By metal-organic vapor-phase epitaxy, we have fabricated InAs quantum dots (QDs) on InGaAs/GaAs metamorphic buffer layers on a GaAs substrate with area densities that allow addressing single quantum dots. The photoluminescence emission from the quantum dots is shifted to the telecom C-band at 1.55 μm with a high yield due to the reduced stress in the quantum dots. The lowered residual strain at the surface of the metamorphic buffer layer results in a reduced lattice mismatch between the quantum dot material and growth surface. The quantum dots exhibit resolution-limited linewidths (mean value: 59 μeV) and low fine-structure splittings. Furthermore, we demonstrate single-photon emission (g(2)(0)=0.003) at 1.55 μm and decay times on the order of 1.4 ns comparable to InAs QDs directly deposited on GaAs substrates. Our results suggest that these quantum dots can not only compete with their counterparts deposited on InP substrates but also constitute an InAs/GaAs-only approach for the development of non-classical light sources in the telecom C-band.

We demonstrate the emission of polarization-entangled photons from a single semiconductor quantum dot in the telecom C-band (1530 nm–1565 nm). To reach this telecommunication window, the well-established material system of InAs quantum dots embedded in InGaAs barriers is utilized with an additional insertion of an InGaAs metamorphic buffer to spectrally shift the system to the desired wavelengths. For the observation of polarization-entangled photon pairs, the biexciton-exciton cascade of a quantum dot displaying an intrinsically low fine-structure splitting is investigated by means of polarization-dependent cross-correlation measurements. A complete set of tomography measurements enables us to reconstruct the two-photon density matrix and therefore to calculate a corresponding fidelity f+ to the maximally entangled Bell state Ψ+ of 0.61 ± 0.07, a concurrence of 0.74 ± 0.11, a tangle of 0.55 ± 0.14, and a negativity of 0.63 ± 0.12, clearly proving the entanglement of the states. Finally, the development of the concurrence is studied in dependency of the post-selected time-gate of the emission events and the progression of the time-delay dependent fidelity to distinct Bell states is displayed.

We present an on-chip beamsplitter operating on a single-photon level by means of a quasi-resonantly driven InGaAs/GaAs quantum dot. The single photons are guided by rib waveguides and split into two arms by an evanescent field coupler. Although the waveguides themselves support the fundamental TE and TM modes, the measured degree of polarization (∼90%) reveals the main excitation and propagation of the TE mode. We observe the preserved single-photon nature of a quasi-resonantly excited quantum dot by performing a cross-correlation measurement on the two output arms of the beamsplitter. Additionally, the same quantum dot is investigated under resonant excitation, where the same splitting ratio is observed. An autocorrelation measurement with an off-chip beamsplitter on a single output arm reveal the single-photon nature after evanescent coupling inside the on-chip splitter. Due to their robustness, adjustable splitting ratio, and their easy implementation, rib waveguide beamsplitters with embedded quantum dots provide a promising step towards fully integrated quantum circuits.

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In the present work, the effect of resonant pumping schemes in improving the photon coherence is investigated on InAs/InGaAs/GaAs quantum dots emitting in the telecom C-band. The linewidths of transitions of multiple exemplary quantum dots are determined under above-band pumping and resonance fluorescence via Fourier-transform spectroscopy and resonance scans, respectively. The average linewidth is reduced from 9.74 GHz in above-band excitation to 3.50 GHz in resonance fluorescence underlining its superior coherence properties. Furthermore, the feasibility of coherent state preparation with a fidelity of 49.2 % is demonstrated, constituting a step towards on-demand generation of coherent, single C-band photons from quantum dots. Finally, two-photon excitation of the biexciton is investigated as a resonant pumping scheme. A deconvoluted single-photon purity value of gHBT (0) = 0.072 ± 0.104 and a degree of indistinguishability of V HOM = 0.894 ± 0.109 are determined for the biexciton transition. This represents an important step towards fulfilling the prerequisites for quantum communication applications like quantum repeater schemes at telecom wavelength.Over the past two decades, semiconductor quantum dots (QDs) have received unceasing attention from researchers in the field of quantum optics due to their outstanding properties in terms of non-classical light emission 1-5 , i.e. bright singlephoton emission, entanglement fidelity, indistinguishability and the simultaneous combination of the aforementioned 4,5 . This designates them as promising candidates for applications like quantum computing and quantum communication 6 . The best performances are currently achieved with GaAs-based dots emitting in the near infrared 7 . However, especially regarding quantum communication schemes, an emission wavelength around 1550 nm (Telecom C-band) is much soughtafter both for satellite-based quantum communication due to an atmospheric transmission window and the possibility to perform it in broad daylight 8 , as well as for its fiber-based counterpart due to the global absorption minimum and low dispersion of standard glass fibers forming the existing global fiber network 9 . However, to extend the range of quantum communication applications such as quantum key distribution 10,11 , quantum relays 12,13 or quantum repeaters 14-17 are needed. The ideal light source for such applications combines bright single-photon and entangled-photon pair emission with a high degree of indistinguishability at 1550 nm.The emission of single and entangled photons in the telecom C-band has been demonstrated in two material systems, namely InAs/InP 18,19 and InAs/InGaAs/GaAs 20,21 . The last requirement, i.e. the indistinguishability of photons, is of major importance because it is necessary for two-photon interference (TPI), enabling linear-optic Bell state measurements and, therefore, entanglement swapping 22,23 in quantum repeater schemes. An experimental demonstration at this wavelength has been elusive in both material systems up to now.Ho...

