1973 Nobel Prize in Physics(2)
Reason for Award
for his theoretical predictions of the properties of a supercurrent through a tunnel junction, in particular those phenomena known as the Josephson effects (Phys. Lett. 1 (1962) 251-253; Adv. Phys. 14 (1965) 419)
Laureates
United Kingdom of Great Britain and Northern Ireland
Explanation
1. In very cold places, electricity can flow through metals with zero resistance; this is called superconductivity. 2. Brian Josephson predicted that even if two superconductors are separated by a thin wall, electricity can tunnel right through. 3. The current flows in a special, coordinated way and continues even when no voltage is applied. 4. It is like water quietly spilling over a dam without needing extra push—very mysterious. 5. The Josephson effect is used in ultra-sensitive voltmeters and as a time standard, helping GPS and communications work accurately. 6. Josephson’s insight, made while he was still a student, continues to support science and technology today.
Related Keywords
Josephson effect
When two superconductors are separated by an ultrathin insulator, Cooper pairs tunnel without applied voltage, producing a current known as the Josephson effect. It has DC and AC forms, the latter converting voltage to frequency using only fundamental constants. The effect underpins SQUID operation, enabling ultra-sensitive magnetometry. It also forms the basis of voltage standards and qubit circuits. It is a classic example of a quantum phase manifesting as a measurable quantity.
SQUID
A Superconducting Quantum Interference Device uses Josephson junctions arranged in a loop to form a quantum interferometer. The loop’s critical current modulates with the enclosed magnetic flux in units of Φ_0, enabling detection of fields down to the picotesla range. Applications include geomagnetic surveys, magneto-encephalography, and dark-matter searches. Reducing flux noise is vital for improved performance. SQUIDs have become essential readout elements in modern quantum computers.
Cooper pair
In superconductors, electrons form bound spin-singlet states mediated by phonons, called Cooper pairs. The pairs condense into a macroscopic quantum state allowing zero-resistance current. Their phase can be probed directly via tunneling and interference. External fields or temperature can break the pairs, and critical parameters characterise different superconductors. In many qubits, Cooper pairs themselves are the controllable quantum entities.
AC Josephson relation
The AC Josephson relation links the applied voltage V to an oscillation frequency f via f = (2e/h) V. Because it ties voltage to frequency through fundamental constants, it forms the basis of electrical metrology. Shapiro steps appearing under microwave irradiation are an experimental confirmation of this relation. Modern voltage standards achieve relative accuracies of 10⁻⁹ using the effect.
Shapiro step
When a Josephson junction is irradiated with an external RF signal, step-like constant-voltage plateaus appear in its I-V curve, known as Shapiro steps. Because frequency and voltage are locked by f = (2e/h) V, quantum interference becomes visible. Step width and appearance depend on junction capacitance, resistance, and bias strength. They are exploited for precision voltage sources and for validating quantum standards. Observation of fractional steps aids in diagnosing many-body effects and junction inhomogeneities.