Quantum concepts: qubit, superposition, entanglement, decoherence
Prelims and MainsCurrent affairs on this: Quantum science and technology
A classical bit is 0 or 1; a qubit, a quantum bit, is a system such as an electron's spin or a superconducting circuit that can be in a superposition, a combination of 0 and 1 at once, until it is measured, when it gives one answer. Two qubits can be entangled, so that measuring one fixes the other however far apart they are, which Einstein called spooky and experiment has confirmed. A quantum computer uses both to work on many possibilities together, which for factoring, searching and simulating molecules promises speed no classical machine can match. The obstacle is decoherence: any disturbance from the environment collapses the superposition, so qubits must be isolated, often near absolute zero, and errors corrected by using many physical qubits for one reliable one. Quantum communication uses the same fragility as a strength: an eavesdropper on a quantum key disturbs it and is detected.
Two answers to the quantum threat to encryption
| Quantum key distribution | Post quantum cryptography | |
|---|---|---|
| What it is | a physical way to share a secret key using photons | new mathematical algorithms run on ordinary computers |
| What protects it | the laws of physics: measuring the key disturbs it | hardness of problems no known quantum algorithm solves |
| Needs | dedicated fibre or line of sight, or a satellite; special hardware | a software update |
| Range | tens to a few hundred km per link; satellites for more | unlimited |
| Ready today | demonstrations and pilot networks | standards published 2024, deployment under way |
| India | National Quantum Mission's 2,000 km network target, DRDO and ISRO demonstrations | adoption by banks and government systems is the next step |
- Hardware routes: superconducting circuits (IBM, Google), trapped ions, photons and semiconductor spins; each trades speed, error rate and ease of scaling. "Quantum advantage" is a task done faster than any classical computer, first claimed by Google in 2019.
- Quantum key distribution sends a key as photons; the BB84 protocol detects interception because measuring a photon changes it. India has demonstrated QKD over 100 km of fibre (DRDO, 2022) and in free space (ISRO, 2021), and the National Quantum Mission targets satellite QKD and a 2,000 km fibre network.
- Post quantum cryptography is the other defence: ordinary mathematics chosen because no known quantum algorithm breaks it; the United States standardised the first three algorithms in August 2024, and the threat it answers is "harvest now, decrypt later".
- Quantum sensing uses the same sensitivity to disturbance to measure time, gravity and magnetic fields more finely than any classical sensor, for navigation without satellites and for medical imaging.
- The National Quantum Mission, approved in April 2023 with Rs 6,003.65 crore to 2031, aims at intermediate scale computers of 50 to 1,000 qubits, QKD over 2,000 km, and four thematic hubs, for computing at IISc Bengaluru, communication at IIT Madras, sensing at IIT Bombay and materials at IIT Delhi.
Mains: Quantum technology is three different bets, computing, communication and sensing, on the same physics; India's mission funds all three, but the near term payoff is in communication and sensing, where the physics is proven and the engineering is the barrier.
UPSC has asked
- Prelims 2022: the context in which the term qubit is used
See also: National Quantum Mission · C-DOT quantum security products · Prompt injection and ASCII smuggling