NISQ (Noisy Intermediate-Scale Quantum) describes the current generation of quantum computers — devices with roughly 50–1000+ qubits that are powerful enough to be interesting but too noisy and error-prone for many theoretically advantageous quantum algorithms.

What NISQ Means

• Noisy: Current qubits are imperfect — they experience decoherence (losing quantum state), gate errors (operations aren't exact), and measurement errors. Error rates of 0.1–1% per gate limit circuit depth.
• Intermediate-Scale: Tens to hundreds of usable qubits — enough to be beyond classical simulation for some tasks, but far fewer than the millions needed for full error correction.
• No Error Correction: NISQ machines operate without full quantum error correction, which would require thousands of physical qubits per logical qubit.

NISQ-Era Algorithms

• VQE (Variational Quantum Eigensolver): Hybrid quantum-classical algorithm for finding ground state energies of molecules. Uses short quantum circuits that tolerate noise.
• QAOA (Quantum Approximate Optimization Algorithm): For combinatorial optimization problems using parameterized quantum circuits.
• Variational Quantum Classifiers: Quantum circuits trained as ML classifiers.
• Quantum Approximate Sampling: Sampling from distributions that may be hard classically.

NISQ Limitations

• Short Circuit Depth: Noise accumulates with each gate, limiting circuits to ~100–1000 operations before results become unreliable.
• Limited Qubit Connectivity: Physical qubits can only directly interact with neighboring qubits, requiring overhead for non-local operations.
• No Proven Practical Advantage: No NISQ algorithm has demonstrated clear practical advantage over classical approaches for real-world problems.

Major NISQ Processors

• IBM Eagle/Condor: 1,121 qubits (Condor, 2023). Superconducting transmon qubits.
• Google Sycamore: 70 qubits. Superconducting qubits.
• IonQ Forte: 36 algorithmic qubits. Trapped ion technology.
• Quantinuum H2: 56 qubits. Trapped ion with industry-leading gate fidelity.

Beyond NISQ

The goal is to reach fault-tolerant quantum computing with error-corrected logical qubits. This requires ~1,000–10,000 physical qubits per logical qubit, meaning millions of physical qubits — likely a decade or more away.

NISQ is the proving ground for quantum computing — demonstrating potential and developing algorithms while hardware catches up to theoretical requirements.