fpm-qsim 0.1.3

Drop-in Lindblad dephasing simulator backed by the Finite Possibility Mechanics affine map

fpm-qsim

Drop-in Lindblad dephasing simulator backed by the Finite Possibility Mechanics affine map.

fpm-qsim is a small, dependency-light Python package that lets you simulate open-system quantum dephasing dynamics using the FPM affine coherence map. The map

c_{t+1} = kappa * c_t + nu

with kappa ∈ [0, 1] is the engine. Theorem 3 of the FPM paper identifies one particular choice — kappa = 1 - gamma*dt — with the Euler-discretized Lindblad dephasing equation. This package uses the exact continuous form

kappa = exp(-gamma * dt)

which is also a valid FPM affine-map coefficient and which makes the integrator machine-precise for pure dephasing: it reproduces the analytic continuous-dephasing solution

rho(t) = exp(-gamma*t) * (rho_0 - diag(rho_0)) + diag(rho_0)

to machine precision (about 5e-16 in the benchmark), matching Kraus and matrix-exponential references for pure dephasing.

The honest positioning is not "uniquely fastest." fpm-qsim is the reference implementation of the FPM affine-map primitives: it provides competitive pure-dephasing simulation, a NumPy-only pure-dephasing path, the FPM falsifiability ceiling, and the conservation-ledger primitives needed by downstream FPM research.

For speed, the important distinction is structural vs constant-factor: pure dephasing can be implemented in O(N^2) per step by any dephasing-aware method. fpm-qsim is competitive with that specialized baseline and much faster than a general matrix-exponential Liouvillian on the same pure-dephasing problem.

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Installation

Install from PyPI:

pip install fpm-qsim

From source:

git clone https://github.com/alxspiker/fpm-qsim.git
cd fpm-qsim
pip install -e .

Requirements: Python ≥ 3.9, NumPy ≥ 1.22. SciPy is optional for tests and for unitary_step.

---

Quick start

import math
import numpy as np
import fpm_qsim as fpm

# Start in the |+><+| state (maximal coherence).
rho0 = fpm.pure_state([1, 1])

# Apply one dephasing step.  Off-diagonal contracts by exp(-gamma*dt):
rho1 = fpm.lindblad_step(rho0, gamma=0.1, dt=1.0)
assert math.isclose(abs(rho1[0, 1]), math.exp(-0.1) * abs(rho0[0, 1]))

# Roll out a full trajectory.
traj = fpm.simulate(rho0, gamma=0.05, dt=1.0, n_steps=200)
assert traj.shape == (201, 2, 2)

Drop-in replacement

If you have an existing Lindblad dephasing loop, swap one import:

# Before
# from your_qsim_library import dephasing_step
# rho = dephasing_step(rho, c_ops=[sqrt(gamma / 2) * sigma_z], dt=0.1)

# After
from fpm_qsim import lindblad_step
rho = lindblad_step(rho, gamma=gamma, dt=0.1)

---

What's distinctive about fpm-qsim

Property	fpm-qsim	QuTiP	matrix-exp	Kraus
Dephasing accuracy	~5e-16	~2e-7 in the benchmark	~5e-16	~5e-16
Pure-dephasing speed	Competitive with dephasing-specialized O(N^2)	General solver overhead	General or specialized	Single-qubit baseline
Dependencies	NumPy for pure dephasing; SciPy only for unitary_step	SciPy + Cython + ...	SciPy for general matrix-exp	NumPy
Falsifiability ceiling	gamma_max = 31.87	---	---	---
Theorem-verified affine map	Theorem 3	---	---	---
Closed-universe ledger	Yes	---	---	---
Combined unitary + dephasing	Explicit unitary_step + user-chosen splitting	Built in	Built in	Channel-specific

The only Lindblad integrator in the Python ecosystem with a built-in falsifiability ceiling: observations with gamma > 32.0 raise FalsificationError rather than silently producing unphysical results. This is the FPM finite-lag-ceiling theorem (paper Test 09) made operational.

Equally important: this package is where the FPM theorem is made operational. kappa_exact, kappa_from_gamma, bounded_gamma, ConservationLedger, and DaemonState are not generic convenience functions; they are the public primitives for building FPM-aligned simulators, auditors, and falsification tests.

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Benchmarks

The benchmark corrected three earlier overclaims:

• The large speedup against general matrix-exp is real, but structural: it comes from exploiting pure-dephasing structure, which any dephasing-aware implementation can also exploit.
• With the gamma convention corrected, fpm-qsim, matrix-exp, and Kraus all match the analytic pure-dephasing solution at machine precision.
• The old H parameter on lindblad_step was removed because it used a naive Euler unitary kick. Use unitary_step and compose the splitting explicitly.

