Why in news?

The 2025 Nobel Prize in Physics went to John Clarke, Michel Devoret, and John Martinis for demonstrating that quantum tunnelling — where particles cross barriers they shouldn’t be able to — can occur not only in subatomic particles but also in macroscopic superconducting circuits.

Their pioneering work proved that quantum phenomena, once thought to exist only at the atomic and subatomic scale, can also occur in man-made electrical circuits visible to the naked eye. It paved the way for technologies that could transform computing, sensing, and communication.

What’s in Today’s Article?

• Quantum Tunnelling and Energy Quantisation Made Visible
• Bridging the Quantum and the Everyday World
• Applications: From Quantum Chips to Sensors

Quantum Tunnelling and Energy Quantisation Made Visible

• The Nobel laureates — John Clarke, Michel Devoret, and John Martinis — demonstrated two of quantum physics’ defining principles, tunnelling and energy quantisation, in a macroscopic electric circuit.
• The Josephson Junction: Heart of the Discovery
  • At the core of their experiments lies the Josephson junction, a device where two superconductors are separated by a thin insulating barrier.
  • The researchers asked whether the phase difference — a measurable electrical property — across this junction could behave like a single quantum particle.
  • By sending current through the circuit, they observed that when it was small, electrons (in Cooper pairs) were trapped, producing no voltage.
    • Cooper pairs are pairs of electrons bound together by an attractive force, mediated by lattice vibrations called phonons, that occurs at low temperatures in superconducting materials. 
    • These pairs, which have opposite spins and total zero spin, behave as a single quantum unit called a boson and can flow through the material without resistance, enabling superconductivity. 
  • But sometimes, the current “tunnelled” through the barrier, suddenly flowing freely and generating a measurable voltage.
  • This confirmed macroscopic quantum tunnelling — a quantum leap happening in an entire electrical circuit.
• Solving the Fragility Problem
  • Early efforts to detect quantum tunnelling failed because of environmental noise and microwave interference.
  • The Berkeley team, led by Clarke, solved this by using special filters, shielding, and ultra-cold, stable setups to isolate the circuit.
  • When cooled to near absolute zero, the system behaved exactly as quantum theory predicted — the rate of tunnelling became independent of temperature, confirming it wasn’t due to thermal noise but a true quantum process.
• Revealing Quantum Energy Levels
  • The team then looked for quantised energy states, a hallmark of quantum behaviour.
  • By shining microwaves of varying frequencies on the junction, they saw that when the frequency matched the energy gap between two levels, the circuit “escaped” more easily from its trapped state.
  • This showed that the circuit absorbed and emitted discrete packets of energy, behaving like a macroscopic atom.
  • For the first time, scientists saw quantum behaviour in a system visible to the naked eye.
• Blueprint for Quantum Control
  • These experiments proved two key ideas:
    • Macroscopic electrical circuits can exhibit quantum properties when isolated from noise.
    • Their behaviour can be described using standard quantum mechanics.
  • The work also established methods for controlling and reading macroscopic quantum states using bias currents and microwaves — techniques that became the foundation for superconducting qubits and quantum measurement systems.

Bridging the Quantum and the Everyday World

• For years, scientists questioned how large a system could be and still exhibit quantum effects. Normally, quantum behaviour disappears when many particles interact.
• But the Nobel laureates — John Clarke, Michel Devoret, and John Martinis — proved that with superconducting materials, extreme cooling, and precision engineering, even a visible electronic chip can display clear quantum phenomena.

Applications: From Quantum Chips to Sensors

• The laureates’ findings underpin many modern quantum technologies:
  • Superconducting qubits: Circuits that act like artificial atoms and are the basis of quantum computers by Google, IBM, and others.
  • Quantum sensors: Devices capable of detecting tiny magnetic fields or gravitational variations, useful in medical diagnostics and geophysical exploration.
  • Quantum amplifiers: Boost faint signals without adding noise, vital for space exploration and dark matter detection.
  • Metrology: Josephson junctions now define electrical standards like the volt and ampere with quantum-level precision.
  • Microwave-to-optical converters: Link quantum processors to optical fibre networks for quantum communication.
• Turning Fragility into Functionality
  • Ultimately, these devices are powerful because even minute external changes cause large, measurable shifts in the circuit’s quantum state.
  • The laureates’ work transformed this sensitivity — once a limitation — into a defining feature, creating tools that bridge quantum theory and real-world technology.