Indian team stabilizes telecom fiber for quantum-secure links, eyeing a 2,000-km network

The coils of glass in the corner of a lab at the Inter-University Centre for Astronomy and Astrophysics (IUCAA) do not look like the backbone of a future quantum network.

They resemble any other spools of telecom cable. But on a recent test in Pune, laser light sent through those fibers and back again returned almost perfectly in step with itself—its rhythm so stable that it barely wavered under temperature swings, vibrations and the usual noise of an urban campus.

That stability, Indian researchers say, is enough to turn ordinary internet fiber into a “quantum-grade” channel—and it could underpin a planned 2,000-kilometer secure quantum communication network across the country.

In a study published Nov. 14, 2025, in Communications Physics (Nature Portfolio), a team led by scientists at IUCAA in Pune, working with Jaypee Institute of Information Technology (JIIT) in Noida, reported a fiber-based system that keeps laser light phase-locked over tens of kilometers of standard telecom cable.

The system, branded PhotonSync, suppresses phase noise in the fiber link by about 47–48 decibels and achieves fractional frequency stability on the order of 2 × 10⁻¹⁶ over relevant averaging times—benchmarks more typical of national metrology links than commercial fiber used for phone calls and video streams.

“PhotonSync transforms an ordinary telecom optical fiber into a highly specialized, ultra-stable link,” said Subhadeep De, a professor at IUCAA who led the work. He said the system preserves the “precisely defined quantum characteristics” of light over long distances, enabling it to carry secure quantum information as well as extremely precise timing signals.

Turning glass into a quantum channel

At its core, PhotonSync is a combination of optics and control electronics wrapped around standard single-mode fiber.

The researchers used a narrow-linewidth laser at the standard telecom wavelength of 1,550 nanometers, along with acousto-optic modulators, Faraday mirrors and optical isolators arranged in a bidirectional configuration. Light is sent down the fiber, reflected back at the far end, and then compared with the original beam.

Any difference between the outgoing and returning light reveals how the fiber and the laser have drifted. A field-programmable gate array (FPGA)-based controller, disciplined by a rubidium atomic clock synchronized to GPS, drives the modulators in real time to cancel out these disturbances.

In effect, the fiber becomes its own sensor. Environmental noise—temperature changes, mechanical stress, acoustic vibrations—that would ordinarily scramble the phase of the light is measured and subtracted almost as soon as it appears.

The team tested the system on two main links: a 3.3-kilometer loop of buried fiber laid across and beyond the IUCAA campus, and a 71-kilometer length of fiber arranged in spools in the lab. In both cases, they reported phase-noise suppression near 47.5 dB and frequency stability around the 10⁻¹⁶ level, with integrated phase fluctuations in the milliradian range.

Crucially, they also addressed a slower, more insidious problem: the gradual drift of the laser’s own frequency as components age and the reference cavity shifts. In the paper, the researchers report that the intrinsic drift of their laser—about 33.8 millihertz per second—was reduced to roughly 0.05 millihertz per second with an optical self-referencing technique that uses the fiber itself as a delay line.

The combination of a phase-coherent fiber (PCF) link and an actively drift-corrected laser is presented as the main technical advance.

“We report the development of a system that produces phase-coherent fiber links and also actively corrects the unavoidable slow frequency drift of the source laser,” the authors wrote, arguing that this paired capability enables a “quantum link” suitable for advanced communication protocols.

Built for twin-field quantum key distribution

While ultra-stable optical links have been demonstrated in Europe, the United States and China for more than a decade—often to compare atomic clocks between national metrology institutes—the IUCAA–JIIT system was designed from the outset with quantum cryptography in mind.

Specifically, the group targeted twin-field quantum key distribution (TF-QKD), an approach proposed in 2018 that allows two distant users to share secret encryption keys via a central measurement station, potentially over hundreds of kilometers of fiber without intermediate “trusted” repeater nodes.

TF-QKD relies on the interference of extremely weak laser pulses from the two users at the central station. If the relative phase between those pulses wanders too much—because the lasers drift or the fibers carrying them fluctuate—the interference visibility drops and the error rate rises, undermining the protocol’s security and range.

By stabilizing both the laser and the fiber link to the levels reported, the IUCAA–JIIT team estimates that the quantum bit error rate in a TF-QKD system could be reduced by a factor of about 73 compared with using an unstabilized link.

