Particle decays do not constitute a spin "measurement" in the quantum-mechanical sense but still modify the spin state, in particular, for an entangled system. We show that for a spin-entangled pair of particles the entanglement of the system can increase after the decay of one particle. This unique phenomenon has no equivalent for stable particles and could be observable in top pair production at a high-energy polarized e^{+}e^{-} collider.

arXiv:2401.06854Physical Review LettersSep 10, 202420 citations

Physics Letters B

Decay of entangled fermion pairs with post-selection

J. A. Aguilar–Saavedra

Published Dec 19, 2023• 8 citations

Publisher

Abstract

DOI: 10.1016/j.physletb.2023.138409

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e^+ e^-

collisions at wide angles.

We investigate the role of time ordering in entanglement experiments involving unstable particles, focusing on

\mu^+ \mu^-

pairs produced in a maximally-entangled spin state. We analyse the correlations between measurements performed by two experimenters, Alice (who measures

\mu^-

spin) and Bob (who measures

\mu^+

decay products). Remarkably, the observed correlations persist irrespective of whether Bob's muon decays before or after Alice's spin measurement. We further discuss different interpretations of the same empirical results depending on the observer's reference frame. The findings reinforce the viewpoint that the Copenhagen interpretation of measurement is a mathematical tool rather than a literal account of physical reality.