A Thermometer for Measuring Quantumness | Hacker News Reader

A Thermometer for Measuring Quantumness

quantamagazine.org

48 pointsrbanffy11 comments

Intro bugs me. Second law is "violated" all the time at small enough scales. Aka, fluctuation theorems. Of course the violation just means you're not really in the thermodynamic regime. Thermodynamics is kind of circularly defined, and that's fine. Thermodynamics is valid when it works. Temperature is emergent, it's only well defined, quantum or classically, for sufficient large system or sufficient strong coupling to bath. Rest of the articles seems fine, but I just hate these kind of set ups trying to make things more mysterious than they are.

I'd be curious if someone who understands exactly what and how they are proposing this measurement works could speak on whether it would contradict the no-signalling theorem (and so, the effect can't be real).

That is, given that destroying the correlation between two entangled particles can't be used to send information between the particles' locations, how could a measurement of whether a system contains entangled particles work? Does it just require all entangled particles to be present for the extra heat transfer to work?

"given that destroying the correlation between two entangled particles" i think this is the assumption that is easy to make without digging deeper into entanglement.

i am still in the process of reading this article (https://arxiv.org/pdf/2408.06418) however entanglement witnesses can be realized in several ways and are one of the underlying aspects of how quantum networking can be made reliable.

under the category of heralded entanglement, one realization uses photons striking photo detectors after they meet in a beamsplitter under the hong-ou-mandel effect scenario. for type 1 entanglement with HOM: if the photons at the two input modes are identical, they always bunch due to quantum interference, and if the photons resulted from emissions in the respective quantum nodes those nodes are now entangled, and the detection is the classical signal that the entangled link was created. the nodes can now transmit information unidirectionally into the entangled qubits. for type 2 entanglement with HOM it's a little bit more complicated although the underlying concept of indistinguishability is what results in the entanglement just the same.

heres one experiment from oxford, https://www.nature.com/articles/s41586-024-08404-x, where they achieved this with high fidelity although the particular details of the beam splitter experiment are not as well detailed.

Although I am not an expert in quantum information, I think the problem you pose is resolved by the fact that the no-signalling theorem is about measurements of a quantum state, which is a microscopic state, and heat transfer is a measurement of a thermodynamic quantity, which is macroscopic. In much the same way that measuring the temperature of a classical gas doesn't give information on the location or momenta of the constituent particles, a thermodynamic probe of entanglement doesn't necessarily furnish precise information on how a state is entangled (e.g., Eq. 2 in https://arxiv.org/pdf/quant-ph/0406040).

Yeah the whole entangled system is present and interacting with the "thermal ancilla" in their setup.

Well, I already knew that I didn't understand quantum mechanics, but I didn't expect entanglement to violate transitivity !

Needs to be called a Steins Gate

I have Absolute Zero interest in this.

Are you sure?

They're referencing the temperature Absolute Zero as a ... pun

I was referencing the Quantumness as a ... pun

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