Fault tolerant quantum computing

In quantum information, fault-tolerant quantum computing (FTQC) is a regime of quantum processors that are both large-scale and that effectively incorporate quantum error correction to achieve arbitrarily low error rates (i.e. their logical error rate is much lower than their physical error rate).

Full-FTQC processors are theoretically possible, but have not yet been realized experimentally. They are often seen as the primary end goal of quantum processor development, and are used to contrast with existing noisy intermediate-scale quantum (NISQ) quantum processors, which are subject to noise and decoherence preventing scalable error correction.

One way FTQC devices can be realized is by grouping together multiple physical qubits to create a single logical qubit, and using error correction methods such as the surface code so that the combined system is fault-tolerant. Proposed FTQC devices generally include hundreds of logical (error-corrected) qubits, which would mean thousands of physical qubits at minimum.

The transition from NISQ-era devices to the FTQC regime is an open research topic in quantum information. It is likely that advances at both the hardware and algorithm level are necessary for the transition to occur.