Imagine you’re about to toss a coin. In our everyday world, this coin represents uncertainty—a fair 50:50 chance of landing heads or tails. However, even though you’re uncertain about the outcome before the coin lands, in classical terms the coin was always in one definite state (either heads or tails); you simply didn’t know which one it would be until you looked. This is similar to how classical bits work.

Classical Bits: Independent, Definite Switches

In classical computing, we picture bits as tiny switches that can be either ON (1) or OFF (0). Think of these switches as independent coins that, once flipped, have a definite outcome—even if we sometimes deal with randomness or error correction later. For example:

• When representing a letter like “S,” a specific sequence of bits (or coin outcomes) is required.
• If one bit flips by accident, error-correction mechanisms are there to flag and fix it, yet each coin’s (or bit’s) result is independent of its neighbor.

Even if we perform millions of coin tosses in parallel (or billions of bit operations per second), each coin is individually defined. There’s no inherent “conversation” or physical connection between them—they just provide a series of 0s and 1s that together encode information.

A Probabilistic Switch: The Classical Coin Toss

Now, let’s reframe our perspective. Consider a coin that we toss, but instead of saying “the coin is definitely heads or tails,” we describe our lack of knowledge by saying there’s a 50:50 chance for each outcome. We might represent our uncertainty mathematically by saying:

$$ \text{Probability(Heads)} = \frac{1}{2} \quad \text{and} \quad \text{Probability(Tails)} = \frac{1}{2}. $$

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Here, the randomness arises from our ignorance of the outcome until the coin lands. Importantly, the coin itself isn’t in a state of “heads and tails at once”—it just appears that way because we don’t know the result yet. If you were to use a high speed camera, you could actually reliably know which side is facing upwards at each point in time, determining the outcome when the coin lands.

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The Quantum Leap: What if the Coin Really Was in Both States?

In the quantum world, however, things are far more intriguing. A qubit—the quantum counterpart of a classical bit—is not just a coin with an unknown outcome; it’s as if the coin actually exists in a blend of both heads and tails simultaneously, until we take a look. So, no peeking with a high speed camera here.

At this point think of the quantum coin as being a sci-fi black box spinning, not revealing its nature until it lands (until you “measure” the “quantum system”)

To capture this, we say that a qubit is in a superposition of states. For a perfectly balanced quantum coin (or qubit), you might think of it as having “amplitudes” for being heads (0) and tails (1) that, when squared, yield a 50:50 chance upon measurement.

The key point is:

• Superposition (before we measure): A qubit can be in a state that represents both possibilities simultaneously. If we imagine a spinning coin in mid-air, its state isn’t just unknown—it’s genuinely a combination of heads and tails.
• Upon measurement: When you finally catch the coin (or measure the qubit), the superposition “collapses” into one of the definite outcomes.