, and quantum-information-theoretic measures such as purity and magic, we analyse how beam polarisation shapes the structure of the spin-density matrix. We derive analytic expressions for a wide range of helicity configurations, including both Standard Model contributions and generic new physics effects parametrised by scalar, vector, and tensor four-fermion operators within an effective field theory framework. We show that beam polarisation unlocks a substantially richer set of spin configurations and significantly enhances sensitivity to non-standard interactions. As a phenomenological application, we study $$ t\overline{t} $$ t t ¯ production at a future linear collider and demonstrate that quantum observables provide a comprehensive and complementary probe of top-quark interactions and stronger constraints on the scale of new physics.

Journal of High Energy PhysicsJul 9, 20263 citations

We study high-energy photons produced at a lepton collider that convert into an

e^+e^-

pair in the detector, as a tool for measuring quantum information observables. We consider single- and double-conversion processes in

e^+e^- \to γγ

and

e^+e^- \to e^+e^-γ

e^+e^-

7.4σ

violation of the Bell inequality could be achieved. The same framework can also probe quantum discord and nonstabilizerness, which could be measured with precisions of 5.6\% and 1.6\%, respectively. All the reconstructed results from photon conversion in the two-qubit framework are found to be consistent with the kinematic approach of real photons, and the formalism can apply to other spin-1 systems in an appropriate two-qubit limit.

Probing entanglement and testing Bell inequality violation with

\textrm{e}^{+}\textrm{e}^{-} \rightarrow τ^{+}τ^{-}

at Belle II

We present a feasibility study to probe quantum entanglement and Belle inequality violation in the process

\textrm{e}^{+}\textrm{e}^{-} \rightarrow τ^{+}τ^{-}

at a center-of-mass energy of

\sqrt{s} = 10.579

GeV. The sensitivity of the analysis is enhanced by applying a selection on the scattering angle

\vartheta

in the

τ^{+}τ^{-}

center-of-mass frame. We analyze events in which both

τ

leptons decay to hadrons, using a combination of decay channels

τ^{-} \rightarrow π^{-}ν_τ

τ^{-} \rightarrow π^{-}π^{0}ν_τ

, and

τ^{-} \rightarrow π^{-}π^{+}π^{-}ν_τ

. The spin orientation of the

τ

leptons in these decays is reconstructed using the polarimeter-vector method. Assuming a dataset of

200

million

τ^{+}τ^{-}

events and accounting for experimental resolutions, we expect the observation of quantum entanglement and Bell inequality violation by the Belle-II experiment will be possible with a significance well in excess of five standard deviations.

We investigate the quantum properties of heavy-fermion pairs, such as

t\bar t

τ^+τ^-

, produced in lepton-lepton collisions with polarised beams. Focusing on spin correlations, entanglement, Bell-inequality violation, and quantum-information-theoretic measures such as purity and magic, we analyse how beam polarisation shapes the structure of the spin-density matrix. We derive analytic expressions for a wide range of helicity configurations, including both Standard Model contributions and generic new-physics effects parametrised by scalar, vector, and tensor four-fermion operators within an effective field theory framework. We show that beam polarisation unlocks a substantially richer set of spin configurations and significantly enhances sensitivity to non-standard interactions. As a phenomenological application, we study

t\bar t

production at a future linear collider and demonstrate that quantum observables provide a comprehensive and complementary probe of top-quark interactions and stronger constraints on the scale of new physics.

A bstract We investigate the quantum properties of heavy-fermion pairs, such as

t\overline{t}

t t ¯ or τ + τ − , produced in lepton-lepton collisions with polarised beams. Focusing on spin correlations, entanglement, Bell-inequality violation, and quantum-information-theoretic measures such as purity and magic, we analyse how beam polarisation shapes the structure of the spin-density matrix. We derive analytic expressions for a wide range of helicity configurations, including both Standard Model contributions and generic new physics effects parametrised by scalar, vector, and tensor four-fermion operators within an effective field theory framework. We show that beam polarisation unlocks a substantially richer set of spin configurations and significantly enhances sensitivity to non-standard interactions. As a phenomenological application, we study

t\overline{t}

t t ¯ production at a future linear collider and demonstrate that quantum observables provide a comprehensive and complementary probe of top-quark interactions and stronger constraints on the scale of new physics.