RAISE-TAQS: Photon-Number-Resolving Integrated Avalanche Photodiodes for Scalable Quantum Computing

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Non-technical description: Quantum computing and quantum communications are emerging fields that harness fundamentally quantum mechanical properties to process and exchange information. They offer transformative potential for applications ranging from solving important classes of problems faster than traditional digital computers, such as integer factorization and simulating complex quantum mechanical systems, to creating secure communications channels that cannot be hacked. Any quantum computer or quantum communication network that uses light must have the ability to count the number of photons in order to correct computation or transmission errors; specifically, a receiver is required to detect and transduce the photons into an electrical signal that is proportional to the exact number of photons that reaches the receiver at any given moment. This project aims to demonstrate such a receiver and harnesses this capability to perform measurements that are of both fundamental scientific and practical importance for future quantum computing and quantum communications systems. This research resonates strongly across several disciplines, ranging from fundamental materials science, to basic physics, through engineering. It provides unique interdisciplinary research opportunities for graduate, undergraduate, and high school students. A key educational goal is to prepare students for the cross-disciplinary challenges they may face as quantum technology intersects engineering via a broadly accessible, self-contained course in "quantum engineering" simultaneously at UT-Austin and UVA, with all course materials and recorded lectures freely available to the general public. The project also integrates research with outreach activities, such as inspiring pre-K-12 students through classroom visits, public lectures, and collaborative exhibits/tours with local museums.Technical description: Quantum photonics is a key quantum technology. A critical element for quantum photonics is the photon-number-resolving photodetector, which produces a signal proportional to the number of incident photons, enabling full access to the corpuscular nature of quantum electromagnetic fields. The latter is key to high photon flux applications in quantum information, such as quantum repeaters in quantum communication, entanglement distillation, quantum error correction, and fault tolerant universal quantum computing over continuous variables using squeezed states. A number of device technologies offer photon number resolution, but typically operate at reduced temperatures, often requiring a large cooling apparatus and large arrays for modest number resolution. This project investigates a novel approach to this challenge, where a single photon avalanche photodiode (SPAD) is decomposed into several waveguide-coupled "nanoSPAD" segments, each detecting at most one photon via individual readout. The goal is photon number resolution at room temperature, with the potential to cover wavelengths from the visible to mid-infrared at high bandwidths - a combination that is essential, yet inaccessible with any existing technology. With design grounded in a fully quantum model and enabled by a new class of low noise III-V avalanche photodetector materials, this structure offers the potential for photon number resolution with high positive-operator-valued measurement purity. The nanoSPADs are integrated monolithically onto InP, the platform of modern long-haul telecommunications. The litmus test of the capabilities of the photon number resolving nanoSPAD is the demonstration of quantum state tomography and Fock-state filtering in real time. The quantum tomography protocol is, in turn, used to characterize the photon number resolution. Beyond enabling these quantum tomography experiments, it is expected that the nanoSPAD will accelerate new scientific breakthroughs in quantum photonics.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.