Well, quantum mechanics describes the universe as a wave function which describes a probability distribution. Most physicists who work on quantum mechanics seem to believe that wave function collapse is a truly nondeterministic process; the only real alternative, from what I understand, is hidden variable theories, which we know imply nonlocality thanks to Bell's theorem.
There's the caveat of Everettian quantum mechanics, where the universe is described exclusively through the wave function which evolves completely deterministically. However, this still causes the observer to experience a measurement outcome as truly nondeterministic; there's just one version of the observer for each measurement outcome.
Here's the Wikipedia article on quantum indeterminacy, which I'm sure you can use as a jumping-off point for further reading: https://en.m.wikipedia.org/wiki/Quantum_indeterminacy
EDIT: Though while this topic is fascinating, I don't think it's extremely relevant to my own thoughts on free will or whatever. I'm perfectly happy to discuss a hypothetical brain which is truly deterministic, and I don't think I would consider a consciousness simulated on a deterministic computer to have any less "free will". The consciousness in the computer would _be_ the circuitry and program. Of course it couldn't make different choices twice given the exact same state and input; it wanted to make the choice it did because of its state and inputs. I don't understand how the choice is any less free just because it's deterministic.
There's the caveat of Everettian quantum mechanics, where the universe is described exclusively through the wave function which evolves completely deterministically. However, this still causes the observer to experience a measurement outcome as truly nondeterministic; there's just one version of the observer for each measurement outcome.
Here's the Wikipedia article on quantum indeterminacy, which I'm sure you can use as a jumping-off point for further reading: https://en.m.wikipedia.org/wiki/Quantum_indeterminacy
EDIT: Though while this topic is fascinating, I don't think it's extremely relevant to my own thoughts on free will or whatever. I'm perfectly happy to discuss a hypothetical brain which is truly deterministic, and I don't think I would consider a consciousness simulated on a deterministic computer to have any less "free will". The consciousness in the computer would _be_ the circuitry and program. Of course it couldn't make different choices twice given the exact same state and input; it wanted to make the choice it did because of its state and inputs. I don't understand how the choice is any less free just because it's deterministic.