The entire subject of quantum computation is subject to error correction schemes. It is also extremely challenging task to do error correction in the quantum world unlike the classical computation. Without error correction, it is abundantly clear that universal quantum computation could not exist or solve anything useful. What happens, soon after calculation starts, there would be a successive accumulation of errors, and the computer would quickly decohere.
So three physicists and mathematicians in 1995, Andrew Steane, Peter Shor, and Robert Calderbank, put forward a revolutionary idea in 1995. The idea is called "parity measurements."
By adding additional qubits in the circuit, they suggested that if we entangle a set of ancilla qubits with the qubit, we want to protect (from error), we can make parity measurements on the entangled state, which allows us to detect and correct errors -- that's clever. Sort of like, we are looking and the nature doesn't know it, thus would not decohere. The reason is that these parity measurements give no information about the state of any individual qubit and, therefore, preserve the coherence of the state. They are, however, projective measurements and will, thus, project a general error undergone by the Qubit onto one of a discrete set of errors, which can then be corrected for.
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