On June 16, 2025, the Defence Research and Development Organisation (DRDO) and IIT-Delhi successfully demonstrated free-space quantum secure communication using quantum entanglement over a distance exceeding one kilometre on the IIT-Delhi campus. This achievement represents a significant leap forward from traditional fibre-optic quantum communication methods, demonstrating the feasibility of atmospheric quantum key distribution that could eventually enable satellite-based quantum networks across India.

Understanding Quantum Communication And Its Revolutionary Nature

Quantum communication represents a fundamental paradigm shift from classical encryption methods, leveraging the peculiar properties of quantum mechanics rather than mathematical complexity to ensure security. Unlike classical cryptography, which relies on computational difficulty and can potentially be broken by sufficiently powerful computers, quantum communication provides theoretically unbreakable security based on the laws of physics themselves. The security stems from quantum entanglement, where pairs of photons become correlated in such a way that measuring one photon instantly affects its partner, regardless of the distance separating them.

The revolutionary aspect of quantum communication lies in its inherent ability to detect eavesdropping attempts. When an unauthorised party attempts to intercept quantum-encrypted information, the very act of measurement disturbs the quantum states of the photons, immediately alerting the legitimate communicating parties to the presence of an intruder. This property, combined with the quantum no-cloning theorem that prevents perfect copying of unknown quantum states, creates an fundamentally secure communication channel.

Quantum Key Distribution (QKD) serves as the primary application of quantum communication technology, enabling two parties to establish shared cryptographic keys with guaranteed security. It's important to note that QKD doesn't encrypt the actual message content; instead, it securely distributes the encryption keys that are then used with traditional algorithms like AES to protect the transmitted data. This approach combines the unbreakable security of quantum physics with the practical efficiency of classical encryption methods.

Technical Achievement And Performance Metrics

The IIT-Delhi demonstration employed entanglement-based QKD, which offers significant advantages over traditional prepare-and-measure quantum communication methods. In entanglement-based systems, a source creates pairs of entangled photons and distributes them to the communicating parties, ensuring that even if the devices are compromised or imperfect, the quantum entanglement itself maintains the security of the key distribution process. This approach provides greater resilience against device-specific attacks and maintains security even under non-ideal conditions.

The achieved performance metrics demonstrate the practical viability of the system for real-world applications. The secure key rate of 240 bits per second, while modest compared to classical communication speeds, represents a significant achievement for free-space quantum communication over kilometer distances. The quantum bit error rate of less than 7% indicates excellent system performance, as this falls well below the theoretical threshold of 11% that marks the boundary between secure and potentially compromised quantum communication channels.

Common sources of error in free-space quantum communication include atmospheric turbulence, detector noise, and interference from artificial lighting. The IIT-Delhi team's ability to maintain such low error rates despite these challenges demonstrates the robustness of their experimental setup and the potential for scaling the technology to longer distances and more challenging environmental conditions.

Progressive Development Timeline

The IIT-Delhi quantum communication program has demonstrated consistent progress through a series of increasingly ambitious experiments over the past several years. In 2022, the team achieved India's first intercity quantum communication link between Vindhyachal and Prayagraj in Uttar Pradesh, covering a distance of more than 100 kilometers using commercial-grade underground dark optical fiber. This initial demonstration established sifted key rates of up to 10 kHz and proved the viability of quantum key distribution over existing telecommunications infrastructure.

Building on this foundation, the team achieved a world record in 2023 by demonstrating secure quantum communication over 380 kilometers using standard telecom fibre, representing the longest distance achieved globally for the Differential Phase Shift (DPS) Quantum Key Distribution protocol. This achievement maintained an impressively low quantum bit error rate of 1.48%, demonstrating the team's advancing expertise in maintaining quantum coherence over extended distances. The research was published in Nature Scientific Reports, highlighting its international significance.

