How Physicists Proved Everything is Quantum - Nobel Physics Prize 2025 Explained

The 2025 Nobel Prize in Physics was conferred upon John Clark, Michael H. Devay, and John M. Martinez for their crucial experimental findings. These findings demonstrated that the peculiar phenomena associated with quantum mechanics are observable not only at the scale of atoms and subatomic particles but also at sizes more relatable to human experience. This achievement bridges the gap between the bizarre quantum realm and the classical world previously thought to govern larger objects.

The central element of this discovery is quantum tunneling, one of the most fundamental and counterintuitive aspects of quantum mechanics. When a macroscopic object strikes a wall, it invariably bounces back. However, when this concept is scaled down to the subatomic level, an electron encountering a very thin barrier possesses a small probability of not reflecting but passing straight through the barrier, appearing on the opposite side without ever having existed within the barrier itself.

Thinking about quantum objects merely as miniature billiard balls proves inaccurate for describing this behavior. Instead, these entities must be conceptualized as waves lacking definite edges. In quantum mechanics, a particle's location is described by a wave function—a mathematical structure indicating the most probable location for detection. The amplitude of the wave corresponds to this likelihood.

The wave fades at its edges, but it never quite reaches zero. Meaning that there is always a small chance that that particle could be somewhere unexpected.

When this probability wave encounters a barrier, a portion of it is reflected, but a segment can seep through the obstruction. This seepage carries the minute possibility that the particle will manifest on the other side, an effect that appears almost magical.

This tunneling effect has practical implications for modern electronics. For an electron traveling down a wire, a break presents a small chance for the electron to hop the gap and continue its path as if the discontinuity did not exist. As computer chips are engineered to increasingly smaller dimensions, electrons jumping between closely spaced circuits, against design intent, is becoming a significant engineering challenge.

For most of the 20th century, quantum effects were strictly confined to the microscopic regime involving electrons and photons. However, during the 1980s, researchers at the University of California, Berkeley, including Clark, Devay, and Martinez, initiated investigations to determine if these strange quantum behaviors could manifest at the macro scale.

The team focused on a specialized application of the electron jumping concept known as the Josephson junction. This setup sandwiches two superconducting wires on either side of an insulating barrier, which acts as the gap or the impenetrable wall that classical electrons should not cross.

In a normal wire, negative electrons repel each other and scatter off atoms, causing resistance, heat, and light. Superconductors, cooled to extremely low temperatures, behave differently. An electron slightly distorts the lattice of metal ions, creating a trailing positive charge region. A second electron is attracted to this region, linking the two into what is termed Cooper pairs. Billions of these pairs move collectively without resistance, described by a single wave function.

When this collective wave function encounters the thin insulating barrier of the Josephson junction, it extends into the barrier, overlapping with the wave function on the opposite side. This overlap permits Cooper pairs to tunnel, establishing a steady supercurrent characterized by zero voltage, the defining feature of superconductivity. This specific effect earned Brian Josephson the Nobel Prize in 1973.

The Berkeley team though wanted to take this one step further. They wanted to see if the whole wave function representing billions of Koopa pairs within the wire could tunnel through the barrier as a single quantum object.

Achieving macroscopic quantum tunneling—tunneling on a scale relevant to human perception—required meticulous experimental control. To measure the minute changes in current and voltage across the junction, the setup was placed inside a dilution refrigerator, achieving temperatures colder than interstellar space. Furthermore, the apparatus was encased in layers of magnetic shielding and microwave filtering to isolate the delicate quantum system.

When a precisely controlled current was passed through the junction at low levels, the system behaved as predicted by superconductivity theory: a steady supercurrent flowed with zero voltage as Cooper pairs tunneled across the barrier. However, as the current increased beyond a specific critical value, a voltage began to appear, counterintuitive to simple superconducting predictions.

This voltage spike provided the evidence that the collective quantum state had successfully escaped its confinement and tunneled across the junction. This change in voltage arises because the relative overlap between the wave function on either side of the barrier continually varies as the macroscopic wave function moves, which is the driving mechanism for the measured voltage.

A crucial aspect of the research involved rigorously confirming that the observed switch was indeed a quantum tunneling process rather than a classical effect. At higher temperatures, random thermal energy can provide enough jolts to push the wave function over the barrier, a process known as thermal activation.

The distinction was made by observing temperature dependence. At higher temperatures, a strong correlation exists between temperature and the current required to observe the phenomenon. Crucially, as temperatures were decreased, the escape rate stopped showing any dependence on temperature whatsoever. This independence from thermal fluctuations confirmed the process was governed by the quantum system representing billions of Cooper pairs tunneling.

When Erwin Schrödinger proposed his famous thought experiment involving a cat in a box, the intent was to illustrate the perceived absurdity of applying quantum theory outside the microscopic world. Thanks to the work of Clark, Devay, and Martinez, it has been experimentally confirmed that quantum effects are indeed possible at macro scales. While not involving a zombie cat, this work laid the essential groundwork for superconducting qubits utilized in contemporary quantum computing.

This discovery garners significant attention because it forms the underpinning of much of modern quantum mechanics and quantum computation infrastructure. It is important to recall that this monumental achievement resulted from decades of pursuit driven purely by scientific curiosity and the fundamental need to understand the universe, rather than immediate practical application.

Testing a theory against the universe's behavior until definitive proof is established is precisely why this research is deemed worthy of the Nobel Prize. It represents a triumph of theoretical validation through rigorous experimentation.