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Physical Review A

Entanglement production in colliding wave packets

C. K. Law

Published Dec 10, 2004• 32 citations
Publisher

Abstract

We study the generation of quantum entanglement in two-particle scattering processes. When two wave packets collide, we discover that entanglement grows appreciably because of the interference between the incident and scattered waves. After the collision, quantum entanglement is mainly governed by the scattering phase shift and the widths of the incident wave packets. For interaction potentials that exhibit scattering resonance, we show that quantum entanglement can be enhanced near resonance energies.

DOI
10.1103/physreva.70.062311

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We study the generation of quantum entanglement in two-particle scattering processes. When two wave packets collide, we discover that entanglement grows appreciably because of the interference between the incident and scattered waves. After the collision, quantum entanglement is mainly governed by the scattering phase shift and the widths of the incident wave packets. For interaction potentials that exhibit scattering resonance, we show that quantum entanglement can be enhanced near resonance energies.
A qubit (a spin-1/2 particle) prepared in the up state is scattered by local spin-flipping potentials produced by the two target qubits (two fixed spins), both prepared in the down state, to generate an entangled state in the latter when the former is found in the down state after scattering. The scattering process is analyzed in three dimensions, both to lowest order and in full order in perturbation, with an appropriate renormalization for the latter. The entanglement is evaluated in terms of the concurrence as a function of the incident and scattering angles, the size of the incident wave packet, and the detector resolution to clarify the key elements for obtaining an entanglement with high quality. The characteristics of the results are also discussed in the context of (in)distinguishability of alternative paths for a quantum particle.
Abstract The generation of quantum entanglement through scattering processes is a fundamental mechanism in quantum information science. In this work, we investigate how coherent-state superpositions, represented as spatial Gaussian wave-packet superpositions, modify nonclassical phase-space features and enhance entanglement production during the scattering of a single particle from a localized spin impurity governed by a Heisenberg-type interaction. We construct even and odd superpositions of coherent states and analyze their time evolution using Fourier-space propagation. The corresponding Wigner distributions are computed analytically, revealing os-cillatory interference fringes and negative-valued regions that quantify nonclassicality. Using the nonclassical volume as a metric, we show that increasing the spatial separation between the Gaussian components enhances nonclassicality, eventually saturating at a stable asymptotic maximum. By incorporating these superposed states as incident wave packets, we compute the post-scattering spin entanglement between the reflected particle and the impurity using the negativity measure. Our results demonstrate that the symmetry of the initial coherent-state superposition fundamentally governs the stability of the generated entanglement: while the even superposition exhibits parameter-sensitive, oscillatory entanglement as the wave-packet separation increases, the odd superpo-sition yields a robust, decay-resistant entanglement plateau across all separation scales. These findings highlight the critical role of engineered nonclassical initial states in optimizing scattering-based quantum technologies
Generation of entanglement between two qubits by scattering an entanglement mediator is discussed. The mediator bounces between the two qubits and exhibits a resonant scattering. It is clarified how the degree of the entanglement is enhanced by the constructive interference of such bouncing processes. Maximally entangled states are available via adjusting the incident momentum of the mediator or the distance between the two qubits, but their fine tunings are not necessarily required to gain highly entangled states and a robust generation of entanglement is possible.
Generation of entanglement between two qubits by scattering an entanglement mediator is discussed. The mediator bounces between the two qubits and exhibits a resonant scattering. It is clarified how the degree of the entanglement is enhanced by the constructive interference of such bouncing processes. Maximally entangled states are available via adjusting the incident momentum of the mediator or the distance between the two qubits, but their fine tunings are not necessarily required to gain highly entangled states and a robust generation of entanglement is possible.
The scattering cross section is the effective area of collision when two particles collide. Quantum mechanically, it is a measure of the probability for a specific process to take place. Employing wave packets to describe the scattering process, we compute the entanglement entropy in 2-to-2 scattering of particles in a general setting using the S -matrix formalism. Applying the optical theorem, we show that the linear entropy E 2 is given by the elastic cross section σ el in unit of the transverse size L 2 of the wave packet, E 2 ∼ σ el / L 2 , when the initial states are not entangled. The result allows for dual interpretations of the entanglement entropy as an area and as a probability. Since σ el is generally believed, and observed experimentally, to grow with the collision energy s in the high energy regime, the result suggests a “second law” of entanglement entropy for high energy collisions. Furthermore, the Froissart bound places an upper limit on the entropy growth.
Einstein, Podolsky, and Rosen discussed their paradox in terms of measuring the positions or momenta of two particles. These degrees of freedom can become entangled upon scattering, but how much entanglement can be created in this process? Here we address this question using fully coherent calculations of bipartite scattering in three-dimensional space, quantifying entanglement by the inverse of the single particle purity. We show that the standard plane-wave description of scattering fails to capture the entanglement properties, due to the essential role of quantum uncertainty in the initial state. For a more realistic description of a scattering setup, we find that the entanglement scales linearly with the scattering cross section, including strong enhancement near shape resonances, for sufficiently narrow initial momentum dispersion. We highlight the differences between scattering in one and higher spatial dimensions and discuss how the generation of motional entanglement can be detected in experiments. Our results open the way to probing, controlling, and eventually using entanglement in quantum collisions.