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Quantum information processing with space-division multiplexing optical fibres
Guilherme B. Xavier, Gustavo Lima
TL;DR
The paper reviews how spatial-division-multiplexing fibres can address quantum information processing needs by supporting multiple spatial channels and high-dimensional states. It surveys fibre components and experiments spanning QKD, entanglement distribution, and coexistence with classical traffic, while identifying propagation, modal-dispersion, and integration challenges. The review concludes that SDM provides hardware for efficient, high-fidelity, high-dimensional quantum information processing, although longer-distance and higher-dimensional validation remains necessary.
Problem
Quantum information systems need fibre-compatible ways to transmit spatially encoded and entangled states while integrating with next-generation SDM optical networks.
Method
The paper reviews SDM fibre designs, multiplexing components, and recent QIP experiments involving QKD, entanglement distribution, and coexistence with classical channels.
Results
Experiments have demonstrated spatial QKD, high-dimensional entanglement distribution, and simultaneous quantum-classical transmission using SDM fibres.
Takeaways & Limitations
SDM offers hardware for efficient, high-fidelity, high-dimensional quantum information processing and is expected to integrate with quantum communication networks.
Takeaways & Limitations
Further work must establish propagation of high-dimensional states over distances comparable to single-mode fibres and address modal dispersion, Raman scattering, and higher-dimensional mode support.
Abstract
from arXiv · showhide
The optical fibre is an essential tool for our communication infrastructure since it is the main transmission channel for optical communications. The latest major advance in optical fibre technology is spatial division multiplexing (SDM), where new fibre designs and components establish multiple co-existing data channels based on light propagation over distinct transverse optical modes. Simultaneously, there have been many recent developments in the field of quantum information processing (QIP), with novel protocols and devices in areas such as computing, communication and metrology. Here, we review recent works implementing QIP protocols with SDM optical fibres, and discuss new possibilities for manipulating quantum systems based on this technology.
A. SDM fibres
SDM fibres increase spatial data density by multiplexing distinct transverse optical modes, using multi-core, few-mode, and OAM-based designs. These designs trade channel density, coupling, and detection complexity against transmission capacity.
- SDM multiplexes data through distinct transverse optical modes, providing a spatial analogue of wavelength division multiplexing as conventional fibres approach capacity limits.
- Multi-core fibres place several single-mode cores within one cladding, while sufficiently separated cores can be treated as approximately independent channels.Core separation above approximately 40 µm reduces cross-coupled power by several tens of dBs.
- Few-mode fibres support only a small number of transverse modes, limiting intermodal crosstalk enough to make MIMO decoding feasible compared with multimode fibres.Typical few-mode fibres support 3 or 6 linearly polarised modes.
- Combining multiple cores with few-mode operation increases spatial channel density and can substantially increase transmission capacity.
- OAM multiplexing uses orthogonal modes labelled by integer topological charge, with each photon carrying lℏ of orbital angular momentum.
B. Multiplexers and demultiplexers
SDM multiplexers and demultiplexers combine and split independent data streams into spatial channels using passive fibre-integrated or photonic-chip components. Their implementations map single-mode inputs onto selected modes of multi-core or few-mode fibres.
- B. Multiplexers and demultiplexers: Multiplexers and demultiplexers map N independent single-mode inputs onto, and back from, selected spatial modes of an SDM fibre.The review focuses on passive components implemented directly in fibres or through photonic chips rather than bulk optical elements.
- B. Multiplexers and demultiplexers: Three-dimensional ultrafast-laser-written silica waveguides can construct multi-core-fibre mux/demux devices by connecting appropriate fibres to an integrated chip.
- B. Multiplexers and demultiplexers: An SDM network can use different spatial technologies concurrently, with multi-core fibres allocating separate cores and few-mode fibres allocating distinct modes for quantum and classical traffic.The illustrated link uses two cores for quantum nodes, three cores for exchange points, and separate LP11 modes for entangled and classical channels.
A. High-dimensional quantum key distribution over SDM fibres
High-dimensional QKD uses path, OAM, and hybrid spatial encodings to transmit more information per quantum state, while SDM fibres have enabled practical-distance demonstrations. Experiments progressed from four-core path encoding to OAM-based demonstrations over ring-core and air-core fibres.
- A. High-dimensional quantum key distribution over SDM fibres: Path-encoded d-dimensional qudits carry log2d bits per quantum state, increasing QKD transmission rate without simply using faster optoelectronic modulation and detection.The path basis represents distinct spatial paths with relative phases between them.
