What Is a Qubit?

While a classical bit is either 0 or 1, a qubit can be in a quantum superposition of both states at once, described by complex probability amplitudes. When measured, it collapses to either 0 or 1 with specific probabilities. Qubits can also be entangled — quantum-correlated so that measuring one instantly determines the state of the other, regardless of distance. These properties allow quantum computers to explore many solutions simultaneously. N qubits can represent 2^N states at once: 300 qubits can represent more states than there are atoms in the observable universe. Qubits are physically implemented using superconducting circuits (Google, IBM), trapped ions (IonQ, Honeywell), photons, or spin qubits in silicon. The biggest challenge is decoherence: qubits are extremely fragile, losing their quantum properties through environmental interactions within microseconds, requiring extreme cooling (near absolute zero) and error correction. Google claimed "quantum supremacy" in 2019 when its 53-qubit Sycamore processor solved a specific problem in 200 seconds that would take a classical supercomputer about 10,000 years.