Physics Nobel 2025: Pioneers Of Quantronics And The Experiment That Made Quantum Real

When the Royal Swedish Academy of Sciences in Stockholm announced the much-awaited winners of the Nobel Prize in Physics for 2025, the honour went not to a recent, headline-grabbing discovery but to a series of quiet, meticulous experiments conducted four decades earlier in a low-temperature laboratory at the University of California, Berkeley.

The laureates were John Clarke, Michel H. Devoret, and John M. Martinis.

The citation for their prize was precise and significant: “for the discovery of macroscopic quantum mechanical tunnelling and energy quantisation in an electric circuit.”

What does this mean?

For decades, a fundamental question had haunted physics. Were the bizarre, counter-intuitive rules of quantum mechanics, the realm where particles can be in multiple places at once or tunnel through solid walls, strictly confined to the infinitesimal world of atoms and electrons? Or could a large, tangible, human-made object be coaxed into obeying these ghostly laws?

In a landmark achievement between 1984 and 1985, Clarke, Devoret, and Martinis answered this question with a definitive “yes”. Their experiments, the Nobel committee declared, “revealed quantum physics in action” on a silicon chip, demonstrating that quantum phenomena could manifest on a scale large enough to be held in the hand.

This led to a new field that combined a crowning glory of 20th-century pure science, quantum mechanics, with one of the greatest technological achievements of the same century, electronics. Thus, the pathway was created that would usher in Quantronics: the science of electrical circuits whose collective properties, such as the flow of charge or the direction of a magnetic field, are not described by the familiar laws of classical electronics. Instead, these circuits behave as a single collective quantum system.

Prelude to Quantronics

To understand this fully, one should start in 1911, when the Dutch physicist Heike Kamerlingh Onnes, studying the properties of mercury at temperatures approaching absolute zero (0 Kelvin, or -273.15°C), discovered that below a “critical temperature” of 4.2 K, the electrical resistance of mercury suddenly and completely vanished. He had discovered superconductivity, a mysterious phenomenon explained only in 1957 with the theory of Bardeen, Cooper, and Schrieffer (BCS theory).

According to this theory, in a superconductor, electrons, which normally repel each other, can form weakly bound pairs. This pairing is mediated by the crystal lattice of the material itself. As one electron moves through the lattice, its negative charge attracts the positive atomic nuclei, creating a slight distortion—a ripple of positive charge. This ripple, in turn, attracts a second electron, binding the two together into what is now called a Cooper pair.

Fun fact: Leon Cooper, who was behind this discovery, won the 1972 Nobel Prize and served as the basis for the famous TV character Sheldon Cooper.

As a material is cooled below its critical temperature, its Cooper pairs begin to condense into the lowest possible energy state, forming a Bose–Einstein condensate.

Incidentally, the Bose–Einstein condensate was first proposed by Indian physicist Satyendra Nath Bose and Albert Einstein in the 1920s. Bose developed the statistical framework for bosons (particles that can share quantum states), and Einstein extended this work to predict that atoms, when cooled to extremely low temperatures, could merge into this unique quantum state, which was then named after them.

In this state, the individual identities of the billions of Cooper pairs merge. They cease to be a chaotic swarm of particles and begin to move in perfect, silent unison, described by a single, shared quantum wave function. This is the essence of a “macroscopic quantum phenomenon”.

In 1962, a 22-year-old graduate student at Cambridge named Brian Josephson made a startling theoretical prediction. He considered what would happen if a superconductor were cut in two and then the pieces separated by an incredibly thin insulating barrier, just a few nanometres thick. In a classical system, no current would flow. Josephson predicted that the Cooper pairs, being quantum objects, could perform a trick forbidden in the classical world: quantum tunnelling.

Here, the Cooper pairs could pass directly through the insulating barrier without any applied voltage, creating a dissipation-less supercurrent. Called the “Josephson effect”, this is a direct manifestation of the wave-like nature of the Cooper pairs, whose wave functions can extend across the insulating gap. Josephson received the Nobel Prize in 1972. Fun fact: Josephson was very much into Hindu Vedanta.

The Josephson junction can be considered a manufactured reality. The properties of a natural atom, like its energy levels, are fixed by fundamental constants. But the properties of a superconducting “artificial atom” can be determined by design. This ability to engineer such a circuit is the core principle of Quantronics. It elevates the work from mere observation of quantum mechanics to the design and construction of a new quantum reality on a chip.

