Entanglement concentration of high-dimensional unknown partially entangled state

High-dimensional quantum systems offer a number of advantages in larger information capacity, stronger noise resiliency, higher efficiency, and accuracy over the qubit systems. In quantum communication, the maximally entangled states will inevitably become mixed states or less-entangled pure states due to the channel noise during practical transmission or storage. We propose a universal scheme to concentrate nonlocal high-dimensional generalized Bell states with unknown parameters. After the cross-Kerr nonlinearities, <i>X</i>-quadrature homodyne measurements, and single-partite projection measurements are performed only at Bob's site, a two-qutrit maximally entangled Bell state can be distilled, while previous entanglement concentration protocols (ECPs) mostly focused on two-level qubit systems. The concentrated partially entangled qubit states, reserved as the by-product, are the fascinating resources for some quantum information processing tasks. Moreover, single-qutrit projection measurement, the key ingredient for our ECP with unknown parameters, is completed by using linear optical elements. Additionally, linear optical high-dimensional ECP with known parameters is also designed.