A quantum-bit encoding converter

From telecommunications to computing architectures, the realm of classical information hinges on converter technology to enable the exchange of data between digital and analogue formats, a process now routinely performed across a variety of electronic devices. A similar exigency also exists in quantum information technology, where different frameworks are being developed for quantum computing, communication and sensing. Thus, efficient quantum interconnects are a major need to bring these parallel approaches together and scale up quantum information systems. So far, however, the conversion between different optical quantum-bit encodings has remained challenging due to the difficulty of preserving fragile quantum superpositions and the demanding requirements for postselection-free implementations. Here we demonstrate such a conversion of quantum information between the two main paradigms, namely discrete- and continuous-variable qubits. We certify the protocol on a complete set of single-photon qubits, successfully converting them to cat-state qubits with fidelities exceeding the classical limit. Our result demonstrates an essential tool for enabling interconnected quantum devices and architectures with enhanced versatility and scalability.