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arXiv:2502.08879 · Newton

Recent advances in high-dimensional quantum frequency combs

Kai-Chi Chang, Xiang Cheng, Murat Can Sarihan, Chee Wei Wong

Published Mar 1, 2025• 38 citations
arXivHTMLPDF

Abstract

High-dimensional entanglement in qudit states offers a promising pathway towards the realization of practical, large-scale quantum systems that are highly controllable. These systems can be leveraged for various applications, including advanced quantum information processing, secure communications, computation, and metrology. In this context, quantum frequency combs have a crucial role as they inherently support multiple modes in both temporal and frequency domains, while preserving a single spatial mode. The multiple temporal and frequency modes of quantum frequency combs facilitate the generation, characterization, and control of high-dimensional time-frequency entanglement in extensive quantum systems. In this review article, we provide an overview of recent technological advancements in high-dimensional energy-time entangled quantum frequency combs. We explore how these time-frequency qudits, achieved using scalable telecommunications-wavelength components, can empower the creation of large-scale quantum states. Advances in quantum frequency combs can unlock new capabilities and versatility for promising developments in quantum science and technology.

DOI
10.1016/j.newton.2025.100024

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High-dimensional entanglement in qudit states offers a promising pathway towards the realization of practical, large-scale quantum systems that are highly controllable. These systems can be leveraged for various applications, including advanced quantum information processing, secure communications, computation, and metrology. In this context, quantum frequency combs have a crucial role as they inherently support multiple modes in both temporal and frequency domains, while preserving a single spatial mode. The multiple temporal and frequency modes of quantum frequency combs facilitate the generation, characterization, and control of high-dimensional time-frequency entanglement in extensive quantum systems. In this review article, we provide an overview of recent technological advancements in high-dimensional energy-time entangled quantum frequency combs. We explore how these time-frequency qudits, achieved using scalable telecommunications-wavelength components, can empower the creation of large-scale quantum states. Advances in quantum frequency combs can unlock new capabilities and versatility for promising developments in quantum science and technology.
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The development of quantum technologies for quantum information science demands the realization and precise control of complex (multipartite and high dimensional) entangled systems on practical and scalable platforms. Quantum frequency combs (QFCs) generated via spontaneous four-wave mixing in integrated microring resonators represent a powerful tool towards this goal. They enable the generation of complex photon states within a single spatial mode as well as their manipulation using standard fiber-based telecommunication components. Here, we review recent progress in the development of QFCs, with a focus on our results that highlight their importance for the realization of complex quantum states. In particular, we outline our work on the use of integrated QFCs for the generation of high-dimensional multipartite optical cluster states and their unidirectional processing, being at the core of measurement-based quantum computation. These results confirm that engineering the time-frequency entanglement properties of QFC may provide a stable, practical, low-cost, and established platform for the development of near-future quantum devices for out-of-the-lab applications, ranging from practical quantum computing to more secure communications.
Entangled optical quantum states are essential towards solving questions in fundamental physics and are at the heart of applications in quantum information science. For advancing the research and development of quantum technologies, practical access to the generation and manipulation of photon states carrying significant quantum resources is required. Recently, integrated photonics has become a leading platform for the compact and cost-efficient generation and processing of optical quantum states. Despite significant advances, most on-chip nonclassical light sources are still limited to basic bi-photon systems formed by two-dimensional states (i.e., qubits). An interesting approach bearing large potential is the use of the time or frequency domain to enabled the scalable onchip generation of complex states. In this manuscript, we review recent efforts in using on-chip optical frequency combs for quantum state generation and telecommunications components for their coherent control. In particular, the generation of bi- and multi-photon entangled qubit states has been demonstrated, based on a discrete time domain approach. Moreover, the on-chip generation of high-dimensional entangled states (quDits) has recently been realized, wherein the photons are created in a coherent superposition of multiple pure frequency modes. The time- and frequency-domain states formed with on-chip frequency comb sources were coherently manipulated via off-the-shelf telecommunications components. Our results suggest that microcavity-based entangled photon states and their coherent control using accessible telecommunication infrastructures can open up new venues for scalable quantum information science.
A key challenge in today’s quantum science is the realization of large-scale complex non-classical systems to enable e.g. ultra-secure communications, quantum-enhanced measurements, and computations faster than classical approaches. Optical frequency combs represent a powerful approach towards this, since they provide a very high number of temporal and frequency modes which can result in large-scale quantum systems. Here, we discuss the recent progress on the realization of integrated quantum frequency combs and reveal how their use in combination with on-chip and fiber-optic telecommunications components can enable quantum state control with new functionalities, yielding unprecedented capability.
High-dimensional entanglement with larger Hilbert spaces enable an encoding of more bits per photon and thus promise increased communication capacities over quantum channels. Quantum frequency combs, which are intrinsically multimode in the temporal and frequency degrees of freedom within a single spatial mode, naturally facilitating the generation and measurement of high-dimensional entanglement. Current challenges include the extension of well-known methods for two qubits to high-dimensional quantum systems and their application in entanglement experiments with photons. More specifically, the major challenge is the certification of high-dimensional entanglement by a number of accessible experimental measurements. In this paper, we increase the Hilbert space dimensionality and provide versatile tools for quantifying and certifying high-dimensional entanglement in a biphoton frequency comb. We quantify the time binned Schmidt number up to 18 and certify entanglement of formation with 1.89 ebits. We have demonstrated a 648- dimensional Hilbert spaces with time-frequency entanglement in a biphoton frequency comb, enabling a computational space up to 13 photonic qubits, and 6.28 bits/photon classical information capacity. This high-dimensional time frequency multimode quantum states of biphoton frequency comb significantly boosting the photon information capacity that is critical for large-scale quantum information processing. Biphoton frequency comb has indeed demonstrated an attractive and powerful approach towards achieving this fundamental goal with applications in high-dimensional quantum information processing, time-frequency cluster-state quantum computation, high-dimensional encoding in quantum networks, and high-dimensional quantum simulations.
Frequency modes of light are one of the most promising platforms that provide access to high-dimensional quantum states amongst different photonic degrees of freedom capable of high-dimensionality, enabling robust, error-tolerant, and scalable quantum optical information systems. We demonstrate engineering of precisely controlled two-photon high-dimensional states entangled in frequency through time-domain Fourier optical synthesis. We generate and convert a continuous broadband frequency-entangled state into a large range of discrete frequency bins suitable for ITU standards, with spacings ranging from 12.5 GHz to 750 GHz, and observe spectral anticorrelations over 38 frequency bins, including intra-bin pure states at a 100 GHz bin spacing. We characterize the full quantum state dimensionality via Schmidt decomposition and observe lower bounds on the frequency-binned Hilbert-space dimensionalities of at least 289, formed by two entangled qudits with dimension 17. Furthermore, we demonstrate quantum nonlocality via frequency correlations in a transmission experiment over a campus-scale two-node fiber network. This work represents a crucial step towards building a versatile and relatively simple way of generating precisely controlled high-dimensional spectral qudits, with the potential of harnessing in wavelength-multiplexed quantum networks, high-dimensional information processing, and communication of quantum states specifically, and fiber-optic quantum remote sensing.