On-demand generation of background-free single photons from a solid-state source
Lucas Schweickert, Klaus D. Jöns, Katharina D. Zeuner, Saimon Filipe Covre da Silva, Huiying Huang, Thomas Lettner, Marcus Reindl, Julien Zichi, Rinaldo Trotta, Armando Rastelli, Val Zwiller
True on-demand high-repetition-rate single-photon sources are highly sought after for quantum information processing applications. However, any coherently driven two-level quantum system suffers from a finite re-excitation probability under pulsed excitation, causing undesirable multi-photon emission. Here, we present a solid-state source of on-demand single photons yielding a raw second-order coherence of g(2)(0)=(7.5±1.6)×10−5 without any background subtraction or data processing. To this date, this is the lowest value of g(2)(0) reported for any single-photon source even compared to the previously reported best background subtracted values. We achieve this result on GaAs/AlGaAs quantum dots embedded in a low-Q planar cavity by employing (i) a two-photon excitation process and (ii) a filtering and detection setup featuring two superconducting single-photon detectors with ultralow dark-count rates of (0.0056±0.0007) s−1 and (0.017±0.001) s−1, respectively. Re-excitation processes are dramatically suppressed by (i), while (ii) removes false coincidences resulting in a negligibly low noise floor.
True on-demand high-repetition-rate single-photon sources are highly sought after for quantum information processing applications. However, any coherently driven two-level quantum system suffers from a finite re-excitation probability under pulsed excitation, causing undesirable multi-photon emission. Here, we present a solid-state source of on-demand single photons yielding a raw second-order coherence of g(2)(0)=(7.5±1.6)×10−5 without any background subtraction or data processing. To this date, this is the lowest value of g(2)(0) reported for any single-photon source even compared to the previously reported best background subtracted values. We achieve this result on GaAs/AlGaAs quantum dots embedded in a low-Q planar cavity by employing (i) a two-photon excitation process and (ii) a filtering and detection setup featuring two superconducting single-photon detectors with ultralow dark-count rates of (0.0056±0.0007) s−1 and (0.017±0.001) s−1, respectively. Re-excitation processes are dramatically suppressed by (i), while (ii) removes false coincidences resulting in a negligibly low noise floor.
Summary form only given. On-demand single-photon sources that produce a single photon as a response to a deterministic input are widely regarded as critical light sources for quantum key distribution, quantum repeaters, and quantum information science generally [1]. The wavepackets from such a source must have low probability to contain more than one photon, exist in a pure state so they may interfere with one another, and have a spatio-temporal profile that is appropriate for their application [2]. A very popular source for generating high quality single-photon wavepackets is the quantum two-level system, an artificial atom with one ground and one excited state, often implemented using solid-state quantum dots embedded within microcavities [3]. The two-level system is excited with a short resonant pulse of width τFWHM that prepares the system in its excited state, called a π-pulse, where it subsequently decays and emits a single photon with almost unity probability. Because the typical pulse lengths are orders of magnitude shorter than the lifetime of the system, it is often not appreciated that there is finite chance for an ideal two-level system to emit more than one photon at a time.
Abstract High-quality sources of single photons are of paramount importance for quantum communication, sensing, and metrology. To these ends, resonantly excited two-level systems based on self-assembled quantum dots have recently generated widespread interest. Nevertheless, we have recently shown that for resonantly excited two-level systems, emission of a photon during the presence of the excitation laser pulse and subsequent re-excitation results in a degradation of the obtainable single-photon purity. Here, we demonstrate that generating single photons from self-assembled quantum dots with a scheme based on two-photon excitation of the biexciton strongly suppresses the re-excitation. Specifically, the pulse-length dependence of the multi-photon error rate reveals a quadratic dependence in contrast to the linear dependence of resonantly excited two-level systems, improving the obtainable multi-photon error rate by several orders of magnitude for short pulses. We support our experiments with a new theoretical framework and simulation methodology to understand few-photon sources.
High-quality sources of single photons are of paramount importance for quantum communication, sensing and metrology. To these ends, resonantly excited two-level systems based on self-assembled quantum dots have recently generated widespread interest. Nevertheless, we have recently shown that for resonantly excited two-level systems, emission of a photon during the presence of the excitation laser pulse and subsequent re-excitation results in a degradation of the obtainable single-photon purity. Here, we demonstrate that generating single photons from self-assembled quantum dots with a scheme based on two-photon excitation of the biexciton strongly suppresses the re-excitation. Specifically, the pulse-length dependence of the multi-photon error rate reveals a quadratic dependence in contrast to the linear dependence of resonantly excited two-level systems, improving the obtainable multi-photon error rate by several orders of magnitude for short pulses. We support our experiments with a new theoretical framework and simulation methodology to understand few-photon sources.
Due to their excellent optical properties, quantum dots are promising for applications in photonic quantum technologies. For on-demand single-photon generation, a two-level system given by an excitonic transition is typically excited with a resonant laser pulse of area π. This prepares the two-level system in its excited state from where it spontaneously emits a single photon. However, emission that occurs already during the presence of the laser pulse allows for re-excitation and, thus, multi-photon emission which limits the single-photon purity [1]. In contrast, when exciting the system with a pulse of area 2π, the system is expected to be returned to the ground state. However, in this case emission during the presence of the pulse is most likely to occur when the system is in its excited state – exactly after an area of π has been absorbed. This restarts the Rabi oscillation with a pulse area of π remaining in the pulse which leads to re-excitation with near-unity probability and the emission of a second photon within the excited state lifetime [2,3]. Finally, we present the generation of single photons with ultra-low multi-photon probability [4]. Using two-photon excitation of the bi-exciton suppresses re-excitation and improves the single photon purity by several orders of magnitude for short pulses. [1] K. A. Fischer, et al., New J. Phys. 18, 113053 (2016) [2] K. A. Fischer, et al., Nature Physics 13, 649-654 (2017) [3] K. A. Fischer, et al., Quantum Sci. Technol. 3, 014006 (2017) [4] L. Hanschke et al., arxiv: 1801.01672 (2018)
High-quality sources of single photons are of paramount importance for quantum communication, sensing and metrology. To these ends, resonantly excited two-level systems have recently generated widespread interest. Nevertheless, for resonantly excited two-level systems, emission of a photon during the presence of the excitation laser pulse and subsequent re-excitation results in a degradation of the obtainable single-photon purity [1]. Here, we investigate a two-photon excitation scheme based on a three-level system formed by the bi-exciton - exciton cascade in a self-assembled quantum dot and demonstrate that it improves the multi-photon error rate by several orders of magnitude [2]. We support our experiments with a new theoretical framework and simulation methodology to understand few-photon sources. For a resonantly excited two-level system the multi-photon error rate scales linear with the pulse length [3]. In contrast, the two-photon excitation scheme exhibits a quadratic dependence, improving the obtainable multi-photon error rate by several orders of magnitude for short pulses. Moreover, the scheme is easy to implement and facilitates fast repetition rates in contrast to schemes involving three-level lambda-type systems that require re-pumping. Unlike resonant excitation of a two-level system, this scheme does not require the measurement technique of cross-polarized suppression to reject the excitation laser and, thus, enables a higher source brightness. Finally, the scheme is directly compatible with increasing the emission rate by Purcell enhancement. [1] K.A. Fischer et al. Nature Physics 13, 649-654 (2017) [2] L. Hanschke, et al. arXiv:1801.01672 (2018) [3] K.A. Fischer, et al. Quantum Science and Technology 3, 1 (2017)
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).