Quantum Tunnelling

The quantum realm is a bizarre world where classical logic and conventional scientific rationality do not work at all. Extremely small particles are connected with each other even if very long distances separate them. Particles move through dense barriers effortlessly as if barriers do not exist at all. Quantum tunnelling is a phenomenon where a subatomic particle or atom can be present on the opposite side of the physical obstacle that is normally impossible for the particle to infiltrate. This is as if a person is walking and reaching a 20-metre tall wall extending a long way on both sides. Without a long enough ladder, it is impossible to reach the other side of the obstacle. However, in the case of quantum tunnelling, the person could easily go through the obstacle without any physical repercussions.

What is Quantum Tunnelling?

Quantum tunnelling is defined as a quantum mechanical process where wavefunctions can penetrate through a potential barrier. The transmission through the potential barrier can be finite and relies exponentially on the barrier width and barrier height. The wave functions have the genuine probability of disappearing on one side and reappearing on the remaining side. The first derivative of the wave functions is continuous. In the steady-state case, the probability flux in the forward trajectory is spatially uniform. No wave or particle is eliminated. Tunnelling happens with barriers of thickness about 1–3 nm and smaller. Quantum tunnelling cannot be explained through the laws of classical mechanics, where a dense potential barrier needs potential energy. It has a crucial role in physical processes such as nuclear fusion. It’s been used in quantum computing, tunnel diodes and scanning tunnelling microscopes. The quantum phenomenon was theorised in the early 20th century, and it was accepted as a practical physical phenomenon in the mid-century.

Quantum tunnelling is forecasted to create physical limits to the dimensions of the transistors employed in microelectronics. This is due to electrons’ ability to tunnel transistors that are too small. Tunnelling can be understood through the concepts of Heisenberg’s uncertainty principle. In other words, the uncertainty in the precise location of electromagnetic particles permits these particles to break the laws of classical physics and propagate in space without going over the potential energy boundary. Both tunnelling and uncertainty principle are mutually compatible as they consider a quantum body as both wave and particle simultaneously.

The video explains the fundamentals of the quantum mechanical model.

Applications of Quantum Tunnelling

Scanning Tunnelling Microscope

Heinrich Rohrer and Gerd Binning developed scanning tunnelling microscopes (STM). It is a type of microscope that helps to observe objects at atomic levels. It functions by utilising the connection between quantum tunnelling with distance. STM analyses the surface by using a sharp conducting tip that can differentiate characteristics smaller than 0.1 nm with a 0.01 nm depth resolution. So, individual atoms can be consistently imaged and manipulated.

Nuclear Fusion

Quantum tunnelling is a crucial part of nuclear fusion. The average temperature of a star’s core is usually not sufficient for atomic nuclei to overcome the Coulomb barrier and kick start thermonuclear fusion. The tunnelling increases the chances of infiltrating this barrier. Though the probability is still low, the huge number of nuclei in the stellar core is enough to drive a steady fusion reaction.

Electronics

Tunnelling is a frequent source of current leakage in very-large-scale integration (VLSI) electronics. The VLSI electronics experience substantial power loss and heating effects that cripple such devices. It is usually considered the lower threshold on how microelectronic device elements can be created. Tunnelling is also a basic technique employed to set the floating gates in flash memory. Cold emission, tunnel junction, quantum-dot cellular automata, tunnel diode, and tunnel field-effect transistors are some of the main electronic processes or devices that use quantum tunnelling.

Quantum Biology

Quantum tunnelling is one of the core quantum phenomena in quantum biology. It is essential for both proton tunnelling and electron tunnelling. Electron tunnelling is a critical factor in numerous biochemical redox reactions (cellular respiration, photosynthesis) and enzymatic catalysis. Proton tunnelling also has a key role in spontaneous DNA mutation.

Quantum Tunnelling and the Light Speed Threshold

There is a theoretical assumption that spin-zero particles could move faster than the speed of light when tunnelling. This seems to break the principle of causality as a reference frame exists in which the particles arrive before they have gone. Physicist Herbert Winful proposed that the wave packet of tunnelling particles moves locally. Particles cannot penetrate through the potential barrier non-locally. He also deduced that the results used to show non-local movement had been misinterpreted. The group velocity of wave packets does not measure its speed but is connected to the amount of time the wave packets are stored in the barrier. However, the issue remains that the wave functions still arise inside the potential wall at all points simultaneously. In any area that is not accessible to calculation, non-local propagation is still theoretically certain. Recent experiments also showed that subatomic particles could tunnel at relative speeds faster than light.

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