Quantum Dot-cavity Research Articles

Recent Advances in Tunable External Cavity Diode Lasers

A narrow linewidth tunable laser source is a critical component in various fields, including laser radar, quantum information, coherent communication, and precise measurement. Tunable external cavity diode lasers (ECDLs) demonstrate excellent performance, such as narrow linewidth, wide tunable range, and low threshold current, making them increasingly versatile and widely applicable. This article provides an overview of the fundamental structures and recent advancements in external cavity semiconductor lasers. In particular, we discuss external cavity semiconductor lasers based on quantum well and quantum dot gain chips. The structure of the gain chip significantly influences laser’s performance. External cavity quantum well laser has a narrower linewidth, higher power, and better mode stability. Conversely, external cavity quantum dot laser provides a wider tunable range and a remarkably lower threshold current. Furthermore, dual-wavelength external cavity tunable diode lasers are gaining importance in applications such as optical switching and terahertz radiation generation. With the continuous optimization of chips and external cavity structures, external cavity diode lasers are increasingly recognized as promising light sources with narrow linewidth and wide tunability, opening up broader application prospects.

Design principles for >90% efficiency and >99% indistinguishability broadband quantum dot cavities

Abstract Quantum dots have the potential to be one of the brightest deterministic single photon sources with high end applications in quantum computing and cluster state generation. In this work, we re-examine the design of simple micropillars by meticulously examining the structural effects of the decay into leaky channels beyond an atom-like cavity estimation. We show that precise control of the side losses with the diameter and avoidance of propagating Bloch modes in the distributed Bragg reflectors can result in easy-to-manufacture broadband (Q≈ 750 − 2500) micropillars, allowing for broad optical coherent control pulses necessary for high single photon purity (> 99.2 − 99.99% achievable) while simultaneously demonstrating extremely high efficiency out the top (90.5% − 96.4%). We also demonstrate that such cavities naturally decouple from the phonon sideband, with the phonon sideband reducing by a factor of 5 − 33 allowing us to predict that the photons should show 98.5% − 99.8% indistinguishability without the need for filtering.

High-fidelity single logical qubit encoding scheme assisted by single-sided quantum dot-cavity systems.

We present an encoding scheme of a single logical qubit with single-sided quantum dot (QD)-cavity systems, which is immune to the collective decoherence. By adjusting the Purcell factor to satisfy the balanced reflection condition, the detrimental effects of unbalanced reflection between the coupled and uncoupled QD-cavity systems can be effectively suppressed. Furthermore, the fidelity of each step can be increased to unity regardless of the strong coupling regime and the weak coupling regime of cavity quantum electrodynamics (QED) with the assistance of waveform correctors. The scheme requires QD-cavity systems and simple linear optical elements, which can be implemented with the currently experimental techniques.

Electrically Tunable Single Polaritonic Quantum Dot at Room Temperature.

Exciton-polaritons confined in plasmonic cavities are hybridized light-matter quasiparticles, with distinct optical characteristics compared to plasmons and excitons alone. Here, we demonstrate the electric tunability of a single polaritonic quantum dot operating at room temperature in electric-field tip-enhanced strong coupling spectroscopy. For a single quantum dot in the nanoplasmonic tip cavity with variable dc local electric field, we dynamically control the Rabi frequency with the corresponding polariton emission, crossing weak to strong coupling. We model the observed behaviors based on the quantum confined Stark effect in the strong coupling regime.

Single site-controlled inverted pyramidal InGaAs QD–nanocavity operating at the onset of the strong coupling regime

Precise positioning of single site-controlled inverted pyramidal InGaAs quantum dots (QDs) at the antinode of a GaAs photonic crystal cavity with nanometer-scale accuracy holds unique advantages compared to self-assembled QDs and offers great promise for practical on-chip photonic quantum information processing. However, the strong coupling regime in this geometry has not yet been achieved due to the low cavity Q-factor based on the (111)B-oriented membrane structures. Here, we reveal the onset of phonon-mediated coherent exciton–photon interaction on our tailored single site-controlled InGaAs QD–photonic crystal cavity. Our results present the Rabi-like oscillation of luminescence intensity between excitonic and photonic components correlated with their energy splitting pronounced at small detuning. Such Rabi-like oscillation is well reproduced by modeling the coherent exchange of the exciton-photon population. The modeling further reveals an oscillatory two-time covariance at QD-cavity resonance, which indicates that the system operates at the onset of the strong coupling regime. Moreover, by using the cavity mode as a probe of the virtual state of the QD induced by phonon scattering, it reveals an increase in phonon scattering rates near the QD–cavity resonance and asymmetric phonon emission and absorption rate of even around 50 K.

