In this paper, we focus on quantum communication systems that facilitate either secure data transfer or quantum key distribution via free-space links. Unlike classical channels where the effects of turbulent media on the optical wave front is well known and can be predicted with existing theoretical models, the mechanism described in the latter cannot be directly applied to quantum states. In our approach that relies on emitting correlated photon pairs with polarization entanglement, another realm of problems is encountered, which is not related to wave front distortions, but rather to integrity of the quantum states. Proper response of the detection system to non-classical features of light requires that photon pairs with proper polarization arrive to the receiver and their correlation characteristics are still preserved. Therefore, it is necessary to research a wide array of operating conditions corresponding to different levels of turbulence and finding proper mechanisms to replicate those on our laboratory testbed. In this paper, we present a system that integrates an atmospheric chamber developed by the AFRL, a link emulating quantum communication and analysis instrumentation. A system is developed that allows scaling the experiments over different ranges and quantitative analysis of entanglement characteristics of the received signals. Integrity of the quantum states is evaluated under practical operating conditions.
Quantum entanglement is an essential element for building the backbone of quantum information systems. Our particular interest lies in long-range distribution of entangled photons to facilitate secure data transfer in free space. To achieve this, we rely on photon pairs generated in such a way that their polarization characteristics represent the corresponding qubit states and have a high degree of correlation in measurements. The main focus of this paper is integrity of the quantum states in free-space channels. When transmitted in atmosphere, classical signals suffer from wave front distortions caused by the spatial and temporal fields of the refractive index. However, this mechanism does not have the same bearing on qubit values and their correlation. We study the effects of turbulence on quantum states by utilizing a laboratory testbed that includes an atmospheric chamber developed by the AFRL. It uses a system of controlled components capable of creating various turbulence conditions. When polarized signals are passed through the atmospheric chamber, we analyze the corresponding quantum states and evaluate the degree of entanglement using our mathematical models and existing metrics.
Free-Space Optical (FSO) communication holds the potential for data communications at high bandwidths security while minimizing size, weight, and power (SWAP). However, the effects of atmospheric turbulence on an optical beam during propagation limits and degrades communication performance and bit-error-rate. Although degradation of beam quality occurs due to many factors, typically unwanted aberrations due to fluctuations in the refractive index n along beam path causing scattering, absorption, and beam wander is the main cause. Randomly distributed cells called eddies are formed in the propagating medium giving rise to turbulence as well. In this paper, we report experimental results from a 3-meter FSO data link. An intensity modulated 10 Gbps beam in the next phase will be analyzed and correlated to real time weather. We study scintillations and deviation of the beam from its original path (beam wander and spread). A phosphor-coated silicon CCD is used to record and study the beam’s intensity profile.
The primary focus of this paper is high-performance quantum communication systems that facilitate secure data transfer via free-space links. We consider an approach that uses correlated photon pairs generated in such a way that their polarizations are entangled and can be used to support quantum encryption protocols. However, when deployed in free space, these links can be affected by channel distortion, primarily via the spatial and temporal fields of the refractive index along the propagation path. In classical links, these fields alter the optical wave front characteristics; however, this mechanism does not directly apply to the quantum states utilized in single-photon or entangled photon protocols. Transmitting signals with quantum-based encryption creates a realm of problems, not related to wave front distortions, but rather to integrity of the quantum states after the signals propagate over free-space channels. We study these phenomena by implementing a laboratory testbed capable of creating a turbulent environment using atmospheric chambers developed by the AFRL. It is then used for experimental investigation of quantum entanglement after photon pairs are propagated both collinearly and via separate paths.
Free-space optical communication (FSOC) holds unmatched potential for high bandwidth and secure communications while minimizing size, weight, and power (SWAP). However, the effects of atmospheric scintillations on high bandwidth signals limits data link performance by degrading OSNR (Optical signal-to-noise ratio) and Q-factor. A critical component due to which a communication signal quality deteriorates is timing jitter. Jitter may be due to timing of the data signal or it may be due to the amplitude variations in the data bit stream as it propagates through free-space. As the data bandwidth increases, these effects become more significant. A small-time deviation in a lower data rate signal which would be tolerable or be above a receiver sensitivity, turns into an intolerable signal at higher data rates as jitter increases. The total jitter (TJ) can be further broken down to deterministic jitter (DJ) and random jitter (RJ). These may help understand signal behavior and the root cause of degradation in a FSOC or any data communication link. Thus, for a system to achieve desired BER (bit-error-rate and bit-error-ratio), an in-depth analysis of jitter by investigating each of the subclass of both timing jitters, DJ and RJ, would be extremely helpful and enhance the robustness of the link. In this paper, we report in-depth jitter analysis from a FSOC data link at 10 Gbps propagating at 1550 nm.
Quantum entanglement is critical to build the backbone of all quantum technologies. Quantum networks, quantum computations, and quantum communication networking is based on long-range distribution of entangled photons and teleportation of photon qubit states. In order to understand quantum entanglement, characterization of atmospheric turbulence and its effects on propagating quantum states in free-space is essential. One method of photon entanglement is using a photon’s polarization. In this paper, we report results using polarization entangled signal and idler photons. The results may be applicable to support various quantum computing, encryption, and other qubit based high-performance communication protocols. Classically, the degradation of beam quality occurs due to many factors but primarily due to the distortion of spatial and temporal fields of refractive index. However, behavior of single photons through similar turbulent media creates a different set of challenges pointing to integrity of quantum states during propagation. We study this behavior by analyzing quantum states and the degree of entanglement in real-time and correlating it to known atmospheric models, (refractive index structure parameter), and relevant propagation path parameters. This experimental study was performed initially in a controlled laboratory environment, and then devised to be implemented outdoors over a 100-meter free space communication link.
KEYWORDS: Free space optics, Data communications, Signal attenuation, Scintillation, Turbulence, Eye, Free space optical communications, Atmospheric turbulence, Atmospheric propagation, Receivers
Free-space optical communication (FSOC) holds an unmatched potential for data communications with high bandwidth and security while minimizing size, weight, and power (SWAP). However, the effects of atmospheric turbulence on an optical wave during propagation limits and degrades communication performance. Although this degradation of beam quality occurs due to many factors, but unwanted aberrations due to scattering and absorption of the propagating electromagnetic wave is typically the primary cause. In this paper, we report experimental results from a free space optical FSO communication data link. Bandwidth up to 10 Gbps at 1550 nm is correlated to 𝑐𝑛2 (refractive index structure parameter), transmission wavelength, transmit and receive parameters. For further random and data dependent analysis, the communication link's transmission and receive data eye with amplitude and time jitter decomposition is performed using multiple NRZ PRBS patterns. Additionally, with the aim of reducing SWAP and cost, the experiment is built and designed mostly using off-the-shelf long-range single mode, small factor pluggable devices.
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