# Qiskit

**Qiskit** is IBM's open-source Python framework for quantum computing. Build quantum circuits using gates, transpile them for hardware, simulate locally (up to ~20 qubits), and run on IBM quantum processors or other backends. Widely used for variational algorithms (VQE, QAOA), quantum simulation, and education.

A **circuit** is a sequence of quantum gates applied to qubits, followed by measurement. A **simulator** runs the circuit classically; a **backend** can be either a simulator (like AerSimulator) or a real quantum processor. Use transpilation to adapt your abstract circuit to the specific gate set and topology of your target backend.

```python
from qiskit import QuantumCircuit
from qiskit_aer import AerSimulator

# Create Bell state: (|00⟩ + |11⟩) / √2
qc = QuantumCircuit(2, 2)
qc.h(0)
qc.cx(0, 1)
qc.measure([0, 1], [0, 1])

# Simulate
sim = AerSimulator()
result = sim.run(qc, shots=1000).result()
print(result.get_counts(qc))  # {'00': ~500, '11': ~500}
```

Circuits are abstract until transpiled—Qiskit adapts gates and layout to your backend's constraints. Run jobs on simulators for debugging, then deploy to hardware via IBM Quantum Platform or custom backends.

## Concepts

 1. [[qiskit-quantum-circuits|Quantum circuits]]
 2. [[qiskit-qubits|Qubits and quantum states]]
 3. [[qiskit-quantum-gates|Quantum gates]]
 4. [[qiskit-measurement|Measurement]]
 5. [[qiskit-backends|Backends]]
 6. [[qiskit-simulators|Simulators]]
 7. [[qiskit-transpilation|Transpilation]]
 8. [[qiskit-parameterized-circuits|Parameterized circuits]]
 9. [[qiskit-variational-algorithms|Variational algorithms]]
 10. [[qiskit-vqe|VQE (Variational Quantum Eigensolver)]]
 11. [[qiskit-qaoa|QAOA (Quantum Approximate Optimization)]]
 12. [[qiskit-ansatz|Ansatz]]
 13. [[qiskit-optimizers|Optimizers]]
 14. [[qiskit-noise-models|Noise models]]
 15. [[qiskit-error-mitigation|Error mitigation]]
 16. [[qiskit-pulse|Pulse programming]]
 17. [[qiskit-qasm|OpenQASM]]
 18. [[qiskit-circuit-optimization|Circuit optimization]]
 19. [[qiskit-ibm-quantum-platform|IBM Quantum Platform]]
 20. [[qiskit-job-execution|Job submission and results]]