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We demonstrate resonance fluorescence from single In-GaAs/GaAs quantum dots embedded in a rib waveguide beamsplitter structure operated under pulsed laser excitation. A systematic study on the excitation laser pulse duration depicts that a sufficiently small laser linewidth enables a substantial improved single-photon-to-laser-background ratio inside a waveguide chip. This manifests in the observation of clear Rabi oscillations over two periods of the quantum dot emission as a function of laser excitation power. A photon cross-correlation measurement between the two output arms of an on-chip beamsplitter results in a g(2)(0)=0.18, demonstrating the generation, guiding and splitting of triggered single photons under resonant excitation in an on-chip device. The present results open new perspectives for the implementation of photonic quantum circuits with integrated quantum dots as resonantly-pumped deterministic single-photon sources.

Self-potential anomalies caused by inclined sheets of infinite horizontal extension may be interpreted in terms of two infinite line poles situated on the body at positions nearest and furthest from the free surface, respectively. The direct interpretation of such pairs of line poles in terms of their positions and strength is possible from the observed anomaly profile. The solution is unique in its own family, i.e., no pair of line poles other than that arrived at by the present method can give rise to the observed anomaly. I-ODUCTIONThe spontaneous-polarisation method for prospecting for minerals, especially metallic sulphides, finds wide application as the field procedure involved is sufficiently simple.and also economic. But, compared with the other geophysical methods, the quantitative interpretation technique of SP data does not yet appear to have been adequately developed.The earliest published aids on the interpretation of SP data are the works of Petrowsky (1928) on polarised spheres as the source body. His method of interpretation utilizes the current densities, i.e., the space derivatives of the observed field, while de Witte (1948) developed a method which involves field SP values only. Yungul (1950) also considered the case of a polarised sphere and offered a method for removing topographic and regional effect.While all these authors included the possibility of inclined axis of polarisation, it still remains to be supplemented with some physicochemical explanation as to how such an axis can develop in a spherical ore body intercepted by a horizontal water table. This question, however, does not arise in the case of a spheroidal body when the theoretical analyses already developed do not, possibly, hold without further modification. Besides this, the aforesaid methods seem to have -very little applications in the cases where the ratio of the major diameters to minor diameters of the equianomaly contours are sufficiently large. In these cases, the causative body may be supposed to be two dimensional (i.e., one of its horizontal axes extending from -g to +x ), which necessitates the development of a separate interpretation technique for a two-dimensional case.The theoretical model for a two-dimensional situation will include two pairs of horizontal parallel infinite line poles, one in the neighbourhood of the upper end and the other in the neighbourhood of the lower end of the causative body. When the thickness of the ore body is so small that we can assume it to be a thin plate, then two of the line poles in each pair merge into a single fine.Edge and Laby (1931) worked out the expression for potential in this case without proceeding further for an interpretation method.To the knowledge of the present author, the only published works offering detailed quantitative interpretation methods are those by Roy and Chowdhury (1959) and Meiser (1962). The method proposed by Roy and Chowdhury is solely dependent on the assumed invariance of the ratio of minimum and maximum SP values with respect to D/b, where, f...