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Cryogenic microwave link between refrigerators ic_cryolink

Superconducting waveguide held below 50 mK along its whole length, carrying itinerant 5–7 GHz photons or squeezed states between two independently cooled dilution refrigerators.

Cryogenic microwave link between refrigerators

ic_cryolink · layer 9 Interconnect · emerging · since 2020
Open on the map

30 m superconducting waveguide at < 50 mK; Bell fidelity 80.4% at 12.5 kHz; 0.55–0.65 dB total loss.

Reading marks
crossing technology — fabricated carrier + far connectivity (transport / long-range)
Brief →

Key references

[772] P. Magnard et al., “Microwave Quantum Link between Superconducting Circuits Housed in Spat… · 2020
[305] S. Storz et al., “Loophole-free Bell inequality violation with superconducting circuits,” … · 2023
[773] J. D. Schär et al., “A Modular Cryogenic Link for Microwave Quantum Communication Over Dis… · 2026
[774] W. K. Yam et al., “Cryogenic microwave link for quantum local area networks,” npj Quantum … · 2025

Design space — attributes

(a) carrier affinity
fabricated
(b) time · entangling
—
(c) readout
—
(d) mobility
long-range static couplers
(e) control
microwave @ millikelvin stage
(f) error structure
loss (erasure), coherent / calibration
(g) manufacturing
superconducting lithography

Evaluation space — dated attributes

30 m link Bell fidelity / rate → 80.4% / 12.5 kHz
defines: error channel · 2023-05 · source
2026 modular link → waveguide loss < 0.03 dB/30 m; end-to-end transfer loss 0.55–0.65 dB, i.e. 86–88% transmission
defines: scaling path · 2026-04 · source

ETH Zürich. A '75–79% transfer efficiency' sometimes quoted is not implied by 0.55–0.65 dB, which is 86–88% transmission; the ETH paper attributes the loss to the 26 demountable module joints, and any residue would be node emission/absorption efficiency.

Actors & goals — annotations

Transmon lattice with tunable couplers (alternate) — IBM (Heron, Nighthawk), Google (Willow), Rigetti, IQM, OQC, USTC/Zhejiang, Fujitsu/RIKEN, QuantWare · G2 G3 G4

Edges

needs: Superconducting-qubit lithography (Al/AlOx/Al junctions, Nb wiring, 300 mm)
alternatives: Multi-chip modules / l-couplers (same cryostat), Microwave–optical transducer (useful efficiency)
° one of several that would do · the usual route, not a strict need

Used by no registered machine

Pictures

Every picture carries the story of what it shows and its credit line.

Fig. 3 — Fig. 3: Cryogenic microwave quantum link.a, Computer-aided design (CAD) model. b–d, Photographs of the 30-m-long cryogenic set-up. Dilution refriger…
A four-part figure of a 30-metre cryogenic link between two refrigerators. Panel (a) is a computer model of the whole set-up: a long horizontal tube joins dilution refrigerators A and B, each holding a superconducting qubit circuit (insets), with a central cooling unit and support stands. Panels (b)–(d) are photographs from end A, the centre and end B, showing the aluminium tube running along a basement corridor on frames. From Nature (2023), 'Loophole-free Bell inequality violation with superconducting circuits', by Simon Storz and co-authors. The waveguide inside is kept below 50 mK over the whole distance, so microwave photons can link qubits in separate cryostats. “Fig. 3 — Fig. 3: Cryogenic microwave quantum link.a, Computer-aided design (CAD) model. b–d, Photographs of the 30-m-long cryogenic set-up. Dilution refrigerators at…” by Storz et al., www.nature.com/articles/s41586-023-05885-0, CC BY 4.0 (creativecommons.org/licenses/by/4.0/)
Fig. 1 — Figure 1: (a) CAD rendering of the transmon emitter/detector (TED) layout and (b) its corresponding circuit, indexed by x∈{s,m}x\{s,m\} to distingui…
A four-panel figure: a CAD rendering of a superconducting chip layout (200 µm scale bar), its circuit diagram, a measured plot of frequency against applied flux, and a block schematic of source and detector devices. It comes from the arXiv paper "Transmon Architecture for Emission and Detection of Single Microwave Photons" by Daniel L. Campbell and co-authors. It shows three coupled transmons — storage, waveguide-linked, and a flux-driven coupler between them — that release or catch single microwave photons; notice the storage transmon's frequency barely moves. “Fig. 1 — Figure 1: (a) CAD rendering of the transmon emitter/detector (TED) layout and (b) its corresponding circuit, indexed by x∈{s et al., arxiv.org/abs/2601.11378, CC BY 4.0 (creativecommons.org/licenses/by/4.0/)
Fig. 1 — Figure 1: Photonic link between refrigerators. (a) Schematic of the photonic link. Felix up-converts microwave photons (ωm) to optical photons
A two-panel figure: a render, not a photograph, of two dilution refrigerators holding transducers named Felix and Albert, joined by a 1-km optical fibre, and a plot of signal transmission against link length. It comes from the arXiv paper "A kilometer photonic link connecting superconducting circuits in two dilution refrigerators" by Yiyu Zhou and co-authors. It illustrates microwave–optical transducers: microwave photons become light to cross fibre losing 0.2 dB/km instead of coaxial cable losing 1 dB/m; notice the transducer pair's −60 dB versus −140 dB for commercial modulators. “Fig. 1 — Figure 1: Photonic link between refrigerators. (a) Schematic of the photonic link. Felix up-converts microwave photons (ωm) to optical photons…” by Yiyu Zhou et al., arxiv.org/abs/2508.02444, CC BY 4.0 (creativecommons.org/licenses/by/4.0/)
ic_cryolink · layer 9 Interconnect · centrality 11.9 · emerging · since 2020 · updated 2026-09-30

