Single-atom waveguide-QED systems have emerged as a leading platform for demonstrating non-linear optics in the few-photon regime. Devices with such non-linear behavior are necessary for developing deterministic gates for photonic quantum computing. Artificial atoms coupled to nanophotonic or superconducting waveguides can be used to engineer highly efficient light-matter interfaces for such non-linear interactions, where every photon in the system interacts with the quantum emitter. However, it has been a challenge to access the reflection port of the waveguide due to the inability of rejecting stray electromagnetic fields that do not interact with the artificial atom. Here, we show coherent reflection of single photons from a semiconductor quantum dot strongly coupled to the fundamental waveguide mode of a nanowire. We utilize the tapered end of the nanowire for ideal mode-matching in a vertical geometry, enabled by a novel extinction technique. This mode-matching allows for efficient transfer of photons from the input to the reflection port while having all interacted with the quantum dot. Such a high-efficiency scalable photonic platform opens up applications in few-photon quantum nonlinear optics and has been proposed for creating key resource states for universal photon-based quantum computing.

Quantum dots embedded in semiconductor photonic nanowires (NW-QDs) can deterministically produce single-photons and entangled photon pairs at high repetition rates. These photons can be efficiently coupled from the photonic nanowire into free space or optical fibers thanks to the sharp tip of the nanowire, which provides impedance matching. However, precise control of the NW-QD emission frequency in a way that is reversible, does not degrade the properties of the emitted photons, and can be used independently for individual NW-QDs on the same chip has so far remained a challenge. Resolving this issue is crucial for applications when interfacing the photons with quantum systems that require MHz to sub-GHz precision, such as atomic ensembles acting as memories in a quantum network. Here, we demonstrate a reversible tuning method that can tune the emission frequency of a NW-QD by more than 300 GHz with sub-GHz precision. We achieve this through gas condensation that is then partially reversed with localized laser ablation. This process finely adjusts stress applied to the quantum dots, thereby tuning their emission frequency. We validate the precision and stability of this method by tuning the frequency of the emitted single-photons across an atomic resonance to probe its absorption and dispersion. We observed up to 80% absorption of the single-photons from NW-QD in hot cesium vapor at the D1-line resonances and a 75-fold decrease in group velocity associated with the hyperfine transitions of the D1-line ground states. We observed no discernible effects in the second-order autocorrelation function, lifetime, or linewidth of the NW-QD emission for up to 300 GHz of tuning and we saw minimal effects on the fine structure splitting of the NW-QD when tuning up to 100 GHz.

We report on a stable form of pulsed electroluminescence in a dopant-free direct band gap semiconductor heterostructure that we coin the "tidal effect." Swapping of an inducing gate voltage in an ambipolar field effect transistor allows incoming and outgoing carriers of opposite charge to meet and recombine radiatively. We develop a model to explain the carrier dynamics that underpins the frequency response of the pulsed electroluminescence intensity. Higher mobilities enable larger active emission areas than in previous reports, as well as stable emission over long timescales.

Coherent control of single photon sources is a key requirement for the advancement of photonic quantum technologies. Among them, nanowire-based quantum dot sources are popular due to their potential for on-chip hybrid integration. Here we demonstrate on-demand single-photon generation (g^(2)(0)(X^*) =0.078 and g^(2)(0)(X)= 0.03) from resonantly excited InAsP/InP nanowire quantum dots and observe Rabi oscillations in the dot emission, indicating successful coherent manipulation of the excitonic states in the nanowire. We also measure a low emission time jitter for resonant excitation as compared to above-band excitation. This work addresses the long-standing challenge of resonantly exciting nanowire-quantum dots. It paves the way for hybrid quantum photonic integration, enabling spin-photon entanglement and matter memories on-chip.

The ability to detect light with high efficiency is an important device metric for single-photon detectors and cameras, essential for applications ranging from quantum communication to biomedical imaging. However, these photodetectors have limited detection efficiency in the 850-1100 nm wavelength range, known as the 'valley of death'. Here, we demonstrate a near-perfect absorber in the 'valley of death' using a semiconductor metasurface with spectral and spatial selectivity on a high refractive index substrate. Our design leverages higher order optical modes of InGaAs resonators to generate Kerker interference at the target wavelength of 920 nm, which leads to a measured peak absorption efficiency of ∼94%. In addition, numerical calculations show that our design enables spatial control of the absorption profile within the resonators, which is promising for improving response time. Our approach offers tunability over a desired spectral range and paves the way for development of high-performance photodetectors.

We present the implementation of an automatic raster scan of 10,000 InAsP quantum dots (QDs) embedded in InP tapered nanowires on a chip. The aim is to identify photon emission from the QDs that is close in frequency to the caesium D 1-line for interfacing single photons with cold atoms for quantum frequency conversion and implementing quantum repeaters.

Abstract Various noncollinear spin textures and magnetic phases have been predicted in twisted two-dimensional CrI3 due to competing ferromagnetic (FM) and antiferromagnetic (AFM) interlayer exchange from moiré stacking—with potential spintronic applications even when the underlying material possesses a negligible Dzyaloshinskii–Moriya or dipole–dipole interaction. Recent measurements have shown evidence of coexisting FM and AFM layer order in small-twist-angle CrI3 bilayers and double bilayers. Yet, the nature of the magnetic textures remains unresolved and possibilities for their manipulation and electrical readout are unexplored. Here, we use tunneling magnetoresistance to investigate the collective spin states of twisted double-bilayer CrI3 under both out-of-plane and in-plane magnetic fields together with detailed micromagnetic simulations of domain dynamics based on magnetic circular dichroism. Our results capture hysteretic and anisotropic field evolutions of the magnetic states and we further uncover two distinct non-volatile spin textures (out-of-plane and in-plane domains) at ≈1° twist angle, with a different global tunneling resistance that can be switched by magnetic field.

