These days it’s easy to take information technology for granted: Our computers will do what they’re programmed to do, the files we download will be intact, and compact discs and DVDs will deliver perfect sound and pictures. That trust is rooted in the pervasiveness of error correction: Errors inevitably creep in, through noisy transmission lines or scratches on CDs, but redundant coding of information and error-correcting procedures minimize the effect of errors on downloading pictures or on our enjoyment of Bach cantatas.
Quantum information is even more susceptible to errors than its classical counterpart. A qubit’s coherent superposition of states, which underlies the potential power of quantum computers, is fragile and can be lost due to unwanted coupling to the environment. Through careful engineering, such decoherence can be reduced, but errors will always creep into a computation at a finite rate. Thus there will come a time in a quantum...
