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Optics Express

Measurement of the inhomogeneous broadening of a bi-exciton state in a quantum dot using Franson-type nonlocal interference

Yong-Nan Sun, Yang Zou, Geng Chen, Jian‐Shun Tang, Haiqiao Ni, Mi-Feng Li, Guowei Zha, Zhichuan Niu, Yong‐Jian Han, Chuan‐Feng Li, Guang‐Can Guo

Published Jan 23, 2017• 7 citations
Publisher

Abstract

The inhomogeneous broadening of the bi-exciton state in quantum dots, i.e., the inhomogeneous broadening of the upper level of the cascade process, is not only a fundamental problem in quantum dots, but also closely related with the coherent control of this complex system and the quality of the entangled photon pairs, especially the time-bin entangled photon pairs. This inhomogeneous broadening is inherently a two-photon correlated phenomenon. In this work, we construct a genuine Franson-type nonlocal interference process to measure the inhomogeneous broadening of the bi-exciton state. The results show that the inhomogeneous broadening of the bi-exciton state is considerably smaller than that of the exciton state, that is why the entangled photon pairs can be generated by the cascade process in the quantum dot.

DOI
10.1364/oe.25.001778

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The inhomogeneous broadening of the bi-exciton state in quantum dots, i.e., the inhomogeneous broadening of the upper level of the cascade process, is not only a fundamental problem in quantum dots, but also closely related with the coherent control of this complex system and the quality of the entangled photon pairs, especially the time-bin entangled photon pairs. This inhomogeneous broadening is inherently a two-photon correlated phenomenon. In this work, we construct a genuine Franson-type nonlocal interference process to measure the inhomogeneous broadening of the bi-exciton state. The results show that the inhomogeneous broadening of the bi-exciton state is considerably smaller than that of the exciton state, that is why the entangled photon pairs can be generated by the cascade process in the quantum dot.
Photons which are generated in a two-photon cascade process have an underlying time correlation since the spontaneous emission of the upper level populates the intermediate state. This correlation leads to a reduction of the purity of the photon emitted from the intermediate state. Here we characterize this time correlation for the biexciton-exciton cascade of an InAs/GaAs quantum dot. We show that the correlation can be reduced by tuning the biexciton transition in resonance to a planar distributed Bragg reflector cavity. The enhanced and inhibited emission into the cavity accelerates the biexciton emission and slows down the exciton emission thus reduces the correlation and increases the purity of the exciton photon. This is essential for schemes like creating time-bin entangled photon pairs from quantum dot systems.
A well controlled, triggered source of entangled photons is desirable for many applications in quantum information processing. The two-photon cascade from a biexciton state, two electrons and holes, confined in a quantum dot can be such a source of polarisation-entangled photons provided the two decay paths from the biexciton carry no “which-path” information. In recent years many techniques have been employed to make the two optical decay paths from the biexciton state indistinguishable. A number of these techniques will be discussed and entangled-photon emission from a single quantum dot with a high fidelity in the expected Bell-state from the cascade will be demonstrated. This source of entangled photons allows optical interferometry beyond the limits imposed by the photon wavelength. Interference fringes of the entangled biphoton state reveals a periodicity half of that obtained with the single photon, and much less than that of the pump laser. High fringe visibility indicates that biphoton interference is less sensitive to decoherence than interference of two sequential single photons. The effect of the exciton fine-structure splitting on our entangled photon source will be shown. Surprisingly the entanglement is found to persist despite relatively large separations between the intermediate energy levels of up to 4µeV. Measurements demonstrate that entanglement of the photon pair is robust to the dephasing of the intermediate exciton state responsible for the first order coherence time of either single photon. We distinguish between the first-order coherence time, and a parameter defined as the cross-coherence time, this is illustrated in the figure.
A well controlled, triggered source of entangled photons is desirable for many applications in quantum information processing. The two-photon cascade from a biexciton state, two electrons and holes, confined in a quantum dot can be such a source of polarisation-entangled photons provided the two decay paths from the biexciton carry no “which-path” information. In recent years many techniques have been employed to make the two optical decay paths from the biexciton state indistinguishable. We will demonstrate the effect of the exciton fine-structure splitting on a single-dot based entangled photon source. Surprisingly the entanglement is found to persist despite relatively large separations between the intermediate energy levels of up to 4µeV. Measurements demonstrate that entanglement of the photon pair is robust to the dephasing of the intermediate exciton state responsible for the first order coherence time of either of the single photons. We distinguish between the first-order coherence time (2*), and a parameter defined as the cross-coherence time (HV) which is illustrated in the figure. The cross-coherence time is found to be at least an order of magnitude greater than the first-order coherence time. We will also report on recent progress towards an ideal source of triggered polarisation-entangled photons with a single quantum dot.
Abstract We report on coherent control of the two‐photon transition between the ground state and the biexciton state on a single quantum dot level. The population of the biexciton is monitored by the cascaded spontaneous emission of the biexciton complex via the exciton states. The signal shows interference pattern at the double laser frequency and its contrast decays non‐exponentially at the time scale of 10 ps. The contrast decrease is attributed to a dynamical inhomogeneous broadening of the energy levels due to long‐term fluctuations in the dot environment. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Coherent control of a strongly inhomogeneously broadened system, namely, InAs self-assembled quantum dots, is demonstrated. To circumvent the deleterious effects of the inhomogeneous broadening, which usually masks the results of coherent manipulation, we use prepulse two-dimensional coherent spectroscopy to provide a size-selective readout of the ground, exciton, and biexciton states. The dependence on the timing of the prepulse is due to the dynamics of the coherently generated populations. To further validate the results, we performed prepulse polarization dependent measurements and confirmed the behavior expected from selection rules. All measured spectra can be excellently reproduced by solving the optical Bloch equations for a 4-level system.