# [SCI] Cryogenics
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**Cryogenics** is the science and technology of producing and using very low temperatures (below −150°C / 123 K), enabling the study of materials at the quantum mechanical limit and the infrastructure for superconducting devices.
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## Overview
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James Dewar liquefied hydrogen (1898) and Heike Kamerlingh Onnes liquefied helium (1908, 4.2 K) — the last and most difficult permanent gas to liquefy. In doing so, Onnes discovered superconductivity in mercury (1911), opening a new domain of physics. The Joule-Thomson effect (throttling expansion cools a gas) and the Linde refrigeration cycle (1895) provided the industrial basis for cryogenic gas production.
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Cryogenics is the experimental prerequisite for:
- **Superconductivity** — most superconductors require temperatures below ~20 K (achieved by liquid helium or modern cryocoolers)
- **Clinical MRI** — superconducting magnets operating at 4 K or 20 K create the 1.5–7 Tesla fields needed
- **Quantum computing** — superconducting qubits require millikelvin temperatures (achieved by dilution refrigerators reaching ~10 mK)
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## Key Figures & Recognition
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- **Heike Kamerlingh Onnes** (1853–1926): Helium liquefaction, discovery of superconductivity. **Nobel Prize 1913**.
- **Carl von Linde** (1842–1934): Industrial gas liquefaction (Linde process), 1895.
- **James Dewar** (1842–1923): Dewar flask (vacuum-insulated container), hydrogen liquefaction.
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## Seminal Papers
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- Kamerlingh Onnes, H. "The Superconductivity of Mercury." *Comm. Phys. Lab. Univ. Leiden* 120b (1911).
- Linde, C. "Verfahren zur Verflüssigung atmosphärischer Luft." DE Patent 88,824 (1895).
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## What This Enables
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- **[SCI] BCS Superconductivity** — Cryogenic experiments revealed superconductivity in 1911; BCS theory (1957) is the explanation of what cryogenics discovered.
- **[TECH] MRI (Magnetic Resonance Imaging)** — Clinical MRI magnets use superconducting wire cooled by liquid helium or cryocoolers to achieve the necessary field strengths.
- **[TECH] Quantum Computing Hardware** — Superconducting qubits (IBM, Google) operate at ~15 mK in dilution refrigerators, only achievable with cryogenic technology.
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# Parents
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* [SCI] Classical Thermodynamics⏎