Physics That Earned the 2025 Nobel Prize
The 2025 Nobel Prize in Physics recognizes one of the most profound discoveries in modern physics: that quantum mechanical phenomena—traditionally associated with individual atoms and subatomic particles—can manifest in circuits large enough to see and manipulate with human hands. This scientific overview explores the fundamental physics of macroscopic quantum tunneling and energy quantization that bridges the quantum and classical worlds.
The Quantum-Classical Divide
For most of the 20th century, physics operated with an implicit boundary. Quantum mechanics governed the microscopic realm of atoms, electrons, and photons, where particles exhibited wave-like behavior, existed in superposition states, and tunneled through barriers. Classical physics described the macroscopic world of everyday objects, where such quantum strangeness disappeared into deterministic, predictable behavior.
This division seemed natural and inevitable. Quantum effects in large objects appeared impossible because environmental interactions—collisions with air molecules, thermal vibrations, stray electromagnetic fields—would destroy quantum coherence almost instantaneously. The larger the system, the more channels for decoherence, and the faster quantum behavior would collapse into classical predictability.
“We essentially had two physics,” explains Professor Eva Olsson of the Nobel Committee. “One for the very small, where quantum mechanics reigned, and one for everything else, where classical mechanics sufficed. The laureates’ work showed this division was artificial—a consequence of our inability to isolate systems, not a fundamental limit of nature.”
Quantum Tunneling
Quantum tunneling ranks among the strangest predictions of quantum mechanics. In classical physics, a particle encountering an energy barrier higher than its kinetic energy simply cannot pass through—a ball rolled toward a hill will roll back if it lacks sufficient energy to reach the summit.
Quantum mechanics tells a different story. Due to the wave nature of quantum particles, there exists a non-zero probability that a particle can “tunnel” through an energy barrier, appearing on the other side despite lacking the classical energy to surmount it. This isn’t science fiction but observable reality, confirmed through countless experiments with individual atoms and electrons.
The mathematics underlying tunneling emerges from the Schrödinger equation, which describes quantum particles as probability waves. When such a wave encounters a potential barrier, it doesn’t stop abruptly at the boundary but instead decays exponentially into the barrier region. If the barrier is sufficiently thin, the wave function maintains non-zero amplitude on the far side, meaning the particle has some probability of appearing there.
The tunneling probability depends exponentially on barrier properties—height, width, and the particle’s mass. For everyday objects, these probabilities become vanishingly small. A tennis ball will never tunnel through a racket, no matter how many times you swing. The mass is too large, making quantum effects negligible.
Or so it seemed.
Josephson Junctions: The Quantum Gateway
The key to observing quantum phenomena in macroscopic circuits proved to be the Josephson junction—a device consisting of two superconductors separated by a thin insulating barrier. British physicist Brian Josephson predicted in 1962 that Cooper pairs—the bound electron pairs responsible for superconductivity—could tunnel through such barriers, producing measurable quantum effects.
Josephson’s prediction, confirmed experimentally shortly thereafter and earning him the 1973 Nobel Prize, revealed something remarkable: quantum tunneling could occur not just for individual particles but for coherent quantum states involving many particles acting collectively.
In a Josephson junction, Cooper pairs tunnel through the insulating barrier, creating a supercurrent that flows without voltage. The junction’s behavior is governed by quantum mechanics, with the phase difference between superconductors on either side acting as a quantum variable subject to Heisenberg uncertainty relations.
“Josephson junctions are quantum objects that you can hold in your hand,” notes Dr. Terry Orlando of MIT. “They operate according to quantum mechanics but at scales accessible to conventional electronics. That makes them the perfect platform for studying quantum phenomena in macroscopic systems.”
Building Macroscopic Quantum Circuits
The laureates’ breakthrough involved integrating Josephson junctions into larger superconducting circuits—loops of superconducting wire interrupted by one or more junctions. These circuits, cooled to temperatures within thousandths of a degree above absolute zero to maintain superconductivity, exhibit collective quantum behavior despite containing billions of electrons.
The crucial insight was that such circuits possess “collective quantum variables”—properties like magnetic flux through a superconducting loop or total charge on a superconducting island—that behave quantum mechanically. These variables obey quantum uncertainty relations, exist in superposition states, and can tunnel through energy barriers, exactly like individual quantum particles.
