This preprint studies how local amplitude damping redistributes two-qubit resources between entanglement and stabilizer nonstabilizerness. The focus is an analytically tractable two-qubit X-state sector in which dissipative dynamics can destroy entanglement while leaving a detectable non-Clifford state resource. The paper constructs a seven-correlator Pauli witness that is valid for the full two-qubit stabilizer polytope and facet-tight after projection to the relevant X-sector coordinates. For the Phi-type amplitude-damped slice, the witness detects nonstabilizerness beyond the boundary lambda = 1 - gamma, whereas the PPT separability boundary is lambda = gamma. Their separation gives an explicit analytic region of separable states with certified stabilizer nonstabilizerness. By contrast, the corresponding Psi-type slice remains stabilizer-convex throughout the considered parameter range by an explicit decomposition into stabilizer projectors. The contribution is deliberately geometric and conservative. The paper does not claim that separable magic is new as a general phenomenon. Instead, it provides a compact stabilizer-witness boundary, a closed-form separable-magic wedge in an amplitude-damped X-state model, and a reproducible finite stabilizer-polytope verification. The result illustrates a resource hierarchy inversion in open quantum dynamics: entanglement may vanish according to the two-qubit PPT criterion while stabilizer nonstabilizerness remains certified. The manuscript package includes reproducibility material that reconstructs the two-qubit stabilizer vertices, evaluates the witness on all pure stabilizer vertices, projects the vertices to the X-sector correlators, verifies the projected affine-rank condition, and regenerates the resource-map data.
This preprint studies how local amplitude damping redistributes two-qubit resources between entanglement and stabilizer nonstabilizerness. The focus is an analytically tractable two-qubit X-state sector in which dissipative dynamics can destroy entanglement while leaving a detectable non-Clifford state resource. The paper constructs a seven-correlator Pauli witness that is valid for the full two-qubit stabilizer polytope and facet-tight after projection to the relevant X-sector coordinates. For the Phi-type amplitude-damped slice, the witness detects nonstabilizerness beyond the boundary lambda = 1 - gamma, whereas the PPT separability boundary is lambda = gamma. Their separation gives an explicit analytic region of separable states with certified stabilizer nonstabilizerness. By contrast, the corresponding Psi-type slice remains stabilizer-convex throughout the considered parameter range by an explicit decomposition into stabilizer projectors. The contribution is deliberately geometric and conservative. The paper does not claim that separable magic is new as a general phenomenon. Instead, it provides a compact stabilizer-witness boundary, a closed-form separable-magic wedge in an amplitude-damped X-state model, and a reproducible finite stabilizer-polytope verification. The result illustrates a resource hierarchy inversion in open quantum dynamics: entanglement may vanish according to the two-qubit PPT criterion while stabilizer nonstabilizerness remains certified. The manuscript package includes reproducibility material that reconstructs the two-qubit stabilizer vertices, evaluates the witness on all pure stabilizer vertices, projects the vertices to the X-sector correlators, verifies the projected affine-rank condition, and regenerates the resource-map data.
Local Markovian noise cannot bring entanglement back, but it can bring magic back. Unlike separability, stabilizer membership is not preserved by local channels, allowing dissipation to push states out of the stabilizer polytope as well as in. Under local amplitude damping, the n-qubit GHZ family α∣0n⟩+β∣1n⟩ (0<α<β) loses its magic at a lower damping strength γ− and regains it at a higher one γ+, while entanglement is irreversibly lost at γe. This magic--entanglement complementarity, γe+γ+=1 for every n, reflects a system--environment duality of amplitude damping. Within real phase-covariant Markovian semigroups the phenomenon is mapped out in full: zero-temperature rebirth occurs if and only if T2>T1, unital dynamics produce no rebirth, and sufficiently weak thermal excitation confines rebirth to a finite magic island ending in a second sudden death. For small α, the reborn magic resides in a fully separable state with all proper marginals stabilizer, yet parity-syndrome extraction concentrates it onto a single qubit for magic-state distillation, without loss of expected robustness and with optimal per-register yield α2/2. Local dissipation further divides pure stabilizer states into magic-generators and magic-insulators: at two qubits, the Bell state ∣Φ+⟩ generates magic immediately, while its Bell-state partner ∣Ψ+⟩ remains stabilizer. Together, magic and entanglement reveal a symmetry invisible to either alone.
Detecting nonstabilizerness requires full tomography and an optimization over exponentially many stabilizer states. A limited Pauli measurement set promises resource-efficient magic certification, yet the resulting reduced stabilizer polytope is generally difficult to characterize. We trace this difficulty into two coupled obstructions: the simultaneous measurability of measurements captured by their frustration graph structure, and the consistency of sign dependencies from stabilizer formalism. We show that the sign dependencies can be discarded exactly whenever active dependencies are absent, and that perfect frustration graphs then make this reduced polytope efficiently solvable. This solvable regime derives a closed form bounded by the clique number of the frustration graph, revealing a tradeoff between witness capacity and simultaneous measurability. Clifford covariance allows rotated measurement sets to enlarge the detectable state space without raising the capacity. Graph structure therefore emerges as both a certificate of tractability and a design principle for scalable magic resource detection.
