Quantum Computing and Quantum Technologies

Quantum technology has been tested twice in four years, in 2022 and again in 2025, and no reference book covers it. The 2022 question was as simple as asking which field the term qubit belongs to. The 2025 question was considerably harder and turned on a specific chip and who made it. Between those two sits the material worth actually understanding.

The bit, and what a qubit does differently

A classical computer stores information in bits, each of which is either 0 or 1. Everything a normal computer does reduces to manipulating those two states.

A quantum computer uses a qubit (quantum bit), which can be 0, or 1, or in a superposition of both at once. The word "at once" does a lot of work and is usually explained badly, so it is worth being careful. A qubit in superposition is not secretly a 0 or a 1 that we merely have not looked at yet. It is genuinely in a combined state, described by the probabilities of each outcome. When you measure it, the superposition collapses and you get a definite 0 or 1. You cannot read a superposition directly; measuring destroys it.

The second property that matters is entanglement. Two or more qubits can be linked so that their states are correlated no matter how far apart they are: measuring one immediately determines what the other will give. Entanglement is what lets qubits work as a system rather than as a collection of independent coins.

Together these produce the property that gets quoted: n qubits can represent 2 to the power n states simultaneously, so a few hundred qubits could hold more states than there are atoms in the observable universe. But the catch matters as much as the claim. You cannot simply read all those states out, because measurement collapses everything to a single answer. A quantum algorithm has to be cleverly constructed so that the wrong answers cancel each other out and the right one is overwhelmingly likely to be what you measure.

This is why a quantum computer is not a faster computer. It is not going to run a spreadsheet, a browser or a video game better than the machine you have. It is dramatically better at a narrow class of problems where that cancellation trick applies: factoring very large numbers, searching unstructured data, simulating molecules and materials, and certain optimisation problems. For everything else, classical computers remain better, and will stay better.

Decoherence, and why building one is so hard

A qubit's quantum state is extraordinarily fragile. Heat, vibration, stray electromagnetic fields, essentially any interaction with the surrounding environment, destroys the superposition. This is decoherence, and it is the central engineering obstacle in the entire field. It is why quantum processors are kept in dilution refrigerators at temperatures near absolute zero, colder than deep space, and heavily shielded.

Because qubits are so error-prone, the field distinguishes physical qubits, the actual hardware, from logical qubits, the error-corrected units an algorithm actually uses. It can take a very large number of physical qubits working together to produce one reliable logical qubit. This is why a headline qubit count is a poor measure of a machine's power, and why "quantum computers with a million qubits" and "quantum computers that can do useful work" are not the same milestone.

Several competing hardware approaches are being pursued, and none has clearly won:

• Superconducting qubits, tiny circuits cooled to near absolute zero. Currently the most widely used approach.
• Trapped ions, individual charged atoms held in electromagnetic fields and manipulated with lasers.
• Photonic qubits, using particles of light, which have the advantage of being less prone to decoherence.
• Topological qubits, a longer-shot approach that aims to encode information in a way that is intrinsically protected from local disturbance, making it far more stable rather than merely better corrected. Microsoft's Majorana 1 processor, unveiled in February 2025, was the first quantum processor built on this topological approach. It is worth attaching the name to the right company, since a 2025 question tested exactly that and offered a plausible wrong one.

The cryptography problem, and why it is urgent now

Most encryption securing the internet, banking and government communication relies on the fact that multiplying two large prime numbers is easy while factoring the result back into those primes is, for a classical computer, effectively impossible within any useful timeframe.

Shor's algorithm, designed for a quantum computer, factors large numbers efficiently. A sufficiently powerful, error-corrected quantum computer would therefore break the widely used public-key encryption standards outright. No such machine exists today.

The reason this is nonetheless an immediate concern is harvest now, decrypt later: an adversary can record encrypted traffic today and simply store it until a machine capable of decrypting it exists. Anything that must stay secret for a decade or more is already exposed. This is what drives work on post-quantum cryptography, which means new classical algorithms designed to resist quantum attack. Note the point that often gets muddled: post-quantum cryptography runs on ordinary computers. It is not quantum technology; it is defence against it.

Quantum communication is a different thing again

Quantum Key Distribution (QKD) is regularly confused with quantum computing, and they are separate technologies solving opposite problems.

QKD uses quantum mechanics to distribute encryption keys securely. Its security rests on the measurement property described at the top of this note: because observing a quantum state disturbs it, any eavesdropper intercepting the key inevitably leaves a detectable trace. The two legitimate parties can therefore tell whether their key exchange was intercepted, which no classical method can guarantee. So quantum computing threatens encryption, while quantum communication defends it, and a country can pursue both at once without contradiction.

India's National Quantum Mission

The digital systems this encryption protects, and India's own stack, are covered in The Digital Rupee and UPI's Global Expansion.

The National Quantum Mission (NQM) was approved by the Union Cabinet on 19 April 2023, with a total outlay of Rs 6,003.65 crore running from 2023-24 to 2030-31. It is implemented by the Department of Science and Technology.

Its stated targets are specific enough to be examinable:

• Developing intermediate-scale quantum computers with 50 to 1,000 physical qubits over eight years, across platforms including superconducting and photonic technology. Note that the target is stated in physical qubits, which connects directly to the physical-versus-logical distinction above.
• Satellite-based secure quantum communication between ground stations over a range of 2,000 kilometres within India, and long-distance secure quantum communication with other countries.
• Inter-city quantum key distribution over 2,000 km, and multi-node quantum networks with quantum memories.

The mission also establishes Thematic Hubs (T-Hubs) in participating institutions, organised around the mission's technology areas.

Quick revision points

• A qubit can be 0, 1, or a superposition of both. Measuring it collapses the superposition to a definite value, so a superposition cannot be read directly. Entanglement links qubits so their outcomes are correlated regardless of distance.
• n qubits represent 2^n states at once, but you cannot read them all out. A quantum algorithm must make wrong answers cancel so the right one is what you measure.
• A quantum computer is not a faster computer. It beats classical machines only on a narrow class of problems: factoring, unstructured search, molecular and materials simulation, some optimisation.
• Decoherence, the loss of quantum state through any interaction with the environment, is the central engineering problem, hence near-absolute-zero operation.
• Physical qubits are hardware; logical qubits are error-corrected units an algorithm uses, and many physical qubits are needed per logical one. A raw qubit count is a poor measure of capability.
• Hardware approaches: superconducting (most common), trapped ion, photonic, and topological. Majorana 1, the first topological quantum processor, was unveiled by Microsoft in February 2025.
• Shor's algorithm would break current public-key encryption. Harvest now, decrypt later makes this urgent before any such machine exists. Post-quantum cryptography is the defence, and it runs on ordinary classical computers.
• Quantum Key Distribution is not quantum computing. It secures key exchange by making eavesdropping detectable. Quantum computing threatens encryption; quantum communication defends it.
• National Quantum Mission: approved 19 April 2023, outlay Rs 6,003.65 crore, period 2023-24 to 2030-31, implemented by the Department of Science and Technology. Targets 50 to 1,000 physical qubits in eight years, satellite-based quantum communication over 2,000 km, and inter-city QKD over 2,000 km.