This year's Nobel Prize in physics went to three scientists who proved that various quantum mechanical phenomena can be observed even in the tangible macroscopic world. Their research is credited with opening a new path to developing next-generation technologies that use quantum mechanical principles, such as quantum computers and quantum cryptography.
The Nobel Committee for Physics at Sweden's Karolinska Institute announced on the 7th (local time) that this year's Nobel Prize in physics will go to John Clarke, 83, a professor at the University of California, Berkeley (UC Berkeley), Michel Devoret, 72, a professor at Yale University, and John Martinis, 67, a professor at the University of California, Santa Barbara (UC Santa Barbara).
Clarke, who is from the United Kingdom, earned his doctorate at the University of Cambridge. Martinis was born in the United States and received his doctorate at UC Berkeley, while Devoret, who is from France, earned his doctorate at Paris-Sud University (now Paris-Saclay University).
◇First achievement of quantum computer advantage
Quantum mechanics is a field of physics that describes the microscopic world smaller than atoms, such as atomic nuclei and electrons. Quantum mechanics explains that in the microscopic world there are phenomena such as quantum superposition, in which a particle exists in multiple states at once, and quantum entanglement, in which particles behave as if they are connected even when far apart.
The three scientists who won this year's Nobel Prize in physics proved through experiments conducted at UC Berkeley in 1984–1985 that quantum mechanical phenomena can be observed in the macroscopic world that humans can see and touch. At the time, Clarke was the supervising professor, Martinis was a doctoral student, and Devoret was a postdoctoral researcher from France.
The three conducted experiments using an electronic circuit large enough to see with the eye and hold in the hand. The silicon chip included a "Josephson junction," in which a thin insulating layer is placed between superconductors with zero electrical resistance.
According to classical physics, in an ordinary circuit, even if current flows, electrons cannot penetrate an insulating layer that does not conduct electricity. In contrast, in a circuit made of superconductors, a "quantum tunneling" phenomenon was observed in which charged particles pass through the insulating layer as if going through a wall.
An "energy quantization" phenomenon also appeared, in which the circuit absorbs and emits only specific amounts of energy. Quantization means that a physical quantity is not continuous but appears in integer multiples of a basic unit (quantum). These experimental results are credited with being a decisive turning point that brought quantum mechanics, once regarded as a theory confined to the microscopic world, into the realm of engineering.
The Nobel Committee said that "their research opened the door to the development of next-generation quantum technologies, including not only quantum computers but also quantum cryptography that is impossible to hack and quantum sensors capable of ultra-precise measurements." In particular, the superconducting circuits used by the three scientists became the prototype of the core technology for creating "qubits," the basic units of computation in today's quantum computers.
Conventional computers represent the absence or presence of electrons as 0 and 1, that is, in units of one bit. In contrast, the unit of a quantum computer is the qubit, in which the 0 and 1 states are superposed. If a classical computer has two bits, it can be one of 00, 01, 10, or 11, but two qubits can be all four at the same time. If there are 300 qubits, 2 to the 300th power states—more than the total number of atoms in the universe—are possible, dramatically boosting computing power.
Martinis joined Google in 2014 to develop quantum computers using superconducting 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. Martinis left Google in 2020 and founded the quantum technology company Qolab two years later.
◇Continuing the Nobel Prize lineage in physics
The achievements of this year's Nobel laureates in physics built on the work of previous Nobel Prize winners. They discovered quantum tunneling through experiments with superconducting circuits. The superconducting phenomenon was first discovered at minus 269 degrees Celsius in 1911 by Dutch physicist Heike Kamerlingh Onnes. He received the Nobel Prize in physics in 1923 for this achievement.
American physicists John Bardeen, Leon Cooper, and John Robert Schrieffer explained the superconducting phenomenon in 1957 with the so-called BCS theory, named after the initials of their surnames. Vibrations of the crystal lattice act as a glue between electrons. The theory explains that electron pairs thus formed move without resistance, allowing current to flow. The three scientists received the Nobel Prize in 1972 for the BCS theory.
This year's laureates confirmed that in superconducting circuits, electrons form so-called "Cooper pairs," named after Leon Cooper, who established the BCS theory, and behave like a "single particle" that fills the entire circuit, exhibiting quantum tunneling in which they pass through the insulating barrier to the other side.
In ordinary conductive circuits, electrons collide with one another. Electrons have strong exclusivity and try to keep their distance. In contrast, in superconducting circuits, electrons form Cooper pairs and create a current with no electrical resistance.
This year's physics laureates conducted experiments with silicon chips containing Josephson junctions. British physicist Brian Josephson theoretically predicted in 1962 that current would flow even if an insulator that should not allow current was inserted between superconductors.
In 1963, researchers at Bell Labs verified Josephson's prediction with an experiment inserting an insulator between superconducting lead. The experimental sample at the time was a few millimeters in size. Josephson received the Nobel Prize in physics in 1973. This year's laureates observed quantum tunneling in silicon chips with Josephson junctions, demonstrating that quantum mechanics can be implemented in electronic circuits.
References
Nature (2019), DOI: https://doi.org/10.1038/s41586-019-1666-5
Physical Review Letters (1985), DOI: https://doi.org/10.1103/PhysRevLett.55.1908
Physical Review Letters (1985), DOI: https://doi.org/10.1103/PhysRevLett.55.1543
Physical Review Letters (1984), DOI: https://doi.org/10.1103/PhysRevLett.53.1260