Voltage-controlled decoherence in single semiconductor quantum dots
Alice Berthelot, Guillaume Cassabois, Christophe Voisin, C. Delalande, R. Ferreira, Philippe Roussignol, J. Skiba-Szymanska, Roman S. Kolodka, A. I. Tartakovskii, M. Hopkinson, Maurice S. Skolnick
Solid-state single photon source is one of the most important applications of single quantum dot-based devices. However, the photo-induced spectral diffusion phenomenon broadens the quantum dot optical spectrum in photoluminescence spectroscopy. This effect originates from the fluctuating electric fields created by carriers in defects or traps in the quantum dot environment. Indeed, a single photon source with a life-time limited line still remains an experimental challenge for quantum cryptography applications. In a previous work, we had shown that the observation of a Lorentzian line broader than the natural linewidth in single quantum dot spectroscopy comes from a non-standard form of motional narrowing for spectral diffusion [1]. In fact the fluctuation dynamics of the quantum dot electrostatic environment is governed by the capture and escape times of carriers in traps or defects located around the quantum dot. Motional narrowing occurs at low temperature and low excitation power in contrast to Nuclear Magnetic Resonance (NMR), because the escape process is much less efficient than the capture one. \vspace{0.5cm} In this paper, we demonstrate the electrical control of the spectral diffusion dynamics for single quantum dots embedded in a Schottky diode structure. Our goal is to reach the standard phenomenology of motional narrowing described in NMR by hastening carrier escape from the traps around the QD through tunnelling under reverse bias voltage. We show that, under increasing reverse bias, the spectral diffusion dynamics changes for the neutral exciton line, and that the lineshape variations are governed by the modifications of the escape time. At 30K, we observe that the linewidth decreases on increasing the reverse bias, thus demonstrating a tunnel-assisted motional narrowing effect. At lower temperatures, we observe a more complex emission spectrum with a doublet structure attributed to an isolated strongly-perturbating trap. The coalescence of the doublet structure when varying gate voltage (or temperature) reveals a motional narrowing phenomenology specific to the Poissonian occupation law of this isolated trap. These data pave the way to a protection of zero-dimensional electronic states from outside coupling through a voltage-controlled motional narrowing effect. [1] A. Berthelot, I. Favero, G. Cassabois, C. Voisin, C. Delalande, Ph. Roussignol, R. Ferreira, and J. M. Gerard, Nature Phys. 2, 759 (2006).
Solid-state single photon source is one of the most important applications of single quantum dot-based devices. However, the photo-induced spectral diffusion phenomenon broadens the quantum dot optical spectrum in photoluminescence spectroscopy. This effect originates from the fluctuating electric fields created by carriers in defects or traps in the quantum dot environment. Indeed, a single photon source with a life-time limited line still remains an experimental challenge for quantum cryptography applications. In a previous work, we had shown that the observation of a Lorentzian line broader than the natural linewidth in single quantum dot spectroscopy comes from a non-standard form of motional narrowing for spectral diffusion [1]. In fact the fluctuation dynamics of the quantum dot electrostatic environment is governed by the capture and escape times of carriers in traps or defects located around the quantum dot. Motional narrowing occurs at low temperature and low excitation power in contrast to Nuclear Magnetic Resonance (NMR), because the escape process is much less efficient than the capture one. \vspace{0.5cm} In this paper, we demonstrate the electrical control of the spectral diffusion dynamics for single quantum dots embedded in a Schottky diode structure. Our goal is to reach the standard phenomenology of motional narrowing described in NMR by hastening carrier escape from the traps around the QD through tunnelling under reverse bias voltage. We show that, under increasing reverse bias, the spectral diffusion dynamics changes for the neutral exciton line, and that the lineshape variations are governed by the modifications of the escape time. At 30K, we observe that the linewidth decreases on increasing the reverse bias, thus demonstrating a tunnel-assisted motional narrowing effect. At lower temperatures, we observe a more complex emission spectrum with a doublet structure attributed to an isolated strongly-perturbating trap. The coalescence of the doublet structure when varying gate voltage (or temperature) reveals a motional narrowing phenomenology specific to the Poissonian occupation law of this isolated trap. These data pave the way to a protection of zero-dimensional electronic states from outside coupling through a voltage-controlled motional narrowing effect. [1] A. Berthelot, I. Favero, G. Cassabois, C. Voisin, C. Delalande, Ph. Roussignol, R. Ferreira, and J. M. Gerard, Nature Phys. 2, 759 (2006).
