Turbulence Modelling Reveals Interference in Quantum Free-Space Optical Links
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Heyang Peng and the colleagues have modelled quantum multiple-input multiple-output (MIMO) channels in free-space optical links experiencing atmospheric turbulence. They developed a new, first-principles model directly based on wave-optical propagation, explicitly considering factors like intermodal crosstalk and detection apertures. The model distinguishes between scenarios with distinguishable and indistinguishable photons, revealing that indistinguishability creates complex interference effects. A key introduction is an erasure-extended encoding which maps turbulence and photon loss into identifiable erasure states, ultimately simplifying the Quantum MIMO channel into a correlated multi-qubit erasure channel and offering insights into correlated Pauli channel approximations. This provides a logical framework for understanding and potentially enhancing quantum communication through turbulent free-space optical links. Modelling atmospheric turbulence unlocks reliable long-distance multi-qubit entanglement Error rates, a key metric for quantum communication fidelity, dropped from previously unquantifiable levels to those allowing for reliable multi-qubit entanglement across 10 kilometres of turbulent free-space. This represents a significant advancement, as atmospheric turbulence traditionally poses a substantial challenge to maintaining the delicate quantum states required for secure communication and distributed quantum computing. The improvement stems from a first-principles model, derived directly from wave-optical propagation, that accurately simulates atmospheric turbulence and detector limitations, a feat impossible with prior methods relying on assumed channel behaviours. These earlier methods often employed simplified models, such as the Gamma-Gamma or Log-Normal distributions, to approximate turbulence, which failed to capture the full complexity of the wavefront distortions and their impact on quantum signals. The current model, however, solves the wave equation directly, accounting for the propagation of light fields through randomly varying refractive index fluctuations in the atmosphere. Representing the Quantum MIMO channel as a correlated n-qubit erasure channel, where ‘n’ signifies the number of qubits in the polarization register, this simplification allows for more efficient calculations of channel capacity and optimal coding strategies. The concept of an erasure channel is crucial; it treats signal loss not as a random bit flip, but as a known absence of information, enabling the design of error-correcting codes specifically tailored to these erasures. This is particularly advantageous in quantum communication, where directly correcting errors can destroy the quantum state itself. Atmospheric turbulence now introduces predictable correlations rather than random errors in quantum communication.
The team modelled the channel, utilising spatial multiplexing to send multiple qubits simultaneously, as a correlated n-qubit erasure channel. Spatial multiplexing, in this context, involves transmitting multiple independent quantum channels through different spatial modes of light, increasing the overall data throughput. The first-principles model accurately accounts for intermodal crosstalk, the mixing of light between different spatial modes, and finite detector apertures, elements ignored by earlier simulations. Intermodal crosstalk arises because atmospheric turbulence causes the light beams to spread and overlap, leading to interference between the different spatial modes. Finite detector apertures further exacerbate this effect, as they only capture a portion of the total light field. These results currently assume ideal polarization alignment and do not yet demonstrate sustained performance in real-world conditions with pointing errors or atmospheric precipitation. Maintaining precise polarization alignment is critical for preserving the quantum information encoded in the photons, and even slight misalignments can introduce errors. Furthermore, atmospheric precipitation, such as rain or snow, can cause significant signal attenuation and scattering, further degrading the performance of the free-space optical link. Distinguishable versus indistinguishable photons underpin turbulence durability in Quantum MIMO Increasingly sophisticated models of quantum communication channels are being built, essential for realising secure networks and distributed computing. The work reveals a reliance on simplifying assumptions about how photons, particles of light, behave when multiple paths are used simultaneously; specifically, the model distinguishes between scenarios with distinguishable and indistinguishable photons, highlighting how the latter introduces complex interference. Distinguishable photons, in this context, are those that can be individually identified and traced through the system, while indistinguishable photons exhibit wave-like behaviour and interfere with each other. The interference patterns created by indistinguishable photons can be both beneficial and detrimental to quantum communication, depending on the specific channel conditions. The authors acknowledge that their current framework assumes ideal polarisation alignment, despite the new erasure-extended encoding elegantly mapping turbulence and photon loss into identifiable states. This erasure-extended encoding is a key innovation, as it allows the model to treat turbulence and photon loss as a form of quantum erasure. The research demonstrated a first-principles model for Quantum MIMO channels using free-space optical links and accounting for atmospheric turbulence. This is important because increasingly complex quantum communication channels require accurate modelling of photon behaviour, particularly when utilising multiple transmission paths simultaneously. The model distinguishes between distinguishable and indistinguishable photons, revealing how the latter creates interference effects and introduces correlations between qubits. Researchers implemented an erasure-extended encoding to map turbulence and photon loss into identifiable states, providing a logical description of the channel. 👉 More information 🗞 Quantum MIMO Channel Modeling in Turbulent Free-Space Optical Links 🧠 ArXiv: https://arxiv.org/abs/2604.06931 Tags: