Integrated Room Temperature Single Photon Source in Hexagonal Boron Nitride for Quantum Key Distribution

Single photons are the key ingredient for many photonic quantum technologies including quantum key distribution and measurement-based quantum computing. However, it remains difficult to create devices with the appropriate specifications for use in non-laboratory environments. The optical microcavity platform provides an attractive route towards a room temperature single photon source device. Our ultra-small focused ion beam (FIB) milled open-access cavities offer enhancement of the spontaneous emission rate, tunability of the emission spectrum and increased light collection. The embedment of solid-state emitters within these cavities enables us to create a robust room temperature single photon source device, with the potential for high efficiencies and single photon purities. Defects such as the nitrogen-vacancy (NV) centre in diamond have been shown to be stable room-temperature sources of single photons. There are new single emitters emerging in two-dimensional materials such as hexagonal boron nitride (hBN). Here we present developments in room-temperature coupling of single defects to open-access microcavities of a planar-hemispherical geometry with mode volumes down to λ3. We report enhancements in the spectral density of photons into a single cavity mode, combined with improved single photon purities. It will be shown that the NV-cavity system provides a ~3% single photon emission efficiency with purities of up to 94%. The hBN-cavity system provides count rates >1Mcts/s into a single cavity mode with purities up to 96%. With these high single photon purities, such devices would be robust against photon number splitting attacks making them attractive for applications in quantum cryptography.

Quantum key distribution (QKD) offers a secure means of communication based on the laws of quantum physics. Despite its remarkable advancements, the current reliance of QKD on attenuated coherent (laser) light sources has imposed a fundamental limit on the secure key rate (SKR) per channel use. This constraint stems from the scarcity of single-photon components within coherent light, inherently bounded by a maximum of 1/e. Here, we report comprehensive demonstrations of single-photon-source-based high-rate QKD, surpassing the fundamental SKR limit imposed by the weak coherent light. By employing an on-demand, bright single-photon source with an efficiency of 0.71(2), coupled with narrow-bandwidth filtering and random polarization modulation, we demonstrated a field QKD trial over a 14.6(1.1)dB-loss free-space urban channel, achieving a SKR of 1.08×10^{-3} bits per pulse. This SKR surpasses the practical limit of weak coherent-light-based QKD by 79%. These findings unequivocally demonstrate the superior performance of single-photon sources over weak coherent light for QKD applications, marking a pivotal stride towards realizing a global quantum internet.

Single photon sources are essential building blocks for fundamental quantum optics, and quantum information networks. Their widespread use is currently hindered by impractical features, such as the need for operation at cryogenic temperatures and emission wavelengths lying outside telecom windows. Taking advantage of telecom technology and point defects in GaN crystals, we present, for the first time, the development of a fully-fibered source of single photons operating at room temperature, emitting photons in the telecom O-band and fulfilling the standards of telecom photonics. We characterise an emitter producting single photons at the wavelength of 1292 nm, a spectral broadening compatible with CWDM channels of 13 nm, and a brightness of 25 kcps per mW of pump power. The source shows a signal-to-noise ratio of 16.5 and a purity g(2)(0) = (5.9 ± 0.5) × 10-2 at room temperature, showing high potential for being integrated transportable quantum cryptography devices.

Stable and bright single photon sources are key components for future quantum applications. A simple fabrication method is an important requirement for such sources. Here, we present a single photon source based on diced ridge waveguides in titanium indiffused LiNbO 3 . These waveguides can be easily fabricated by combining planar titanium in-diffusion without lithographic patterning and easy-to-handle precision dicing. Such devices have the potential to generate high single photon rates because ridge structures are typically less prone to the photorefractive effect. We achieve waveguide propagation losses <0.4dBcm and a SHG conversion efficiency of about 81%Wcm 2 . Harnessing a type-0 SPDC process to generate 1550 nm photons, we obtain a SPDC brightness of 3⋅10 5 1s⋅mW⋅nm, with a heralding efficiency of η h =45% ( η h,wg =77.5% for the waveguide itself excluded setup losses) and a heralded second-order correlation function of g h 2 (0)<0.003 at low pump powers.