In 2024, the team successfully distributed quantum keys using entanglement over a 100-kilometer spool of telecom-grade optical fibre, further refining their techniques and preparing for the transition to free-space quantum communication. This progression from fibre-based to free-space quantum communication represents a crucial technological advancement, as free-space systems eliminate the need for expensive optical fibre infrastructure and enable quantum communication to satellites and remote locations.

Strategic Implications And Defence Applications

The military and strategic implications of quantum communication technology cannot be overstated, particularly in an era of increasing cyber warfare and electronic surveillance. Defence Minister Rajnath Singh's characterization of this achievement as a "game-changer in future warfare" reflects the transformative potential of quantum-secured communications for military operations. Quantum communication provides fundamentally unbreakable encryption that remains secure even against future quantum computers, which pose a significant threat to current classical encryption methods.

The free-space nature of the IIT-Delhi demonstration is particularly significant for military applications, as it eliminates dependence on vulnerable fibre-optic infrastructure that can be physically compromised or severed during conflicts. Free-space quantum communication enables secure communications in border areas, remote military installations, and mobile platforms where laying optical fiber would be expensive, difficult, or strategically inadvisable. This capability is crucial for maintaining command and control communications in contested environments where traditional communication channels might be compromised or denied.

The technology also supports the development of quantum networks with multiple nodes, enabling secure communication across entire military command structures. Such networks could revolutionize military communications by providing guaranteed secure channels for intelligence sharing, operational planning, and real-time battlefield coordination. The ability to detect any eavesdropping attempt instantly provides an additional layer of security assurance that is impossible to achieve with classical communication methods.

Future Prospects And Technological Roadmap

The ultimate goal of the IIT-Delhi quantum communication program extends far beyond the current 1-kilometer demonstration to encompass satellite-based quantum key distribution that could serve the entire Indian subcontinent. Satellite-based quantum communication would enable secure key distribution to any location within India by beaming entangled photons through the atmosphere from orbiting satellites. This capability would be particularly valuable for securing communications to remote areas, mobile platforms, and locations where terrestrial infrastructure is unavailable or vulnerable.

India is planning to intensify its satellite quantum communication efforts over the next 6 to 12 months, with the goal of launching dedicated quantum communication satellites. The challenges of satellite-based quantum communication, particularly with Low Earth Orbit satellites that provide only 15-20 minutes of coverage per day, would likely require a constellation of 4-7 satellites to ensure continuous coverage. The Indian Space Research Organisation (ISRO) is expected to play a crucial role in developing and deploying these quantum communication satellites.

The development of quantum communication technology also supports India's broader quantum technology objectives outlined in the National Quantum Mission. These objectives include developing intermediate-scale quantum computers with 50-1000 physical qubits, establishing satellite-based secure quantum communications over 2000 kilometers, and creating multi-node quantum networks with quantum memories. The mission also encompasses quantum sensing applications, including magnetometers and atomic clocks for precision timing and navigation.

Conclusion

IIT-Delhi's quantum communications breakthrough represents a watershed moment in India's technological development, demonstrating the country's growing capabilities in one of the most strategically important emerging technologies of the 21st century. The successful demonstration of free-space quantum key distribution over one kilometre not only advances the science of quantum communication but also establishes India as a serious competitor in the global race to develop practical quantum technologies.

The achievement's significance extends beyond its technical merits to encompass strategic, economic, and scientific dimensions that could reshape India's position in the global technology landscape. With continued investment through the National Quantum Mission and the commitment of institutions like DRDO and IIT-Delhi, India is well-positioned to develop the satellite-based quantum communication networks that could provide unbreakable security for both military and civilian applications.

As the world moves toward an era where quantum technologies will determine national competitiveness and security, India's quantum communications breakthrough establishes a foundation for future developments that could have profound implications for cybersecurity, national defence, and economic growth. The progression from laboratory experiments to practical demonstrations suggests that quantum-secured communications may become a reality sooner than many anticipated, potentially transforming how sensitive information is protected and transmitted in the digital age.

IDN