Quantum Entanglement Concentration Based on Nonlinear Optics for Quantum Communications

Quantum hyperentanglement and its applications in quantum information processing

Hyperentanglement is a promising resource in quantum information processing with its high capacity character, defined as the entanglement in multiple degrees of freedom (DOFs) of a quantum system, such as polarization, spatial-mode, orbit-angular-momentum, time-bin and frequency DOFs of photons. Recently, hyperentanglement attracts much attention as all the multiple DOFs can be used to carry information in quantum information processing fully. In this review, we present an overview of the progress achieved so far in the field of hyperentanglement in photon systems and some of its important applications in quantum information processing, including hyperentanglement generation, complete hyperentangled-Bell-state analysis, hyperentanglement concentration, and hyperentanglement purification for high-capacity long-distance quantum communication. Also, a scheme for hyper-controlled-not gate is introduced for hyperparallel photonic quantum computation, which can perform two controlled-not gate operations on both the polarization and spatial-mode DOFs and depress the resources consumed and the photonic dissipation.

Universal remote quantum computation assisted by the cavity input–output process

Quantum computing may provide potential superiority to solve some difficult problems. We propose a scheme for scalable remote quantum computation based on an interface between the photon and the spin of an electron confined in a quantum dot embedded in a microcavity. By successively interacting auxiliary photon pulses with spins charged in optical cavities, a prototypical quantum controlled–controlled flip gate (Toffoli gate) is achieved on a remote three-spin system using only one Einstein–Podolsky–Rosen entanglement, and local operations and classical communication. Our proposed model is shown to be robust to practical noise and experimental imperfections in current cavity–quantum electrodynamics techniques.

Efficient hyperconcentration of nonlocal multipartite entanglement via the cross-Kerr nonlinearity

We propose two schemes for concentration of hyperentanglement of nonlocal multipartite states which are simultaneously entangled in the polarization and spatial modes. One scheme uses an auxiliary single-photon state prepared according to the parameters of the less-entangled states. The other scheme uses two less-entangled states with unknown parameters to distill the maximal hyperentanglement. The procrustean concentration is realized by two parity check measurements in both the two degrees of freedom. Nondestructive quantum nondemolition detectors based on cross-Kerr nonlinearity are used to implement the parity check, which makes the unsuccessful instances reusable in the next concentration round. The success probabilities in both schemes can be made to approach unity by iteration. Moreover, in both schemes only one of the N parties has to perform the parity check measurements. Our schemes are efficient and useful for quantum information processing involving hyperentanglement.

Entanglement distillation for quantum communication network with atomic-ensemble memories

Atomic ensembles are effective memory nodes for quantum communication network due to the long coherence time and the collective enhancement effect for the nonlinear interaction between an ensemble and a photon. Here we investigate the possibility of achieving the entanglement distillation for nonlocal atomic ensembles by the input-output process of a single photon as a result of cavity quantum electrodynamics. We give an optimal entanglement concentration protocol (ECP) for two-atomic-ensemble systems in a partially entangled pure state with known parameters and an efficient ECP for the systems in an unknown partially entangled pure state with a nondestructive parity-check detector (PCD). For the systems in a mixed entangled state, we introduce an entanglement purification protocol with PCDs. These entanglement distillation protocols have high fidelity and efficiency with current experimental techniques, and they are useful for quantum communication network with atomic-ensemble memories.