In view of real world applications of quantum information technologies, the combination of miniature quantum resources with existing fibre networks is a crucial issue. Among such resources, on-chip entangled photon sources play a central role for applications spanning quantum communications, computing and metrology. Here, we use a semiconductor source of entangled photons operating at room temperature in conjunction with standard telecom components to demonstrate multi-user quantum key distribution, a core protocol for securing communications in quantum networks. The source consists of an AlGaAs chip emitting polarization entangled photon pairs over a large bandwidth in the main telecom band around 1550 nm without the use of any off-chip compensation or interferometric scheme; the photon pairs are directly launched into a dense wavelength division multiplexer (DWDM) and secret keys are distributed between several pairs of users communicating through different channels. We achieve a visibility measured after the DWDM of 87% and show long-distance key distribution using a 50-km standard telecom fibre link between two network users. These results illustrate a promising route to practical, resource-efficient implementations adapted to quantum network infrastructures.
Within the ambitious quest for an electrically pumped version of the optical parametric oscillator (OPO), we demonstrate
the first near-infrared integrated OPO in a direct gap semiconductor. This nonlinear device is based on a selectively
oxidized GaAs/AlAs heterostructure, the same “AlOx” technology that is at the heart of VCSEL fabrication. The
heterostructure and waveguide design allows for type-I form-birefringent phase matching, with a TM<sub>00</sub> pump around 1 μm and TE<sub>00</sub> signal and idler around 2 μm. Relying on the high non-resonant χ<sup>(2)</sup> of GaAs, relatively weak guided-wave optical losses, and monolithic SiO<sub>2</sub>/TiO<sub>2</sub> dichroic Bragg mirrors, we observe a threshold of 210 mW at degeneracy in the continuous-wave regime, with a single-pass-pump doubly resonant scheme. Further improvement can be achieved by adopting a double-pump-pass scheme and, in a more fundamental way, by further optimizing the waveguide optical
losses. The latter are induced by a not entirely mastered AlAs oxidation process and are of two distinct types: Rayleighlike
scattering at signal and idler wavelength (α ≤ 1cm<sup>-1</sup>), due to the interface roughness between GaAs and AlOx layers; and absorption at pump wavelengths (α ≈ 3cm<sup>-1</sup>), due to volume defects in the GaAs layers adjacent to the aluminum oxide. This result marks a milestone for integrated nonlinear photonics and represents a significant step toward the goal of a broadly tunable coherent light source on chip.
In recent years, great efforts have been devoted to the miniaturization of quantum information technology on semiconductor chips. In the context of photon pair sources, the bi-exciton cascade of a quantum dot and the four wave mixing in a Silicon waveguide have been used to demonstrate the generation of entangled states. Spontaneous parametric down-conversion in III-V semiconductor waveguides combines the advantages of room temperature and telecom wavelength operation, while keeping open the possibility of electrically pumping of the device. Here we present a source consisting of a multilayer AlGaAs waveguide grown on a GaAs substrate and then chemically etched to achieve lateral confinement in a ridge. The structure design is such that a pump beam (around 759 nm), impinging on the waveguide surface with an incidence angle <i>θ</i> generates two counterpropagating orthogonally polarized beams (around 1518 nm). The waveguide core is surrounded by distributed Bragg reectors to enhance the pump field within the device. We demonstrate the direct emission of polarization entangled photons by pumping the device at two symmetric angles of incidence corresponding to frequency degeneracy and performing a quantum tomography measurement. Most common entanglement witnesses are satisfied and a raw fidelity of <i>F</i> = 0:86 to a Bell state is obtained. These results open the route to the demonstration of other interesting features of our device such as the generation of hyper-entangled states via the control of the frequency correlation degree through the spatial and spectral pump beam profile, leading to a new generation of completely integrated devices for quantum information.
We report on the design, fabrication and optical investigation of AlGaAs microcavities for THz Difference Frequency
Generation (DFG) between Whispering Gallery Modes (WGMs), where the pump and DFG wavelengths (λ ≈ 1.3 μm and λ ≈ 75-150 μm, respectively) lie on opposite sides of the Restrahlen band. For the pump modes, we demonstrate CW lasing of quantum-dot layers under electrical injection at room temperature. We control the number of lasing WGMs via vertical notches on the pillars sidewalls, providing a selection mechanism for funneling the power only to the modes contributing to DFG. In parallel with the optimization of the pump lasers and in order to validate design and material parameters before the DFG experiments, we have performed linear measurements on two sets of passive samples. For the telecom range, the micropillars have been integrated with waveguides for distributed coupling and characterized via transmission measurements. In the THz range we have measured reflectivity spectra on 2D arrays of identical cylinders. In both cases, we demonstrate a good agreement between experimental results and simulations. On a more speculative note, we numerically show that etching a hole along the pillar axis can facilitate phase matching, while single-lobe farfield pattern can be obtained for the THz mode by micro-structuring the metallic ground plane around the microcavity. Finally, we suggest a real-time fine-tuning mechanism for the forthcoming active devices.
