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Objectives
The MULTIVERSE project’s objective is to conceive, design, and validate a complete high-dimensional Quantum Key Distribution (QKD) system tailored for satellite-to-ground optical links, with quantum state dimensionality ≥4. It seeks to push beyond the one-bit-per-photon limit of traditional QKD by exploiting photonic degrees of freedom in tandem (polarisation and time-bin) to encode qubits. This approach promises higher secure key rates and improved noise tolerance compared to standard two-dimensional (qubit) protocols.
MULTIVERSE develops both a decoy-state (prepare-and-measure) QKD transmitter and an entanglement-based QKD source, enabling flexibility for different mission scenarios. The project ultimately strives to advance the technology readiness of multi-dimensional QKD to Technology Readiness Level (TRL) 4 through rigorous design, breadboard implementation, and testing in relevant free-space conditions.
Benefits
By increasing the quantum state dimensionality, MULTIVERSE offers substantial benefits over existing QKD systems, which typically use qubit states. A 4-D qubit encoding can potentially double the secret bits extracted per photon, boosting the secure key rate for a given photon flux. Moreover, high-dimensional protocols are inherently more resilient to noise and loss, extending operational distances or tolerances beyond those of standard BB84 schemes. The chosen hybrid polarisation and time-bin approach provides versatility: the system can interoperate with legacy two-dimensional QKD receivers (using only polarization or only time-bin encoding when needed), and it can even run parallel dual 2D QKD channels to maximise throughput in suitable conditions.
Compared to the first satellite QKD demonstrations (e.g. China’s Micius experiment, which was limited to polarization qubits) and current QKD mission proposals, MULTIVERSE’s technology promises significantly higher key rates and robust performance in the challenging satellite-to-ground channel.
Features
MULTIVERSE’s quantum transmitter is built around two cutting-edge encoders that together deliver unprecedented stability, speed, and versatility in free-space quantum key distribution. The first is the iPOGNAC polarisation encoder, which uniquely leverages a fibre-Sagnac loop and a single phase modulator to produce all required polarisation states directly in a free-space reference frame without any need for post-launch alignment or calibration. In laboratory tests, this design sustained over 24 hours with an intrinsic quantum bit error rate well below 0.2% and supports modulation rates scalable into the gigahertz regime. Complementing this, the MacZac time-bin encoder embeds an unbalanced Mach–Zehnder interferometer within a Sagnac loop so that a single modulator both carves early/late time-bins and sets decoy-state intensities, all while passively maintaining phase stability.
Bench-top demonstrations have achieved record-low intrinsic error rates (below 2×10⁻⁵) without any active thermal or phase control, and the same nested-loop topology can be extended to four or more time-slots simply by adding interferometric arms. Together, these encoders enable MULTIVERSE to switch seamlessly between qubits and 4-D qudit protocols, maximising secure key rates under a wide range of channel conditions and telescope configurations.
Challenges
Establishing a high-dimensional quantum link from a satellite to the ground involves significant challenges. The long free-space propagation distance (on the order of 500–1000 km) introduces high losses due to geometric diffraction, while atmospheric turbulence causes signal fading and wavefront distortions. Additionally, the relative motion of the satellite results in Doppler shifts and rapidly changing pointing requirements. Ensuring, precise timing, stable polarization states and interferometric phase alignment under these dynamic conditions further complicates the system design.
System Architecture
MULTIVERSE offers two complementary source designs that share these compact, space-ready encoding modules. In its prepare-and-measure configuration, pulsed laser outputs are routed through the MacZac and the iPOGNAC encoders before being launched through a fine-pointing telescope toward the ground station. A parallel classical link handles synchronisation, basis reconciliation, and key-distillation protocols.
For entanglement-based operation, a hyperentangled photon-pair generator produces simultaneous polarisation-and-time-bin entanglement which are directed through two independent fine-pointing telescopes directed toward for downlink transmission. Each ground receiver then combines polarisation analysis optics with an unbalanced interferometer and high-efficiency single-photon detectors to recover the full multi-dimensional quantum state. By packaging all optical and electronic subassemblies, MULTIVERSE paves the way for a TRL-4 demonstration of high-dimensional QKD from space.
Plan
For its 24-month duration, the project is structured into sequential phases. It begins with a comprehensive study phase (survey of the state-of-the-art, user requirements analysis, and baseline concept selection). Next, a detailed design phase defines the system architecture and components through trade-off analyses and simulations, resulting in a finalised technical specification and implementation plan. In the subsequent phase, laboratory breadboard models of the QKD transmitter and receiver are developed and tested to verify performance against the specifications. Finally, a field trial is conducted using a ~600 m free-space urban optical link to validate the system under real atmospheric conditions, concluding with a technology assessment and roadmap for further development.
Current Status
The project has recently kicked off and is currently in its early implementation stage. The initial state-of-the-art review has been completed, and a baseline high-dimensional QKD solution – combining polarisation and time-bin encodings – has been selected as the starting point. Detailed design and procurement of key components are underway, leveraging the consortium’s prior expertise in polarisation and time-bin modulation technologies. The next major milestone is the integration and testing of the laboratory breadboard system. The project is progressing on schedule with no significant issues to date
