Xinan Chen

Entanglement distillation and entanglement swapping have been extensively researched in the context of perfect quantum memories. However, near-term quantum networks will be fundamentally limited by quantum memories with finite coherence time, resulting in complex choices for the timing and ordering of these operations. In this work, we study entanglement distillation and entanglement swapping at the level of the elementary building blocks of noisy quantum repeater networks, with the goal of isolating the basic tradeoffs induced by memory decoherence. First, we focus on a minimal single-hop setting, where we analytically compare "distill-as-soon-as-possible" and "distill-as-late-as-possible" strategies against a benchmark protocol that simply discards the older entangled state. We find that delaying distillation until the end consistently outperforms the other two strategies. We then extend our analysis to two-hop repeater chains using Monte Carlo simulation. In this setting, we find that entanglement swapping should be performed as soon as possible, while distillation should again be delayed to the end. Together, these results clarify how decoherence reshapes optimal operation timing in quantum networks and provide insight into the basic repeater-level rules that govern larger-scale architectures.

Quantum catalysis enables transformations of quantum states that are otherwise impossible. However, catalyzing transformations of quantum dynamics has remained largely unexplored. In this work, we initiate the study of correlated catalysis in dynamical resource theories and investigate whether resourceful quantum channel can be broadcast to another system. Specifically, we propose two frameworks for broadcasting dynamical resources: output broadcasting and input-output broadcasting. We establish no-go theorems that rule out output broadcasting of non-Gibbs-preserving channels. We also rule out input-output broadcasting of a variety of dynamical resources, including entanglement and coherence. Conversely, we construct general methods for output broadcasting applicable to a wide range of dynamical resource theories, including but not limited to entanglement, coherence, and non-stabilizerness.

Communication using quantum channels generally requires encoding and decoding on many identical uses of a quantum channel. In practical settings, decoherence may severely limit our ability to do so, potentially rendering each channel use nonidentical. Given $n$ nonidentical channels, we present an entanglement-assisted encoding and decoding strategy that yields a channel with higher entanglement fidelity than all of the $n$ given channels. The strategy is universal, in the sense that the improvement holds regardless of what channels are given, as long as they satisfy mild assumptions on their initial entanglement fidelities. This idea can also be extended to classical channels, where shared randomness between the sender and the receiver allows universal enhancement of the probability of correct transmission. Finally, we prove that such universal improvement is always possible in affine resource theories, which we believe to be an interesting result in its own right.