A qubit isn't literally a classical 0 and a classical 1 existing simultaneously. Instead, it is a single quantum state that is a linear combination of the two classical states, |0\rangle and |1\rangle.
Here is a breakdown of what that means:
1. The Mathematical State (Superposition)
The state of a qubit, denoted |\psi\rangle, is represented by the formula:
* |0\rangle and |1\rangle are the two basis states, analogous to the two states of a classical bit.
* \alpha and \beta are complex numbers called probability amplitudes.
* The system exists in this single combined state as long as it is not measured (to determine if it's 0 or 1). It's an entirely new state, not just two classical states side-by-side.
2. The Role of Probability Amplitudes
The complex numbers \alpha and \beta determine the likelihood of observing either state when you measure the qubit:
* The probability of measuring the state as \mathbf{0} is |\alpha|^2 (the magnitude squared of \alpha).
* The probability of measuring the state as \mathbf{1} is |\beta|^2.
* Crucially, the probabilities must sum to 1: |\alpha|^2 + |\beta|^2 = 1.
3. The Collapse upon Measurement
The counter-intuitive part of quantum mechanics is what happens when you measure the qubit:
* The moment you perform a measurement, the superposition state collapses instantaneously.
* The qubit snaps out of its combined state and lands irreversibly on a single, definite classical value: either \mathbf{0} or \mathbf{1}, with the probability dictated by the amplitudes.
* After the measurement, the quantum information contained in the superposition is lost, and the qubit acts just like a classical bit.
Analogy: A Spinning Coin 🪙
A common way to visualize superposition is with a spinning coin:
* Classical Bit: A flipped coin must be either Heads (0) or Tails (1).
* Qubit in Superposition: A coin spinning in the air is like a qubit. Before it lands, it is not definitely Heads or Tails; it is a potential for both, or more precisely, it embodies the probability of either outcome.
* Measurement: When the spinning coin hits the table, it collapses from its combined "potential" state into a single, definite outcome (Heads or Tails).
This ability to embody multiple potential states simultaneously is what gives quantum computers their immense power, as they can process information on all those possibilities in parallel before the final measurement is made.
--Gemini
https://youtu.be/lypnkNm0B4A
https://fossbytes.com/microsoft-quantum-computer-topoligical-qubit/
https://www.forbes.com/sites/jasonbloomberg/2017/08/11/this-is-why-quantum-computing-is-more-dangerous-than-you-realize/amp/
Thai introduction to quantum computing's areas and use cases
