High-dimensional Frequency Gates in Integrated Photonics for Scalable Quantum Interconnects

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Within the overall landscape of quantum science and technology, the development of quantum networks is critical for applications such as distributed quantum computing, connected quantum sensors, and blind quantum computing. While there has been progress in facilitating entanglement and communication between end nodes using satellite-based free space links, for dense and short reach networks these modes of communication are unrealistic given the need for line-of-sight access. In contrast, optical fiber offers tremendous bandwidth and low loss over lengths up to about 100 km, making it the logical choice for local area and metropolitan area quantum networks. However, much of the previous work in quantum networking has utilized photonic degrees of freedom, like polarization, which cannot be easily preserved in standard single-mode fiber. On the other hand, frequency encoding provides natural stability in optical fiber, straightforward measurement with high-efficiency filters and detectors, and compatibility with wavelength-division multiplexing. We propose to harness quantum interference in the spectral domain to implement high-dimensional mode transformations that support increased information per photon for direct quantum communication protocols. To realize the functionality required for these schemes, we will leverage recent advances in silicon photonics and organic electro-optic materials to develop a quantum frequency processor in an integrated photonic platform. A key outcome of the proposed work will be a demonstration of entanglement swapping with spectrally distinguishable photons – a milestone important for moving to a networking paradigm based on spectrally multiplexed and/or frequency-encoded quantum information. In addition, a team of undergraduates will be integrated into this research through long term projects under an experiential learning initiative supported by the College of Engineering. The team will progress from developing lab automation skills and repeating foundational quantum optics experiments to photonic device design and system-level testing toward the end of the project. TechnicalThis project will tackle three critical challenges in the field of quantum interconnects – (i) high-dimensional encoding for information transport more robust to loss, (ii) high-speed, low-loss, and broadband optical switches for nanosecond scale scheduling and routing, and (iii) photon-photon interconnects for entangling heterogeneous nodes/sources. To do so, we will develop a silicon photonics-based quantum frequency processor (QFP) that can implement high-dimensional mode transformations and logic gates. We will overcome the lack of a second order nonlinearity in silicon/silicon nitride by building on progress in organic electro-optic materials, which have been harnessed to realize low-power and high bandwidth modulators. Process advances at silicon photonics foundries will be leveraged to realize pulse shapers with narrow spectral channels, thereby making it possible to then drive a QFP with many RF harmonics over a limited analog bandwidth. This, in turn, will facilitate implementation of high-dimensional quantum frequency gates (d 6). On a parallel track, we will carry out proof-of-concept experiments to validate the generalization of the QFP protocol to higher dimensions. Both tracks will converge toward the realization of a two-state Bell state analyzer for frequency qubits, which will be used in a heralded entanglement generation protocol where photons participating in the joint measurement are spectrally distinguishable. This protocol is generalizable to higher dimensions and may facilitate entanglement swapping of qudits. At the network level, this has the potential to relax constraints on spectral indistinguishability and will be useful in situations where qubits from quantum memories are intentionally shifted (via quantum frequency conversion) to different spectral bins in the telecom band for spectrally multiplexed fiber transmission or when trying to generate entanglement between different types of matter-based qubits or between qubits in different local environments.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.