---
title: "Josephson’s prediction: phase could drive a zero-voltage current"
description: "What was new about a supercurrent through an insulating barrier?"
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language: "en"
page_type: "story"
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  zh-CN: "https://sandboxphysics.com/zh/stories/condensed-matter/josephson-phase-current/"
last_modified: "2026-10-23"
---

[Cold liquids and superconducting order](https://sandboxphysics.com/stories/condensed-matter/#collective)

Materials · Measurements · Explanation

# Josephson’s prediction: phase could drive a zero-voltage current

What was new about a supercurrent through an insulating barrier?

Sandbox Physics · 2026-10-23

Nobel contribution: 1973-2

1. Couple two superconductors
2. Set a relative phase
3. Predict pair current
4. Test current, voltage and magnetic interference

A superconducting phase difference is a collective circuit variable. Its response requires more than a single-electron transmission probability.

## The prediction concerned coherent pair transfer

Josephson predicted in 1962 that two weakly coupled superconductors could carry a phase-dependent supercurrent without a dc voltage. The electrodes possess collective phase coherence, so the coupling has consequences beyond quasiparticle tunneling.

Experiments soon tested zero-voltage current, magnetic interference and voltage–frequency relations. The 1973 award recognized the theoretical prediction as its own contribution.

Sources: [1]

## Phase and voltage are linked

In the simplest sinusoidal junction, phase sets the pair current. A voltage causes phase to advance in time. A steady voltage can consequently produce an oscillating supercurrent, with frequency set by the pair charge and Planck’s constant.

$$
I_{\mathrm s}=I_{\mathrm c}\sin\varphi,\qquad\frac{d\varphi}{dt}=\frac{2eV}{\hbar}
$$

Sources: [1]

## A loop adds magnetic interference

Two junctions in a superconducting loop constrain their phase differences through magnetic flux. Their currents combine constructively or destructively, producing a periodic flux response. Inductance, junction asymmetry and noise modify the ideal pattern.

Sources: [1]

## Different circuit Labs ask different questions

The SQUID Lab examines loop interference, the switching Lab examines escape from a biased state, and the Transmon Lab examines bound energy levels. They share Josephson coupling but have different geometries, controls and evidence. None substitutes for all the others.

Sources: [1] [2]

Continue with the physics

## Try the related models

### [SQUID flux response](https://sandboxphysics.com/labs/squid-flux-response/)

Scan magnetic flux and compare interference with junction asymmetry.

The small-inductance teaching loop assumes sinusoidal current–phase relations and is not a calibrated magnetometer.

### [Transmon energy levels](https://sandboxphysics.com/labs/transmon-artificial-atom/)

Compare anharmonic levels and charge sensitivity.

This spectrum is not the current-biased escape experiment honored in 2025.

### [Josephson switching statistics](https://sandboxphysics.com/labs/josephson-switching-statistics/)

Compare repeated switching under different bias ramps and temperatures.

Separate thermal and ground-state tunneling approximations use a synthetic junction; a plateau alone does not establish macroscopic quantum tunneling.

## Sources and reading notes

1. [Josephson: The discovery of tunneling supercurrents (1973)](https://www.nobelprize.org/uploads/2018/06/josephson-lecture_new.pdf)

   The phase-dependent pair current, voltage-driven phase evolution and subsequent experimental tests.

2. [Nobel Prize in Physics 1973: official award](https://www.nobelprize.org/prizes/physics/1973/summary/)

   Official attribution and award year. The observation or theoretical paper may be much earlier.

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