Abstract Generation of an entangled spin state as a result of a low-energy elastic collision of two protons is considered. The scattering process is described by a theoretical model that is in good agreement with the available experimental data on the angular differential cross sections of proton–proton collisions at low energies. The entanglement of the spin state of final protons as a function of the scattering angle is analyzed using Bell’s inequality and pair concurrence. The numerical results for different values of the collision energy are compared with the results without considering nuclear forces. It is shown that nuclear interaction significantly enhances the effects of spin entanglement.
Abstract Generation of an entangled spin state as a result of a low-energy elastic collision of two protons is considered. The scattering process is described by a theoretical model that is in good agreement with the available experimental data on the angular differential cross sections of proton–proton collisions at low energies. The entanglement of the spin state of final protons as a function of the scattering angle is analyzed using Bell’s inequality and pair concurrence. The numerical results for different values of the collision energy are compared with the results without considering nuclear forces. It is shown that nuclear interaction significantly enhances the effects of spin entanglement.
In this Letter, I work out spin entanglement properties of neutron-proton scattering using the exact S-matrix, considering both S-wave and higher partial wave contributions. The dependence of spin entanglement on momentum, scattering angle, and initial spin configuration is investigated for realistic nuclear forces, while low-energy properties of spin entanglement are analyzed within the framework of pionless effective field theory at leading order. New connections are found between spin entanglement and symmetry emergence of strong interactions. These results lead to a more complete understanding of how spin entanglement is generated via neutron-proton interaction. They also lay the theoretical foundation for controllable production of entangled nucleon-nucleon pairs in future experiments.
We discuss the feasibility of quantum spin-state teleportation in a three-proton system at an energy of 151 MeV, where, similarly to the low-energy case, a single Bell-state term dominates the proton-proton scattering matrix. We find that, in contrast to the low-energy regime, where unpolarized proton-proton scattering produces strongly entangled outgoing proton pairs, at higher energies a pair of protons, each with an energy of 151~MeV, must be produced in an unpolarized, exclusive proton-deuteron breakup reaction under complete final-state-interaction kinematics. The subsequent interaction of one of the entangled protons with a polarized hydrogen target triggers, as in the low-energy case, the teleportation process, whereby the polarization of the target proton is transferred to the second member of the entangled pair within the very narrow angular region of strong entanglement centered around a laboratory scattering angle of 45∘. We also find that scattering one member of a strongly correlated proton pair forming a Bell state from an unpolarized hydrogen target leads to Einstein-Podolsky-Rosen-like correlations between the polarization of the scattered proton and that of the unscattered second entangled proton. The polarization of the scattered proton varies with its scattering angle. An identical polarization, following the angular dependence of thepolarization of the first proton, is induced in the second member of the pair, whose initial polarization was zero and whose momentum remains unchanged. The sign of the polarization of the second proton is determined by the sign of the spin correlation in the Bell state.
The theory of quantum information constitutes the functional value of the quantum entanglement, i.e., quantum entanglement is essential for high fidelity of quantum protocols, while fundamental physical processes behind the formation of quantum entanglement are less relevant for practical purposes. In the present work, we explore physical mechanisms leading to the emergence of quantum entanglement in the initially disentangled system. In particular, we analyze spin entanglement of outgoing electrons in a nonrelativistic quantum (e,2e) collision on a target with one active electron. Our description exploits the time-dependent scattering formalism for typical conditions of scattering experiments, and contrary to the customary stationary formalism operates with realistic scattering states. We quantify the spin entanglement in the final scattering channel through the pair concurrence and express it in terms of the experimentally measurable spin-resolved (e,2e) triple differential cross sections. Besides, we consider Bell's inequality and inspect the regimes of its violation in the final channel. We address both the pure and the mixed initial spin state cases and uncover kinematical conditions of the maximal entanglement of the outgoing electron pair. The numerical results for the pair concurrence, entanglement of formation, and violation of Bell's inequality obtained for the (e,2e) ionization process of atomic hydrogen show that the entangled electron pairs indeed can be formed in the (e,2e) collisions even with spin-unpolarized projectile and target electrons in the initial channel. The positive entanglement balance---the difference between entanglements of the initial and final electron pairs---can be measured in the experiment.
Abstract In two recent papers, Blum and Lohmann (2016 Phys. Rev. Lett. 116 033201) and Lohmann et al (2016 Phys. Rev. A 94 032331), the possibility of continuously varying the degree of entanglement between an elastically scattered electron and the valence electron of quasi-one electron targets was discussed. Here we present results for elastic electron scattering from atomic hydrogen in the energy regime of 1−10 eV and the full range of scattering angles 0 ° − 180 ° . We confirm previous calculations at very low energies, which predicted that the hydrogen target is not a promising candidate for Bell correlations through electron collisions. This finding remains unchanged in the near-resonance regime of incident electron energies just below 10 eV. In addition to the spin-correlation parameter P , we present the angle-integrated total cross section, as well as the angle-differential cross section at a few representative collision energies.
In the framework of nonstationary scattering theory, we study the formation of an entangled state of two identical nonrelativistic spin- 1/2 particles as a result of their elastic scattering. The measure of particle entanglement in the final channel is described using pair concurrence. For the indicated quantitative criterion, we obtain general expressions in terms of the direct and exchange scattering amplitudes in the cases of pure and mixed spin states of the pair in the initial channel. We consider the violation of Bell’s inequality in the final channel. We show that as a result of a collision between unpolarized particles, a Werner spin state of the pair forms, which is entangled if the singlet component of the angular differential scattering cross section in the center-of-mass reference frame exceeds the triplet component. We use the process of free electron-electron scattering as an example to illustrate the developed formalism.
We investigate spin entanglement generated in neutron–proton scattering by evaluating the concurrence from the scattered two-nucleon density matrix. While previous studies mainly discussed pure initial spin states, the present work extends the analysis to polarization-induced mixed states, including partially polarized and unpolarized cases, within a unified framework. Initial polarization directions along the x, y, and [Formula: see text] axes are considered. Here, the [Formula: see text] axis is along the incident momentum, the y axis is normal to the scattering plane, and the x axis completes a right-handed system. We then examine the dependence of the concurrence on the incident energy, scattering angle, polarization degree, and polarization direction. The results show that the angular dependence of the concurrence becomes more pronounced as the incident energy increases. Although the concurrence tends to decrease overall as the polarization degree is reduced, this behavior is not uniform over all scattering angles, and in some regions unpolarized mixed states yield larger concurrence than fully polarized pure states. The concurrence is also found to be sensitive to the initial polarization direction. These features indicate that the entanglement generated in neutron–proton scattering reflects the interference structure of the scattering amplitudes. The present framework provides a basis for discussing spin entanglement under more realistic experimental conditions beyond pure-state idealizations.