26th Australian Institute of Physics Congress and the Australian and New Zealand Conference on Optics and Photonics

Entanglement distillation rates beyond the direct transmission using noiseless linear amplification

Not scheduled

20m

Belinda Hutchinson Building (The University of Sydney )

Contributed Oral ANZOS | Quantum Computing and Quantum Information (ANZCOP QCQI)

Description

Entanglement distribution over lossy quantum channels is a fundamental challenge in quantum communication, where channel loss and decoherence progressively degrade the quality of shared entangled states. Noiseless linear amplification (NLA) offers a probabilistic but heralded solution to this problem, enabling entanglement distillation without violating the no-cloning theorem. However, the probability of success of conventional NLA — where the heralding station is placed at the receiver's end — scales linearly with channel transmissivity η, imposing the same fundamental constraints as direct transmission and falling below the repeaterless bound in high-loss regimes.

Here we experimentally demonstrate that repositioning the NLA heralding station to the midpoint of the lossy channel yields a markedly improved scaling of the success probability, proportional to √η rather than η. This improvement arises because both Alice's and Bob's travelling modes traverse only half the total channel before interfering at the heralding station, breaking the assumptions underlying the repeaterless bound without requiring quantum memory at the midpoint. Using an engineered spontaneous parametric down-conversion (SPDC) source producing spectrally pure, indistinguishable photons at 1550 nm with heralding efficiency above 80% and Hong-Ou-Mandel interference visibility of (99.962 ± 0.011)%, we implement both NLA configurations in a polarisation-encoded free-space optical setup with superconducting nanowire single-photon detectors.

In the high-loss regime (~20 dB), we demonstrate that midpoint NLA achieves entanglement distillation rates that surpass the direct transmission bound — the first experimental demonstration of a heralded distillation protocol exceeding this benchmark without state renormalisation. The amplified state maintains an average fidelity of 0.956 ± 0.038 with a maximally entangled state, while the photon-to-vacuum state ratio remains stable across all loss values, in contrast to direct transmission where it degrades rapidly. These results represent a critical step toward scalable quantum repeaters and long-distance quantum communication architectures.