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Frequently asked questions
Basics
What is superposition, in one sentence?
Before a measurement, a quantum system doesn't have to decide on a single result, but can carry several possibilities within it at the same time. This does not mean that we simply don't know the answer. It really isn't fixed yet and is only determined by the measurement.
Is Schrödinger's cat really dead and alive?
No. Schrödinger came up with the example in 1935 as a criticism. He wanted to show how absurd it gets if you apply the rules of tiny particles to big things. For single atoms, superposition can be measured precisely. A cat doesn't have it, because it consists of unimaginably many particles and is always clearly alive or clearly dead.
What is a qubit?
A qubit is the smallest quantum system with exactly two possible measurement results, usually 0 and 1. Unlike an ordinary bit, it can be in a superposition of both values before the measurement. To describe its state completely, you need two angles. That's why it can be shown as a point on a sphere, the Bloch sphere.
What is interference?
Waves overlap and add up their amplitudes. Where crest meets crest, the wave gets stronger. Where crest meets trough, they cancel out. In quantum mechanics, the possibilities themselves overlap. That's why a result can vanish even though both paths to it are open, while another becomes all the more likely.
What does “measurement destroys the state” mean?
Measuring is not passive looking, but an interaction with the system. Afterwards it is in exactly one of the possible results and the original superposition is irretrievably lost. You can try this yourself on the Bloch sphere. After every measurement the arrow jumps to one of the two poles.
What is entanglement?
Two particles can be so closely connected that neither of them has a state of its own. Only the pair can be described together. If you measure both, the results always match, even though each one on its own comes out completely at random. You can make such a pair yourself with the Bell state at the QuantumTable.
What is the uncertainty principle?
Certain pairs of quantities, such as the position and momentum of a particle, cannot both be fixed with arbitrary precision at the same time. The more precisely the position is known, the less determined the momentum becomes, and vice versa. This is not down to imprecise measuring devices. It is a property of nature itself and would remain even with perfect technology.
Quantum computers
Are quantum computers simply faster computers?
No. For certain tasks they really are much faster, but for most others they are slower than your laptop. For videos, chats or word processing they are no use. A quantum computer is a specialist tool for selected problems, not a better all-rounder.
Does a quantum computer try all possibilities at once?
That's the most common misconception. During the calculation it does have many possibilities in play at once, but in the end it may only measure once and gets a single result. If you just measured away, you'd only get chance. The real art is to design the calculation so that the wrong possibilities cancel out through interference and the right one remains.
What are they actually good for?
The most promising use is simulating molecules and materials, for example for new medicines or better batteries. Then there are certain optimisation problems. Well known, but still far away, is breaking today's encryption. Much of this is still at the research stage and not a finished product.
When can I buy one?
For your home, probably never, because the devices need extreme cold and elaborate shielding. Access via the cloud is more realistic. It already exists today, partly even free of charge for small experiments. Your program runs on a device in a lab and you just get the result back.
Is my encryption going to be unsafe soon?
Not yet today. That would require much larger and more stable quantum computers than exist at present. The risk is rather that someone stores encrypted data now and decrypts it later. That's why new, quantum-safe methods are already being developed and introduced today.
Why are quantum computers so sensitive?
Every disturbance from outside destroys the superposition of the qubits. Experts call this decoherence. Heat, vibrations or stray light are already enough. That's why many quantum computers are cooled almost to absolute zero and elaborately shielded, and why error correction is such a big research topic.
Big questions
Is the randomness real, or do we just not know the details?
As far as we know today, it is real. Experiments on Bell's inequalities, which were awarded the Nobel Prize in Physics in 2022, rule out obvious explanations with “hidden variables”. That doesn't fully settle the interpretation of quantum mechanics, though, and research on it continues.
Are there parallel universes?
That is an interpretation of quantum mechanics, the many-worlds interpretation, not a measurement. It makes the same predictions as other interpretations. Physics decides between theories through experiments, and so far all interpretations give the same results here. So at present nobody can prove whether parallel universes exist.
Does consciousness have anything to do with measurement?
No. A measurement doesn't need consciousness. A detector, a photographic plate or even the air in the room are completely enough, because they interact with the system. The idea goes back to speculation by individual physicists, such as Eugene Wigner in the 1960s, and was spread mainly by popular books. It plays no role in physics today.
Can you communicate faster than light with entanglement?
No, and this can even be proven. Your measurement result is pure chance that you can't control. That both sides match only becomes apparent when they compare their results. For that they have to exchange information over a perfectly ordinary channel, which is never faster than light.
Everyday life
Do I come across quantum physics in everyday life?
All the time. LEDs, lasers, solar cells, the chips in your phone, MRI in hospitals and the atomic clocks in GPS all rely on quantum mechanics. So it isn't exotic, but the best-confirmed theory in all of physics.
Why does everyday life never seem “quantum”?
The reason is decoherence. The more particles an object has, the faster its surroundings destroy its superposition. For everyday objects this happens so unimaginably fast that we never notice any of it.
What about “quantum healing” or “quantum energy”?
That has nothing to do with physics. A good test is to ask whether precise numbers are predicted that could be proven wrong. Real quantum physics makes numerical predictions. When “quantum” only appears as a mood word, it's marketing.
I just don't get it. Am I too stupid?
No. Quantum mechanics contradicts our everyday experience and your brain wasn't built for it. Even Richard Feynman, one of the best-known physicists of the 20th century, is said to have remarked that nobody really understands it. We can still calculate and predict with it extremely well, and understanding grows as you try things out.
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Questions we like to hear
“Can I ever understand this?”
Yes. Not completely today, but the core is more accessible than all the talk about it suggests.
“What is it good for?”
A completely fair question. Honest answer: for some things we know, for some we hope, for some we don't know yet.
“Do you really believe this?”
We don't believe — we measure. And the measurements agree with the theory to many decimal places.
And if we don't know?
Then we say so. In quantum physics there are questions experts have been arguing about for a hundred years — “that's open” is a legitimate and pretty exciting answer.