Generation of vortex particles via weak measurements

Abstract

While twisted photons with orbital angular momentum can be generated in several ways, their massive counterparts -vortex electrons, neutrons, or atoms -can be obtained so far only via diffraction techniques, not applicable for relativistic energies. Here we show that particles with phase vortices, including hadrons, ions, and nuclei, can be generated during emission in helical undulators, via Cherenkov radiation, in collisions with intense laser beams, in such scattering and annihilation processes as eµ → eµ, ep → ep, etc. We elucidate that the key element in obtaining them is not the process itself but the post-selection protocol due to quantum entanglement. A final particle acquires a phase vortex if the other final particle's momentum is weakly measured with no post-selection of its azimuthal angle. This technique can be used for generating all types of vortex particles, it can be adapted for ultrarelativistic beams of modern colliders for hadronic and spin studies, and it can also facilitate the development of sources of hard X-ray and γ-range twisted photons.

Promises and challenges of high-energy vortex states collisions

“…The price to pay is the necessity of a post-selection protocol, a rather exotic requirement for particle physics. Although this idea was already used in the first publications on high-energy processes with vortex particles [32,33,274,153], its general description was presented only very recently [47,48]. Below, we follow the main arguments from these recent publications.…”

“…These arguments were recently developed in [47,48] in a more formal way in terms of the strong postsection protocol which is always implicitly applied when we discuss measurements with traditional local detectors, see more discussion in Section 5.2.2.…”

“…At present, no such detectors exist, at least for high-energy particles. As we discussed in Section 2.2, a standard detector instrumentation will be of little use: as long as one detects an outgoing particle in a certain direction, one loses information about its ϕ-dependent phase [47,48]. These challenge may be less severe for processes featuring very narrow angular distributions of one of the final particles, such as Compton backscattering from ultrarelativistic electrons [32,33].…”

“…In [47,48], this final state was called pre-selected, or evolved state, to clearly distinguish it from what a detector will eventually record. This outgoing state may include several reaction channels which are kinematically possible as well as a non-scattered contribution of the initial particles [146].…”

The role of post-selection in generating vortex particles of arbitrary mass, spin, and energy

“…In this paper, which is complementary to our Letter [61], we describe in detail a universal and versatile method to generate vortex particles of arbitrary mass, spin, and energy, including hard X-ray and γ-range twisted photons, relativistic muons, protons, ions and nuclei. Such particles can be obtained during emission of photons in electromagnetic fields of accelerators and freeelectron lasers, via scattering or annihilation processes with freely propagating particles, such as…”