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Graphical representation of the two-photon polarization density matrix describing the 2X and 1X collected photons under 20 µW excitation power.
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Polarization correlation in a linear basis, but not entanglement, is observed between the biexciton and single-exciton photons emitted by a single InAs quantum dot in a two-photon cascade. The results are well described quantitatively by a probabilistic model that includes two decay paths for a biexciton through a non-degenerate pair of one-exciton...
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Citations
... Various types of single-photon emitters have been proposed and fabricated, and their performances have been reported. [1][2][3][4][5][6] Many of them are basically a combination of quantum dots (QDs) as light-emitting sources and resonant structures to control the light-emitting properties. Typical resonant structures are photonic crystals and metamaterials. ...
We propose a polarization-controlled ultrasmall single-photon emitter that combines a single photon source and an elliptical split ring (SR) type metamaterial element. Simulations using the finite difference time domain method showed that in elliptical metamaterial elements, the annular mode is suppressed and axial electric field oscillations occur preferentially, resulting in upward light emission from the photon source with controlled polarization. We fabricated prototype devices by focused ion beam lithography and PbS QDs. Evaluation of the optical properties of the devices revealed that the emission spectrum width was narrower and the emission lifetime was shorter than those of QD ensemble, and that the polarization was controlled along the minor axis of ellipse. These suggested that the QD emission and the metamaterial element resonated as expected. The combination of a single QD and an elliptical SR-type metamaterial element was shown to operate as a polarization-controlled single-photon emitter.
... Semiconductor quantum dots are used to generate single photons [14][15][16][17], correlated photon pairs [18], cluster states [9,19,20], and entangled photon pairs [21][22][23]. Recently, coherent driving of a two-level system quantum dot (a charged exciton under zero magnetic field) has allowed to demonstrate the coherent superposition [24][25][26] and time entanglement [12,13] encoded in the photon-number basis, a basis that has been scarcely exploited due to the difficulty to manipulate photonic states containing vacuum [13,27,28]. ...
We propose a general scheme to generate entanglement encoded in the photon-number basis, via a sequential resonant two-photon excitation of a three-level system. We apply it to the specific case of a quantum dot three-level system, which can emit a photon pair through a biexciton–exciton cascade. The state generated in our scheme constitutes a tool for secure communication, as the multipartite correlations present in the produced state may provide an enhanced rate of secret communication with respect to a perfect GHZ state.
... Semiconductor quantum dots are used to generate single photons [13][14][15][16], correlated photon pairs [17], cluster states [9,18,19] and entangled photon pairs [20][21][22]. Recently, the coherent driving of a two-level system quantum dot (a charged exciton under zero magnetic field) has allowed to demonstrate the coherent superposition [23] and entanglement [12] in the photon-number basis, an encoding basis that has been scarcely exploited due to the difficulty to manipulate photonic states containing vacuum [24][25][26]. ...
We propose a general scheme to generate entanglement encoded in the photon number basis, via a sequential resonant two-photon excitation of a three-level system. We apply it to the specific case of a quantum dot three-level system, which can emit a photon pair through a biexciton-exciton cascade. The state generated in our scheme constitutes a tool for secure communication, as the multipartite correlations present in the produced state may provide an enhanced rate of secret communication with respect to a perfect GHZ state.
... Note that the FSS between exciton levels is typically of the order of several tens of μeV 57 and only causes minor quantitative changes to the results; however, the entangled polarized photon pairs have a lower degree of entanglement. 58 [60][61][62][63][64][65] to resolve this issue, so we ignore it in this work and assume that the two states are degenerate. ...
Motivated by recent advances in the development of single photon emitters for quantum information sciences, here we design and formulate a quantum cascade model that describes cascade emission by a quantum dot (QD) in a cavity structure while preserving entanglement that stores information needed for single photon emission. The theoretical approach is based on a photonic structure that consists of two orthogonal cavities in which resonance with either the first or second of the two emitted photons is possible, leading to amplification and rerouting of the entangled light. The cavity–QD scheme uses a four-level cascade emitter that involves three levels for each polarization, leading to two spatially entangled photons for each polarization. By solving the Schrodinger equation, we identify the characteristic properties of the system, which can be used in conjunction with optimization techniques to achieve the “best” design relative to a set of prioritized criteria or constraints in our optical system. The theoretical investigations include an analysis of emission spectra in addition to the joint spectral density profile, and the results demonstrate the ability of the cavities to act as frequency filters for the photons that make up the entanglements and to modify entanglement properties. The results provide new opportunities for the experimental design and engineering of on-demand single photon sources.
