Free-space Communication Research Articles

An Overview of Geometrical Optics Restricted Quantum Key Distribution.

Quantum key distribution (QKD) assures the theoretical information security from the physical layer by safely distributing true random numbers to the communication parties as secret keys while assuming an omnipotent eavesdropper (Eve). In recent years, with the growing applications of QKD in realistic channels such as satellite-based free-space communications, certain conditions such as the unlimited power collection ability of Eve become too strict for security analysis. Thus, in this invited paper, we give a brief overview of the quantum key distribution with a geometrical optics restricted power collection ability of Eve with its potential applications.

Controlling polarization entanglement in biphotons generated with partially spatially coherent pump beam

We theoretically study the polarization entanglement and the non-local correlations manifested by biphotons using measures as concurrence and CHSH Bell test, generated in the spontaneous parametric down-conversion (SPDC) process for partially spatially coherent pump, affected by the spatial walk-off effects of the extraordinary beam. The study has been carried out for a well-known scheme for generating polarization entanglement utilizing non-collinear, frequency degenerate SPDC process in paired type-I uniaxial crystals. It is found that the polarization entanglement changes drastically with variation in spatial coherence of the pump, offering a possibility to realize a tunable source of polarization entanglement. The study reveals the overriding role of aperture size in compensation of the effect of spatial coherence of pump on the polarization entanglement and non-local correlations, especially in the conservation of polarization entanglement for incoherent and partially coherent pump beams. Free-space quantum secure communication utilizing these quantum states possessing maximal entanglement and partially spatially coherent properties would essentially offer robustness to the losses on propagation through atmospheric turbulence, resulting in a significant enhancement in the key rate.

Influence of Microbial Agglomerated Particles on the Performance of Free-Space Quantum Communication

Influence of Microbial Agglomerated Particles on the Performance of Free-Space Quantum Communication

Performance investigation of diffractive optical elements effect and rain attenuation on BER of an optical free space communication based system

Performance investigation of diffractive optical elements effect and rain attenuation on BER of an optical free space communication based system

Private communication with quantum cascade laser photonic chaos

Mid-infrared free-space optical communication has a large potential for high speed communication due to its immunity to electromagnetic interference. However, data security against eavesdroppers is among the obstacles for private free-space communication. Here, we show that two uni-directionally coupled quantum cascade lasers operating in the chaotic regime and the synchronization between them allow for the extraction of the information that has been camouflaged in the chaotic emission. This building block represents a key tool to implement a high degree of privacy directly on the physical layer. We realize a proof-of-concept communication at a wavelength of 5.7 μm with a message encryption at a bit rate of 0.5 Mbit/s. Our demonstration of private free-space communication between a transmitter and receiver opens strategies for physical encryption and decryption of a digital message.

Investigation of 1064-nm Pumped Type II SPDC in Potassium Niobate for Generation of High Spectral Purity Photon Pairs

The generation and detection of nonclassical light of about 2 μm has good potential in an emerging field of high-sensitivity metrology, especially gravitational wave detection, as well as free-space quantum communication. A pair of photons is generated through a spontaneous parametric down-conversion (SPDC) process in a nonlinear optic crystal, which can be properly entangled in a spatial region where two beams with each polarization overlap or in a Sagnac-loop interferometer configuration. We investigated theoretically and numerically Type II SPDC in a potassium niobate (KNbO3, KN) crystal, which is useful as a material platform for generating photon pairs of high spectral purity in the 2-μm range. The technique is based on the frequency degenerate SPDC under Type II extended phase matching (EPM). We described the EPM characteristics of KN and showed that it is practically feasible for a 1064-nm pumped SPDC under moderate temperature conditions. The effective nonlinear optic coefficient of KN is at least four-times larger than those of other crystals using the Type II EPM approach, which implies a significant improvement in SPDC efficiency. The joint spectral analysis showed that a pair of photons can be generated with a high purity of 0.995 through proper pump filtering.

Satellite quantum communications: Fundamental bounds and practical security

Satellite quantum communications are emerging within the panorama of quantum technologies as a more effective strategy to distribute completely-secure keys at very long distances, therefore playing an important role in the architecture of a large-scale quantum network. In this work, we apply and extend recent results in free-space quantum communications to determine the ultimate limits at which secret (and entanglement) bits can be distributed via satellites. Our study is comprehensive of the various practical scenarios, encompassing both downlink and uplink configurations, with satellites at different altitudes and zenith angles. It includes effects of diffraction, extinction, background noise and fading, due to pointing errors and atmospheric turbulence (appropriately developed for slant distances). Besides identifying upper bounds, we also discuss lower bounds, i.e., achievable rates for key generation and entanglement distribution. In particular, we study the composable finite-size secret key rates that are achievable by protocols of continuous variable quantum key distribution, for both downlink and uplink, showing the feasibility of this approach for all configurations. Finally, we present a study with a sun-synchronous satellite, showing that its key distribution rate is able to outperform a ground chain of ideal quantum repeaters.

