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arXiv:1605.02053 · Optica

Topological protection of photonic path entanglement

Mikael C. Rechtsman, Yaakov Lumer, Yonatan Plotnik, Armando Pérez-Leija, Alexander Szameit, Mordechai Segev

Published Aug 22, 2016• 111 citations
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Abstract

The recent advent of photonic topological insulators has opened the door to using the robustness of topologically protected transport—originated in the domain of condensed matter physics—in optical devices and in quantum simulation. Concurrently, quantum walks in photonic networks have been shown to yield exponential speedup for certain algorithms, such as Boson sampling. Here we theoretically demonstrate that photonic topological insulators can robustly protect the transport of quantum information through photonic networks, despite the presence of disorder.

DOI
10.1364/optica.3.000925

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The recent advent of photonic topological insulators has opened the door to using the robustness of topologically protected transport—originated in the domain of condensed matter physics—in optical devices and in quantum simulation. Concurrently, quantum walks in photonic networks have been shown to yield exponential speedup for certain algorithms, such as Boson sampling. Here we theoretically demonstrate that photonic topological insulators can robustly protect the transport of quantum information through photonic networks, despite the presence of disorder.
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Topological insulators combine insulating properties in the bulk with scattering-free transport along edges, supporting dissipationless unidirectional energy and information flow even in the presence of defects and disorder. The feasibility of engineering quantum Hamiltonians with photonic tools, combined with the availability of entangled photons, raises the intriguing possibility of employing topologically protected entangled states in optical quantum computing and information processing. However, while two-photon states built as a product of two topologically protected single-photon states inherit full protection from their single-photon "parents", a high degree of non-separability may lead to rapid deterioration of the two-photon states after propagation through disorder. In this work, we identify physical mechanisms which contribute to the vulnerability of entangled states in topological photonic lattices. Further, we show that in order to maximize entanglement without sacrificing topological protection, the joint spectral correlation map of two-photon states must fit inside a well-defined topological window of protection.
Photon entanglement is a fundamental resource for quantum technologies, yet it is extremely fragile especially in large quantum circuitry, which is necessary for quantum information processing and computing. We propose an integrated scheme for direct topological protection of entanglement, enabling robust transport of entangled photonic states, even when the photon dynamically evolves during propagation. Our platform relies on a two-dimensional discrete-time quantum walks in synthetic dimensions, supporting topological edge states at the interface between distinct domains. Thus far, in all previous approaches for exploiting topology to protect entanglement, the protection was linked to the position of the photon in the actual structure. In contrast, our scheme relies on a synthetic temporal dimension (time bins) where the entangled state is always reproduced, despite fluctuations or variations that may emerge in the system. Thus, our scheme provides precise temporal control important for synchronization and timing in quantum computation. By suppressing phase-sensitive errors—the most critical noise source in photonic platforms—our approach can lower the physical error rate below fault-tolerance thresholds and reduce the overhead typically required for scalable quantum information processing. The robustness to errors arises from the fact that the entangled state is preserved through correlations in the synthetic (temporal) dimension. As a result, we ensure that at specific designated times, the emergent state faithfully reproduces the initial entangled state, despite the photon having taken any dynamically evolving paths. This approach establishes a robust framework for directly protecting entanglement.
Abstract Entangled multiphoton states lie at the heart of quantum information, computing, and communications. In recent years, topology has risen as a new avenue to robustly transport quantum states in the presence of fabrication defects, disorder, and other noise sources. Whereas topological protection of single photons and correlated photons has been recently demonstrated experimentally, the observation of topologically protected entangled states has thus far remained elusive. Here, we experimentally demonstrate the topological protection of spatially entangled biphoton states. We observe robustness in crucial features of the topological biphoton correlation map in the presence of deliberately introduced disorder in the silicon nanophotonic structure, in contrast with the lack of robustness in non‐topological structures. The topological protection is shown to ensure the coherent propagation of the entangled topological modes, which may lead to robust propagation of quantum information in disordered systems.
Quantum entanglement, as the strictly non-classical phenomenon, is the kernel of quantum computing and quantum simulation, and has been widely applied ranging from fundamental tests of quantum physics to quantum information processing. Meanwhile, the topological phase is found inherently capable of protecting physical fields from unavoidable fabrication-induced disorder, which inspires the potential application of topological protection to quantum states. Here, we present the experimental demonstration of topologically protected quantum entangled states on a photonic chip. The process tomography shows that quantum entanglement can be well preserved by the topological states even when the chip material introduces disorder and relative polarization rotation in phase space. Our work links the fields of materials, topological science and quantum physics, opening the door to wide applications of topological enhancement in quantum regime.