#quantumtunneling

EPFL News

For years, quantum time has been one of science' biggest mysteries. The exact time of a quantum transition, such an electron tunneling over a barrier, is notoriously difficult to establish, even if basic quantum physics may predict its possibility. A team of EPFL researchers has developed a new method to quantify these extremely fast events, showing that quantum tunneling's "speed" depends on the material's symmetry and form.

The Newton journal published the groundbreaking subatomic research. By using electrons as a "clock," the scientists avoided interference issues that hinder high-precision quantum experiments.

The Attosecond Timing Challenge

Humans cannot comprehend quantum processes like tunneling and photon absorption. Occurrences often occur in quintillionths or tens of attoseconds (10-18 seconds). Consider that light cannot flow through a virus in the time it takes for these conversions.

Hugo Dil, EPFL professor and physicist, says the observer effect has prevented quantification of these gaps. “Measuring time intervals of this magnitude is notoriously difficult,” explains Dil, “particularly because any external timing device can interfere with the very phenomenon one aims to observe”. The 2023 Nobel Prize in Physics proved that humans can access these short periods, although utilizing an external reference can introduce “artefacts” or mistakes.

The “Spin Clock” Method

EPFL solved this with SPERS, a clever method. Synchrotron light excites electrons and drives them away.

As they are evacuated, these electrons carry quantum spin information. When light hits the electron, numerous interfering quantum paths form a superposition. This interference gives the electron's spin a distinctive pattern that changes with the quantum process.

By observing these small spin pattern variations in relation to electron energy, the researchers were able to establish the transition length without a clock. This internal “spin clock” shows how the electron's wavefunction evolves from initial to greater energy.

Symmetry and Quantum Leap Speed

The most surprising discovery of the study is that the material's atomic-level “shape” or symmetry considerably affects these quantum events' length.

Researchers examined many materials with varied geometries:

3D Copper: This typical, highly symmetric three-dimensional structure's quantum transition took 26 attoseconds.

Bilayer Materials (TiSe2 and TiTe2): Loosely linked layers behave as flat sheets. This procedure took 140–175 attoseconds, a significant delay.

Copper Telluride (CuTe) is a one-dimensional chain. The transition time approached 200 attoseconds in this low-symmetry scenario.

The model was clear: the longer the quantum transition, the simpler and smaller the material. Lower symmetry “slows down” quantum leaps.

Future Impact

This discovery sheds light on whether quantum transitions are immediate or have a time. Understanding these timescales can benefit technology beyond theoretical physics.

Researchers said “understanding the duration of a quantum transition can assist researchers in creating materials with targeted quantum characteristics.” Future technologies like high-speed electronics and quantum computing require precision quantum state manipulation.

Quantum Tunnelling

A particle can bypass an energy barrier through “quantum tunnelling” even without energy. Classical physics states that a ball needs energy to climb a hill or roll back down. Particles behave differently in quantum space.

How Quantum Tunnelling Works

Quantum tunnelling is caused by the Schrödinger equation's wave-particle duality. Wave functions, which predict the possibility of finding a particle at a given place, characterise particles in quantum physics.

Wave Function Penetration: A particle's wave function does not stop when it hits a potential barrier. Instead, it decays rapidly inside the barrier. Importantly, the wave function exhibits a small but non-zero amplitude over the barrier. This suggests the particle will seem far away.

Penetration likelihood: The square of the wave function's amplitude gives the likelihood of finding the particle across the barrier. This chance decreases exponentially with barrier height, width, and tunnelling particle mass. Tunnelling is particularly visible when low-mass particles like protons or electrons move through microscopically tight barriers, which are usually 0.1 nm for heavier particles and 1-3 nm for electrons.

This phenomenon breaks classical physics, which stipulates that a particle must have energy larger than the barrier height to pass through. The finite transmission is exponentially affected by the potential barrier's width and height.

Quantum tunnelling can be explained using Heisenberg's uncertainty principle, which states that electromagnetic particles can bypass classical physics and propagate without crossing the potential energy boundary due to uncertainty in their location. Tunnelling and the uncertainty principle are compatible by characterising a quantum material as a wave and particle.

Quantum Tunnelling Features

Non-Classical: It contradicts the classical physics rule that a particle must have more energy than the barrier height to cross.

The fact that matter is a wave causes wave-particle duality.

The particle may tunnel or reflect because the result is probabilistic. We eliminate no particles or waves during the procedure.

