Single Photon Extraction Research Articles

Topology-optimized Plasmonic Nanoantenna for Efficient Single-photon Extraction

Topology-optimized Plasmonic Nanoantenna for Efficient Single-photon Extraction

On-demand Single-photon Extraction for Underwater Quantum Communication

On-demand Single-photon Extraction for Underwater Quantum Communication

Enhanced Single-Photon Emission from GaN Quantum Dots in Bullseye Structures

We present the design, fabrication, and detailed characterization of a photonic bullseye structure to enhance the single-photon extraction efficiency from self-assembled GaN/AlN quantum dots (QDs) ...

Broadband enhancement of on-chip single-photon extraction via tilted hyperbolic metamaterials

On-chip single-photon sources with high repetition rates are a fundamental block for quantum photonics and can enable applications such as high-speed quantum communication or quantum information processing. Ideally, such single-photon sources require a large on-chip photon extraction decay rate, especially over a broad spectral range, but this goal has remained elusive so far. Current approaches implemented to enhance the spontaneous emission rate include photonic crystals, optical cavities, metallic nanowires, and metamaterials. These approaches either have a strong reliance on the frequency resonance mechanisms, which unavoidably suffer from the issue of narrow working bandwidth, or are limited by poor outcoupling efficiency. Here, we propose a feasible scheme to enhance the on-chip photon extraction decay rate of quantum emitters through the tilting of the optical axis of hyperbolic metamaterials with respect to the end-facet of nanofibers. The revealed scheme is applicable to arbitrarily orientated quantum emitters over a broad spectral range extending up to ∼80 nm for visible light. This finding relies on the emerging unique feature of hyperbolic metamaterials if their optical axis is judiciously tilted. Hence, their supported high-k (i.e., wavevector) hyperbolic eigenmodes, which are intrinsically confined inside them if their optical axis is un-tilted, can now become momentum-matched with the guided modes of nanofibers, and more importantly, they can efficiently couple into nanofibers almost without reflection.

Integration of single photon emitters in 2D layered materials with a silicon nitride photonic chip

Photonic integrated circuits (PICs) enable the miniaturization of optical quantum circuits because several optic and electronic functionalities can be added on the same chip. Integrated single photon emitters (SPEs) are central building blocks for such quantum photonic circuits. SPEs embedded in 2D transition metal dichalcogenides have some unique properties that make them particularly appealing for large-scale integration. Here we report on the integration of a WSe2 monolayer onto a Silicon Nitride (SiN) chip. We demonstrate the coupling of SPEs with the guided mode of a SiN waveguide and study how the on-chip single photon extraction can be maximized by interfacing the 2D-SPE with an integrated dielectric cavity. Our approach allows the use of optimized PIC platforms without the need for additional processing in the SPE host material. In combination with improved wafer-scale CVD growth of 2D materials, this approach provides a promising route towards scalable quantum photonic chips.

Tilted-potential photonic crystal cavities for integrated quantum photonics.

We propose and investigate a new type of photonic crystal (PhC) cavity for integrated quantum photonics, which provides tailored optical modes with both confined and extended spatial components. The structures consist of elongated PhC cavities in which the effective index of refraction is varied quasi-linearly along their axis, implemented by systematic lateral shifts of the PhC holes. The confined modes have approximately Airy-function envelopes, exhibiting single peaks and extended tails, which is useful for optimizing single photon extraction and transmission in integrated quantum photonic devices. The measured spectrally resolved near-field patterns of such devices show the expected spatial and resonance wavelength behavior, in agreement with numerical simulations of the Airy-Bloch modes. The effects of fabrication-induced disorder on the mode features are also analyzed and discussed. Selective excitation of specific Airy-Bloch modes using integrated, site-controlled quantum dots as localized light sources is demonstrated. Based on the tilted-potential cavity, multiple-QD single photon emitters exploiting wavelength division multiplexing are proposed.

A solid-state source of strongly entangled photon pairs with high brightness and indistinguishability.

The generation of high-quality entangled photon pairs has been a long-sought goal in modern quantum communication and computation. So far, the most widely used entangled photon pairs have been generated from spontaneous parametric down-conversion (SPDC), a process that is intrinsically probabilistic and thus relegated to a regime of low rates of pair generation. In contrast, semiconductor quantum dots can generate triggered entangled photon pairs through a cascaded radiative decay process and do not suffer from any fundamental trade-off between source brightness and multi-pair generation. However, a source featuring simultaneously high photon extraction efficiency, high degree of entanglement fidelity and photon indistinguishability has been lacking. Here, we present an entangled photon pair source with high brightness and indistinguishability by deterministically embedding GaAs quantum dots in broadband photonic nanostructures that enable Purcell-enhanced emission. Our source produces entangled photon pairs with a pair collection probability of up to 0.65(4) (single-photon extraction efficiency of 0.85(3)), entanglement fidelity of 0.88(2), and indistinguishabilities of 0.901(3) and 0.903(3) (brackets indicate uncertainty on last digit). This immediately creates opportunities for advancing quantum photonic technologies.