Magic -- the resource enabling quantum computational advantage beyond stabilizer circuits -- has a clean phase-space characterization in odd prime dimensions that qubits notoriously lack. We study C(rho), the l_1 distance from a state's discrete Wigner function to the stabilizer polytope, and determine its exact geometry. For a single qubit with Bloch vector r we prove the closed form C = max(0, ||r||_1 - 1)/2, so the maximum is (sqrt(3)-1)/2, attained at the eight face states. For products of single-qubit states, C(rho_1 x ... x rho_n) <= prod_i (1 + C(rho_i)) - 1, with equality whenever every factor has Bloch sign product r_x r_y r_z <= 0; this includes the exact tensor powers ((1+sqrt(3))/2)^n - 1 and the exact maximum of C over fully separable n-qubit states.
Because no discrete Wigner function for qubits is Clifford covariant, any such measure is frame dependent, and we identify which of its features are not. For a single qubit there are exactly two valid frames up to phase-space translation and C is identical on both. Pauli-covariant two-qubit frames number 2048 up to translation, and across them the ratio C(rho_Rx)/C(rho_Ry) takes the values 1/2, 1 and 2. In every one of these frames some stabilizer state has negative Wigner entries, a consequence of the Mermin-Peres square, so the two-qubit bound Gamma >= 1 + C/2 against the robustness of magic Gamma is frame independent.
Within the Wootters frame we prove integer values 1, 2, 1 of the tightness ratio kappa := (Gamma-1)/C for three families in the two-qubit repetition codespace, with explicit certificates. For n-qubit GHZ phase states we prove Gamma = |sin phi| + |cos phi| for all n and C >= 2^(floor(n/2)-1) (|sin phi| + |cos phi| - 1), tight for n = 2 and, by exact linear programming, for n = 3, 4, so kappa decays exponentially. Finally, C is not a magic monotone, so it cannot replace Gamma in distillation bounds.
We systematically characterize quantum entanglement in all sixteen combinations of two-qubit Bell states across three operational regimes: pure superposition, noiseless mixing, and asymmetric mixing of one noisy and one pure Bell state. For pure superpositions, we calculate the von Neumann entropy and show that entanglement depends critically on the relative phase phi between superposed states. For mixed states, we use concurrence C under three physically motivated decoherence channels. The Depolarizing (DP) and Phase Damping (PD) channels are modeled as global, collective operations on the two-qubit Hilbert space, while Amplitude Damping (AD) is treated as independent local decay on each qubit, reflecting the asymmetric, energy-dissipating nature of spontaneous emission. A central and counterintuitive result is that increasing noise can raise concurrence in specific parameter regimes across all three channels. We further map the boundary between mathematical entanglement (C > 0) and operational quantum non-locality using the Horodecki criterion, finding this boundary to be strongly channel-dependent. Under Phase Damping, the Bell-local entanglement regime vanishes entirely for identical and same-bases mixtures, meaning any surviving entanglement guarantees a CHSH violation. Under Depolarizing noise, a large Bell-local region requires mixing probability r > 1/sqrt(2) to recover non-locality at maximum noise. For Amplitude Damping, certain mixtures exhibit a non-monotonic Horodecki parameter M(T): concurrence decreases monotonically with noise, yet the capacity for CHSH violation is lost at intermediate noise levels and restored at high noise, as maximal damping repurifies the noisy branch. These results provide a unified analytical reference for entanglement and non-locality management across Bell-state combinations and decoherence channels relevant to NISQ architectures.
The nonstabilizerness of quantum states is a necessary resource for universal quantum computation, yet its characterization is notoriously demanding. Quantifying nonstabilizerness typically requires an exponential number of measurements and a doubly exponential classical post-processing cost to evaluate its standard monotones. In this work, we show that nonstabilizerness is, to a large extent, in the eyes of the beholder: it can be witnessed and quantified using any set of mn-qubit Pauli measurements, provided the set contains anti-commuting pairs. We introduce a general framework that projects the stabilizer polytope onto the subspace defined by these observables and provide an algorithm that estimates magic from Pauli expectation values with runtime exponential in the number of measurements m and polynomial in the number of qubits n. By relating the problem to a stabilizer-restricted variant of the quantum marginal problem, we also prove that deciding membership in the corresponding reduced stabilizer polytope is NP-hard. In particular, unless P=NP, no algorithm polynomial in m can solve the problem in full generality, thus establishing fundamental complexity-theoretic limitations. Finally, we employ our framework to compute nonstabilizerness in different Hamiltonian ground states, demonstrating the practical performance of our method in regimes beyond the reach of existing techniques.
Nonstabilizerness (magic) is the quantum resource that, together with stabilizer operations, makes universal quantum computation possible. However, quantifying nonstabilizerness is difficult. The standard measure sums over exponentially many Pauli operators, and for quantum codes no quantitative theory has been available. A structural observation removes this obstacle: the Clifford sector carries no magic and can be removed by a Clifford transformation, leaving an order-three object whose magic is an exactly evaluable fourth moment. Removing this inert background yields closed forms for several code families: a formula for all Dicke states, reducing a 100-qubit case from 4100 terms to a short binomial sum; a bound for cubic-phase codes (twisted quantum doubles and non-Abelian topological order), saturated only by the D4 code; and a cyclic/zero criterion for group multiplication states. For codeword-stabilized (CWS) codes, the companion paper \cite{Liu26arXiv} carries this reduction to its conclusion: the most magical codes are exactly the Sidon sets, and the nonstabilizerness bounds the code's transversal non-Clifford power. Together these results reduce the computation of nonstabilizerness for a broad class of codes to finite classical counting problems, and identify the maximally magical codes among them.