Finite-temperature transition-window weights on a fault-tolerant quantum computer: duplex filtering on a thermal purification

Abstract

The excite-filter-readout framework of Kharazi et al. computes zero-temperature response windows on a fault-tolerant quantum computer from a pure ground state. We extend it to Gibbs states, where the two objects decouple: filters act on absolute energies, a thermal response on transition frequencies. The extension rests on a Liouville-space/TFD identity, explicit in the block-encoding/QSP setting: the thermally weighted transition-window weight is a spectral projection of H_0 (x) I - I (x) H_0^T onto the Schmidt (thermofield-double) purification, instance of Rall's framework. Around it we assemble a routed-SELECT block encoding (one Hamiltonian query per filter query), a conjugate-pair readout, and a four-route (duplex, bin, Fourier-LCU, METTS) compatibility analysis. The cost is multiplicative and conditional: a per-window query bound M_"AE" * (2*sqrt(d/Z_"lb")*d_"vt" + 4*d_W), with a tight signal-adaptive count, the optimal sqrt(alpha beta) degree, and a window-edge term. The preparation factor sqrt(d/Z) grows exponentially with system size at fixed temperature in positive-entropy-density systems (2.08 -> 25.7, N = 8 to 14 at beta = 2), relieved only as beta -> 0. Zero temperature is recovered for beta G >= ln(2(d-1)||D||^2/epsilon). Identities are cross-checked between independent formula paths to 10^(-16)-10^(-12) and the readout assembly is validated at matrix level under a block-encoding promise.