A Modular Cryogenic Link for Microwave Quantum Communication Over Distances of Tens of Meters
Josua D. Schär, Simon Storz, Paul Magnard, Philipp Kurpiers, Janis Lütolf, Melvin Gehrig, Jean-Claude Besse, Anatoly Kulikov
Abstract
Quantum technologies promise a radically new way to solve classically intractable computing problems. Superconducting circuits as a platform are at the forefront of this field. The cryogenic operation temperatures of superconducting circuits however impose challenges for the further scaling to many connected quantum information processing units into a local area or global network. In this work, we present a hardware solution for connecting quantum devices operating at microwave frequencies into local area networks, which enable the exchange of quantum information between spatially separated parties. Specifically, we demonstrate a modular system spanning distances of 5, 10 and 30 meters operated at cryogenic temperatures and connecting two superconducting circuit systems, located in individual dilution refrigerators, through a quantum communication channel. We develop a thermal model to evaluate the heat transfer processes in the setup, optimize the design and select appropriate materials for its construction. The assembled 30-meter-long system achieves operating temperatures of below 50 mK after a cooldown time of about six and a half days. This link enables the execution of distributed quantum computing and communication algorithms. It also adds the resource of non-locality, certified by a loophole-free Bell test, to the field of quantum science and technology with superconducting circuits.
AI Impact Assessments
(3 models)Scientific Impact Assessment
Core Contribution
This paper presents the engineering design, thermal modeling, and experimental demonstration of a modular cryogenic link capable of connecting two dilution refrigerators over distances of 5, 10, and 30 meters for microwave quantum communication between superconducting circuits. The core innovation is a scalable, modular architecture comprising link modules, adapter modules, braid modules, and an intermediate cooling unit that maintains base temperatures below 50 mK along the entire 30-meter length. The work also develops a validated thermal model that enables extrapolation to systems up to ~120 meters using additional cooling units every 15 meters.
The problem addressed is fundamental: superconducting qubits operate at millikelvin temperatures and communicate via microwave photons, which unlike optical photons cannot propagate through room-temperature channels without prohibitive thermal noise and loss. This paper provides the critical infrastructure enabling spatial separation of quantum processors—a prerequisite for distributed quantum computing and foundational tests like loophole-free Bell tests with superconducting circuits.
Methodological Rigor
The thermal engineering is thorough and systematic. The authors develop a cascaded 1D thermal model that accounts for radiative heat loads (Stefan-Boltzmann law with adapted emissivities), conductive loads through support posts, heat flow through radiation shields, and contact resistances at module interfaces. They validate this model against measurements from three progressively longer systems (5, 10, 30 m), showing good agreement across all temperature stages.
The material characterization is extensive: thermal conductivity measurements of three copper grades (ETP, OF, OFE) using a custom dipstick apparatus; mechanical and thermal characterization of Bluestone (a 3D-printed nano-composite) for support posts; and quantification of multi-layer insulation performance. The characterization of braid module thermal resistance, decomposed into bulk and contact contributions with distinct temperature scalings (contact resistance ∝ T⁻²), provides actionable engineering insight.
The paper systematically addresses all major design constraints: thermal contraction management (up to 125 mm over 30 m for the aluminum waveguide), radiation shielding optimization (MLI reducing heat flux from ~6.4 to ~1.0 W/m²), and material selection based on the yield-strength-to-thermal-conductivity ratio for support posts.
One limitation is that the thermal model uses several simplifications (1D heat flow, neglecting circumferential temperature variations, cascaded stage-by-stage calculation). While justified and validated, the extrapolation to 120 meters relies on these approximations holding at scales where new thermal challenges might emerge. The 50K stage temperature drift (~0.2 K/day due to gas diffusion through O-rings degrading MLI) limits continuous operation to ~6 months, which is acknowledged but not fully resolved.
Potential Impact
The immediate impact is enabling a new class of experiments with superconducting circuits. The paper explicitly notes this infrastructure has already been used for: deterministic quantum state transfer over 5 m, a loophole-free Bell test over 30 m, device-independent self-testing, and device-independent randomness amplification—each representing landmark results in the field.
