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How do Josephson junctions convert direct voltage into alternating current?

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Figure 1: Specific heat in the normal (Cen) and superconducting (Ces) states of a classic superconductor as a function of absolute temperature. The two functions are identical at the transition temperature (Tc) and above Tc. — superconductivity Figure 1: Specific heat in the normal (C_en) and superconducting (C_es) states of a classic superconductor as a function of absolute temperature. The two functions are identical at the transition temperature (T_c) and above T_c.

In a Josephson junction, the conversion of direct current (DC) voltage into alternating current (AC) is known as the AC Josephson effect.12

When a DC voltage $V$ is applied across two superconductors separated by a thin insulating barrier, the energy of the Cooper pairs changes by an amount of $2 e V$ as they tunnel across the junction. According to the Planck relationship ( $E = h ν$ ), this energy change causes the supercurrent to oscillate at a specific frequency $ν$ , defined by the equation:12

$ν = \frac{2 e V}{h}$

In this formula, $e$ represents the elementary charge of an electron and $h$ is Planck’s constant. Because the frequency of this oscillation is directly proportional to the applied voltage, the junction effectively acts as a frequency-to-voltage converter. This phenomenon is so precise that it is utilized in metrology to establish an international voltage standard, allowing scientists to determine voltage by measuring frequency with high accuracy.12