Generalized efficiency mismatch attack to bypass the detection-scrambling countermeasure.

The ability of an eavesdropper to compromise the security of a quantum communication system by changing the angle of the incoming light is well-known. Randomizing the role of the detectors has been proposed to be an efficient countermeasure to this type of attack. Here we show that the proposed countermeasure can be bypassed if the attack is generalized by including more attack variables. Using the experimental data from existing literature, we show how randomization effectively prevents the initial attack but fails to do so when Eve generalizes her attack strategy. Our result and methodology could be used to scrutinize a free-space quantum communication receiver against detector-efficiency-mismatch type attacks.

Transmission characters of wide-spectrum OAM beam in tunable atmospheric turbulence

In this paper, a wide-spectrum orbital angular momentum (OAM) based free space communication system is experimentally demonstrated. The wide-spectrum laser can be generated using narrow-linewidth pulses to pump nonlinear fiber and the OAM beam can be generated with spatial light modulator (SLM). In the experiment, a controllable atmospheric turbulence simulation device was used to ensure a unified test environment. The results show that with the combination of wide-spectrum laser and OAM beam, the received sensitivity can achieve a 4.23 dB gain compared with narrow-linewidth Gaussian beam-based communication system in weak atmospheric turbulence environment.

Se-doped NH2-functionalized graphene quantum dot for single-photon emission at free-space quantum communication wavelength

Se-doped NH2-functionalized graphene quantum dot for single-photon emission at free-space quantum communication wavelength

Integrated Optical Phased Arrays for Beam Forming and Steering

Integrated optical phased arrays can be used for beam shaping and steering with a small footprint, lightweight, high mechanical stability, low price, and high-yield, benefiting from the mature CMOS-compatible fabrication. This paper reviews the development of integrated optical phased arrays in recent years. The principles, building blocks, and configurations of integrated optical phased arrays for beam forming and steering are presented. Various material platforms can be used to build integrated optical phased arrays, e.g., silicon photonics platforms, III/V platforms, and III–V/silicon hybrid platforms. Integrated optical phased arrays can be implemented in the visible, near-infrared, and mid-infrared spectral ranges. The main performance parameters, such as field of view, beamwidth, sidelobe suppression, modulation speed, power consumption, scalability, and so on, are discussed in detail. Some of the typical applications of integrated optical phased arrays, such as free-space communication, light detection and ranging, imaging, and biological sensing, are shown, with future perspectives provided at the end.

Mid-infrared homodyne balanced detector for quantum light characterization.

We present the characterization of a novel balanced homodyne detector operating in the mid-infrared. The challenging task of revealing non-classicality in mid-infrared light, e. g. in quantum cascade lasers emission, requires a high-performance detection system. Through the intensity noise power spectral density analysis of the differential signal coming from the incident radiation, we show that our setup is shot-noise limited. We discuss the experimental results with a view to possible applications to quantum technologies, such as free-space quantum communication.

2 Gbps free-space ultraviolet-C communication based on a high-bandwidth micro-LED achieved with pre-equalization.

In this Letter, we experimentally achieve high-speed ultraviolet-C (UVC) communication based on a 276.8 nm UVC micro-LED. A record ${-}{{3}}\;{\rm{dB}}$ optical bandwidth of 452.53 MHz and light output power of 0.854 mW at a current density of ${{400}}\;{\rm{A/c}}{{\rm{m}}^2}$ are obtained with a chip size of 100 µm. A UVC link over 0.5 m with a data rate of 2 Gbps is achieved using 16-ary quadrature amplitude modulation orthogonal frequency division multiplexing and pre-equalization, and an extended distance over 3 m with a data rate of 0.82 Gbps is also presented. The demonstrated high-speed performance shows that micro-LEDs have great potential in the field of UVC communication.

Insensitivity of higher order topologically charged Laguerre–Gaussian beams to dynamic turbulence impact

The propagation characteristics of topologically charged Laguerre–Gaussian​ beams through a dynamic (rotating) pseudo random phase plate mimicking atmospheric turbulence, like beam wandering, scintillation etc are measured in laboratory level. Our findings clearly show that the beam wandering and scintillation effects are higher for Laser Gaussian beams compared to topologically charged Laguerre–Gaussian beams. Also, the impact of turbulence reduces considerably when the topological charges of LG beams increase. This may help to use higher order topologically charged Laguerre–Gaussian beams for optical communications in free space.

Study of Type II SPDC in Lithium Niobate for High Spectral Purity Photon Pair Generation

Recent advances of high-quality lithium niobate (LN) on insulator technology have revitalized the progress of novel chip-integrated LN-based photonic devices and accelerated application research. One of the promising technologies of interest is the generation of entangled photon pairs based on spontaneous parametric down-conversion (SPDC) in LNs. In this paper, we investigated, theoretically and numerically, Type II SPDC in two kinds of LNs—undoped and 5-mol% MgO doped LNs. In each case, both non-poled and periodically poled crystals were considered. The technique is based on the SPDC under Type II extended phase matching, where the phase matching and the group velocity matching are simultaneously achieved between interacting photons. The proposed approach has not yet been reported for LNs. We discussed all factors required to generate photon pairs in LNs, in terms of the beam propagation direction, the spectral position of photons, and the corresponding effective nonlinearities and walk-offs. We showed that the spectral positions of the generated photon pairs fall into the mid-infrared region with high potential for free-space quantum communication, spectroscopy, and high-sensitivity metrology. The joint spectral analyses showed that photon pairs can be generated with high purities of 0.995–0.999 with proper pump filtering.

