We report a state- and observable-dependent retention advantage in operational fidelity proxies under dephasing-dominated noise across multiple quantum platforms. We do not claim enhanced off-diagonal GHZ coherence; rather, we compare measured observables with different channel sensitivities under identical physical noise: GHZ population fidelity Fpop = P(|00…0⟩) + P(|11…1⟩) (diagonal in the computational basis) versus an X-basis product-state control implemented as H⊗N → delay → H⊗N → measure. On IonQ Forte (5–20 qubits), parity visibility follows V(N) = 1.041 exp(−0.030N) with χ²/dof = 0.86 across 5–15 qubits. At 20 qubits, V departs by −8.2σ, consistent with correlated-noise onset; model comparison (AIC) favors a piecewise description with regime separation. On IBM Heron (3-qubit GHZ experiments), the fidelity advantage ΔF reaches 69.7 percentage points at maximum idle delay, with the population survival ratio G = 12.5× from raw counts without error mitigation. Three platforms with average two-qubit gate fidelities in the 94–98% range do not show a detectable advantage within our dataset, consistent with a graded performance dependence on hardware fidelity. The effect is GHZ-specific within our data: W-states and 1D cluster states show reduced fidelities (−12% and −38% relative to product controls), with no indication of protection. All fidelities are from raw shot counts; no ZNE, PEC, or post-selection was applied. These results motivate regime-aware benchmarks that treat the measured observable as a first-class experimental variable.
We report a state- and observable-dependent retention advantage in operational fidelity proxies under dephasing-dominated noise across multiple quantum platforms. We do not claim enhanced off-diagonal GHZ coherence; rather, we compare measured observables with different channel sensitivities under identical physical noise: GHZ population fidelity Fpop = P(|00…0⟩) + P(|11…1⟩) (diagonal in the computational basis) versus an X-basis product-state control implemented as H⊗N → delay → H⊗N → measure. On IonQ Forte (5–20 qubits), parity visibility follows V(N) = 1.041 exp(−0.030N) with χ²/dof = 0.86 across 5–15 qubits. At 20 qubits, V departs by −8.2σ, consistent with correlated-noise onset; model comparison (AIC) favors a piecewise description with regime separation. On IBM Heron (3-qubit GHZ experiments), the fidelity advantage ΔF reaches 69.7 percentage points at maximum idle delay, with the population survival ratio G = 12.5× from raw counts without error mitigation. Three platforms with average two-qubit gate fidelities in the 94–98% range do not show a detectable advantage within our dataset, consistent with a graded performance dependence on hardware fidelity. The effect is GHZ-specific within our data: W-states and 1D cluster states show reduced fidelities (−12% and −38% relative to product controls), with no indication of protection. All fidelities are from raw shot counts; no ZNE, PEC, or post-selection was applied. These results motivate regime-aware benchmarks that treat the measured observable as a first-class experimental variable.
We report experimental measurements comparing the coherence decay of three-qubit Greenberger-Horne-Zeilinger (GHZ) entangled states versus locally coherent product states (|+++⟩) under idle time evolution on IBM Heron quantum processors. Across three different processor generations (Heron r1, r2, r3), we consistently observe that the GHZ state maintains higher fidelity than the product state as idle delay increases. Decay rate analysis reveals that |+++⟩ decays 2.4–13.8× faster than GHZ depending on the backend. A control experiment with |000⟩ confirms the decay mechanism is dominated by dephasing (T₂-type noise) rather than energy relaxation (T₁) or readout errors. These results demonstrate a structural dependence of coherence stability on the topology of the quantum state. Backends: ibm_fez (Heron r2), ibm_torino (Heron r1), ibm_boston (Heron r3)Configuration: 3 qubits, 4000 shots, 20 repetitions per delay pointDelays: 0, 500, 1000, 2000, 4000 dt
We provide a minimal analytic framework, entirely within standard open-quantum-system theory, that explains the protocol-robust GHZ advantage observed experimentally across heterogeneous quantum platforms. The framework rests on three results: (1) the Hamming Decay Theorem, showing that off-diagonal density matrix elements decay as lambda^{d_H} under any local quantum channel; (2) a binomial derivation of the product-state fidelity F_prod = [(1+exp(-gamma*t))/2]^N under the IBM protocol; and (3) a unification of the two experimental regimes (IBM dephasing-dominated and IonQ amplitude-damping-dominated) via T2-blindness of the GHZ observable. A three-parameter noise model calibrated to IonQ 5-qubit data reproduces the selectivity pattern GHZ+, W-, Cluster- with correct signs. The structural invariant S(rho) captures the channel-independent topology underlying the advantage. Includes Appendix A: Revised Empirical Basis with cross-platform convergence data, selectivity tables, hardware threshold, and kill switches.
