This thesis deals with the optical characterization of single quantum dot devices emitting at 1300nm. Thanks to the development and optimization of the growth technique we were able to achieve at the same time emission at 1300nm and ultra low QD densities. Our single QD devices present clear and reproducible spectral signatures in which we can identify exciton, biexciton and charged exciton transitions. Quasi-resonant excitation at 70K demonstrates background free single exciton transitions, which is very promising for the realization of a single photon device operating at temperatures in easy reach of thermoelectric coolers. A time-correlated single photon counting setup was built and used to measure the radiative lifetimes of single exciton transitions. These measurements also present new evidence on a background emission superposed to the narrow spectral transitions. Demonstration of single photon emission at these wavelengths required building a setup to measure the correlations between fiber-coupled single photons in a 300ps time window, emitted from a nano-device in free space at cryogenic temperatures, and with the capability of maintaining the optical alignment on a micrometer scale for several hours. With such a setup we have demonstrated that our QDs can generate single photon states at 1300nm. We used this single photon source to characterize novel detectors based on superconducting nanowires and measured for the first time the intensity correlation function at 1300nm on single photons from a QD. These detectors show at least 2 orders of magnitude improvement on the signal to noise ratio as compared to InGaAs APDs; this is very important since, for a QKD system, the detector noise, amongst others, determines the maximum distance over which a secure key can be exchanged. It should be noted that, due to the di±culties in these measurements, to date there has been only one other clear demonstration of single photon emission at 1300nm.
This thesis deals with the optical characterization of single quantum dot devices emitting at 1300nm. Thanks to the development and optimization of the growth technique we were able to achieve at the same time emission at 1300nm and ultra low QD densities. Our single QD devices present clear and reproducible spectral signatures in which we can identify exciton, biexciton and charged exciton transitions. Quasi-resonant excitation at 70K demonstrates background free single exciton transitions, which is very promising for the realization of a single photon device operating at temperatures in easy reach of thermoelectric coolers. A time-correlated single photon counting setup was built and used to measure the radiative lifetimes of single exciton transitions. These measurements also present new evidence on a background emission superposed to the narrow spectral transitions. Demonstration of single photon emission at these wavelengths required building a setup to measure the correlations between fiber-coupled single photons in a 300ps time window, emitted from a nano-device in free space at cryogenic temperatures, and with the capability of maintaining the optical alignment on a micrometer scale for several hours. With such a setup we have demonstrated that our QDs can generate single photon states at 1300nm. We used this single photon source to characterize novel detectors based on superconducting nanowires and measured for the first time the intensity correlation function at 1300nm on single photons from a QD. These detectors show at least 2 orders of magnitude improvement on the signal to noise ratio as compared to InGaAs APDs; this is very important since, for a QKD system, the detector noise, amongst others, determines the maximum distance over which a secure key can be exchanged. It should be noted that, due to the di±culties in these measurements, to date there has been only one other clear demonstration of single photon emission at 1300nm.
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).
In the fields of quantum communications and quantum computing, the transmission of information using single photons guarantees the security of the communication and enables calculations that would be impossible with our "classical" computers. One of the key elements in implementing such protocols is the photon source. It must emit, on demand, one and only one photon with specific quantum properties.The aim of this project is to characterise a source of single and indistinguishable photons, designed and manufactured at CEA-Grenoble, and consisting of an indium arsenide (InAs) semiconductor quantum dot integrated into a gallium arsenide (GaAs) nanocavity. With its low quality factor, the nanocavity enables photons to be extracted efficiently over a wide range of wavelengths (30nm). In addition, thanks to its small mode volume, the spontaneous emission of quantum dots within it can be accelerated by Purcell effect (Purcell factor up to 6).In this manuscript, we first detail the implementation and the optimisation of the experimental setups used to characterise the photons emitted by our sample : microphotoluminescence, radiative lifetime measurement, Hanbury-Brown and Twiss (HBT) experiment and Hong-Ou-Mandel (HOM) experiment. Then, we present the experimental results obtained for photons emitted by two distinct quantum dots : One containing neutral exciton states, and the second, containing a charged exciton state. In order to photo-create these exciton states in the quantum dots, two pulsed optical excitation schemes were implemented: the two-photon resonant excitation of the neutral biexciton, and the phonon-assisted quasi-resonant excitation of the neutral and charged excitons. We present the measurement of the three figures of merit of a single photon source for these two quantum dots : the brightness, the single-photon purity and the indistinguishability of the emitted photons. In a second phase, we will focus on the charged exciton state and measure the profile of its emission line and its autocorrelation function under continuous resonant excitation. These results, achieved within the nano-photonics group of the University of Basel, constitute the first demonstration of continuous excitation on these nanostructures, which is a good surprise given their small size.Finally, we will look at the effects of the application of mechanical stress on the energy separation of the two neutral exciton states, known as the "fine structure splitting" (FSS). During the radiative cascade of the neutral biexciton state, the two photons emitted are entangled in polarisation, and the fidelity of the entanglement depends in particular on the FSS. Here, we will consider a second sample, where the InAs quantum dots are embedded in a GaAs photonic nanowire. By bending the nanowire, mechanical stresses are applied to the quantum dots, which will modify the energy of the exciton states. We will present an experimental study aimed at observing a possible effect on the fine structure splitting.In conclusion, we have developed and optimised various experimental setups for characterising photon sources for applications in the fields of quantum communications and quantum computing. For a first version of these photonic nanostructures, the brightness, the single-photon purity and the indistinguishability of the photons emitted by our quantum dots integrated into nanocavities are very encouraging for the future. The next objectives are to improve the nanocavities and to characterise the decoherence phenomena and the mechanical coupling between the quantum dots and these nanostructures.
