Burke, P. J.; Aghaei, P.; Noh, S.; Ramos-Silva, J. N.; Jiang, M. J.; chen, P.-L.; Chen, Y.; Goodarzinia, F.; Usselman, R. J.; Hemmer, P.; Wallace, D. C.

Recent work has shown that genetically engineered proteins can serve as quantum bits in living systems. These quantum bits arise from the photochemistry of protein-bound flavins: blue-light excitation drives electron transfer to form a spin-correlated radical pair whose coherent singlet-triplet interconversion makes the protein's fluorescence sensitive to weak magnetic fields. Because this radical-pair reaction depends on the redox state of the flavin, itself a central electron carrier in cellular metabolism, the magneto-fluorescence of a biological qubit is intrinsically coupled to the biochemistry around it. This suggests a powerful application of fundamental significance in biology, until now an unsolved problem in the field of quantum sensing. Here we show a new class of quantum sensor, mtMagLOV2, that interfaces directly to a defining feature of life itself: the bioenergetic state of the cell. We genetically engineer flavin mononucleotide (FMN)-containing, magnetic-field-sensitive fluorescent proteins (biological qubits) to be expressed and translocated into the key bioenergetic machinery of the cell: the mitochondrial matrix. Using confocal and super-resolution microscopy, mtMagLOV2 localizes to the mitochondrial cristae, home of the electron transport chain complexes I-V and ATP synthase, the site of oxidative phosphorylation (OXPHOS). By pharmacological manipulation of OXPHOS, we show that the sensor's magneto-fluorescence tracks the redox (oxidation, reduction) state of the mitochondrial flavins, providing a quantum readout of redox status. The response differs between cancer cells (which rely heavily on glycolysis) and cardiomyocytes (which rely predominantly on OXPHOS), demonstrating quantum bioenergetic profiling. Together, these results establish biological qubits as quantum sensors capable of probing mitochondrial bioenergetics, opening a quantum window into the energetic machinery of living cells. More broadly, we anticipate that coupling quantum redox sensitivity to the specific biochemical targets will extend the reach of quantum technologies across the life sciences.