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Single photon sources operating on-demand at telecom wavelengths are required in fiber-based quantum secure communication technologies. In this work we demonstrate single photon emission from position-controlled nanowire quantum dots emitting at {\lambda} > 1530 nm. Using above-band pulsed excitation, we obtain single photon purities of g(2)(0) = 0.062. These results represent an important step towards the scalable manufacture of high efficiency, high rate single photon emitters in the telecom C-band.

Single photon emitters are a key component for enabling the practical use of quantum key distribution protocols for secure communications. For long-haul optical networks it is imperative to use photons at wavelengths that are compatible with standard single mode fibers: 1.31 {\mu}m and 1.55 {\mu}m. We demonstrate high purity single photon emission at 1.31 {\mu}m using deterministically positioned InP photonic waveguide nanowires containing single InAsP quantum dot-in-a-rod structures. At 4 K the detected count rate in fiber was 1.9 Mcps under above-band pulsed laser excitation at 80 MHz corresponding to a single photon collection efficiency at the first lens of 25%. At this count rate, the probability of multiphoton emission is g(2)(0) = 0.021. We have also evaluated the performance of the source as a function of temperature. Multiphoton emission probability increases with temperature with values of 0.11, 0.34 and 0.57 at 77 K, 220 K and 300 K, respectively, which is attributed to an overlap of temperature-broadened excitonic emission lines. These results are a promising step towards scalably fabricating telecom single photon emitters that operate under relaxed cooling requirements.

We demonstrate that, even when employing above-band excitation, photons emitted from semiconductor quantum dots can have linewidths that approach their transform-limited values. This is accomplished by using quantum dots embedded in bottom-up photonic nanowires, an approach which mitigates several potential mechanisms that can result in linewidth broadening: (i) only a single quantum dot is present in each device, (ii) dot nucleation proceeds without the formation of a wetting layer, and (iii) the sidewalls of the photonic nanowire are comprised not of etched facets, but of epitaxially grown crystal planes. Using these structures we achieve linewidths of 2x the transform limit, unprecedented for above-band excitation. We also demonstrate a highly nonlinear dependence of the linewidth on both excitation power and temperature which can be described by an independent Boson model that considers both deformation and piezoelectric exciton-phonon coupling. We find that for sufficiently low excitation powers and temperatures, the observed excess broadening is not dominated by phonon dephasing, a surprising result considering the high phonon occupation that occurs with above-band excitation.

We report on a platform for the production of single photon devices with a fabrication yield of 100%. The sources are based on InAsP quantum dots embedded within position-controlled bottom-up InP nanowires. Using optimized growth conditions, we produce large arrays of structures having highly uniform geometries. Collection efficiencies are as high as 83% and multiphoton emission probabilities as low as 0.6% with the distribution away from optimal values associated with the excitation of other charge complexes and re-excitation processes, respectively, inherent to the above-band excitation employed. Importantly, emission peak lineshapes have Lorentzian profiles indicating that linewidths are not limited by inhomogeneous broadening but rather pure dephasing, likely elastic carrier-phonon scattering due to a high phonon occupation. This work establishes nanowire-based devices as a viable route for the scalable fabrication of efficient single photon sources and provides a valuable resource for hybrid on-chip platforms currently being developed., Comment: 9 pages, 5 figures

We demonstrate a method to monolithically integrate nanowire-based quantum dot single photon sources on-chip using evanescent coupling. By deterministically placing an appropriately tapered III-V nanowire waveguide, containing a single quantum dot, on top of a silicon-based ridge waveguide, the quantum dot emission can be transferred to the ridge waveguide with calculated efficiencies close to 100%. As the evanescent coupling is bidirectional, the source can be optically pumped in both free-space and through the ridge waveguide. The latter onfiguration provides a self-contained, all-fiber, single photon source suitable as a plug-and-play solution for applications requiring bright, on-demand single photons. Using InAsP quantum dots embedded in InP nanowire waveguides, we demonstrate coupling efficiencies to a SiN ridge waveguide of 74% with a single photon purity of 97%.

We present a quantum optics theory, numerical calculations, and experiments on coupled quantumdots in semiconductor nanowire waveguides. We first present an analytical Green function theory tocompute the emitted spectra of two coupled quantum dots, treated as point dipoles, fully accountingfor retardation effects, and demonstrate the signatures of coherent and incoherent coupling througha pronounced splitting of the uncoupled quantum dot resonances and modified spectral broadening.In the weak excitation regime, the classical Green functions used in models are verified and justifiedthrough full 3D solutions of Maxwell equations for nanowire waveguides, specifically using finite-difference time-domain techniques, showing how both waveguide modes and near-field evanescentmode coupling is important. The theory exploits an ensemble-based quantum description, and andan intuitive eigenmode-expansion based Maxwell theory. We then demonstrate how the molecularresonances (in the presence of coupling) take on the form of bright and dark (or quasi-dark) reso-nances, and study how these depend on the excitation and detection conditions. To go beyond theweak excitation regime, we also introduce a quantum master equation approach to model the non-linear spectra from an increasing incoherent pump field, which shows the role of the pump field onthe oscillator strengths and broadening of the molecular resonances, with and without pure dephas-ing. Next, we present experimental photoluminescence spectra for spatially-separated quantum dotmolecules (InAsP) in InP nanowires, which show clear signatures of pronounced splittings, thoughthey also highlight additional mechanisms that are not accounted for in the dipole-dipole couplingmodel. Two different approaches are taken to control the spatial separation of the quantum dotmolecules, and we discuss the advantages and disadvantages of each., Comment: 20 pages, 16 figures

We demonstrate an in-situ technique to tune the emission energy of semiconductor quantum dots. The technique is based on laser-induced atomic intermixing applied to nanowire quantum dots grown using a site-selective process that allows for the deterministic tuning of individual emitters. A tuning range of up to 15 meV is obtained with a precision limited by the laser exposure time. A distinct saturation of the energy shift is observed, which suggests an intermixing mechanism relying on grown-in defects that are subsequently removed from the semiconductor material during annealing. The ability to tune different emitters into resonance with each other will be required for fabricating remote quantum dot-based sources of indistinguishable photons for secure quantum networks.

We report on the site-selected growth of bright single InAsP quantum dots embedded within InP photonic nanowire waveguides emitting at telecom wavelengths. We demonstrate a dramatic dependence of the emission rate on both the emission wavelength and the nanowire diameter. With an appropriately designed waveguide, tailored to the emission wavelength of the dot, an increase in count rate by nearly two orders of magnitude (0.4kcps to 35kcps) is obtained for quantum dots emitting in the telecom O-band. Using emission-wavelength-optimised waveguides, we demonstrate bright, narrow linewidth emission from single InAsP quantum dots with an unprecedented tuning range from 880nm to 1550nm. These results pave the way towards efficient single photon sources at telecom wavelengths using deterministically grown InAsP/InP nanowire quantum dots.