Single-photon Sources Research Articles

Monolayer-Based Single-Photon Source in a Liquid-Helium-Free Open Cavity Featuring 65% Brightness and Quantum Coherence.

Solid-state single-photon sources are central building blocks in quantum information processing. Atomically thin crystals have emerged as sources of nonclassical light; however, they perform below the state-of-the-art devices based on volume crystals. Here, we implement a bright single-photon source based on an atomically thin sheet of WSe2 coupled to a tunable optical cavity in a liquid-helium-free cryostat without the further need for active stabilization. Its performance is characterized by high single-photon purity (g(2)(0) = 4.7 ± 0.7%) and record-high, first-lens brightness of linearly polarized photons of 65 ± 4%, representing a decisive step toward real-world quantum applications. The high performance of our devices allows us to observe two-photon interference in a Hong-Ou-Mandel experiment with 2% visibility limited by the emitter coherence time and setup resolution. Our results thus demonstrate that the combination of the unique properties of two-dimensional materials and versatile open cavities emerges as an inspiring avenue for novel quantum optoelectronic devices.

Graphene‐Enhanced Single Ion Detectors for Deterministic Near‐Surface Dopant Implantation in Diamond

AbstractDiamond color centers with applications to single photon sources, quantum computation, and magnetic field sensing down to the nanoscale have been investigated using ensembles of near‐surface implanted atoms. Deterministic ion implantation for ions stopping between 30 and 130 nm deep is demonstrated by configuring an electronic‐grade diamond substrate with a biased surface graphene electrode connected to charge sensitive electronics. The thin graphene electrode has a negligible surface dead layer, so implantation events are signaled from the drift of electron–hole pairs induced by the dissipation of ion kinetic energy in the substrate. Ion beam induced charge maps from a scanned 1 MeV He microbeam show the graphene electrode is highly effective with a charge collection efficiency up to 86% compared to an O‐terminated diamond surface of 30%. For lower energy ions, applicable to near surface implantation, single ion detection confidence is >99(1)% for 19.5 keV H and 85(2)% for 24 keV N ions to allow maps of device surface features by employing a focused ion beam microscope or an atomic force microscope cantilever incorporating a nanostencil beam collimator. In the near‐term, this system can be used to investigate the annealing dynamics for the conversion of single implanted ions into diamond color centers.

Free-Space and Fiber-Integrated Measurement-Device-Independent Quantum Key Distribution under High Background Noise.

Measurement-device-independent quantum key distribution (MDI QKD) provides immunity against all attacks targeting measurement devices. It is essential to implement MDI QKD in the future global-scale quantum communication network. Toward this goal, we demonstrate a robust MDI QKD fully covering daytime, overcoming the high background noise that prevents BB84 protocol even when using a perfect single-photon source. Based on this, we establish a hybrid quantum communication network that integrates free-space and fiber channels through Hong-Ou-Mandle (HOM) interference. Additionally, we investigate the feasibility of implementing HOM interference with moving satellites. Our results serve as a significant cornerstone for future integrated space-ground quantum communication networks that incorporate measurement-device-independent security.

Nonlinearity Measurement of Si Transferring Photodetector in the Low Radiation Flux Range

In order to establish a transferring chain from a photon flux of a single-photon source in quantum radiometry, the nonlinearity of the photodetector needs to be accurately measured. Using the flux superposition method, a nonlinearity measurement setup has been designed. The measurement setup consists of two tungsten halogen lamps, parent–child integrating spheres, an adjustable aperture, a diaphragm tube, and an optical filter. It has the advantage of low polarization error, low interference error, and low stray light effect. The Si photodiode to be measured is cooled to −40 °C to obtain a low noise level for low-flux radiation measurement. The nonlinearity of the Si photodetector is measured for photocurrent ranges from 10−12 A~10−6 A level, with a relative standard uncertainty from 0.0092~0.023%. The relative standard uncertainty of the nonlinearity correction factor ranged from 0.023~0.049%.

