Quantum Dot Single Photon Sources Research Articles

Violation of Bell's inequality with quantum-dot single-photon sources

We investigate the possibility of realizing a loophole-free violation of Bell's inequality using deterministic single-photon sources. We provide a detailed analysis of a scheme to achieve such violations over long distances with immediate extensions to device-independent quantum key distribution. We investigate the effect of key experimental imperfections that are unavoidable in real-world single-photon sources including the finite degree of photon indistinguishability, single-photon purity, and the overall source efficiency. We benchmark the performance requirements to state-of-the-art deterministic single-photon sources based on quantum dots in photonic nanostructures and find that experimental realizations appear to be within reach. We also evaluate the requirements for a post-selected version of the protocol, which relaxes the demanding requirements with respect to the source efficiency.

Numerical optimization of single-mode fiber-coupled single-photon sources based on semiconductor quantum dots.

We perform extended numerical studies to maximize the overall photon coupling efficiency of fiber-coupled quantum dot single-photon sources emitting in the near-infrared and O-band and C-band. Using the finite element method, we optimize the photon extraction and fiber-coupling efficiency of quantum dot single-photon sources based on micromesas, microlenses, circular Bragg grating cavities and micropillars. The numerical simulations which consider the entire system consisting of the quantum dot source itself, the coupling lens, and the single-mode fiber, yield overall photon coupling efficiencies of up to 83%. Our work provides objectified comparability of different fiber-coupled single-photon sources and proposes optimized geometries for the realization of practical and highly efficient quantum dot single-photon sources.

Unity yield of deterministically positioned quantum dot single photon sources

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.

Tailoring solid-state single-photon sources with stimulated emissions.

The coherent interaction of electromagnetic fields with solid-state two-level systems can yield deterministic quantum light sources for photonic quantum technologies. To date, the performance of semiconductor single-photon sources based on three-level systems is limited mainly due to a lack of high photon indistinguishability. Here we tailor the cavity-enhanced spontaneous emission from a ladder-type three-level system in a single epitaxial quantum dot through stimulated emission. After populating the biexciton (XX) of the quantum dot through two-photon resonant excitation, we use another laser pulse to selectively depopulate the XX state into an exciton (X) state with a predefined polarization. The stimulated XX-X emission modifies the X decay dynamics and improves the characteristics of a polarized single-photon source, such as a source brightness of 0.030(2), a single-photon purity of 0.998(1) and an indistinguishability of 0.926(4). Our method can be readily applied to existing quantum dot single-photon sources and expands the capabilities of three-level systems for advanced quantum photonic functionalities.

In-plane resonant excitation of quantum dots in a dual-mode photonic-crystal waveguide with high β-factor

A high-quality quantum dot (QD) single-photon source is a key resource for quantum information processing. Exciting a QD emitter resonantly can greatly suppress decoherence processes and lead to highly indistinguishable single-photon generation. It has, however, remained a challenge to implement strict resonant excitation in a stable and scalable way, without compromising any of the key specs of the source (efficiency, purity, and indistinguishability). In this work, we propose a novel dual-mode photonic-crystal waveguide that realizes direct in-plane resonant excitation of the embedded QDs. The device relies on a two-mode waveguide design, which allows exploiting one mode for excitation of the QD and the other mode for collecting the emitted single photons with high efficiency. By proper engineering of the photonic bandstructure, we propose a design with single-photon collection efficiency of β > 0.95 together with a single-photon impurity of ϵ < 5 × 10−3 over a broad spectral and spatial range. The device has a compact footprint of and would enable stable and scalable excitation of multiple emitters for multi-photon quantum applications.

