Development of semiconductor light sources for photonic-enabled quantum communication

Quantum information technologies have attracted tremendous attentions and development efforts by worldwide research organizations and governments in the past decades. It comprises the generation, manipulation, and transfer of quantum bits `qubits' based on the laws of quantum mechanics, enabling the applications of quantum metrology, quantum computation, quantum communication, etc. As one of the frontier quantum technologies, quantum communication features unconditionally secure data transfer between parties over long distance in theory, which can be accomplished through quantum state of light photons, due to their weak interaction with the environment and their remaining coherence over long distance. Meanwhile, quantum repeaters, similar as amplifier in classical communication are believed to be indispensable components to address the photon absorption and decoherence in noisy quantum channels, which scales exponentially with the distance. Quantum repeaters generally consist of three basic elements, namely entanglement swapping, entanglement purification, and quantum memories. In spite of significant breakthroughs achieved with a variety of optical protocols theoretically and experimentally, lack of near-perfect deterministic light sources with fast repetition rates, high degree of single photon purity, indistinguishability, and entanglement still impedes the practical applications. Semiconductor quantum dots are one of the leading system that have exhibited their potential for on-demand generation of high-quality single and entangled photon pairs for above applications. In this work, epitaxially grown III-V semiconductor quantum dots are investigated for driving their application in future quantum networks. First, an individual quantum dot emitting two pairs of entangled photons under pulsed two-photon resonant excitation has been utilized for realization of entanglement swapping, with the swapped photon pairs yielding a fidelity of 0.81 ± 0.04 to the Bell state Ψ+. To explore the practical limits of future quantum networks featuring multiple semiconductor based sources, we scrutinize the consequences of device fabrication, dynamic tuning techniques, time evolution of entanglement, and statistical effects on two separated quantum dot devices adapted in an entanglement swapping scheme. A numerical model based on the observed experimental data is proposed, serving not only as a benchmark for scalability of quantum dot devices, but also laying a roadmap for optimization of solid-state quantum emitters in quantum networks. For real-world quantum applications envisioned with quantum dots, the brightness of the quantum light sources is one of the key enabling factors, which is determined by the source excitation and extraction efficiency, as well as system detection system efficiency. Usually, the primary issue restricting the extraction of photons from III-V semiconductor quantum dots is the high-refractive index material of the host matrix which causes at the semiconductor-vacuum interface. To improve the photon extraction efficiency, a simple and efficient structure based on the principle of optical antennas is developed, resulting in an observed extraction of 17% of single photons in the telecom O-band, and a broadband enhancement of up to 180 times compared to the as-grown sample. A further limiting factor in the source efficiency is caused by the presence of charges in the solid-state environment. Charge fluctuation occur that quench radiative emission processes in resonant excitation schemes and induce fluorescence intermittence (blinking) that deteriorates the quantum yield. The photo-neutralization of GaAs/AlGaAs quantum dots excited by two-photon resonant pumping is investigated. Applying weak gate laser light to the quantum dot allows for controlling the charges capture processes. By adjusting the gate laser power and wavelength, an increase in excitation efficiency of 30% is observed compared to the two-photon resonant excitation without optical gating. The transition rates between the neutral and charged ground state are investigated by means of auto-/cross- correlation measurements. Furthermore, by studying a series of surface-passivated samples with different dot-to-surface distance as close to 20 nm, ODT was found to be an effective compound to neutralize the surface states, leading to reduced formation of non-radiative transition channels. It is anticipated that such a passivation method paves the way of near-field coupling related nano-photonic devices, or elimination of surface states for well-preserved emission properties towards the development of uncapped structure, fundamentally getting rid of total internal reflection to the maximum extent.European Research Council (ERC)/Starting Grant/QD-NOMS/E

On Number of Circles Intersected by a Line

AbstractConsider a set U of circles in the plane such that any line intersects at least one of those circles. For a given natural number m, is there a line in the plane intersecting at least m circles in U? In this paper this problem is solved. Our result is also generalized to compact convex subsets and to higher dimensional cases

A solid-state source of single and entangled photons at diamond SiV−^--center transitions operating at 80K

Large-scale quantum networks require the implementation of long-lived quantum memories as stationary nodes interacting with qubits of light. Epitaxially grown quantum dots hold great potential for the on-demand generation of single and entangled photons with high purity and indistinguishability. Coupling these emitters to memories with long coherence times enables the development of hybrid nanophotonic devices incorporating the advantages of both systems. Here we report the first GaAs/AlGaAs quantum dots grown by droplet etching and nanohole infilling method, emitting single photons with a narrow wavelength distribution (736.2 $\pm$ 1.7 nm) close to the zero-phonon line of Silicon-vacancy centers. Polarization entangled photons are generated via the biexciton-exciton cascade with a fidelity of (0.73 $\pm$ 0.09). High single photon purity is maintained from 4 K (g$^($$^2$$^)$(0) = 0.07 $\pm$ 0.02) up to 80 K (g$^($$^2$$^)$(0) = 0.11 $\pm$ 0.01), therefore making this hybrid system technologically attractive for real-world quantum photonic applications

