Experimental challenges and prospects for quantum-enhanced energy conversion: Stationary Fano coherence in V-type qutrits interacting with polarized incoherent radiation

Quantum coherence offers potential for energy conversion technologies. It influences light absorption and emission, affecting energy conversion limits and efficiency. As a result, quantum coherence is being harnessed to boost performance in quantum heat engines, photocells, and photosynthetic-inspired platforms. Of particular interest in this context is the generation of Fano coherences, i.e., the formation of quantum coherences due to the interaction with the continuum of modes characterizing an incoherent process. We aim to formalize mathematically the possibility of achieving steady-state Fano coherence in a V-type three-level quantum system using polarized incoherent radiation, without requiring the energy difference between the excited levels to tend to zero. We perform this analysis by deriving the Bloch-Redfield equation from first-principles by quantizing the incoherent radiation. The resulting reduced dynamics of the system are analysed, so as to determine the lifetime of Fano coherence and identify the conditions under which it becomes stationary. We characterise distinct dynamical regimes, ranging from weak to strong pumping, in which steady-state Fano coherence emerges, and we quantitatively determine its magnitude. For each regime, we analyse the generation of Fano coherence as a function of both the intensity of the incoherent pumping and the energy splitting between the excited levels. We also assess how obtaining Fano coherence is modified by symmetric or asymmetric decay rates. These findings indicate that a three-level quantum system driven by polarized incoherent light can act as a robust resource for coherence-assisted energy conversion and storage. Finally, we discuss the experimental challenges associated with the implementation of the proposed model using an ensemble of Rubidium atoms.

💡 Research Summary

The paper investigates the generation and stabilization of stationary Fano coherence in a V‑type three‑level quantum system driven by polarized incoherent radiation, with an eye toward quantum‑enhanced energy conversion and storage. Starting from a fully quantized description of the electromagnetic reservoir, the authors derive a Bloch‑Redfield master equation for the reduced dynamics of the system. They carefully apply the Born‑Markov approximation, a partial secular approximation, and the Weisskopf‑Wigner treatment to retain off‑diagonal (coherence) terms that would otherwise be eliminated under isotropic illumination.

A central result is that the polarization and anisotropy of the incoherent field preserve phase correlations between the two optical transitions |c⟩→|a⟩ and |c⟩→|b⟩. Consequently, even when the excited‑state splitting Δ is comparable to or larger than the average spontaneous decay rate (\bar\gamma), a non‑zero steady‑state coherence (\rho_{ab}) can be maintained without requiring Δ→0. The steady‑state value of (|\rho_{ab}|) depends on three key parameters: the mean photon number (\bar n) (which quantifies the pumping strength), the splitting Δ, and the decay asymmetry between the two excited states.

The authors identify two dynamical regimes. In the weak‑pumping limit ((\bar n\ll1)), the coherence scales linearly with (\bar n) and is suppressed by the factor (\Delta/(\bar\gamma^2+\Delta^2)). In the strong‑pumping limit ((\bar n\gg1)), the coherence saturates to a value that is maximized when Δ≈(\bar\gamma) and the decay rates are symmetric. They also derive an effective decoherence rate (\Gamma_{\text{eff}} = (\gamma_a+\gamma_b)/2 + (r_a+r_b)/2); strong pumping can offset this rate, leading to a practically infinite coherence lifetime in the steady state.

To bridge theory and experiment, the paper proposes using an ensemble of ^87Rb atoms. The V‑type configuration can be realized within the hyperfine manifold of the D1/D2 lines, selecting appropriate Zeeman sublevels for |a⟩, |b⟩, and |c⟩. Polarized incoherent radiation could be generated by passing broadband thermal emission through high‑extinction polarizers or by employing a spectrally broadened laser whose phase is randomized while preserving a fixed polarization. The main experimental challenges identified are: (i) achieving a spectral width larger than Δ while maintaining polarization purity, (ii) suppressing inhomogeneous dephasing in the atomic ensemble (e.g., via cooling and magnetic field control), and (iii) detecting the tiny off‑diagonal density‑matrix element, which may require quantum state tomography, heterodyne detection of emitted fluorescence, or high‑resolution Raman spectroscopy.

Overall, the work demonstrates that polarized incoherent light is a powerful resource for engineering steady‑state quantum coherences without external coherent drives. This opens new avenues for designing quantum heat engines, photovoltaic devices, and quantum batteries where coherence‑assisted processes can surpass classical performance limits. Future directions include extending the analysis to many‑body systems, exploring non‑Markovian reservoirs, and quantifying the actual gain in work extraction or power output that stationary Fano coherence can provide in realistic devices.