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.
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.
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 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.
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 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)
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.
Single-photon sources are at the heart of quantum-optical networks, with their uniquely quantum emission and phenomenon of two-photon interference allowing for the generation and transfer of nonclassical states. Although a few analytical methods have been briefly investigated for describing pulsed single-photon sources, these methods apply only to either perfectly ideal or at least extremely idealized sources. Here, we present the first complete picture of pulsed single-photon sources by elaborating how to numerically and fully characterize non-ideal single-photon sources operating in a pulsed regime. In order to achieve this result, we make the connection between quantum Monte-Carlo simulations, experimental characterizations, and an extended form of the quantum regression theorem. We elaborate on how an ideal pulsed single-photon source is connected to its photocount distribution and its measured degree of second- and first-order optical coherence. By doing so, we provide a description of the relationship between instantaneous source correlations and the typical experimental interferometers (Hanbury-Brown and Twiss, Hong–Ou–Mandel, and Mach–Zehnder) used to characterize such sources. Then, we use these techniques to explore several prototypical quantum systems and their non-ideal behaviors. As an example numerical result, we show that for the most popular single-photon source—a resonantly excited two-level system—its error probability is directly related to its excitation pulse length. We believe that the intuition gained from these representative systems and characters can be used to interpret future results with more complicated source Hamiltonians and behaviors. Finally, we have thoroughly documented our simulation methods with contributions to the Quantum Optics Toolbox in Python in order to make our work easily accessible to other scientists and engineers.