Quantum Superposition

- Overview

Quantum superposition is a fundamental phenomenon of quantum mechanics where two or more quantum states can be added together “superposed,” and the result will be another valid quantum state. 

In quantum physics, waves are everywhere. Click or tap above to create a wave, and watch as it travels outward from a central point. When two waves overlap, they interfere and either add together or cancel each other out—an effect called superposition. 

Atoms, electrons and many other inhabitants of the quantum world can be described by waves. But these waves don’t represent the movements of physical things like water or air. Rather, their rolling peaks and valleys represent the probability that a quantum property like position or energy will have a certain value when it’s measured. 

For example, an electron orbiting an atom doesn’t sit at a definite point in space like the Earth does as it orbits the sun. Rather, it gets smeared out into a cloud of possibilities called an orbital. That orbital cloud is really a three-dimensional quantum wave, with peaks and valleys that fluctuate in time and represent the chance of finding an electron at a particular spot. 

The shape of this wave changes depending on the electron’s energy. It’s possible to create a superposition in which two quantum waves---representing two electron energy levels---get added together, which leads to a new pattern of peaks and valleys. This changes where the electron is most likely to be found and can affect the physical properties of an atom. 

In this kind of superposition, it’s common to say that the electron has two different energies at the same time, or that it’s in many places at once. This can be pretty confusing if you’re thinking of the electron as a particle. But if you imagine the electron as a wave, which is already an extended thing, superposition is a little easier to understand. Waves---including superpositions of quantum waves---are in many places at once.  

Quantum bits, or qubits, can be in a superposition of multiple states at the same time. This ability is called superposition, and it's a fundamental property of quantum computing systems. 

A qubit can represent a 0, a 1, or any proportion of 0 and 1 in superposition, with a certain probability of being a 0 and a certain probability of being a 1. For example, a set of two qubits can be in a superposition of four states. 

When a quantum computer with qubits in superposition is used, it can process all possible states of the qubits at once, allowing it to perform multiple computations in parallel. This is different from classical computers, which can only perform one computation at a time because classical bits can only be in one of two states, 0 or 1. The final result of a calculation only emerges once the qubits are measured, which causes their quantum state to "collapse" to either 1 or 0. 

Qubits can represent numerous possible combinations of 1 and 0 at the same time. This ability to simultaneously be in multiple states is called superposition. To put qubits into superposition, researchers manipulate them using precision lasers or microwave beams.

Please refer to the following for more information:

• Wikipedia: Quantum Superposition

- Quantum Superposition

Quantum superposition is a fundamental principle of quantum mechanics where a quantum system, such as an electron, can exist in multiple states or properties simultaneously, rather than in a single, defined state. 

2. Examples and Analogies

• Schrödinger's Cat: This famous thought experiment illustrates superposition by imagining a cat that is both alive and dead at the same time until the box is opened and the cat is observed.
• Double-Slit Experiment: In this experiment, particles like electrons behave as waves and pass through both slits simultaneously, creating an interference pattern. However, if you try to observe which slit the electron passes through, the interference pattern disappears, and the electron behaves like a classical particle.

- Understanding Superposition in Quantum Technology

Quantum superposition is the principle that a quantum object, like an electron, can exist in multiple states or locations simultaneously until measured. 

This concept is illustrated by wave-like properties of particles, where quantum waves describe probabilities of existence, similar to how water waves overlap. 

While seemingly abstract, this principle allows for complex patterns like interference and is crucial for advanced technologies such as quantum computing. 

1. How Quantum Superposition Works: 

• Wave-Particle Duality: Quantum objects exhibit both wave and particle characteristics.
• Probability Waves: Unlike visible waves, quantum waves are mathematical descriptions of probabilities, indicating the likelihood of a particle being in a certain state or location.
• Combination of States: A quantum object can be in a combination of all its possible states at once, a state known as superposition.
• Observation and Collapse: When an observer measures a quantum object, its superposition "collapses," and the object settles into a single, definite state.

2. Analogies for Understanding: 

• Water Waves: Waves from two points on a pond's surface overlap, creating a more complex pattern.
• A Coin Toss: Before it lands, a coin is in a superposition of both heads and tails, but once observed, it becomes either one or the other.
• Schrödinger's Cat: This famous thought experiment imagines a cat in a sealed box with a poison that is equally likely to kill it or not. The cat is considered both alive and dead in superposition until the box is opened.

3. Importance in Quantum Technology: 

• Quantum Computing: Superposition is a foundational concept for quantum computers, where quantum bits (qubits) can represent both 0 and 1 simultaneously.
• Interference and Entanglement: The principle of superposition enables other quantum phenomena like quantum interference and entanglement, leading to new technological possibilities.

- How can Scientists Observe Superposition?

Scientists observe superposition indirectly by setting up systems where particles in superposition exhibit wave-like interference. 

The classic double-slit experiment is a prime example: when particles are fired one at a time, an interference pattern appears, indicating each particle passed through both slits simultaneously in a state of superposition. Another method involves using multiple polarizing filters, where the probabilities of light passing through each filter reveal the presence of superposition, even though individual photons are detected only once. 

Double-Slit Experiment: 

1. The Setup: 

• Particles like photons or electrons are sent towards a barrier with two narrow slits. A detector screen is placed behind the barrier to record where the particles land. 

2. The Observation: 

• If no detector is placed at the slits to "watch" the particles, an interference pattern forms on the screen. This is a series of bright and dark bands, similar to what happens with waves.

3. The Implication: 

• This interference pattern is evidence of superposition because it suggests that each particle somehow went through both slits at the same time, interfering with itself as a wave. This would not happen if the particle only went through one slit or the other.

4. Polarizing Filters: 

• The Setup: Unpolarized light, which is a mix of many different polarized states, is sent through a horizontal polarizing filter.
• The Observation: The horizontal filter blocks all light that isn't polarized horizontally. If the light then encounters a second filter that is rotated toward a vertical orientation, the amount of light that passes through decreases steadily.
• The Implication: The fact that some light passes through a diagonal filter, and then some through a vertical one, is a direct result of the light being in a superposition of horizontal and vertical states after the first filter. If the light were strictly horizontally polarized after the first filter, it would be completely blocked by any rotation of the second filter, but this isn't the case.

5. Key takeaway: 

• Scientists cannot directly "see" a particle in a superposition of states. Instead, they observe the consequences of superposition, such as interference patterns or specific probability distributions of outcomes, which only make sense if the particle was in multiple states at once until it was "measured" or interacted with the environment.

<More to come ..>