Quantum optical properties of a single ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}\text{−}\mathrm{Ga}\mathrm{As}$ quantum dot two-level system

We report on a two-level system, defined by the ground-state exciton of a single $\mathrm{In}\mathrm{Ga}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ quantum dot. Saturation spectroscopy combined with ultrahigh spectral resolution gives us a complete description of the system in the steady-state limit. Rabi oscillations and quantum interference experiments, on the other hand, provide a detailed insight into the coherent high excitation regime. All fundamental properties of the two-level system show an excellent quantitative agreement in both domains, even though obtained under entirely different experimental conditions. We thus are able to demonstrate control over an almost ideal two-level system, suitable for possible applications in quantum information processing.

Figure 1

Photocurrent resonance of the ground-state single exciton at a fixed laser wavelength. Bias voltage levels can be transferred into energy scales via the Stark effect, as indicated by the upper axis. A linewidth of $5\mu \mathrm{eV}$ and an asymmetry splitting of $30\mu \mathrm{eV}$ are observed.

Figure 2

Exciton Rabi oscillation at excitation with picosecond laser pulses (bias voltage $0.6\mathrm{V}$). The oscillation is only slightly damped towards high pulse areas.

Figure 3

Quantum interference experiment with varying delay between two $\pi ∕2$ pulses (bias voltage $0.4\mathrm{V}$). The crosses correspond to a measurement at which only one of the asymmetry split levels is addressed. Circles on the other hand, correspond to an excitation of both lines, resulting in quantum beats. Solid lines represent fit curves, from which we deduce a dephasing time of $322\mathrm{ps}$ and an asymmetry splitting of $31\mu \mathrm{eV}$.

Figure 4

Comparison of dephasing times derived by linewidth analysis (circles) and quantum interference experiments (triangles). Both sets of data show very good agreement at voltage levels up to $0.7\mathrm{V}$. At higher bias voltage, quantum beats could not be suppressed, independent of the polarization of the excitation beam. Dephasing times here are derived from the ratio of the initial signal vs the first maximum of the beats (open triangles).