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The book An Introduction to Quantum Computing (section 10.4.1 Error Models for Quantum Computing) by Phillip Kaye, Raymond Laflamme and Michele Mosca states the following:

Errors occur on a qubit when its evolution differs from the desired one. This difference can occur due to imprecise control over the qubits or by interaction of the qubits with an environment. By ‘environment’, we mean everything external to the qubit under consideration. A ‘quantum channel’ is a formal description of how qubits in a given setting are affected by their environment.

I would like to know what is the environment and what are the physical characteristics that describe it.

Furthermore, does the qubit alone generate noise within itself (assuming that no noise is generated from its environment)?

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The whole point of the system/environment description is that the "system" is the bit that you can characterise and control. The environment is the bit that you cannot characterise or control. It could be anything. Ultimately, it's the whole of the rest of the Universe. For your particular physical realisation, you might have some approximation of what the environment might be doing, but it is very much case dependent.

Furthermore, does the qubit alone generate noise within itself (assuming that no noise is generated from its environment)?

That might come down to a matter of definition. All non-unitary evolution can be described by reference to a larger system, i.e. an environment, on which the evolution is unitary. If no noise is generated from the environment, i.e. if the qubit and environment are decoupled, then the evolution of the qubit should be unitary, and there is no noise.

On the other hand, what about things like the spontaneous emission of an atom? That sounds like a qubit alone generating its own noise. I'm not an expert on this (I'm relying on Wikipedia!) but, basically, if you want to describe this using regular quantum, you have to introduce an environment (e.g. Jaynes-Cummings model). A proper description uses quantum electrodynamics, at which point you possibly could describe it as a qubit generating its own noise.

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