Macroscopic Quantum Tunneling in Superconducting Circuits

The 2025 Nobel Prize was awarded to John Clarke, Michel H. Devoret, and John M. Martinis for their discovery of macroscopic quantum mechanical tunneling and energy quantization in an electric circuit. Their experiments proved that quantum effects, which was once thought to exist only in atoms and subatomic particles, can emerge in circuits visible without the use of a microscope. This research became the foundation for modern superconducting qubits used in quantum computing.

The behavior of particles at the smallest scales, where energy quantization and tunneling are common, is described by quantum mechanics. However, it was long believed that these effects disappear at larger, macroscopic scales.

The 2025 Nobel laureates challenged this belief. Using superconducting circuits built with Josephson junctions, they demonstrated that a large, engineered system could still behave according to the laws of quantum mechanics. This discovery provided the first ever clear evidence that quantum tunneling and energy quantization can occur in electrical systems made of trillions of particles.

Key Experiment and Discovery

The researchers used superconducting loops which were cooled to temperatures close to absolute zero in their experiments. Every loop had a Josephson junction, which allowed electron pairs (also known as Cooper pairs) to tunnel through a thin layer of insulation.

They measured the system's transition between energy states by applying microwave radiation and then carefully controlling the current. Thermal activation caused these switches to occur at high temperatures. However, the switching continued as the temperature dropped, indicating that quantum tunneling rather than thermal escape was taking place.

The circuit could only absorb energy at specific frequencies when exposed to microwave radiation, exposing discrete energy levels, which are a defining feature of quantum mechanics.

These findings demonstrated that a superconducting circuit can work as a single quantum object even though it is macroscopic.

Significance

The difference between the quantum and classical worlds became unclear as a result of this discovery. It demonstrated that quantum laws hold true for both engineered macroscopic systems and particles.

Superconducting qubits, the fundamental components of modern quantum computers, also became possible by their hard work. Similar Josephson junctions are used by these qubits to store and control quantum data.

Conclusion

Their discovery that transformed modern day physics was recognized with the 2025 Nobel Prize in Physics. By proving that the bizarre effects of quantum mechanics may occur in systems big enough to see, Clarke, Devoret, and Martinis changed our insight of reality and opened the door to potential change for quantum technology.