To show the feasibility of a long distance partial Bell-State measurement, a Hong-Ou-Mandel experiment with coherent photons is reported. Pairs of degenerate photons at telecom wavelength are created by parametric down conversion in a periodically poled lithium niobate waveguide. The photon pairs are separated in a beam-splitter and transmitted via two fibers of 25 km. The wavepackets are relatively delayed and recombined on a second beam-splitter, forming a large MachZehnder interferometer. Coincidence counts between the photons at the two output modes are registered. The main challenge consists in the trade-off between low count rates due to narrow filtering and length fluctuations of the 25 km long arms during the measurement. For balanced paths a Hong-Ou-Mandel dip with a net visibility of 47.3 % is observed, which is close to the maximal theoretical value of 50% developed here. This proves the practicability of a long distance Bell state measurement with two independent sources, as e.g. required in an entanglement swapping configuration in the scale of tens of km.

In the realization of ultrasmall semiconductor lasers, cavity-QED effects are used to enhance spontaneous emission and enable the lasing threshold to be crossed with gain contributions from only a few solid-state emitters. Operation in this regime fosters correlation effects that leave their fingerprint especially in the emission dynamics of nanolasers. Using time-resolved photon-correlation spectroscopy, we show that in a quantum-dot photonic-crystal nanolaser emitting in the telecom band, second-order coherence associated with lasing is established on a different timescale than the emission itself. By combining measurements with a microscopic semiconductor laser theory, we attribute the origin to carrier-photon correlations that give rise to non-Markovian effects in the emission dynamics that are not captured by laser rate-equation theories. Our results have direct implications with respect to the modulation response, repetition rate, noise characteristics, and coherence properties of nanolasers for device applications. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Quantum propagation of indistinguishable photons in coupled waveguide lattices is analyzed in the asymptotic limit. Such an approach reveals the existence of two main relevant families of modes supported by the photonic lattice: localized side modes and Floquet-Bloch eigenmodes delocalized over the array. Under two-photon injection, the modal framework unravels the photon bunching and antibunching effects occurring in coupled waveguide lattices. In particular, photon correlations in the Floquet-Bloch modes are exhibited. Engineering of the propagation in such delocalized eigenmodes through patterning of the array coupling constant may open the way to discrete photonics platforms implementing efficient modal manipulation of quantum light involving large photon-number states.

The turn-on delay jitter in pulsed lasers in which a large fraction (β) of spontaneous emission is channeled into the lasing mode is measured by use of a photon correlation technique. This jitter is found to significantly increase with β, reaching values of the order of the pulse width at threshold. This is due to the increase in the relative value of the discretization noise when the number of photons at threshold becomes small, as is the case in high-β lasers.

For the past fifteen years, single semiconductor quantum dots, often referred to assolid-state artificial atoms, have been at the forefront of various research directionlines for experimental quantum information science, in particular in the development ofpractical sources of quantum states of light. Here we review the research to date, on thetailoring of the emission properties from single quantum dots producing single photons,indistinguishable single photons and entangled photon pairs. Finally, the progress andfuture prospects for applications of single dots in quantum information processing isconsidered.

We present a novel experimental technique that can differentiate unequivocally between chaotic light and coherent light with amplitude fluctuations, and thus permits us to characterize unambiguously the output of a laser. This technique consists of measuring the second-order intensity cross correlation at the outputs of an unbalanced Michelson interferometer. It is applied to a chaotic light source and to the output of a semiconductor nanolaser whose "standard" intensity correlation function above threshold displays values compatible with a mixture of coherent and chaotic light. Our experimental results demonstrate that the output of such lasers is not partially chaotic but is indeed a coherent state with amplitude fluctuations.

We quantitatively explore the ability of coupled waveguide arrays to characterize and manipulate two-photon and NOON states. We emphasize in particular the potential of patterned arrays in which the coupling is structured.

We report a fully fibered source emitting cross time-bin-entangled photons at 1540 nm from type-II spontaneous parametric down-conversion. Compared to standard time-bin-entanglement realizations, the preparation interferometer requires no phase stabilization, simplifying its implementation in quantum key distribution experiments. Bell-type tests of such a cross time-bin state are performed in the Franson configuration and lead to two-photon interference raw visibilities greater than 95$%$, which are only limited by the dark counts in the detectors and imperfections in the analysis system. Just by trusting the randomness of the beam splitters, the correlations generated by the source can be proved of nonclassical origin even in a passive implementation. The obtained results confirm the suitability of this source for time-bin-based quantum key distribution.

Interferometric photon-correlation measurements, which correspond to the second-order intensity cross-correlations between the two output ports of an unbalanced Michelson interferometer, are sensitive to both amplitude and phase fluctuations of an incoming beam of light. Here, we present the theoretical framework behind these measurements and show that they can be used to unambiguously differentiate a coherent wave undergoing dynamical amplitude and phase fluctuations from a chaotic state of light. This technique may thus be used to characterize the output of nanolasers and monitor the onset of coherent emission.

A plasmonic microcavity providing broadband control of spontaneous emission for large and sparse semiconductor quantum dots emitting at telecommunications wavelengths is proposed. By designing and fabricating such a cavity, we demonstrate a broadband Purcell effect with spontaneous emission enhancement over a broad spectral range of Δλ≃20 nm with a 3.9-fold maximum enhancement, as well as inhibition over Δλ≃100 nm around 1.3 μm. The broadband feature relaxes the constraint on spectral matching between the dot emission and the cavity mode, favourable for implementing efficient non-classical light sources or nanoscale lasers.

