# Research on Intraparticle to Interparticle Entanglement Swapping Protocols
## 1 Introduction  
Quantum technology is currently receiving substantial worldwide attention and accounts for a major national strategy. Quantum technology is a new interdisciplinary field combining quantum physics and information technology. Entanglement is one of the most striking features of quantum systems, whereby its non-classical correlation is an essential resource in quantum technology. Many quantum protocols rely on non-classical correlations, such as quantum teleportation (QT) [1], quantum cryptography [2, 3], and quantum random number generators [4], etc. In addition to entwining at one degree of freedom, two systems can also entangle at various degrees of freedom (hybrid entanglement) [5, 6, 7]. Hybrid entanglement opens the door to intraparticle entanglement. Intraparticle entanglement was first proposed by [PERSON] in 1998 [8]. The study highlighted entanglement involving at least two commutation observables, which can also belong to a particle or multi-particle system. The commutation observables of intraparticle entanglement belong to a particle. Various degrees of freedom in a single system are commutation observables, which can form intraparticle entangled states, such as the polarization-momentum entanglement of a photon [9, 10], the spin-pathentanglement of a neutron [11; 12; 13], and the spin-orbit entanglement of a photon [14; 15; 16], etc. Intraparticle entanglement has a stronger robustness property under decoherence and dephasing and consumes fewer resources than interparticle entanglement. It can also be applied to numerous endeavors, such as superdense coding [15], QT [17; 18], information transfer [19], etc. Interparticle and intraparticle entanglements are two types of entanglement. While interparticle entanglement has received the most attention in current research, intraparticle entanglement has also received considerable attention. The two forms of entanglement have differences and similarities. Analyzing and contrasting these similarities and differences can help us understand entanglement. Entanglement swapping is a technique based on quantum entanglement. It can be applied in many quantum protocols. It especially plays a vital role in the quantum repeater [20], as it can distribute the entanglement over a greater distance. Entanglement consists of two types: intraparticle and interparticle entanglement. Entanglement swapping also consists of various kinds, including interparticle to interparticle and intraparticle to interparticle. Interparticle entanglement swapping has been extensively studied, such as in [21; 22; 23; 24]. The following studies have been conducted to swap entanglement from intraparticle to interparticle. A swapping protocol was established from path-spin intraparticle to spin-spin interparticle entanglement by using the Mach-Zehnder interferometer, the Stern-Gerlach measuring device, and the corresponding unitary operations [25]. Another swapping protocol was accomplished from the intraparticle to interparticle hybrid entanglement by using two indistinguishable particles and suitable unitary operations [26]. Another swapping protocol was achieved from intraphoton path-polarization (spin) to interphoton entanglement by using linear optical devices [27]. The authors of [28] achieved an entanglement swapping protocol from path-spin intraparticle to spin-spin interparticle entanglement by using simple optical devices. The spin-orbit entanglement of photons has significant academic and practical value and has acquired extensive attention. It is frequently used in quantum protocols based on the experimental progression of manipulating photon spin-orbit degrees of freedom [29; 30]. Thus, this study focuses on the entanglement swapping from orbit-spin intraparticle to orbit-orbit interparticle entanglement. This study parallels previous studies about intraparticle to interparticle entanglement swapping and does not contrast with prior studies.