How close are we to scalable, fault-tolerant quantum computers?

We are not close. We are currently in the NISQ (Noisy Intermediate-Scale Quantum) era. This means our current quantum computers are "noisy" (prone to errors) and "intermediate-scale" (a few hundred to a few thousand qubits).

The two main challenges are:

Fault-Tolerance: Qubits are incredibly fragile. The slightest vibration, temperature change, or electrical fluctuation can cause them to "decohere" and lose their quantum state, creating an error. A "fault-tolerant" machine is one that can correct these errors faster than they occur.

Scalability: We need to be able to add more and more high-quality qubits.

The solution to fault-tolerance is Quantum Error Correction (QEC). This involves creating a "logical qubit" (one stable, error-corrected qubit) by bundling together many physical qubits (perhaps 1,000 or more).

As of 2025, teams at Google, IBM, and various universities are making breakthroughs in demonstrating these QEC principles, but the overhead is massive. We are likely 5-10 years away from creating a single, useful, logical qubit. A scalable, fault-tolerant computer built from thousands of these logical qubits (representing millions of physical qubits) is likely still a decade or more away.