A quantum key distribution testbed using a plug&amp;play telecom-wavelength single-photon source

Deterministic solid state quantum light sources are considered key building blocks for future communication networks. While several proof-of-principle experiments of quantum communication using such sources have been realized, most of them required large setups—often involving liquid helium infrastructure or bulky closed-cycle cryotechnology. In this work, we report on the first quantum key distribution (QKD) testbed using a compact benchtop quantum dot single-photon source operating at telecom wavelengths. The plug&amp;play device emits single-photon pulses at O-band wavelengths (1321 nm) and is based on a directly fiber-pigtailed deterministically fabricated quantum dot device integrated into a compact Stirling cryocooler. The Stirling is housed in a 19 in. rack module including all accessories required for stand-alone operation. Implemented in a simple QKD testbed emulating the BB84 protocol with polarization coding, we achieve an multiphoton suppression of g(2)(0)=0.10±0.01 and a raw key rate of up to (4.72 ± 0.13) kHz using an external pump laser. In this setting, we further evaluate the performance of our source in terms of the quantum bit error ratios, secure key rates, and tolerable losses expected in full implementations of QKD while accounting for finite key size effects. Furthermore, we investigate the optimal settings for a two-dimensional temporal acceptance window applied on the receiver side, resulting in predicted tolerable losses up to 23.19 dB. Not least, we compare our results with previous proof-of-concept QKD experiments using quantum dot single-photon sources. Our study represents an important step forward in the development of fiber-based quantum-secured communication networks exploiting sub-Poissonian quantum light sources.

Quantum-dot single-photon sources for the quantum internet.

Quantum-dot single-photon sources for the quantum internet.

Unidirectional output from a quantum-dot single-photon source hybrid integrated on silicon

We report a quantum-dot single-photon source (QD SPS) hybrid integrated on a silicon waveguide embedding a photonic crystal mirror, which reflects photons and enables efficient unidirectional output from the waveguide. The silicon waveguide is constituted of a subwavelength grating so as to maintain the high efficiency even under the presence of stacking misalignment accompanied by hybrid integration processes. Experimentally, we assembled the hybrid photonic structure by transfer printing and demonstrated single-photon generation from a QD and its unidirectional output from the waveguide. These results point out a promising approach toward scalable integration of SPSs on silicon quantum photonics platforms.

A broad-band planar-microcavity quantum-dot single-photon source with a solid immersion lens

The integration of single quantum dots (QDs) into a planar Fabry–Pérot microcavity has been established as a direct and viable approach to vertically steer photons emitted from the quantum emitters, resulting in a strong increase in the source brightness, which becomes particularly evident when a lens with a low numerical aperture is used. However, the spectral bandwidth of QD–microcavity structures is limited and determined by their intrinsic quality factor; these structures are, thus, not ideal for the extraction of entangled photon pairs or for studies of exciton dynamics. We have found that, when a deterministic low-index solid immersion lens is placed on top of the QD in a QD–microcavity structure, the structure exhibits an enhancement in the bandwidth to 27 nm and a source brightness of 23%. The solid immersion lens is deterministically fabricated via two-photon absorption and can be remade several times without perturbing the QD, ensuring that the QD's intrinsic properties are preserved and ensuring its long-term reliability.

Design optimization for bright electrically-driven quantum dot single-photon sources emitting in telecom O-band.

A combination of advanced light engineering concepts enables a substantial improvement in photon extraction efficiency of micro-cavity-based single-photon sources in the telecom O-band at ∼1.3 µm. We employ a broadband bottom distributed Bragg reflector (DBR) and a top DBR formed in a dielectric micropillar with an additional circular Bragg grating in the lateral plane. This device design includes a doped layer in pin-configuration to allow for electric carrier injection. It provides broadband (∼8-10 nm) emission enhancement with an overall photon-extraction efficiency of ∼83% into the upper hemisphere and photon-extraction efficiency of ∼79% within numerical aperture NA=0.7. The efficiency of photon coupling to a single-mode fiber reaches 11% for SMF28 fiber (with NA=0.12), exceeds 22% for 980HP fiber (with NA=0.2) and reaches ∼40% for HNA fiber (with NA=0.42) as demonstrated by 3D finite-difference time-domain modeling.

Hong-Ou-Mandel Interference with Imperfect Single Photon Sources.

Hong-Ou-Mandel interference is a cornerstone of optical quantum technologies. We explore both theoretically and experimentally how unwanted multiphoton components of single-photon sources affect the interference visibility, and find that the overlap between the single photons and the noise photons significantly impacts the interference. We apply our approach to quantum dot single-photon sources to access the mean wave packet overlap of the single-photon component. This study provides a consistent platform with which to diagnose the limitations of current single-photon sources on the route towards the ideal device.

