Quantum Measurement, How Reading a Qubit Works

Quantum measurement is the act of pulling a definite, classical answer out of a quantum state, which sounds simple enough until you look at what it costs. It is where quantum mechanics is at its strangest, because the act of looking changes the thing you are looking at and forces a random outcome. Every quantum computation ends in a measurement, and on real hardware that read is a specification in its own right, with its own speed and its own error rate. Measurement is an active event that destroys information, not a passive reading. One rule, named after Max Born, turns the quantum state’s amplitudes into the odds of each outcome, and everything else about reading a qubit follows from it. The act is irreversible. It takes many repetitions to learn anything about an unknown state, and it needs no conscious observer, only a detector that ends up correlated with the qubit. It extracts a classical answer. A measurement maps a quantum state to a single classical outcome plus a changed state, turning quantum information into ordinary bits. The Born rule sets the odds. The probability of each outcome is the squared magnitude of its amplitude, the bridge from quantum…