Adventures in High Dimensions: Unveiling the Power of High-Dimensional Entanglement

The phenomenon of quantum entanglement enables the most secure form of communication possible—one where the communication devices themselves can be in the hands of an adversary, and security is still guaranteed. However, generating and distributing entanglement over long distances and through a noisy environment is no easy task. Entangled photons can be lost while propagating through fibres, and large amounts of noise can compromise detectors. As a result, tests of quantum nonlocality—the most stringent form of entanglement—have only been performed under very controlled conditions. Quantum steering relaxes the strict technical requirements of nonlocality by assuming an untrusted device only on one side of an (untrusted) channel.

In this talk, I will briefly introduce high-dimensional (qudit) entanglement and its merits over qubit entanglement. Then, I will show how we harness the advantages of qudit entanglement to demonstrate two kinds of quantum steering. The first set showcases genuine high-dimensional quantum steering. The second set of quantum steering is with the detection-loophole closed under extreme conditions of noise and loss. Here we capitalize on high-dimensional entanglement's noise robustness and loss tolerance to significantly surpass qubit-based systems. This enables us to demonstrate quantum steering in up to 53 dimensions under challenging conditions equivalent to 79 km of telecommunication fibre loss and 36% of white noise. Surprisingly, the use of high dimensions also significantly reduces the measurement time required, achieving a quantum steering violation nearly two orders of magnitude faster by simply doubling the Hilbert space dimension.

Building on this progress, we explore expanding dimensionality in the temporal degree of freedom. We identify the challenges in certifying high-dimensional time-bin entanglement due to the arduous nature of current measurement techniques. To overcome this, we introduce a novel technique for conducting generalized measurements on high-dimensional time-bin states using space-time coupling inside a long multi-mode fibre. We experimentally validate this approach in a 4-dimensional Hilbert space. Our work demonstrates that the fundamental phenomenon of entanglement can transcend the limits imposed by a realistic environment and opens a clear pathway towards quantum communication protocols with unconditional security.