Determining the output port from the distance

Abstract

In this paper we discuss the following scenario. Suppose that two distant observers share an entangled state of two $D$-level systems. The two observers, Alice and Bob, have to send randomly their particles through a multi-input multi-output device. The task is to determine the nature of this device or black-box, such that Alice will be able to determine with certainty the output port of Bob's particle. We prove that this black-box is the multi-input-port quantum sorter. Further, we investigate if it is possible to determine the output port in the case when three observers are involved by analyzing the cases when the Greenberger-Horne-Zeilinger (GHZ) state and the W state are used.