High-quality quantum light sources are crucial components for the implementation of practical and reliable quantum technologies. The persistent challenge, however, is the lack of scalable and deterministic single photon sources that can be synthesized reproducibly. Here, we present a combination of droplet epitaxy with selective area epitaxy to realize the deterministic growth of single quantum dots in nanowire arrays. By optimization of the single quantum dot growth and the nanowire cavity design, single emissions are effectively coupled with the dominant mode of the nanowires to realize Purcell enhancement. The resonance-enhanced quantum emitter system boasts a brightness of millions of counts per second with nanowatt excitation power, a short radiation lifetime of 350 ± 5 ps, and a high single-photon purity with g(2)(0) value of 0.05 with continuous wave above-band excitation. Finite-difference time-domain (FDTD) simulation results show that the emissions of single quantum dots are coupled into the TM01 mode of the nanowires, giving a Purcell factor ≈ 3. Our technology can be used for creating on-chip scalable single photon sources for future quantum technology applications including quantum networks, quantum computation, and quantum imaging.

Lanthanoid-doped Gallium Nitride (GaN) semiconductors are a promising class of materials with the potential to be used across a range of novel quantum optelectronic all-locations including room temperature single photon sources. However, to date there is relatively little understood about the properties of small lanthanoid ensembles, which is required before this class of materials can reach their full potential. This paper reports room temperature optical properties of submicron extent implanted-Praseodymium (Pr) in GaN. Thermal annealing at 1200 °C is conducted for the Pr implanted GaN using SiN cap technique to activate implanted-Pr as luminescent centers. The photoluminescence shows two main peaks at 650.3 and 652.0 nm originating from the 3P0 → 3F2 transition in 4f-shell of Pr3+. We explore the saturation behavior of the photoluminescence intensity. The implanted-Pr activation ratio in a 100 nm × 100 nm region is estimated to be 1.7% in the case of 532 nm excitation.

We apply an InGaAs quantum dot based single-photon source for the absolute detection efficiency calibration of a silicon single-photon avalanche diode operating in Geiger mode. The single-photon source delivers up to (2.55 ± 0.02) × 106 photons per second inside a multimode fiber at the wavelength of 929.8 nm for above-band pulsed excitation with a repetition rate of 80 MHz. The purity of the single-photon emission, expressed by the value of the 2nd order correlation function g(2)(τ = 0), is between 0.14 and 0.24 depending on the excitation power applied to the quantum dot. The single-photon flux is sufficient to be measured with an analog low-noise reference detector, which is traceable to the national standard for optical radiant flux. The measured detection efficiency using the single-photon source remains constant within the measurement uncertainty for different photon fluxes. The corresponding weighted mean thus amounts to 0.3263 with a standard uncertainty of 0.0022.

The authors present a comparison between the basic BB84 Quantum Key Distribution (QKD) and its optimised version based on Quantum Dense Coding, in terms of theoretical modelling and feasibility of circuits implementations. These circuits are mainly composed of probabilistic quantum gates, single photon sources, beam splitters (BSs), polarising BS and single photon detectors. They assume that the photonic devices composing the circuits are not perfect. They assess the average success probabilities of the two protocols depending on the success probability of the photonic sources used and the distance between Alice and Bob. They demonstrate that typical real-world photonic devices permit the implementation of secure communication with 0.65 of success probability over 120 km, while it could reach more than 0.9 over 148 km if just deterministic sources are provided.

Hexagonal boron nitride is a wide band gap semiconductor exhibiting various luminescence bands in visible and near ultraviolet range, which can be used as single photon source. The luminescence band with zero phonon line at 4.1 eV is commonly ascribed to the carbon impurity introduced during crystal growth. In this paper we provide experimental evidence that carbon-related luminescent centers can be introduced in hBN by local electron irradiation in the chamber of scanning electron microscope at room temperature that can be used as a technique for the nanofabrication of single photon source devices with desired pattern.

Single-photon sources (SPSs) are essential for quantum technologies. Colloidal quasi two-dimensional (2D) quantum wells (QWs) possess high emission uniformity, photoluminescence quantum yields (PLQYs), and narrow linewidths at room temperature, all contributing to ideal SPSs. However, their large lateral dimensions make excitons sensitive to environments and enhance multiexciton emission, casting long-standing doubt on their viability as SPSs. Here, we demonstrate bright room-temperature single-photon emission (SPE) with high purity and minimal blinking from in-plane-engineered (IPE) 2D QWs incorporating a doubly gradient architecture. A compositionally graded CdSe/CdSexS1-x core in the in-plane direction tailors electron-hole wavefunctions to control multiexciton Auger dynamics, while a graded CdyZn1-yS shell in the thickness direction suppresses interfacial strain and nonradiative defects. This design unites 0D and 2D structural advantages, achieving near-unity ensemble PLQY and bright SPE (0.6-1.2 × 105 counts/s, NA = 0.65). Controlling the CdSe core size via IPE, we attain 92% single-photon purity (2 nm core), and suppressed blinking with a 96.9% ON-time fraction (8 nm core). This work establishes deterministic design rules for colloidal 2D QWs as high-performance SPSs for scalable quantum technologies.