I report a hands-on, systematic investigation of Greenberger–Horne–Zeilinger (GHZ) state generation on a 127-qubit supercon-ducting quantum processor, covering system sizes from 4 to 80 qubits. I measured both computational-basis populations andglobal Pauli-X parity oscillations to decompose the state fidelity into its population purity and phase-coherence components. Mydata reveal an exponential collapse of fidelity with qubit number: a 4-qubit GHZ state achieves a fidelity of F = 0.9307, wellabove the threshold for entanglement-based quantum key distribution, yet by 32 qubits the fidelity has plummeted to 0.0963 andthe parity signal is extinguished. At 80 qubits, the state is indistinguishable from a maximally mixed state (F = 0.0453). I isolatethe dominant noise channels—CNOT gate errors, dephasing, readout crosstalk—and map the current usability frontier for rawmultipartite entanglement. I then contrast my results with mitigation-enhanced benchmarks from other platforms, showing thatadaptive compilation and dynamical decoupling can recover fidelity above the genuine multipartite entanglement (GME) thresholdeven at 120 qubits. This work provides a deeply personal, quantitative perspective on the challenges and near-term solutions forscaling entanglement in quantum communication, sensing, and error-mitigated computation.
Version 3.1: Extended cross-platform validation of structural coherence protection in GHZ entangled states. KEY FINDINGS:- GHZ states show 2.4-13.8× slower decay (IBM Heron) and +5-9% fidelity advantage (IonQ Forte) vs product states- Structural protection is GHZ-specific: W-states (-12%) and Cluster states (-38%) underperform product states on IonQ- Non-Markovian memory (N1) detected in trapped ions: Dynamical decoupling with Z-pulses improves GHZ protection by +2.99%, 5× stronger than IBM's +0.6%- Scaling confirmed from 3 to 7 qubits with 100% consistency across all measurements PLATFORMS:- IBM Heron: ibm_torino (r1), ibm_fez (r2), ibm_boston (r3)- IonQ Forte 1 (trapped ions, 36 qubits) EXPERIMENT DATES: January 15-18, 2026TOTAL INVESTMENT: ~$2,350 USD (IBM + AWS/IonQ) This version extends v2.1 (DOI: 10.5281/zenodo.18284373) with IonQ cross-platform validation, entanglement-type dependence analysis, and non-Markovian memory detection via dynamical decoupling.
Version 4.1 PLATINUM: Extended validation of structural coherence protection (N2) in GHZ entangled states, now including 9-qubit scaling results across two fundamentally different quantum technologies. KEY BREAKTHROUGH - TECHNOLOGY INDEPENDENCE:- IBM Heron (9q): +51.75% GHZ advantage (GHZ 52.05% vs Product 0.30%)- IonQ Forte (9q): +50.12% GHZ advantage (GHZ 83.00% vs Product 32.88%)- Cross-platform difference: <2% This near-identical convergence (~50%) across superconducting transmons and trapped ions strongly suggests N2 is a fundamental property of GHZ topology under dephasing-dominated noise, not a hardware artifact. PLATINUM HIGHLIGHTS:- Protection ratio ~173× on IBM Heron at maximum stress (4000 dt)- N2 persists across scaling: 5q (+7.23%), 7q (+4.88%), 9q (+50.12%) on IonQ- Statistical foundation: p = 7.17×10⁻²⁷ at 5 qubits (from v4.0) FIVE DEFINITIVE FINDINGS:1. N2 is real and scales to 9 qubits2. N2 is technology-independent (~50% in both superconducting and trapped-ion systems)3. N2 requires high-fidelity hardware (>99.5% 2Q gate fidelity threshold)4. N2 is GHZ-specific (W-states and Cluster states show no protection)5. N2 is statistically incontrovertible PLATFORMS TESTED:- IBM Heron r2 (ibm_fez): 3-9 qubits ✓- IonQ Forte 1: 3-9 qubits ✓- Rigetti Ankaa-3: 5 qubits ✗ (low fidelity)- IQM Garnet: 5 qubits ✗ (low fidelity)- IQM Emerald: 5 qubits ✗ (low fidelity) Total experimental investment: ~$3,800 USDExperiment dates: January 16-19, 2026 This version extends v4.0 (DOI: 10.5281/zenodo.18295213) with 9-qubit PLATINUM results demonstrating cross-platform convergence. "Two dragons of different technologies, tamed by the same physics"IBM Heron: +51.75% | IonQ Forte: +50.12% | Difference: <2%
Version 2.1: Extended experimental analysis comparing coherence decay of GHZ, W, and product states on IBM Heron quantum processors (r1, r2, r3). Key findings:- GHZ states decay 2.4–13.8× slower than product states across three processor generations- Dynamical decoupling with virtual Z-pulses (DD-Z) provides marginal improvement (+0.6%), while physical X-pulses (DD-X) degrade coherence by -24.4%- W-state performance is architecture-dependent: superior on r1/r3, inferior on r2- Weak non-Markovian noise signatures detected at 5 µs delay window This version extends v1.0 (DOI: 10.5281/zenodo.18274447) with W-state analysis, comprehensive DD protocol testing, and the DD-Z vs DD-X discovery. Backends: ibm_torino (r1), ibm_fez (r2), ibm_boston (r3)Configuration: 3-5 qubits, 4000 shots, delays 0-100,000 dt