Alexander Ling

We present the strategic framework and technical foundations behind the development of the National Quantum-Safe Network (NQSN) in Singapore—a resilient, fully interoperable quantum-safe network. The testbed features a star-topology architecture built on production-grade fiber infrastructure, supporting multi-protocol quantum key distribution (QKD) and diverse applications from multiple vendors. A centralised key and network management system underpins its interoperability, enabling seamless integration across technologies. We further explore a range of quantum-secured use cases, including data center connectivity, edge computing, hybrid QKD–post-quantum cryptography (PQC) encryption, and multi-layer integration within the OSI stack. These technical insights demonstrate the feasibility and flexibility of deploying quantum-safe capabilities in a multi-input, multi-output network environment.

The security of prepare-and-measure satellite-based quantum key distribution (QKD), under restricted eavesdropping scenarios, is addressed. We particularly consider cases where the eavesdropper, Eve, has limited access to the transmitted signal by Alice, and/or Bob’s receiver station. For instance, Eve can only receive an attenuated version of the transmitted signals. This results in settings where an uncharacterized bypass channel, inaccessible to Eve, can also carry signals to Bob. We obtain generic bounds on the key rate in the presence of bypass channels and apply them to continuous-variable QKD protocols with Gaussian encoding as well as to the family of BB84 protocols. We find regimes of operation in which the above restrictions on Eve can considerably improve system performance. Our work opens up new security frameworks for spaceborne quantum communications systems.

Future quantum networks depend entirely on the ability to distribute quantum resources \cite{kimble2008quantum, wehner2018quantum}. Although terrestrial optical fibers provide the foundation for such networks, fundamental attenuation limits their utility over intercontinental distances, making satellite-based links essential for global connectivity. A critical prerequisite for this infrastructure is the reliable distribution of entanglement, which enables information-theoretic secure communication via Quantum Key Distribution (QKD) and facilitates advanced protocols, such as entanglement routing via quantum repeaters. Historically, entanglement-based QKD has relied on bespoke, ad hoc experimental configurations tailored for proof-of-concept laboratory demonstrations \cite{rozenman2026free}. However, realizing an operational space-based quantum network necessitates transitioning from these specialized setups to robust, space-qualified hardware \cite{vergoossen2020spooqy}. Bridging the gap between terrestrial prototypes and orbital deployments requires the rigorous characterization of flight-representative payloads and automated optical ground stations under representative field conditions. Such a validation is vital to ensure the scalable distribution of quantum resources necessary for a global quantum internet. We deployed a horizontal free-space QKD experiment spanning a 1.8 km link in a semi-urban desert environment, effectively mimicking the architecture planned for future space-to-ground networks. The transmitter node integrated the space-qualified engineering model of the quantum light source and receiver designed for the upcoming SpeQtre satellite mission, functioning as a highly stable source of polarization-entangled photon pairs. At the receiver node, the experiment utilized the Abu Dhabi Quantum Optical Ground Station (ADQOGS) \cite{amairi2024versatile}, an automated facility engineered for free-space optical and quantum communications. The ADQOGS employs an 80 cm Ritchey–Chrétien telescope equipped with a precision acquisition and tracking system, single-photon detectors, and stringent spatial and spectral filtering stages optimized for background noise suppression. Together, this combination of a flight-ready transmitter and a highly specialized ground station creates a rigorous, real-world testbed for evaluating the performance of operational quantum hardware outside the laboratory. We report nighttime operations under stable atmospheric conditions. The integrated system demonstrated sustained operational stability, yielding a sifted coincidence rate of approximately 24,000 photon pairs per second. We recorded a mean quantum bit error rate (QBER) of 4.78\% and extracted a finite-size secure key rate of 7565 bps, validating the efficacy of the entanglement-based link. Furthermore, extrapolating these metrics to a realistic Low Earth Orbit (LEO) scenario indicates that the hardware can successfully overcome the substantial diffraction and atmospheric losses expected during a LEO satellite downlink. Ultimately, the successful field integration of a flight-representative payload with an automated ground station verifies the operational readiness of the hardware. These results mark a decisive step toward the execution of the SpeQtre mission and provide critical empirical foundations for the deployment of large-scale, satellite-based quantum communication networks.

Satellite-based quantum communication has emerged as a promising solution to overcome the range limitations of ground-based systems. In this implementation, network nodes in space can connect different global ground points coherently. Building an optical ground station (OGS) is a precursor towards satellite-to-ground quantum communication that can operate in an uplink or downlink configuration.

In satellite-based quantum key distribution (QKD), the number of secret bits that can be generated in a single satellite pass over the ground station is severely restricted by the pass duration and the free-space optical channel loss. High channel loss may decrease the signal-to-noise ratio due to background noise, reduce the number of generated raw key bits, and increase the quantum bit error rate (QBER), all of which have detrimental effects on the output secret key length. Under finite-size security analysis, higher QBER increases the minimum raw key length necessary for non-zero secret key length extraction due to less efficient reconciliation and post-processing overheads. We show that recent developments in finite key analysis allow three different small-satellite-based QKD projects CQT-Sat, UK-QUARC-ROKS, and QEYSSat to produce secret keys even under very high loss conditions, improving on estimates based on previous finite key bounds. This suggests that satellites in low Earth orbit can satisfy finite-size security requirements, but remains challenging for satellites further from Earth. We analyse the performance of each mission to provide an informed route toward improving the performance of small-satellite QKD missions. We highlight the short and long-term perspectives on the challenges and potential future developments in small-satellite-based QKD and quantum networks. In particular, we discuss some of the experimental and theoretical bottlenecks, and improvements necessary to achieve QKD and wider quantum networking capabilities in daylight and at different altitudes.

The National Quantum-Safe Network (NQSN) in Singapore is a nationwide collaborative platform and a field-deployed test-bed aimed at demonstrating quantum-safe cryptography solutions. NQSN links up academic, public and private members, targets trials for quantum key distribution (QKD) network with different QKD protocols, post-quantum cryptography (PQC) and classical symmetric key technologies.

In this work, we present the design considerations and architecture of an optical ground station being developed on National University of Singapore campus. The primary objective of the station is to enable quantum key distribution and facilitate other free space communication protocols. The development of the optical ground station is underway and it is projected to be commissioned by 2023. We elaborate on the building blocks and design techniques of the optical ground station in Singapore that can receive i.e downlink weak quantum signals from a satellite and perform necessary analysis to generate secret keys in a quantum key distribution experiment. We emphasize on the different subsystems namely the telescope system, quantum receiver, polarization correction system, and the pointing, acquisition and tracking system. We envision our ground station to support a range of beacon wavelengths to ensure its compatibility with various similar satellite missions. The working lab-configuration of the station is able to receive and analyse state of photons around 800 nm. To achieve a global quantum network, cross-compatibility among optical ground stations and quantum satellites is crucial. To facilitate this, we have initiated a collaboration with various academic groups involved in satellite based quantum key distribution research to standardize the configuration of an optical ground station. This collaboration aspires to create cross-compatibility among multiple optical ground stations and quantum satellites to enhance the efforts of a global quantum network.

Verifying the quality of a random number generator involves performing computationally intensive statistical tests on large data sets commonly in the range of gigabytes. Limitations on computing power can restrict an end-user's ability to perform such verification. There are also applications where the user needs to publicly demonstrate that the random bits they are using pass the statistical tests without the bits being revealed. We report the implementation of an entanglement-based protocol that allows a third party to publicly perform statistical tests without compromising the privacy of the random bits.