qubit-playground

Hands-on studies of differentiable open-quantum-system simulation with dynamiqs/JAX, motivated by superconducting cat-qubit platforms. Current content: a validated lossy harmonic oscillator; see Roadmap.

The physics example — lossy harmonic oscillator

A single cavity mode with Hamiltonian H = ω a†a under single-photon loss (jump operator √κ a) obeys the Lindblad master equation. Starting from a coherent state |α₀⟩, the mean photon number decays purely exponentially:

⟨a†a⟩(t) = |α₀|² · exp(−κ·t)

The dynamiqs simulation (dq.mesolve) reproduces this to within a maximum absolute error of ~5e-6 over the full time window. This agreement is enforced by the test suite in CI.

Setup

Requires Python 3.12+ and uv.

uv sync                                                    # create env, install deps
uv run python -m qubit_playground.plot_lossy_oscillator   # regenerate the figure
uv run pytest                                              # run the test suite

Layout

src/qubit_playground/
  lossy_oscillator.py       # model + simulation, analytic reference, error metric
  plot_lossy_oscillator.py  # figure generation + validation report
tests/                      # analytic, decay-rate, truncation, and plot tests
figures/                    # generated plots

Roadmap

• Fock-space and cat-state studies.
• Notes on solver internals (Diffrax).
• A semiclassical light–matter module driven by an external 2-D EM solver.