Quantum technology

The first principle

At the scale of atoms, electrons and photons, nature does not behave like billiard balls: a particle's state is described by amplitudes for each possible outcome, these amplitudes can add or cancel like waves, and measuring the particle forces one definite outcome with a probability set by those amplitudes. Quantum technology deliberately uses these rules, superposition, interference, entanglement and the disturbance caused by measurement, as resources. A quantum computer uses them to compute, quantum communication uses them to detect eavesdropping, and quantum sensing uses them to measure, which is why one mission funds all three.

The core

The ideas in plain terms

• The qubit. A classical bit is either 0 or 1. A quantum bit, or qubit, is any two state quantum system, such as the spin of an electron, two energy levels of a trapped ion, the polarisation of a photon or a current in a superconducting circuit, and it can be in a superposition: a combination of 0 and 1 with a definite amplitude for each. A measurement always gives 0 or 1, with probabilities equal to the square of each amplitude's size, so a qubit is not "both at once" in any usable sense until the amplitudes are made to interfere.
• Interference. Amplitudes, unlike ordinary probabilities, can cancel. A quantum algorithm is a sequence of operations arranged so that the amplitudes of wrong answers cancel and those of right answers add up, which is where any speed up comes from.
• Entanglement. Two qubits can share a joint state in which neither has a state of its own, so their measured results are correlated more strongly than any classical system allows, however far apart they are. Entanglement cannot be used to send a message faster than light, because each result taken alone is random; experiments testing this, by Alain Aspect, John Clauser and Anton Zeilinger, won the Nobel Prize in Physics 2022.
• Scaling. Describing n entangled qubits needs 2ⁿ amplitudes, so 50 qubits already need about a thousand million million numbers, which is why classical computers cannot simulate large quantum systems and why quantum computers might.
• Decoherence. Any stray interaction with the surroundings, heat, vibration or a passing field, effectively measures the qubit and destroys its superposition. Qubits must therefore be isolated, often cooled to near absolute zero, and errors corrected by spreading one reliable logical qubit over many physical qubits.

Three technologies from one physics

• Quantum computing. A quantum computer applies sequences of operations, called gates, to many entangled qubits. It is not faster at everything; it is expected to excel at specific problems, such as factoring large numbers by Shor's algorithm, which would break today's public key encryption, and simulating molecules and materials for drugs, batteries and fertilisers. Leading hardware routes are superconducting circuits, trapped ions, neutral atoms and photons. The Nobel Prize in Physics 2025 went to John Clarke, Michel Devoret and John Martinis for showing quantum behaviour in electrical circuits large enough to hold in the hand, the basis of superconducting qubits.
• Quantum communication. Quantum key distribution (QKD) sends the bits of a secret key encoded in single photons. Under the BB84 protocol, any eavesdropper must measure the photons and so disturbs them, and the two users detect the errors and discard the key; security therefore rests on physics, not on a hard mathematical problem. Photons are lost in fibre, so links reach only tens to a few hundred kilometres unless relayed by trusted nodes or satellites.
• Quantum sensing. Because quantum states are so sensitive to their surroundings, they make superb sensors: atomic clocks keep time to better than a second in millions of years, atom interferometers measure gravity and acceleration for navigation without satellites, and magnetometers detect the faint fields of the heart and brain.

Dimensions

• Security. A large error corrected quantum computer running Shor's algorithm would break the public key systems that protect banking, government and internet traffic. Since encrypted data can be stored today and decrypted later, the threat called "harvest now, decrypt later" makes migration urgent for secrets that must last for decades.
• Strategic. Quantum computing, communication and sensing are seen as dual use, so countries restrict exports of components such as dilution refrigerators and invest in national missions; quantum navigation could reduce dependence on satellite signals that can be jammed.
• Economic. The early value lies in sensing, secure communication and simulation for chemistry and materials rather than general computing, and a quantum industry needs cryogenics, lasers, photonics and precision electronics that India must also build.
• Scientific. Quantum devices test the foundations of physics and serve as laboratories for new materials, and the same skills feed advanced electronics and metrology.
• Ethical and social. If only a few states or firms hold working quantum computers, the power to read others' encrypted data would be unequal, which is why open post quantum standards matter for every country.

