Stable and efficient collection of single photons emitted from a semiconductor quantum dot into a single-mode optical fiber

Abstract

We study stable and efficient coupling of single photons generated from a quantum dot (QD) into a single-mode fiber (SMF) prepared in a fiber couple module (FCM). We propose a method to focus the objective lens to a sample surface without imaging with the help of laser reflection. By assembling all the constituents, i.e., a pair of lenses, the SMF, and the optical alignment to the QD source, we demonstrate stable single-photon count rate and a high collection efficiency of 43.5 % of the photons emitted in air from the QD into the collection lens in the FCM.The Internet of Things (IoT) 1) has been rapidly progressing recently, and everything will be involved in the network in near future. In such a society, secure and trustable communication is getting increasingly important, and cryptography works as a fundamental platform to provide a privacy preserving authentication and confidentiality to legitimate users. However the present cryptography relies on the computational difficulty, and we cannot completely eliminate the latent risk of the system being disabled by some emerging technologies. Quantum key distribution (QKD) 2) is expected to be a method to cope with such situations, since it can offer absolute security that cannot be breached, even with high-performance computers or algorithms. When information is encoded on single photons, the security is assured quantum mechanically on the condition that ideal single photon sources (SPSs) and detectors can be employed. Implementation of solid-state SPSs is one of the limited choices to realize deterministic SPSs and for future large-scale system integration, with the probability of generating higher number of photons at a time being nearly zero.From this perspective, semiconductor quantum dots (QDs) have been intensively studied and the SPSs 3, 4) working in O-band 5) and C-band 6) have been demonstrated. However, for a QD embedded in a bulk semiconductor, a large difference of the refractive indices at the semiconductor-air interface tends to prevent photon extraction to the air-side. When the critical angle for total internal reflection is θ CR , the photon extraction efficiency is given by *

Exciton Dynamics in Droplet Epitaxial Quantum Dots Grown on (311)A-Oriented Substrates

“…In other QDs we measured values between 30 and 140 eV (not shown). These values are very large if compared with QDs grown on (111) substrates, where the three-fold symmetry of the crystal provides a more isotropic surface diffusion and a corresponding triangular or hexagonal nanostructure shape [ 26 , 27 , 35 , 36 , 41 , 42 , 46 , 61 , 68 , 69 , 70 , 71 ]. The origin of this splitting in (311)A QDs is attributed to asymmetries in the QDs shape that is affected by the anisotropy of the underlying crystal.…”

“…Among the different categories, epitaxial quantum dots (QDs) stand as the best alternative for quantum devices thanks to their brightness, stability, and compatibility with photonic and electronic devices [ 7 , 8 , 10 , 11 , 12 ]. Within this class of QDs, droplet epitaxy (DE) [ 10 , 13 , 14 ] and droplet etching [ 9 , 15 , 16 ] (alternative growth protocols to Stranski–Krastanov for strain-free III–V-based semiconductor nanostructures), enabled the fabrication of state-of-the-art devices such as lasers [ 17 , 18 , 19 , 20 , 21 ] and quantum emitters, including single-photon sources [ 22 , 23 , 24 , 25 , 26 ] and entangled photons [ 9 , 27 , 28 , 29 , 30 ] with electrical injection [ 31 ]. The versatility of this method allowed to grow many different semiconductor alloys (GaInSb [ 32 ], AlGaAs [ 33 , 34 , 35 , 36 , 37 ], InGaAs [ 38 , 39 , 40 , 41 , 42 , 43 , 44 ], and InGaP [ 26 , 45 , 46 ]), forming a plethora of nanostructures [ 47 ] such as quantum dots ...…”

“…Another aspect relevant for applications is the possibility to grow high-quality nanostructures on different substrate orientation, providing the ground for highly symmetric QDs on (111)A surfaces [ 26 , 27 , 35 , 36 , 41 , 42 , 46 , 61 , 68 , 69 , 70 , 71 ] (e.g., for entangled photon pairs generation) and for ultra-high density quantum wires [ 60 ] and QDs [ 19 , 32 , 38 , 63 , 64 , 65 , 72 , 73 ] formation on the highly anisotropic (311)A surface (e.g., for laser emission). This latter class of nanostructures grown on (311)A surface has not yet been thoroughly investigated and a clear assessment of the corresponding excitonic dynamics has not yet been reported.…”

Single Quantum Dot with Microlens and 3D-Printed Micro-objective as Integrated Bright Single-Photon Source

“…The underlying mechanism can be explained by additional charge carriers, which are created by the nonresonant excitation and remain in wetting layer states or are captured by charge traps. 36 , 37 These states repopulate the QD after the initial emission event with characteristic time delay τ cap . 38 The subsequent decay with a time constant τ dec results in a secondary photon emission, which explains the correlation counts for small delay times.…”

Bright Single-Photon Source at 1.3 μm Based on InAs Bilayer Quantum Dot in Micropillar

“…Self-assembled individual quantum dots (QDs) are potential to emit real single photons and thus have attracted great interest [1–4]. The integration of a distributed Bragg reflector (DBR) cavity to a single QD will enhance its directional emission.…”