For distributed quantum computing, this work provides a concrete pathway to connecting multiple superconducting quantum processors into local area networks. The modular design philosophy—standardized modules that can be manufactured, tested, and assembled independently—is practical for real deployment. The waveguide channel achieves remarkably low loss (<0.03 dB over 30 m, or <1 dB/km), though the dominant loss currently comes from chip-to-waveguide interfaces (0.55-0.65 dB total).
The thermal model and design principles are transferable to other cryogenic systems requiring extended low-temperature environments, including particle physics experiments and other quantum technology platforms operating at millikelvin temperatures.
Timeliness & Relevance
This work is highly timely. The quantum computing community widely recognizes that scaling beyond single-cryostat qubit counts will likely require modular, networked architectures. While microwave-to-optical transduction remains an active research area, no current system achieves the required combination of efficiency, bandwidth, and low noise. The cryogenic microwave link provides a working alternative for local-area quantum networks today.
The 30-meter distance is particularly significant as it exceeds the threshold needed for space-like separation in Bell tests with superconducting circuits, enabling foundational physics experiments previously inaccessible to this platform. The extrapolation to 120 meters would enable inter-building quantum links.
Strengths
1. Complete engineering solution: The paper covers the full design-build-test cycle, from material selection through thermal modeling to multi-length demonstration, providing a comprehensive reference for the community.
2. Validated thermal model: Agreement between model and experiment across three system lengths builds confidence in extrapolations and enables informed design of future systems.
3. Practical modularity: The modular architecture with standardized 2.5 m link modules addresses real-world concerns of manufacturing, assembly, maintenance, and extensibility.
4. Demonstrated scientific utility: The system has already enabled multiple high-impact experiments, proving its practical value beyond engineering demonstration.
5. Extensive appendices: The detailed characterization data (copper conductivity, Bluestone properties, MLI performance, contact resistances) constitutes a valuable resource for the cryogenic engineering community.
Limitations
1. Operation lifetime: The 6-month continuous operation limit due to MLI degradation from gas diffusion is a practical constraint, though mitigable with improved vacuum seals.
2. Interface losses dominate: The chip-to-waveguide connection contributes more loss than the 30 m waveguide itself, limiting quantum communication fidelity—though this is not a limitation of the link per se.
3. No dilution-unit cooling along the link: The intermediate cooling unit provides only 50K/4K cooling; extending to >120 m would require additional dilution refrigerators, significantly increasing cost and complexity.
4. Scalability to networks: The paper discusses point-to-point links but does not address the engineering challenges of creating multi-node grid topologies.
Overall Assessment
This is a strong engineering physics paper that solves a concrete infrastructure problem enabling new quantum science with superconducting circuits. While the physics concepts (thermal radiation, heat conduction, thermal contraction) are well-established, the careful optimization, characterization, and demonstration at the 30-meter scale represents significant experimental achievement with clear scientific impact already demonstrated through multiple landmark experiments.
Generated Apr 20, 2026
Comparison History (50)
Paper 2 presents a significant hardware breakthrough for scaling quantum computers through distributed networks, addressing a major bottleneck in superconducting quantum computing. While Paper 1 offers a valuable computational method for simulating quantum systems, Paper 2 provides a tangible, real-world infrastructure (a 30-meter cryogenic link) that directly enables distributed quantum computing and long-distance entanglement, promising broader and more immediate technological impact across the quantum information field.
While Paper 1 presents an impressive hardware engineering feat for scaling quantum networks, Paper 2 tackles the most critical bottleneck in modern quantum computing: real-time quantum error correction (QEC). By successfully demonstrating ultra-low latency FPGA-based neural network decoding for surface codes, Paper 2 paves the immediate pathway to scalable fault-tolerant quantum computation. Real-time feedback is universally essential for non-Clifford logical operations, making this methodological breakthrough highly impactful and broadly applicable to the survival and advancement of the entire quantum computing field.
Paper 1 demonstrates real-time quantum error correction with an FPGA-based neural network decoder on actual hardware, addressing a critical bottleneck for fault-tolerant quantum computing. The 550 ns closed-loop latency within a 1.25 μs QEC cycle is a significant engineering and scientific milestone. While Paper 2 presents an impressive cryogenic interconnect enabling distributed quantum computing and a loophole-free Bell test with superconducting circuits, Paper 1 tackles the more fundamental and immediately pressing challenge of scalable QEC—a prerequisite for practical quantum computation—with demonstrated real-time performance comparable to offline decoding.
