v0.7.0: solitons and crystals, supermodes, thermal baths

Every item of the original roadmap is now implemented, each pinned to exact anchors (66 tests total, up from 47; CI on Python 3.9 through 3.13).

Solitons and soliton crystals by verified Newton continuation (newton_state, soliton_seed, continuation): localized steady states of the LLE, solved with the exact discrete form of the package's own fluctuation matrix as the Jacobian -- the operator that damps a perturbation and the operator Newton inverts are the same linearization. The solver refuses to return anything whose stationary residual is not verified below tolerance, and the tests close the loop through independent code paths: a flat seed lands on the exact cubic root; the converged soliton barely moves under the independent split-step evolver, sits on the exact lower-branch background, and carries the exact translation (Goldstone) zero mode with eigenvector d psi / d theta; a two-pulse crystal equals, grid point for grid point, the single soliton of the equation with dispersion scaled by four (the exact rescaling of the periodic domain); and a truncated stable mode set around the soliton shows sideband squeezing below vacuum.

Marginal vs unstable, distinguished: the Goldstone mode makes a soliton's drift matrix marginally rather than asymptotically stable, so the spectra now refuse a marginal matrix with an explanation unless allow_marginal=True says the translation mode is understood -- while a genuinely positive growth rate stays refused regardless of the flag.

Supermode decomposition (principal_quadratures): the eigendecomposition of any multimode covariance matrix, whose smallest eigenvalue is -- exactly, by linear algebra -- the deepest squeezing any generalized quadrature of the state attains, with its supermode as eigenvector. Pinned to the two-mode squeezed vacuum closed form ((hbar/2) e^(-/+2r), EPR supermodes), flat on vacuum, and verified to lower-bound a scan of quadratures on the photonic molecule. Deliberately distinct from the Williamson spectrum, which measures mixedness.

Thermal input noise: Bose occupations for the extraction-port and loss baths in output_quadrature_variance, per-mode occupations in intracavity_covariance, and thermal_occupation computing the physical number from the exact SI constants (h and k_B are exact by definition since 2019). Anchors are closed forms: a passive cavity with baths at n_bar emits exactly (2 n_bar + 1)/2 at every frequency, coupling and phase; the parametric oscillator's whole spectrum scales by exactly (2 n_bar + 1); a passive mode holds exactly n_bar photons; the hot-loss/cold-port mixture matches its hand-derived form; and n_bar crosses 1 exactly at hbar omega = k_B T ln 2.

Future-proofing: a CHANGELOG covering every release, and the CI matrix widened to Python 3.9, 3.11, 3.12 and 3.13.

The scope that remains -- non-Gaussian quantum dynamics, thermorefractive/Raman technical noise, pulsed pumping, Bloch-Messiah for pure states -- is named in the README as deliberate boundaries with their reasons, so a designed-out feature can be told from a gap.

Install: pip install sqzcomb -- archived on Zenodo (DOI badge in the README).

two-mode Gaussian entanglement of the comb

New module entangle: two-mode Gaussian entanglement computed directly from the covariance matrices the package already produces.

• ppt_symplectic_eigenvalue: partial-transpose symplectic eigenvalue from the local-symplectic invariants (Simon, PRL 84, 2726 (2000)) - a necessary and sufficient entanglement test for 1x1-mode Gaussian states.
• • logarithmic_negativity: E_N = max(0, -ln(2 nu_tilde / hbar)) (Vidal and Werner, PRA 65, 032314 (2002)).
• • duan_epr_sum: the symmetric Duan-Simon EPR sum, with its separability bound.
• • entanglement_report: all of the above for every sideband pair of a comb state.
• • ppt_eigenvalue_direct: an independent cross-check via an explicit partial transpose, asserted in the tests to agree with the invariant route to 1e-9.
    Validation in the test suite: the two-mode squeezed vacuum reproduces E_N = 2r exactly; the photonic-molecule twin beams are PPT-entangled below threshold in a regime where the Duan sum alone misses the entanglement.

