# scqubits

**scqubits** is a Python library for designing and analyzing superconducting qubit systems. Simulate [[scqubits-circuit-quantization|circuit quantization]] for [[scqubits-transmons|transmons]], [[scqubits-fluxoniums|fluxoniums]], and custom circuits; compute eigenspectra and transition frequencies; model couplings and parameter sweeps; study anharmonicity and dispersive shifts. Used for hardware design, qubit characterization, and optimizing circuit parameters.

A **superconducting qubit** is a nonlinear oscillator built from Josephson junctions and capacitances. scqubits starts with the classical circuit (schematic), performs circuit quantization to derive the quantum Hamiltonian, then solves for energy levels and matrix elements. Understanding your qubit's Hamiltonian is essential for gate design and noise characterization.

```python
from scqubits import *

# Transmon qubit: two junctions, two capacitors
transmon = Transmon(
    EJ=15.0,    # Josephson energy (GHz)
    EC=0.3,     # Charging energy (GHz)
    ng=0.0,     # Offset charge
    ncut=30     # Hilbert space cutoff
)

# Compute first few energy levels
eigenvals = transmon.eigenvals(n=5)
print(eigenvals)

# Anharmonicity (difference between transition frequencies)
print(transmon.anharmonicity())
```

scqubits handles the Hamiltonian and spectral analysis; pair with [[qiskit|Qiskit]] for gate construction or [[qutip|QuTiP]] for dynamics simulation.

## Concepts

 1. [[scqubits-superconducting-qubits|Superconducting qubits]]
 2. [[scqubits-circuit-quantization|Circuit quantization]]
 3. [[scqubits-transmons|Transmons]]
 4. [[scqubits-fluxoniums|Fluxoniums]]
 5. [[scqubits-charge-qubits|Charge qubits]]
 6. [[scqubits-phase-qubits|Phase qubits]]
 7. [[scqubits-hamiltonian|Hamiltonian construction]]
 8. [[scqubits-eigenspectra|Eigenspectra and energy levels]]
 9. [[scqubits-anharmonicity|Anharmonicity]]
 10. [[scqubits-transition-frequencies|Transition frequencies]]
 11. [[scqubits-couplings|Couplings and interactions]]
 12. [[scqubits-parameter-sweeps|Parameter sweeps]]
 13. [[scqubits-dispersive-shift|Dispersive shifts]]
 14. [[scqubits-ac-stark|AC Stark shift]]
 15. [[scqubits-noise-decoherence|Noise and decoherence]]
 16. [[scqubits-hilbert-space|Hilbert space truncation]]
 17. [[scqubits-numerical-methods|Numerical methods]]
 18. [[scqubits-visualization|Plotting and visualization]]
 19. [[scqubits-hierarchy|Hierarchy and composite systems]]
 20. [[scqubits-optimization|Parameter optimization]]