... This greatly reduces timing jitter in the preparation of the exciton state caused by the biexciton population lifetime, and consequently, reestablishes the high indistinguishability for photons emitted from the exciton to ground state decay. The scheme allows us to deterministically program the polarization of the emitted photon (H or V) via the polarization of the stimulation pulse [28,29]. Moreover, this allows us to obtain all emission of interest to occur in the polarization of the detection channel enabling higher brightness than crosspolarized resonant excitation. ...
We propose a scheme for the generation of highly indistinguishable single photons using semiconductor quantum dots and demonstrate its performance and potential. The scheme is based on the resonant two-photon excitation of the biexciton followed by stimulation of the biexciton to selectively prepare an exciton. Quantum-optical simulations and experiments are in good agreement and show that the scheme provides significant advantages over previously demonstrated excitation methods. The two-photon excitation of the biexciton suppresses re-excitation and enables ultralow multiphoton errors, while the precisely timed stimulation pulse results in very low timing jitter of the photons, and consequently, high indistinguishability. In addition, the polarization of the stimulation pulse allows us to deterministically program the polarization of the emitted photon (H or V). This ensures that all emission of interest occurs in the polarization of the detection channel, resulting in higher brightness than cross-polarized resonant excitation.
... The first demonstrations of polarization correlations from a biexciton cascade were published soon after the initial proposal [192][193][194]. However, the presence of large excitonic FSS prohibited the observation of quantum entanglement. ...
Quantum dots are a key building block for quantum technologies as they are able to generate single photons or entangled photon pairs. The deterministic fabrication of sources based on quantum dots in cavities paves the way towards scalability: we show for the first time that it is possible to obtain homogeneous properties, by studying in detail 15 of our sources. Each characteristic of the quantum dots’ emission is measured with a specific setup. Among them, the indistinguishability is accessible through the visibility of the Hong-Ou-Mandel interference. We study how this latter quantity is affected by the presence of additional photons, depending on the nature of that noise. We thus derive a formula to deduce the true single-photon indistinguishability from the measured visibility and apply it to our sources. Finally, the properties of the emission from quantum dots is deeply related to their charge state and symmetry. We propose a new way to control the exciton’s symmetry by applying three voltages via a specific structure. This experimental study opens the way to the reproduciblegeneration of entangled photon pairs with a high brightness as well as frequency-encoded qubits.
... [cond-mat.mes-hall] 7 Jul 2021 pulse [28]. Experiments and simulations exploring the system dynamics are in very good agreement and confirm the validity of the theoretical model. ...
We propose a scheme for the generation of highly indistinguishable single photons using semiconductor quantum dots and demonstrate its performance and potential. The scheme is based on the resonant two-photon excitation of the biexciton followed by stimulation of the biexciton to selectively prepare an exciton. Quantum-optical simulations and experiments are in good agreement and show that the scheme provides significant advantages over previously demonstrated excitation methods. The two-photon excitation of the biexciton suppresses re-excitation and enables ultra-low multi-photon errors, while the precisely timed stimulation pulse results in very low timing jitter of the photons, and consequently, high indistinguishability. Since both control laser fields are detuned from the emission energy, the scheme does not require polarization filtering, facilitating high brightness approaching unity. Moreover, the polarization of the emitted single photons is controlled by the stimulation laser field, such that the polarization of the quantum light is deterministically programmable.
... In Fig. 1(d) the cross-correlation between the X 0 S and X − S + XX 0 S emission lines is shown. As expected from the XX 0 S → X 0 S cascade a strong bunching is observed, while the X 0 S → XX 0 S process is anti-bunched [28]. Due to the nature of the cascade for each emission of a XX 0 S photon the X 0 S state is inherently coherently prepared. ...
