Macroscopic Quantum Tunnelling: Quantum Effects at Visible Circuit Scales

Context:

• The 2025 Nobel Prize in Physics was awarded to John Clarke, Michel Devoret, and John Martinis.

• They were recognised for experimentally demonstrating macroscopic quantum tunnelling in electrical circuits, proving that quantum mechanics is not confined to atomic or subatomic scales.

• Their work laid the foundational basis for superconducting quantum computers and ultra-sensitive quantum devices.

Key Highlights:

What is Macroscopic Quantum Tunnelling?

• A phenomenon where quantum tunnelling occurs in large, visible systems, such as electrical circuits.

• Contradicts classical expectations where current should stop at an insulating barrier.

• Demonstrates that quantum behaviour can persist at macroscopic scales, provided conditions are controlled.

Josephson Junctions – Core Experimental System

• A Josephson junction consists of:

  • Two superconductors

  • Separated by a thin insulating layer

• Despite the insulator, Cooper pairs (paired electrons) can tunnel through the barrier.

• This creates a supercurrent governed by the phase difference of quantum wave functions.

Key Scientific Findings

• Observed quantum tunnelling of current in macroscopic circuits.

• Confirmed that superconductivity is maintained through tunnelling.

• Established a working physical model for superconducting qubits.

• Required:

  • Extreme cooling (near absolute zero)

  • Isolation from environmental noise
    to preserve quantum coherence.

Why This Discovery Matters

• Bridged classical and quantum physics, showing continuity rather than separation.

• Proved that quantum laws apply to engineered, human-scale devices.

• Enabled:

  • Superconducting quantum processors

  • Quantum microwave circuits

  • Superconducting resonators

Technological Applications

• Quantum Computing

  • Superconducting qubits form the backbone of many modern quantum processors.

• Quantum Sensors

  • Development of SQUIDs (Superconducting Quantum Interference Devices) for detecting extremely weak magnetic fields.

• Energy-efficient Electronics

  • Near-zero energy loss due to superconductivity.

• Precision Measurement

  • Ultra-sensitive detection in astronomy, medicine, and materials science.

Experimental Challenges

• Maintaining quantum coherence at macroscopic scales.

• Achieving:

  • Ultra-low temperatures

  • Radiation shielding

  • Precise control of electrical noise

Scientific Significance

• Validated quantum mechanics beyond microscopic systems.

• Established the physical foundation of modern quantum technologies.

• Accelerated progress in quantum information science and engineering.

UPSC Relevance (GS-wise):

• GS Paper 3 – Science & Technology

  • Prelims:

    • Quantum tunnelling, superconductivity, Josephson effect.

    • Macroscopic vs microscopic quantum phenomena.

  • Mains:

    • Role of fundamental physics discoveries in enabling emerging technologies.

    • Importance of superconducting systems in quantum computing and sensing.