Welcome to my personal website! I am currently pursuing my PhD in Computer Science at the University of Massachusetts, Amherst as part of the ACQuIRe Lab and advised by Professor Don Towsley. My research focuses on using simulations and realistic weather data to investigate how large clusters of satellites should be arranged to best enable distributed quantum applications such as quantum key distribution, distributed quantum computing, and quantum sensing. Here, you can find information about my publications, teaching experience, and a collection of my favorite recipes.
Publications
Multi-Mode Quantum Memories for High-Throughput Satellite Entanglement Distribution
Authors: Connor Casey, Albert Williams, Catherine McCaffrey, Eugene Rotherham, Nathan Darby
Conference: International Astronautical Congress
Year: 2025
Abstract: Quantum networking seeks to enable global entanglement distribution through terrestrial and free space channels; however, the exponential loss in these channels necessitates quantum repeaters with efficient, long lived quantum memories (QMs). Space based architectures, particularly satellite assisted links, offer a path to truly global connectivity, yet they demand QMs that are compatible with orbital factors such as infrared radiation and the unique challenges of operating aboard a satellite. In this work, we propose a multimode quantum memory (MMQM) for low Earth orbit (LEO) repeaters based on the atomic frequency comb (AFC) protocol. Our design integrates a hybrid alkali noble gas ensemble in an optical cavity, using alkali atoms for strong photon matter coupling and noble gas nuclear spins for minutes to hours coherence, all without the need for cryogenics. The architecture natively supports temporal and spectral multiplexing, enabling the storage of 100 modes to parallelize probabilistic operations and overcome light limited round trip times. Representative link budgets at km with realistic apertures, , and of several minutes predict improvements of up to two orders of magnitude in per pass success probability and instantaneous SKR relative to a memoryless dual downlink, with clear scaling in . Our contributions are (i) a non cryogenic, space ready multimode memory, (ii) a systems analysis coupling mode count, storage time, and orbital geometry to achievable rate, and (iii) a near term implementation roadmap. Together, these results indicate feasibility with current to near term technology and provide a practical path toward a high rate, space enabled quantum internet.
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Airborne reflectors for satellite-based quantum entanglement and key distribution
Authors: Kavindu Sellahewa, Nitish K. Panigrahy, Albert Williams, Don Towsley, Deirdre Kilbane
Conference: Nature: Scientific Reports
Year: 2025
Abstract: These effects can lead to lower entanglement distribution rates, secret key rates, and increased quantum bit error rates, especially in direct satellite-to-ground communication. This paper proposes a practical solution: the design of an airborne gold-coated parabolic reflector to be placed in the stratosphere directly above the ground station. This reflector effectively acts as a second virtual transmitter, a novel concept introduced in this work. The proposed method brings about a substantial increase in the distributed entanglement rate, boosting it by up to 25 times at zenith compared to direct satellite-to-ground communication. It also reduces the minimum elevation angle for secure communication, approximately from to for BB84 and from to for E91 when using the proposed reflector method compared to direct satellite-to-ground communication. Furthermore, the proposed reflector method extends the communication time window by for the BB84 protocol and for the E91 protocol. These enhancements underscore the potential of our approach to significantly extend the duration of secure communication and improve performance, particularly at lower elevation angles where direct satellite-to-ground communication is not feasible.
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Scalable Scheduling Policies for Quantum Satellite Networks
Authors: Albert Williams, Nitish K. Panigrahy, Andrew McGregor, Don Towsley
Conference: IEEE International Conference on Quantum Computing and Engineering (QCE)
Year: 2024
Abstract: As Low Earth Orbit (LEO) satellite mega constellations continue to be deployed for satellite internet and recent successful experiments in satellite-based quantum entanglement distribution emerge, a natural question arises: How should we coordinate transmissions and design scalable scheduling policies for a quantum satellite internet? In this work, we consider the problem of transmission scheduling in quantum satellite networks subject to resource constraints at the satellites and ground stations. We show that the most general problem of assigning satellites to ground station pairs for entanglement distribution is NP-hard. We then propose four heuristic algorithms and evaluate their performance for Starlink mega constellation under various amount of resources and placements of the ground stations. We find that the maximum number of receivers necessary per ground station grows very slowly with the total number of deployed ground stations. Our proposed algorithms, leveraging optimal weighted b-matching and the global greedy heuristic, outperform others in entanglement distribution rate, entanglement fidelity, and handover cost metrics. While we develop these scheduling algorithms, we have also designed a software system to simulate, visualize, and evaluate satellite mega-constellations for entanglement distribution.
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Optimizing Flows in Changing Tree-based Sensor Networks
Authors: Albert Williams, Don Towsley
Conference: IEEE Military Communications Conference (MILCOM)
Year: 2021
Abstract: Military sensor networks often operate in resource challenged environments. This poses the problem of how to allocate resources to sensors flow to accomplish a mission. In this paper we consider a set of sensors that communicate observations up a tree to a fusion center. The value of the mission is modeled by a separable increasing concave functions and we develop a low complexity one step algorithm that allocates link capacities to each sensor so as to maximize this function. By limiting ourselves to a tree topology, we derive several important benefits, including the ability to quickly adapt to changes in utility functions or topology, and in a straightforward way to run our algorithm in a parallel, distributed manner over the network with little communication overhead and no centralized planning.
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