Quantum Networks Demonstrate Losses Exceeding 100 Percent through Spatial-Mode Mixing

Stephan Grebien and colleagues at the Institut für Quantenphysik and Zentrum für Optische Quantentechnologien, Universität Hamburg, reveal that coherent spatial-mode mixing within quantum-correlated networks can lead to ‘hyperloss’, where apparent loss exceeds 100% of the initial quantum squeezing. A degree of mode mismatch as little as 8% can destroy the quantum advantage of a 5.8dB squeezed state, effectively rendering it thermal. The team also show that hyperloss is not an insurmountable obstacle, as lost correlations can be recovered and mode mismatch mitigated through careful control of spatial-mode phases, offering a pathway to improved design and performance in future photonic quantum processors, interferometers and quantum-sensing networks.

Hyperloss reveals rapid quantum state degradation via mode mismatch

A 5.8dB squeezed state was reduced to an effectively thermal state with only 8% mode mismatch, revealing a previously unrecognised phenomenon termed ‘hyperloss’ in quantum networks. This threshold signifies a complete loss of quantum advantage, as the squeezed light, a key resource for quantum technologies, behaves indistinguishably from ordinary light under these conditions. Before this discovery, mode mismatch was assumed to cause only minor, incoherent signal degradation, but this research reveals it can be a coherent effect, exceeding 100% loss of the initial quantum squeezing.

The team successfully recovered lost correlations by tuning the phase of spatial modes, offering a pathway to improved network design. A slight misalignment of light beams, at 8% mode mismatch, was sufficient to reduce a 5.8dB squeezed state to a completely thermal state, effectively eliminating any quantum benefit. This finding challenges the conventional understanding of mode mismatch as simply causing minor signal degradation; instead, it demonstrates a coherent effect where loss can exceed 100% of the initial quantum squeezing.

Lost quantum correlations were successfully recovered by precisely tuning the phase of the spatial modes, showing a method to counteract hyperloss. Further analysis revealed that a 15% geometric mismatch could be reduced to an effective loss of only 2.8% through this phase tuning. However, these results were obtained in a minimal two-node network, and whether this recovery method scales with increased network complexity and higher squeezing levels remains an open question.

Differential phase tuning mitigates hyperloss in spatially mismatched quantum channels

This investigation into quantum signal loss was underpinned by precise control of spatial-mode phases. Spatial modes describe the ways light can travel within a network, akin to different vibration patterns on a guitar string each producing a unique sound. To simulate imperfections common in real-world quantum devices, a small misalignment of 8% mode mismatch between these spatial modes was deliberately introduced. Rather than simply measuring the resulting signal loss, a technique of tuning the differential phases of these light paths was employed, effectively altering how the modes interfered with one another.

This allowed not only observation of the phenomenon of hyperloss, but also manipulation and ultimate recovery of the lost quantum correlations, demonstrating a pathway towards more robust quantum networks. Hyperloss, an unexpected form of signal degradation in quantum networks using squeezed states of light, a strong quantum resource, was investigated. Deliberately introducing an 8% mismatch between spatial modes, different ways light can travel, replicated imperfections found in quantum devices. This approach allowed detailed observation of hyperloss, unlike simply measuring overall signal loss which would obscure the underlying mechanism; a 5.8dB squeezed state was reduced to a thermal state, demonstrating significant quantum signal loss within the two-node network.

Exceeding one hundred percent loss reveals flaws in quantum network modelling

Quantum networks promise major advances in computing and sensing, but maintaining the delicate quantum states within them remains a formidable challenge. Mode mismatch degrades signals, yet a far more insidious effect has now been revealed: ‘hyperloss’, where signal loss exceeds one hundred percent. This isn’t merely a matter of diminishing returns; it’s a fundamental flaw in how these systems have been modelled, treating misalignment as simple attenuation rather than a coherent interaction.

Acknowledging that hyperloss arises from established physics, specifically, coherent mixing of light modes, does not diminish the importance of identifying and mitigating it. This research clarifies a previously overlooked mechanism degrading quantum signals in networks; treating misalignment as simple signal reduction proved inadequate. Understanding hyperloss allows engineers to move beyond basic loss calculations and actively control spatial modes, improving performance in sensitive applications like gravitational wave detection and quantum computing. Seemingly small misalignments in quantum networks can induce ‘hyperloss’, exceeding one hundred percent signal reduction.

Engineers can now design more durable systems by actively controlling spatial modes and compensating for these effects. This research establishes that spatial mode mixing, how light travels within a quantum network, induces a coherent form of signal loss exceeding simple attenuation. Previously, misalignment of light paths was treated as minor, incoherent degradation; however, this work demonstrates ‘hyperloss’, where quantum information isn’t simply lost but interacts destructively. The team proved lost quantum correlations are recoverable by precisely controlling the phase of these light paths, effectively turning a design limitation into a parameter for optimisation.

The research demonstrated that spatial mode mismatch in a two-node quantum network could induce ‘hyperloss’, exceeding 100% signal reduction in a 5.8dB squeezed state. This finding matters because current models of quantum networks incorrectly assume misalignment causes only simple signal loss, overlooking destructive interference between light modes. By actively controlling the phase of these modes, researchers recovered lost quantum correlations and even reduced the effective loss from 15% geometric mismatch to approximately 2.8%. This work suggests future quantum network designs should prioritise precise spatial mode control to maintain signal integrity and enhance performance in applications such as quantum computing and gravitational-wave detection.