Impact of the phonon environment on the nonlinear quantum-dot–cavity QED: Path-integral approach

Impact of the phonon environment on the nonlinear quantum-dot–cavity QED: Path-integral approach

Mollow triplets under few-photon excitation

Resonant excitation is an essential tool in the development of semiconductor quantum dots (QDs) for quantum information processing. One central challenge is to enable transparent access to the QD signal without post-selection information loss. A viable path is through cavity enhancement, which has successfully lifted the resonantly scattered field strength over the laser background under weak excitation. Here, we extend this success to the saturation regime using a QD-micropillar device with a Purcell factor of 10.9 and ultra-low background cavity reflectivity of just 0.0089±0.0001. We achieve a signal to background ratio of 55 and overall system responsivity of 3.01±0.08%, i.e., we detect on average 0.03 resonantly scattered single photons for every incident laser photon. Raising the excitation to the few-photon level, the QD response is brought into saturation where we observe Mollow triplets as well as the associated cascaded single photon emissions, without resorting to any laser background rejection technique. Our work offers a perspective on a QD cavity interface that is not restricted by the laser background.

Cavity transmission in a QD-cavity system using Cluster Expansion: Beyond the mean-field approach

We theoretically study the temporal evolution of the mean number of photons in a strongly coupled cavity quantum dot system driven by a symmetric laser pulse using the Cluster Expansion method. The quantum dot is modeled as a two-level system that interacts with a single mode of the quantized electromagnetic field according to the Jaynes–Cummings model. We compare our results with those obtained through the Master Equation method and find that second-order Cluster Expansion is sufficient to reproduce the transmission spectrum without losing the fundamental physical processes and with a relatively low computational cost.

Exciton-polariton dynamics of the single site-controlled quantum dot-nanocavity in the coexisting strong-weak coupling regime

Deterministic positioning single site-controlled high symmetric InGaAs quantum dots (QDs) in (111)B-oriented GaAs photonic crystal cavities with nanometer-scale accuracy provides an idea component for building integrated quantum photonic circuits. However, it has been a long-standing challenge of improving cavity Q-factors in such systems. Here, by optimizing the trade-off between the cavity loss and QD spectral quality, we demonstrate our site-controlled QD-nanocavity system operating in the intermediate coupling regime mediated by phonon scattering, with the dynamic coexistence of strong and weak coupling. The cavity-exciton detuning-dependent micro-photoluminescence spectrum reveals concurrence of a trend of exciton-polariton mode avoided crossing, as a signature of Rabi doublet of the strongly coupled system. Meanwhile, a trend of keeping constant or slight blue shift of coupled exciton–cavity mode(CM) energy across zero-detuning is ascribed to the formation of collective states mediated by phonon-assisted coupling, and their rare partial out-of-synchronization linewidth-narrowing is linked to their coexisting strong-weak coupling regime. We further reveal the pump power-dependent anti-bunching photon statistical dynamics of this coexisting strong-weak coupled system and the optical features of strongly confined exciton-polaritons, and dark-exciton-like states. These observations demonstrate the potential capabilities of site-controlled QD-cavity systems as deterministic quantum nodes for on-chip quantum information processing and provide guidelines for future device optimization for achieving the strong coupling regime.

