Quantum superposition is a fundamental principle of quantum mechanics that describes how a quantum system can exist in multiple states or configurations simultaneously. Unlike classical bits that are either 0 or 1, a quantum bit (qubit) can be in a combination—called a superposition—of both 0 and 1 states at the same time until it is measured. The superposition state is described mathematically as a linear combination:
∣ψ⟩=α∣0⟩+β∣1⟩|\psi\rangle =\alpha|0\rangle +\beta|1\rangle ∣ψ⟩=α∣0⟩+β∣1⟩
where ∣ψ⟩|\psi\rangle ∣ψ⟩ is the state of the qubit, ∣0⟩|0\rangle ∣0⟩ and ∣1⟩|1\rangle ∣1⟩ are the basis states, and α\alpha α and β\beta β are complex numbers called probability amplitudes that determine the probability of measuring the qubit in either state. When measured, the superposition collapses into one of the basis states with a probability proportional to the amplitude squared. Superposition allows quantum systems to perform multiple computations in parallel, which is a foundation for the power of quantum computing. It is often illustrated with analogies like Schrödinger's cat, which is simultaneously alive and dead until observed, or a coin that is both heads and tails until measured. Physically, superposition can be understood through experiments like the double-slit experiment, where a particle such as a photon passes through two slits simultaneously and interferes with itself, demonstrating wave-like behavior that arises from superposition. Importantly, superposition applies only while the system is unobserved; once an observation or measurement occurs, the superposition collapses to a definite state. This principle distinguishes quantum superposition from classical wave superposition, with the quantum version involving probability amplitudes and the quantum states being fundamentally probabilistic and non-classical. In summary, quantum superposition is the ability of quantum particles or qubits to exist in multiple possible states at once, only collapsing into a single state upon measurement, enabling unique quantum behaviors and technologies like quantum computing.