![]() Experimental Preparation of a 2-D Four-Qubit Entangled State The experimental results proved that the prepared 2-D four-photon system is an entangled state with high fidelity. Finally, the properties of the prepared four-qubit entangled state were analyzed by three methods: quantum state tomography, entanglement witness, and the violation of Ardehali inequality against local realism (LR). Next, we designed the composite interferometer and prepared two-dimensional (2-D) four-qubit Greenberger–Horne–Zeilinger (GHZ) entanglement states for the polarization and spatial path of photons. As a basis, two photon entangled states with high brightness and high fidelity were prepared using SPDC technology. ![]() ![]() In this paper, motivated by facilitating the generation of multiqubit entangled states, we have designed an interferometer that can couple the polarization and spatial paths of photons and prepare an entangled state of the two modes. However, some systems are complex, and some require harsh experimental conditions such as low temperature. These works provide a good entanglement source for the research and application of quantum information technology. Many methods have been devised to prepare multiparticle entangled states through H-D entangled states. For example, it can improve the channel capacity of quantum communication and speed up quantum computation significantly. H-D entanglement is a fascinating resource for quantum communication and quantum computation. These H-D entangled systems have also been demonstrated experimentally. With this method, many different types of H-D entangled states can be prepared, such as the polarization–spatial H-D entangled state, polarization–orbital angular momentum H-D entangled state, etc. In experiments, multidegree of freedom entanglement can be generated by the combination of the techniques used for creating entanglement in a single degree of freedom. A single photon is able to carry more than just a qubit of quantum information, and when two photons are entangled in more than one degree of freedom, higher-dimensional (H-D) entanglement can be realized. ![]() However, the difficulty of preparing a multiparticle entanglement increases exponentially with the number of particles because of the low multiphoton coincidence count rate and the high double-pair emission noise effect. In the conventional protocols for quantum information processing, the entanglement in one degree of freedom of photon systems is selected in the SPDC process. In experiments, the entangled photon systems are usually prepared by the spontaneous parametric down-conversion (SPDC) process in nonlinear crystal. At the same time, many efforts are also being made to study the theoretical predictions of quantum mechanics based on polarization-entangled photons. Over the past years, great efforts have been devoted to generating and manipulating more qubits. The quantum information processing theory and experiments have been developed rapidly, especially the multiphoton entanglement, which plays an important role not only in the basic test of quantum nonlocality, but also in optical quantum computing, quantum teleportation quantum key distribution (QKD), and many other aspects. In recent years, a variety of entanglement schemes have been proposed and verified, such as multiphoton schemes, cold atom schemes, quantum dot schemes, etc. Quantum entanglement is the basic resource in quantum information processing.
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