Free-Space Quantum Communications Trials Set the Stage for Satellite Missions

SpeQtral’s entangled photon source and receiver payload is now fully integrated into the UKRI RAL Space’s quantum comms satellite as final preparations are made for launch! However, this UK-Singapore collaborative mission, SpeQtre, won’t be the first time our entangled photon sources have been used for quantum comms through the air. Over the last few years we have been working with CQT, the Centre for Quantum Technologies at the National University of Singapore, to demonstrate quantum communications links between our two locations using the same entangled photon source and receiver hardware that is being flown on the satellite.

Why Free-Space Quantum Communications?

The driving application for quantum communications is Quantum Key Distribution (QKD) which promises the highest levels of security by leveraging the fundamental laws of physics. Yet, the promise of QKD hinges on our ability to send delicate quantum signals—often single photons—across vast distances without them succumbing to noise or interception. While fibre-optic networks are effective on the ground, their reach is fundamentally limited by loss and decoherence. To achieve truly global secure communication, we must look skyward.

This is where free-space quantum communication comes in. By transmitting quantum signals through open air—between buildings, across city blocks, and from next year, between ground stations and orbiting satellites—we can leap beyond the confines of terrestrial fibres. Our demonstrations in the urban environment of Singapore are designed to help test, validate, and refine the various elements of this process, as we prepare for the rigours of the ultimate test: space.

Engineering for the Unexpected: Urban to Orbital Challenges

Reproducing the conditions of space within a laboratory is impossible; real-world environments offer a spectrum of challenges, from atmospheric turbulence to variable lighting and even rain. Our demonstration campaign helps to bridge this gap, deploying equipment across rooftops and open terraces, exposing our systems to the unpredictable.

Site Deployments and Lessons Learned:

• We set up transmitter and receiver terminals—named Alice and Bob—across urban rooftops, from our Office at Fusionopolis to a rooftop at NUS. These locations offered the necessary separation and line-of-sight for quantum transmission, while presenting practical hurdles like variable weather, limited manpower, and logistical constraints.
• From the earliest trials, we encountered the realities of outdoor deployment: equipment transport required thoughtful planning; extension cables, chairs, and headlamps quickly became essentials; and cable lengths and mounting solutions had to be optimized for reliable alignment.
• Each outing brought new insights—whether improving tripod logistics, correcting collimation of beacon lasers, or devising better ways to visually locate faint quantum beams outdoors.

Refining the Optical Link: From Beacons to Quantum Photons

At the heart of our setup is a sophisticated arrangement of lasers, telescopes, and single-photon detectors. Initial tests focused on aligning beacon lasers (red and green) between Alice and Bob, using them as reference points for the more elusive quantum signal at 780 nm. Achieving and maintaining precise alignment over hundreds of meters—up to 900 metres on the main link—was both an art and a science.

We iteratively refined our methods:

• Manual searches gave way to automated spiral scan algorithms, maximizing optical throughput between the terminals.
• Live feedback from photodiodes and single-photon detectors enabled continuous adjustments, while wide-field cameras provided coarse alignment and troubleshooting support.

Yet, the pursuit of perfection continues. Collimation errors, mechanical stability, and the need for precise X-Y-Z adjustments on all lenses kept our team on its toes and with every challenge came a solution that refined the system further.

From Single Photons to Secure Keys: Testing QKD Protocols

With the link established, we moved to the next step: running complete end-to-end quantum key distribution. This involved not just transmitting photons, but integrating advanced software for alignment, polarization correction, and real-time data processing.

Key milestones included:

• Executing auto-alignment and polarisation correction algorithms that mimicked the autonomy required for satellite operation.
• Efficient single photon detection with robust dark count subtraction.
• Generating and distilling secret keys.
• Testing under varying pump powers, and source parameters.

Bridging Earth and Space: The Road to SpeQtre

So, how does this urban demonstration connect to an orbital future?

• Miniaturisation: Our hardware is highly miniaturised with low SWaP to be compatible with almost all satellites, including low resource CubeSats. This also supports multiple units to be accommodated on board larger satellites. This also makes it very convenient to operate in the field.
• Space Qualification: The systems are designed and tested to survive the vibrations of rocket launch and the vacuum conditions of space, which also improves their ruggedness on the ground.
• Autonomous Operation: Just as our ground-based system must maintain alignment and compensate for disturbances, so too must a satellite autonomously acquire and track ground stations, with no opportunity for manual intervention.
• Environmental Variability: Atmospheric turbulence, temperature swings, and stray light in the city present a taste of the dynamic conditions SpeQtre will face in low-Earth orbit—albeit with the added complexity of high-velocity movement and vastly increased distances.
• Optical Precision: The art of aligning beams to within a fraction of a degree, over hundreds of meters, directly translates to the requirement for centimetre-scale pointing accuracy over thousands of kilometres in space.
• System Integration: By testing the interplay of hardware, software, and protocol in a real-world environment, we build confidence that the same integrated approach will succeed in the more formidable theatre of space.
• Performance Validation: Our QKD trials are not just demonstrations—they are stress tests, exposing weaknesses and validating improvements that can be applied to SpeQtre’s mission.

What Comes Next?

The journey from city rooftops to the vacuum of space is long and complex, but every photon counted and every key distilled brings us closer. With each outdoor deployment, we sharpen our tools and deepen our understanding. The SpeQtre satellite will inherit a legacy of lessons hard-won on the ground—lessons that will help ensure its success as it brings quantum-secure communications to the world.

The ground-based tests also continue as we use the setup as a ground station testing and calibration service, as well as testing with newer quantum light sources and explore tests during daylight hours. Further details of these and our previous tests will be published together with our various collaborators in due course and we are very much interested in exploring demonstrations of commercial applications of ground-based free-space links.

Stay tuned as we continue to advance the frontiers of quantum technology, and watch as today’s skyline experiments become tomorrow’s spaceborne breakthroughs. The age of quantum space networking is just beginning, and our journey is one step closer to orbit.

For more information on SpeQtral's products, check out https://speqtralquantum.com/ or reach out to us at info@speqtral.space.