Single photon extraction and propagation in photonic crystal waveguides incorporating site-controlled quantum dots

Deterministic integration of site-controlled quantum dots with photonic crystal waveguides is demonstrated, which allows positioning the dots for optimal overlap with the waveguide modes. The coupling efficiency (β-factor) of quantum dot emission to propagating waveguide modes ranging from 0 to 88% is measured accounting for statistical variations of quantum dot properties. Using site controlled quantum dots permits us to distinguish between the spectral and spatial origins of fluctuations in β. The role of Fabry-Pérot modes that prevent reaching a deterministic coupling between quantum dots and photonic crystal waveguides is revealed, and ways to overcome this problem are proposed. The results are useful for constructing high-flux single photon emitters based on multiplexed single photon sources.

Integrated nanoplasmonic quantum interfaces for room-temperature single-photon sources

We describe a general analytical framework of a nanoplasmonic cavity-emitter system interacting with a dielectric photonic waveguide. Taking into account emitter quenching and dephasing, our model directly reveals the single photon extraction efficiency, $\eta$, as well as the indistinguishability, $I$, of photons coupled into the waveguide mode. Rather than minimizing the cavity modal volume, our analysis predicts an optimum modal volume to maximize $\eta$ that balances waveguide coupling and spontaneous emission rate enhancement. Surprisingly, our model predicts that near-unity indistinguishability is possible, but this requires a much smaller modal volume, implying a fundamental performance trade-off between high $\eta$ and $I$ at room temperature. Finally, we show that maximizing $\eta I$ requires that the system has to be driven in the weak coupling regime because quenching effects and decreased waveguide coupling drastically reduce $\eta$ in the strong coupling regime.

Single photon extraction from self-assembled quantum dots via stable fiber array coupling

We present a direct fiber output of single photons from self-assembled quantum dots (QDs) realized by a stable fiber array-QD chip coupling. The integration of distributed Bragg reflector cavity and the etching of micropillar arrays isolate QDs and enhance their normal emission. The matched periods and mismatched diameters of the pillar array and the single-mode fiber array with Gaussian-shaped light spots enable a large alignment tolerance and a stable, efficient (i.e., near-field), and chip-effective (i.e., parallel) coupling of single QD emission, as compared to the traditional “point-based” coupling via a confocal microscope, waveguide, or fiber. The single photon counting rate at the fiber end reaches 1.87 M counts per second (cps) with a time correlation g2(0) of 0.3 under a saturated excitation, and 485 K cps with a g2(0) of 0.02 under a weak excitation, demonstrating a nice “all-fiber” single-photon source.

An electrically driven cavity-enhanced source of indistinguishable photons with 61% overall efficiency

We report on an electrically driven efficient source of indistinguishable photons operated at pulse-repetition rates f up to 1.2 GHz. The quantum light source is based on a p-i-n-doped micropillar cavity with integrated self-organized quantum dots, which exploits cavity quantum electrodynamics effects in the weak coupling regime to enhance the emission of a single quantum emitter coupled to the cavity mode. We achieve an overall single-photon extraction efficiency of (61 ± 11) % for a device triggered electrically at f = 625 MHz. Analyzing the suppression of multi-photon emission events as a function of excitation repetition rate, we observe single-photon emission associated with g(2)HBT(0) values between 0.076 and 0.227 for f ranging from 373 MHz to 1.2 GHz. Hong-Ou-Mandel-type two-photon interference experiments under pulsed current injection at 487 MHz reveal a photon-indistinguishability of (41.1 ± 9.5) % at a single-photon emission rate of (92 ± 23) MHz.

Deterministic generation of bright single resonance fluorescence photons from a Purcell-enhanced quantum dot-micropillar system.

We report on the observation of bright emission of single photons under pulsed resonance fluorescence conditions from a single quantum dot (QD) in a micropillar cavity. The brightness of the QD fluorescence is greatly enhanced via the coupling to the fundamental mode of a micropillar, allowing us to determine a single photon extraction efficiency of (20.7 ± 0.8) % per linear polarization basis. This yields an overall extraction efficiency of (41.4 ± 1.5) % in our device. We observe the first Rabi-oscillation in a weakly coupled quantum dot-micropillar system under coherent pulsed optical excitation, which enables us to deterministically populate the excited QD state. In this configuration, we probe the single photon statistics of the device yielding g(2)(0) = 0.072 ± 0.011 at a QD-cavity detuning of 75 μeV.