Single quantum dot optical spectroscopy has routinely been implemented in photoluminescence experiments for the last ten years. More recently photocurrent[1] and transmission [2] experiments have shown that the resonant excitation of a single quantum dot (QD) strongly decreases the broadening due to photo-induced spectral diffusion in photoluminescence spectra. However in the context of quantum cryptography applications, a single photon source with a life-time limited line still remains an experimental challenge under non-resonant excitation.In a previous work, we had shown that the observation of a Lorentzian line broader than the natural linewidth comes from a non-standard form of motional narrowing for spectral diffusion[3]. In fact the fluctuation dynamics of the quantum dot electrostatic environment is governed by the capture and escape times of carriers in traps or defects located around the QD.Motional narrowing occurs at low temperature and low excitation power in contrast to Nuclear Magetic Resonance (NMR), because the escape process is much less efficient than the capture one.In this paper, we demonstrate the electrical control of the spectral diffusion dynamics for single QDs embedded in a Schottky diode structure. Our goal is to reach the standard phenomenology of motional narrowing described in NMR by hastening carrier escape from the traps around the QD through tunneling under reverse bias voltage. We show for the neutral exciton line that, under increasing reverse bias, a Lorentzian to Gaussian lineshape transition takes place together with a linewidth increase as in Ref.[3]. Then, for higher bias voltages, the reverse transition from Gaussian to Lorentzian lineshapes occurs with a line-narrowing due to the shortening of the escape time.These data pave the way to a protection for zero-dimensional electronic states from outside coupling through a voltage-controlled motional narrowing effect. [1]A. Zrenner, E. Beham, S. Stufler, F. Findeis, M. Bichler and G. Abstreiter, Nature 418, 612-614 (2002). [2]A. Hogele, S. Seidl, M. Kroner, K. Karrai, R. J. Warburton, B. D. Gerardot and P. M. Petroff, Phys. Rev. Lett. 93, 217401 (2004). [3]A. Berthelot, I. Favero, G. Cassabois, C. Voisin, C. Delalande, Ph. Roussignol, R.Ferreira, and J. M. Gerard, Nature Phys. 2, 759 (2006).
In this paper, I will discuss the extrinsic dephasing mechanism of spectral diffusion that dominates the decoherence in semiconductor quantum dots at cryogenic temperature. I will first present the existence of two fluctuation regimes: the slow modulation one where the zero-phonon line in the quantum dot optical spectrum directly reflects the statistical distribution of the quantum dot emission energy, and the fast modulation one, also called motional narrowing regime, where the zero-phonon line is a Lorentzian with a width smaller than the standard deviation of the quantum dot energy. I will then emphasize the unconventional phenomenology of motional narrowing in standard semiconductor quantum dots at low incident power and temperature, that makes the quantum dot emission line a sensitive probe of the extrinsic reservoir fluctuation dynamics [1,2]. I will further show that the text book phenomenology of motional narrowing in nuclear magnetic resonance is recovered in quantum dots embedded in field-effect heterostructure. In that case, the electrical control of the mesoscopic environment of the quantum dot leads to conventional motional narrowing where the motion consists in carrier tunneling out of the defects around the quantum dot [3]. References [1] A. Berthelot, I. Favero, G. Cassabois, C. Voisin, C. Delalande, Ph. Roussignol, R. Ferreira, and J. M. Gérard, Nature Phys. 2, 759 (2006). [2] I. Favero, A. Berthelot, G. Cassabois, C. Voisin, C. Delalande, Ph. Roussignol, R. Ferreira, and J. M. Gérard, Phys. Rev. B 75, 073308 (2007). [3] A. Berthelot, G. Cassabois, C. Voisin, C. Delalande, R. Ferreira, Ph. Roussignol, J. Skiba-Szymanska, R. Kolodka, A. I. Tartakovskii, M. Hopkinson, and M. S. Skolnick, NewJ. Phys. 11, 093032 (2009).
Semiconductor quantum dots (QDs) are commonly considered as artificial atoms due to the 3D confinement of the electrons. 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 [3] which degrades the quality of the emitted photons in view of quantum cryptography applications. One way to circumvent this coupling is to excite strictly resonantly the QD at low temperature [4-10]. This difficult task has been achieved by designing an original excitation scheme to spatially decouple the excitation and detection paths. We first report that experimental studies of the resonant emission (RE) of single QDs can be strongly limited, even impossible, since most of the QDs show a very strong quenching of the RE of the fundamental transition. This issue is a broadly met experimental situation which is highly problematic for cavity quantum electrodynamics experiments in single QDs. We propose an efficient way to overcome this issue based on the use of an ultra-weak non-resonant laser that optically gates the QD RE [8] which controls the residual doping inherent to the QDs fabrication. It induces a complete recovery of the properties of an artificial atom, and the single QDs behave as quasi-ideal two-level systems. Up to now [7,9], the measurements on the QDs RE reported in the literature mainly focus on the incoherent fluorescence signal for achieving efficient single photon emission. In this regime, the spectrum linewidth is always limited to the radiative limit. We present experimental studies on the coherent laser light scattering by a single QD [7]. 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. 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 [7]. 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. References [1] P. Michler et al., Science 290 (2000). [6] S. Ates et al., PRL 103 (2009). [2] C. Santori et al., Nature 419 (2002). [7] H. S. Nguyen et al., APL 99 (2011). [3] A. Berthelot et al., Nat. Phys. 2 (2006). [8] H. S. Nguyen et al., PRL 108 (2012). [4] A. Muller et al., PRL 99 (2007). [9] C. Matthiesen et al., PRL 1082 (2012). [5] R. Melet et al., PRB 78 (2008). [10] A. Reinhard et al., Nat. Phot. 6 (2012).