Impact of the oxidation temperature on the density of single-photon sources formed at SiO2/SiC interface

The impact of oxidation temperature on the formation of single photon-emitting defects located at the silicon dioxide (SiO2)/silicon carbide (SiC) interface was investigated. Thermal oxidation was performed in the temperature range between 900 and 1300 °C. After oxidation, two different cooling processes—cooling down in N2 or O2 ambient—were adopted. Single photon emission was confirmed with second-order correlation function measurements. For the samples cooled in an N2 ambient, the density of interface single photon sources (SPSs) increased with decreasing oxidation temperature with a density that could be controlled over the 105 to 108 cm−2 range. For the O2 cooled samples, on the other hand, many interface SPSs were formed irrespective of the oxidation temperature. This is attributed to the low-temperature oxidation during the cooling process after oxidation.

Experimental realization of a quantum classification: Bell state measurement via machine learning

The Bell state is a crucial resource for the realization of quantum information tasks, and when combined with orbital angular momentum (OAM), it enables a high-dimensional Hilbert space, which is essential for high-capacity quantum communication. In this study, we demonstrate the recognition of OAM Bell states using interference patterns generated by a classical light source and a single-photon source from a Sagnac interferometer-based OAM Bell state evolution device. The interference patterns exhibit a one-to-one correspondence with the input Bell states, providing conclusive evidence for the full recognition of OAM Bell states. Furthermore, we introduce machine learning to the field of Bell state recognition by proposing a neural network model capable of accurately recognizing higher order single-photon OAM Bell states, even in the undersampling case. In particular, the model’s training set includes interference patterns of OAM Bell states generated by classical light sources, yet it is able to recognize single-photon OAM Bell states with high accuracy, without relying on quantum resources during training. Our innovative application of neural networks to the recognition of single-photon OAM Bell states not only circumvents the resource consumption and experimental difficulties associated with quantum light sources but also facilitates the study of OAM-based quantum information.

Cavity-enhanced excitation of a quantum dot in the picosecond regime

A major challenge in generating single photons with a single emitter is to excite the emitter while avoiding laser leakage into the collection path. Ideally, any scheme to suppress this leakage should not result in a loss in the efficiency of the single-photon source. Here, we investigate a scheme in which a single emitter, a semiconductor quantum dot, is embedded in a microcavity. The scheme exploits the splitting of the cavity mode into two orthogonally-polarised modes: one mode is used for excitation, the other for collection. By linking the experiment to theory, we show that the best population inversion is achieved with a laser pulse detuned from the quantum emitter. The Rabi oscillations exhibit an unusual dependence on pulse power. Our theory describes them quantitatively, enabling us to determine the absolute population inversion. By comparing the experimental results with our theoretical model, we determine a population inversion of for optimal laser detuning. The Rabi oscillations depend on the sign of the laser-pulse detuning, a phenomenon arising from the non-trivial effect of phonons on the exciton dynamics. The exciton–phonon interaction is included in the theory and gives excellent agreement with all the experimental results.

Deterministic single-photon source in the ultrastrong-coupling regime

Deterministic single-photon source in the ultrastrong-coupling regime

Inverse design of distributed bragg reflector targeting a sharp reflectivity spectrum

Distributed Bragg reflectors (DBRs) have found applications in various fields, including optical communications, light generation, solar cells, and sensors. DBRs are also crucial in open-access Fabry‐Pérot microcavities for enhanced and spectrally tunable light-matter interactions. This document presents the inverse design, fabrication, and optical characterization of aperiodic Ta2O5/SiO2 multilayer DBR mirrors targeting a sharp reflectivity spectrum. The results were confirmed through simulations and experimental measurements. The proposed inverse design methodology can be used to develop DBRs for important technologies such as single-photon sources, polaritonic systems, and random laser generation.

Single-photon sources based on incomplete binary-tree multiplexers with optimal structure.

We consider novel types of spatially multiplexed single-photon sources based on output-extended incomplete binary-tree multiplexers containing general asymmetric routers where the construction of the multiplexers takes into account the total transmission efficiencies of the multiplexer arms at which a novel router can be added to the system. After selecting the multiplexer that outperforms the others, we identify the ranges of the loss parameters for which the application of the selected multiplexer leads to single-photon sources with higher single-photon probabilities and lower multiphoton noise than that can be achieved by using asymmetric multiplexers. We show that using the selected multiplexer is especially advantageous in the case of single-mode sources characterized by thermal statistics of the input photon pairs. We also reveal that the application of this multiplexer yields high performance single-photon sources even for suboptimal system sizes that is a typical situation in current experiments.