Mathematics Foundation

The time-independent Schrödinger equation describes quantum tunnelling mathematically. This equation can be solved in several ways depending on the particle's potential energy V(x) and total energy E. Solutions where V(x) – E is negative represent travelling waves. When V(x) – E is positive, the particle is inside the barrier where its energy is smaller than the potential energy, causing rising and falling exponentials, or evanescent waves. The semiclassical WKB approximation helps change potential barriers by approximating this challenging mathematical issue. This helps calculate the transmission coefficient, which indicates tunnelling likelihood.

History Of Quantum Tunnelling

In 1926, Schrödinger published his equation. Friedrich Hund originally utilised it in 1927 to analyse double-well potentials and molecular spectra for tunnelling over a potential barrier. In 1928, Mikhail Leontovich and Leonid Mandelstam independently discovered tunnelling.

Tunnelling theory triumphed after George Gamow's 1928 mathematical explanation of alpha decay, which Ronald Gurney and Edward Condon independently explained. By solving the Schrödinger equation for a nuclear potential model, they found a correlation between particle half-life and emission energy, which affects tunnelling. Walter Schottky coined “wellenmechanischer Tunneleffekt” in 1931, and Yakov Frenkel's textbook popularised “tunnel effect” in 1932.

After Leo Esaki demonstrated electron tunnelling in a semiconductor structure in 1957, the tunnel diode was invented. Brian Josephson predicted Cooper pairs would tunnel in 1962, and Ivar Giaever demonstrated superconductor tunnelling in 1960. Solids quantum tunnelling won Esaki, Giaever, and Josephson the 1973 Physics Nobel Prize. 1981 STM inventors Heinrich Rohrer and Gerd Binnig awarded the 1986 Physics Nobel.

Quantum Tunnelling Uses

Many natural and technological processes involve quantum tunnelling.

Nuclear Fusion: Stars like the Sun need it. For atomic nuclei to overcome their mutual electrostatic repulsion (Coulomb barrier) and fuse conventionally, star cores are usually too cold. Despite the modest individual chance, quantum tunnelling dramatically increases the chances of crossing this barrier and continuing star-driving fusion events.

Alpha decay is explained by quantum tunnelling, in which alpha particles leave the atomic nucleus through the strong nuclear force barrier. This provides astrobiology with energy in various cases.

Quantum tunnelling generates atomic-level surface pictures in scanning tunnelling microscopes (STM). Using a voltage bias, a conductive surface and a sharp conducting tip can be placed close together to detect tunnelling current. This current is increasingly sensitive to distance, allowing STMs to resolve surface features with 0.001 nm precision.

Tunnelling leaks current in very-large-scale integration (VLSI) circuits, causing power loss and warmth. It sets a minimum microelectronic device component size. Programming flash memory floating gates requires it. The following electronics employ tunnelling:

Tunnel diodes: Quantum tunnelling gives these devices unique current-voltage features. High-speed applications can have a voltage range where current declines with voltage. Different tunnelling is utilised in resonant tunnelling diodes.

Cold Emission (Field Electron Emission): Strong electric fields tunnel electrons out of atomic states, creating an exponentially changing current. This includes vacuum tubes, flash memory, and electron microscopes.

Tunnel Junctions: Josephson junctions use thin insulators as barriers between conductors, which require tunnelling for precise measurements.

Tunnel Field-Effect Transistors (TFETs): Quantum tunnelling regulates its gate (channel) instead of thermal injection, lowering gate voltage and power usage.

Quantum tunnelling helps explain electron collisions and electron transport through metals, especially impurities. Chemistry:

In interstellar clouds, chemical reactions occur at extremely low energy. Quantum tunnelling sustains hydrogen ion and molecule processes.

Quantum tunnelling is needed to explain large isotopic effects in chemical kinetics that classical theories cannot, especially when a heavier isotope is replaced with a lighter one.

Biology: Quantum biology's quantum tunnelling is important.

Enzymatic catalysis, photosynthesis, and cellular respiration depend on electron tunnelling.

Source: scientificamerican.com

The Unseen Pathways of Particles

When I first read about Quantum Tunnels, it reminded me of a concept from Vedic philosophy, how the soul can transcend physical limitations. In the Bhagavad Gita, it is said that the soul is eternal and can pass through different bodies, much like particles in quantum tunneling that move through barriers without being stopped. This made me realize how ancient wisdom and modern science sometimes share fascinating parallels. The idea that tiny particles can defy physical limits shows how mysterious and interconnected the universe truly is. In this article, we will explore the meaning of Quantum Tunnels, how they work, their real-world applications, and their potential impact on future technology.

What Are Quantum Tunnels?

Quantum Tunnels are a quantum mechanical phenomenon in which particles pass through potential barriers that they would not be able to cross in classical physics. According to classical mechanics, if a particle lacks the necessary energy to overcome a barrier, it should be reflected or stopped. However, in quantum mechanics, particles also behave like waves, allowing them to penetrate and cross barriers, even when their energy is insufficient.

