John Clarke UC Berkeley, CC BY 4.0 , via Wikimedia Commons |
This year the Nobel Prize in Physics was awarded to John Clarke, Michel Devoret, and John M. Martinis “for the discovery of macroscopic quantum mechanical tunnelling and energy quantization in an electric circuit.”
I will focus on one member of this trio, John Clarke. Russ Hobbie and I mention Clarke in Intermediate Physics for Medicine and Biology in our chapter on biomagnetism.
Sensitive detectors are constructed from superconducting materials. Some compounds, when cooled below a certain critical temperature, undergo a sudden transition and their electrical resistance falls to zero. A current in a loop of superconducting wire persists for as long as the wire is maintained in the superconducting state. The reason there is a superconducting state is a well-understood quantum-mechanical effect that we cannot go into here. It is due to the cooperative motion of many electrons in the superconductor (Eisberg and Resnick 1985, Sect. 14.1; Clarke 1994). The [line integral of the electric field] around a superconducting ring is zero, which means that [the change in magnetic flux] is zero, and the magnetic flux through a superconducting loop cannot change. If one tries to change the magnetic field with some external source, the current in the superconducting circuit changes so that the flux remains the same.This was not the first Nobel Prize related to the SQUID. In 1973 Brian Josephson shared the Nobel Prize “for his theoretical predictions of the properties of a supercurrent through a tunnel barrier, in particular those phenomena which are generally known as the Josephson effects.” Now, over fifty years later, it’s Clarke’s turn.
The detector is called a superconducting quantum interference device (SQUID). The operation of a SQUID and biological applications are described in the Scientific American article by Clarke (1994).
A Lawrence Berkeley Laboratory announcement stated
Clarke joined Berkeley Lab in 1969 and retired as a faculty senior scientist in the Materials Sciences Division in 2010. At the time of their prize-winning research, Martinis worked as a graduate student researcher, and Devoret as a postdoctoral scholar, in the Clarke group at Berkeley Lab and UC Berkeley….Clarke describes his first SQUID-like circuit in his Scientific American article that Russ and I cite.
[Clarke’s circuit using a tunnel barrier] is the foundation for an ultrasensitive detector called a SQUID or a superconducting quantum interference device. Clarke has pioneered and used SQUIDs in many applications, including detection of nuclear magnetic resonance (NMR) signals at ultralow frequencies; geophysics; nondestructive evaluation of materials; biosensors; detection of dark matter; and observing qubits, the fundamental unit of information in a quantum computer.
In my early days as a research student at Cambridge, my supervisor, Brian Pippard, proposed that I use a SQUID to make a highly sensitive voltmeter. In those days, procedures for making Josephson junctions were in their infancy and not practicable for manufacturing instruments. One day early in 1965, over the traditional afternoon tea at the Cavendish Laboratory, I was discussing this problem with Paul C. Wraight, a fellow student. He suggested that a molten blob of solder (an alloy of lead and tin that becomes superconducting in liquid helium) deposited onto a niobium wire might just conceivably make a Josephson junction. His rationale was that niobium has a native oxide layer that might behave as a suitable tunnel barrier.
We rushed back to the laboratory, begged a few inches of niobium wire from a colleague, melted a blob of solder onto it, attached some leads and lowered it into liquid helium. As we hoped, Josephson tunneling! The fact that Wraight’s idea worked the first time was important. If it had not, we would never have bothered to try again. Because of its appearance, we christened the device the SLUG. Later I was able to make a voltmeter that could measure 10 femtovolts (10-14 volt), an improvement over conventional semiconductor voltmeters by a factor of 100,000.
Clarke’s article goes on to describe many of the biological applications of SQUIDs, including for measuring the magnetocardiogram (magnetic field of the heart) and the magnetoencephalogram (magnetic field of the brain).
Congratulations to John Clarke and this colleagues on their Nobel Prize. It’s another wonderful example of physics applied to biology and medicine.
UC Berkeley press conference 10/7/2025: Professor Emeritus John Clarke 2025 Nobel Prize in Physics