Consider a superconducting loop interrupted by a Josephson junction, threaded by magnetic flux. In classical physics, the flux could take any value. Quantum mechanically, however, the flux is quantized—it can only take on discrete values, multiples of the magnetic flux quantum Φ₀ = h/2e (where h is Planck’s constant and e is the electron charge).
This quantization represents a macroscopic manifestation of quantum mechanics. The flux quantum Φ₀ ≈ 2 × 10⁻¹⁵ Weber is tiny, but the principle—energy levels separated by discrete jumps rather than varying continuously—applies to a circuit large enough to photograph and manipulate with tweezers.
Macroscopic Quantum Tunneling
The most dramatic demonstration of quantum behaviour in large circuits came with observations of macroscopic quantum tunnelling. In these experiments, a superconducting circuit is prepared in a metastable state—trapped in a local energy minimum separated from the global minimum by an energy barrier.
Classically, the system would remain indefinitely in this metastable state, lacking the thermal energy to overcome the barrier. Quantum mechanically, however, the system can tunnel through the barrier to the lower-energy state, even at temperatures approaching absolute zero where thermal activation is negligible.
John Clarke’s pioneering experiments in the 1980s definitively observed this phenomenon. He prepared superconducting circuits in metastable magnetic flux states and measured the rate at which they tunnelled to lower-energy configurations. The results agreed quantitatively with quantum mechanical predictions while remaining utterly inexplicable classically.
“What made the experiments compelling was their precision,” recalls Professor Paul Richards, Clarke’s Berkeley colleague. “John could measure tunnelling rates varying over many orders of magnitude and show that quantum theory predicted them exactly. There was no wiggle room for alternative explanations.”
The tunneling rates exhibited characteristic quantum signatures: they remained finite at zero temperature (impossible for classical thermal activation), depended exponentially on system parameters in ways matching quantum predictions, and showed interference effects characteristic of quantum phenomena.
Energy Quantization in Macroscopic Systems
Complementing the tunneling observations were demonstrations of energy quantization—the existence of discrete energy levels rather than continuous energy spectra—in macroscopic circuits. This represents perhaps the most fundamental signature of quantum mechanics, originating in Planck’s revolutionary hypothesis that energy comes in discrete quanta.
Michel Devoret’s contributions were central to demonstrating energy quantization in superconducting circuits. Using sophisticated microwave spectroscopy techniques, his group showed that circuits containing Josephson junctions exhibited sharp spectral lines corresponding to transitions between discrete energy levels—exactly analogous to atomic spectra but in human-scale devices.
The energy level spacings could be precisely engineered by designing circuit parameters—junction capacitance, inductance, and critical current. This tunability transformed superconducting circuits from systems displaying quantum phenomena into quantum systems that could be designed and optimized for specific applications.
“Demonstrating energy quantization was crucial,” explains Dr. Robert Schoelkopf of Yale. “It proved these weren’t just quantum effects appearing in large systems—they were fully quantum systems that could be engineered with the precision we need for quantum information processing.”
Circuit Quantum Electrodynamics
The experimental demonstrations required accompanying theoretical development. Circuit quantum electrodynamics (circuit QED), pioneered by Devoret and collaborators, provided the conceptual framework for understanding macroscopic quantum circuits.
Circuit QED treats superconducting circuits as “artificial atoms”—quantum systems with discrete energy levels that can interact with quantized electromagnetic fields (microwave photons). The mathematical formalism parallels cavity quantum electrodynamics in atomic physics but applies to engineered circuits rather than natural atoms.
This framework revealed profound insights. Superconducting circuits could be designed to mimic various quantum systems—two-level atoms, harmonic oscillators, or more exotic configurations. They could be coupled to resonators that confine microwave photons, enabling strong light-matter interactions. And crucially, their properties could be engineered rather than being fixed by nature.
“Circuit QED provided the Rosetta Stone for translating between quantum mechanics and electrical engineering,” notes Professor Steven Girvin, who contributed to the theory’s development. “It allowed us to design quantum circuits the way engineers design classical electronics, but with full quantum control.”
