Quantum computing

1. Scientific foundations: bits, qubits and measurement

Quantum computing applies the laws of quantum mechanics to information processing. A classical computer represents information using bits, each of which is either 0 or 1. A quantum computer uses quantum bits, or qubits. A qubit can be represented as a superposition of two basis states, conventionally written as |0⟩ and |1⟩. Physical implementations include superconducting electrical circuits, trapped ions, neutral atoms, photons and electron or nuclear spins.

Superposition does not mean that a user can read every possible answer simultaneously. When a qubit is measured in the computational basis, the outcome is 0 or 1, with probabilities determined by its quantum state. An n-qubit system requires 2ⁿ complex amplitudes to describe a general pure state mathematically, but measurement does not reveal all these amplitudes. Useful computation therefore requires carefully designed algorithms rather than merely creating many superposed states.

Entanglement creates correlations between quantum systems that cannot generally be described as independent states of their parts. It is an important resource, but cannot transmit usable information faster than light. Interference allows quantum amplitudes to reinforce or cancel one another. Algorithms exploit this behaviour to increase the likelihood of useful measurement outcomes. Superposition, entanglement and interference should therefore be understood together, not as interchangeable terms.

• Quantum gates transform qubit states; a sequence of gates forms a quantum circuit.
• The no-cloning theorem prohibits perfect copying of an arbitrary unknown quantum state.

2. Capabilities, algorithms and practical limitations

Quantum computers are specialised computational tools, not automatic replacements for laptops, servers or supercomputers. Their advantages arise only where a suitable algorithm exploits quantum properties. Quantum simulation is especially promising because molecules and materials themselves obey quantum mechanics. Potential applications include understanding chemical reactions, catalysts, battery materials and strongly interacting particles. However, translating a theoretical advantage into commercially useful performance requires reliable hardware and comparison with the best classical methods.

Shor’s algorithm can factor integers and solve discrete-logarithm problems efficiently on an ideal quantum computer. These problems underpin important public-key cryptographic systems. Grover’s algorithm offers a quadratic reduction in the number of queries needed for unstructured search; it does not provide an exponential speed-up or make every search problem easy. Proposed applications in optimisation, finance and machine learning remain active research areas, with many practical advantages still unproven.

Quantum advantage means outperforming classical methods on a specified task under stated conditions. It does not establish superiority across all problems, and a benchmark may have little direct commercial value. Present experimental devices suffer from noise and errors, often discussed under the term noisy intermediate-scale quantum, or NISQ, computing. Many useful algorithms are expected to require fault-tolerant machines operating with error-corrected logical qubits.

• Hybrid approaches combine classical processing with quantum subroutines.
• Quantum annealing and universal gate-based quantum computing are different computational approaches.

3. Hardware, decoherence and error correction

Qubits are difficult to control because interaction with their environment can destroy the quantum coherence needed for computation. This process is called decoherence. Imperfect gates, unwanted interactions between qubits and measurement errors further reduce accuracy. A useful processor must initialise qubits, manipulate them precisely, maintain coherence for sufficient time and measure results reliably. Increasing the number of qubits without improving their quality may not produce a more capable computer.

Different hardware platforms involve different engineering trade-offs. Superconducting processors usually require dilution refrigerators operating at extremely low temperatures. Trapped-ion systems use electromagnetic fields to confine ions and commonly use lasers for control. Photonic systems encode information in properties of light, while neutral-atom systems can use optical tweezers to arrange and manipulate atoms. Not every quantum-computing platform requires the same cooling arrangements, and there is no universally accepted winning architecture.

Quantum error correction encodes a logical qubit across multiple physical qubits and detects error information without directly measuring the encoded computational state. This does not violate the no-cloning theorem because it is not simple copying of an unknown state. Fault tolerance seeks to prevent errors from spreading uncontrollably during computation. The overhead can be substantial and depends on hardware error rates, the code used and the algorithm.

• Physical qubit count and logical qubit count are not equivalent measures of computing capability.
• Important performance indicators include gate fidelity, coherence time, connectivity and error-corrected operation.