Photoblinking and photobleaching are commonly encountered problems for single-photon sources. Numerous methods have been devised to suppress these two impediments; however, either the preparation procedures or the operating conditions are relatively harsh, making them difficult to apply to practical applications. Here, we reported giant suppression of both photoblinking and photobleaching of a single fluorescent molecule, terrylene, via the utilization of hexagonal boron nitride (h-BN) flakes as substrates. Experimentally, a much-prolonged survival time of terrylene has been determined, which can have a photostable emission over 2 h at room temperature under ambient atmospheres. Compared with single molecules on a SiO2/Si substrate or glass coverslip, a more than 100-fold increase in the total number of photons collected from each terrylene on h-BN flakes has been demonstrated. We also proved that the photostability of terrylene molecules can be well maintained for more than 6 months even under ambient conditions without any further protection. Our results demonstrate that the utilization of h-BN flakes to suppress photoblinking and photobleaching of fluorescent molecules has promising applications in the production of high-quality single-photon sources at room temperature.

A demonstration of the principles of quantum key distribution (QKD) is performed using a single-photon source in a proof of concept test-bed over a distance of 2 km in standard telecommunications optical fiber. The single-photon source was an optically-pumped quantum dot in a microcavity emitting at a wavelength of 895 nm. Characterization of the QKD parameters was performed at a range of different optical excitation powers. An investigation of the effect of varying the optical excitation power of the quantum dot microcavity on the quantum bit error rate and cryptographic key exchange rate of the system are presented.

Single-photon sources and their optical spin readout are at the core of applications in quantum communication, quantum computation, and quantum sensing. Their integration in photonic structures such as photonic crystals, microdisks, microring resonators, and nanopillars is essential for their deployment in quantum technologies. While there are currently only two material platforms (diamond and silicon carbide) with proven single-photon emission from the visible to infrared, a quantum spin–photon interface, and ancilla qubits, it is expected that other material platforms could emerge with similar characteristics in the near future. These two materials also naturally lead to monolithic integrated photonics as both are good photonic materials. While so far the verification of single-photon sources was based on discovery, assignment and then assessment and control of their quantum properties for applications, a better approach could be to identify applications and then search for the material that could address the requirements of the application in terms of quantum properties of the defects. This approach is quite difficult as it is based mostly on the reliability of modeling and predicting of color center properties in various materials, and their experimental verification is challenging. In this paper, we review some recent advances in an emerging material, low-dimensional (2D, 1D, 0D) hexagonal boron nitride (h-BN), which could lead to establishing such a platform. We highlight the recent achievements of the specific material for the expected applications in quantum technologies, indicating complementary outstanding properties compared to the other 3D bulk materials.

Quantum key distribution (QKD) enables long‐distance secret keys transmission with information‐theoretic security. However, weak‐randomness obviously limits the security of practical QKD systems. This may leak the key information by considering the imperfect devices. In this work, we proposed a practical security analysis model that simultaneously considers the bit‐weak‐randomness and the basis‐weak‐randomness parameters, which we refer to as the weak‐randomness model. First, we establish a novel bit‐weak‐randomness model based on the virtual mutually unbiased bases method. Subsequently, under the framework of information‐theoretic security proof, we proposed a comprehensive weak‐randomness model by incorporating the basis‐weak‐randomness model. Then, we characterize the relationship between typical quantum attacking strategies and weak‐randomness parameters. Based on our proposed weak‐randomness model, we perform a security analysis against these attacks. Finally, we perform numerical simulations of the weak‐randomness model based on the advantage distillation method, considering both single photon sources and weak coherent sources. Our results can provide a model capable of characterizing security parameters to simultaneously account for the impacts of both imperfect devices and quantum hacking. This enables a unified standard for evaluating the security levels of practical QKD systems and offers a theoretical foundation for QKD system testing and assessment.

Quantum cryptography, and especially quantum key distribution (QKD), is steadily progressing to become a viable tool for cryptographic services. In recent years we have witnessed a dramatic increase in the secure bit rate of QKD, as well as its extension to ever longer fibre- and air-based links and the emergence of metro-scale trusted networks. In the foreseeable future even global-scale communications may be possible using quantum repeaters or Earth–satellite links. A handful of start-ups and some bigger companies are already active in the field. The launch of an initiative to form industrial standards for QKD, under the auspices of the European Telecommunication Standards Institute, described in the paper by Laenger and Lenhart in this Focus Issue, can be taken as a sign of the growing commercial interest.