Field test of quantum key distribution with high key creation efficiency.

Quantum key distribution (QKD) promises unconditional security for communication. However, the random choices of the measurement basis in QKD usually result in low key creation efficiency. This drawback is overcome in the differential-phase-shift QKD, provided that each photon can be prepared in a large number of time slots with a proper waveform. In this work we develop a miniature room-temperature 1550-nm single-photon source to generate narrowband single photon in 50 time slots with a nearly optimal waveform for achieving unity key creation efficiency. By utilizing these single photons in the field test, we demonstrate the differential-phase-shift QKD with a key creation efficiency of 97%. Our work shows that the practical QKD can benefit from the narrowband single photons with controllable waveforms.

Theoretical Modeling of sp3 Quantum Defects in Boron Nitride

Atomic defects in solids are intensively investigated as promising candidates for use as single photon sources. However, except for several remarkable exceptions, the atomic structures of these defects are largely unknown. We performed density functional theory calculations to predict SP3 quantum defects in boron nitride that can behave as bright emitters. The results point to possible new designs of quantum emitters in low-dimensional BN materials.

Direct-write projection lithography of quantum dot micropillar single photon sources

We have developed a process to mass-produce quantum dot micropillar cavities using direct-write lithography. This technique allows us to achieve mass patterning of high-aspect ratio pillars with vertical, smooth sidewalls maintaining a high quality factor for diameters below 2.0 μm. Encapsulating the cavities in a thin layer of oxide (Ta2O5) prevents oxidation in the atmosphere, preserving the optical properties of the cavity over months of ambient exposure. We confirm that single dots in the cavities can be deterministically excited to create high-purity indistinguishable single photons with interference visibility (0.941 ± 0.008).

(Invited) Coupled Quantum Emitters in Carbon Nanotubes

The development of covalent diazonium functionalization of carbon nanotubes brings unique opportunities in terms of original quantum sources of light. In particular, it should be possible to create a linear ensemble of closely packed single-photon sources along the carbon nanotube backbone. The physical proximity of several dopants would make it possible to couple them simultaneous to a resonant cavity mode and to induce original dynamics and light emission properties. Here, we investigate experimentally the case of a series of four quantum emitters attached to the same carbon nanotube coupled to a high finesse fiber cavity. We explore their properties through PLE, polarization and super-localization measurements and their light emission properties using time-correlation techniques and photo-luminescence decay measurements. We show that two pairs show qualitatively different behaviors in terms of spectral diffusion and luminescence saturation, which we interpret using two models of electronically coupled or uncoupled two-level systems.

Mid-infrared Spectrally Pure Single-Photon States Generation from 22 Nonlinear Optical Crystals

We theoretically investigate the preparation of pure-state single-photon source from 14 birefringent crystals (CMTC, THI, LiIO3, AAS, HGS, CGA, TAS, AGS, AGSe, GaSe, LIS, LISe, LGS, and LGSe) and 8 periodic poling crystals (LT, LN, KTP, KN, BaTiO3, MgBaF4, PMN-0.38PT, and OP-ZnSe) in a wavelength range from 1224 nm to 11650 nm. The three kinds of group-velocity-matching (GVM) conditions, the phase-matching conditions, the spectral purity, and the Hong-Ou-Mandel interference are calculated for each crystal. This study may provide high-quality single-photon sources for quantum sensing, quantum imaging, and quantum communication applications at the mid-infrared wavelength range.

Electric and Magnetic Fields Effects in Vertically Coupled GaAs/AlxGa1−xAs Conical Quantum Dots

Vertically coupled quantum dots have emerged as promising structures for various applications such as single photon sources, entangled quantum pairs, quantum computation, and quantum cryptography. We start with a structure composed of two vertically coupled GaAs conical quantum dots surrounded by AlxGa1−x, and the effects of the applied electric and magnetic fields on the energies are evaluated using the finite element method. In addition, the effects are evaluated by including the presence of a shallow-donor impurity. The electron binding energy behavior is analyzed, and the effects on the photoionization cross-section are studied. Calculations are carried out in the effective mass and parabolic conduction band approximations. Our results show a notable dependence on the electric and magnetic fields applied to the photoionization cross-section. In general, it has been observed that both the electric and magnetic fields are useful parameters for inducing blueshifts of the resonant photoionization cross-section structure, which is accompanied by a drop in its magnitude.

