The Science Behind the 2025 Nobel Discovery: Macroscopic Quantum Tunneling

Understanding Quantum Tunneling: From Particles to Macroscopic Systems

Quantum tunneling has long been understood as a distinctly quantum phenomenon, where particles spontaneously pass through energy barriers that classical physics says should be impenetrable. When a particle encounters an energetic barrier, classical mechanics predicts it will bounce back unless it possesses sufficient energy to overcome the obstacle. However, quantum mechanics reveals a fundamentally different reality: particles have a probability of appearing on the opposite side of the barrier, even when lacking the classical energy to breach it. This counterintuitive behavior stems from the wave-like nature of matter, wherein a particle's quantum wave function extends through and beyond the barrier, maintaining a non-zero amplitude on the far side.

For decades, scientists observed this phenomenon exclusively in microscopic systems—alpha particles escaping radioactive nuclei, electrons tunneling through semiconductor barriers, and individual atoms exhibiting quantum behavior. The prevailing assumption held that quantum effects would remain confined to atomic and subatomic scales, with macroscopic objects adhering strictly to classical physics rules.

The revolutionary insight of Clarke, Devoret, and Martinis challenged this fundamental assumption. Their experiments demonstrated that quantum tunneling and quantized energy states could manifest in systems large enough to hold in one's hand, involving billions upon billions of particles behaving in concert as a single quantum entity.

The Experimental Breakthrough: Josephson Junctions and Superconducting Circuits

The key to observing macroscopic quantum effects lay in leveraging superconductivity—a phenomenon where certain materials cooled below critical temperatures exhibit zero electrical resistance. The laureates constructed an elegant experimental apparatus: a superconducting electrical circuit incorporating a Josephson junction, a device consisting of two superconducting materials separated by an ultrathin insulating barrier.

Named after British physicist Brian Josephson, who predicted the effect theoretically in 1962, a Josephson junction permits Cooper pairs—electron pairs with opposite spins that move coherently as bosons—to tunnel quantum mechanically through the insulating barrier. When cooled to near absolute zero (below 20 millikelvin), this setup creates conditions where billions of electrons behave not as independent particles but as a unified quantum system characterized by a single macroscopic wave function.

The elegance of this system lies in its dual quantum behaviors. First, the researchers demonstrated macroscopic quantum tunneling: the entire electrical circuit spontaneously transitioned from a stable zero-voltage state to a voltage-bearing state without possessing the classical energy to surmount the energy barrier separating these configurations. This transition, observable as a sudden voltage measurement, directly evidenced quantum tunneling occurring at a macroscopic scale involving trillions of electrons acting cooperatively.

Second, the team revealed energy quantization in this macroscopic system. By irradiating their circuit with microwave photons of specific frequencies, they observed that the system could only absorb energy in discrete packages—quanta—rather than continuously as classical physics predicts. The circuit exhibited precisely defined energy levels, much like an atom with its characteristic spectral lines, confirming that the macroscopic system obeyed quantum mechanical rules.

The Physical Mechanism: From Wave Functions to Collective Behavior

At the heart of this discovery lies a fundamental principle of quantum mechanics: the wave function, which describes the probability distribution for finding a particle at any given location. In the Berkeley experiments, the millions of trillions of electrons within the superconducting circuit did not maintain individual wave functions but instead shared a single, coherent macroscopic wave function spanning the entire circuit.

This collective wave function embodied the hallmark feature of quantum systems—superposition. Just as a microscopic quantum particle exists simultaneously in multiple states until measurement occurs, the macroscopic circuit existed in superposition between its zero-voltage and voltage-bearing states. The circuit could tunnel between these configurations not through classical transitions but through quantum mechanical probability pathways.

The Josephson junction itself operates through quantum tunneling of Cooper pairs. At the junction, these electron pairs maintain a phase relationship across the insulating barrier. When the junction is part of a larger superconducting circuit, this phase relationship extends throughout the entire system, unifying billions of particles into a single quantum entity.

