Quantum state transfer between photons preloaded with quantum information

Quantum mechanics provides a ``disembodied'' way to transfer an unknown quantum state from one quantum system to another. However, all experiments of quantum state transfer to date are limited to cases where the target quantum system contains no prior quantum information. Here we propose a scheme for transferring a quantum state to a quantum system preloaded with quantum information. By using an optical qubit-ququart entangling gate, we have experimentally demonstrated this new protocol -- transferring a qubit to a photon preloaded with one qubit of quantum information. After the state transfer, the target photon contains two qubits of quantum information, one from the qubit being transferred and the other from the pre-existing qubit. Furthermore, we have also experimentally realized the inverse operation of the aforementioned quantum state transfer, which is called the partial quantum state transfer, namely transferring one qubit of quantum information from a photon preloaded with two qubits of quantum information to another photon. The fidelities of the quantum state transfer range from $0.700$ to $0.917$, all above the classical limit of $2/3$. Our work sheds light on a new direction for quantum state transfer and demonstrates our ability to implement entangling operations beyond two-level quantum systems.