Single-photon sources (SPS) are essential for secure quantum communication and quantum computing. Solid-state systems based on quantum dots (QDs) are particularly attractive for emitting pure, on-demand single photons, making them ideal candidates for flying qubits in quantum key distribution protocols. Integrating a SPS into photonic circuits is a key step toward scalable and "plug-and-play" quantum technologies. However, the SPS with the best performances (III-V QDs) operate only at cryogenic temperatures, a significant drawback for practical implementation. Alternatively, CdSe QDs (II-VI) are operational at 300 K and emit in the blue-green range, thus allowing for free-space long-distance communication in seawater and air.The thesis aims to develop an on-chip single-photon source that can operate at non-cryogenic temperatures. The SPS consists of a CdSe QD embedded within a ZnSe nanowire (NW) shell grown by molecular beam epitaxy on a patterned GaAs (111)B substrate. The NW geometry enhances light extraction by acting as a single-mode waveguide, while the conical ending of the shell improves emission control.We investigated the optical performance of the as-grown QD-NW emitter at room temperature, achieving a promising brightness of 0.17 photons per pulse and antibunching behavior with g(2)(0) < 0.3 within a reduced spectral window. However, the emission spectrum at 300 K features overlapping broad lines, leading to a trade-off between brightness and purity. At 6 K, we used a broadband filter to capture entirely the narrow exciton line and measured directly the photon count rate with an avalanche photodiode. In addition, we observed strong antibunching with a few percent purity through cathodoluminescence.Phonons impact the performance of the SPS at high temperatures, where broad lines overlap, and at cryogenic temperatures, where they reduce the photon indistinguishability. Understanding and controlling the influence of phonons has increased the need for high-accuracy temperature measurements at cryogenic temperatures. We present a calibration-free method to extract the local temperature from the emission spectrum, which we used to investigate the heating effects induced by non-resonant excitation. Additionally, we analysed the line-broadening mechanisms from cryogenic to room temperature. An extension of the Huang-Rhys model is developed to analytically describe the acoustic exciton-phonon interactions, accounting for multi-phonon processes. The model accurately fits emission spectra from 6 K to 300 K using parameters obtained from low-temperature data.The tapered nanowire shape is also an advantage for integration into a photonic device, enabling efficient evanescent coupling when the emitter is positioned next to a waveguide. The direct transfer using micromanipulators in a scanning electron microscope onto pre-fabricated waveguides is very challenging due to the tiny size of the waveguide at an emission wavelength of 550 nm. We developed a new approach, where the emitter is deposited within a large zone and then embedded within a resist, which serves as a mask during waveguide etching. This method significantly reduces the need for micro-manipulation and allows for promising prospects. We used finite-difference time-domain simulations to support the design and fabrication of the waveguides for optimal coupling.
Semiconductor quantum dots (QDs) are commonly considered as artificial atoms and these nanostructures are very promising for the realization of integrated devices such as single photon sources for quantum cryptography applications [1,2]. However, contrarily to genuine atoms, QDs are condensed matter systems and they consequently suffer from the coupling to their environment that acts as a fluctuating reservoir for spectral diffusion [3] and degrades the indistinguishability of the emitted photons. One way to circumvent this coupling in order to reach the so-called fundamental radiative limit is to excite strictly resonantly the QD at low temperature [4-9]. Up to now, the measurements reported in the literature mainly 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. We present an original type of single photon source based on the coherent laser light scattering by a single QD [10]. 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 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 what we name an ultra-coherent character [10]. 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). [6] S. Ates et al., PRL 103, 167402 (2009). [2] C. Santori et al., Nature 419, 594 (2002). [7] H. S. Nguyen et al., PRL 108, 057401 (2012). [3] A. Berthelot et al., Nat. Phys. 2, 759 (2006). [8] C. Matthiesen et al., PRL 108, 093602 (2012). [4] A. Muller et al., PRL 99, 187402 (2007). [9] A. Reinhard et al., Nat. Phot. 6, 93 (2012). [5] R. Melet et al., PRB 78, 073301 (2008). [10] H. S. Nguyen et al., APL 99, 261904 (2011).
We present time integrated and time-resolved photoluminescence (PL) measurements on a single InAs∕GaAs quantum dot (QD), embedded in a planar microcavity, emitting in the 1300nm telecom band. The results of both measurements clearly identify the exciton and biexciton transitions from a single QD. By optimizing the extraction efficiency of the QD PL into the single mode fibers and carefully tuning two InGaAs avalanche photodiodes, we were able to measure the second order correlation function with integration times comparable to those made with silicon based technology. These measurements demonstrate that our single QDs are efficient sources of triggered single photons for quantum key distribution in the O band.
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]. 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 limited to the so-called 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 [2]. We present an original type of single photon source based on the coherent laser light scattering by a single QD [3]. Besides the antibunching effect showing the nonclassical nature of the emitted light, we observe that the QD emission spectrum is determined by the spectrum of the resonant excitation laser. Figure (a,b,c) shows 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 in figure (d,e,f). By reducing the resonant excitation power, the 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 [3]. References [1] P. Michler et al, Science 290, 2282 (2000). [2] H. S. Nguyen et al, Phys. Rev. Lett. (accepted, in press). See arXiv 1109.4304. [3] H. S. Nguyen et al, Appl. Phys. Lett. 99, 261904 (2011).