2025 is being recognized as the century year of the quantum mechanics by the UNESCO, and a new beginning for the quantum driven science and technology; an area that is emerging to make the daily life better, strengthening the strategic sectors and also uplifting the science exploration capability. IUCAA has developed a critical technology, namely the PhotonSync that uses normal telecom fibers and uplift its capability to build a quantum network among sensors, computers, and communication. The normal fibers are affected due to the temperature fluctuations, acoustic, seismic noises and other environmental effects. These unavoidable effects add phase noise to photons transported through the fibers therefore lacks in carrying signals with unprecedented accuracy, which brings certain limitation for carrying quantum information. The Precision and Quantum Measurement Laboratory, commonly known as the PQM-lab (https://pqmlab.iucaa.in/) at IUCAA, have designed, developed and demonstrated working of an ultra-stable “Phase Coherent Fiber (PCF)” link using indigenously developed hardware PhotonSync. The related scientific results are published in Communications Physics and also trademark is obtained for the PhotonSync. The hardware makes a normal telecom fiber to a special ultrastable fiber link, thereby makes it capable to transfer data in the form of photons preserving their precisely predetermined characteristics from one end to another distant location. A potential application of the PCF link developed by IUCAA are analyzed theoretically by the Jaypee Institute of Information Technology (JIIT) team with a specific focus on twin field quantum key distribution (TFQKD).

Scientific Content

Phase coherent fibers (PCF) are essential to distribute nearly monochromatic optical photons along with ultra-stable in their frequency and phases, which have demanding requirements for state-of-the-art networked experiments, quantum as well as very high-speed communications. IUCAA have developed a novel PhotonSync system that is capable to produce the ultrastable PCF links, also the hardware actively corrects the unavoidable slow frequency drift of the source laser. This all-in-one PhotonSync solution producing a quantum link which has wide range of applications, as following,

• Enhance the effectiveness of the twin field quantum key distribution (TF-QKD) by nearly a 73-fold reduction of the quantum bit error rate (QBER) that arises from using unstabilized fiber links, as well as relaxes the laser frequency drift correction constraints by severalfold.
• Time, frequency and phase synchronization of the geographically separated nodes: Alice Bob, Charlie etc. Therefore, useful for optical fiber-based quantum communication.
• Intercomparing atomic clocks with unprecedented accuracy those are situated distant apart.
• Very highspeed ~100 Tb/s optical communication e.g., advanced 6G.

The developed system is tested using field deployed optical fibers up to a length of 3.3 km as well as using fiber spools up to 71 km. The PCF follows white phase noise limited σ_o × τ⁻¹ stability behavior having σo values 1.9(2) × 10⁻¹⁶ and 2.6(1) × 10⁻¹⁶ for a 3.3 km field-deployed and 71 km spool fibers, respectively, with up to 47.5 dB suppression of the phase noise compared to a normal fiber.

Additionally, the system is featured to correct the source laser’s 33.8 mHz/s frequency drift to as lowas ≃ 0.05 mHz/s.

Frequency drift corrected ultra-stable laser through phase-coherent fiber producing a quantum channel
Stanley Johnson, Sandeep Mishra, Anirban Pathak, Subhadeep De
Published in Nature portfolio journal: Communications Physics volume 8, Article number: 508 (2025) [DOI: 10.1038/s42005-025-02412-7]

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