How It Works

Imagine you are rolling a marble toward a hill. In classical physics, if the marble lacks the energy to climb over the hill, it will stop. However, in quantum physics, due to the wavelike nature of particles, there is a small probability that the marble will “tunnel” through the hill and appear on the other side without physically climbing over it.

This bizarre behavior is a direct result of the wave-particle duality, a fundamental concept in quantum mechanics. It means that particles exhibit both particle-like and wave-like properties. The wave associated with the particle can extend beyond the barrier, allowing it to “leak” through.

The Science behind Quantum Tunnels

To understand Quantum Tunnels better, let’s break it down into three key scientific concepts:

1. Wave Function and Probability

Source – newatlas.com

In quantum mechanics, particles are described by a wave function, which gives the probability of finding the particle in a particular location. When a particle approaches a barrier, part of its wave function passes through it, even if its energy is too low. This creates a small but finite chance that the particle will be found on the other side of the barrier.

2. Tunneling Probability

The probability of tunneling depends on factors such as:

The height and width of the barrier: Thinner or lower barriers increase the chance of tunneling.

The energy of the particle: Higher-energy particles have a greater tunneling probability.

3. Quantum Superposition

Particles can exist in multiple states at once. This means that while a particle may seemingly be blocked by a barrier, it also exists in a “tunneled” state simultaneously. This superposition allows the particle to reappear on the other side of the barrier.

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Real-World Applications of Quantum Tunnels

Even though Quantum Tunnels sound abstract, they have real-world applications that impact our daily lives. Here are some of the most common and impactful uses:

1. Semiconductors and Transistors

The technology behind modern electronics heavily relies on quantum tunneling. Transistors in microchips use this effect to control electrical currents. As transistors shrink in size, electrons can tunnel through thin insulating layers, enabling faster and more efficient computing.

2. Tunnel Diodes

Tunnel diodes are semiconductor devices that exploit quantum tunneling to achieve ultra-fast switching speeds. They are used in high-frequency applications, such as microwave amplifiers and oscillators, due to their rapid response.

3. Scanning Tunneling Microscope (STM)

The STM is an advanced imaging device that uses quantum tunneling to visualize individual atoms on a surface. It works by bringing an extremely sharp tip close to a sample, allowing electrons to tunnel between the tip and the surface. This produces high-resolution images of atomic structures.

4. Nuclear Fusion

Quantum tunneling plays a key role in nuclear fusion reactions, where hydrogen nuclei combine to form helium. Despite the electrostatic repulsion between positively charged nuclei, quantum tunneling allows them to overcome this barrier, enabling the fusion process that powers stars and experimental fusion reactors.

5. Flash Memory Devices

Flash memory, used in USB drives and SSDs, uses quantum tunneling to store and erase data. Electrons tunnel through a thin insulating layer, allowing information to be written and read efficiently.

The Future of Quantum Tunnels in Technology

As technology advances, Quantum Tunnels are expected to play an even larger role. Here are some potential future applications:

1. Quantum Computing

Source – orfonline.org

Quantum computers leverage quantum tunneling to process information in ways that classical computers cannot. Tunneling enables qubits to exist in multiple states at once, significantly boosting computational power and speed. This can potentially revolutionize fields like cryptography, data analysis, and artificial intelligence.

2. Quantum Encryption

Tunneling-based encryption methods could lead to more secure communication systems. Quantum key distribution (QKD) uses the properties of tunneling to transmit data securely, making it nearly impossible for hackers to intercept.

3. Medical Imaging and Treatment

Researchers are exploring the use of quantum tunneling in advanced medical imaging techniques. For example, magnetic resonance imaging (MRI) and positron emission tomography (PET) could benefit from more precise quantum-level interactions, improving diagnosis accuracy.

4. Energy-Efficient Devices

Tunneling transistors are being developed to create more energy-efficient electronic devices. These transistors use tunneling effects to reduce power consumption while maintaining high performance, potentially extending battery life in portable electronics.

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Challenges and Limitations of Quantum Tunnels

Source – newscientist.com

While Quantum Tunnels offer incredible possibilities, they also come with challenges:

Error Rates in Quantum Computing: Quantum tunneling can lead to decoherence and errors in quantum computers. Scientists are working on error correction techniques to minimize these effects.

Energy Leakage: In some semiconductor applications, unintended quantum tunneling can cause energy leakage, reducing efficiency.

Difficulties in Control: Controlling quantum tunneling precisely is complex and requires highly controlled environments, making practical applications challenging.

Conclusion