Cryogenic microwave link between refrigerators

Superconducting waveguide held below 50 mK along its whole length, carrying itinerant 5–7 GHz photons or squeezed states between two independently cooled dilution refrigerators.

Verdict. Two groups hold the headlines: ETH Zürich for photon transfer and Bell tests — 30 m, Bell fidelity 80.4% at 12.5 kHz, loophole-free — and TU Munich with VTT for squeezed states over 6.6 m. Loss sits in the demountable joints, not the waveguide, and no architecture puts this on a fault-tolerance critical path.

Key references

Λ = error-suppression factor per code-distance step; QBI = DARPA Quantum Benchmarking Initiative (Stage A concept → B R&D plan → C government V&V); G1–G7 = the report's goal classes (see Actors & economics).

A superconducting waveguide cooled far below aluminium's superconducting transition (≈1.2 K) along its whole length, carrying itinerant ~5–7 GHz photons between two separately cooled dilution refrigerators — not between chips in one cryostat. Wallraff's group at ETH Zürich introduced it in 2020: 5 m, state transfer 85.8%, remote Bell pairs 79.5% D[772]. The 30 m successor carried the first loophole-free superconducting Bell test in 2023 D[305]; the 2026 paper documents it as a reproducible modular assembly D[773]. TU Munich with VTT runs the only independent second line, sending squeezed Gaussian states over 6.6 m D[774]. Attributes: d = long-range inter-cryostat connectivity, the only layer entry spanning separate refrigerators; e/f/g = millikelvin microwave drive at both fridges, loss and coherent phase rather than Pauli errors, superconducting lithography with demountable interfaces (graph record).

The waveguide is not the limit: attenuation is α < 1 dB/km, under 0.03 dB across the full 30 m — the same figure in the 2023 and 2026 papers D[305], [773]. End-to-end photon transfer loss is nonetheless 0.55–0.65 dB (12–14%), which the 2026 paper attributes to the interfaces between modules, not the line D[773]: every one of the 26 demountable joints costs transmission.

Second, thermal occupation. At 5.6 GHz a photon is worth 0.27 K, so a 50 mK channel holds 4.6×10⁻³ residual photons, while the 1 K channel centre at which Munich/VTT still resolved entanglement holds ≈3.3 D[774]. Fock transfer degrades roughly linearly in that occupation; Gaussian squeezed states tolerate it, so squeezing is the cheaper carrier to lengthen. Third, phase: two mechanically independent cryostats need a common reference, and no source quantifies the residual phase noise — the term that would bound a repeatered link. What moves the floor: welded or bump-bonded joints replacing demountable flanges.

Interface loss dominates, ~20× the waveguide's own contribution D[773]; no chained (≥3-fridge) link, no link run during gate execution, and no independent 30 m replication as of 2026-09-04.

Every link is bespoke. The ETH 30 m assembly is 26 modules plus two nodes and a central cooling unit, holding the 4 K stage under 6 K across the span; the 6.5-day cooldown, not fidelity, is what makes iteration slow, and simulation puts one cooling unit per ~15 m out to 120 m D[773]S[773]. Base cryostats come from a concentrated market: Bluefors closed its Cryomech acquisition (pulse-tube and Gifford-McMahon cryocoolers) on 2023-03-28, giving ~600 staff and revenue above EUR 160 M C[524]; ULVAC is a Japanese third source built with IBM input C[525]fact. Export exposure is inherited: ECCN 3A904 (BIS, 2024-09-06) covers refrigerators sustaining ≥600 µW at 0.1 K for 48 h; no ECCN names a link G[301]fact.