Entanglement Swapping with Semiconductor-Generated Photons Violates Bell’s Inequality

Transferring entangled states between photon pairs is essential in quantum communication. Semiconductor quantum dots are the leading candidate for generating polarization-entangled photons deterministically. Here we show for the first time swapping of entangled states between two pairs of photons emitted by a single dot. A joint Bell measurement heralds the successful generation of the Bell state Ψ+, yielding a fidelity of 0.81±0.04 and violating the CHSH and Bell inequalities. Our photon source matches atomic quantum memory frequencies, facilitating implementation of hybrid quantum repeaters.BMBF/Q.comERC/QD-NOMSIFW Excellence Progra

Statistical limits for entanglement swapping with semiconductor entangled photon sources

Semiconductor quantum dots are promising building blocks for quantum communication applications. Al- though deterministic, efficient, and coherent emission of entangled photons has been realized, implementing a practical quantum repeater remains outstanding. Here we explore the statistical limits for entanglement swapping with sources of polarization-entangled photons from the commonly used biexciton-exciton cascade. We stress the necessity of tuning the exciton fine structure, and explain why the often observed time evolution of photonic entanglement in quantum dots is not applicable for large quantum networks. We identify the critical, statistically distributed device parameters for entanglement swapping based on two sources. A numerical model for benchmarking the consequences of device fabrication, dynamic tuning techniques, and statistical effects is developed, in order to bring the realization of semiconductor-based quantum networks one step closer to reality. ©2022 American Physical Societ

Quantum dot-based broadband optical antenna for efficient extraction of single photons in the telecom O-band

Long-distance fiber-based quantum communication relies on efficient non-classical light sources operating at telecommunication wavelengths. Semiconductor quantum dots are promising candidates for on-demand generation of single photons and entangled photon pairs for such applications. However, their brightness is strongly limited due to total internal reflection at the semiconductor/vacuum interface. Here we overcome this limitation using a dielectric antenna structure. The non-classical light source consists of a gallium phosphide solid immersion lens in combination with a quantum dot nanomembrane emitting single photons in the telecom O-band. With this device, the photon extraction is strongly increased in a broad spectral range. A brightness of 17% (numerical aperture of 0.6) is obtained experimentally, with a single photon purity of (2)(0)=0.049±0.02 at saturation power. This brings the practical implementation of quantum communication networks one step closer

Strain control of exciton and trion spin-valley dynamics in monolayer transition metal dichalcogenides

The electron-hole exchange interaction is a fundamental mechanism that drives valley depolarization via intervalley exciton hopping in semiconductor multi-valley systems. Here, we report polarization-resolved photoluminescence spectroscopy of neutral excitons and negatively charged trions in monolayer MoSe$_2$ and WSe$_2$ under biaxial strain. We observe a marked enhancement(reduction) on the WSe$_2$ triplet trion valley polarization with compressive(tensile) strain while the trion in MoSe$_2$ is unaffected. The origin of this effect is shown to be a strain dependent tuning of the electron-hole exchange interaction. A combined analysis of the strain dependent polarization degree using ab initio calculations and rate equations shows that strain affects intervalley scattering beyond what is expected from strain dependent bandgap modulations. The results evidence how strain can be used to tune valley physics in energetically degenerate multi-valley systems

Photoneutralization of charges in GaAs quantum dot based entangled photon emitters

Semiconductor-based emitters of pairwise photonic entanglement are a promising constituent of photonic quantum technologies. They are known for the ability to generate discrete photonic states on-demand with low multiphoton emission, near-unity entanglement fidelity, and high single photon indistinguishability. However, quantum dots typically suffer from luminescence blinking, lowering the efficiency of the source and hampering their scalable application in quantum networks. In this paper, we investigate and adjust the intermittence of the neutral exciton emission in a GaAs/AlGaAs quantum dot under two-photon resonant excitation of the neutral biexciton. We investigate the spectral and quantum optical response of the quantum dot emission to an additional wavelength tunable gate laser, revealing blinking caused by the intrinsic Coulomb blockade due to charge capture processes. Our finding demonstrates that the emission quenching can be actively suppressed by controlling the balance of free electrons and holes in the vicinity of the quantum dot and thereby significantly increasing the quantum efficiency by 30%. ©2022 American Physical Societ