We present a first comprehensive study on deterministic spin preparation employing excited state resonances of droplet etched GaAs quantum dots. This achievement facilitates future investigations of spin qubit based quantum memories using the GaAs quantum dot material platform. By observation of excitation spectra for a range of fundamental excitonic transitions the properties of different quantum dot energy levels, i.e. shells, are revealed. The innovative use of polarization resolved excitation and detection in quasi-resonant excitation spectroscopy facilitates determination of $85$ $\%$ maximum spin preparation fidelity - irrespective of the relative orientations of lab and quantum dot polarization eigenbases. Additionally, the characteristic non-radiative decay time is investigated as a function of ground state, excitation resonance and excitation power level, yielding decay times as low as $29$ ps for s-p-shell exited state transitions. Finally, by time resolved correlation spectroscopy it is demonstrated that the employed excitation scheme has a significant impact on the electronic environment of quantum dot transitions thereby influencing its charge and coherence.
... By doing this, the formation of a wetting layer is avoided, only a 'quasi-wetting layer' exists where the droplets lattice match to the substrate. With no strain-driven growth, it is possible to reach much higher QD symmetries, demonstrated by [89,90], which is crucial for entangled light emission [91,92]. ...
Semiconductor quantum dot (QD) quantum light sources have long been established as suitable candidates for many quantum information applications, due to the on-demand emission of highly pure and highly indistinguishable single and entangled photons. A key factor in the development of this technology is the operation over the standard telecommunication optical fibre network infrastructure, where the minimum absorption wavelength window is centred on the telecom C-band (1530 – 1565 nm). Initial experiments in this work demonstrated single-photon emission of a QD light source emitting directly in the telecom C-band, under both continuous wave (CW) and 1-GHz pulsed excitation regimes. The QDs were further characterised in terms of fine-structure splitting (FSS) and coherence time, in order to determine their suitability for quantum entanglement and interference-based applications. Long coherence times were observed in the majority of the QDs considered, allowing the demonstration of Hong-Ou-Mandel-type two-photon interference of subsequently emitted photons under CW excitation. The post-selected interference visibility was found to be limited by only the detector resolution and single-photon purity. A further demonstration of high-visibility interference under the same limitations was then made using QD photons and dissimilar photons from a laser, forming the basis of a fibre-based quantum relay. Working further towards a quantum relay, polarisation-entangled photon pairs in the telecom C-band were then generated using the radiative cascade of the biexciton, where a record high fidelity to the ©+ Bell state was observed under both CW and 1-GHz pulsed excitation regimes. While an anomalous effect of the FSS was observed in a majority of the studied QDs, a further characterisation of the FSS in terms of the QD polarisation eigenstates confirmed the emission of entangled photon pairs from such an anomalous-splitting QD. Finally, the work of this thesis was combined to demonstrate a proof-of-principle quantum relay using a QD light source in the telecom C-band. The relay was operated first under CW excitation where polarisation encoded laser input qubits were used and high-fidelity quantum teleportation was observed. In an effort to demonstrate a more technologically relevant application, the quantum relay was subsequently operated at 1 GHz in order to demonstrate the teleportation of initially time-bin encoded laser input qubits. A high mean teleportation fidelity was again observed, demonstrating the potential of this telecom C-band QD quantum light source in the future of long-distance quantum information applications.
... This feature will lead to the introduction of a which-path information in the XX À X cascade that will degrade the entanglement between the two emitted photons. For this reason, the early measurements on entanglement in QDs [36,37] only led to detection of classical correlations; nonclassical correlations were only observed by improving the growth techniques and choosing QDs with FSS ≈ 0 [38,39]. In the recent past, several techniques have been proposed and demonstrated in order to erase the FSS of QDs using electric fields [40][41][42], strain [43], and an optical approach not requiring nanofabrication [44]. ...
The generation of on-demand, optimally entangled photon pairs remains one of the most formidable challenges in the quantum optics and quantum information community. Despite the fact that recent developments in this area have opened new doors leading toward the realization of sources exhibiting either high brightness or near-unity entanglement fidelity, the challenges to achieve both together persist. Here, we will provide a historical review on the development of quantum dots (QDs) for entangled photon generation, with a focus on nanowire QDs, and address the latest research performed on nanowire QDs, including measuring entanglement fidelity, light-extraction efficiency, dephasing mechanisms, and the detrimental effects of detection systems on the measured values of entanglement fidelity. Additionally, we will discuss results recently observed pertaining to resonant excitation of a nanowire QD, revealing the potential of such sources to outperform spontaneous parametric down-conversion (SPDC) sources, providing a viable solution to the current challenges in quantum optics and quantum information.
