High-dimensional quantum state transfer in decoherence-free subspaces

A qubit is the fundamental unit of quantum information, but there is no reason to limit the number of states to just two. Qudits can be formed by ensembles of qubits interacting with each other. Photon-mediated interactions between the qubits lead to bright states and dark states, the latter of which creates a decoherence-free subspace that can be used as a qudit. The properties of the qudit can be programmed by the positions and individual properties of the emitters, allowing for the construction of complex high-dimensional spaces optimized for particular tasks. We show that careful design of the decoherence-free subspace allows for complete transfer of the state of the qudit to an outgoing photon. With local driving of the emitters and manipulation of the resonance frequencies, qudit state populations can be moved into a bright state, leading to photon emission. The qudit is carried by the photon encoded in its phase, in a time-bin encoding, or a hybrid of both encodings, and can then be absorbed by a second identical qudit.