An on-demand source of bright entangled photon pairs is desirable for quantum key distribution (QKD) and quantum repeaters. The leading candidate to generate such pairs is based on spontaneous parametric down-conversion (SPDC) in non-linear crystals. However, its pair extraction efficiency is limited to 0.1% when operating at near-unity fidelity due to multiphoton emission at high brightness. Quantum dots in photonic nanostructures can in principle overcome this limit, but the devices with high entanglement fidelity (99%) have low pair extraction efficiency (0.01%). Here, we show a measured peak entanglement fidelity of 97.5% ± 0.8% and pair extraction efficiency of 0.65% from an InAsP quantum dot in an InP photonic nanowire waveguide. We show that the generated oscillating two-photon Bell state can establish a secure key for peer-to-peer QKD. Using our time-resolved QKD scheme alleviates the need to remove the quantum dot energy splitting of the intermediate exciton states in the biexciton-exciton cascade.

With recent developments in the field of quantum computing and cryptography, establishing quantum networks would allow for the implementation of post-quantum cryptographic protocols, distributed quantum computing, and quantum sensor networks. Though, quantum networks require the use of quantum repeaters to preserve the transmitted quantum information over long distances. This work focuses on the implementations of quantum frequency conversion which is used to ensure the signal is of a suitable frequency for transmission between the different optical components in the system.

Here we show that incorporating broadband metamaterial perfect absorbers into a photodetector's active area can improve device efficiency and speed. We show an optical absorption of 93% across the spectral region where commercially available Si and InGaAs detectors have poor efficiencies. Combining the metamaterial perfect absorber with an avalanche photodiode layer stack, we aim to realize a high efficiency portable single photon avalanche diode with high timing resolution ideal for quantum ranging, quantum communication, and medical imaging applications.

Recently several machine learning methods have been proposed to estimate the SNR, based on launch data and other system factors. These data-driven methods typically require a large number of datasets for training and generally are not interpretable. In this paper, we propose an alternative approach that requires less data and is interpretable, specifically a hybrid algorithm combining a physical model with Gaussian process regression. We develop a measurement-informed physical model, systematically reducing the number of independent parameters based on the underpinning physics and improve the overall performance of the physical model marginally. The model is validated using measurements performed on a 15-channel wavelength-division multiplexed system propagating over 1,000 km of standard single-mode fiber. The proposed hybrid model is not only interpretable but also obtains better agreement with measurements than a Gaussian process regression model and a simple neural network model for a given number of training datapoints.

Quantum communications and remote sensing protocols rely on preparing individual quanta of light, or photons, as carriers of quantum information. While it is easy to generate many-photon classical states of light (e.g., a light bulb or a laser), it is challenging to generate single quanta or pairs of entangled quanta on-demand. Yet, only in this latter regime can the fundamentally quantum nature of light be exploited to achieve performance exceeding classical bounds. We are developing a single-photon source based on combining a one-parameter single electron pump (Figure 1a,b) – an electrically controlled, on-demand, high-fidelity emitter of single electrons [1] – with a lateral p-n junction [2] (Figure 1c,d) in which injected single electrons recombine with holes to produce single photons. These devices are realized in undoped GaAs/AlGaAs heterostructures and designed to be ambipolar. Such a quantum light source could lead to a paradigm shift in metrology, providing a quantum redefinition of two of the seven SI base units, the ampere and the candela. It is also possible in principle to realize scalable source arrays on a single chip. I will discuss progress towards realizing and characterizing these sources, including work on optimizing collection efficiency and Purcell enhancement using a lateral distributed Bragg reflector in a concentric ring geometry. Beyond fundamental studies, the proposed photon source will find practical use in quantum communications protocols (QKD), where it can offer fast data transmission rates, and quantum sensing with LiDAR, where it could outperform laser-based systems in the few-photon regime for low-power, stealth applications. [1] B. Buonacorsi et al, “Non-adiabatic single-electron pumps in a dopant-free GaAs/AlGaAs 2DEG”, Appl. Phys. Lett. 119 , 114001 (2021). [2] L. Tian et al, “Stable electroluminescence in ambipolar dopant-free lateral p-n junctions”, Appl. Phys. Lett. 123 , 061102 (2023). Figure 1

We report a reduced complexity single polarization all-pass modulation scheme that uses overlap and discard to implement continuous transmission. We find that transmitter NSRs of −25 dB and −21 dB can be achieved with 1/3 and 1/6 the complexity, respectively, of reported solutions. We compare this modulation scheme with a conventional Mach-Zehnder modulator solution by measures of throughput, spectral efficiency, DSP and hardware complexity as well as back-to-back modem margin.

We report local reversible tuning of nanowire quantum dot emission to cesium D 1 line (~ 894.6 nm) and demonstrate slow light and resonant absorption of single photons in hot cesium vapor.

Entangled photons are an important resource for quantum optics. Quantum dots are a source of on-demand and highly entangled photon pairs at a high repetition rate. However, fine structure splitting (FSS) in the biexciton-exciton cascade causes the photons to be emitted in a time-dependant state instead of an ideal Bell state. Current techniques to remove the FSS include applying a strain, electric, or magnetic field and require post-processing of the quantum dots which reduces device yield. We use a novel all-optical approach implemented by emulating a fast-rotating half-wave plate in a Lithium Niobate waveguide using a electro-optic modulator. This method allows us to frequency shift single photons and produce a time-independant entangled photon.