Vacuum in string theory How is vacuum described in string theory? The same as in the QFT? Does string theory have quantum fluctuations of the vacuum, are the processes of creation and destruction of virtual strings considered, or is there nothing in the vacuum state?
 A: String fields exist and they have a vacuum as any other quantum field theory. There is an entire subject known as string field theory that studies perturbative string theory via string fields. In string field theory you can define off-shell string states, study the creation of a string, or compute amplitudes for a finite time string scattering process.
An interesting application of the second quantized perspective concerns the perturbative string finiteness is the UV/IR connection. I strongly recommend Ultraviolet and Infrared Divergences in Superstring Theory to gain an intuition of this connection. After the identification of UV divergences as IR effects, soft theorems are needed to demonstrate that the IR divergences can be cured Of course the latter is subtle in perturbative string theory (where adjectives like "soft" and "off-shell" are a little bit mysterious). It is convenient to highlight the outstanding String Field Theory as World-sheet UV Regulator. I am not aware of any other beautiful application of string field theory to ordinary perturbative string vacua of this type. A truly lovely paper that rigorously exhibit the perturbative healthy of string theory.
References:
If you are interested in string field theory, then you will love the truly wonderful paper Four Lectures on Closed String Field Theory
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For an spectacularly marvelous introduction to closed string field theory see  String Field Theory – A Modern Introduction.
For an overview see developments in perturbative string theory.
What does second quantization mean in the context of string theory?
A: The vacuum described by string theory is significantly different to that in QFT. In fact, there is no one “vacuum” in string theory similar to what we have in QFT.
There is actually a very large number of vacuum states according to string theory. This is described by the string theory landscape or landscape of vacua refers to the collection of possible false vacua in string theory, together comprising a collective "landscape" of choices of parameters governing compactifications.
See this article for more.