Paper 1 claims an order-of-magnitude reduction in the physical qubits required to break RSA-2048, drastically accelerating the projected timeline for cryptographically relevant quantum computers. This has profound global security implications across all digital infrastructure, forcing faster adoption of post-quantum cryptography. While Paper 2 presents a crucial engineering milestone for quantum networking, Paper 1's architectural breakthrough poses a paradigm-shifting threat to current cryptographic standards, giving it a broader and more urgent scientific and real-world impact.
Paper 2 likely has higher impact: it delivers a concrete enabling hardware capability—tens-of-meters cryogenic microwave links between separate dilution refrigerators—directly addressing a major bottleneck for scaling superconducting quantum processors into networks. The work appears methodologically rigorous (thermal modeling, materials/design optimization, demonstrated 5/10/30 m systems, achieved <50 mK) and highly timely for distributed QC and modular architectures, with broad relevance across superconducting QC, quantum networking, and fundamental tests (loophole-free Bell). Paper 1 is novel but more speculative, especially on cryptographic primitives, and its practical adoption may be less immediate.
Paper 2 demonstrates a concrete hardware breakthrough—a 30-meter cryogenic microwave link enabling loophole-free Bell tests with superconducting circuits and distributed quantum computing. This addresses a critical scaling bottleneck for superconducting quantum processors and opens practical paths toward quantum networks. Its experimental nature, real-world applicability, and significance for both distributed quantum computing and fundamental tests of non-locality give it broader and more immediate impact. Paper 1 presents interesting theoretical ideas about LCU circuits but its applications (matrix completion, trapdoor functions) remain speculative and preliminary.
Paper 1 demonstrates a groundbreaking experimental achievement by physically connecting quantum systems over 30 meters at millikelvin temperatures. This hardware breakthrough directly solves the critical physical scaling bottleneck of single dilution refrigerators, enabling immediate distributed quantum computing and local quantum networks. Experimental realizations of this magnitude generally yield broader and more immediate scientific impact than the architectural and algorithmic optimizations presented in Paper 2.
Paper 1 demonstrates a concrete, modular cryogenic microwave link over up to 30 m connecting two dilution refrigerators, backed by thermal modeling/engineering optimization and culminating in an operating platform (<50 mK) that enables distributed superconducting-qubit networking and even supports loophole-free Bell nonlocality. This is a timely, high-rigor advance addressing a major scalability bottleneck with clear near-term real-world applications across quantum computing and networking hardware. Paper 2 is conceptually novel and potentially broad, but appears more theoretical/architectural and may face greater practical/experimental validation hurdles.
Paper 1 presents a significant experimental breakthrough by demonstrating a 30-meter cryogenic link for superconducting circuits. This addresses a major, immediate bottleneck in scaling quantum computing and local area quantum networks. While Paper 2 offers an innovative theoretical concept for utilizing noise in entanglement generation, Paper 1's tangible hardware solution and proven implementation at scale provide more direct and immediate practical impact on advancing current quantum technologies.
Paper 2 likely has higher near-term scientific impact due to a concrete, scalable hardware advance enabling cryogenic microwave quantum links over 10–30 m between dilution refrigerators, directly addressing a key bottleneck for superconducting-networked quantum computing. It demonstrates system-level engineering (thermal modeling, material choices) with clear performance metrics (<50 mK over 30 m) and broad applicability to distributed quantum computing, communication, and loophole-free Bell tests. Paper 1 is conceptually novel but more speculative and may require substantial validation and adoption before comparable cross-community impact.
Paper 1 presents a critical hardware breakthrough in quantum networking by demonstrating a 30-meter cryogenic link connecting superconducting circuits. This directly addresses a major bottleneck in scaling quantum computers—connecting multiple dilution refrigerators—paving the way for distributed quantum computing. While Paper 2 offers a strong theoretical advancement, Paper 1's tangible, real-world application to scaling quantum infrastructure gives it exceptionally high timeliness, relevance, and immediate impact across the rapidly growing quantum technology sector.
Paper 2 demonstrates a practical hardware solution for connecting superconducting quantum processors over tens of meters at cryogenic temperatures—a critical infrastructure challenge for scaling quantum computing. The achievement of a loophole-free Bell test with superconducting circuits and enabling distributed quantum computing represents a landmark experimental milestone. Its breadth of impact spans quantum computing, quantum networking, and fundamental physics. While Paper 1 presents a sophisticated theoretical framework for quantum magnetometry with notable innovations (transient sensing, noise cancellation, scalability), Paper 2 addresses a more pressing bottleneck in quantum technology scaling with demonstrated experimental results.
