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Science

Topological Hong-Ou-Mandel interference

Max Ehrhardt, Christoph Dittel, Matthias Heinrich, Alexander Szameit

Published Jun 20, 2024• 21 citations
Publisher

Abstract

The interplay of topology and optics provides a route to pursue robust photonic devices, with the application to photonic quantum computation in its infancy. However, the possibilities of harnessing topological structures to process quantum information with linear optics, through the quantum interference of photons, remain largely uncharted. Here, we present a Hong-Ou-Mandel interference effect of topological origin. We show that this interference of photon pairs-ranging from constructive to destructive-is solely determined by a synthetic magnetic flux, rendering it resilient to errors on a fundamental level. Our implementation establishes a quantized flux that facilitates exclusively destructive quantum interference. Our findings pave the way toward the development of next-generation photonic quantum circuitry and scalable quantum computing protected by virtue of topologically robust quantum gates.

DOI
10.1126/science.ado8192

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The interplay of topology and optics provides a route to pursue robust photonic devices, with the application to photonic quantum computation in its infancy. However, the possibilities of harnessing topological structures to process quantum information with linear optics, through the quantum interference of photons, remain largely uncharted. Here, we present a Hong-Ou-Mandel interference effect of topological origin. We show that this interference of photon pairs-ranging from constructive to destructive-is solely determined by a synthetic magnetic flux, rendering it resilient to errors on a fundamental level. Our implementation establishes a quantized flux that facilitates exclusively destructive quantum interference. Our findings pave the way toward the development of next-generation photonic quantum circuitry and scalable quantum computing protected by virtue of topologically robust quantum gates.
Topological insulators are materials that have a gapped bulk energy spectrum but contain protected in-gap states appearing at their surface. These states exhibit remarkable properties such as unidirectional propagation and robustness to noise that offer an opportunity to improve the performance and scalability of quantum technologies. For quantum applications, it is essential that the topological states are indistinguishable. We report high-visibility quantum interference of single-photon topological states in an integrated photonic circuit. Two topological boundary states, initially at opposite edges of a coupled waveguide array, are brought into proximity, where they interfere and undergo a beamsplitter operation. We observe Hong-Ou-Mandel interference with 93.1 ± 2.8% visibility, a hallmark nonclassical effect that is at the heart of linear optics-based quantum computation. Our work shows that it is feasible to generate and control highly indistinguishable single-photon topological states, opening pathways to enhanced photonic quantum technology with topological properties, and to study quantum effects in topological materials.
Structured photons are nowadays an important resource in classical and quantum optics due to the richness of properties they show under propagation, focusing, and in their interaction with matter. Vectorial modes of light in particular, a class of modes where the polarization varies across the beam profile, have already been used in several areas ranging from microscopy to quantum information. One of the key ingredients needed to exploit the full potential of complex light in the quantum domain is the control of quantum interference, a crucial resource in fields like quantum communication, sensing, and metrology. Here we report a tunable Hong-Ou-Mandel interference between vectorial modes of light. We demonstrate how a properly designed spin-orbit device can be used to control quantum interference between vectorial modes of light by simply adjusting the device parameters and no need of interferometric setups. We believe our result can find applications in fundamental research and quantum technologies based on structured light by providing a new tool to control quantum interference in a compact, efficient, and robust way.
Structured light provides high-dimensional Hilbert spaces holding tremendous potential for fundamental quantum optics and quantum technologies. However, existing characterization methods, like Hong-Ou-Mandel (HOM) interference, typically assume perfectly tuned conditions, overlooking the geometric physics governing spatial mode evolution. Here, we establish topological quantum interferometry driven by an interaction-based geometric phase, the exchange Berry phase (BPX). Our formalism generalizes qqq-plate state generation and characterization to arbitrary topological charges and (de)tuning conditions, demonstrating that BPX acts as a geometric marker governing spatial interference. We show BPX serves as a deterministic control parameter, decomposing two-photon spatial patterns into geometry-dictated fundamental modes. This mapping reveals topological invariants and phase singularities that function as a non-tomographic witness for state dimensionality estimation, circumventing full-state reconstruction. Being device-independent and highly scalable, this approach enables scalable high-dimensional characterization and topologically protected state selection, with direct applicability to quantum metrology and high-capacity quantum networks.
Controlling light at the level of individual photons has led to advances in fields ranging from quantum information and precision sensing to fundamental tests of quantum mechanics. A central development that followed the advent of single-photon sources was the observation of the Hong–Ou–Mandel (HOM) effect, a novel two-photon path interference phenomenon experienced by indistinguishable photons. The effect is now a central technique in the field of quantum optics, harnessed for a variety of applications such as diagnosing single-photon sources and creating probabilistic entanglement in linear quantum computing. Recently, several distinct experiments using atomic sources have realized the requisite control to observe and exploit Hong–Ou–Mandel interference of atoms. This chapter provides a summary of this phenomenon and discusses some of its implications for atomic systems. Transitioning from the domain of photons to atoms opens new perspectives on fundamental concepts, such as the classification of entanglement of identical particles. It aids in the design of novel probes of quantities such as entanglement entropy by combining well-established tools of AMO physics—unity single-atom detection, tunable interactions, and scalability—with the Hong–Ou–Mandel interference. Furthermore, it is now possible for established protocols in the photon community, such as measurement-induced entanglement, to be employed in atomic experiments that possess deterministic single-particle production and detection. Hence, the realization of the HOM effect with atoms represents a productive union of central ideas in quantum control of atoms and photons.
Structured photons are a crucial resource in both classical and quantum technologies, particularly in spin–orbit hybrid photons, enabling various practical applications ranging from ultra-sensitive metrology techniques to quantum-enhanced information processing tasks. However, the two-photon interference of spin–orbit hybrid photons, which combines polarization modes and complex transverse spatial structures across the beam profile, remains unexplored. Here, we present an experimental observation of Hong–Ou–Mandel (HOM) interference of spin–orbit hybrid photons. The tunable q-plates that work as spin–orbit coupler devices are used to prepare various forms of spin–orbit hybrid entangled photons. By harnessing the match degree in the temporal domain, the coalescence and anti-coalescence effects resulting from the symmetric and anti-symmetric properties of the incident quantum states are observed. Moreover, we demonstrated the feasibility of quantum-enhanced photon polarization gears through HOM interference and theoretically analyze the noise-resilient advantages based on coherent HOM measurements. These results provide an alternative route toward quantum experiments with structured photons that allows for controlling their quantum interference in a compact, stable, and efficient way.