We don’t call your laptop a fault-tolerant laptop, but it is. And we don’t call your smartphone a fault-tolerant smartphone, but it is. What we want and need is for quantum computers to become as reliable as laptops and smartphones so that we can refer to them, simply, as quantum computers.
To live up to their potential, quantum computers need to execute algorithms with computational speed-ups over classical algorithms. To solve real-world problems, these algorithms, such as Shor’s and Grover’s (unstructured search), will need the execution of millions, billions, or even more operations. Given that advances in hardware are unlikely to limit errors beyond one per million operations, that’s a lot of cumulative errors. Also keep in mind that one error early in a computation exhibits a domino effect, negatively affecting all the computation that follows it.
In addition to hardware advances, we need Quantum Error Correction (QEC). A fault-tolerant quantum computer will detect and correct errors in real time, not only correcting individual errors but preventing uncorrected errors from cascading throughout the computation. In other words, the fault-tolerant quantum computer will be able to tolerate a fault and still complete the computation accurately.