Paper 1 presents a profound hardware breakthrough by demonstrating a 30-meter cryogenic link, solving a critical physical bottleneck for scaling superconducting quantum computers. While Paper 2 offers valuable algorithmic optimizations for quantum circuit design using reinforcement learning, Paper 1 directly enables distributed quantum computing networks and macroscopic quantum communication. Overcoming the thermal and engineering challenges of connecting multiple dilution refrigerators opens an entirely new paradigm for quantum local area networks, granting it a much higher potential for foundational scientific and technological impact.
Paper 2 presents a significant hardware breakthrough by demonstrating a 30-meter cryogenic link for superconducting circuits. This directly addresses the critical bottleneck of scaling quantum computers beyond a single cryostat, paving the way for distributed quantum computing and local area quantum networks. While Paper 1 provides a highly useful software framework for QEC evaluation, Paper 2's physical demonstration of macroscopic quantum communication infrastructure has a more profound and immediate impact on the physical realization of large-scale quantum technologies.
Paper 1 demonstrates a groundbreaking hardware achievement—a modular cryogenic link enabling quantum communication over tens of meters between superconducting circuits, including a loophole-free Bell test. This addresses a fundamental scaling bottleneck for superconducting quantum computing and opens entirely new experimental capabilities (distributed quantum computing, certified non-locality). Paper 2 presents valuable architectural optimizations for neutral atom fault-tolerant computing with concrete speedups, but is more incremental in nature. Paper 1's broader impact across quantum networking, distributed computing, and fundamental physics gives it higher potential scientific impact.
Paper 1 likely has higher near- to mid-term scientific impact due to a concrete, scalable hardware advance: a modular tens-of-meters cryogenic microwave link between dilution refrigerators, with thermal modeling, material optimization, and demonstrated <50 mK operation. This directly enables distributed superconducting-qubit networking and experiments (e.g., loophole-free Bell tests) and is timely for scaling quantum processors. Paper 2 is highly novel and rigorous theoretically, but its impact is more specialized and may influence error-correction theory and noise modeling rather than immediately enabling new experimental capabilities.
Paper 1 presents a significant hardware breakthrough by demonstrating a 30-meter cryogenic link for superconducting circuits, directly addressing the critical physical scaling bottleneck of quantum computers. While Paper 2 offers valuable theoretical insights into the barren plateau problem in variational quantum algorithms, Paper 1's tangible experimental achievement enables distributed quantum computing and quantum networking, promising a broader and more immediate real-world impact on the physical realization of large-scale quantum technologies.
Paper 1 demonstrates a breakthrough hardware infrastructure for connecting superconducting quantum processors over tens of meters at cryogenic temperatures, enabling distributed quantum computing and a loophole-free Bell test with superconducting circuits. This addresses a critical scaling bottleneck for one of the leading quantum computing platforms, with immediate practical applications for quantum networking. While Paper 2 makes a meaningful theoretical contribution to hybrid CV-DV error correction, Paper 1's experimental demonstration of modular quantum interconnects has broader and more immediate impact on the field's trajectory toward scalable quantum computing.
Paper 2 demonstrates a modular cryogenic link enabling microwave quantum communication over tens of meters between superconducting circuits, including a loophole-free Bell test—a landmark achievement for this platform. This addresses a critical scaling bottleneck for superconducting quantum computing, which is the leading quantum computing platform. The infrastructure enabling distributed quantum computing and certified non-locality with superconducting circuits opens new experimental capabilities across multiple research directions. Paper 1 presents a valuable decoherence suppression scheme, but it is more incremental in nature, refining existing loss-mitigation approaches in optical systems.
Paper 1 demonstrates a major experimental breakthrough by building a 30-meter cryogenic link, directly addressing the critical bottleneck of scaling superconducting quantum computers. This hardware innovation enables distributed quantum computing and local area networks, offering immediate and practical impacts. While Paper 2 presents a strong theoretical framework for quantum metrology, Paper 1's experimental realization of large-scale quantum networking infrastructure is likely to drive wider real-world applications and immediate technological advancements across the quantum sector.