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.

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