Explainer series 2 of 5: What is quantum superposition and why does it make quantum computers different

[VidiTechnica]

This is the second in our five part beginner series on quantum concepts. Superposition is the idea that trips people up first when they encounter quantum computing. It sounds like magic and is often described badly. This thread will try to explain it accurately without pretending you need a physics degree.

A normal computer bit is like a light switch. It is either off, which means zero, or on, which means one. That is the only two options it has, ever. A quantum bit, called a qubit, is different. Before you measure it, it is in a state called superposition, which means it is described by a combination of both zero and one at the same time. Not flickering between them quickly. Not randomly one or the other. Actually both, held together in a mathematical description until the moment something interacts with it and forces it to resolve into a definite answer. The famous and slightly misleading analogy is Schrodinger's cat, a cat in a box that is both alive and dead until you open the box. The analogy is imperfect but the weirdness it points at is real.

Here is why this matters for computing. If you have three normal bits you can represent one of eight possible combinations at a time: 000, 001, 010, and so on up to 111. With three qubits in superposition you can represent all eight combinations simultaneously within the quantum state. Add a fourth qubit and you represent sixteen combinations. The number doubles with every qubit you add. With 300 qubits in superposition you are representing more states than there are atoms in the known universe. This exponential scaling is what gives quantum computers their theoretical power for specific types of problems.

The crucial word is specific. Superposition alone does not make a quantum computer faster at everything. A quantum computer running a word processor would be no better than a laptop. The power only appears for problems where you can use the mathematics of superposition, combined with other quantum properties like interference and entanglement, to steer toward correct answers and away from wrong ones. Shor's algorithm is one such problem. Finding the structure of complex molecules for drug discovery is another. Most things you do on a computer every day are not on that list, which is why quantum computers will complement classical computers rather than replace them

[Holly]

The light switch analogy followed by the actual correction is the clearest version of this explanation I have read. Most explanations stop at the analogy

[SpinState]

The part about it not being faster at everything is the thing that gets lost in every news article about quantum computing. Every headline implies it will replace normal computers and that is just wrong

[Courier53]

I always assumed quantum computers were just faster computers. Finding out they are a different category of tool for specific problem types is more interesting actually

[BigDog92]

Same. It reframes everything. The question is not when will quantum computers be faster than my laptop. The question is which specific problems are in the class that quantum helps with

[One-One-Five]

Can you say more about interference because you mentioned it but did not explain it. How does that work with superposition

[Ria99]

Good question. Interference is when you use the wave-like mathematics of superposition to make wrong answers cancel out and right answers reinforce each other. It is similar to how noise cancelling headphones work, same principle at the quantum level

[ReacherBadger]

That is a helpful way to put it. The headphones analogy makes interference feel much more concrete

[IronQuarry48]

The 300 qubits representing more states than atoms in the universe statistic is the one I always use when trying to explain this to people. It lands every time

[Candle]

It lands but it can also mislead. Having those states represented is not the same as being able to access them all usefully. The challenge is steering the computation toward the right answer among all those possibilities

[TeaAndCode72]

Fair point. The power is in the steering not just the representation. The representation is a necessary condition not a sufficient one

[CMPunk_Mike]

What physically is a qubit. Like what object in the real world is actually being used

[Reacher Quarry]

Several different physical things depending on the company. IBM and Google use superconducting circuits cooled to near absolute zero. IonQ uses individual trapped ions. Others use photons. Each has different tradeoffs on stability, speed, and error rate

[RayOfLight32]

Near absolute zero means colder than outer space for context. The engineering required to maintain qubit superposition is extraordinary