Nobel Physics 2025: How Quantum Circuits Became Real

Nobel Physics 2025 Latest 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.

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.

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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.

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