Why does quantum computing need so many qubits if one qubit can be in multiple states?

[KnotKnull]

I thought quantum qubits can be 0 and 1 simultaneously through superposition. So why do we need thousands of qubits to do useful computation? Shouldn't a few qubits be enough?

[TheGame]

Superposition is useful but alone it's not enough. A qubit in superposition can represent 0 and 1 but when you measure it you get one answer. You lose the superposition. That's the measurement problem

[TealBear]

Quantum advantage comes from entanglement not just superposition. Entangled qubits are correlated in ways that have no classical equivalent. To explore large problem spaces you need many entangled qubits working together

[Phil]

Technically N qubits in superposition can represent 2^N states simultaneously. But error correction requires massive overhead. To get one reliable logical qubit you need 100-1000 physical qubits. Then multiply by the qubits you actually need for the algorithm

[Router53]

Different algorithms need different qubit counts. Factoring a 2000-bit number with Shor's algorithm requires millions of qubits after error correction. Simulating molecular behavior needs hundreds to thousands depending on accuracy required

[Slay40]

Superposition lets you explore many paths. But you can't keep all paths. When you measure the quantum state collapses to one answer. To solve hard problems you need interference patterns that amplify correct answers and cancel wrong ones. That requires entanglement

[Matticus]

It's like lottery tickets. One ticket is you guessing one number. Superposition is holding all tickets simultaneously. But you only collect prize on one winner. Quantum algorithms design interference so the right answer shows up with high probability

[Sam92]

Error rates matter enormously. Even small error rates compound across computations. You need extra qubits for error correction which overshadows the qubits needed for the actual algorithm. That's the overhead crisis

[KnotKnull]

Current quantum computers have 100-1000 qubits but mostly unstable. Practical useful quantum computers probably need millions of stable qubits. We're nowhere near that

[Candle]

The surprising part is error correction overhead scales with qubit count. More qubits means more places for errors so you need more correction overhead. It's not linear it's something like cubic scaling in some approaches

[AEWNoah32]

The beginner mistake is thinking superposition solves the scaling problem. It doesn't. Quantum advantage comes from interference and entanglement which require specific problem structure and many qubits