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We have shown theoretically that efficient multiple exciton generation (MEG) by a single photon can be observed in small nanocrystals (NCs). Our quantum simulations that include hundreds of thousands of exciton and multi-exciton states demonstrate that the complex time-dependent dynamics of these states in a closed electronic system yields a saturated MEG effect on a picosecond timescale. Including phonon relaxation confirms that efficient MEG requires the exciton-biexciton coupling time to be faster than exciton relaxation time. PACS numbers:Solar light would be an important source of clean and renewable energy if the efficiency of inexpensive solar cells could be increased. Increased efficiency can be achieved through carrier multiplication: Photo-generated carriers, whose excess energy is greater than the energy gap, can create secondary electron-hole pairs via impact ionization of the filled band. Through this process, two (or more) electron-hole pairs are collected from each photon instead of just one. This process is very inefficient in bulk semiconductors, where impact ionization has a very low probability and carrier thermalization, which always competes with impact ionization, is much faster.As first suggested by Nozik, impact ionization may effectively compete with cooling in NCs, due to the enhanced rate of inverse Auger processes for carrier multiplication and the "phonon bottleneck" suppression of carrier relaxation, leading to efficient MEG [19,20].Previous attempts at a theoretical understanding of the enhanced MEG provide only estimations of the MEG efficiency observed in NCs [3,[21][22][23][24][25][26]. A selfconsistent theory of this phenomena requires a currently non-existent theoretical description of both the relaxation mechanisms for and couplings between the highly excited exciton and multi-exciton states in NCs [24]. To explain the high efficiency of MEG in NCs, the coherent superposition model , based on the strong quasi-resonant coupling between exciton and multi-exciton states in a NC, was proposed [3,24]. Non-coherent models for efficient MEG in NCs [21][22][23]26] are based on the important observation that the density of biexiton states is significantly larger than the density of exciton states at the same energy [21], and the density of trion states is particularly important for efficient MEG [25]. The calculations of MEG efficiency in all of these non-coherent models are based on Fermi's Golden Rule, which requires the final biexciton state to decay much faster than the rate of the exciton-biexciton transition, an assumption that has not been justified either experimentally or theoretically.In this letter we unify both approaches and consider a single-photon excitation coherently coupled with multiexciton-states in a NC within a full quantum-state evolution approach. The time-dependent dynamics of our modeled systems is described using a large multipleconfiguration basis representation of the many-electron Hamiltonian, including energy non-conserving exciton and biexciton decay c...

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