Extraction of a single photon from an optical pulse

Removing a single photon from a pulse is one of the most elementary operations that can be performed on light, having both fundamental significance and practical applications in quantum communication and computation. So far, photon subtraction, in which the removed photon is detected and therefore irreversibly lost, has been implemented in a probabilistic manner with inherently low success rates using low-reflectivity beam splitters. Here we demonstrate a scheme for the deterministic extraction of a single photon from an incoming pulse. The removed photon is diverted to a different mode, enabling its use for other purposes, such as a photon number-splitting attack on quantum key distribution protocols. Our implementation makes use of single-photon Raman interaction (SPRINT) with a single atom near a nanofibre-coupled microresonator. The single-photon extraction probability in our current realization is limited mostly by linear loss, yet probabilities close to unity should be attainable with realistic experimental parameters.

Metal-coated semiconductor nanostructures and simulation of photon extraction and coupling to optical fibers for a solid-state single-photon source

We have realized metal-coated semiconductor nanostructures for a stable and efficient single-photon source (SPS) and demonstrated improved single-photon extraction efficiency by the selection of metals and nanostructures. We demonstrate with finite-difference time-domain (FDTD) simulations that inclination of a pillar sidewall, which changes the structure to a nanocone, is effective in improving the photon extraction efficiency. We demonstrate how such nanocone structures with inclined sidewalls are fabricated with reactive ion etching. With the optimized design, a photon extraction efficiency to outer airside as high as ∼97% generated from a quantum dot in a nanocone structure is simulated, which is the important step in realizing SPS on-demand operations. We have also examined the direct contact of such a metal-embedded nanocone structure with a single-mode fiber facet as a simple and practical method for preparing fiber-coupled SPS and demonstrated practical coupling efficiencies of ∼16% with FDTD simulation.

Bright Single-Photon Emission From a Quantum Dot in a Circular Bragg Grating Microcavity

Bright single-photon emission from single quantum dots (QDs) in suspended circular Bragg grating microcavities is demonstrated. This geometry has been designed to achieve efficient <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX"> $(&gt;\!50\%)$</tex></formula> single-photon extraction into a near-Gaussian-shaped far-field pattern, modest <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(\approx 10 \times)$</tex></formula> Purcell enhancement of the radiative rate, and a spectral bandwidth of a few nanometers. Measurements of fabricated devices show progress toward these goals, with collection efficiencies as high as <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\approx\!{10}\%$</tex></formula> demonstrated with moderate spectral bandwidth and rate enhancement. Photon correlation measurements are performed under above-bandgap excitation (pump wavelength = 780 to 820 nm) and confirm the single-photon character of the collected emission. While the measured sources are all antibunched and dominantly composed of single photons, the multiphoton probability varies significantly. Devices exhibiting tradeoffs among collection efficiency, Purcell enhancement, and multiphoton probability are explored and the results are interpreted with the help of finite-difference time-domain simulations. Below-bandgap excitation resonant with higher states of the QD and/or cavity (pump wavelength = 860 to 900 nm) shows a near-complete suppression of multiphoton events and may circumvent some of the aforementioned tradeoffs.

Efficient single-photon extraction from quantum-dots embedded in GaAs micro-pyramids

We demonstrate an easy method to fabricate efficient single-photon sources based on In(Ga)As quantum-dots embedded in reversed GaAs micro-pyramids. It relies on a single wet-chemical etching step utilizing an AlAs sacrificial layer. Due to the pyramidal shape of the cavities, we have been able to separate a small number of quantum-dots from the self-assembled ensemble and improve the extraction efficiency for single photons. The latter is predicted by finite difference time domain and finite elements method simulations to be about 80%–90% over a broad spectral range of 40 nm. Single-photon emission has been proven experimentally by means of auto-correlation measurements.

Single photon source using confined Tamm plasmon modes

We evaluate the potential of confined Tamm plasmon structures for single photon extraction. A single quantum dot is inserted in a structure consisting of a gold microdisk deposited on a distributed Bragg reflector. The quantum dot exciton line experiences acceleration of spontaneous emission while the biexciton line experiences inhibition. This property leads to non-linearities in the emission dynamics and the photon statistics as a function of the excitation rate. We show that the device operates as a single photon source with a measured brightness of 0.20 ± 0.02 collected photons per pulse. By modeling the extraction efficiency, we show that efficiencies around 60% can be reached in optimized structures.

Efficient quantum dot single photon extraction into an optical fiber using a nanophotonic directional coupler

We demonstrate a spectrally broadband and efficient technique for collecting emission from a single InAs quantum dot directly into a standard single mode optical fiber. In this approach, an optical fiber taper waveguide is placed in contact with a suspended GaAs nanophotonic waveguide with embedded quantum dots, forming a broadband directional coupler with standard optical fiber input and output. Efficient photoluminescence collection over a wavelength range of tens of nanometers is demonstrated, and a maximum collection efficiency of 6% (corresponding single photon rate of 3.0 MHz) into a single mode optical fiber is estimated for a single quantum dot exciton.