Nobel Prize in Physics 2025: Putting Quantum Mechanics in the Palm of our Hands

In the International Year of Quantum Science and Technology – celebrating 100 years since the birth of quantum mechanics – it seems apt that the 2025 Nobel Prize in Physics went to three quantum physicists. On 7 October, John Clarke, Michel H. Devoret, and John M. Martinis were jointly recognized “for the discovery of macroscopic quantum mechanical tunnelling and energy quantisation in an electric circuit.”

The trio led a series of experiments in the mid-1980s at the University of California, Berkeley, USA, demonstrating that quantum phenomena – often at odds with everyday experience and usually on the scale of particles – can be observed in macroscopic superconducting electric circuits.

This included showing that the system is quantized, which means it only absorbs or emits energy in specific, discrete amounts, and demonstrating quantum tunnelling – a quantum-mechanical phenomenon where the wave-like behaviour of electrons allows them to pass through classically impenetrable barriers; the equivalent of a ghost walking through a solid wall.

Quantum Foundations

As with all Nobel discoveries, the work of Clarke, Devoret, and Martinis built on numerous earlier breakthroughs. For example, the quantum-mechanical depiction of the atom was revolutionary 100 years ago, and integral to the trio’s research.

Quantum mechanics was first recognized by the Nobel Committee when they awarded the Nobel Prize in Physics to Werner Heisenberg in 1932, and Erwin Schrödinger and Paul Dirac in 1933 (the former and latter of whom were regular Lindau attendees), for their foundational roles in developing what is now one of the primary pillars of modern science.

Since these early awards, a raft of others have followed for quantum mechanics breakthroughs, most recently in the 2022 Nobel Prize in Physics, which went to Alain Aspect, John Clauser and Anton Zeilinger “for their experiments with entangled photons, establishing the violation of Bell’s inequalities and pioneering quantum information science.”

But the list directly relevant to this year’s Prize – involving quantum tunnelling in superconducting electric circuits – is shorter. 1972 Nobel Prize in Physics recipients John Bardeen (the only person to be awarded the Nobel Prize in Physics twice, 1956 being his first), Leon N. Cooper and John R. Schrieffer developed their eponymous BCS theory to explain the strange properties of superconductors; materials that when cooled to extreme levels can conduct current without any electrical resistance. Meanwhile, 1973 Nobel Prize in Physics recipients Leo Esaki and Ivar Giaever led fundamental theoretical and experimental research into quantum tunnelling in semiconductors and superconductors, respectively; the latter confirming BCS theory predictions.

Most relevant, however, is the work of Brian D. Josephson (1973 Nobel Prize in Physics) and Anthony Leggett (2003 Nobel Prize in Physics), who really laid the groundwork for Clarke, Devoret, and Martinis’ 1980s experiments.

The Right Pair

In 1962, Josephson predicted how the Cooper pairs of electrons predicted by BCS theory to carry current in a superconductor can tunnel across a Josephson junction – which consists of an insulating barrier separating two superconductors. Named the Josephson effect, this phenomenon is a quantum effect observable as a current with zero voltage at an ordinary, macroscopic scale.

The effect was confirmed experimentally in 1963 by Philip W. Anderson (1977 Nobel Prize in Physics) and John Rowell. They observed a Josephson junction producing a steady, non-zero current when no voltage was applied. And then when they applied a constant DC voltage, the current oscillated at a frequency determined by the fundamental Cooper pair charge.

In 1978, Leggett claimed that an even more astonishing phenomenon was possible: that a classical device exhibiting a macroscopic manifestation of quantum mechanics, such as a Josephson junction, could itself be pushed beyond its classical description, where the billions of Cooper pairs constituting the entire electric circuit behave as if they are a single particle with a shared wavefunction.

Macroscopic Nucleus

Together with his senior PhD student Martinis and postdoc Devoret, Clarke carefully designed a superconducting circuit/Josephson junction setup etched on a microchip that would test Leggett’s idea in the real world. Their setup allowed electrical current to flow without resistance across the junction, while reducing noise to an absolute minimum to avoid any interference from the environment, resulting in zero voltage.

If the system was indeed quantum and behaved like a single particle, it would occasionally and randomly produce a voltage across the junction; which was exactly what the researchers observed. They then obtained further proof by firing microwaves of varying wavelengths into the zero-voltage state, producing excited states with discrete quantized energy levels. Their superconducting circuit was indeed a single large quantum system, a “macroscopic nucleus”.

These discoveries that the research trio made in the 1980s represent the beginning of quantum engineering, and have provided a promising path towards useful quantum computing – an endeavour still being hotly pursued today.

Towards Quantum Computing

Like the bits of a regular computer, a quantum computer’s basic unit of information is the qubit. But unlike classical bits that can represent either a 0 or a 1, a qubit takes advantage of the quantum phenomenon of superposition to be able to represent a 0, a 1, or a state where it is any proportion of both 0 and 1 simultaneously. This trait should enable a quantum computer to solve certain problems that would stump even the most powerful supercomputer.

Superconducting circuits, which can trace their lineage back to those 1980s experiments, are one of the most promising candidates for qubits. And Martinis and Devoret are leading efforts to build quantum computers with them.

In 2019, Martinis and his team built a 53-qubit quantum processor at Google Quantum AI using superconducting qubits that debatably achieved a quantum advantage (i.e. solving a problem that no classical computer can). More recently, he co-founded QoLab to significantly accelerate the timeline toward achieving a million-qubit system. Meanwhile, Devoret spent significant time in the 2010s developing new types of qubits – including the exotic-sounding quantronium, fluxonium and transmon – before being hired by Google Quantum AI as Chief Scientist of Quantum Hardware in 2023.

The ongoing work of Martinis and Devoret showcases the true impact of the trio’s foundational discovery. They didn’t just confirm a theoretical prediction of quantum mechanics, they provided a blueprint for engineering quantum phenomena on a controllable, macroscopic scale – a blueprint that continues to find use in developing the quantum technologies of the future.