Collaborative Research: FET: Small: Towards full photon utilization by adaptive modulation and coding on quantum links

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Secure communication has long been an indispensable part of numerous systems, ranging from the more traditional such as finance and defense to the emerging ones such as the internet of (battlefield) things and health data management. Traditional data encryption methods based on using public keys are threatened by the advances in quantum computing algorithms promising to efficiently solve otherwise intractable problems which make public key encryption secure. However, it is precisely quantum information processing advances that are also expected to enable secure communications by allowing efficient and secure private key distribution. The main advantage of private key encryption is that as long as the key strings are truly secret, it is provably secure, that is, insensitive to advances in computing. A Quantum Key Distribution (QKD) protocol describes how two parties, commonly referred to as Alice and Bob, can establish a secret key by communicating over a quantum and a public classical channel that both can be accessed by an eavesdropper Eve. For the widespread adoption of QKD, it is mandatory to provide high key rates over long distances. What has appeared as a bottleneck in practice is the inability to maximize the utility of information-bearing quantum states. This project seeks to solve this inefficiency problem. The results will pave the way for practical quantum networks in which multiple receivers communicate with a source simultaneously though multi-channel entanglement distribution.This project focuses on maximizing the utility of photons in frequency-time entanglement based QKD, through a combination of innovations in adaptive photon generation-aware modulation and coding, and a state of the art experimental validation. QKD offers a physically secure way for establishing an encryption key over a quantum and a public communication channel, both of which are observed by an eavesdropper. Because of the growing demand for quantum communications, research on improving QKD protocols has steeply intensified. One recent breakthrough is the experimental observation of continuous-variable frequency-time hyperentangled photons. This high-dimensional large Hilbert-space approach promises high information efficiency by potentially carrying multiple bits per an entangled photon pair. However, to ensure unconditional security in QKD, the biphotons (whether carrying single qubit or multiple qubits per photon), must be transmitted under photon-starved conditions, creating an immediate need to maximize utility of all generated biphotons. The project will offer an integrated solution consisting of photon-aware modulation and coding schemes, and will be the first such to be demonstrated on time-bin encoded multi-dimensional biphotons.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.