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Physical Review B

Quantum optical properties of a single InxGa1−xAs−GaAs quantum dot two-level system

Stefan Stufler, Patrick Ester, A. Zrenner, Max Bichler A

Published Sep 1, 2005• 118 citations
Publisher

Abstract

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.

DOI
10.1103/physrevb.72.121301

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We report on a two-level system, defined by the ground-state exciton of a single InGaAs∕GaAs\mathrm{In}\mathrm{Ga}\mathrm{As}∕\mathrm{Ga}\mathrm{As}InGaAs∕GaAs 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.
Abstract In a single self‐assembled InGaAs quantum dot, the one‐exciton ground‐state transition defines a two‐level system, which appears as an extremely narrow resonance of only a few μeV width. The resonant interaction of this two‐level system with cw laser fields can be studied in detail by photocurrent spectroscopy, revealing the fine structure of the excitonic ground state as well as the effects of nonlinear absorption and power broadening. For the case of pulsed laser fields and in the absence of decoherence, the two‐level system represents a qubit. Excitations with ps laser pulses result in qubit rotations, which appear as Rabi oscillations in photocurrent experiments. Double pulse experiments further allow us to infer the decoherence time and to perform coherent control on a two level system.
Transient nonlinear optical spectroscopy, performed on excitons confined to single GaAs quantum dots, shows oscillations that are analogous to Rabi oscillations in two-level atomic systems. This demonstration corresponds to a one-qubit rotation in a single quantum dot which is important for proposals using quantum dot excitons for quantum computing. The dipole moment inferred from the data is consistent with that directly obtained from linear absorption studies. The measurement extends the artificial atom model of quantum dot excitonic transitions into the strong-field limit, and makes possible full coherent optical control of the quantum state of single excitons using optical pi pulses.
Potential application of elementary excitation in semiconductor quantum dot to quantum computation is discussed. Construction of scalable hardware and all optical implementation of logical gate that exploits discrete nature of electron-hole states and their well concentrated oscillator strenth for ultrafast gate operation are proposed. Rabi population oscillation of an excitonic two-level system in an isolated single InxGa1-xAs quantum dot is manifested by a quantum wave function interferometry in time domain, demonstrating a long lived coherence of zero-dimensional excitonic states and revealed the coherent population flopping under strong optical field. A phase-sensitive coherent gate operation on single-dot exciton qubit is demonstrated.
Excitons in quantum dots represent a remarkably good two-level system with a large optical dipole transition moment. Although most research currently focuses on enhancing their already outstanding properties as single photon emitters they become also increasingly interesting for experimental fundamental research due to their large dipole transition moment. This thesis presents studies which explore fundamental phenomena in quantum dots using coherent spectrocopy methods. First, we will first examine the optical properties of site-controlled quantum dots which were grown using a buried stressor via resonance fluorescence. Subsequently, we will show results that were obtained using photocurrent spectroscopy to study single quantum dots that are embedded in micropillar resonators. This is followed by a study of the Wigner time delay induced by a single quantum dot and which shows that the maximum delay is larger than its dephasing time T2 which is the theoretical limit for coherently scattered fraction. The next section demonstrates that the response of a resonantly driven two-level system which in our case is represented by an exciton in a seminconductor quantum dot depends significantly on the photon statistics of the exciting light field. Most strikingly, the Mollow triplet which we observe under coherent excitation vanishes under thermal excitation. The second part presents results concerned with the resonantly driven biexciton cascade via two- photon excitation. The coherent interaction between light field and excitons gives rise to a complex five-level system. Here, we show for the first time two-photon Rabi oscillations in the temporal domain. Furthermore, we examine the correlations between the different transitions. The measurements show that we can manipulate the time-ordering of the emitted photons by changing for example the laser power. All these studies share the resonant excitation scheme which allows for a coherent manipulation of the states of the quantum dot and grants access to phenomena typical for resonance fluorescence such as the Mollow triplet and Rabi oscillations. In particular the experiments on photon excitation spectroscopy and the studies on the dressed biexciton cascade show that fundamental effects of quantum optics can be explored in a scalable and application oriented setting using quantum dots.