Each attempt needs a flux-tunable coupler at both ends and microwave sources phase-locked across two cryostats. The 2023 Bell test carries this report's hardest real-time constraint: 32.824 m of separation leaves a 109 ns window in which basis choice, gate, readout and record must all complete D[305]. The 12.5 kHz repetition rate — 80 µs per attempt — is set by reset and pulse shaping, not time of flight. Nothing is published at 10³–10⁶ qubits; the binding quantities are cryoplant count and independent phase references.

The only architecture is superconducting transmon (Google, IBM, Rigetti, IQM, OQC, USTC/Zhejiang, Fujitsu); it requires superconducting-qubit lithography. It competes with same-fridge modularity — IBM coupled two cryogenic cells in 2026-08, l-couplers at "one meter scale", with no qubits and no fidelity reported C[50]fact — and with microwave-optical transduction, best total efficiency 47% and nothing at η > ½ with sub-unity added noise P[775]fact. Derived clock: 80 µs per entanglement attempt (2 s.f.) against a 1.1 µs surface-code cycle — ~73× slower, under two orders of magnitude. Adjacent empty slot: a cryogenic microwave switch, absent which N fridges need N(N−1)/2 waveguides.

A loophole-free violation of the CHSH (Clauser–Horne–Shimony–Holt) Bell inequality with space-like separated measurements is device-independent, not tomographic — unusually strong verification for this layer D[305]. Munich rests on homodyne tomography D[774]; the two are not comparable, neither replicated, and both ran with local processors idle. Conflict: the graph record's 75–79% transfer efficiency is not what 0.55–0.65 dB implies alone (86–88%).

Not published: the residual phase noise between the two cryostats.

Who.
| Organisation | Role | Country | What exactly they do with this technology | Evidence |
|---|---|---|---|---|
| ETH Zürich | Research | Switzerland | Built the 5 m, 30 m and modular links; holds the photon-transfer and Bell-test headlines | D[305], [772], [773] |
| TU Munich | Research | Germany | 6.6 m squeezed-state link with VTT — the independent second line, with its own headline | D[774] |
| Bluefors | Supplier | Finland | Refrigerator and cryocooler vendor; KIDE is the rival path | C[303]fact |
| ULVAC Cryogenics | Supplier | Japan | ~10 mK refrigerator with IBM input, tested at Poughkeepsie | C[525]fact |
| IBM | Developer | USA | Runs the competing intra-fridge modular-cryogenics line | C[50]fact |

Money. 2023-03-28 · Bluefors · M&A (Cryomech) · undisclosed · combined revenue > EUR 160 M · closed C[524]. 2024-09-06 · US BIS · ECCN 3A904 rule · n/a · in force G[301]. 2025-03-20 · ULVAC · ~10 mK refrigerator programme with IBM · undisclosed · "early 2026" target passed, no shipment confirmed as of 2026-09-30 C[525]. No round, grant or acquisition specific to an inter-fridge link exists as of 2026-09-04.

Market & supply chain. There is no link market, only a refrigerator market in which a link consumes two units instead of one; Oxford Instruments is the second merchant source C[525]. Bluefors plus Cryomech supplies the dilution stage and the pulse tubes beneath it with no alternative at that volume C[524]. G6 (networking) pays directly; G4 pays only once a cryostat's line budget runs out, which KIDE's >4,000 RF lines are designed to postpone C[303]fact.

IP & standards. No patent family specific to cross-cryostat microwave links in any named database and no standards body addressing cryogenic microwave interfaces — no dated fact found; module geometry is published openly D[773].

Roadmaps & track record. ETH Zürich: (2023-05 · 30 m Bell test · delivered); (2026-04 · 120 m via cooling units per ~15 m · simulated only) S[773]. ULVAC/IBM: (2025-03 · refrigerator "early 2026" · target passed, unconfirmed as of 2026-09-30) C[525]. IBM: (2026-08 · two coupled cells · delivered, no qubits in them) C[50]. ETH has met every milestone it announced; both vendor timelines are unverified.

Strategic reading. If this works the cryostat suppliers capture the value, not the QPU vendor. If it fails nothing breaks — every published superconducting roadmap scales inside one cryostat first. Substitution runs both ways: l-couplers remove the need below ~1,000 qubits per cell C[50], transduction removes the distance limit above it. No independent link vendor exists to bargain with.

Falsifiable (12–24 months): confirm if any group publishes a three-cryostat chained link, or a 30 m link run while both processors execute gates, by end-2027; demote if interface loss is not below 0.3 dB by then. Best case 2029: per-hop efficiency above 90% and an architecture that needs two fridges. Worst case: intra-cryostat modularity absorbs all growth through 2030 and this stays a Bell-test instrument. Open questions: residual inter-cryostat phase noise; whether joints can be non-demountable and still serviceable; whether anyone builds a cryogenic microwave switch. Watch: ETH follow-ons, IBM's first coupled cells with qubits in them.

References