Glossary term
Physical Layer
The part of the AI stack that sells compute — chips, memory, packaging, optical interconnect, power, cooling and data-centre construction — as opposed to the application layer that buys it. The two halves of the AI trade move in opposite directions when the discount rate or the pace of the frontier changes.
AI-generated — produced automatically by Closelook’s systems under this site’s editorial policy.
What it means
Every AI product runs on hardware that somebody manufactured, powered and cooled. The physical layer is that hardware and the companies that make it: accelerators and their foundries, high-bandwidth memory, advanced packaging, optical transceivers and cable, power semiconductors, transformers, cooling, racks and the contractors that build data centres. Its revenues are capital expenditure by the buyers of compute; its valuations carry whatever scarcity premium the build-out’s pace justifies.
The application layer — models, software, security, agents — buys compute and sells what runs on it. Cheaper compute is a cost for the first group and a margin for the second.
Why it matters for the AI trade
The physical layer is the longest-duration asset in the market: its cash flows sit in backlogs and in bonds maturing in 2050. A rise in the real yield, or a slower frontier, lowers their present value first. On 14 September 2026 the chip index fell 5.6%, the Rubin Build-Out 100 5.25% with all eight layers red, while the software index rose 5% and the Agentic Winners gained 4%. The trade was sorted by which side of the compute market each company sits on.
How Closelook uses it
Rubin is the physical layer as an index: eight layers from architects and foundry through equipment, materials, memory, interconnect and power to data-centre systems, each with its own sub-index. The six-layer model is the framework the split comes from; the hyperscaler and neocloud entries describe the buyers.
Common questions
- Which companies are in the physical layer?
- Chip designers and foundries (Nvidia, AMD, TSMC), memory makers (SK Hynix, Samsung, Micron, SanDisk), equipment (ASML, Applied Materials, Lam, Tokyo Electron, Advantest), optical and interconnect (Coherent, Lumentum, Corning), power and cooling (Vertiv, Schneider, Siemens Energy) and the data-centre builders.
- Why does the physical layer fall when rates rise?
- Its value depends on cash flows years away — backlogs, multi-year contracts, long bonds — and a higher real yield discounts distant cash flows more than near ones. Software collected this quarter is less exposed.
- Is the physical layer the same as the AI trade?
- It was, for two years, in most portfolios. Since September 2026 the two halves have diverged, and Closelook reads them as a pair rather than as one position.