Optical fiber coupling of quantum dot single photon sources

Semiconductor quantum dot (QD) at low temperature will create excitons with sharp spectral lines for single photon emission. Optical fiber coupling avoids scanning for positioning and vibration influence in low-temperature confocal setup, and is a key technology in realizing the plug-play and componentization of QD single photon sources. For the fiber coupling techniques, the lateral coupling of a photonic crystal cavity or waveguide with a tapered fiber, or normal coupling of a QD chip with a tapered facet fiber in a large numerical aperture has been developed based on mask in a micro-region; however, the above techniques require multi-dimensional precise adjusting in order to avoid abnormally bending a soft fiber to realize alignment and high-efficiency coupling. Ceramic ferrule or silica V-shaped groove-mounted fiber has a large smooth facet and no bending; it can collect light in the normal direction by being aligned with bonding QD chip; V-shaped groove-mounted fiber array also enables a random adhesion and avoid scanning for alignment, which is simple in technique. This work is based on the previous realization of single photon output by random adhesion of few-pair DBR micropillar chip with V-shaped groove-mounted fiber array, and uses many-pair DBR cavity chip with theoretical simulation optimization to improve the normal light extraction and its fiber collection efficiency, and greatly improves the fiber output of single photon count rate.

Deterministically fabricated strain-tunable quantum dot single-photon sources emitting in the telecom O-band

Most quantum communication schemes aim at the long-distance transmission of quantum information. In the quantum repeater concept, the transmission line is subdivided into shorter links interconnected by entanglement distribution via Bell-state measurements to overcome inherent channel losses. This concept requires on-demand single-photon sources with a high degree of multi-photon suppression and high indistinguishability within each repeater node. For a successful operation of the repeater, a spectral matching of remote quantum light sources is essential. We present a spectrally tunable single-photon source emitting in the telecom O-band with the potential to function as a building block of a quantum communication network based on optical fibers. A thin membrane of GaAs embedding InGaAs quantum dots (QDs) is attached onto a piezoelectric actuator via gold thermocompression bonding. Here, the thin gold layer acts simultaneously as an electrical contact, strain transmission medium, and broadband backside mirror for the QD-micromesa. The nanofabrication of the QD-micromesa is based on in situ electron-beam lithography, which makes it possible to integrate pre-selected single QDs deterministically into the center of monolithic micromesa structures. The QD pre-selection is based on distinct single-QD properties, signal intensity, and emission energy. In combination with strain-induced fine tuning, this offers a robust method to achieve spectral resonance in the emission of remote QDs. We show that the spectral tuning has no detectable influence on the multi-photon suppression with g(2)(0) as low as 2%–4% and that the emission can be stabilized to an accuracy of 4 μeV using a closed-loop optical feedback.

Large-range frequency tuning of a narrow-linewidth quantum emitter

A hybrid system of a semiconductor quantum dot single photon source and a rubidium quantum memory represents a promising architecture for future photonic quantum repeaters. One of the key challenges lies in matching the emission frequency of quantum dots with the transition frequency of rubidium atoms while preserving the relevant emission properties. Here, we demonstrate the bidirectional frequency tuning of the emission from a narrow-linewidth (close-to-transform-limited) quantum dot. The frequency tuning is based on a piezoelectric strain-amplification device, which can apply significant stress to thick bulk samples. The induced strain shifts the emission frequency of the quantum dot over a total range of 1.15 THz, about three orders of magnitude larger than its linewidth. Throughout the whole tuning process, both the spectral properties of the quantum dot and its single-photon emission characteristics are preserved. Our results show that external stress can be used as a promising tool for reversible frequency tuning of high-quality quantum dots and pave the wave toward the realization of a quantum dot–rubidium atom interface for quantum networking.

Carrier–phonon interaction of GaAs/Al$$_{0.3}$$Ga$$_{0.7}$$As quantum dots grown by droplet epitaxy

We examined several types of GaAs/Al $$_{0.3}$$ Ga $$_{0.7}$$ As quantum dots (QDs) grown by the droplet epitaxy (DE) technique. Using comparative optical analyses using multi-oscillator models, we investigated the individual exciton–phonon coupling channels of several QD types with different temperature dispersions. Each phonon dispersion was calculated for up to three different coupled modes in a phonon field. Nanoscale phonon engineering can exploit the dynamics of exciton–phonon interactions for the design of efficient acousto-excitonic devices and engineered QD single-photon sources.