Color Centers in Hexagonal Boron Nitride.

Atomically thin two-dimensional (2D) hexagonal boron nitride (hBN) has emerged as an essential material for the encapsulation layer in van der Waals heterostructures and efficient deep ultraviolet optoelectronics. This is primarily due to its remarkable physical properties and ultrawide bandgap (close to 6 eV, and even larger in some cases) properties. Color centers in hBN refer to intrinsic vacancies and extrinsic impurities within the 2D crystal lattice, which result in distinct optical properties in the ultraviolet (UV) to near-infrared (IR) range. Furthermore, each color center in hBN exhibits a unique emission spectrum and possesses various spin properties. These characteristics open up possibilities for the development of next-generation optoelectronics and quantum information applications, including room-temperature single-photon sources and quantum sensors. Here, we provide a comprehensive overview of the atomic configuration, optical and quantum properties, and different techniques employed for the formation of color centers in hBN. A deep understanding of color centers in hBN allows for advances in the development of next-generation UV optoelectronic applications, solid-state quantum technologies, and nanophotonics by harnessing the exceptional capabilities offered by hBN color centers.

Dissipative dynamics of optomagnonic nonclassical features via anti-Stokes optical pulses: squeezing, blockade, anti-correlation, and entanglement

We propose a feasible experimental model to investigate the generation and characterization of nonclassical states in a cavity optomagnonic system consisting of a ferromagnetic YIG sphere that simultaneously supports both the magnon mode and two whispering gallery modes of optical photons. The photons undergo the magnon-induced Brillouin light scattering, which is a well-established tool for the cavity-assisted manipulations of magnons as well as magnon spintronics. At first, we derive the desired interaction Hamiltonian under the influence of the anti-Stokes scattering process and then proceed to analyze the dynamical evolution of quantum statistics of photons and magnons as well as their intermodal entanglement. The results show that both photons and magnons generally acquire some nonclassical features, e.g., the strong antibunching and anti-correlation. Interestingly, the system may experience the perfect photon and magnon blockade phenomena, simultaneously. Besides, the nonclassical features may be protected against the unwanted environmental effects for a relatively long time, especially, in the weak driving field regime and when the system is initiated with a small number of particles. However, it should be noted that some fast quantum-classical transitions may occur in-between. Although the unwanted dissipative effects plague the nonclassical features, we show that this system can be adopted to prepare optomagnonic entangled states. The generation of entangled states depends on the initial state of the system and the interaction regime. The intermodal photon-magnon entanglement may be generated and pronounced, especially, if the system is initialized with low intensity even Schrödinger cat state in the strong coupling regime. The cavity-assisted manipulation of magnons is a unique and flexible mechanism that allows an interesting test bed for investigating the interdisciplinary contexts involving quantum optics and spintronics. Moreover, such a hybrid optomagnonic system may be used to design both on-demand single-photon and single-magnon sources and may find potential applications in quantum information processing.

Electric-field-induced colour switching in colloidal quantum dot molecules at room temperature.

Colloidal semiconductor quantum dots are robust emitters implemented in numerous prototype and commercial optoelectronic devices. However, active fluorescence colour tuning, achieved so far by electric-field-induced Stark effect, has been limited to a small spectral range, and accompanied by intensity reduction due to the electron-hole charge separation effect. Utilizing quantum dot molecules that manifest two coupled emission centres, we present a unique electric-field-induced instantaneous colour-switching effect. Reversible emission colour switching without intensity loss is achieved on a single-particle level, as corroborated by correlated electron microscopy imaging. Simulations establish that this is due to the electron wavefunction toggling between the two centres, induced by the electric field, and affected by the coupling strength. Quantum dot molecules manifesting two coupled emission centres may be tailored to emit distinct colours, opening the path for sensitive field sensing and colour-switchable devices such as a novel pixel design for displays or an electric-field-induced colour-tunable single-photon source.

Improve the performance of reference-frame-independent measurement-device-independent quantum key distribution with heralded single-photon sources

Improve the performance of reference-frame-independent measurement-device-independent quantum key distribution with heralded single-photon sources