Add ACircuit.compute_fock_matrix

[anthonyrtw]

Summary

Adds compute_fock_matrix to linear optical circuits, allowing users to compute transition-amplitude matrices over a supplied Fock-state basis.

Changes

• Computes Fock-space transition amplitudes using permanents of repeated-row/repeated-column submatrices.
• Supports exact permanent methods: ryser and glynn.
• Supports approximate Gurvits permanent estimation via n_iter.
• Handles mixed photon-number bases by returning zero amplitudes between sectors.
• Rejects polarized circuits for now.

To use

from exqalibur import FSArray
from perceval import *

m = 5
n  = 3
circuit = Unitary.random(5)
basis = FSArray(m, n)

exact_unitary = circuit.compute_fock_matrix(basis, perm_method="ryser")
est_unitary = circuit.compute_fock_matrix(basis, perm_method="gurvits", n_iter=10_000)

Tests

• Added coverage comparing compute_fock_matrix amplitudes against SLOS simulation output.