What Happened
- Researchers from S. N. Bose National Centre for Basic Sciences (an autonomous institution under India's Department of Science and Technology) proved that no finite amount of classical communication can faithfully simulate a quantum communication channel
- The study, published in Proceedings of the Royal Society A (2026), was conducted by Sahil Gopalkrishna Naik and Manik Banik from SNBNCBS, in collaboration with Mani Zartab (Universitat Autònoma de Barcelona) and Nicolas Gisin (University of Geneva)
- The research established a formal "no-go theorem" — a mathematically rigorous impossibility proof — showing that quantum communication channels cannot be classically simulated, even with unlimited multi-round, bidirectional classical messaging
- The key finding: when multiple distant senders try to reproduce quantum measurement outcome statistics at a central location using only classical communication, it is fundamentally impossible for systems involving entangled measurements
- The study reinforces quantum advantage as a matter of principle, not merely current engineering limitations
Static Topic Bridges
Quantum Communication: Principles and Significance
Quantum communication uses the laws of quantum mechanics — particularly the principles of superposition and entanglement — to transmit information in ways that are fundamentally more secure and information-rich than classical communication.
- A qubit (quantum bit) is the basic unit of quantum information; unlike a classical bit (0 or 1), a qubit can exist in a superposition of both states simultaneously
- Quantum entanglement: When two particles are entangled, measuring one instantly determines the state of the other, regardless of the distance between them (Einstein called it "spooky action at a distance")
- Quantum Key Distribution (QKD): The most advanced practical application of quantum communication; enables theoretically unbreakable encryption (any eavesdropping disturbs the quantum state, alerting the communicating parties)
- No-cloning theorem: Quantum information cannot be copied perfectly — a fundamental law that makes quantum communication inherently secure
- Classical communication uses bits (0/1 based electrical/optical signals); quantum communication uses quantum states that can carry exponentially more information
Connection to this news: The study formally proves that even if we had infinite classical bandwidth, we could not replicate what a quantum channel does — establishing the absolute, irreducible necessity of quantum infrastructure for quantum communication tasks.
Quantum Advantage and Why It Matters
Quantum advantage (also called quantum supremacy in specific contexts) refers to the ability of quantum systems to perform tasks that classical computers or communication systems cannot, or can only perform with resources growing exponentially with problem size.
- Computational quantum advantage: Demonstrated by Google's Sycamore processor in 2019 (performed a specific calculation in 200 seconds that would take classical computers 10,000 years)
- Communication quantum advantage: This study proves that certain communication tasks (faithfully simulating a qubit channel) require quantum channels — no amount of classical communication suffices
- The distinction matters: previous demonstrations showed quantum advantage in practice; this study proves it in principle (a no-go theorem)
- Entangled measurements — which cannot be decomposed into separate local measurements — are the key mechanism behind this impossibility
- The result has implications for quantum networking, distributed quantum computing, and quantum cryptography protocols
Connection to this news: The no-go theorem proved by Indian and international researchers closes a theoretical question that had remained open: quantum channels are not merely more efficient than classical channels — they are categorically different and irreplaceable.
India's Quantum Technology Mission
India launched the National Quantum Mission (NQM) in April 2023 with a budget of ₹6,003 crore over 8 years (2023-2031) to develop quantum technologies across four verticals: quantum computing, quantum communication, quantum sensing, and quantum materials.
- NQM targets: Develop intermediate-scale quantum computers with 50-1,000 physical qubits in 8 years; satellite-based QKD over 2,000 km; quantum communication networks between cities; quantum metrology for precision timing
- Key institutions: IISc, IITs, TIFR, and DST autonomous institutions like S. N. Bose National Centre for Basic Sciences (authors of this study)
- India's first long-distance quantum communication: IIT Delhi demonstrated secure 1 km free-space quantum communication (QKD) using entangled photons
- Quantum secure satellite communication is part of ISRO's roadmap
- The DST has also supported quantum initiatives through the Quantum Enabled Science and Technology (QuEST) programme
Connection to this news: The study directly emerges from institutions funded under India's quantum research ecosystem. The finding — that quantum channels cannot be classically replaced — validates the strategic importance of India's NQM investment: quantum communication networks must use genuine quantum infrastructure, not classical approximations.
Entanglement and Entangled Measurements
Quantum entanglement is a phenomenon where two or more particles are correlated such that the quantum state of one cannot be described independently of the others. Entangled measurements extend this to measurement operations — measuring an entangled state requires global, joint operations that cannot be decomposed into local measurements.
- Entanglement is a resource: like energy, it can be created, stored, consumed, and quantified (measured in "ebits" — entanglement bits)
- Bell states: The four maximally entangled two-qubit states — foundational to quantum teleportation, superdense coding, and QKD protocols like E91
- Entangled measurements (joint measurements): Operations that measure multiple entangled particles simultaneously; their outcomes cannot be predicted from separate measurements of individual particles
- The study shows entangled measurements are the core obstacle for classical simulation — no classical protocol can produce their statistics
- This distinguishes quantum channels from classical ones at the most fundamental mathematical level
Connection to this news: The impossibility result in the study hinges specifically on entangled measurements — proving that the quantum nature of measurement, not just transmission, is irreducible. This has direct implications for designing quantum networks: genuine quantum measurement apparatus, not classical hardware, is non-negotiable.
Key Facts & Data
- Study published in: Proceedings of the Royal Society A, 2026 (DOI: 10.1098/rspa.2025.0831)
- Lead institution: S. N. Bose National Centre for Basic Sciences, Kolkata (DST autonomous institution)
- Indian researchers: Sahil Gopalkrishna Naik, Manik Banik
- International collaborators: Mani Zartab (Universitat Autònoma de Barcelona), Nicolas Gisin (University of Geneva)
- Core finding: No finite classical communication — even multi-round, bidirectional — can simulate a qubit channel
- The impossibility arises from entangled measurements, which have no classical equivalent
- India's National Quantum Mission: ₹6,003 crore over 8 years (2023-2031)
- NQM target: Quantum computers with 50-1,000 qubits; satellite QKD over 2,000 km
- Practical application: Validates the necessity of genuine quantum infrastructure for secure quantum communication networks