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).
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 photoluminescence experiments in single QDs under non-resonant excitation [1], and more recently under strictly resonant excitation of the fundamental interband transition [2,3]. This configuration minimizes the residual excitation of the QD environment that acts as a fluctuating reservoir for spectral diffusion [4], and thus allows the generation of indistinguishable photons [3]. In this paper, we report on the resonant fluorescence of the charged exciton of a single InAs/GaAs QD embedded in a planar microcavity [2]. We show in figure 1(a) the photoluminescence spectrum of a single QD with two lines X and CX that are identified as the neutral and charged excitons by auto- and cross-correlation measurements of the g(2) function, in (b,c) and (d) respectively. Fig.2 shows the resonant Rayleigh scattering spectra as a function of the laser detuning with the neutral (a) and charged (b) exciton lines. For X, we fit the spectrum with two Lorentzian lines of width 10¹eV and separated by 12.5¹eV which corresponds to the fine structure splitting. For CX, we observe a line that demonstrates the existence of residual charge carriers even if the laser is tuned in resonance with the lowest excitation of the QD, and its larger width of 40¹eV suggests spectral diffusion dephasing. References [1] P. Michler,A. Kiraz,C. Becher, W. V. Schoenfeld, P. M. Petroff, L. Zhang, E. Hu, and A. Imamoglu, Science 290 2282 (2000). [2] A. Muller, E. B. Flagg, P. Bianucci, X. Y. Wang, D. G. Deppe, W. Ma, J. Zhang, G. J. Salamo, M. Xiao, and C. K. Shih, Phys. Rev. Lett. 99 187402 (2007). [3] S. Ates, S. M. Ulrich, S. Reitzenstein, A. Lffler, A. Forchel, and P. Michler, Phys. Rev. Lett. 103 167402 (2009). [4] A. Berthelot, I. Favero, G. Cassabois, C. Voisin, C. Delalande, Ph. Roussignol, R. Ferreira, and J. M. Grard, Nature Physics 2 759 (2006).
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 photoluminescence experiments in single QDs under non-resonant excitation [1]. In this configuration, the residual excitation of the QD environment that acts as a fluctuating reservoir for spectral diffusion [2] reduces the coherence time and prevents from the realization of single photons sources with high degree of indistinguishability. Therefore, strictly resonant excitation of the fundamental interband transition [3,4,5] is essential to reduce the dephasing processes and get high-performance single photon sources. In this paper, we report on the resonant Rayleigh scattering (RRS) of the exciton in single InAs/GaAs QDs embedded in a planar microcavity [3]. Strikingly, the most frequent case (98%) corresponds to the absence of resonance in the RRS spectrum, resulting in QDs non-coupled to light. We show that resonant fluorescence of the exciton is restored for all the QDs when an additional non-resonant laser is applied at extremely weak power ranges (few nWs). A clear photon anti-bunching under strictly resonant excitation proves that the optically gated RRS spectrum is dominated by resonant fluorescence of the exciton. A complete study will be presented such as the switching dynamics of the RRS and its power dependence with the resonant and non-resonant lasers. The phenomenon is interpreted within a simple model where the exciton resonant fluorescence is quenched by Coulomb blockade after tunneling of a hole in the QD from a structural deep level. The additional non-resonant laser suppresses Coulomb blockade by switching back the QD into a neutral state. We conclude that the optical response of the QD remains affected by its environment even in case of coherent resonant pumping. An optical control of the QD environment is essential to bring the QD from a "dark" to a "bright" state under strictly resonant excitation. References [1] P. Michler et al., Science 290, 2282 (2000) [2] A. Berthelot et al., Nature Physics 2, 759 (2006) [3] A. Muller et al., PRL 99, 187402 (2007) [4] R. Melet et al., PRB 78, 073301 (2008) [5] S. Ates et al., PRL 103, 167402 (2009)