DC SQUID — Quantum Interference

Superconducting Quantum Interference Device — Flux Quantization Φ₀ = h/2e

Applied Flux Φ/Φ₀ Φ/Φ₀ = 0.50

Bias Current I/Ic0 I/Ic0 = 0.90

Junction Asymmetry α α = 0.00

Inductance β_L = 2LIc/Φ₀ β_L = 0.20

Critical current:

Ic(Φ) = Ic0 |cos(πΦ/Φ₀)|

Flux quantum:

Φ₀ = h/2e ≈ 2.07 fWb

Josephson relation:

V = (Φ₀/2π) dφ/dt

Phase constraint:

φ₁ − φ₂ = 2πΦ/Φ₀

A DC SQUID has two Josephson junctions in a superconducting loop. Quantum interference between the two junction order parameters makes the critical current oscillate with period Φ₀ = h/2e ≈ 2.07×10⁻¹⁵ Wb. SQUIDs are the most sensitive magnetic field detectors ever made (~1 fT/√Hz).