What is a quantum computer and how does it work?

What a quantum computer is, how qubits differ from bits, what quantum computers are good for and where they stand today. A plain-language guide, no physics needed.

The short answer

A quantum computer processes information with qubits instead of bits and uses quantum effects such as superposition, entanglement and interference to compute. It is not faster at everything, but for certain problems (simulating molecules, factoring, some search and optimisation tasks) it is expected to reach speeds no classical computer can.

Bits versus qubits

A classical computer represents everything with bits, and every bit is either 0 or 1 at any moment. The basic unit of a quantum computer is the qubit. A qubit also gives 0 or 1 when measured, but before measurement it can be in a state that is a weighted mixture of both.

Describing the joint state of n qubits exactly takes 2ⁿ numbers (amplitudes). For 50 qubits that is more than a quadrillion. A quantum computer carries this huge state physically, while simulating it on a classical computer quickly becomes impossible as the number of qubits grows.

Three ideas that make quantum computers powerful

Superposition: a qubit can be in a mixture of 0 and 1 weighted by amplitudes. Put a qubit into an equal mixture with a Hadamard gate and measure it, and you see 0 or 1 half of the time each.

Entanglement: several qubits can share a joint state that cannot be described qubit by qubit. Two qubits in a Bell state always give the same result when measured (00 or 11), yet which one appears is not decided until the measurement.

Interference: amplitudes have signs and, like waves, can reinforce or cancel each other. Quantum algorithms use interference to cancel the amplitudes of wrong answers and amplify the right one. This is the real source of quantum speed-ups; “trying every possibility at once” is a popular but incomplete explanation.

How is a quantum computer programmed?

Quantum programs are written as quantum circuits: quantum gates are applied to qubits one after another and the qubits are measured at the end. The same circuit is run hundreds or thousands of times and the distribution of results is analysed.

Today these circuits are mostly written in Python with open-source libraries such as IBM’s Qiskit, Google’s Cirq or Xanadu’s PennyLane. Small circuits run on a simulator on your own computer; larger ones can be sent to real quantum processors over the cloud.

What are quantum computers good for?

Chemistry and materials: molecules obey quantum physics, so simulating them on a quantum computer is a natural fit. Designing drugs, catalysts and battery materials are the most anticipated applications (with algorithms such as VQE).

Cryptography: Shor’s algorithm, on a large enough error-corrected quantum computer, could break the RSA and elliptic-curve encryption widely used today. This is why the world is moving to quantum-resistant (post-quantum) cryptography. Quantum key distribution (BB84) offers an alternative whose security rests on physics.

Search and optimisation: Grover’s algorithm gives a quadratic speed-up for unstructured search; methods such as QAOA are being studied for logistics and scheduling. A practical advantage in these areas has not been proven yet.

Not for everything: quantum computers will not replace classical computers for email, games or the web. They are expected to work next to classical computers as accelerators for special problems.

Where do quantum computers stand today?

Today’s devices have between tens and a few hundred qubits, all of them noisy. John Preskill named this era NISQ (noisy intermediate-scale quantum). Because qubits are disturbed by their environment and make errors, circuits have to be kept short.

Long computations require quantum error correction: one logical qubit is protected by many physical qubits. In 2024 Google showed on its Willow processor that the surface code reduces errors as the code grows. IBM, Google, Quantinuum, IonQ, QuEra and others are working towards fault-tolerant machines with different hardware (superconducting circuits, ions, neutral atoms, photons).

How can I learn quantum computing?

You can start without advanced physics or mathematics. Begin with the quantum programming guide to install Qiskit and write your first circuit, then read about quantum algorithms. Quantum Store’s interactive lessons and in-browser circuit simulator are currently in Turkish, but the circuits work the same in any language.

Frequently asked questions

Is a quantum computer faster than a normal computer?

Not at everything. Large speed-ups are expected only for specific problems with known quantum algorithms, such as simulating molecules or factoring. For everyday tasks classical computers are faster and cheaper.

Can I use a quantum computer at home?

No; most quantum processors run close to absolute zero or in a vacuum. But with cloud services such as IBM Quantum and Amazon Braket you can send circuits to real quantum computers over the internet, and you can simulate small circuits on your own computer.

Can quantum computers break encryption?

Today’s devices cannot. Breaking RSA-2048 with Shor’s algorithm needs much larger, error-corrected machines. Because of this risk, quantum-resistant encryption standards are already being rolled out.

Do I need to know physics to start quantum programming?

No. Basic probability and some Python are enough; you can learn superposition, entanglement and interference by experimenting with circuits.

Concepts in this guide

Qubit · A qubit is the basic unit of information in a quantum computer: measured, it gives 0 or 1, but before measurement it can be in a superposition of both.
Superposition · Superposition is the quantum state in which a qubit, before it is measured, holds the states 0 and 1 together with definite amplitudes.
Quantum entanglement · Entanglement means several qubits share a joint quantum state, so their measurement results are correlated even when the qubits are far apart.
Quantum interference · Quantum interference is the way amplitudes add up like waves, raising the probability of some results and cancelling others to zero.
Quantum error correction · Quantum error correction spreads one logical qubit over many physical qubits so that errors caused by noise can be found and fixed without measuring the qubits directly.
NISQ (noisy intermediate-scale quantum) · NISQ is the current era of quantum computing, named by John Preskill in 2018: devices with tens to a few hundred noisy qubits and no error correction.
Shor’s algorithm · Shor’s algorithm factors large numbers exponentially faster than known classical methods, which is why it threatens RSA encryption.

Full quantum computing glossary →