Hypercomputation
Hypercomputation is the theory of methods for the computation of nonrecursive functions. The classes of functions which they can compute is studied in the field known as recursion theory. A similar recent term is superTuring computation, which has been used in the neural network literature to describe machines with various expanded abilities, possibly including the ability to compute directly on real numbers, the ability to carry out uncountably many computations simultaneously, or the ability to carry out computations with exponentially lower complexity than standard Turing machines.Hypercomputation was first introduced by Alan Turing in his 1939 paper Systems of logic based on ordinals, which investigated mathematical systems in which an oracle was available to compute a single arbitrary (nonrecursive) function from naturalss to naturals.
Other posited kinds of hypercomputer include:
 A quantum mechanical system which somehow uses (for example) an infinite superposition of states to compute a nonrecursive function^{1}. Such a system could not be an ordinary qubit quantum computer.
 A Turing machine which runs for an infinite period of time (perhaps the observer is being dropped into a black hole).
 A Turing machine which increases its speed exponentially over time. In a Newtonian universe, such a gadget might operate by manufacturing a clone of itself which was only half the size and operated at twice the speed.
 A nondeterministic Turing machine which has a preference ordering over its final states.
 A "real computer" (a sort of idealized analog computer) might be able to perform hypercomputation if physics admits general real variables (not just computable reals), and these are in some way "harnessable" for computation. This might require quite bizarre laws of physics (for example, a measurable physical constant with an oracular value, such as Chaitin's constant), and would at minimum require the ability to measure a realvalued physical value to arbitrary precision despite thermal noise and quantum effects.
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Notes
 There have been some claims to this effect; see Tien Kieu, Quantum Algorithm for Hilbert's Tenth Problem : [1]. & the ensuing literature. Errors have been pointed out in Kieu's paper (cf. [4]) and, in more recent papers, Kieu has downgraded his claims from "a quantum algorithm that solves an uncomputable problem" to "a quantum algorithm that might be solving an uncomputable problem".
References
 Alan Turing, Systems of logic based on ordinals, Proc. London math. soc., 45, 1939
 Tien Kieu, Quantum Algorithm for the Hilbert's Tenth Problem, Int. J. Theor. Phys., 42 (2003) 14611478, eprint archive quantph/0110136 (PDF)
 Toby Ord, Hypercomputation: computing more than the Turing machine (PDF), Honours Thesis, University of Melbourne, 2002.
 Boris Tsirelson. The quantum algorithm of Kieu does not solve the Hilbert's tenth problem. eprint archive quantph/0111009 (PDF)