Gaussian Purification Quotients and Fixed Nielsen Penalties
Abstract
Information distance and circuit complexity are both obtained by minimizing lengths, but they minimize over different objects. We make this distinction explicit for faithful one-mode Gaussian states. First, invariant-form uniqueness implies that no positive-definite quadratic gate cost can be invariant under the full adjoint action of the noncompact symplectic group; a positive Cartan majorant necessarily introduces additional reference data. The Uhlmann purification quotient realizes the Bures metric, and the radial covariance direction requires a system-ancilla coupling because system-only Gaussian unitaries preserve the Williamson eigenvalue. We then minimize fixed right-invariant quadratic norms on the minimal two-mode Gaussian gate algebra sp(4,ℝ). For the unweighted Frobenius norm, the quotient coefficients for radial and traceless covariance tangents are G0=[ℏ2(u−1)]−1 and G2=[ℏ2(3u−1)]−1, where u=(2ν/ℏ)2. Their ratio does not equal the Bures ratio. The radial coefficient, however, reproduces the Bures value exactly at every u; the mismatch is confined to the traceless sector. More generally, a constant block-diagonal two-weight schedule gives G0/G2=1+2(β/α)u/(u−1); matching Bures throughout the isotropic family would require the state-dependent relation β/α=1/u. At the Bures-Fisher determinant crossing u=φ, pointwise matching is possible only by inserting β/α=φ−1. Thus the Bures purification quotient is an exact state-geometric cost, but it is neither an unweighted symplectic gate cost nor a member of this fixed two-weight Nielsen family. The existence of a more general fixed positive gate norm realizing the quotient remains open.
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BibTeX
@misc{kerskens2026gaussian,
title = {{Gaussian Purification Quotients and Fixed Nielsen Penalties}},
author = {Christian Kerskens},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.17574},
eprint = {2609.17574},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.17574},
}
RIS
TY - UNPB
TI - Gaussian Purification Quotients and Fixed Nielsen Penalties
AU - Christian Kerskens
PY - 2026
DA - 2026/09/17
PB - arXiv
DO - 10.48550/arxiv.2609.17574
UR - https://arxiv.org/abs/2609.17574
AB - Information distance and circuit complexity are both obtained by minimizing lengths, but they minimize over different objects. We make this distinction explicit for faithful one-mode Gaussian states. First, invariant-form uniqueness implies that no positive-definite quadratic gate cost can be invariant under the full adjoint action of the noncompact symplectic group; a positive Cartan majorant necessarily introduces additional reference data. The Uhlmann purification quotient realizes the Bures metric, and the radial covariance direction requires a system-ancilla coupling because system-only Gaussian unitaries preserve the Williamson eigenvalue. We then minimize fixed right-invariant quadratic norms on the minimal two-mode Gaussian gate algebra sp(4,ℝ). For the unweighted Frobenius norm, the quotient coefficients for radial and traceless covariance tangents are G_0=[ℏ^2(u−1)]^−1 and G_2=[ℏ^2(3u−1)]^−1, where u=(2ν/ℏ)^2. Their ratio does not equal the Bures ratio. The radial coefficient, however, reproduces the Bures value exactly at every u; the mismatch is confined to the traceless sector. More generally, a constant block-diagonal two-weight schedule gives G_0/G_2=1+2(β/α)u/(u−1); matching Bures throughout the isotropic family would require the state-dependent relation β/α=1/u. At the Bures-Fisher determinant crossing u=φ, pointwise matching is possible only by inserting β/α=φ^−1. Thus the Bures purification quotient is an exact state-geometric cost, but it is neither an unweighted symplectic gate cost nor a member of this fixed two-weight Nielsen family. The existence of a more general fixed positive gate norm realizing the quotient remains open.
ER -