(This is point number 3) I'm not sure about what you mean with "source of photons from nothing": you can't create energy, so for example you need to provide the energy needed to your quantum system for going to $E_2$ first. Said that, the photon wasn't "inside" the system, it was created, 'from nothing' if you want.

1. See the comment for lower energy limit. For an upper limit, if you give too much energy to your oscillator (how much depends on the system) you'll end up destroying it (tearing apart its costituents), so any theoretical upper limit (see the question you linked) is not very relevant.

2. Yes, for example using Compton backscattering en.wikipedia.org/wiki/Compton_scattering. Notice that there you don't create photons directly, but instead transform already existent photons.

(This is point number 3) I'm not sure about what you mean with "source of photons from nothing": you can't create energy, so for example you need to provide the energy needed to your quantum system for going to $E_2$ first. Said that, the photon wasn't "inside" the system, it was created, 'from nothing' if you want.

1. See the comment for lower energy limit. For an upper limit, if you give too much energy to your oscillator (how much depends on the system) you'll end up destroying it (tearing apart its costituents), so any theoretical upper limit (see the question you linked) is not very relevant.

2. Yes, for example using Compton backscattering en.wikipedia.org/wiki/Compton_scattering. In this process, low energy photons scatter on high energy charged particles (electrons are best suited) and some of them gain a lot of energy. Notice that there you don't create photons 'from nothing', but instead transform already existent photons (altough is most correct to state that you are destroying the old photon and creating a new one).

The limit here is basically the speed your electrons can reach (so it's about particle accelerators max energy, which grows as technology improves): for backscattering (the new photon goes in opposite direction than the old one, with the biggest possible energy), given the electron $\beta = v/c$, the new photon energy $E'_\gamma$ relates to the old $E_\gamma$ as $E'_\gamma = \frac{(1 + \beta)^2}{1 - \beta^2} E_\gamma$ (the formula is approximated but very good in this setting). E.g., 1.5 GeV electrons scattering on 2.5 eV photons yelds photons up to 100 MeV, so GeV photons and beyond should be achievable today.

(This is point number 3) I'm not sure about what you mean with "source of photons from nothing": you can't create energy, so for example you need to provide the energy needed to your quantum system for going to $E_2$ first. Said that, the photon wasn't "inside" the system, it was created, 'from nothing' if you want.

1. See the comment for lower energy limit. For an upper limit, if you give too much energy to your oscillator (how much depends on the system) you'll end up destroying it (tearing apart its costituents), so any theoretical upper limit (see the question you linked) is not very relevant.

2. Yes, for example using Compton backscattering en.wikipedia.org/wiki/Compton_scattering. Notice that there you don't create photons directly, but instead accelerate already existent photons.

(This is point number 3) I'm not sure about what you mean with "source of photons from nothing": you can't create energy, so for example you need to provide the energy needed to your quantum system for going to $E_2$ first. Said that, the photon wasn't "inside" the system, it was created, 'from nothing' if you want.

1. See the comment for lower energy limit. For an upper limit, if you give too much energy to your oscillator (how much depends on the system) you'll end up destroying it (tearing apart its costituents), so any theoretical upper limit (see the question you linked) is not very relevant.

2. Yes, for example using Compton backscattering en.wikipedia.org/wiki/Compton_scattering. Notice that there you don't create photons directly, but instead transform already existent photons.