Schrödinger's Cat with 20 Qubits | Hacker News Reader

Schrödinger's Cat with 20 Qubits

fz-juelich.de

40 pointsrbanffy13 comments

> it is not clear whether the cat is dead or alive. It would be both at the same time until an experimenter takes a look. A single state would only be obtained starting from the time of this observation.

> Qubits, which have several states simultaneously due to the superposition principle, can store and process several values in parallel in one step.

This is a popular science and not very accurate version of how qubits work, no?

Can you give a bit more info on that? What is not accurate?

Let's take the first quote which is talking about superposition. From an information theoretic perspective, superposition is not different from a classical statistical model. A weather model can be in the state (20% chance of rain tomorrow, 80% dry). Its when you measure or observe the weather tomorrow you will see either rain or dryness. In other words, while it is technically correct to say "a qubit is in two states at the same time", it's a very boring and non-deep statement.

Now, the second quote. This one is heavily misleading. I don't even know where to begin. I will just leave this here https://www.scottaaronson.com/blog/?p=198

I always liked this comic as an explanation https://www.smbc-comics.com/comic/the-talk-3

There must be a Reporters' Union regulation against getting it right. But the cartoonists are not bound by it.

I don't know if I agree with your first paragraph. The weather model analogy is at least misleading since the corresponding thing in QM would be a mixture of states, not a superposition.

Not really. For the weather model, instead of recording my state as (20,80), I could just store it in memory as (-sqrt(20),sqrt(80))/normalization, with the prescription that to regain probabilities, I must square each entry. The fact that there are negative or complex numbers in the state of a single particle is not concerning. Only when you have entanglement for two or more particles is when something non-classical happens. Please see my other comments under my parents comment.

It is very different than the classical statistical model. I can create quantum gates that manipulate all the superimposed qbits; There is nothing like this in the classical model where you can only manipulate a single probability.

What you might be referring to is entanglement, which is different from mere superposition. Classical models can't easily simulate entangled quantum sytems, but can very easily simulate unentangled superimposed states.

No, I'm talking about superposition. Classical models can only simulate superposition with an exponential amount of information to model each possibility of a sequence of qbits. Shor's algorithms can only factor large integers efficiently because it uses a quantum fourier transform that manipulates the superimposed states. If you could do that with a classical model, then you would have a superpower that no one else has without a quantum computer.

> Classical models can only simulate superposition with an exponential amount of information to model each possibility of a sequence of qbits.

This is not true at all when the qubits are only in superposition - not in entanglement. The state of n unentangled qubits is just 2n numbers, compared to the 2^n numbers required to describe an arbitrary entangled state of n qubits. To simulate 2n real numbers, you can encode them in the (amplitude,phase) of n electric fields etc.

Shor's algorithm works because it uses entanglement - as does every quantum algorithm that shows a speedup.

Let me try in a different way. If you don't know whether you are living in a universe whose laws are classical-non-deterministic or quantum, you do a Bell experiment. To to do a Bell experiment in the quantum world you need entangled states - mere superposed states won't do. If you have superposed states, you won't be able to tell the difference between classical and quantum.

To put it yet another way. You design any quantum experiment which may use superposed states, but not entangled states. I will look at your design and design a corresponding classical non-deterministic experiment. Then we will go to our trusted friend Charlie and ask him to run the two experiments for us, and then return the datasets for the two experiments, without telling us which is which. I assure you, there is no algorithm that will allow you to look at the experimental datasets and tell which was from the classical experiment and which from the quantum one.

So that blog post quotes a demo of passing 1024 qubits by 2008. Anyone know where we are at on reality now?

Or is that d wave claim material for/r/agedlikemilk?

Reply on news.ycombinator.com