In this paper, we investigate biphoton generation via spontaneous four-wave mixing in Doppler-broadened atomic vapors. Thermal motion breaks the zero-temperature symmetry between copropagating and counterpropagating geometries and introduces competition between the biphoton generation rate and propagation loss. A temperature-dependent pump-detuning scheme restores an approximate mirror symmetry between the frequency-resolved responses of the two geometries, yielding nearly identical biphoton generation rates and correlation strengths over a broad temperature range. We find that increasing atomic thermal velocity drives the temporal correlation function from damped oscillations at low temperatures to a purely exponential decay at higher temperatures by enhancing decoherence and pushing the effective Rabi frequency into the imaginary regime. This behavior is captured analytically through the temperature-dependent modification of the poles governing the oscillatory correlations. The generated biphotons retain strong nonclassical character, accompanied by spectral narrowing and temperature-induced temporal broadening. Numerical simulations incorporating Doppler averaging validate these predictions, quantify the model's accuracy, and confirm that the same qualitative trends persist at higher optical depths. These results identify a simple and tunable route to narrowband biphoton generation in warm vapors, with direct relevance to quantum memories and long-distance quantum communication.

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• Effects of atomic coherence on light propagation
• Electric dipole-dipole interactions
• Nonlinear optical susceptibility
• Optical coherence
• Photon pairs & parametric down-conversion
• Quantum description of light-matter interaction
• Third order nonlinear optical processes
• Atomic ensemble
• Polarizability
• Dipole approximation
• Four-wave mixing
• Perturbative methods
• Rotating wave approximation