Robust quantum communication through lossy microwave links

Abstract: Entanglement generation lies at the heart of many quantum networking protocols as it enables distributed and modular quantum computing. For superconducting qubits, entanglement fidelity is typically limited by photon loss in the links that connect these qubits together. We propose and realize a new scheme for heralded entanglement generation that almost entirely circumvents this limit. We produce Bell states with $92\pm1\%$ state fidelity, including state preparation and measurement (SPAM) errors, between separated superconducting bosonic qubits in a high-loss regime where direct deterministic state transfer fails. Our scheme exploits simple but fundamental physics found in microwave links, specifically the ability to treat our communication channel as a single standing wave mode. Combining this with local measurements on bosonically encoded qubits allows us to herald entanglement with success probabilities approaching the scheme's upper limit of 50% per attempt. We then use the heralded Bell state as a resource to deterministically teleport a qubit between modules with an average state transfer fidelity of $90\pm1\%$. This is achieved despite the link possessing a direct single photon transfer efficiency of 2%. Our work informs the design of future superconducting quantum networks, by demonstrating fast coupling rates and low loss links are no longer strict requirements for high-fidelity quantum communication in the microwave regime.