The tunneling probability for this macroscopic system depends exponentially on the barrier's height and width—the same relationship predicted by quantum mechanics for individual particles. However, what distinguished the Berkeley experiments was scale: they operated this quantum phenomenon not in a single atom but in an engineered circuit visible to the naked eye, containing trillions of particles.

From Fundamental Physics to Quantum Computing Revolution

While Clarke, Devoret, and Martinis conducted their experiments primarily as fundamental physics research—investigating whether quantum mechanics truly extended to macroscopic scales—the practical applications transformed the field. Their discoveries established the theoretical foundation upon which modern superconducting quantum computing rests.

Superconducting qubits, the quantum bits used in today's leading quantum computers from companies including Google, IBM, and others, directly employ the principles demonstrated in the 1984-1985 experiments. These qubits use Josephson junctions as their core component, encoding quantum information in the quantized energy levels that the Berkeley team first revealed.

In a superconducting qubit, the two lowest energy levels of a Josephson junction circuit serve as the quantum bit's 0 and 1 states. By applying microwave pulses of precise frequencies, researchers manipulate these energy states, creating quantum superposition and entanglement—the essential resources enabling quantum computation. The very mechanism of quantum tunneling that the laureates demonstrated now allows qubits to transition between states through inherently quantum pathways, enabling computations impossible for classical systems.

Martinis, one of the three laureates, directly translated these fundamental discoveries into practical quantum computing. Leading Google's quantum computing development, he spearheaded research on superconducting qubits and in 2019 helped demonstrate "quantum supremacy"—a quantum computer solving a problem faster than the best classical supercomputers. Notably, Google's latest Willow quantum chip, announced in December 2024, advanced quantum error correction to unprecedented levels, solving complex calculations in under five minutes that would require classical supercomputers 10^25 years.

Broader Implications Beyond Computing

The significance of macroscopic quantum effects extends far beyond quantum computing. The discovery has catalyzed development of quantum sensors based on Josephson junctions, which Clarke himself pioneered through his work on SQUIDs (superconducting quantum interference devices). These ultrasensitive detectors enable unprecedented measurement capabilities in geophysics, magnetometry, and biomagnetic sensing.

According to the World Economic Forum, quantum technologies grounded in these principles—encompassing quantum computing, quantum sensing, and quantum communication—could generate economic value between $900 million and $2 trillion by 2035. More recent estimates from McKinsey & Company project that quantum technologies could collectively generate up to $97 billion in revenue worldwide by 2035, with quantum computing capturing approximately $72 billion of that figure.

The macroscopic quantum effects discovered in 1985 have also informed quantum communication and cryptography systems, utilizing the same principles of quantum coherence and tunneling to create theoretically unbreakable encryption protocols.

The Recognition in Historical Context

2025 marks the International Year of Quantum Science and Technology, the centennial of quantum mechanics' formal establishment. Within this context, honoring the discovery of macroscopic quantum tunneling represents recognition of quantum mechanics' continued capacity to surprise and inspire, nearly a century after its foundations were laid.

The Nobel Committee acknowledged that Clarke, Devoret, and Martinis built upon earlier foundations laid by previous Nobel laureates, particularly Brian Josephson (who predicted the Josephson effect) and Anthony Leggett (who theoretically explored macroscopic quantum phenomena). Yet their achievement transcended prior work by demonstrating experimentally what many thought impossible: that quantum mechanics could be engineered into systems of comprehensible scale, containing vast numbers of particles, yet maintaining their inherently quantum nature.

Upon learning of the award, John Clarke expressed his astonishment, noting that the discovery underlying their recognition "is the basis of quantum computing" and constitutes "one of the underlying reasons that cellphones work." This characterization underscores the profound interplay between fundamental research and technological transformation: a curiosity-driven investigation into the boundaries of quantum mechanics, pursued without immediate practical application in mind, ultimately catalyzed a revolution in quantum technology that now shapes modern computing and will increasingly define the technological landscape of the coming decades.

The 2025 Nobel Prize thus celebrates not merely a historical achievement but recognition of ongoing paradigm shifts in how humanity harnesses the quantum world, transitioning from theoretical understanding to engineered systems that solve problems inaccessible to classical approaches to classical approaches.