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Physical Review Letters

Photon Beats from a Single Semiconductor Quantum Dot

Timur Flissikowski, A. Hundt, Martin Lowisch, Martin Rabe, F. Henneberger

Published Apr 2, 2001• 191 citations
Publisher

Abstract

Single-photon interference is observed on the ultranarrow long-term stable exciton resonance of an individual semiconductor quantum dot. This interference is related to the fine-structure splitting and allows direct conclusions about the coherence properties of the exciton. When selectively addressing a particular dot by quasiresonant phonon-assisted excitation, despite a rapid orientation relaxation on a 1-ps time scale, coherence is partly maintained. No significant further decoherence occurs when the ground state is reached until the exciton recombines radiatively (approximately 300 ps).

DOI
10.1103/physrevlett.86.3172

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We report on coherent control of the resonantly driven exciton state in a single quantum dot in the strong coupling regime. With picosecond laser pulses, the two-level system undergoes Rabi oscillations and a given coherent superposition of states can be addressed. Using a pair of phase-locked pulses, optical manipulation is achieved during the coherence time of the qu-bit. The dephasing processes are also studied and we show that in typical quantum dots, the coherence time T 2 can be of the same order of magnitude as the effective lifetime of the exciton T 1 (a few hundreds of ps) although not reaching the upper theoretical limit of 2T 1 . We conclude that energy relaxation and pure dephasing processes due to virtual scattering with phonons contribute on an equal footing to the loss of coherence.
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We report on coherent control of the fundamental exciton state in a single quantum dot by means of two phase-locked picosecond laser pulses. By analyzing the experiments performed in the Rabi regime and comparing them to numerical simulations, we show that we can address the exciton coherence and get an insight into the involved dephasing processes. The total decoherence time T2{T}_{2}T2​ (170 ps) is comparable to the effective exciton lifetime T1{T}_{1}T1​ (200 ps), although not reaching the upper theoretical limit of 2T12{T}_{1}2T1​. We conclude that energy relaxation and pure dephasing processes due to virtual scattering with phonons both contribute on an equal footing to the loss of coherence in this kind of monolayer-step-induced GaAs quantum dots.
We present a method for time-resolving the coherent precession of the spin of a single neutral exciton confined to an InAs/GaAs quantum dot. A pair of circularly polarized π pulses are used to pump and probe the exciton spin, rather than a pair of interferometrically stable π/2-pulses in a quantum interference type experiment, faster, technically simpler, measurements of the small 15 μ eV exciton fine-structure can be made in the photocurrent regime. A small change in the fine-structure splitting with vertically applied electric-field is observed.
We investigate the coherent exciton spin dynamics in a single InAlAs/AlGaAs quantum dot using time-resolved quantum state tomography. Under two-LO-phonon quasi-resonant excitation of neutral exciton, we observe pronounced quantum beats in the circular and diagonal polarization components, reflecting the fine-structure splitting (Δ≈19.6 μeVΔ\approx 19.6 \ μ\text{eV}Δ≈19.6 μeV). By employing a global fitting procedure across three orthogonal polarization bases, we demonstrate that the spin evolution is consistently described by a unified Hamiltonian dominated by the anisotropic exchange interaction. While the initial degree of circular polarization is limited to ≈0.28\approx 0.28≈0.28 due to fast relaxation processes during carrier cooling, the subsequent dynamics reveal a long-lived spin coherence (1.1±0.21.1 \pm 0.21.1±0.2 ns) that exceeds the exciton lifetime (∼767\sim 767∼767 ps). Our analysis reveals that the spin-formation time is significantly shorter than the instrument response function, and the absence of a discernible Overhauser shift confirms a negligible influence from the local nuclear environment under the present conditions. These results provide a quantitative benchmark for the three-dimensional reconstruction of spin trajectories using differential polarization signals, demonstrating the feasibility of using quasi-resonant excitation for stable spin initialization in semiconductor nanostructures.