Berkeley Team

In the early 1980s, in the sub-basement of Birge Hall at UC Berkeley, a team started working on superconductivity. It was led by John Clarke, a British physicist, and included Michel H. Devoret, a French postdoctoral researcher, and John M. Martinis, then a graduate student who embodied the spirit of an “engineer among physicists”.

Their experimental apparatus was centred on a single, tiny Josephson junction fabricated from thin films of superconducting material on a silicon chip. This chip was mounted inside a dilution refrigerator, a complex machine that used a mixture of helium isotopes to cool the circuit to temperatures just a few thousandths of a degree above absolute zero.

The goal was to isolate the quantum state of this junction—a variable known as the “phase difference” across the insulating barrier—from the thermal and electrical chaos of the macroscopic world. In this pristine, ultra-cold environment, the collective phase of the billions of Cooper pairs in the junction could be treated as the position of a single quantum “particle” trapped in a potential energy well. To climb the well, this “particle” needs energy. But the team showed that even without the required energy, the “particle” tunnelled directly through the energy barrier.

Soon, the team was able to show that in this artificially created, engineered quantum system, important non-classical, typical quantum properties were preserved. This was the first unambiguous experimental confirmation that a macroscopic variable, representing the collective behaviour of an astronomical number of electrons, obeyed the probabilistic rules of quantum tunnelling.

The success of the Berkeley experiment was more than a confirmation of quantum tunnelling. It was a landmark victory over a formidable foe known as quantum decoherence.

Quantum states like superposition and tunnelling are exquisitely fragile. The slightest interaction with the outside world—a stray photon, a tiny vibration, a fluctuation in a magnetic field—can destroy the delicate quantum coherence and force the system to “collapse” into a single classical state. A macroscopic object, with its countless atoms, has exponentially more ways to interact with its environment than a single electron. Most physicists believed that decoherence would happen so rapidly in such a system that its quantum nature would be instantly washed away.

The fact that Clarke, Devoret, and Martinis were able to observe these effects in such an engineered macrosystem was a testament to their masterful experimental design.

Their success proved that decoherence in an engineered, solid-state device was not an insurmountable barrier. It was a technical challenge that could be met and overcome. This demonstration of quantum control over an artificial macroscopic system was the true, foundational breakthrough. It showed that the fight against decoherence, which remains the central challenge in building a useful quantum computer today, was a winnable one.

The 1985 experiment was a beginning, not an end. In the decades that followed, the three physicists embarked on separate but interconnected paths, each taking a piece of their foundational discovery and forging it into an essential tool for the nascent field of quantum computing. They personally built the toolkit that would be used to construct the next generation of quantum machines. Then, in 2019, their works converged. The culmination was an experiment that, for the first time, demonstrated a quantum computer performing a task beyond the practical reach of even the most powerful classical supercomputers on the planet.

The leader of the Google Quantum AI team that conducted this landmark experiment was John Martinis. The machine at the heart of the experiment, the Sycamore processor, was a direct descendant of the work of all three: the two physicists and the engineer. It was a 53-qubit processor composed of transmon qubits, the noise-resilient design co-invented by Devoret's group, arranged in the scalable grid layout pioneered by Martinis. The state of each qubit was measured using techniques derived from the methods that Clarke had perfected. The Sycamore chip was the physical embodiment of 35 years of progress in the field of Quantronics.

The 53-qubit Sycamore processor generated one million samples from the output distribution of a complex random circuit in just 200 seconds. It is estimated that for the world's most powerful supercomputer in 2019, IBM's Summit, to perform the same task would take approximately 10,000 years. This was triumphantly called “quantum supremacy”, now replaced by the less confrontational “quantum advantage”.

The forty-year journey from a single Josephson junction in a Berkeley basement to a 53-qubit processor challenging a supercomputer represents a fundamental shift in humanity's relationship with the quantum world. The pioneering work of John Clarke, Michel Devoret, and John Martinis did more than just confirm a subtle prediction of quantum mechanics. It provided a new blueprint for technology.

Their 1985 experiment was the first to prove that the laws of quantum mechanics were not merely a descriptive science for the natural, microscopic world but a prescriptive engineering manual for building a new class of macroscopic technology.

The Nobel Prize for Physics in 2025 celebrates this 1985 achievement and also the subsequent progressive journey.