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Comment by daveguy | Hacker News Reader

Thank you. It seemed off to me too. Even the first image caption:

> In the classical world, if you tried to charge a battery using two chargers, you would have to do so in sequence, limiting the available options to just two possible orders.

Uhhhh. Of course you can charge in parallel, just like you can power something from batteries in parallel. How do they think a solar panel array charges a battery at a specific voltage while increasing wattage of the array? -- parallel solar panels => parallel charging.

I don't know if it's a translation peculiarity or what, but the whole thing seems off.

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Quantum mechanics are a little dense for me, but assuming the laws of entropy have not been broken, I think this means a more efficient "concurrent" charging process, as opposed to sequentially charging. Perhaps current "parallel" charging differs in approach to this new process.

> Earlier methods to charge a quantum battery involved a series of charging stages one after the other. However, here, the team instead used a novel quantum effect they call indefinite causal order, or ICO. In the classical realm, causality follows a clear path, meaning that if event A leads to event B, then the possibility of B causing A is excluded.

I don't blame the authors for interpreting this as causality being broken, as most programmers know how difficult causality is to think about once you have concurrent processes in play.

I read the press release a few times and it's very confusing. The research paper is paywalled.

My guess is:

The "battery" is an atom that can be in the ground state or exited.

The two chargers are laser, let's call them A and B

A classic "battery" can go:

* Empty (total = 0) -> absorb a photon from A (total = A) -> absorb a photon from B (total = A+B)

* Empty (total = 0) -> absorb a photon from B (total = B) -> absorb a photon from A (total = A+B

But a quantum "battery" can go:

* Empty (total = 0) -> 50% absorb a photon from A and 50% absorb a photon from B and (total = (A+B)/2) -> absorb the other photon (total = A+B)

where 50%+50% is actually 1/sqrt(2)±1/sqrt(2) for technical reasons. (They are not absorbing the 50% of the photon.)

I don't understand the part of the weaker charger. If the laser A and B have the same frequency/color, then it does not make sense. If they are different, it is almost obvious because fine tuning the energy of the photons in the laser to the energy of the transitions of the levels of an atom is a good idea to promote absorption.

As I said, I'm guessing too much, so I'd be happy if anyone can post a better explanation.

Empty (total = 0) -> photon A -> charge battery element 1 & entangled battery element 2 where battery element is/are the atoms storing the charge.

One cannot assume the charge will be split 50/50 because its probabilistic and how the atoms are impacted by the photos may impact charge. Also, it may be the case that - probabilistically - the charge is no better or more efficient than a classic battery. Anyway, I think this explains how a lesser charge may result in a higher stored charge.

It looks like they use an initial auxiliary entangled system with a state like 1/sqrt(2)A+1/sqrt(2)B and then A and B decide how the "battery" charges. So the final state of the "battery" is 1/sqrt(2)C1C2+1/sqrt(2)C2C1, that is a superposition of the two ways to charge the "battery", that is a 50% for each one.

there is a paper on arxiv.org which may shed some light: https://arxiv.org/pdf/2105.12466.pdf

Not too much light :( , but let's try again:

It looks like a previous work. It looks like it's a single qubit going from 0 to 1.

Charger 1 is connected using the Pauli matrix σx and Charger 2 is connected using the Pauli matrix σy. σx and σy does not conmute, so using the chargers in different order get a different result.

From the preprint:

> A quantum switch that realizes indefinite causal order admits higher-order processes where a control qubit plays the role of determining the order of occurrence of C1 and C2. Depending on the control qubit’s initial state, infinitely many configurations become possible, e.g., when the control qubit is prepared in a coherently balanced state, the battery undergoes the charging dynamics determined by C1C2 and C2C1 in a balanced superposition. A measurement will be performed on the control qubit following the dynamical evolutions of the target system.

It looks like standard superposition with strange names.

IIUC they make measurements and repeat the operation, and that increase the probability that the final state of the qubit is 1. This second part may be interesting, but the unusual names make me decide to forget it.

it's more likely a failure of translation from scientist to journalist than from Japanese to English.

Ah, Scientist to Journalist, the classical source for the term "Hallucinations".

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