# Backends

**Backends** are the execution targets for [[qiskit-quantum-circuits|quantum circuits]]. A backend can be a classical [[qiskit-simulators|simulator]] or a real quantum processor. Qiskit abstracts the details: you design once, then run on different backends without changing code.

Each backend has constraints: a gate set (which gates it natively supports), qubit connectivity (which pairs can interact), and error rates. [[qiskit-transpilation|Transpilation]] adapts your circuit to meet these constraints.

## Backend Selection

Qiskit provides several built-in backends:
- **AerSimulator**: fast classical simulator, supports arbitrary gate sets, perfect fidelity
- **QasmSimulator**: OpenQASM 2.0 simulator
- **StatevectorSimulator**: tracks the full quantum state (for small circuits)
- **IBM backends**: real quantum processors via IBM Quantum Platform

```python
from qiskit_aer import AerSimulator, QasmSimulator
from qiskit_ibm_runtime import QiskitRuntimeService

# Local simulators
sim = AerSimulator()
result = sim.run(qc, shots=1000).result()

# IBM Quantum hardware (requires account and token)
service = QiskitRuntimeService()
backend = service.backend('ibmq_qx5')
job = backend.run(qc, shots=1000)
result = job.result()
```

## Backend Properties

Query a backend's properties: qubit count, gate times, error rates, connectivity:

```python
backend = AerSimulator()
print(backend.num_qubits)
print(backend.configuration().basis_gates)
```

Different backends have different performance characteristics. Simulators are perfect but slow for large circuits; real hardware is noisy but available now.