The miniaturization of quantum information technology is a subject attracting a growing attention. The exploitation of
spontaneous parametric down conversion in AlGaAs waveguides to generate photon pairs presents several advantages:
high nonlinear susceptibility, room-temperature operation and high emission directionality in the telecom range. In this
work we will present our recent results on three different kinds of AlGaAs devices: a selectively oxidized source based
on form birefringence, a waveguide based on modal phase matching and a microcavity-based source based on
counterpropagating phase matching. We will discuss and compare the figures of merit characterizing the three devices
for quantum communication applications.
We discuss the generation of THz radiation at room temperature by the exploitation of a nonlinear optical process taking
place in a high quality factor AlGaAs microcavity. The approach is grounded on 1) a novel quasi-phase-matching
scheme for parametric processes involving whispering-gallery modes circulating in nonlinear microcylinders; and 2)
recent advances concerning quantum dots microcylindrical lasers. After a brief summary of the theory used to describe
the nonlinear process, we present the results of our modeling in the case of a passive device pumped by two lasers at
wavelengths close to 1.3 μm. Finally, we conclude with preliminary measurements performed with a tapered fiber.
In the last few years considerable effort has been devoted to the miniaturization of quantum information technology on
semiconductor chips; in addition, recent developments in quantum information theory have roused a growing interest in
'generalized' states of frequency correlation. Parametric generation in semiconductor waveguides allows roomtemperature
operation in the telecom range. We propose and compare some microcavity-based schemes for the
generation of counterpropagating photon pairs and we experimentally demonstrate a bright source emitting 1.2 × 10<sup>-11</sup>pairs/pump photon for a 1.8 mm long waveguide. The indistiguishability of the photons of the pair is measured via a
Hong-Ou-Mandel two-photon interference experiment showing a visibility of 85 %. The versatility of the source to
control the generated two-photon state is also discussed.
We demonstrate an integrated semiconductor ridge microcavity source of counterpropagating twin photons at room
temperature in the telecom range. Based on type II parametric down conversion in a counterpropagating phase matching
scheme with transverse pump, the device generates around 10<sup>-11 </sup>pairs/pump photons having a 0.3 nm bandwidth for a 1
mm long waveguide. The emission spectrum shows the existence of two equally probable processes, which is a
preliminary step to the direct generation of Bell states. The twin character of the photons of each pair is demonstrated via
a temporal correlation measurement. These results open the way to the demonstration of several interesting features
associated to the counterpropagating geometry, such as the control of the frequency correlation degree via the spatial and
spectral properties of the pump beam.
Continuously tunable sources with room-temperature operation are required in the mid-infrared region for applications
such as spectroscopy or pollutants monitoring. In this spectral range, optical parametric oscillators (OPOs) are more
versatile than laser diodes.
Guided-wave OPOs constitute a promising perspective, thanks to higher conversion efficiency provided by the
confinement of the interacting waves. While LiNbO3 has been the crystal of choice for a long time, GaAs is a good
alternative thanks to higher nonlinearity, broader transparency range, and optoelectronic integrability. So far, a GaAs
integrated OPO has not yet been demonstrated due to technology induced propagation losses.
Here we present a detailed investigation of the propagation losses in partially oxidized multilayer GaAs/AlAs
waveguides. We have studied the impact of oxidation on the roughness of the multilayer interfaces, via transmission
electron microscopy. While the roughness of our MBE-grown GaAs/AlAs heterostructures is the standard 0.3 nm, it
increases to at least 0.53 nm after AlAs oxidation. Semi-analytical modeling shows that this level of roughness is
responsible for scattering losses, in fair agreement with the measured values. Optimization of the oxidation process is
currently under way with the aim of reaching the OPO oscillation threshold.
GaAs micro-nanodisks (typical disk size 5 μm × 200 nm in our work) are good candidates for boosting optomechanical
phenomena thanks to their ability to confine both optical and mechanical energy in a sub-micron interaction volume. We
present results of optomechanical characterization of GaAs disks by near-field optical coupling from a tapered silica
nano-waveguide. Whispering gallery modes with optical Q factor up to a few 10<sup>5</sup> are observed. Critical coupling, optical
resonance doublet splitting and mode identification are discussed. We eventually show an optomechanical phenomenon
of optical force attraction of the silica taper to the disk. This phenomenon shows that mechanical and optical degrees of
freedom naturally couple at the micro-nanoscale.
A semiconductor ridge microcavity is designed to generate counterpropagating twin photons by parametric fluorescence.
This device is suitable as a narrow bandwidth source of twin photons at 1.55 μm working at room temperature. A
sensible efficiency improvement due to the presence of the vertical cavity is demonstrated. The degree of frequency
correlation can be controlled through the pump field spatial and spectral profiles.
We demonstrate an integrated semiconductor source of counterpropagating twin photons in the telecom range. A pump
beam impinging on top of an AlGaAs waveguide generates parametrically two counterpropagating, orthogonally
polarized signal/idler guided modes. A 2 mm long waveguide emits at room temperature one average photon pair per
pump pulse, with a spectral linewidth of 0.15 nm. The twin nature of the emitted photons is tested through a time-correlation
A third-order-mode-emitting laser diode is demonstrated. The AlGaAs/GaAs hetero-structure is engineered to emit a photon pair through intra-cavity modal phase-matched parametric down-conversion. Device operations and twin photon generation experimental issues are discussed.