Phonon-induced transition between entangled and nonentangled photon emission in constantly driven quantum-dot–cavity systems

Entangled photon pairs are essential for many applications in quantum technologies. Recent theoretical studies demonstrated that different types of entangled Bell states can be created in a constantly driven four-level quantum emitter-cavity system. Unlike other candidates for the realization of the four-level emitter, semiconductor quantum dots unavoidably interact with their environment, resulting in carrier-phonon interactions. Surprisingly, phonons change the entanglement of emitted photon pairs in a qualitative way, already at low temperatures on the order of 4 K. While one type of Bell state can still be generated using small driving strengths, the other type is suppressed due to phonon interactions in strongly-confined quantum dots. The degree of entanglement decreases with rising temperature and driving strength until it vanishes at a certain parameter value. Because it remains zero afterward, we encounter a phonon-induced transition between entangled and nonentangled photon emission that resembles a phase transition. The transition occurs at temperatures below 30 K and, independent of the driving strength, the concurrence as a function of the reduced temperature is found to obey a power law with exponent one near the transition point.

Low power optical bistability from quantum dots in a nanobeam photonic crystal cavity

We demonstrate a low power thermally induced optical bistability at telecom wavelengths and room temperature using a nanobeam photonic crystal cavity embedded with an ensemble of quantum dots. The nanobeam photonic crystal cavity is transfer-printed onto the edge of a carrier chip for thermal isolation of the cavity with an efficient optical coupling between the nanobeam waveguide and optical setup. Reflectivity measurements performed with a tunable laser reveal the thermo-optic nature of the nonlinearity. A bistability power threshold as low as 23 μW and an on/off response contrast of 6.02 dB are achieved from a cavity with a moderately low quality factor of 2830. Our device provides optical bistability at power levels an order of magnitude lower than previous quantum-dot-based devices.

Normal mode splitting and four-wave mixing in Kerr-nonlinear optical system containing coupled double quantum dot

We theoretically investigate the four wave mixing (FWM) and normal mode splitting (NMS) in coupled double quantum dots cavity system in the presence of a third-order Kerr nonlinear medium. The FWM generated in the system has narrow spectral width and this can be tuned by nonlinear medium strength, the coupling strength between two coupled quantum dots and pump strength. Further, we study NMS that can also be tuned by various physical parameters and show that energy exchange between different modes is possible. The proposed model have potential applications in the field of quantum information processing.

Performance analysis of all optical-based quantum internet circuits

A Quantum dot implanted within a double-sided optical microcavity is considered and investigated as a critical component for all optical-based quantum internets. Due to the duality as the photonic quantum transistor and gate, the QD cavity system repsents a sturdy base for the future photonic quantum network. With the help of the analytical investigation and review presented in this paper, a quantum dot cavity unit can be developed for implementing deterministic quantum gates and transistors. The maximum fidelity observed for quantum diode, router, and storage is 95.24, 62.06, and 90.42%without considering a noisy environment, respectively, and 62.12, 60.36, and 43.66% under a noisy environment, respectively. Fidelity is also calculated with varying coupling conditions (strong and weak coupling), and optimized cavity parameters are calculated.

Three-Dimensional Electrical Control of the Excitonic Fine Structure for a Quantum Dot in a Cavity.

The excitonic fine structure plays a key role for the quantum light generated by semiconductor quantum dots, both for entangled photon pairs and single photons. Controlling the excitonic fine structure has been demonstrated using electric, magnetic, or strain fields, but not for quantum dots in optical cavities, a key requirement to obtain high source efficiency and near-unity photon indistinguishability. Here, we demonstrate the control of the fine structure splitting for quantum dots embedded in micropillar cavities. We propose and implement a scheme based on remote electrical contacts connected to the pillar cavity through narrow ridges. Numerical simulations show that such a geometry allows for a three-dimensional control of the electrical field. We experimentally demonstrate tuning and reproducible canceling of the fine structure, a crucial step for the reproducibility of quantum light source technology.

Squeezed light generated with hyperradiance without nonlinearity.

