The big "if" in quantum computing is in our ability to engineer error correction and fault tolerance. Implementing fault tolerance requires a physical qubit ove

The big "if" in quantum computing is in our ability to engineer error correction and fault tolerance. Implementing fault tolerance requires a physical qubit overhead of 10-10,000 qubits per logical qubit, depending on the technology, base error rate and whose analysis you look at. It's not entirely accurate to say nobody knows how to do this. The theory of quantum error-correction has been rigorously developed since the 1990s, and experimental implementations have shown that improved fidelity is possible with known error-correction schemes. I think we'll know definitively within the next 3-5 years how hard it will be to engineer fault tolerance, and workable solutions will emerge from those findings.