Quantum computing

Quantum computing is a model of computation that harnesses the principles of quantum mechanics, first articulated as a feasible idea by Richard Feynman (1981–82) and formalised by David Deutsch (1985), who described a universal quantum Turing machine. Whereas a classical computer stores information in bits that are strictly 0 or 1, a quantum computer uses qubits that exist in a superposition of both states simultaneously. Through entanglement, the state of one qubit becomes correlated with another regardless of distance, and through quantum interference, computational paths reinforcing the correct answer are amplified while wrong paths cancel. An n-qubit register can represent 2ⁿ states at once, giving quantum machines an exponential state space that underlies their theoretical advantage for specific problems. The discipline rests on landmark algorithms: Peter Shor's 1994 algorithm for integer factorisation (which threatens RSA cryptography) and Lov Grover's 1996 algorithm for unstructured search (a quadratic speed-up).

In practice, qubits are realised through several competing hardware approaches: superconducting circuits (IBM, Google), trapped ions (IonQ, Quantinuum), photonics (Xanadu, PsiQuantum), neutral atoms, and topological qubits (Microsoft's Majorana programme). The central engineering obstacle is decoherence—qubits lose their quantum state through environmental noise within microseconds—necessitating cryogenic cooling near absolute zero and elaborate quantum error correction, where many physical qubits encode one stable logical qubit. The present era is described as NISQ (Noisy Intermediate-Scale Quantum), a term coined by John Preskill in 2018, denoting devices with tens to hundreds of imperfect qubits insufficient for fault-tolerant computation. In 2019 Google's Sycamore processor claimed quantum supremacy, completing in 200 seconds a sampling task it argued would take classical supercomputers millennia, a claim contested by IBM. In December 2024 Google unveiled its Willow chip, demonstrating below-threshold error correction.

For India, quantum computing is a strategic priority. The Union Government launched the National Quantum Mission (NQM) in April 2023 with an outlay of ₹6,003.65 crore (2023–2031), targeting intermediate-scale quantum computers of 50–1000 physical qubits across superconducting and photonic platforms, alongside thematic hubs in computing, communication, sensing/metrology, and materials. This complements the earlier QuEST programme of the Department of Science and Technology. Globally, the US National Quantum Initiative Act (2018), the EU Quantum Flagship (2018, €1 billion), and China's substantial investment—including Pan Jianwei's Micius satellite (2016) for quantum key distribution and the Jiuzhang photonic processor—define an intensifying technology competition. The associated security threat of "harvest now, decrypt later" has driven NIST's 2024 standardisation of post-quantum cryptography algorithms.

For the UPSC examination, quantum computing recurs in the General Studies Paper III science and technology section, and increasingly in Prelims current-affairs questions. The most common angles are: the basic distinction between qubits and bits and the meaning of superposition and entanglement; the institutional facts of the National Quantum Mission (year, ministry, budget, the four thematic verticals); the cryptographic implications of Shor's algorithm and post-quantum cryptography; and the strategic dimension of quantum communication, satellite QKD, and the global race. Candidates should pair the conceptual mechanism with these dated Indian and international policy milestones, since questions typically blend the two.