Optical implementation of the encoding of two qubits to a single qutrit

Abstract

We have devised an optical scheme for the recently proposed protocol for encoding two qubits into one qutrit. In this protocol, Alice encodes an arbitrary pure product state of two qubits into a state of one qutrit. Bob can then restore error-free any of the two encoded qubit states but not both of them simultaneously. We have successfully realized this scheme experimentally using spatial-mode encoding. Each qubit (qutrit) was represented by a single photon that could propagate through two (three) separate fibers. We theoretically propose two generalizations of the original protocol. We have found a probabilistic operation that enables to retrieve both qubits simultaneously with the average fidelity above 90% and we have proposed extension of the original encoding transformation to encode N qubits into one (N+1)-dimensional system.