Memory is assumed to be stored in the brain as a cellular ensemble consisting of a set of neurons that is activated during learning. Although optical stimulation of a cellular ensemble is known to trigger the retrieval of the corresponding memory, it is unclear how the association of distinct information occurs at the cell ensemble level. Here, we show in mice that activation of a cell ensemble corresponding to two distinct memory events generates an artificial association between initially non-related events. In the context pre-exposure and immediate shock (IS) paradigm, mice failed to associate the shock with the pre-exposed context when the IS was delivered to their foot in a different context. Cells activated during the context pre-exposure and the IS in hippocampal CA1 and the basolateral amygdala (BLA) were targeted with channelrhodopsin-2, a light-activated cation channel. These cells were later simultaneously activated by optical stimulation in the mice's home cage. The next day, these mice exhibited freezing behaviour, an indicator of a fear response, in the pre-exposed context that was not originally associated with the shock. Thus, the artificial activation of distinct cell ensembles, without any sensory input, is capable of generating an artificially associated memory. Furthermore, our finding suggests that the association of distinct units of information is achieved through the synchronous activity of distinct cell ensembles. This mechanism may underlie memory update by incorporating novel information into pre-existing networks to form qualitatively new memories.