Multi-frequency Comb System Generates Long-range Entanglement for Advanced Signal Multiplexing Applications

Frequency combs, systems that generate light at many distinct frequencies, underpin a wide range of precision measurements and technological applications. Sahil Pontula from MIT, Debasmita Banerjee and Yannick Salamin from the University of Central Florida, along with colleagues, now demonstrate a theoretical pathway to create entanglement and quantum correlations across multiple frequency combs. The team reveals how a system of nonlinearly coupled combs, driven by a unique upconversion and downconversion process, generates squeezed light and entanglement spanning an exceptionally broad spectrum, from ultraviolet to mid-infrared wavelengths. This achievement unlocks the potential for advanced techniques like tunable ghost spectroscopy, improved pump-probe measurements, and the creation of entangled states across a wide range of frequencies, paving the way for significant advances in quantum technologies and optical metrology.

Chip-Based Comb Generation and Nonlinear Optics

This collection of research papers focuses on optical frequency combs, technologies generating precise frequencies of light, and related areas like nonlinear optics and quantum physics. Studies explore how these combs can be created using compact chip-based devices and applied to precision measurements and potentially quantum technologies. A central theme is the use of microresonators, tiny structures that enhance light-matter interactions, to generate these combs efficiently. Researchers are also investigating the generation of squeezed light, a special state of light with reduced noise, crucial for enhancing the sensitivity of measurements and enabling secure quantum communication.

A significant trend is integrating these technologies onto photonic chips, paving the way for compact, stable, and cost-effective devices. Cutting-edge research explores non-Hermitian physics and topological photonics, concepts allowing unprecedented control over light propagation and the creation of novel optical devices. This research delves into the idea of treating frequency as a spatial dimension, opening new possibilities for manipulating light and creating complex optical structures. Combining non-Hermitian physics with topological concepts enables the creation of robust and controllable optical devices with unique properties. Scientists are focused on generating solitons and frequency combs in systems with both energy loss and gain, and achieving broadband squeezing with minimal noise, promising improvements in quantum-enhanced metrology, sensing, and the development of integrated quantum photonic circuits.

Cascaded Combs and Quantum Correlation Simulations

Scientists have engineered a system to generate multiple frequency combs and explore the resulting quantum correlations, which describe the relationships between different light frequencies. The process relies on cascaded three-wave mixing, where light interacts nonlinearly to create new frequencies, facilitated by a single “idler” comb. Researchers modeled this system using equations simulating how light pulses propagate through nonlinear materials, allowing representation of experimental setups. Simulations used parameters approximating commercially available laser systems, revealing that cascaded pulse generation becomes significant over centimeter-scale distances.

To understand the system’s behavior, the team examined scenarios with weak and strong initial signals. While the overall spectra appeared similar, researchers anticipated differences in the quantum noise properties. To efficiently calculate these properties, scientists developed a method extracting fluctuation information from a single simulation of the system’s dynamics. This technique enabled the calculation of a covariance matrix, revealing the intensity differences between different light beams, visualized as heatmaps. Analysis of these heatmaps demonstrated strong correlations between different combs and within each comb, spanning the entire multi-pulse system. These variations, previously observed in nonlinear fiber propagation, are likely due to the sensitivity of the nonlinear phase shift at high peak powers. A stronger initial signal broadened these correlations across a larger portion of the spectrum, revealing strong connections extending across multiple pulses, indicating potential for quantum-enhanced measurements and generating entangled light for quantum technologies.

Cascaded Frequency Combs Extend Spectral Coverage

Scientists have achieved a breakthrough in generating multiple frequency combs through a novel cascaded process within a nonlinear optical cavity. This work demonstrates a method for creating a series of interconnected frequency combs, mediated by a single “idler” comb, enabling the generation of light spanning from ultraviolet to mid-infrared frequencies. The research establishes a framework for harnessing quantum correlations in these complex optical systems, paving the way for new quantum technologies. The team explored a mechanism where a pump comb and a seed comb are injected into a cavity, initiating three-wave mixing to generate the idler comb.

This idler comb then acts as a catalyst, facilitating the creation of additional “sub-combs” through upconversion and downconversion processes. The system was designed with a pump comb spacing smaller than the idler comb spacing, allowing strong coupling between the sub-combs and the emergence of nontrivial correlations and entanglement. Experiments revealed significant depletion of the pump comb as energy cascaded into the sub-combs, indicating a regime where strong quantum correlations are expected. Measurements demonstrate the generation of inter- and intracomb two-mode squeezing and entanglement.

By analyzing the fluctuations of light within the cavity, scientists calculated the output fluctuations of amplitude and phase for each mode. Results show that a system with five sub-combs, each containing three comb lines, can be effectively generated and controlled. The team observed that the modulation rate, which sets the spacing between modes within each subcomb, plays a crucial role in the system’s behavior. Specifically, the system was designed to operate with a modulation rate corresponding to a multiple of the cavity’s free spectral range, ensuring resonance across all comb modes. This breakthrough delivers a platform for exploring novel forms of entanglement-assisted spectroscopy and spectral multiplexing in quantum computing, confirming that this cascaded comb generation method provides a pathway for creating complex optical systems with enhanced quantum properties.

Broadband Entanglement via Cascaded Frequency Combs

This research presents a theoretical exploration of a novel system for generating multiple frequency combs, interconnected through nonlinear optical processes. The team demonstrates how cascaded three-wave mixing, mediated by a single idler comb, can produce inter- and intracomb two-mode squeezing and entanglement across a broad spectral range, spanning from ultraviolet to mid-infrared frequencies. Importantly, the system allows for the engineering of on-demand multimode light through optimization of its covariance matrix. These findings offer potential advancements in several areas of quantum optics and spectroscopy.

The generated entangled states could be utilized in tunable broadband ghost spectroscopy protocols, squeezing-enhanced pump-probe measurements, and broadband entanglement schemes for multiplexed quantum information processing. The authors acknowledge that realizing this system experimentally presents significant challenges, particularly in controlling the nonlinear interactions and maintaining coherence across multiple combs. Future research directions include investigating the interplay between second and third-order nonlinearities within the multi-comb system, potentially leading to the realization of photonic higher-order topological insulators and chirally protected quantum states, establishing a promising framework for exploring multi-dimensional quantum optics and developing advanced technologies.