We elaborate InP1-xAsx quantum dots embedded in InP nanowires by Au-catalyzed molecular beam epitaxy. Each nanowire contains a quantum dot well positioned on its axis and presenting a cylindrical geometry with sharp interfaces. The quantum dot emission wavelength can be tuned in the optical fiber transmission range and the final shape of the nanowire is tailored for efficient wave-guiding and light extraction. These objects, obtained in a single growth run, open a new route to the fabrication of efficient single photon sources. (c) 2013 Elsevier B.V. All rights reserved.

Lasers of diffraction-limited volumes involve the interaction of small numbers of particles (photons and dipoles). We demonstrate that these small populations of discrete particles induce large intensity noise in the output of the laser.

(a) AFM image from a sample with (211)B InAs QDs on the surface. (b) Characteristic µ-PL spectrum from a single PZ QD having the biexciton (XX) state at higher energy compared to the exciton (X) state. Temperature dependent study of carrier dynamics in piezoelectric quantum dots.Evidence for non-radiative recombination.Solution for improved photoluminescence efficiency at high temperatures proposed. The carrier dynamics in self-assembled (211)B InAs quantum dots grown by Stranski-Krastanow method has been investigated by time-resolved and temperature-dependent photoluminescence experiments. The radiative recombination times are found to be larger than 1.7ns and remain practically constant with temperature in line with the zero degree of dimensionality of the system. Above 100K, the decay time is dominated by non-radiative channels. The main activation mechanism for non-radiative recombination is associated with carrier escape from the quantum dots to the wetting layer.

(a) AFM image from a sample with (211)B InAs QDs on the surface. (b) Characteristic µ-PL spectrum from a single PZ QD having the biexciton (XX) state at higher energy compared to the exciton (X) state. Temperature dependent study of carrier dynamics in piezoelectric quantum dots.Evidence for non-radiative recombination.Solution for improved photoluminescence efficiency at high temperatures proposed. The carrier dynamics in self-assembled (211)B InAs quantum dots grown by Stranski-Krastanow method has been investigated by time-resolved and temperature-dependent photoluminescence experiments. The radiative recombination times are found to be larger than 1.7ns and remain practically constant with temperature in line with the zero degree of dimensionality of the system. Above 100K, the decay time is dominated by non-radiative channels. The main activation mechanism for non-radiative recombination is associated with carrier escape from the quantum dots to the wetting layer.

Nonlinear dynamical systems involving small populations of individuals may sustain oscillations in the population densities arising from discrete changes in population numbers due to random events. By applying these ideas to nanolasers operating with small numbers of emitting dipoles and photons at threshold, we show that such lasers should display photon and dipole population cycles above threshold, which should be observable as a periodic modulation in the second-order correlation function of the nanolaser output. Such a modulation was recently reported in a single-mode vertical-cavity surface-emitting semiconductor laser.

The topography and the electronic structure of InAsP/InP quantum dots are probed by cross-sectional scanning tunneling microscopy and spectroscopy. The study of the local density of states in such large quantum dots confirms the discrete nature of the electronic levels whose wave functions are measured by differential conductivity mapping. Because of their large dimensions, the energy separation between the discrete electronic levels is low, allowing for quantization in both the lateral and growth directions as well as the observation of the harmonicity of the dot lateral potential.

Polarization-resolved single-dot spectroscopy reveals that the exciton fine-structure splitting in piezoelectric (211)B InAs/GaAs quantum dots is smaller than 10 mu eV in the vast majority of examined dots. These values are significantly reduced compared to as-grown (100)-oriented InAs dots. Time-resolved measurements confirm the high oscillator strength of these dots, in spite of the internal piezoelectric field, suggesting good quantum efficiency at 4 K, comparable with that of (100) InAs/GaAs dots. Lastly, photon correlation measurements demonstrate single-photon emission from exciton levels of these dots. All these features make this intriguing dot system promising for implementing solid-state entangled photon sources.

We report the realization of a fiber-coupled time-bin entangled photon-pair source at 1538 nm. Unlike previous time-bin sources based on type-0 spontaneous parametric down conversion, our approach takes advantage of cross-polarized paired photons generated in a type-II periodically poled lithium niobate waveguide (PPLN/W). We use a fibered birefingent delay line (BDL) in a Michelson configuration to introduce a controllable delay between the paired photons. After the BDL, these photons are separated by a polarizing beam splitter (PBS) at 45° and sent to Alice and Bob. This way we create the non common time-bin entangled Bell state |Ψ+⟩ = 1/√2 [|s,l⟩ − |l,s⟩], where s and l represent the short and long time-bins, respectively.

Polarization-resolved single dot spectroscopy performed on (211)B InAs/GaAs quantum dots reveals that the fine structure splitting of the excitonic levels in these dots is much lower compared to the usual (100)-grown InAs dots. Time-resolved measurements confirm the high oscillator strength of these dots, and thus their good quantum efficiency at 4 K, comparable with that of (100) InAs/GaAs dots. Last, photon correlation measurements demonstrate single photon emission out of the excitonic optical transition of these dots. All these features make this novel dot system very promising for implementing solid-state entangled photon sources.