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Particle in superposition of energy eigenstates and conservation of energy. 
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#1
Dec1012, 05:23 PM

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When a particle is in superposition of energy eigenstates and has a probability of being found in either state, what does that say about the energy of the particle and conservation of energy.
What I mean is, since the energy eigenstates have different energy values, where's the rest of the energy of the particle, if it is found to be in the lower energy eigenstate? Let's say [itex]\psi> = c_1 E_1> + c_2 E_2>[/itex] Take the system to be a free particle in a box where [itex]\hat{H} = \frac{\hat{p}^2}{2m} + V(\hat{x})[/itex] and [itex]V(x)=\{ \begin{array}{1 1} 0 \quad x<a/2\\ V_0 \quad x>a/2 \end{array}[/itex] I guess my questions could be rephrased as, how do you calculate the total energy of a quantum particle? 


#2
Dec1012, 06:38 PM

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The energy is uncertain. If you measure it, you have probability c1^2 to get E1, and probability c2^2 to get E2. These probabilities do not change with time. This is what it means for energy to be conserved in QM.



#3
Dec1012, 06:53 PM

P: 2

Thanks for your reply.
I was just expecting quantum mechanics to always give me definite answer for the total energy of the system. 


#4
Dec1112, 12:52 AM

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P: 5,464

Particle in superposition of energy eigenstates and conservation of energy.
A conservation law in quantum mechanics is usually formulated in terms of an operator algebra. In the Heisenberg picture which corresponds to the Hamiltonian framework in classical mechanics the timedependence is shifted from the states to the operators; the state vectors are timeindependent, therefore there is no conservation law on the level of the states (trivial). The conservation law (timeindependence) of a certain observable Q is formulated in terms of its commutator with the Hamiltonian H, i.e. [H,Q] = 0 which corresponds to the Heisenberg equations of motion for a conserved quantity Q with dQ/dt = 0. Here I always assume that we start with timeindep. Hamiltonian operator in the Schrödinger picture. In that case the timeindependence of the Hamiltonian in the Heisenberg piucture is trivial: [H,H] = 0



#5
Dec1112, 04:32 AM

P: 133

I think you should also take into account that your particle has to enter the superposition state in some way. If you start with an excited atom, for example, after a while the atom will be in a superposition of being excited or not. But the full superpositon state has a photon taking up the energy in one case and not in the other, so in any case the total energy is conserved.
Thsi leads to a followup question which I cannot answer: Is there any way to prepare a particle in an energy superposition state that does not involve entanglement with another system that is in a complementary superposition? 


#6
Dec1112, 04:53 AM

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#7
Dec1112, 09:12 AM

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L. E. Ballentine, Quantum Mechanics: A Modern Development Sec. 12.4 Quantum Beats 


#8
Dec1112, 09:27 AM

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#9
Dec1112, 09:50 AM

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Exciting an atom with these states leaves it in a superposition of energy eigenstates and does not change the coherent state, so there is also no entanglement created. 


#10
Dec1112, 10:09 AM

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#11
Dec1112, 10:24 AM

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Yes, but the coherent states are eigenstates of the photon anihilation operator. So the removal of a photon does not change the coherent state.



#12
Dec1112, 10:40 AM

P: 381

Correct, and that's because a coherent state involves an infinite number of photons in superposition. However such a coherent state is unnatural. In reality you can find approximate coherent states that involve a very large number of photons in superposition, but still it's a finite number and a subtraction of a photon makes a (small to negligible) difference.
Coherent states are created by lasers, and the process of their creation exactly preserves the total energy. So we are always 'tracing back' to find the closed system that includes all the subsystem involved. I think that the correct assertion would be: In a closed system with well defined energy, all its subsystems that interact with each other unitarily are entangled. Each subsystem is in a superposition of eigenstates, but the overall energy is conserved. If you assume the universe to be closed and with well defined total energy (that's a big assertion to make!:P ) and described by QM then energy is always exactly conserved. 


#13
Dec1112, 11:14 AM

P: 133

@JK423 &others
Thanks  that's what I always assumed, but I've never found it discussed anywhere. 


#14
Dec1112, 11:53 AM

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I always find the assumption that the whole universe is a closed quantum system quite dubious. The  even approximate  construction of closed systems is de facto impossible and they are much more unnatural than coherent states. All measurements are ultimately performed with instruments that are classically or arbitrarily closely so. These systems can swallow and delete any kind of correlation. The coherent states show how this becomes possible in the limit of large system size. 


#15
Dec1212, 02:49 AM

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As long as a closed system (e.g., the whole Universe) evolves unitarily, an energy eigenstate must necessarily evolve into an energyeigenstate. This seems to imply that creation of coherent superpositions of different energies is impossible. But for such a system the state does not change with time, so where does the dependence on time come from?
The catch is that in all interpretations of QM, the system as a whole does NOT evolve unitarily. (For example, the wave function collapse is not unitary.) The only exception is manyworld interpretation, but even there the physical time needs to be identified with some clockconfiguration variable, which again leads to an effective nonunitary "evolution" with respect to the clock time. As a consequence, creation of coherent energy superpositions is possible, in all interpretations. For more details see also Appendix A of http://arxiv.org/abs/1209.5196 


#16
Dec1212, 02:52 AM

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#18
Dec1212, 03:02 AM

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P: 2,470

The only way you can have dynamics in the universe is if universe is not in an eigen state of the Hamiltonian. And why should it be in an eigen state? There are infinitely more states than there are eigen states. So we don't need to discuss collapse here. The universe is in some superposition of eigen states, and that's enough. Note that regardless of what kind of superposition you have, [H, H] = 0. So expectation value of energy does not change in time. Yes, energy of the universe is not an eigen energy. But it's always the same, so there are no issues with conservation laws. 


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