How Do Quantum Computers Work?

Quantum computers work by leveraging the principles of quantum mechanics to perform computations in ways that classical computers cannot. Central to quantum computing is the concept of qubits, the quantum equivalent of classical bits (the smallest unit of data used in computer operations). While classical bits can either hold the value 0 or 1, qubits can hold multiple possible values at once through a property called superposition.

In quantum mechanics superposition is a property through which particles at the atomic scale can occupy multiple possible energy states simultaneously. When an observation, or measurement, is made, the particles “collapse” into a single definite state. Qubits can exist in a superposition of states and hold the value 0, 1, or both simultaneously. The state of superposition allows quantum computers to process a greater amount of information simultaneously compared to classical computers. For instance, a 4-qubit quantum computer can hold 16 different numbers at the same time, allowing it to perform multiple calculations simultaneously. This makes quantum computers potentially much faster than classical computers for tasks such as factoring large numbers or simulating quantum systems. When a quantum calculation is complete, measuring the qubits—which is needed to extract a usable result—“collapses” them to one value.

Additionally, qubits can be entangled, a phenomenon where the state of one qubit is directly related to the state of another, no matter the distance between them. A state of quantum entanglement between qubits is called coherence. Each qubit can hold many more values than a classical bit, and entanglement enables quantum computers to connect multiple qubits to perform operations on an exponentially larger set of data than classical computers and with fewer resources. These computers then provide ranges of possible answers to these operations, reducing calculation times greatly.

Although quantum computers have great potential, building them is challenging. It is difficult for a quantum computer to maintain coherence among its qubits because of their interactions with the environment, a problem known as decoherence. Quantum computers must run at extremely low temperatures and use sophisticated error-correction techniques to keep qubits entangled long enough to perform calculations. Moreover, measuring qubits to record a usable result can disturb their state, making it challenging to extract information at the right stage of the calculation without affecting the computation.

Quantum computers are mostly experimental as of 2025. Scaling the methods used to build them to practical, large-scale systems remains a significant hurdle. However, the potential benefits, such as breaking current encryption schemes and enabling new forms of secure communication, drive ongoing research and development in the field.