4. Cybersecurity and related quantum technologies

A sufficiently powerful fault-tolerant quantum computer could undermine RSA and elliptic-curve cryptography using Shor’s algorithm. These systems support secure communications, authentication and digital signatures. This does not mean that today’s quantum computers can routinely break modern encryption. Nevertheless, information requiring long-term confidentiality faces a harvest-now, decrypt-later risk: adversaries may store encrypted traffic today for possible decryption in the future.

Post-quantum cryptography, or PQC, uses mathematical algorithms designed to resist known classical and quantum attacks. It runs on conventional computers and networks, so it does not require quantum communication hardware. In August 2024, the US National Institute of Standards and Technology finalised its first three principal PQC standards: ML-KEM for key establishment, and ML-DSA and SLH-DSA for digital signatures. Migration requires inventories of cryptographic systems, testing and crypto-agility.

Quantum key distribution, or QKD, instead uses quantum states to establish shared secret keys. Under appropriate assumptions, disturbance and measurement statistics can expose eavesdropping. It requires an authenticated classical channel and remains vulnerable to implementation flaws. QKD is therefore not synonymous with unhackable communication. Quantum sensing uses quantum effects for precision measurements, including magnetic fields, gravity and time, and can deliver benefits independently of universal quantum computers.

• PQC changes cryptographic algorithms; QKD changes how keys are distributed.
• Grover’s algorithm affects symmetric-key security differently from Shor’s attack on public-key systems.

5. India’s National Quantum Mission and strategic significance

The Union Cabinet approved the National Quantum Mission on 19 April 2023. Implemented by the Department of Science and Technology, it has an approved outlay of ₹6,003.65 crore for 2023–24 to 2030–31. The mission aims to strengthen research, technology development and an indigenous quantum ecosystem. One stated objective is developing intermediate-scale quantum computers with 50–1,000 physical qubits over eight years across platforms such as superconducting and photonic technologies.

The mission also targets satellite-based secure quantum communication between ground stations over a range of 2,000 kilometres within India, long-distance secure communication with other countries, inter-city QKD and multi-node quantum networks. Other priorities include high-sensitivity magnetometers, precision atomic clocks, quantum materials, single-photon sources and detectors. These are programme targets, not evidence that all systems have already been deployed or that their security is unconditional.

Four thematic hubs have been established: quantum computing at the Indian Institute of Science, Bengaluru; quantum communication at IIT Madras; quantum sensing and metrology at IIT Bombay; and quantum materials and devices at IIT Delhi. India’s policy challenge extends beyond processor development to skilled personnel, fabrication, cryogenics, photonics, standards and cybersecurity preparedness. Public investment should support fundamental research while assessing demonstrable performance and avoiding exaggerated commercial claims.

• Quantum technology has strategic relevance for defence, telecommunications, healthcare, materials research and secure digital infrastructure.
• For Prelims, distinguish announced mission targets from achieved capabilities, and physical qubits from fault-tolerant logical qubits.

Quantum computing: from qubits to national capability

The supplied metadata identifies an event marking the opening of the United Kingdom’s National Quantum Computing Centre. It is directly relevant as an institutional example, not as evidence of a particular processor’s design or performance. Quantum computing encodes information in qubits and uses controlled operations and interference to shape measurement outcomes. Superposition does not mean that every possible answer can simply be read out; useful computational advantage requires suitable algorithms and reliable hardware.

Research centres help connect experimental hardware, software, specialist skills and industrial applications. Their existence, however, does not establish fault-tolerant computing or commercially useful quantum advantage. Noise, decoherence and operational errors remain major constraints, while error correction generally requires multiple physical qubits to support a protected logical qubit. For India, the comparison is relevant to the National Quantum Mission, whose scope includes quantum computing, communication, sensing and metrology, and materials and devices. The UK example illustrates international institution-building; it is not evidence of Indian achievements or proof that quantum technologies have reached equal maturity.

• Capability assessment: physical qubit counts alone are insufficient; error rates, connectivity, coherence and logical-qubit performance also matter.
• Cryptographic distinction: sufficiently capable quantum computers could threaten widely used public-key schemes; post-quantum cryptography uses classical algorithms and differs from quantum key distribution.
• Mains linkage: assess India’s quantum ecosystem through research capacity, skilled personnel, component supply chains and demonstrated applications, rather than treating mission announcements as technological outcomes.

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