We studied Ramsey interference of quantum states inside a single crystal of GaAs using femtosecond transient reflectivity and attosecond phase-locked femtosecond pulses. Modulation due to coherent optical phonons has been observed in transient reflectivity. Phonon amplitudes changed according to a delay between two pump pulses and showed interference patterns of both the electronic states and phonons. Time evolution of visibility of the electronic interference showed that electronic coherence remained in GaAs until approximately 45 fs even at room temperatures.