Hyperloss from coherent spatial-mode mixing in quantum-correlated networks.

Quantum-correlated networks distribute quantum resources such as squeezed and entangled states. They are central to modern quantum technology, including photonic quantum computing, quantum communications, biological sensing and gravitational-wave detection. Even for squeezed light - the most robust quantum-correlated resource - loss-induced decoherence remains the dominant obstacle to strong quantum advantage. A common design assumption is that spatial-mode mismatch acts as an incoherent loss. Coherent spatial-mode mixing with higher-order modes, however, can produce an apparent loss exceeding the full initial squeezing, a regime we term hyperloss. Here, we show experimentally that a minimal two-node network exhibits hyperloss, with 8 per cent mode mismatch converting 5.8 decibels of observable squeezing into an effectively thermal state, and that the lost correlations can be recovered by tuning differential spatial-mode phases, establishing hyperloss as a practical design constraint for future quantum technologies.