Quantum Gate Operation of Quantum Dot Exciton Qubit
H. Kamada and H. Gotoh
Physical Science Laboratory
Ā@Control of quantum mechanical superposition of an exciton in a single InxGa1-xAs quantum dot was demonstrated. Coherent control of a quantum mechanical system is an ultimate challenge, and it is essential toward realization of quantum computing. Rapidly growing maturity of nanofabrication technologies have made quantum two-level system in solids promising for such approaches. Exciton confined within mesoscopic dot is such a candidate: The similarity between atoms and QD excitons together with the long-lived coherence and the mesoscopically enhanced oscillator strength may offer a great opportunity of coherent manipulation of a well-defined single localized quantum system by optical field.
Ā@As an InxGa1-xAs quantum dot is exposed to electromagnetic radiation, the periodic oscillation of electric field of the light is imprinted to exciton polarization. While this coherent dipole oscillation persists, the periodic energy exchange between the light and the exciton representing light absorption followed by coherent stimulated light emission, results in a periodic exciton population oscillation: Or equivalently, the superposition of the exciton existence oscillates. This is Rabi oscillation, which was confirmed previously. It is then feasible to generate any superposition of the exciton two-level wavefunction in an isolated quantum dot with control of phase and strength of the light pulses. To demonstrate this, two-laser pulse excitation experiment with a precision delay control was undertaken. The first pulse induced the exciton dipole oscillation and created a 30-40 % superposition state (Fig.1). The second pulse delayed by 10 ps exactly in-phase with the first pulse added up the population/superposition to a value twice as large as that created by first one. In contrast, as the second pulse exactly out-of-phase to the first one erased the exciton population to null. Such results are summarized in Fig. 1. Apparently, this demonstrates a phase rotation gate operation on the quantum dot exciton as quantum bit and reinforces the potential of QD excitons in quantum logic application.
 H. Kamada, H. Gotoh, J. Temmyo, T. Takagahara, and H. Ando, Phys. Rev. Lett. 87 (2001) 246401.
Fig. 1. Experimental demonstration of coherent control of exciton population by phase-locked optical pulse pair: The first pulse creates an exciton superposition that is either doubled or erased depending on the relative phase.