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.
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.
Orbital angular momentum modes are a common choice for higher-dimensional encoding of photonic qubits. When transported across large distances, these qubits (or qudits) will pass through atmospheric turbulence and suffer aberrations leading to qubit errors, information loss, entanglement degradation, and loss of quantum behavior. In order to quantify some of these effects, we have developed an atmospheric turbulence emulator which allows us to finely control parameters that determine turbulence strength. In this paper, we describe our turbulence emulator and our methods of coupling orbital angular momentum qubits into the system. We present a quantitative demonstration of the degradation of classical orbital angular momentum modes in turbulence. Finally, we discuss ongoing efforts to measure the loss of quantum behavior in orbital angular momentum qubits due to atmospheric turbulence and the use of adaptive optics to mitigate some of the negative effects of atmospheric turbulence.
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.
To study the effects of atmospheric turbulence on the quality of entanglement of photons propagating from a ground station to a satellite and to verify if coincidence measurements of entangled photons can be used to back out atmospheric turbulence parameters for long-distance propagation or moderate-to-strong turbulence , we built an atmospheric turbulence simulator (ATS) in a laboratory setting. The ATS comprised of two afocal systems with a Lexitek phase wheel and a Meadowlark spatial light modulator representing discrete layers of atmospheric turbulence. The ATS could represent propagation distances on the order of 1km and could theoretically simulate Rytov variances as high as 5.16 and Fried parameters as low as 0.5cm for a 1m telescope. The design parameters, numerical simulations, and experimental setup is detailed in this proceeding.
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.
Entanglement is key quantum information processing feature enabling quantum communication, networking, and sensing [1]. Unfortunately, long-distance quantum communication is affected by the photon loss, given that quantum signals cannot be amplified. As an alternative, free-space optical (FSO) links can be used instead for entanglement distribution. However, the FSO links are severely affected by atmospheric turbulence effects, in addition to scattering effects. In this paper, we demonstrate that adaptive optics [2] is a useful tool to improve fidelity of entanglement when entangled states are distributed over turbulent FSO links. We present results from our FSO communication experiment where we generated entangled photons pairs, launched them into free-space, and later collected them for coincidence count measurement. We demonstrate that adaptive optics can be used to make improvements in coincidence counts.
We developed a laboratory-scale testbed to simulate atmospheric turbulence effects and to investigate the role of turbulence on the quantum information content of a free-space qubit transport. We report the development, characterization, and calibration of the testbed for the statistical parameters of the turbulence employing two phase screens, which produced weak to strong turbulence replicating long distances in the atmosphere for single photon propagation. By taking a long-exposure image of the point-spread function of a pinhole and analyzing the associated modulation transfer function, we measured a D / r 0 ranging from 1.04 to 23.27. Using irradiance fluctuations, we measured an on-axis scintillation index ranging from 0.25 to 2.02. The TASQ was designed to integrate a biphoton source, along with a single photon detection module to test the effect of turbulence on qubit transport. The calibrated setup will provide critical data for field experiments involving long-distance quantum networks.