# Backends and Execution

**Backends** are the execution targets: simulators (CPU, GPU) or real quantum hardware. CUDA-Q abstracts backends—same kernel code runs on any target.

```cpp
#include "cudaq.h"
#include <string>

struct SimpleKernel {
  void operator()() __qpu__ {
    cudaq::qvector q(2);
    h(q[0]);
    cx(q[0], q[1]);
    mz(q);
  }
};

int main() {
  // List available backends
  auto backends = cudaq::available_backends();
  for (const auto& backend : backends) {
    printf("Available: %s\n", backend.c_str());
  }
  
  // Set backend at runtime
  cudaq::set_target("nvidia");  // GPU simulator (cuQuantum)
  auto gpu_result = cudaq::sample<SimpleKernel>(1000);
  printf("GPU result:\n");
  for (auto& [bits, count] : gpu_result) {
    printf("  %s: %lu\n", bits.c_str(), count);
  }
  
  // Switch to CPU simulator
  cudaq::set_target("qpp");
  auto cpu_result = cudaq::sample<SimpleKernel>(1000);
  printf("CPU result:\n");
  for (auto& [bits, count] : cpu_result) {
    printf("  %s: %lu\n", bits.c_str(), count);
  }
  
  // Connect to real hardware (if available)
  cudaq::set_target("iqm");  // IQM backend
  // auto hw_result = cudaq::sample<SimpleKernel>(1000);
  
  return 0;
}
```

## Backend Types

**GPU Simulators** (nvidia, cuQuantum): fast for 20–30 qubits, perfect fidelity

**CPU Simulators** (qpp, stim): universal but slower

**Hardware Providers** (IQM, IonQ, etc.): real quantum processors