This year's Nobel Prize in physics went to professors John Clarke (83, UC Berkeley), Michel Devoret (72, Yale University), and John Martinis (67, UC Santa Barbara), who in the 1980s demonstrated quantum tunneling and energy quantization in palm-sized superconducting circuits.
Quantum tunneling has previously produced Nobel Prizes in physics. British physicist Brian Josephson won the 1973 Nobel Prize in physics for predicting the "Josephson effect," a quantum tunneling phenomenon in superconductors. Why did the Nobel Committee again give the physics prize to scientists who studied quantum tunneling? The difference lies in the "micro" and the "macro."
A superconductor is a material in which, when cooled below a certain temperature, electrical resistance disappears completely and current flows without loss. The Josephson effect, named after Brian Josephson, refers to the phenomenon in which current can flow without resistance even when there is a thin insulating layer between two superconductors.
Predicted in 1962 by the young British physicist Josephson, this effect was first confirmed the following year, 1963, through experiments at Bell Labs. At that time, however, the sample with a thin insulating layer between lead superconductors was on the order of a few millimeters, and the observed quantum phenomena were limited to a very small region rather than the entire circuit.
By contrast, Clarke and colleagues realized quantum phenomena across an entire circuit a few centimeters in size that you could hold in your hand. In that the whole circuit moved as a single quantum state, it is regarded as the first case of handling quanta at the macroscopic level, unlike previous experiments. The scientific community says, "If Josephson opened the door, Clarke and his colleagues stepped through that door into the vast realm of modern quantum technology."
◇ Proving macroscopic quantum tunneling with Josephson junctions
Quantum mechanics is a field of physics that describes the microscopic world smaller than atoms, such as atomic nuclei or electrons. Quantum mechanics explains that in the microscopic world there are quantum superposition phenomena, in which a particle exists in multiple states at the same time, and quantum entanglement phenomena, in which particles behave as if they are connected even when far apart.
There are other strange phenomena described by quantum mechanics. In the macroscopic world, a ball made of an enormous number of molecules bounces back every time it is thrown at a wall. In the microscopic world, by contrast, a single particle can sometimes pass straight through a barrier and appear on the other side. This is the tunneling phenomenon. Tunneling had long been thought to apply only to extremely small particles like electrons or atomic nuclei.
If so, is such a thing impossible in a large circuit where billions of electrons are entangled? Most physicists thought it was impossible. Clarke, Devoret, and Martinis directly challenged this "common sense."
After joining UC Berkeley as a professor in 1969, Clarke, together with Martinis—then a graduate student in the 1980s—and Devoret, a postdoctoral researcher from the Saclay Nuclear Research Center in France, devised an experiment to verify macroscopic quantum tunneling using Josephson junctions.
A Josephson junction is a physical device that makes it possible to observe the Josephson effect in practice. It is a structure made by inserting an extremely thin insulating layer between two superconductors, with thickness on the nanometer (nm, one-billionth of a meter) scale. In this structure, electrons form pairs and can move back and forth across the insulating layer barrier. Scientists call these electron pairs Cooper pairs.
Cooper pairs do not move individually; they all oscillate like one giant wave and behave as if they were a single lump of a particle. A system in which billions of electron pairs move as one is a macroscopic quantum system.
Clarke's idea that Josephson junctions would be useful for realizing macroscopic quantum tunneling came from his connection with Josephson during his doctoral studies. In an interview with the Nobel Committee right after the prize announcement, Professor Clarke said, "In graduate school I studied with Brian Josephson, who was two years ahead of me," and "we became very close when he started his (Josephson effect) research." Professor Clarke said he was greatly influenced by Josephson.
Clarke's team sent a very weak current through an electronic circuit. In the superconducting state, current flows but the voltage remains at zero. If we liken this situation to a landscape of energy, it is like a small ball trapped in a deep valley. The ball does not have enough energy to get over the hill, so it naturally stays put. But during the experiment, they captured the moment when the ball "popped" over the hill on its own into a neighboring valley. A tiny voltage appeared in the circuit. It was the first experimental proof of macroscopic quantum tunneling.