Statistical model for the weak turbulence-induced attenuation and crosstalk in free space communication systems with orbital angular momentum

A novel statistical model connected with turbulence strength is proposed to describe the attenuation and crosstalk in a vortex-based multi-channel free space optical (FSO) communication system. In this model, self-channel fading and interference between different orbital angular momentum (OAM) modes are characterized by the mixture exponential-generalized-gamma (EGG) distribution, and the analytical relations between turbulence strength and the distribution function's parameters are expressed by piecewise functions. The problems of obtaining parameters of this model are converted into optimization problems, and the algorithms based on the trust trigon algorithm are proposed to achieve more optimized parameters. This model is confirmed to have a good fit with the emulated data of OAM attenuation and crosstalk calculated by the square of the scalar product between the fields of two OAM modes. Furthermore, the application of the statistical model to the OAM-multiplexing FSO system with quadrature-phase-shift-keying modulation is presented, in which the theoretical average bit-error rate results match well with Monte Carlo simulation. This model can be used for FSO system design with OAM for continuous weak turbulence condition.

A metasurface performance enhancer

Metasurfaces The detection and emission of light plays a key role in applications such as free-space communications, biosensing, and optical imaging. Fluorescent dye molecules or nanoparticles in a cavity are typically used for photodetection but can be limited in sensitivity and efficiency. Traverso et al. fabricated a low-loss plasmonic metasurface made from silver nanocubes scattered across the surface of the cavity. Modification of the local optical density of states results in a device that can collect fast-modulated light (>14 gigahertz) with a high acceptance angle (120°) and convert it to a directional source with enhanced overall efficiency (>30%). The approach could be incorporated into the design of other ultrafast optoelectronic components to enhance performance. Optica 8 , 202 (2021).

Limits and security of free-space quantum communications

The study of free-space quantum communications requires tools from quantum information theory, optics and turbulence theory. Here we combine these tools to bound the ultimate rates for key and entanglement distribution through a free-space link, where the propagation of quantum systems is generally affected by diffraction, atmospheric extinction, turbulence, pointing errors, and background noise. Besides establishing ultimate limits, we also show that the composable secret-key rate achievable by a suitable (pilot-guided and post-selected) coherent-state protocol is sufficiently close to these limits, therefore showing the suitability of free-space channels for high-rate quantum key distribution. Our work provides analytical tools for assessing the composable finite-size security of coherent-state protocols in general conditions, from the standard assumption of a stable communication channel (as is typical in fiber-based connections) to the more challenging scenario of a fading channel (as is typical in free-space links).

Monolithic infrared silicon photonics: The rise of (Si)GeSn semiconductors

(Si)GeSn semiconductors are finally coming of age after a long gestation period. The demonstration of device-quality epi-layers and quantum-engineered heterostructures has meant that tunable all-group IV Si-integrated infrared photonics is now a real possibility. Notwithstanding the recent exciting developments in (Si)GeSn materials and devices, this family of semiconductors is still facing serious limitations that need to be addressed to enable reliable and scalable applications. The main outstanding challenges include the difficulty to grow high-crystalline quality layers and heterostructures at the desired content and lattice strain, preserve the material integrity during growth and throughout device processing steps, and control doping and defect density. Other challenges are related to the lack of optimized device designs and predictive theoretical models to evaluate and simulate the fundamental properties and performance of (Si)GeSn layers and heterostructures. This Perspective highlights key strategies to circumvent these hurdles and hopefully bring this material system to maturity to create far-reaching opportunities for Si-compatible infrared photodetectors, sensors, and emitters for applications in free-space communication, infrared harvesting, biological and chemical sensing, and thermal imaging.

Towards combined quantum bit detection and spatial tracking using an arrayed single-photon sensor.

Experimental quantum key distribution through free-space channels requires accurate pointing-and-tracking to co-align telescopes for efficient transmission. The hardware requirements for the sender and receiver could be drastically reduced by combining the detection of quantum bits and spatial tracking signal using two-dimensional single-photon detector arrays. Here, we apply a two-dimensional CMOS single-photon avalanche diode detector array to measure and monitor the single-photon level interference of a free-space time-bin receiver interferometer while simultaneously tracking the spatial position of the single-photon level signal. We verify an angular field-of-view of 1.28° and demonstrate a post-processing technique to reduce background noise. The experimental results show a promising future for two-dimensional single-photon detectors in low-light level free-space communications, such as quantum communications.