Nobel Prize 2025: Quantum Circuits and Tunnelling

Why in the News?

The 2025 Nobel Prize in Physics was awarded to John Clarke, Michel Devoret, and John Martinis. They proved in the 1980s that the strange laws of quantum mechanics do not just apply to tiny particles like electrons or atoms but can also apply to entire electrical circuits visible to the eye. Their experiments opened the way for modern quantum computers and many other technologies.

Key Highlights

1. What is Quantum?
   1. The word “quantum” means the smallest possible unit of energy or matter that cannot be divided further. In quantum mechanics, energy levels are discrete (like steps on a ladder) rather than continuous (like a ramp). This is different from classical physics, where energy can vary smoothly.
2. The Experiments of the 1980s
   1. In the 1980s, Clarke, Devoret, and Martinis performed experiments at the University of California.
   2. They studied a special circuit called a Josephson junction. This is made of two superconductors (materials that carry current without resistance) separated by an ultrathin insulating barrier.
3. What Classical Physics Predicted
   1. According to classical physics, current flowing in this system should get stuck unless it has enough energy to cross the barrier.
   2. In other words, without sufficient energy, the current could not pass through.
4. The Quantum Surprise
   1. At extremely low temperatures (near absolute zero), something amazing happened.
   2. The current did not need extra energy (classical activation). Instead, it “tunnelled” through the barrier because of its wave-like properties.
   3. This phenomenon is called Quantum Tunnelling – a uniquely quantum effect where particles or currents pass through barriers they should not cross in classical physics.
   4. Tunnelling rates depend on barrier height, width, and the effective mass of the tunnelling coordinate.
5. Circuits Behaving like Particles
   1. The researchers also found that the entire circuit behaved as if it were a single giant particle.
   2. Instead of having a continuous range of energies, the system had discrete energy levels (like steps on a staircase).
   3. This was proof that quantum laws applied to macroscopic objects (big enough to see).
6. Ensuring the Results Were Genuine
   1. To ensure these effects were not due to experimental error or outside noise, the scientists carefully shielded the circuits from stray microwave radiation.
   2. The scientists proved that in a superconductor, trillions of electrons move together in such perfect coordination that they behave like a single object, and this collective behaviour can be described by just one quantum rule i.e., superconducting phase difference.
   3. Superconducting phase difference: The flow of current across the Josephson junction depends on the quantum phase difference between the two superconductors.
7. Technological Applications of the Work on Josephson Junctions
   1. Foundation for Quantum Computers (Circuit QED):
      1. The circuits behave like artificial atoms with quantised energy levels.
      2. Microwaves can make the system “jump” between these energy levels.
      3. When linked to a resonator (like an echo chamber for microwaves), scientists can measure the system’s state without disturbing it.
      4. This setup is called Circuit Quantum Electrodynamics (Circuit QED). It is the basis of today’s superconducting quantum computers.
   2. Quantum Amplifiers:
      1. These circuits can amplify extremely weak signals without adding extra noise.
      2. This is very useful in medical diagnostics, radio astronomy, and dark matter detection experiments.
   3. Ultra-Precise Measurements:
      1. Josephson-based circuits can measure electric current and voltage with very high precision.
      2. They are used in metrology laboratories to define electrical standards.
   4. Quantum Communication Networks:
      1. The circuits can act as microwave-to-optical converters, linking quantum processors to fibre-optic networks.
      2. This helps in developing quantum internet and long-distance secure communication.
   5. Quantum Simulators:
      1. These circuits can model complex physical and chemical systems at the atomic level.
      2. Scientists use them to simulate materials, molecules, and chemical reactions, helping design new materials or drugs.
   6. Some other modern technologies are:
      1. Superconducting qubits – the heart of today’s leading quantum computers.
      2. Quantum magnetometers (SQUIDs) – devices that measure extremely weak magnetic fields.
      3. Quantum voltage standards – used for precise electrical measurements.
      4. Single-photon detectors – used in astronomy and biomedical imaging.
8. Today’s Challenge
   1. The question today is not whether macroscopic quantum behaviour exists, but how to preserve it for practical use.
   2. Quantum states are very fragile and can collapse if disturbed by the environment. This problem is called Quantum Decoherence.
   3. Current research focuses on:
      1. Developing better materials with fewer energy losses.
      2. Improving cryogenic (ultra-low temperature) control systems.
      3. Designing hybrid architectures that combine superconducting circuits with light (photonic systems), mechanical devices, or spin-based systems.
9. The Larger Lesson
   1. When Clarke, Devoret, and Martinis began their experiments, they were simply curious about whether quantum mechanics applied to large systems.
   2. No one at that time imagined their work would help build quantum computers decades later.
   3. Their discovery shows how curiosity-driven basic science can lead to unexpected technological revolutions and bring prestige to the countries that support such research.

Implications

1. Scientific: Proved that quantum mechanics applies to macroscopic systems, not just atoms.
2. Technological: Enabled quantum computing, ultrasensitive detectors, and precision measurement tools.
3. Research Direction: Shifted focus from proving quantum effects to preserving them for applications.
4. Policy: Highlights the importance of investing in basic research, which can later power revolutions in technology.
5. India’s Context: Strengthens the case for India’s National Quantum Mission to develop indigenous quantum technologies.

Conclusion

The Nobel Prize 2025 celebrates a discovery that bridged the gap between tiny quantum particles and visible macroscopic systems. By showing that entire circuits could follow quantum laws, the laureates opened the door to quantum computing and advanced technologies. Their work is a reminder that fundamental curiosity-driven science can lead to revolutionary applications.