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In the present work, we investigate the coupling of deterministically pre-selected In(Ga)As/GaAs quantum dots (QDs) to low Q circular Bragg grating cavities by employing a combination of state-of-the-art low-temperature in-situ optical lithography and electron-beam lithography. The spatial overlap between the cavity mode and quantum emitter is ensured through the accurate determination of the QD position via precise interferometric position readout. Simultaneously, the high precision of the electron-beam lithography is exploited for the cavity fabrication. In order to optimize the spectral overlap, prior to cavity fabrication, finite-difference time-domain simulations are performed to estimate the spectral position of the cavity mode. A Purcell factor of 2 together with an increased count rate is reported for a deterministically positioned cavity where the emission line is detuned by 3.9 nm with respect to the cavity mode. This non-negligible Purcell enhancement for large detunings and, thus, the large range where this can be achieved points towards the possibility of using the cavity for the simultaneous enhancement of spectrally distinct transitions from the same quantum emitter located spatially in the mode maximum. Furthermore, investigations on the bending of the cavity membrane and the effects on the cavity mode and QD emission are presented. [ABSTRACT FROM AUTHOR]

The GaAs-based material system is well-known for hosting InAs quantum dots (QDs) with outstanding optical properties, typically emitting at a wavelength of around 900 nm. The insertion of a metamorphic buffer (MMB) can shift this emission to the technologically attractive telecom C-band range centered at 1550 nm. However, the thickness of common MMB designs (>1 μm) limits their compatibility with most photonic resonator types. Here, we report on the metal–organic vapor-phase epitaxy (MOVPE) growth of a novel InGaAs MMB with a nonlinear indium content grading profile designed to maximize plastic relaxation within minimal layer thickness. This allows us to achieve the necessary transition of the lattice constant and to provide a smooth surface for QD growth within 180 nm. Single-photon emission at 1550 nm from InAs QDs deposited on top of this thin-film MMB is demonstrated. The strength of the new design is proven by integrating it into a bullseye cavity via nano-structuring techniques. The presented advances in the epitaxial growth of QD/MMB structures form the basis for the fabrication of high-quality telecom nonclassical light sources as a key component of photonic quantum technologies.

The quasi-one-dimensional molecular conductors (TMTTF)2X commonly undergo a charge-order transition upon cooling. For tetrahedral anions (X=BF4-, ClO4-, and ReO4-), in addition, anion ordering is observed. In a comprehensive infrared study, we trace the resonance frequencies of the charge-sensitive vibrational modes to determine the charge disproportionation 2δ=ρrich-ρpoor as a function of temperature. In combination with analog investigations for the centrosymmetric anions, we conclude that charge order does not depend on the anion shape and symmetry, but is intrinsic to the TMTTF stack. Since correlation effects are of paramount importance, we find a universal relation between the charge imbalance 2δ and ordering temperatures TCO and gaps ΔCO, respectively. Below TAO the charge disproportionation gradually changes. Ordering of the tetrahedral anions minimizes the electrostatic energy for the anions and organic molecules as a whole, resulting in a 0110 pattern. This way we can understand that deuteration of the methyl end groups enhances TCO but does not affect the anion ordering, which is linked to the anion-sulfur distance. [ABSTRACT FROM AUTHOR]

Long-range, terrestrial quantum networks require high-brightness single-photon sources emitting in the telecom C-band for maximum transmission rates. For solid-state quantum emitters, the underlying pumping process, i.e., coherent or incoherent excitation schemes, impacts several photon properties such as photon indistinguishability, single-photon purity, and photon number coherence. These properties play a major role in quantum communication applications, the latter in particular for quantum cryptography. Here, we present a versatile telecom C-band single-photon source that is operated coherently and incoherently using two complementary pumping schemes. The source is based on a quantum dot coupled to a circular Bragg grating cavity, whereas coherent (incoherent) operation is performed via the novel SUPER scheme (phonon-assisted excitation). In this way, high end-to-end-efficiencies (η end ) of 5.36% (6.09%) are achieved simultaneously with a small multiphoton contribution g (2) (0) of 0.076 ± 0.001 [ g (2) (0) of 0.069 ± 0.001] for coherent (incoherent) operation.

Quantum key distribution (QKD) enables the transmission of information that is secure against general attacks by eavesdroppers. The use of on-demand quantum light sources in QKD protocols is expected to help improve security and maximum tolerable loss. Semiconductor quantum dots (QDs) are a promising building block for quantum communication applications because of the deterministic emission of single photons with high brightness and low multiphoton contribution. Here we report on the first intercity QKD experiment using a bright deterministic single photon source. A BB84 protocol based on polarisation encoding is realised using the high-rate single photons in the telecommunication C-band emitted from a semiconductor QD embedded in a circular Bragg grating structure. Utilising the 79 km long link with 25.49 dB loss (equivalent to 130 km for the direct-connected optical fibre) between the German cities of Hannover and Braunschweig, a record-high secret key bits per pulse of 4.8 × 10 -5 with an average quantum bit error ratio of ~ 0.65% are demonstrated. An asymptotic maximum tolerable loss of 28.11 dB is found, corresponding to a length of 144 km of standard telecommunication fibre. Deterministic semiconductor sources therefore challenge state-of-the-art QKD protocols and have the potential to excel in measurement device independent protocols and quantum repeater applications., (© 2024. The Author(s).)

The combination of semiconductor quantum dots with photonic cavities is a promising way to realize nonclassical light sources with state-of-the-art performances regarding brightness, indistinguishability, and repetition rate. Here we demonstrate the coupling of InGaAs/GaAs QDs emitting in the telecom O-band to a circular Bragg grating cavity. We demonstrate a broadband geometric extraction efficiency enhancement by investigating two emission lines under above-band excitation, inside and detuned from the cavity mode, respectively. In the first case, a Purcell enhancement of 4 is attained. For the latter case, an end-to-end brightness of 1.4% with a brightness at the first lens of 23% is achieved. Using p-shell pumping, a combination of high count rate with pure single-photon emission (g (2) (0) = 0.01 in saturation) is achieved. Finally, a good single-photon purity (g (2) (0) = 0.13) together with a high detector count rate of 191 kcps is demonstrated for a temperature of up to 77 K.