Comparative Chemico-Physical Analyses of Strain-Free GaAs/Al0.3Ga0.7As Quantum Dots Grown by Droplet Epitaxy.

We investigate the quantum confinement effects on excitons in several types of strain-free GaAs/AlGaAs droplet epitaxy (DE) quantum dots (QDs). By performing comparative analyses of energy-dispersive X-ray spectroscopy with the aid of a three-dimensional (3D) envelope-function model, we elucidate the individual quantum confinement characteristics of the QD band structures with respect to their composition profiles and the asymmetries of their geometrical shapes. By precisely controlling the exciton oscillator strength in strain-free QDs, we envisage the possibility of tailoring light-matter interactions to implement fully integrated quantum photonics based on QD single-photon sources (SPSs).

Proof-of-principle demonstration of compiled Shor's algorithm using a quantum dot single-photon source.

We report a proof-of-principle demonstration of Shor's algorithm with photons generated by an on-demand semiconductor quantum dot single-photon source for the first time. A fully compiled version of Shor's algorithm for factoring 15 has been accomplished with a significantly reduced resource requirement that employs the four-photon cluster state. Genuine multiparticle entanglement properties are confirmed to reveal the quantum character of the algorithm and circuit. The implementation realizes the Shor's algorithm with deterministic photonic qubits, which opens new applications for cluster state beyond one-way quantum computing.

Progress in quantum-dot single photon sources for quantum information technologies: A broad spectrum overview

Semiconductor quantum dots (QDs) of various material systems are being heavily researched for the development of solid state single photon emitters, which are required for optical quantum computing and related technologies such as quantum key distribution and quantum metrology. In this review article, we give a broad spectrum overview of the QD-based single photon emitters developed to date, from the telecommunication bands in the IR to the deep UV.

In situ wavelength tuning of quantum-dot single-photon sources integrated on a CMOS-processed silicon waveguide

Silicon quantum photonics provides a promising pathway to realize large-scale quantum photonic integrated circuits (QPICs) by exploiting the power of complementary-metal-oxide-semiconductor (CMOS) technology. Toward scalable operation of such silicon-based QPICs, a straightforward approach is to integrate deterministic single-photon sources (SPSs). To this end, hybrid integration of deterministic solid-state SPSs, such as those based on InAs/GaAs quantum dots (QDs), is highly promising. However, the spectral and spatial randomness inherent in the QDs poses a serious challenge for scalable implementation of multiple identical SPSs on a silicon CMOS chip. To overcome this challenge, we have been investigating a hybrid integration technique called transfer printing, which is based on a pick-and-place operation and allows for the integration of the desired QD SPSs on any locations on the silicon CMOS chips at will. Nevertheless, even in this scenario, in situ fine tuning for perfect wavelength matching among the integrated QD SPSs will be required for interfering photons from dissimilar sources. Here, we demonstrate in situ wavelength tuning of QD SPSs integrated on a CMOS silicon chip. To thermally tune the emission wavelengths of the integrated QDs, we augmented the QD SPSs with optically driven heating pads. The integration of all the necessary elements was performed using transfer printing, which largely simplified the fabrication of the three-dimensional stack of micro/nanophotonic structures. We further demonstrate in situ wavelength matching between two dissimilar QD sources integrated on the same silicon chip. Our transfer-printing-based approach will open the possibility for realizing large-scale QPICs that leverage CMOS technology.

Phonon effects in quantum dot single-photon sources

Semiconductor quantum dots are inevitably coupled to the vibrational modes of their host lattice. This interaction reduces the efficiency and the indistinguishability of single-photons emitted from semiconductor quantum dots. While the adverse effects of phonons can be significantly reduced by embedding the quantum dot in a photonic cavity, phonon-induced signatures in the emitted photons cannot be completely suppressed and constitute a fundamental limit to the ultimate performance of single-photon sources based on quantum dots. In this paper, we present a self-consistent theoretical description of phonon effects in such sources and describe their influence on the figures of merit.