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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Single quantum dots as photon pair emitters

Ana Predojević, Tobias Huber, Miroslav Ježek, Harishankar Jayakumar, Thomas Kauten, Glenn S. Solomon, Radim Filip, Gregor Weihs

Published May 1, 2013• 0 citations
Publisher

Abstract

Summary form only given. A realization of time-bin entanglement obtained from quantum dots would join the strength of long distance transmission that characterizes this type of entanglement with the single photon state purity of the quantum dot emission. Compared with polarization entanglement from quantum dots this scheme does not require elimination of the fine structure splitting responsible for partial distinguishability of the quantum dot cascades [1].Here, we present our results on coherent and resonant excitation of quantum dots, creation of photon pairs, scattering free emission and measurement of the high purity of the emitted photons [2, 3]. In addition, we will show our measurements of the generated time-bin entanglement. To excite a single quantum dot we performed two-photon resonant excitation of the biexciton state. Here, we exploited the biexciton binding energy in order to use the laser light which was not resonant to any photon emitted from the quantum dot (Fig. 1a). Our measurements were performed on a single self-assembled InAs/GaAs quantum dot. We confirmed the resonant nature of the excitation by observing Rabi oscillations (Fig. 1b). Additionally, we performed Ramsey interference measurement and determined the coherence time of the ground-biexciton state superposition. These measurements show that we can coherently transfer the phase of the excitation laser onto the quantum dot system, a necessary requirement to obtain time-bin entanglement.To test the statistics of the emitted state we measured the auto-correlation of the emitted photons (Fig. 3c). Here, the auto-correlation parameter at zero delay was measured to be 0.012(1) without and 0.0073(8) with background subtraction. In addition, we performed a measurement to characterize the state emitted from a quantum dot using a witness-based criterion described in [4]. With this measurement we experimentally confirmed that the resonantly excited quantum dot emits a non-Gaussian state of light (Fig. 1d). This measurement is the first of this nature ever performed on a semiconductor single photon emitter.

DOI
10.1109/cleoe-iqec.2013.6801613

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The strong confinement of semiconductor excitons in a quantum dot gives rise to atomlike behavior. The full benefit of such a structure is best observed in resonant excitation where the excited state can be deterministically populated and coherently manipulated. Because of the large refractive index and device geometry it remains challenging to observe resonantly excited emission that is free from laser scattering in III/V self-assembled quantum dots. Here we exploit the biexciton binding energy to create an extremely clean single photon source via two-photon resonant excitation of an InAs/GaAs quantum dot. We observe complete suppression of the excitation laser and multiphoton emissions. Additionally, we perform full coherent control of the ground-biexciton state qubit and observe an extended coherence time using an all-optical echo technique. The deterministic coherent photon pair creation makes this system suitable for the generation of time-bin entanglement and experiments on the interaction of photons from dissimilar sources.
Summary form only given. Entanglement, a fundamental resource of quantum technologies, is a unique quantum mechanical attribute. In various systems it can be achieved in various individual degrees of freedom, however some of those systems are able to create entanglement in multiple degrees of freedom simultaneously: hyper-entanglement [1]. Here, we report on demonstration of polarization [2] and time-bin [3] hyper-entangled photons emitted from a single quantum dot. By applying two-photon resonant and coherent excitation on a quantum dot system with marginal fine structure splitting we yield fidelities to the maximally entangled state of 0.80(3) and 0.87(4) in polarization and time-bin, respectively. Quantum enhanced communication schemes rely on Bell-state measurements, which find their simplest realization in the interference of two photons at a beam splitter. However, this method is efficiency limited [4] and a complete Bell state analysis of a photon pair entangled in one degree of freedom using only linear optics is impossible. This limitation can be overcome if the photon pair is entangled in more than one degree of freedom, a hyper-entangled state. Using such states in such schemes does not only reduce the resource overhead [5] it also offers the possibility to increase the success rate significantly [6].
We report a study on coherence of excitation of single quantum dots. We address the coherent excitation of biexcitons, the process that is indispensable for deterministic photon pair generation in quantum dots. Based on theoretical modeling we optimized the duration of the excitation pulse in our experiment to minimize the laser-induced dephasing and increase the biexciton-to-background single-exciton occupation probability. An additional effect of this approach is a high degree of time-bin entanglement with a concurrence of up to 0.78(6) and a 0.88(3) overlap with a maximally entangled state.
Single photon emission is one of the most striking optical properties of semiconductor quantum dots (QDs), and the corresponding photon anti-bunching effect was observed by non-resonant and resonant photoluminescence experiments in single QDs [1,2,3,4]. Other experiments demonstrated the possibility to use single QDs for the generation of indistinguishable single photons [3]. An important general issue is to reach the so-called fundamental radiative limit. Several strategies have been followed in that sense such as the shortening of population lifetime induced by the Purcell effect [5] or the reduction of pure dephasing by resonant excitation [2,3,4]. Up to now, all the measurements reported in the literature focus on the incoherent photoluminescence signal for achieving single photon emission. In this regime, the linewidth of the photoluminescence spectrum is always limited to the radiative limit. In this paper, we report on the resonant emission (RE) of the exciton in single InAs/GaAs QDs embedded in a planar microcavity, in an orthogonal excitation-detection geometry [4]. We present an original type of single photon source based on the coherent laser light scattering by a single QD [6]. Besides the antibunching effect showing the non-classical nature of the emitted light, we observe that the QD emission spectrum is determined by the spectrum of the resonant excitation laser. We present high-resolution measurements by Fourier transform spectroscopy of the RE where the data are fitted to the first-order correlation function g(1)(τ) of the RE of a two-level system. The inverse Fourier transform of the g(1)(τ) function allows a direct comparison of the different RE emission profiles. We show that, by reducing the resonant excitation power, the so-called Mollow triplet shrinks to a single narrow Lorentzian line, while the relative intensity of this narrow peak increases and tends to overwhelm the RE spectrum. This implies that single QDs can produce single photons with a coherence time that is not limited anymore by the QD electronic properties, resulting in an ultra-coherent character [6]. In conclusion, the resulting " ultra-coherent single photon source " promises high degrees of indistinguishability of the emitted photons which is a crucial requirement for quantum information applications. [1] P. Michler et al., Science 290, 2282 (2000). [2] A. Muller et al., Phys. Rev. Lett. 99, 187402 (2007). [3] S. Ates et al., Phys. Rev. Lett. 103, 167402 (2009). [4] H. S. Nguyen et al., Phys. Rev. Lett. 108, 057401 (2012). [5] C. Santori et al., Nature 419, 594 (2002). [6] H. S. Nguyen et al., Appl. Phys. Lett. 99, 261904 (2011).
A hyperentangled state of light represents a valuable tool capable of reducing the experimental requirements and resource overheads, and it can improve the success rate of quantum information protocols. Here, we report on demonstration of polarization and time-bin hyperentangled photon pairs emitted from a single quantum dot. We achieved this result by applying resonant and coherent excitation on a quantum dot system with marginal fine structure splitting. Our results yield fidelities to the maximally entangled state of 0.81(6) and 0.87(4) in polarization and time bin, respectively.