◇ Achieving quantum advantage surpassing supercomputers
The three did not stop there. This time, they shot microwaves—very short electromagnetic waves—into the circuit and observed how the circuit responded. Surprisingly, the circuit did not respond to just any energy. It responded only at specific wavelengths and specific energies.
On the graph, lines appeared at regular intervals, as in an atomic spectrum. This meant that the entire circuit absorbed and emitted only fixed energy levels like an "artificial atom." In other words, it showed that energy does not change continuously but exists only in units of quanta of a fixed size. This phenomenon is energy quantization.
The three scientists' research opened the way for quantum mechanics to be used in the real world rather than remaining theory. In particular, the superconducting circuits they used became the prototype of the core technology for making the "qubit," the basic computational unit of today's quantum computers.
Conventional computers represent the absence or presence of electrons as 0 and 1, that is, in units of 1 bit. By contrast, the unit of a quantum computer is the qubit, in which the 0 and 1 states are superposed. If an ordinary computer has 2 bits, it can be one of 00, 01, 10, or 11, but 2 qubits can be all four at the same time, dramatically increasing computing power.
Professor Martinis joined Google in 2014 and developed a quantum computer operating with qubits. In Oct. 2019, the Google quantum artificial intelligence (AI) team he led announced in the international journal Nature that it had solved a random number verification problem in 200 seconds with a quantum computer, which would take 10,000 years on the best existing supercomputer. It was the first achievement of so-called "quantum advantage," in which a quantum computer surpasses a supercomputer.
Devoret advanced techniques to precisely measure and control quantum states in circuits, laying the groundwork for experiments on quantum superposition and entanglement. He became chief scientist of Google Quantum AI in 2023, and the following year unveiled the 105-qubit superconducting quantum chip "Willow," which can solve a calculation in under 5 minutes that would take 10 ja years (秭年, 1 ja year is 10 to the 24th power years) on existing supercomputers.
◇ Applied to dark matter research and surgeries for epilepsy patients
Confirming the potential of the Josephson junction, Clarke focused in earnest on improving the performance of SQUIDs (superconducting quantum interference devices), which detect extremely small magnetic fields. Early SQUID models developed in the 1960s and 1970s had low sensitivity and high noise. Because the measurable signals are extremely weak in practice, precision and stability were greatly lacking for measuring ultralow currents such as changes in Earth's magnetic field or brain signals. In addition, temperature fluctuations, external electromagnetic interference, and junction quality issues easily made the devices unstable or reduced their reproducibility.
Clarke's goal was to turn SQUIDs from mere research devices into technology that operates with enough stability and high sensitivity for real applications. He comprehensively improved junction materials and circuit structures, minimized noise in cryogenic environments, and enhanced signal amplification techniques.
Clarke applied SQUIDs across diverse fields. In particular, by arraying hundreds of SQUIDs in magnetoencephalography (MEG) devices, which measure the brain's electrical signals, surgeons can precisely locate the seizure focus of epilepsy patients and operate while preserving language and motor functions. SQUIDs are also applied to research on neurological diseases such as Alzheimer's and Parkinson's.
Recently, Clarke has been helping with research to find axions, a dark matter candidate, using SQUIDs. Scientists believe that only 5% of the universe's matter emits light that we can observe, while 70% is dark energy that drives the expansion of the universe and 25% is dark matter that does not emit light but pulls on objects.
Jeong Yeon-uk, a professor in the Department of Nano Engineering at Sungkyunkwan University, said, "In semiconductor terms, the achievements of Clarke, Devoret, and Martinis are on the level of having developed the transistor that ended the era of vacuum tubes." He added, "In particular, Clarke is the person who raised SQUIDs from simple research equipment to technology ready for commercialization," and "the physics community has long regarded him as 'someone who will surely win a Nobel Prize someday.'"
References
Nature (2020), DOI: https://doi.org/10.1038/s41567-020-0829-5
Nature (2008), DOI: https://doi.org/10.1038/nature07128
Nature (2011), DOI: https://doi.org/10.1038/nmat2996