Quantum decoherence

Quantum decoherence is the loss of a special property called quantum coherence in tiny particles. When this happens, a quantum system starts to behave more like everyday objects we see around us, which follow the rules of classical mechanics. Scientists study quantum decoherence to understand how tiny quantum effects turn into the normal world we experience.

This idea began as researchers tried to expand our understanding of quantum mechanics. Over time, the theory grew in many directions, and experiments have shown that some important parts of it are true. One big area where quantum coherence matters a lot is in quantum computing. This technology depends on keeping quantum coherence to work properly, making decoherence a key challenge for scientists and engineers.

Concept

In quantum mechanics, everything is described by something called a quantum state. This helps us figure out what might happen when we test or measure a system.

When a system is all by itself, its state changes in a smooth, predictable way. But if something else interacts with it, like when we measure it, the system shares its state with the world around it. This makes the system seem to lose its special quantum properties — this is called quantum decoherence. The information isn't really gone; it’s just spread out into the environment, kind of like how heat spreads out when we rub our hands together.

History and interpretation

Relation to interpretation of quantum mechanics

An interpretation of quantum mechanics tries to explain how the math of quantum physics matches what we see in the real world. Decoherence can be studied using any interpretation because it uses standard quantum math. It has been closely linked to understanding how quantum physics might turn into the physics we see every day.

Decoherence helps us understand how a quantum system might seem to "collapse" into one state, even though it doesn’t really collapse. Instead, parts of the system get mixed up with their surroundings, making it look like only one thing happens. Even though it looks like one outcome, all possibilities still exist together in a bigger system, but we can’t see them all.

Origin of the concepts

The idea of quantum decoherence first appeared in 1929 in work by Nevill Mott, though he didn’t use that name. In 1951, David Bohm described it as “destruction of interference in the process of measurement.” Later, in 1970, H. Dieter Zeh helped make it a major topic of study. Since the 1980s, many scientists have worked on it, though some still debate if it fully solves big questions in quantum mechanics.

Mechanisms

Quantum decoherence is when a quantum system loses its special quantum properties and starts behaving more like a regular, classical system. This happens because the system interacts with its surrounding environment, causing it to lose information about its quantum state.

One way to understand this is by thinking of the system and its environment as separate parts. When they interact, the system's quantum state becomes mixed with the environment's state. This mixing makes it hard to see the quantum effects, like interference, which are usually visible in isolated quantum systems.

In simpler terms, imagine a spinning coin that can land on heads or tails in a very precise way when it’s isolated. But when it’s dropped in a busy room with lots of people moving around, the exact way it spins gets lost in all the activity. This is similar to how quantum systems lose their unique behaviors when they interact with a lot of other things around them.

Non-unitary modelling examples

Decoherence happens when a quantum system interacts with its surroundings. This interaction causes the system to lose some of its quantum properties, making it behave more like a classical system.

When a system interacts with its environment, information about the system can be lost to the surroundings. This loss of information leads to decoherence. For example, in a system of tiny particles called qubits that are rotated, random changes can occur, causing the qubits to lose their distinct states over time. This process is called decoherence because the qubits can no longer be distinguished from each other.

Timescales

Quantum decoherence happens very quickly for big objects because they interact with many tiny particles. This helps explain why we don’t see quantum effects in everyday items and why classical physics works for large amounts of matter. The time it takes for these quantum effects to disappear is called the decoherence time, and it is usually very short for things we see and use every day.

Mathematical details

When we study a tiny part of a bigger system, like a single atom in a room, the atom can act in special ways that we call "quantum." But when this atom touches or interacts with the air, light, or anything around it, it loses these special quantum behaviors. This happens because the atom shares information with its surroundings.

Scientists describe this using math with special symbols and equations. Even though these equations look complicated, the main idea is simple: when a quantum system like an atom meets its environment, it stops acting in uniquely quantum ways and starts behaving like everyday objects we see. The time it takes for this change to happen is called the decoherence time.

Experimental observations

Scientists have done many tests to measure how quickly quantum systems lose their special properties, called decoherence. This rate can change based on things like temperature or how exact we know a particle's position.

In 1996, researchers in Paris were the first to measure this loss of quantum properties for individual atoms. They sent atoms through a special box filled with microwave waves and watched how the atoms and the waves interacted. Later, in 2011, other scientists used strong magnetic fields to reduce some causes of decoherence in tiny magnetic molecules, helping us understand how temperature and magnetic fields affect this process.

Prevention

Decoherence makes a quantum system lose its special quantum properties. This change turns quantum behavior into the more familiar, everyday behavior we see around us. It is a big problem for building quantum computers.

Quantum systems are very sensitive to noise from their surroundings, such as tiny changes in temperature, electromagnetic fields, and other outside influences. Because of this, scientists need to protect these systems carefully. They must finish their work before the system loses its quantum properties. To help with this, researchers have created many methods to reduce these problems and keep the system stable.

One way to help is by isolating the system from its environment. This can be done by placing the system in a very empty space called a high vacuum, cooling it to very low temperatures, or using special materials to block outside electromagnetic fields. Using better materials and designing circuits carefully also helps.

Another important method is Quantum error correction (QEC). This technique stores information in many small parts called qubits so that mistakes can be found and fixed without disturbing the system. Examples include the Shor code, Steane code, surface codes, and Bosonic codes.

A third method is Dynamical decoupling, which uses special signals to control the system and reduce the effects of outside noise. Examples include Spin echo (SE) and the Carr–Purcell–Meiboom–Gill (CPMG) sequence.