The Role of Superconductivity
None of these demonstrations would be possible without superconductivity—the phenomenon where certain materials conduct electricity without resistance below critical temperatures. Superconductivity provides two essential requirements for macroscopic quantum behaviour.
First, superconductors support dissipationless currents, eliminating a major decoherence mechanism. Electrical resistance causes energy loss and fluctuations that rapidly destroy quantum coherence. Superconducting circuits avoid this, allowing quantum states to persist far longer.
Second, superconductivity involves macroscopic quantum coherence—all Cooper pairs in a superconductor share a common quantum phase, acting as a single quantum entity. This collective behaviour extends quantum mechanics from individual particles to vast numbers acting in concert.
“Superconductivity is itself a macroscopic quantum phenomenon,” explains Dr. Daniel Estève of the French national research centre. “It provided the platform where quantum mechanics naturally appears at large scales, waiting for clever experimentalists to exploit it.”
Quantum Coherence and Decoherence
A central challenge in observing macroscopic quantum behaviour involves maintaining quantum coherence—the preservation of quantum superposition and phase relationships—long enough to make measurements. Environmental interactions cause decoherence, collapsing quantum superpositions into classical mixtures and destroying quantum behavior.
The laureates’ experiments required extraordinary efforts to minimize decoherence. Samples were cooled to millikelvin temperatures, placed in magnetically shielded enclosures, isolated from mechanical vibrations, and measured with ultra-low-noise electronics. Even stray cosmic rays could disrupt quantum states, necessitating careful shielding.
John Martinis made crucial contributions to extending coherence times through improved circuit designs and environmental isolation. His work showed that with sufficient care, superconducting qubits could maintain quantum coherence for milliseconds—brief on human scales but sufficient for thousands of quantum operations.
“The coherence time improvements were essential,” notes Dr. Jay Gambetta of IBM Quantum. “They transformed superconducting qubits from physics demonstrations into computational elements that could execute quantum algorithms.”
Quantum Superposition at Macroscopic Scales
Among the most striking demonstrations of macroscopic quantum behaviour was the creation of quantum superposition states in superconducting circuits—states where a system simultaneously occupies multiple classical configurations.
Researchers prepared circuits in superpositions where currents flowed simultaneously clockwise and counterclockwise, or where magnetic flux had two different values simultaneously. These represent genuine quantum superpositions of macroscopically distinct states—often called “Schrödinger’s cat” states after Erwin Schrödinger’s famous thought experiment.
“Creating a Schrödinger’s cat state in a superconducting circuit is remarkable,” reflects Professor Irfan Siddiqi of UC Berkeley. “You have billions of electrons coherently superposed in two distinct flow patterns. It’s quantum mechanics operating at scales previously thought impossible.”
These superposition states remain fragile, collapsing into classical states within microseconds to milliseconds. But their very existence demonstrates that the quantum-classical boundary is not absolute—with sufficient isolation, quantum behavior persists even in large systems.
Quantum Measurement and Back-Action
The laureates’ work also illuminated fundamental aspects of quantum measurement. Observing quantum systems inevitably disturbs them—the famous “quantum measurement back-action” that lies at the heart of quantum uncertainty.
Measuring a superconducting circuit’s quantum state requires coupling it to a measurement apparatus (typically a microwave resonator and amplifier). The measurement process collapses quantum superpositions, projecting the system into a definite classical state. Understanding and minimizing this back-action proved essential for quantum information applications.
The research revealed that measurement back-action could be analyzed quantitatively and even exploited for quantum control. “Measurement isn’t just observation—it’s interaction,” explains Dr. Aashish Clerk, a quantum measurement theorist. “The laureates’ work showed how to understand and harness that interaction for quantum technology.”
From Physics to Technology: The Quantum Computing Connection
While driven by fundamental physics questions, the laureates’ research directly enabled quantum computing. A quantum computer processes information using quantum bits (qubits) that can exist in superposition states and exhibit quantum entanglement—properties allowing quantum computers to solve certain problems exponentially faster than classical computers.
Superconducting circuits exhibiting macroscopic quantum behavior provide the leading platform for building quantum computers. The energy quantization demonstrated by the laureates allows circuits to act as qubits—two-level quantum systems analogous to classical bits but with quantum superposition. The long coherence times achieved through improved designs enable complex quantum computations.
