COLDFIRE-GY1.0 Readout and Control of Molecular Qubits via a Superconducting Niobium Cavity Gyrotron "The salvaged klystron becomes a quantum interface"

We present COLDFIRE-GY1.0, an architecture for coherent control of molecular qubits (triplet-spin Pd-TPPI-I on SiC) using a superconducting niobium (Nb) cavity gyrotron as a pulsed microwave/THz source. Unlike conventional laboratory sources (klystrons, YIG oscillators), a Nb gyrotron provides field-tunable frequency across 10–700 GHz, exceptional spectral purity (Δν/ν < 10⁻⁶), and high peak power enabling nanosecond quantum gates (τ_π ≈ 1–10 ns). We analyse cyclotron resonance conditions for matching molecular spin transitions, thermal compatibility within a shared cryogenic environment (4–77 K), dispersive readout protocols via a Nb cavity in the circuit-QED regime, and write sequences (π / π-half pulses). We estimate a gate fidelity of 15–40% for a frugal prototype and identify the primary technical bottlenecks. This document is an open theoretical and experimental roadmap — no experimental data are claimed.