NISQ

Noisy Intermediate-Scale Quantum: the current era of quantum computing with imperfect qubits and no error correction.

NISQ (Noisy Intermediate-Scale Quantum) describes the current generation of quantum computers. The term was coined by John Preskill in 2018.

What NISQ Means

Noisy

• Qubits have errors
• Gates are imperfect (~0.1-1% error)
• Coherence times limit computation depth
• No error correction

Intermediate-Scale

• 50-1000+ qubits (current devices)
• Too many to simulate classically
• Too few/noisy for fault tolerance

Quantum

• Genuinely quantum behavior
• Entanglement, superposition
• Potential for quantum advantage

NISQ Characteristics

NISQ Algorithms

Algorithms designed for noisy devices:

Common theme: Variational/hybrid quantum-classical approaches.

NISQ Challenges

The NISQ Question

Can NISQ devices achieve quantum advantage?

Optimistic view: Yes, for specific problems Pessimistic view: Noise always wins; need error correction

The jury is still out.

Post-NISQ: What’s Next?

NISQ (Now)
  │
  ├── More qubits, better fidelity
  │
  ↓
Early Fault-Tolerant (Coming)
  │
  ├── Logical qubits demonstrated
  ├── Partial error correction
  │
  ↓
Full Fault-Tolerant (Future)
  │
  └── Arbitrary computations

Why NISQ Matters

Even if NISQ can’t achieve full advantage:

• Learning to operate quantum systems
• Developing software and algorithms
• Understanding error behavior
• Training the workforce
• Building toward fault tolerance

NISQ Timeline

See also: VQE, QAOA, Quantum Advantage, Fault-Tolerant Quantum Computing