Quantum technology, in plain language.

What a quantum computer is

An ordinary computer stores information in bits. A bit is a switch: it is either 0 or 1.

A quantum computer stores information in qubits. A qubit can be prepared in a superposition, which means the calculation can use a blend of 0 and 1 until it is measured. Measuring it gives a definite 0 or 1. Superposition is not a hidden ordinary bit. It is a different way of holding information while the calculation runs.

Entanglement links two or more qubits so that their results are related. Learning one tells you something about the others. That link lets a quantum computer treat a group of qubits as one system, rather than as separate switches.

These effects help with some problems, such as simulating molecules or searching certain kinds of possibilities. They do not make every program faster. A quantum computer is a specialised machine. It still relies on ordinary computers to control it and to read out the result.

Physical qubits are not logical qubits

The targets on this page count physical qubits, not fault-tolerant logical qubits. A physical qubit is noisy hardware. A logical qubit needs many physical qubits, plus the overhead of error correction.

A higher physical-qubit count is not the same thing as a working, error-corrected computer.

QKD, post-quantum cryptography, and a quantum internet are different

• Quantum key distribution (QKD) uses quantum physics to generate keys that reveal an eavesdropper. It secures a link.
• Post-quantum cryptography (PQC) uses new classical algorithms designed to resist quantum attacks. It secures software everywhere else.
• A long QKD network is a major step. It is not a quantum internet. That also needs quantum memories, repeaters, and entanglement distribution, and none of those are solved.

Glossary

Qubit

The basic unit of information in a quantum computer. Unlike a bit, it can be prepared in a superposition of 0 and 1, and it can be entangled with other qubits.

Physical qubit

A real, noisy device that holds one qubit. Public qubit counts are usually counts of these devices.

Logical qubit

An error-corrected qubit built from many physical qubits. It is the unit that would be needed for a reliable, large calculation.

QKD

Quantum key distribution. A way to share a secret key so that eavesdropping shows up in the result. It protects the key for a link. It does not encrypt the message by itself.

PQC

Post-quantum cryptography. New ordinary algorithms, running on ordinary computers, designed so that a future quantum computer should not be able to break them.

QRNG

Quantum random number generator. A device that draws random numbers from a quantum process. Cryptography, including QKD, needs randomness of this kind.

QBER

Quantum bit error rate. In QKD, the share of key bits that do not match at the two ends. Noise can cause it. So can an eavesdropper.

Quantum repeater

A device that would extend entanglement over long distances. Without repeaters, quantum signals fade in fibre. Repeaters are not a solved technology.

Quantum memory

A device that stores a quantum state and releases it later. A quantum internet would need memories. They are not yet good enough for a long network.

NISQ

Noisy intermediate-scale quantum. A label for today's machines: enough qubits to be interesting, and still too noisy for full error correction.

Quantum advantage

A quantum computer doing a useful task better than the best ordinary computer. A higher qubit count, on its own, is not evidence of it.

TRL

Technology Readiness Level. A scale for how far a technology is from a lab idea to something that can be used outside the lab.

T-Hub

A thematic hub of India's National Quantum Mission. There are four: computing, communication, sensing and metrology, and materials and devices.

Technical Group

A specialist group working under a thematic hub. Public summaries say these groups exist. They do not always name them.

QPU

Quantum processing unit. The processor at the heart of a quantum computer, in the same role a CPU has in an ordinary computer.

NV centre

Nitrogen-vacancy centre. A tiny defect in diamond that can sense very small magnetic fields. It is one tool used in quantum sensing.

Atomic clock

A clock that uses the steady rhythm of atoms to keep time more accurately than an ordinary clock.

Quantum sensing

Measuring magnetic fields, gravity, position, or time with quantum effects, often more precisely than ordinary instruments.

Workforce Pipeline & AICTE Model Curriculum

India's National Quantum Mission mandates training a national cohort of over 25,000 quantum engineers, researchers, and technical specialists by 2031 to prevent workforce dependency on foreign talent.

Over 23 premier institutions across India are setting up designated instructional quantum hardware laboratories, training engineering students directly on optical breadboards, laser controllers, and cryogenic systems.

Key questions to ask about any quantum claim

1. Physical or logical qubits?Ask which count is being quoted. A physical qubit is the noisy device. A logical qubit is an error-corrected unit made from many physical ones. Most public counts are physical.
2. What is the gate fidelity, error rate, or coherence time?Fidelity is how often an operation does what it should. The error rate is how often it does not. Coherence time is how long a qubit keeps its quantum state before noise washes it out. A qubit count without these numbers does not say whether the machine can finish a real calculation.
3. Is the hardware indigenous, or which components are imported?A system assembled in India can still depend on imported chips, lasers, cryogenics, or control electronics. Ask which parts were made locally.
4. Is it a prototype or a deployable product? What TRL?A demonstration in a lab and a product someone can use are different stages. Technology Readiness Level is the usual way to say how far along that path something is.
5. Does it work outside a controlled lab?Many quantum devices need extreme cold, isolation from vibration, or careful shielding. Ask whether it has been shown working in the place it is meant to be used.
6. Is a higher qubit count alone evidence of quantum advantage?No. Quantum advantage means a quantum computer doing a useful task better than the best ordinary computer. More physical qubits do not, by themselves, mean that has happened. Error correction, and the problem being solved, matter as much as the count.

The qubit distinction above is the one used on the Quantum Computing hub. The three-way split of QKD, post-quantum cryptography, and a quantum internet is the one used on the Quantum Communication hub.