Google’s 2019 quantum supremacy demonstration—performing a calculation beyond classical computer reach—used 53 superconducting qubits based directly on the laureates’ research. IBM, Rigetti, IonQ, and numerous other companies building quantum computers rely on the same fundamental physics.
“Without the foundational science recognized by this Nobel Prize, quantum computing would remain science fiction,” observes Dr. John Preskill of Caltech. “The laureates proved it was possible to control quantum mechanics at scales relevant for computation.”
Quantum Sensors and Metrology
Beyond computing, macroscopic quantum circuits enable ultra-sensitive measurements. SQUIDs (Superconducting Quantum Interference Devices), based on Josephson junction physics, detect magnetic fields with sensitivity surpassing any classical sensor by orders of magnitude.
These quantum sensors find applications in brain imaging (magnetoencephalography), geological surveying, submarine detection, and fundamental physics experiments searching for dark matter or testing relativity. The quantum advantage arises from exploiting quantum interference and noise reduction approaching fundamental quantum limits.
“The same physics that enables quantum computing enables quantum sensing,” notes Dr. Konrad Lehnert of JILA. “Both exploit macroscopic quantum coherence for capabilities impossible classically.”
Bridging Quantum and Classical Worlds
The laureates’ work fundamentally revised our understanding of the quantum-classical boundary. Rather than a sharp dividing line between quantum microscopic and classical macroscopic realms, the boundary proves to be gradual and controllable.
Quantum behavior persists in large systems if decoherence can be sufficiently suppressed. Conversely, quantum systems transition to classical behavior when environmental interactions cause decoherence. The boundary location depends not on fundamental physics but on engineering—how well we can isolate systems from their environment.
“This is a profound conceptual shift,” reflects Professor Anton Zeilinger, the 2022 Nobel laureate in Physics. “Quantum mechanics isn’t confined to the microscopic world—it’s the fundamental description of nature at all scales. We just need to work harder to see it in large systems.”
Fundamental Physics Implications
Beyond technological applications, macroscopic quantum phenomena illuminate fundamental physics questions. How does the quantum world transition to classical reality? What role does measurement and observation play? Can quantum mechanics apply to truly macroscopic objects, or does it break down at some scale?
The laureates’ experiments address these questions empirically. They demonstrate that quantum mechanics correctly describes systems containing 10²³ particles acting coherently—scales where many physicists expected quantum theory might fail or require modification.
“These experiments test quantum mechanics in regimes it was never designed for,” explains Professor Angelo Bassi, who studies quantum foundations. “The fact that quantum theory works perfectly at these scales strengthens confidence in its fundamental validity.”
The Quantum Measurement Problem
Superconducting circuits also provide platforms for studying the quantum measurement problem—the question of how quantum superpositions collapse to definite classical outcomes during measurement. By precisely controlling measurement processes, researchers can study the transition from quantum to classical behavior.
“We can now watch quantum states collapse in real time,” notes Dr. Kater Murch of Washington University. “That provides empirical data about measurement processes that were previously purely philosophical questions.”
While controversies about quantum measurement’s ultimate interpretation remain, macroscopic quantum circuits offer unprecedented experimental access to the measurement process, potentially informing future theoretical developments.
Technical Challenges and Solutions
Achieving macroscopic quantum behavior required overcoming formidable technical challenges. Superconducting circuits must be fabricated with nanometer precision, cooled to millikelvin temperatures, shielded from electromagnetic interference, isolated from vibrations, and measured without destroying quantum states.
The laureates pioneered solutions to each challenge. Improved fabrication techniques reduced defects that cause decoherence. Dilution refrigerators cool samples to temperatures below 10 millikelvin. Magnetic shielding blocks external fields. Custom low-noise amplifiers measure quantum states with minimal back-action.
“Every technical advance enabled new physics,” recalls Dr. David DiVincenzo, a quantum computing theorist. “The laureates didn’t just discover phenomena—they invented the techniques making discovery possible.”
