Fault-Tolerant Quantum Computers

What is fault-tolerant quantum computing, or FTQC? It is quantum computing on a machine that finds and fixes its own errors faster than they build up, so a long calculation ends with an answer you can trust. Nobody has built one yet. Labs have shown the parts at small scale, and target dates run from 2027 to 2033. Fault-tolerant quantum computing, or FTQC, is about one problem, errors. The best machines today still slip often. The most accurate, by its maker’s account, gets a basic two-qubit step wrong in about 8 of every 10,000 tries. Useful jobs, such as designing a new catalyst for industry or breaking a code that guards bank data, need millions or billions of those steps in a row. So the errors have to be caught and fixed while the program runs. The idea is almost as old as quantum hardware. In December 1995 a team including Christopher Monroe reported a working two-qubit logic gate in a trapped ion. In 1996 Peter Shor showed how to compute with error-correcting codes. By 1998 Aharonov and Ben-Or, Kitaev, and Knill, Laflamme and Zurek had proved what is now called the threshold theorem. It says that if faults are…