- A. High-dimensional quantum key distribution over SDM fibres: QKD generates a shared secret key between separated parties by transmitting encoded single photons, with security relying on the impossibility of faithfully cloning an unknown quantum system.
- A. High-dimensional quantum key distribution over SDM fibres: SDM fibres enabled the longest reported transmission of path-encoded qudit states: a 300 m four-core fibre supported a high-dimensional QKD session.The experiment used deformable mirrors for phase modulation and improved on earlier spatial-light-modulator efforts.
- A. High-dimensional quantum key distribution over SDM fibres: Integrated silicon photonic circuits also implemented a four-core path-encoded HD-QKD session over 3 m, using on-chip thermal elements for active modulation.
- A. High-dimensional quantum key distribution over SDM fibres: OAM-based QKD was demonstrated in a 2-dimensional space over 60 m of vortex fibre without active state preparation and with hybrid polarisation/OAM ququarts over 1.2 km of air-core fibre.
B. Entanglement distribution
Spatial entanglement distribution over fibre was constrained by single-mode propagation and mode-coupling-induced scrambling, but SDM fibres now support higher-dimensional demonstrations. Recent work includes four-dimensional entanglement over four-core fibre and 3-dimensional OAM entanglement over 1 km few-mode fibre.
- B. Entanglement distribution: Long-distance spatial-entanglement distribution was historically difficult because single-mode fibres support only one spatial mode and mode coupling scrambles multimode states.Recent experiments therefore commonly use fibres supporting only a few spatial modes to reduce coupling.
- B. Entanglement distribution: Spatial entanglement is operationally useful because fibre propagation can combine physical separation with higher-dimensional Hilbert spaces for quantum-information tasks.
- B. Entanglement distribution: SDM experiments distributed 4-dimensional spatial entanglement through four-core fibres and verified it using tomography followed by a Bell inequality test.
- B. Entanglement distribution: A 3-dimensional OAM-entangled state was propagated over 1 km of step-index fibre supporting up to 6 LP modes.The experiment required careful input-axis alignment, modal-dispersion compensation, and decoding of the OAM modes.
III. INTEGRATION WITH CLASSICAL TELECOMMUNICATION OPTICAL NETWORKS
SDM fibres can support coexistence between quantum and classical channels, but wavelength allocation, inter-core crosstalk, and spontaneous Raman scattering constrain system design.
- QKD and classical data have been demonstrated together in separate cores of a multi-core fibre, with the centre core reserved for QKD.The experiment used a 7-core fibre carrying opposing 10 Gbit/s streams in side cores.
- −60 dB and −80 dB forward and backward inter-core crosstalk still prevented quantum and classical channels from sharing wavelengths across separate cores.The reported crosstalk levels were low but remained sufficient to require wavelength separation.
- Spontaneous Raman scattering from classical channels produces broadband photons that contaminate QKD bands and lower secret-key rates after filtering.This noise must be included in coexistence-system design even when in-band crosstalk is removed.
- Further experiments improved classical data rates alongside a centre-core QKD channel and sent parallel keys through 37 cores while carrying 10 Gbit/s per core.
IV. OUTLOOK AND OPEN CHALLENGES
SDM fibres have produced promising QIP results and may support integrated, high-dimensional quantum processing, but distance, dimensionality, modal dispersion, and telecom-noise challenges remain.
- SDM fibres have manipulated and propagated high-dimensional quantum states over long distances, while supporting integration of quantum and classical network systems.
- Future work must test whether multi-core fibres support high-dimensional states over hundreds of kilometres and with larger core counts such as 7 or 19.
- Telecom-compatible SDM experiments must address nonlinear noise, including Raman scattering and four-wave mixing from classical channels.
- OAM fibres require further high-dimensional demonstrations, while improved mode isolation is needed because classical multimode experiments required MIMO detection.Only one cited experiment exceeded OAM qubits, using OAM qutrits; a 36-core, three-mode-per-core fibre demonstrated a possible 108-dimensional space.
- Modal dispersion is especially critical for entangled states, and longer-distance compensation across wider mode ranges remains necessary.Pre-compensation of mode delays has already been used before fibre transmission.
- SDM is envisioned as hardware for efficient, high-fidelity, high-dimensional quantum information processing integrated with future communication networks.
Additional information
The paper includes author correspondence information and a declaration concerning competing financial interests.
- Correspondence and material requests should be addressed to Guilherme B. Xavier or Gustavo Lima.
- The authors declare no competing financial interests.
- The passage provides administrative information rather than scientific findings.