Quantum Local Area Networks For Practical Quantum Advantage
Quantum local area networks New Quantum Network Architectures Enable Robust Communication and Scalable Computing
The pursuit of practical quantum advantage is driving the rapid development of quantum local area networks (QLANs), which are essential for scaling quantum information processing outside of labs. Recent breakthroughs in cryogenic microwave QLANs for superconducting circuits and telecommunications-band QLANs for heterogeneous quantum networking reveal complementary methodologies. Both demonstrate advances in field-deployable, dependable quantum infrastructure that links quantum systems.
Superconducting Circuit Cryogenic Microwave QLANs
Researchers connected two dilution cryostats over 6.6 meters to create a microwave QLAN prototype for superconducting technology. This “cryolink” creates a quantum communication channel between distributed network nodes for superconducting circuit-based distributed quantum computing. The prototype has two dry dilution refrigerator quantum nodes, “Alice” and “Bob,” and an intermediary “cold node” named “Eve”. Bob and Eve are commercial Oxford Instruments NanoScience (OINS) cryostats, with Eve cooling the outer radiation shields without a dilution unit. Alice is a home-built cryostat. Layered low-emissivity radiation screens and vacuum insulation limit heat absorption throughout the system. A full 80-hour cooldown delivers base temperatures of 35 mK at the Alice mixing chamber (MC) stage, 21 mK at the Bob MC stage, and 52 mK at the centre of the MC tube.
The quantum communication channel is three 6-meter-long superconducting NbTi coaxial cables in the cryolink's deepest stage, the MC tube. Because they are strategically supported by PEEK holders and loosely thermally coupled to the Alice and Bob MC stages, these cables have a steady-state centre temperature of 110 mK.
This arrangement can intensely heat the cables in the centre of the cryolink, allowing researchers to study how thermal noise impacts quantum features without changing Alice and Bob's base temperatures. At a carrier frequency of 5.65 GHz, NbTi cables exhibit extremely low dielectric losses of 1.01 dB km−1, crucial for quantum microwave propagation. With this inventive setup, the group demonstrated: Remote dilution refrigerators' continuous-variable entanglement distribution. This was achieved by obtaining exceptional two-mode compressed microwave states. High temperatures preserve quantum entanglement: Quantum entanglement persisted at 1 K channel centre temperature (Tcenter). This matters because “sudden death of entanglement” would ordinarily occur at 1 K, which is equivalent to 3.3 thermal photons per mode if noise were coupled traditionally.
This robustness comes from the fluctuation-dissipation theorem and superconducting coaxial cables' tiny losses. According to theory, superconducting cables have a very small dissipation spectrum at frequencies below the superconductor gap frequency (370 GHz for NbTi). While the wire is superconducting, this decouples the electromagnetic modes inside it from the neighbouring thermal baths, preventing the hot environment from affecting quantum states.
Our study reveals that microwave quantum state transfer and entanglement dispersion throughout the transmission line do not require millikelvin temperatures, which has substantial technological implications. If the transmission lines remain superconducting and have low absorption losses, they can operate at liquid helium or nitrogen temperatures. This work lays the framework for future networked quantum computer architectures and enables quantum teleportation, secret sharing, multipartite entanglement research, and new dark matter axion searches.
Telecom-Band QLANs for Heterogeneous Quantum Networking
Another group of U.S. Air Force Research Laboratory researchers, led by Erin Sheridan, Nicholas J. Barton, and Richard Birrittella, has been developing and implementing telecommunications wavelength QLANs. Because they work with several quantum technologies, heterogeneous networks can link different quantum systems. Their hybrid QLANs connect tough outside environments to controlled laboratory settings via optical fibre and free-space communication. This flexible platform was designed to evaluate quantum communication methods in realistic settings. The team proved these networks worked in tough outdoor conditions like forests by spreading entangled photons throughout the fibre. Performance measurements showed a high degree of entanglement with a Clauser-Horne-Shimony-Holt (CHSH) inequality violation of 2.700, which is close to the theoretical maximum of 2.828. This shows that the networks can preserve delicate quantum states over extended distances, which is essential for future quantum applications.
The successful field-deployment of quantum networks using optical fibre and free-space links shows their practicality under varied scenarios. These multifunctional testbeds' environmental monitoring and signal quality data will help stabilise solutions, guide quantum network simulations, and improve network behaviour forecasts.
To achieve true interoperability, this research will connect matter-based quantum devices like trapped ions and superconducting qubits to boost QLAN capabilities. This effort makes it possible to build powerful and adaptive quantum networks for secure communications and distributed quantum computation.
A Unified Quantum Internet Vision
Both research advance quantum networking, a fast-growing field. The cryogenic microwave QLAN shows that transmission lines may maintain quantum coherence at much higher temperatures than previously thought, easing the communication backbone's tight cooling requirements. This affects the scalability and practicality of superconducting circuit-based distributed quantum computing systems. However, telecommunications-band QLANs demonstrate the ability to seamlessly integrate many quantum technologies into a network and the adaptability and robustness of quantum communication in various circumstances. Overall, these advancements accelerate the creation of a worldwide quantum internet where quantum networks are vital to sensing, communication, and computing. The feasibility of such advanced quantum infrastructure is growing.