Also: README Status section reconciled with the release history.

sqzcomb 0.5.0: imperfect detection

Every spectrum the package computed so far was the noise at the extraction port; this release models what the photodiodes report. The detection module applies the standard beamsplitter model of optical loss and quantum efficiency plus additive electronic noise, both as a scalar map on quadrature variances (detected_variance, detected_squeezing_db, with dark_from_clearance_db converting a receiver's dark clearance in dB into a variance) and as the lossy Gaussian channel on xxpp covariance matrices (lossy_channel_xxpp, per-mode efficiencies allowed). required_efficiency inverts the model into the loss budget of an experiment: the minimum efficiency that still delivers a target squeezing from a given source. The tests assert the vacuum fixed point, exact composition of loss stages, physicality of the channel (symplectic eigenvalues never fall below hbar over 2), agreement of the scalar and matrix forms on a squeezed mode, and closed-form numbers. References: Leonhardt, Measuring the Quantum State of Light (Cambridge, 1997); Weedbrook et al., Rev. Mod. Phys. 84, 621 (2012).

v0.4.0: Gaussian-state interop with the open quantum stack

This release turns the linearized machinery into ecosystem infrastructure. A new gaussian module computes the steady-state covariance matrix of the intracavity Gaussian state with a numpy-only Lyapunov solve, exports it in the standard xxpp quadrature ordering with an explicit hbar convention (vacuum exactly the identity at hbar = 2), and returns Williamson symplectic spectra for purity and entanglement checks. A QuTiP adapter, drift_from_qutip, turns any quadratic QuTiP Hamiltonian into a drift matrix the existing spectra machinery accepts, and refuses non-quadratic Hamiltonians rather than silently linearizing them. Install sqzcomb[interop] for the adapter.

New tests, all closed-form or exact-equivalence: vacuum covariance is the identity for a passive mode and a passive molecule at every coupling; the degenerate parametric oscillator covariance matches its decoupled-sector closed form at three pump strengths; the single-mode symplectic eigenvalue equals 1/sqrt(1 - mu^2) in hbar = 2 units and never violates the Heisenberg bound; and the released two-ring photonic_molecule drift matrix is reproduced exactly, including the rotating-frame sign convention, from a QuTiP Hamiltonian. 29 tests run without qutip (3 skip cleanly); all 32 pass with the interop extra installed.

v0.3.0: multimode comb molecule

Adds the multimode comb molecule: every retained comb line of the LLE fluctuation matrix coupled to a matching auxiliary-ring mode (molecule_fluctuation_matrix), with multi-line bus detection and joint twin-beam quadratures through the auxiliary ring (output_variance_ports now accepts several port modes and joint read-out).

Asserted in the test suite, not claimed: at one retained line the builder equals the released v0.2 two-ring matrix to machine precision; at zero coupling it reduces exactly to the plain fluctuation matrix and the v0.1 spectra (single-line and twin-beam); a passive multimode molecule returns exact vacuum through any bus; the resonant auxiliary ring at zero frequency is exactly the single ring with J^2/gamma_b extra loss per line, including the quarter-turn quadrature rotation; twin-beam squeezing through the auxiliary bus is strictly deeper than through the main bus of the same device once J^2/gamma_b exceeds one; and the stability guard refuses a flat state that is above a pair's modulational-instability threshold when the molecule's added loss is removed. 21 tests total.

Install: pip install sqzcomb

v0.2.0

Adds the photonic molecule: a two-ring coupled-mode model (photonic_molecule), output spectra with per-mode port couplings (output_variance_ports), and the exact instability threshold (molecule_threshold).

Every stated property is asserted in the test suite rather than claimed: exact reduction to the single ring at zero coupling; exact vacuum from a passive molecule at every port, coupling and frequency; supermode splitting of exactly 2J; the resonant threshold mu = 1 + J^2/gamma (Hopf branch 1 + gamma beyond J = gamma); the quarter-turn quadrature rotation of the -iJ hop; the exact zero-frequency equivalence of the auxiliary-ring port to an effective single mode; and a J^2/gamma = 3 molecule reaching 6 dB detected squeezing through the auxiliary port while the Kerr ring has no extraction port. 15 tests total.

Also corrects cff-version in CITATION.cff to 1.2.0 (the format version; the file previously carried the package version there).

Install: pip install sqzcomb

v0.1.1: PyPI packaging release

Packaging release: complete PyPI metadata, CONTRIBUTING.md, and a Trusted-Publishing workflow that tests, builds and publishes to PyPI on release. No changes to the physics code beyond the version number.

After this release: pip install sqzcomb

v0.1.0

Initial release. Lugiato-Lefever steady states (exact Kerr and linear-plus-pump split steps), linearized Bogoliubov fluctuation matrix around arbitrary steady states with a stability guard, and detected quadrature-noise spectra through input-output theory for an extraction port plus intrinsic loss. The test suite reproduces the closed-form degenerate-parametric-oscillator spectrum to 1e-10 and recovers the 3 dB extraction limit at critical coupling from the machinery itself. Photonic-molecule extraction is the v0.2 roadmap. Test suite (7 tests) runs on CI.