what is Measurement in quantum computing explain with examples

 Measurement is the specific action in quantum computing where you "look" at a qubit to check its value.¹

This sounds simple, but in the quantum world, the act of looking changes everything. Before you measure a qubit, it can exist in a "superposition" (a mix of 0 and 1).² The moment you measure it, it collapses into a single, definite state: either a 0 or a 1.³

Here is an explanation of how it works, using analogies and technical examples.

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1. The Core Concept: "Collapse"⁴

In classical computing (like your laptop), measuring a bit is like reading a page in a book. The words are already there; you just look at them. Reading doesn't change the words.

In quantum computing, measuring a qubit is like catching a spinning coin.

• Before Measurement: The coin is spinning. It is not Heads, and it is not Tails—it is a blur of both.

• The Measurement: You slap your hand down on the coin to stop it.

• After Measurement: The coin is forced to be flat. It is now definitely Heads OR definitely Tails. You cannot go back to the spinning state just by looking at it.⁵

2. Example 1: The Spinning Coin Analogy

Imagine you have a quantum coin.

• State: It is spinning on a table (Superposition).

• Action: You decide to measure it. You stop the coin.

• Result: It lands on Heads.

• Consequence: The coin is now "Heads." Even if you look away and look back, it remains "Heads." The "spinning" capability is gone for that moment. The quantum information (the spin) has collapsed into classical information (Heads).

3. Example 2: Schrödinger's Cat (The Famous Thought Experiment)

This is the most famous example used to explain measurement.

• Scenario: A cat is placed in a sealed box with a radioactive atom that has a 50% chance of decaying and releasing poison.

• Before Opening the Box (Before Measurement): According to quantum mechanics, the atom is both decayed and not decayed. Therefore, the cat is both Alive AND Dead at the same time.

• Opening the Box (The Measurement): You open the lid to look.

• The Result: Nature is forced to choose. You will see either a living cat OR a dead cat. You will never see a "half-dead/half-alive" cat. The act of looking forced the outcome.

4. Technical Example (The Math)

In technical terms, a qubit is often represented by a wave function equation.6

Let's say you have a qubit in this state:

$$|\psi\rangle = 0.6|0\rangle + 0.8|1\rangle$$

• The Numbers: The ⁷$0.6$ and ⁸$0.8$ represent the "probability amplitudes" (weights) of the qubit being 0 or 1.⁹

• The Probability: To find the chance of getting a specific result, you square these numbers.¹⁰

  • Chance of getting 0: $0.6^2 = 0.36$ (or 36%)

  • Chance of getting 1: $0.8^2 = 0.64$ (or 64%)

• The Measurement: You run the circuit. The computer rolls the dice based on these percentages.

• The Outcome: Let's say it lands on 1.

• The New State: The qubit is now $100\% |1\rangle$. The $0.6$ part (the connection to 0) has vanished.

Summary Table

Feature	Classical Measurement	Quantum Measurement
State Before	Definite (0 or 1)	Superposition (Mix of 0 and 1)
Action	Just reading	Forcing a collapse
Effect on State	No change	Irreversible change (State alters)
Outcome	Deterministic (Always same)	Probabilistic (Based on chance)

Why is this important?

Measurement is how we get answers from a quantum computer.

A quantum computer performs massive calculations while the qubits are in superposition (spinning).11 However, we humans cannot read "superposition." We can only read 0s and 1s.

Therefore, the Measurement is the final step where we ask the computer, "Okay, stop spinning and tell me—what is the answer?"¹²

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Would you like me to explain "Superposition" or "Entanglement" next to clarify how the qubit gets into that spinning state?