In an interview with The Times of India, Anirban Pathak, a professor at JIIT who leads its Centre for Quantum Science and Technology and co-authored the study, said India’s near-term quantum communication plans assume a 2,000-kilometer fiber network connected by trusted nodes. With strong phase stabilization such as PhotonSync, he said, TF-QKD could support secure links of roughly 1,000 kilometers without those intermediate trusted stations.

Trusted nodes are a sensitive point in quantum networking. At such stations, keys are decrypted and re-encrypted, creating potential vulnerabilities if the node is compromised or its operators are coerced. Protocols like TF-QKD, if backed by sufficiently stable optical infrastructure, can shift more trust back to end users.

National mission, existing infrastructure

The work lands as India ramps up its National Quantum Mission, approved by the Union Cabinet and announced by the Prime Minister’s Office with a mandate to support quantum computing, communications, sensing and materials research.

Official mission documents list as major goals satellite-based secure quantum links between ground stations over 2,000 kilometers and inter-city quantum key distribution over comparable distances, along with multi-node quantum networks using quantum memory devices.

According to the Department of Science and Technology, the earlier Quantum-Enabled Science and Technology (QuEST) program, launched in 2018 with a budget of about ₹250 crore, seeded more than 50 academic projects in quantum technologies. PhotonSync grew out of that ecosystem, with support from QuEST and related funding streams.

What makes the IUCAA–JIIT demonstration notable for policymakers and industry is its reliance on existing physical infrastructure.

Both IUCAA and national media accounts emphasize that the system operates over the same type of optical fiber already deployed across the country’s telecom backbone. No exotic “quantum fiber” is required; the upgrades are at the endpoints, in the form of laser sources, stabilization optics and controller hardware.

That opens several potential pathways. State-owned and private telecom operators could, in principle, offer “quantum-ready” leased lines by installing PhotonSync-like units at select nodes on their networks. Defense public sector undertakings such as Bharat Electronics Ltd. could adapt the design into ruggedized units for military and strategic communications.

De told The Indian Express that the technology, developed and patented in India, contributes to the broader goal of “Viksit Bharat” (developed India) by supporting secure national infrastructure, ultra-precise clock comparisons and distributed sensor networks.

The name “PhotonSync” has been trademarked, and IUCAA-affiliated sources say patent filings cover key aspects of the design. Independent searches of public patent databases were not immediately available.

Limits and open questions

Despite the strong performance figures, the system remains at the prototype stage.

The longest links demonstrated so far are 3.3 kilometers of buried campus fiber and roughly 71–80 kilometers of spooled cable in the laboratory. Scaling to hundreds or thousands of kilometers will require chaining multiple stabilized segments, managing amplification and loss, and integrating with higher-level quantum network protocols and—ultimately—quantum repeaters or memory units where necessary.

No public report has yet described a full end-to-end TF-QKD demonstration over national-scale distances using PhotonSync. The technology is better understood as a piece of the physical transport layer—the plumbing that keeps light stable—rather than a complete quantum communication service.

There are also policy questions. Quantum key distribution promises information-theoretic security, which could protect defense, diplomatic and critical infrastructure communications even against future quantum computers. But such security could also complicate lawful interception and oversight.

Government agencies have not yet detailed how access to quantum-safe links will be prioritized or regulated. Earlier Indian pilots in QKD, including Army-tested links developed with private startups, suggest defense and strategic sectors are likely to be first in line.

Internationally, competition is intense. China’s University of Science and Technology and European consortia have reported long-distance TF-QKD experiments and coherent phase-transfer links, while other strategies focus on satellite-based QKD or on post-quantum classical cryptography that does not require changes to physical infrastructure.

The IUCAA–JIIT team does not claim an outright world record across all metrics, but their reported stability and phase-noise figures compare favorably with several published long-haul links. The work also stands out as an indigenous system tied directly to a national deployment agenda.

Back in Pune, the lab’s spools of glass and quiet racks of electronics are easy to overlook. Yet the light circulating through them—locked in phase to one part in ten quadrillion—hints at a different way of thinking about the internet’s physical layer.

If systems like PhotonSync move from lab benches into field-deployed hardware, India’s existing web of telecom fiber could double as the nervous system of a quantum-secure backbone, carrying not just data but the precisely controlled photons on which the next generation of cryptography and timekeeping may depend.