Quantum Error Correction: Protecting Quantum Information
A crucial application of macroscopic quantum circuits involves quantum error correction—protecting quantum information from decoherence and operational errors. This requires encoding quantum information redundantly across multiple physical qubits and detecting errors without measuring (and thus destroying) the quantum information itself.
John Martinis’s group demonstrated surface code error correction in superconducting circuits—a landmark achievement showing that quantum information could be protected as long as individual qubit error rates remained below thresholds. This validated the theoretical foundations of fault-tolerant quantum computing.
“Error correction transforms quantum computing from physics demonstration to engineering discipline,” explains Dr. Barbara Terhal, a quantum error correction expert. “John’s experiments proved the concepts work in real hardware, not just theory.”
Scaling to Larger Systems
Current research focuses on scaling macroscopic quantum circuits to larger sizes while maintaining quantum coherence. Modern superconducting quantum processors integrate hundreds of qubits, with roadmaps targeting millions of qubits within the next decade.
Scaling introduces new challenges: fabricating many nearly identical qubits, routing control signals without crosstalk, managing heat loads at millikelvin temperatures, and implementing error correction efficiently. Each challenge requires innovative solutions building on foundations the laureates established.
“We’re in the scaling regime now,” observes Dr. Jerry Chow of IBM Quantum. “The physics is proven—the challenge is engineering systems complex enough for practical applications while maintaining quantum performance.”
Alternative Quantum Systems
While this overview focuses on superconducting circuits, macroscopic quantum phenomena appear in other systems as well. Atomic ensembles, superconducting mechanical resonators, and even carefully prepared molecular systems exhibit collective quantum behavior involving many particles.
These alternative platforms benefit from the conceptual frameworks and experimental techniques pioneered in superconducting circuits. The principles of circuit QED, for instance, have been adapted to optomechanical systems and cold atom experiments.
“The laureates’ work established templates applicable across quantum physics,” notes Professor Markus Aspelmeyer of the University of Vienna. “Their insights transcend the specific systems they studied.”
Educational Impact
The laureates’ research has profoundly influenced physics education. Macroscopic quantum circuits provide tangible demonstrations of quantum principles previously accessible only through abstract mathematics or microscopic experiments. Students can now see quantum mechanics operating in circuits they can photograph and manipulate.
“Teaching quantum mechanics changed when we could show students macroscopic quantum systems,” reflects Professor David Pekker of the University of Pittsburgh. “It makes quantum mechanics concrete rather than abstract, engineering rather than philosophy.”
Universities worldwide now offer courses on superconducting qubits and circuit QED, training the next generation of quantum engineers using principles the laureates established.
Looking Forward: Quantum Technologies
As quantum technologies mature, the fundamental physics recognized by the Nobel Prize takes on increasing practical importance. Every quantum computer, quantum sensor, and quantum communication system relies on maintaining quantum coherence in engineered systems—the central achievement of the laureates’ work.
Future quantum technologies will likely exploit even more exotic quantum phenomena—topological states, non-Abelian anyons, quantum error-correcting codes encoded in hardware—all building on the foundation that quantum mechanics can be engineered at macroscopic scales.
“We’re just beginning,” concludes Professor Mikhail Lukin of Harvard University. “The laureates showed us quantum mechanics works at scales we can engineer. Now we need to discover what else becomes possible with that understanding.”
A New Quantum Paradigm
The 2025 Nobel Prize in Physics honors research that fundamentally revised our understanding of quantum mechanics’ domain. By demonstrating quantum tunneling and energy quantization in macroscopic electrodynamic circuits, Clarke, Devoret, and Martinis proved that quantum behavior extends beyond the microscopic world into engineerable systems approaching human scales.
This discovery bridges fundamental physics and transformative technology. It illuminates deep questions about quantum mechanics’ foundations while enabling practical quantum computers, sensors, and communication systems. It represents the kind of breakthrough that defines scientific eras—changing both how we understand nature and how we apply that understanding to reshape our technological capabilities.
The quantum revolution these discoveries enabled continues unfolding. As quantum technologies transition from laboratory demonstrations to practical tools, they rest on foundations established by the 2025 Nobel laureates—proof that quantum mechanics, properly understood and engineered, opens possibilities limited only by our imagination and technical ingenuity.
– Srinivasa Rao Srikantam
