Squidra

Squidra, a superconducting sorcerer, cradles a glowing ring of current that can feel a fraction of a single flux quantum.

The Physics

A dc SQUID is a superconducting loop broken by two Josephson junctions. The current waves through the two junctions interfere, so the voltage across the device swings periodically as the magnetic flux through the loop changes. One full period is one flux quantum. R. C. Jaklevic, J. Lambe, A. H. Silver and J. E. Mercereau first showed this interference (“Quantum Interference Effects in Josephson Tunneling”, Phys. Rev. Lett. 12, 159, 1964).

Φ₀ = h/2e ≈ 2.067833848 × 10⁻¹⁵ Wb

• Φ₀: magnetic flux quantum (weber, Wb)
• h: Planck constant
• 2e: charge of a Cooper pair

Since the 2019 SI redefinition, h and e have exact values, so Φ₀ is exact too. Measuring a small fraction of a period makes SQUIDs the most sensitive magnetometers known, used to read signals from the brain.

Honest caveat: The voltage is periodic in Φ₀, but its exact shape depends on the junctions and bias current. The 2e is there because the charge carriers are electron pairs.

On the sticker

Squidra holds a glowing cyan ring between his hands; the two junctions are not clearly drawn. Printed text: SQUIDRA with the line PERIOD Φ0. Pixel-art kiss-cut vinyl sticker, white, in 2″, 3″, 4″ and 6″ sizes.