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Semiconductor quantum dots are excellent candidates for ultrafast coherent manipulation of qubits by laser pulses on picosecond timescales or even faster. In inhomogeneous ensembles a macroscopic optical polarization decays rapidly due to dephasing, which, however, is reversible in photon echoes carrying complete information about the coherent ensemble dynamics. Control of the echo emission time is mandatory for applications. Here, we propose a concept to reach this goal. In a two-pulse photon echo sequence, we apply an additional resonant control pulse with multiple of 2 π area. Depending on its arrival time, the control slows down dephasing or rephasing of the exciton ensemble during its action. We demonstrate for self-assembled (In,Ga)As quantum dots that the photon echo emission time can be retarded or advanced by up to 5 ps relative to its nominal appearance time without control. This versatile protocol may be used to obtain significantly longer temporal shifts for suitably tailored control pulses. Ultrafast optical manipulation of excitons in semiconductor nanostructures offers practical access to quantum phenomena in condensed matter. The authors demonstrate a new scheme to control the exciton dephasing in large ensembles of quantum dots by application of resonant optical pulses that enable promising routes in the quest for quantum memories for information and communication technologies.

Accurate photon echo timing by optical freezing of exciton dephasing and rephasing in quantum dots