However, two independent theoretical studies published in 1998 and 2008 proved that fault-tolerant quantum computers can be built.
The biggest challenge in achieving fault-tolerant quantum computing is the need for many qubits. QECCs themselves require a lot of qubits to detect and correct errors. Additionally, fault-tolerant gates also require a large number of qubits. However, two independent theoretical studies published in 1998 and 2008 proved that fault-tolerant quantum computers can be built. This has come to be known as the threshold theorem, which states that if the physical error rates of a quantum computer are below a certain threshold, the logical error rate can be suppressed to arbitrarily low values.