Speed results for 1,000 pure-dephasing steps:

Qubits	Dim	fpm-qsim	general matrix-exp	dephasing-specialized matrix-exp	scipy solve_ivp	QuTiP	Kraus
1	2	3.38 ms	1.88 ms	1.18 ms	13.28 ms	10.44 ms	5.47 ms
2	4	3.31 ms	1.98 ms	1.45 ms	13.76 ms	---	---
3	8	3.39 ms	15.03 ms	1.86 ms	28.58 ms	---	---
4	16	4.03 ms	136.08 ms	3.19 ms	147.54 ms	---	---
5	32	7.28 ms	---	6.19 ms	---	---	---
6	64	17.30 ms	---	18.34 ms	---	---	---

At 4 qubits, fpm-qsim is about 34x faster than the general matrix-exp baseline, but about 25% slower than a matrix-exp baseline that is specialized for pure dephasing. The fair claim is that fpm-qsim is competitive with the best dephasing-specialized approach while also carrying the FPM research API and falsifiability checks.

Accuracy results, measured as max absolute error vs the analytic solution after 1,000 steps:

Qubits	Dim	fpm-qsim	general matrix-exp	dephasing-specialized matrix-exp	scipy solve_ivp	QuTiP	Kraus
1	2	4.6e-16	4.6e-16	4.6e-16	4.00e-12	2.04e-07	4.6e-16
2	4	3.5e-16	3.5e-16	3.5e-16	1.57e-12	---	---
3	8	3.4e-16	3.4e-16	3.4e-16	1.33e-12	---	---
4	16	1.9e-16	1.9e-16	1.9e-16	1.16e-12	---	---
5	32	9.7e-17	---	9.7e-17	---	---	---
6	64	7.4e-17	---	7.4e-17	---	---	---

Benchmark configuration: gamma = 0.02, dt = 1.0, Haar-random pure initial state, wall time reported as the minimum of three repeats.

---

API reference

Core FPM primitives (fpm_qsim.core)

Symbol	Description
GAMMA_MAX = 31.8738...	Falsifiable Lorentz-factor ceiling derived from the finite-lag theorem.
FALSIFICATION_THRESHOLD = 32.0	Observations above this falsify FPM.
ENERGY_FLOOR_FRACTION = 0.03138...	v5.0 zero-energy floor (Test 07).
ISOTROPIC_WEIGHT_LIMIT = 1/3	Spectral-gap isotropic limit weight (Test 04).
kappa_from_gamma(gamma, dt=1.0)	Euler-form contraction coefficient 1 - gamma*dt (Theorem 3 form).
kappa_exact(gamma, dt=1.0)	Exact continuous-form contraction coefficient exp(-gamma*dt). This is what lindblad_step uses.
gamma_from_kappa(kappa, dt=1.0)	Inverse of kappa_from_gamma.
fpm_affine_step(c, kappa, nu=0.0)	One tick of the affine map c_{t+1} = kappa*c_t + nu.
fpm_affine_trajectory(c0, kappa, nu=0.0, n_steps=1)	Closed-form rollout of the affine map.
bounded_gamma(gamma_raw, gamma_max=GAMMA_MAX)	Clip a rate to the ceiling; raise if it would falsify.
FalsificationError	Raised when an observation would falsify FPM.

Lindblad-equivalent API (fpm_qsim.lindblad)

Symbol	Description
lindblad_step(rho, gamma, dt=1.0, *, bounded=False)	Advance a density matrix by one exact FPM-affine dephasing step.
unitary_step(rho, H, dt=1.0)	Apply one exact Hamiltonian step, rho -> U rho U^dagger, using a matrix exponential.
simulate(rho0, gamma, dt=1.0, n_steps=1, *, bounded=False, record=True)	Roll out a pure-dephasing trajectory.

State utilities (fpm_qsim.states)

Symbol	Description
basis_state(index, dim)	Computational basis column vector.
pure_state(amplitudes)	Build `
maximally_mixed(dim)	I_dim / dim.
partial_trace(rho, keep, dims)	Partial trace over a tensor-product Hilbert space.
is_density_matrix(rho, tol=1e-9)	Validate Hermiticity, trace, positivity.
trace_distance(rho, sigma)	`0.5 *
fidelity(rho, sigma)	Uhlmann fidelity.

Closed-universe conservation (fpm_qsim.conservation)

Symbol	Description
DaemonState	Per-daemon bookkeeping (energy, coherence, cumulative flows).
ConservationLedger	Closed-universe ledger; tracks replenish == spend + landauer.

---

The math, in one page

The FPM coherence variable c_t evolves under the affine map

c_{t+1} = kappa * c_t + nu

where kappa ∈ [0, 1] is the contraction coefficient and nu is a bounded innovation noise. The FPM framework constrains kappa to [0, 1] but does not mandate a specific form; different kappa choices correspond to different numerical integrators of the same physics:

kappa choice	What it computes	Accuracy
1 - gamma*dt	Euler-discretized Lindblad dephasing	O(dt) per step
exp(-gamma*dt) (this package)	Exact continuous Lindblad dephasing	machine precision

Theorem 3 (Lindblad Correspondence). The Euler form (kappa = 1 - gamma*dt) is algebraically identical to the Euler discretization of the Lindblad master equation for a dephasing channel with H = 0:

rho_{t+1} = (1 - gamma*dt) * rho_t + gamma*dt * diag(rho_t)

under the identification gamma_t = (1 - kappa_t) / dt. Verified numerically to RMSE 3.6 × 10⁻¹⁷ on off-diagonal density-matrix elements over 600 ticks and 10 random paths (paper Test 02; reproduced in tests/test_lindblad_correspondence.py::test_theorem3_lindblad_correspondence).

