We present the solution to a model of fermions hopping between neighbouring sites on a line with random Brownian amplitudes and open boundary conditions driving the system out of equilibrium. The average dynamics reduces to that of the symmetric simple exclusion process. However, the full distribution encodes for a richer behaviour entailing fluctuating quantum coherences which survive in the steady limit. We determine exactly the steady statistical distribution of the system state. We show that the out of equilibrium quantum coherence fluctuations satisfy a large deviation principle and we present a method to recursively compute exactly the large deviation function. As a byproduct, our approach gives a solution of the classical symmetric simple exclusion process based on fermion technology. Our results open the route towards the extension of the macroscopic fluctuation theory to many body quantum systems.Introduction.-Non-equilibrium phenomena are ubiquitous in Nature. Understanding the fluctuations of the flux of heat or particles through systems is a central question in non equilibrium statistical mechanics. Last decade has witnessed tremendous conceptual and technical progresses in this direction for classical systems, starting from the exact analysis of simple models [1-3], such as the Symmetric Simple Exclusion Process (SSEP) [4][5][6][7], via the understanding of fluctuation relations [8][9][10] and their interplay with time reversal [11,12], and culminating in the formulation of the macroscopic fluctuation theory (MFT) which is an effective theory adapted to describe transport and its fluctuations in diffusive classical systems [13]. Whether MFT may be extended to the quantum realm is yet unexplored.In parallel, the study of quantum systems out of equilibrium has received a large amount of attention in recent years [19][20][21][22]. Experimentally, unprecedented control of cold atom gases gave access to the observation of many body quantum systems in inhomogeneous and isolated setups [14][15][16][17][18]. Theoretically, results about closed, quantum systems have recently flourished, with a better perception of the roles of integrability, chaos or disorder [23][24][25][26][27][28][29][30][31][32][33][34]. In critical or integrable models, a good understanding has been obtained with a precise description of entanglement dynamics, quenched dynamics, as well as transport [35][36][37][38][39][40][41][42][43][44][45][46]. These efforts culminated in the development of a hydrodynamic picture adapted to integrable systems [47,48]. However, these understandings are restricted to closed, predominantly ballistic, systems.Many quantum transport processes are diffusive rather than ballistic [49] and, to some extends, physical systems are generically in contact with external environments. It is thus crucial to extend the previous studies by developing simple models for fluctuations in open, quantum many body, locally diffusive, out of equilibrium systems. Putting aside the quantum nature of the environments leads to co...