  The ability to induce, observe and control quantum coherent interactions in
room temperature, electrically driven optoelectronic devices is of outmost
significance for advancing quantum science and engineering towards practical
applications. We demonstrate here a quantum interference phenomena, Ramsey
fringes, in an inhomogeneously broadened InAs/InP quantum dot (QD) ensemble in
the form of a 1.5 mm long optical amplifier operating at room temperature.
Observation of Ramsey fringes in semiconductor QD was previously achieved only
at cryogenic temperatures and only in isolated single dot systems. A
high-resolution pump probe scheme where both pulses are characterized by cross
frequency resolved optical gating (X-FROG) reveals a clear oscillatory behavior
both in the amplitude and the instantaneous frequency of the probe pulse with a
period that equals one optical cycle at operational wavelength. Using nominal
input delays of 600 to 900 fs and scanning the separation around each delay in
1 fs steps, we map the evolution of the material de-coherence and extract a
coherence time. Moreover we notice a unique phenomenon, which can not be
observed in single dot systems, that the temporal position of the output probe
pulse also oscillates with the same periodicity but with a quarter cycle delay
relative to the intensity variations. This delay is the time domain
manifestation of coupling between the real and imaginary parts of the complex
susceptibility.
