Using measurement to extract energy?

[nashed]

I've just read an article (link to the discussed paper at the end) talking about a newly proposed Maxwell's demon where they use a quantum measurement to extract energy from the system, which made me think, isn't energy conserved in QM as well (at least in the average sense)? and seeing that a measurement should fundamentally be a quantum interaction one would conclude that upon performing a measurement, the measuring system would either lose or gain energy such as the average energy of both systems is conserved.

Now back to the article at hand, from what I gather they begin with a qubit in some state, extract energy from it by coupling to an engine then perform a projective measurement that places it either back in the same state that they began the process with or in some other state which they then let evolve freely until it reaches the desired state so they'd be able to extract energy again.

So using the rationale of the first paragraph how is that engine/protocol possible? doesn't the fact that the measurement restores it to a higher energy state mean that they should be providing the energy from somewhere? or is my understanding of energy conservation incorrect?

link to the paper: https://arxiv.org/pdf/1702.01917.pdf

 

[Mentz114]

I've just read an article (link to the discussed paper at the end) talking about a newly proposed Maxwell's demon where they use a quantum measurement to extract energy from the system, which made me think, isn't energy conserved in QM as well (at least in the average sense)? and seeing that a measurement should fundamentally be a quantum interaction one would conclude that upon performing a measurement, the measuring system would either lose or gain energy such as the average energy of both systems is conserved.

Now back to the article at hand, from what I gather they begin with a qubit in some state, extract energy from it by coupling to an engine then perform a projective measurement that places it either back in the same state that they began the process with or in some other state which they then let evolve freely until it reaches the desired state so they'd be able to extract energy again.

So using the rationale of the first paragraph how is that engine/protocol possible? doesn't the fact that the measurement restores it to a higher energy state mean that they should be providing the energy from somewhere? or is my understanding of energy conservation incorrect?

link to the paper: https://arxiv.org/pdf/1702.01917.pdf

My understanding is their system as a whole is a heat engine and does not actually break any conservation laws. Only a subsystem looks as if it extracts heat where there is no 'temperature' difference.

I enjoyed the paper earlier (from VanHees link ) and I can recommend it for a bit of fun.