Sub-femtosecond Stabilization of Multicore Fiber Achieves 100% Duty Cycle for High-fidelity Quantum Networking

Multicore fibres, originally developed to increase the capacity of telecommunications networks, now offer exciting possibilities for precision science and quantum technologies. Takuma Nakamura, Nazanin Hoghooghi, and Nicolas Fontaine, along with colleagues from the National Institute of Standards and Technology, Nokia Bell Labs, and Sumitomo Electric Industries, demonstrate a significant advance in stabilising these fibres for ultra-reliable quantum networking. The team achieves unprecedented stability, measuring jitter in the attosecond range, less than a billionth of a billionth of a second, over a 40-kilometre fibre. This breakthrough enables continuous, 100% duty cycle operation of a quantum channel while simultaneously minimising unwanted noise, paving the way for more robust and efficient quantum communication networks.

Deployed Fiber Enables Attosecond Timing and Frequency Transfer

This research details the successful transfer of ultrastable optical frequencies and achievement of attosecond timing through a deployed multicore fiber. The study overcomes challenges in maintaining quantum coherence and enabling the simultaneous transmission of quantum and classical signals, crucial for building future quantum networks. Researchers successfully transferred an optical frequency with high stability over the deployed fiber, vital for synchronizing remote quantum nodes, and achieved timing resolution down to the attosecond level, demonstrating the potential for precise control of quantum operations. Importantly, the study demonstrated the ability to transmit both quantum and classical signals simultaneously within the same fiber, paving the way for practical hybrid networks.

The use of a multicore fiber allowed for the separation of quantum and classical signals, minimizing interference and improving performance. The deployed fiber exhibited ultra-low crosstalk and loss, contributing to the high stability and fidelity of the transferred signal. High system detection efficiency, achieved with superconducting nanowire single-photon detectors operating at 98% efficiency, further enhanced the experiment’s sensitivity. The method involves transferring a highly stable laser frequency through the fiber while employing techniques to cancel phase noise introduced by the fiber and other time-varying path lengths.

This research represents a significant step towards realizing practical quantum networks. The ability to transfer optical frequencies with attosecond precision and maintain coherence over deployed fiber opens up possibilities for secure quantum communication, enabling long-distance quantum key distribution. It also facilitates distributed quantum computing by connecting remote quantum processors to create a powerful distributed computer, and enables precise synchronization of remote sensors and instruments for applications in metrology and fundamental physics. This demonstrates the feasibility of building robust and scalable quantum networks using multicore fiber technology.

Multicore Fiber Stabilizes Quantum Light Transmission

Scientists engineered a novel approach to co-existing classical and quantum light transmission using a 40 kilometer spool of 7-core multicore fiber, effectively addressing challenges in quantum network stability and throughput. The study harnessed the high degree of noise correlation between adjacent cores within the fiber to achieve ultra-stable synchronization, enabling a 100% duty cycle on the quantum channel while minimizing spurious photon rates. Researchers utilized fan-in/fan-out devices to connect each of the seven cores to individual single mode fibers, carefully characterizing the total link loss at 1550nm to be approximately 10 decibels. The core innovation involved transmitting stabilization light for active fiber noise cancellation at 1550nm through a separate core from the 1542nm quantum signal, leveraging the inherent correlation of environmental instabilities between cores.

This technique dramatically reduced relative timing jitter, achieving an integrated jitter of only 400 attoseconds across the 40 kilometer fiber length, further refined to 100 attoseconds after accounting for laser source contributions. The method involves carefully controlling the power of the stabilization light and utilizing the inherent noise correlation between the fiber cores to minimize timing fluctuations. To quantify noise reduction, scientists employed a superconducting nanowire single photon detector to meticulously measure the Raman-scattered photon rate coupling into the quantum channel. By minimizing the launched stabilization light power to approximately 400 picowatts, the team demonstrated a spurious photon rate of less than 0. 01 photons per second within a 100 gigahertz optical bandwidth, representing a four-order-of-magnitude improvement over single-core wavelength-multiplexed systems. This level of noise suppression is comparable to the dark count rate of state-of-the-art single photon detectors, paving the way for high-fidelity, ultra-stable quantum networks capable of maintaining 100% duty cycle operation.

Stable Quantum Light via Multicore Fibers

Scientists have demonstrated a groundbreaking approach to co-existing classical and quantum light within optical fibers, utilizing a multicore fiber design to achieve unprecedented stability and minimize noise. The research team investigated a 7-core fiber over a 40km span, successfully employing one core for quantum signals and a separate core for stabilization light, leveraging the inherent noise correlation between cores. This innovative method allows for a 100% duty cycle on the quantum channel while maintaining a remarkably low spurious photon rate stemming from crosstalk. Experiments revealed an integrated jitter of only 400 attoseconds across the 40km fiber length, effectively stabilizing the optical path length.

Further refinement, by isolating the contribution from the source laser, reduced this jitter to an exceptional 100 attoseconds, demonstrating sub-femtosecond synchronization potential for quantum networking applications. The technique involves transmitting the stabilization light through a separate core, allowing for active cancellation of noise and stabilization of the quantum signal. The team achieved cycle-slip-free stabilization lasting over 6 hours, maintaining precise timing despite environmental fluctuations. Measurements confirmed a Raman scattering-induced spurious photon rate of just 0. 01 photons per second within a 100GHz bandwidth, a significant reduction compared to traditional single-mode fiber systems. This was accomplished through a combination of frequency detuning between the stabilization and quantum channels, and additional rejection of noise photons due to the low optical crosstalk between the fiber cores. The results demonstrate that multicore fibers offer a promising pathway towards ultra-stable quantum networks, enabling high throughput and maintaining signal integrity for sensitive quantum applications.

Multicore Fiber Enables Stable Quantum Networks

Researchers have demonstrated a promising approach to building high-fidelity quantum networks by leveraging the unique properties of multicore fiber. Their work addresses a significant challenge, the co-existence of quantum and classical light within the same network, by separating these signals into different, yet highly correlated, cores of a seven-core fiber. Over a 40-kilometer fiber length, the team achieved remarkably stable performance, maintaining sub-femtosecond timing jitter and, crucially, a very low rate of spurious photons that could interfere with quantum information. The key to this achievement lies in minimizing the power used for fiber stabilization to an extremely low 400 picowatts, combined with 40 decibels of optical isolation between the fiber cores.

This careful control resulted in a spurious photon rate of less than 0. 01 photons per second, a value far below the detector’s background noise. The method involves carefully controlling the power levels and optical isolation to minimize interference.