The exact form (kappa = exp(-gamma*dt)) is also a valid FPM affine-map coefficient (in [0, 1] for all gamma, dt >= 0) and is what the public lindblad_step uses. It reproduces the analytic continuous-dephasing solution to machine precision, matching Kraus and matrix-exp references. The Euler form remains available in fpm_qsim._reference.euler_lindblad_step for Theorem 3 research.

Hamiltonian evolution

lindblad_step is deliberately scoped to pure dephasing. Earlier versions accepted an H parameter and applied a naive Euler unitary kick before dephasing. That silently reintroduced splitting error for H != 0 and broke the machine-precision guarantee.

For combined unitary + dephasing dynamics, compose the exact Hamiltonian step explicitly. The recommended Strang splitting is:

rho = fpm.unitary_step(rho, H, dt / 2)
rho = fpm.lindblad_step(rho, gamma=gamma, dt=dt)
rho = fpm.unitary_step(rho, H, dt / 2)

This has O(dt^3) per-step splitting error. unitary_step requires SciPy for the matrix exponential; the pure-dephasing path remains NumPy-only.

The falsifiable ceiling

The finite-lag ceiling theorem (paper Test 09) caps the physically admissible gamma at

gamma_max = 31.8738...

The CERN muon (gamma = 29.3) sits below this ceiling. Any observation of gamma > 32.0 falsifies the FPM framework. The package refuses to silently clip such observations; pass bounded=True to lindblad_step and a FalsificationError will be raised — log the observation instead.

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Reproducing the paper's tests

The tests/ directory reproduces four of the paper's numerical experiments and adds a machine-precision regression test:

Test file	Paper test	Reference
tests/test_lindblad_correspondence.py::test_theorem3_lindblad_correspondence	Test 02	RMSE 3.6e-17
tests/test_lindblad_correspondence.py::test_public_lindblad_step_machine_precision	new in v0.1.1	max err 1.1e-16
tests/test_dispersion_contraction.py	Test 01	D* = 1.8e-4
tests/test_conservation.py	Test 03	Drift < 5%
tests/test_bounded_gamma.py	Test 07 + Test 09	gamma_max = 31.87

Run them with:

pip install -e ".[test]"
pytest -v

---

Scope and honest limitations

This package implements pure dephasing channels with H = 0, the regime where the FPM theorem provides an algebraic correspondence.

• lindblad_step does not accept H. For H != 0, compose unitary_step and lindblad_step explicitly, preferably with Strang splitting. Accuracy then depends on the caller's chosen splitting strategy.
• General Lindblad channels (amplitude damping, depolarizing, etc.) are not directly equivalent to the FPM affine map. Future versions may extend the correspondence.
• The ConservationLedger is a faithful bookkeeping layer but does not, by itself, enforce the semantic-entropy conservation gate (paper Section 6.6). Callers must implement that gate in their application logic.

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Changelog

0.1.3 (2026-06-17)

• Reframed the README around the audit-corrected benchmark: competitive pure-dephasing performance, not unique speed dominance.
• Added benchmark tables for speed and accuracy.
• Documented the removal of H from lindblad_step and the explicit unitary_step composition path for H != 0.

0.1.2 (2026-06-17)

• Clarified the PyPI installation command in the README.

0.1.1 (2026-06-17)

• Breaking: lindblad_step now uses the exact continuous form kappa = exp(-gamma*dt) instead of the Euler form 1 - gamma*dt. The public API now matches Kraus / matrix-exp / QuTiP to machine precision (max abs error 1.1e-16 vs analytic). The Euler form is preserved in the private fpm_qsim._reference module for Theorem 3 verification.
• Added kappa_exact(gamma, dt) to the public API.
• Removed reference_lindblad_step from the public API (moved to fpm_qsim._reference.euler_lindblad_step).
• Added machine-precision regression test.
• Added test confirming the exact form handles gamma*dt > 1 without positivity violation (a regime the Euler form cannot reach).
• Updated README with honest speed and accuracy claims.

0.1.0 (2026-06-17)

Initial release. Euler-form affine map, Lindblad-equivalent API, bounded gamma falsifiability, density-matrix utilities, closed- universe conservation ledger.

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Citation

If you use fpm-qsim in published research, please cite:

@misc{spiker2026fpm,
  title  = {Finite Possibility Mechanics: A Unified Information-Theoretic Framework},
  author = {Spiker, Alx},
  year   = {2026},
  note   = {See Theorem 3 (Lindblad Correspondence) and the Finite-Lag-Ceiling Theorem.}
}

@misc{fpm-qsim,
  title  = {fpm-qsim: Drop-in Lindblad dephasing simulator backed by the FPM affine map},
  author = {Spiker, Alx},
  year   = {2026},
  url    = {https://github.com/alxspiker/fpm-qsim},
}

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License

MIT. See LICENSE.