# NISQ
**NISQ** (Noisy Intermediate-Scale Quantum) refers to the current era of quantum computing, characterized by devices with tens to a few thousand qubits that are too noisy for full fault-tolerant quantum error correction. The term was coined by John Preskill in 2018.

NISQ devices have finite coherence times, imperfect gate operations with error rates typically around $10^{-3}$ to $10^{-2}$ per gate, and limited qubit connectivity. These noise levels are too high to run the large fault-tolerant algorithms like [[shor|Shor's algorithm]] or [[grover|Grover's algorithm]], which require error-corrected logical qubits and millions of physical qubits. Despite these limitations, NISQ devices can perform computations that are classically difficult to simulate — a milestone called **quantum advantage** or **quantum supremacy**.

## NISQ algorithms
Algorithms designed for NISQ devices aim to extract useful computation despite noise. They use shallow circuits (to stay within coherence time), variational approaches (to avoid deep gate sequences), and classical-quantum hybrid loops. The two main NISQ algorithmic paradigms are the [[vqe|Variational Quantum Eigensolver (VQE)]] for quantum chemistry and the [[qaoa|Quantum Approximate Optimization Algorithm (QAOA)]] for combinatorial optimization.

## Prospects
Whether NISQ devices can achieve a practical advantage for commercially useful problems remains an open question. Demonstrated quantum advantages (such as Google's 2019 random circuit sampling experiment) solve problems specifically crafted to be hard for classical simulation, not necessarily useful problems. The path from NISQ to fully fault-tolerant quantum computation requires substantial advances in qubit quality, qubit count, and quantum error correction overhead.