We propose that the squeezed light accompanied by hyperradiance is induced by quantum interference in a linear system consisting of a high-quality optical cavity and two coherently driven two-level qubits. When two qubits are placed in the cavity with a distance of integer multiple and one-half of wavelengths (i.e., they have the opposite coupling coefficient to the cavity), we show that squeezed light is generated in the hyperradiance regime under the conditions of strong coupling and weak driving. Simultaneously, Klyshko's criterion alternates up and down at unity when the photon number is even or odd. Moreover, the orthogonal angles of the squeezed light can be controlled by adjusting the frequency detuning between the driving field and the qubits. It can be implemented in a variety of quantum systems, including but not limited to two-level systems such as atoms, ions, quantum dots in single-mode cavities.

Optical response in a double quantum dot molecule inside a nonlinear photonic crystal cavity

In this work, we theoretically analyze optical bistability and absorption in coupled double quantum dot (CDQDs) cavity system in the presence of a third-order Kerr nonlinear χ(3) medium. We find that the optical bistability can be tuned by the coupling strength between the two quantum dots, nonlinear medium strength and other system parameters. The possibility of making the system an efficient optical switch is also studied. We further show that the system parameters can also tune the absorption spectrum which shows both positive and negative absorption (emission). Such a study of the CDQDs system should be helpful for various applications in optical logic devices.

Deterministic Photon Storage and Readout in a Semimagnetic Quantum Dot–Cavity System Doped with a Single Mn Ion

AbstractLight trapping is a crucial mechanism for synchronization in optical communication. Especially on the level of single photons, control of the exact emission time is desirable. In this paper, a single‐photon buffering device composed of a quantum dot doped with a single Mn atom in a cavity is theoretically proposed. A method to detain a single cavity photon as an excitation of the dot is presented. The storage scheme is based on bright to dark exciton conversion performed with an off‐resonant external optical field and mediated via a spin‐flip with the magnetic ion. The induced Stark shift brings both exciton states to resonance and results in an excitation transfer to the optically inactive one. The stored photon can be read out on demand in the same manner by repopulating the bright state, which has a short lifetime. The results indicate the possibility to suspend a photon for almost two orders of magnitude longer than the lifetime of the bright exciton.

Mini-LEDs with Diffuse Reflection Cavity Arrays and Quantum Dot Film for Thin, Large-Area, High-Luminance Flat Light Source.

Mini-light-emitting diodes (mini-LEDs) were combined with multiple three-dimensional (3D) diffuse reflection cavity arrays (DRCAs) to produce thin, large-area, high-brightness, flat light source modules. The curvature of the 3D free-form DRCA was optimized to control its light path; this increased the distance between light sources and reduced the number of light sources used. Experiments with a 12.3-inch prototype indicated that 216 mini-LEDs were required for a 6 mm optical mixing distance to achieve a thin, large-area surface with high brightness, uniformity, and color saturation of 23,044 cd/m2, 90.13%, and 119.2, respectively. This module can serve as the local dimming backlight in next generation automotive displays.

Bright Polarized Single-Photon Source Based on a Linear Dipole.

Semiconductor quantum dots in cavities are promising single-photon sources. Here, we present a path to deterministic operation, by harnessing the intrinsic linear dipole in a neutral quantum dot via phonon-assisted excitation. This enables emission of fully polarized single photons, with a measured degree of linear polarization up to 0.994±0.007, and high population inversion-85% as high as resonant excitation. We demonstrate a single-photon source with a polarized first lens brightness of 0.50±0.01, a single-photon purity of 0.954±0.001, and single-photon indistinguishability of 0.909±0.004.

Magnetic control of biexcitons in a quantum dot-cavity system

This paper study the effect of an external magnetic field on the steady-state observables of a multi-excitonic quantum dot embedded in a bimodal optical cavity. In particular, we perform a detailed analysis of the occupations and dispersion relations of the dressed states of the system as a function of both the intensity and tilt angle of the external magnetic field, and it is found that the dispersion relation reveals the presence of anti-crosses as a signature of coupling light-matter in the quantum system. Additionally, we demonstrate that by varying in the tilt angle and intensity of the applied magnetic field, the steady-state occupations can be manipulated to achieve magnetic control in this solid-state quantum system. We explore an operation regime of the quantum system, where it is possible to find optimal parameters for which the highest biexciton state could lead to the emission of entangled photons.