This paper presents a theoretical investigation and experimental analysis of wavefront sensors based on the ShackHartmann configuration and the Talbot effect. A comparative analysis of their characteristics indicates that the Talbot effectbased wavefront sensor is comparable to the Shack-Hartmann scheme in terms of input resolution, measurement frequency, and accuracy. The experiment demonstrates the feasibility of employing the Talbot effect-based wavefront sensor in adaptive optics systems for atmospheric and space optical communication channels, including those utilizing quantum key distribution technology.
This paper presents a theoretical investigation and experimental analysis of wavefront sensors based on the ShackHartmann configuration and the Talbot effect. A comparative analysis of their characteristics indicates that the Talbot effectbased wavefront sensor is comparable to the Shack-Hartmann scheme in terms of input resolution, measurement frequency, and accuracy. The experiment demonstrates the feasibility of employing the Talbot effect-based wavefront sensor in adaptive optics systems for atmospheric and space optical communication channels, including those utilizing quantum key distribution technology.
A major field of research at the current time is that of implementing Quantum Key Distribution over large distances using satellites. If this protocol works with this technology, it will have huge implications on future information security. In order for a satellite to implement this idea, there are many aspects that must be taken into account. One of the big issues that comes up for this type of system is that of propagating light through the turbulent atmosphere and its effects on the acquisition, pointing and tracking system. The projects studied in this thesis study some of the effects of the atmosphere on certain detectors, try to develop pointing schemes for better accuracy as well as develop knowledge in free space propagation of other single photon experiments. \n \nIn the first experiment, I study the spatial correlations of the daughter photons created in spontaneous parametric down conversion. I look at the effect of altering the pump beam on the positions of the down converted photons and see if the pump can be manipulated in a way to control the directions of the daughter photons. I begin to utilize a deformable mirror and Shack-Hartmann wavefront sensor which are generally used in adaptive optics, but we plan to use them to alter the pump beam in the spontaneous parametric down conversion process to analyze the correlations between the pump and down converted photons. \n \nThe second experiment investigates the effects of laser scintillation on the performance of a possible tracking device that could be implemented on a satellite. This quad sensor tracks the position of a beam and a system will be developed to move the sensor to keep the beam in the center where there is a hole for the quantum single photons to stream through. In order to create the effects of scintillation, a turbulence simulator box was built and characterized. This box combines wind turbulence with a heat gradient to mimic atmospheric turbulence on a small scale. \n \nFinally, my contributions to a large scale, long distance free space quantum optics experiment are explained and the overall goal of the experiment is discussed. This experiment exposed me to actual free space transmission issues as well as many fundamental techniques for performing long distance optics experiments. In this experiment there was no correction for atmospheric turbulence, but in the future, techniques could be implemented which might increase the efficiencies of the free space links.
The task of wavefront sensing is to measure the phase of the optical field. Here, we demonstrate that the widely used Shack-Hartmann wavefront sensor detects the weak value of transverse momentum, usually achieved by the method of quantum weak measurement. We extend its input states to partially coherent states and compare it with the weak measurement wavefront sensor, which has a higher spatial resolution but a smaller dynamic range. Since weak values are commonly used in investigating fundamental quantum physics and quantum metrology, our work would find essential applications in these fields.
This article offers a solution to the fragmentation problem through a unified model and simulation showing how Adaptive Optics (AO) improves both classical and quantum communications. Free-space optical links can carry Quantum Key Distribution (QKD) over distances that cannot be reached with fiber; however, atmospheric turbulence corrupts the wavefront. For a single-mode quantum receiver, this drives the quantum bit error rate above the security threshold before detector noise can even be considered. AO addresses this by restoring the wavefront and recovering the coupling loss. The theory, mitigation techniques, and quantum security analysis needed to understand this are spread across three research communities that rarely cite one another: atmospheric turbulence and AO theory, classical modulation over fading channels, and BB84 quantum key distribution. We consolidate and develops all three topics in order to trace the chain from wavefront error to bit error rate and secret key rate. We survey six families of turbulence mitigation and provide an assessment of their suitability for single-mode quantum links. In addition, we survey the free-space QKD experimental record, from terrestrial hops to satellite downlinks. A wave optics case study of a representative 500-km Low Earth Orbit (LEO) downlink, corrected by a closed-loop Shack–Hartmann and deformable-mirror receiver at 1.55 μm ties the review together. Across a turbulence sweep, the correction stabilizes single-mode coupling near its physical ceiling, while the uncorrected channel varies by more than an order of magnitude. When carried into the downlink budget, this lifts the quantum link across the 11% security bound at 12 of 15 turbulence conditions where the uncorrected link yields no key, thereby restoring a secure quantum bit error rate in a narrow 4.2–4.9% band throughout and a secret key rate of 7.6–11.2 kbit/s while simultaneously widening the margin of the co-propagating classical channel above its forward error correction threshold.
Abstract Future quantum communication infrastructures will rely on both terrestrial and space-based links integrating high-performance optical systems engineered for this purpose. In space-based downlinks in particular, the loss budget and the variations in the signal propagation due to atmospheric turbulence effects impose a careful optimization of the coupling of light in single-mode fibers required for interfacing with the receiving stations and the ground networks. In this work, we perform a comprehensive study of the role of adaptive optics (AO) in this optimization, focusing on realistic baseline configurations of prepare-and-measure quantum key distribution, with both discrete and continuous-variable encoding, and including finite-size effects. Our analysis uses existing experimental turbulence datasets at both day and night time to model the coupled signal statistics following a wavefront distortion correction with AO, and allows us to estimate the secret key rate for a range of critical parameters, such as turbulence strength, satellite altitude and ground telescope diameter. The results we derive illustrate the interest of adopting advanced AO techniques in several practical configurations.
Long-range quantum communication requires sending qubits, usually photonic cubits, over long distances through free space. Photons propagating through free space encounter optical turbulence, causing scattering and entanglement degradation. In recent decades, adaptive optics has been used to correct imaging errors introduced by turbulence. An atmospheric turbulence simulator has been constructed to measure the effect of optical turbulence on single photon propagation, and an adaptive optics system has been incorporated. The turbulence simulator uses two rotating phase plates to emulate the effects of random dispersive media and two afocal systems to scale long propagation distances down to lengths realizable in a laboratory. The adaptive optics system consists of a Shack-Hartman wavefront sensor used to measure wavefront error, and a deformable mirror used to correct this error. The use of adaptive optics allows us to correct wavefront errors introduced by turbulence. Adding an adaptive optics system into this simulator facilitates more robust characterization of the strength of the simulated turbulence, as well as correction of some of the negative effects the turbulence introduces. This presentation will discuss the effects of turbulence on single photon propagation when an adaptive optics system is used and how these results compare to previously measurements taken without the use of adaptive optics, allowing us to quantify the improvement adaptive optics introduces.
We introduce an application of the Hartmann sensor, traditionally designed for wavefront sensing, to measure the coherence properties of optical signals. By drawing an analogy between the coherence matrix and the density matrix of a quantum system, we recast the sensor operation as a quantum estimation problem. We experimentally demonstrate its effectiveness in the regime where signals from different apertures significantly overlap, enabling information extraction beyond the reach of standard wavefront sensing.