Optical Science Laboratory
Semiconductor quantum dots have discrete density of states
and exhibit strong interaction effects with light. Moreover, optically created
excitons and biexcitons exist stably in quantum dots because of confinement
effects from all spatial directions. These features promise possible future
applications of quantum dots for quantum information processes. In quantum
dots, there may be coherent interactions between excitons and biexcitons and
the effects of these interactions have not yet been clearly observed. These
effects provide a source of quantum correlations by exciton-biexciton interactions
and a principle of optical functions by quantum interference effects.
We examined the photo-absorption properties of excitons and biexcitons in
a single InGaAs quantum dot. The coherent effects may appear in these absorption
properties. We employed a micro-photoluminescence (PL) method. Figure 1 shows
measured absorption spectra of exciton (X) and biexciton (XX) for two excitation
conditions. At a low excitation, we obtained usual peak shaped absorption
spectra. However, these spectra change greatly with a high excitation. The
XX spectrum has a broadened peak. In contrast, the X spectrum has an unusual
dip-shaped structure. Figure 2 reveals the physical origin of the unusual
spectrum. The figure shows the energy level structure for the measurement
result. The energy structure is a coherently interacting three-level system
with ground, exciton and biexciton states. This structure varies greatly with
excitation. With a high excitation, the exciton and biexciton states split
due to Rabi oscillation between the exciton and biexciton states. This causes
the exciton state to vanish resulting in the dip-shaped exciton absorption.
This Rabi oscillation is a coherent effect and our result confirms that there
is a strong coherent effect in the exciton and biexciton states.
Our results are an important step towards achieving a quantum two-bit gate
with an exciton and a biexciton. Moreover, this provide a possible way to
demonstrate optical device functions with quantum interference effects.
[1] H.Gotoh, et al., Appl. Phys. Lett. 85 (2004) 3480.
[2] H.Gotoh, et al., Phys. Rev. B in press
 |
||
|
Fig. 1. Exciton and biexciton
absorption. |
Fig. 2. Energy level structure |
|