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Single-photon avalanche diodes

Quantum Leap: Cheaper, Stronger Free-Space Graded-Index Multimode Fibres Enable Quantum Key Distribution

According to a recent study, optical fibre selection can considerably affect Quantum Key Distribution (QKD) system performance and economic viability, especially in free-space applications. The results reveal that graded-index multimode fibres can dramatically reduce quantum bit error rate (QBER), enabling safe, cost-effective communication networks.

Quantum Key Distribution, a secure key-sharing technique, is nearing commercialisation. Fiber-based networks are widely used, although dispersion and losses limit their long-distance performance. With air or vacuum channels, free-space network linkages are a crucial supplement to optical fibre, giving access to distant stations, mobile platforms, and even global coverage through satellites.

However, QKD in free space presents distinct challenges. To couple into single-mode fibre, receivers need a free-space receiver with multimode fibre or adaptive optics. Because it reduces coupling loss, the latter is often used. Single-photon avalanche diodes (SPADs) detectors are compact, light, and low-power for free-space QKD, especially in the visible or near-infrared wavelength area where silicon SPAD technology excels.

QKD performance depends on the single-photon detector's dark count rate, detection efficiency, after-pulsing, and timing responsiveness, often evaluated as full-width at half-maximum (FWHM) time-jitter. SPAD timing jitter is the delay between a photon's arrival and its electronic readout. Jitter limits QKD system speed. At high operating frequencies, timing jitter increases the risk of an incorrect photon being captured inside a time-bin window, increasing QBER. QBER is affected by dark counts, encoding, decoding, and most significantly temporal jitter. QBERjitter describes this temporal jitter contribution.

SPADs' temporal jitter response with single-mode fibres has been the main focus of previous research, while QBER with larger, multimode core sizes has not. This is a large gap considering the growing importance of free-space QKD, which uses multimode fibres to reduce coupling losses. Timing jitter is already affected by SPAD spot size and active region placement.

Innovation in Study Method The researchers' current study statistically examined how multimode fibres affect high repetition rate QKD systems' QBER to fill this knowledge gap. The researchers simulated responses at a 1 GHz operational rate, which is possible for commercial QKD systems using silicon SPAD technology, and performed empirical testing at 1 MHz repetition rate.

The experiment utilised custom-made and COTS multimode optical fibres, including step-index and graded-index types, with core sizes ranging from 10 µm to 400 µm and lengths up to 150 cm, linked to an 850 nm free-space Pico quant laser. Excelsis used its 180 µm active area SPCM-AQRH-12 silicon SPAD single-photon detector. A time-correlated single-photon counter detected high-resolution photon arrival times. The research quantified QBER by comparing photons observed in erroneous temporal bins to those expected in the right bin within a gate width.

Unveiled Key Results:

For short multimode fibres (less than a few meters), the timing jitter response and QBER contribution were substantially independent of fibre length. This suggests that prolonging the fibre within this range won't affect the single-photon detector's response when situated near the primary free-space optical receiver. Core Diameter, QBER: With larger core diameters, step-index multimode fibres' QBER contribution increased. Increasing core diameter increases the long diffusion tail of the recorded pulse and the spot size on the active region of the SPAD due to modal dispersion. A Graded-Index Advantage: The study found that graded-index multimode fibres were beneficial. Even at greater core diameters, these fibres had QBERjitter comparable to single-mode fibres and often less. Graded-index fibres' FWHM broadens, but their FW10M and FW100M are narrower than step-index fibres, minimising optical cross-talk across time-bins. Graded-index fibres outperform step-index fibres due to their larger modal bandwidth and lower modal dispersion. The Kerr effect (also known as spatial mode self-cleaning), in which higher-order modes compress towards equilibrium, may also contribute to this improved performance, albeit its size is uncertain. Graded-index fibres improve data rates in telecommunications, and our discoveries extend them to single-photon applications.

Free-Space QKD Transformation:

This finding has major implications for QKD's future, especially in free space:

Cost reduction and simplified design: Bigger core multimode fibres have wider numerical apertures and acceptance angles than single-mode fibres, improving coupling efficiency. Reducing the requirement for expensive adaptive optics and specialised pointing and tracking components makes receiver system design cheaper and simpler. Better Performance at High Repetition Rates Graded-index cores are critical for maintaining system performance in scenarios demanding high operating frequencies to provide appropriate secret key rates, notably in free space with variable channel loss and time-limited communication windows. Wider QKD Access: These findings could accelerate fiber-coupled receiver development, cutting complexity and cost and making QKD more accessible and alienable. This knowledge will help build QKD receiver connection efficiency technology.

In conclusion

The study concludes that a single-photon detector's full timing jitter response can considerably boost QBER in high repetition rate QKD systems. However, carefully using graded-index multimode fibres, which have larger core sizes and operate like single-mode fibres, can solve this problem. This breakthrough enables more dependable, effective, and cost-efficient QKD systems for secure satellite and ground communication networks.