In practice

• National Quantum Mission (NQM). Approved by the Union Cabinet in April 2023 with Rs 6,003.65 crore for 2023-24 to 2030-31 and run by the Department of Science and Technology (DST), it aims at quantum computers of 50 to 1,000 physical qubits on superconducting and photonic platforms, satellite based quantum communication over 2,000 kilometres, fibre based quantum key distribution between cities, and magnetometers and atomic clocks of high sensitivity. It works through four thematic hubs: computing at the Indian Institute of Science (IISc) Bengaluru, communication at the Indian Institute of Technology (IIT) Madras with the Centre for Development of Telematics (C-DOT), sensing and metrology at IIT Bombay, and materials and devices at IIT Delhi.
• Demonstrations. The Indian Space Research Organisation (ISRO) demonstrated free space quantum communication over about 300 metres in 2021, the Defence Research and Development Organisation (DRDO) demonstrated QKD between Prayagraj and Vindhyachal, about 100 kilometres apart, in 2022, and DRDO with IIT Delhi demonstrated entanglement based free space quantum secure communication over more than a kilometre in June 2025.
• Hardware. QpiAI, a Bengaluru start up supported under the mission, released a 25 qubit superconducting quantum computer, QpiAI-Indus, in April 2025.
• Global benchmarks. Google claimed quantum advantage on a sampling task in 2019, and its 105 qubit Willow chip showed in December 2024 that errors fell as more qubits were added to an error corrected code, a key threshold. The United States National Institute of Standards and Technology (NIST) published the first post quantum cryptography standards in August 2024.

Issues and way forward

• Hardware dependence. Dilution refrigerators, single photon detectors, special lasers and high purity materials are mostly imported, so the materials and devices hub and partnerships with industry must build a domestic supply chain.
• Error correction is the wall. Useful quantum computing needs thousands of reliable logical qubits, each built from many physical ones; India should pursue niche platforms and applications where it can lead, rather than racing on qubit counts alone.
• Migrating encryption. Government, banking and power systems need an inventory of where public key encryption is used and a timetable to move to post quantum algorithms, with QKD reserved for the most sensitive links.
• People. Quantum work needs physicists, engineers and computer scientists trained together, so courses in universities beyond the hubs and fellowships to retain researchers are as important as equipment.
• From laboratory to market. Start ups need early government orders, testbeds and standards, so that demonstrations become products.

Common confusions

• Superposition and "both at once". A qubit in superposition has amplitudes for 0 and 1, but a measurement yields only one, so the advantage comes from interference among amplitudes, not from reading out many answers in parallel.
• Entanglement and faster than light signalling. Entangled results are correlated, but each side alone sees random results, so no message travels faster than light.
• QKD and post quantum cryptography. QKD is a physical way of sharing a key that needs special hardware, while post quantum cryptography is new mathematics that runs as software on ordinary computers.
• Physical and logical qubits. Physical qubits are the noisy hardware; a logical qubit is a reliable qubit built from many physical ones by error correction.
• Quantum advantage and useful advantage. Beating a classical computer on a contrived benchmark is not the same as solving a problem anyone needs solved faster.

For the exam

• Keyword bank: qubit, superposition, interference, entanglement, decoherence, quantum error correction, logical qubit, quantum advantage, Shor's algorithm, quantum key distribution, post quantum cryptography, harvest now decrypt later, quantum sensing, National Quantum Mission, thematic hubs.
• Previous year questions: Practise the previous year questions on this topic →
• Use in an answer:
  • Introduction with data: The National Quantum Mission, approved in April 2023 with Rs 6,003.65 crore up to 2030-31, aims at quantum computers of 50 to 1,000 physical qubits and satellite quantum links over 2,000 kilometres.
  • Introduction with a source: The Nobel Prize in Physics 2022 recognised experiments on entangled photons, and the 2025 prize recognised quantum effects in electrical circuits, the two foundations of quantum communication and superconducting quantum computers.
  • Conclusion: Quantum technology is three bets on one physics; communication and sensing will pay first, while computing needs error correction at scale, so India's mission must balance near term products with long term research.
  • Conclusion: Whatever the pace of quantum computers, the move to post quantum encryption must begin now, because data stolen today can be read tomorrow.