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Remarks on the undecidability of the quantum halting problem

Daegene Song

Published Jan 22, 2007• 1 citation
OpenAlex

Abstract

The halting problem is a decision problem first posed and proved by Alan Turing in 1936. With the recent surge of interest in quantum computation, one is led to ask if the problem can also be considered for a quantum computer. It is reported that the halting problem may not be solved consistently in both the Schrodinger and Heisenberg pictures of quantum dynamics. The assumption of the existence of the quantum halting machine leads to a contradiction when a vector representing an observable is the system that is to be unitarily evolved in both pictures.

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The halting problem is a decision problem first posed and proved by Alan Turing in 1936. With the recent surge of interest in quantum computation, one is led to ask if the problem can also be considered for a quantum computer. It is reported that the halting problem may not be solved consistently in both the Schrodinger and Heisenberg pictures of quantum dynamics. The assumption of the existence of the quantum halting machine leads to a contradiction when a vector representing an observable is the system that is to be unitarily evolved in both pictures.
It is shown that the halting problem cannot be solved consistently in both the Schrodinger and Heisenberg pictures of quantum dynamics. The existence of the halting machine, which is assumed from quantum theory, leads into a contradiction when we consider the case when the observer's reference frame is the system that is to be evolved in both pictures. We then show that in order to include the evolution of observer's reference frame in a physically sensible way, the Heisenberg picture with time going backwards yields a correct description.
Quantum computing allows us to solve some problems much faster than existing classical algorithms. Yet, the quantum computer has been believed to be no more powerful than the most general computing model—the Turing machine. Undecidable problems, such as the halting problem, and unrecognizable inputs, such as the real numbers, are beyond the theoretical limit of the Turing machine. I suggest a model for a quantum computer, which is less general than the Turing machine, but may solve the halting problem for any task programmable on it. Moreover, inputs unrecognizable by the Turing machine can be recognized by the model, thus breaking the theoretical limit for a computational task. A quantum computer is not just a successful design of the Turing machine as it is widely perceived now, but is a different, less general but more powerful model for computing, the practical realization of which may need different strategies than those in use now.
Incompleteness and undecidability theorems have to be revised in view of quantum information and computation theory. qrt.tex 1 As has already been pointed out in Gödel’s centennial paper on the incompleteness af arithmetic [1], the classical undecidability theorems of formal logic [2] and the theory of computable functions [4, 5] are based on semantical pardoxes such as the liar [6] or Richard’s paradox. The method of diagonalization, which was first applied by Cantor for a proof of the undenumerability of real numbers [7], has been applied by Turing for a proof of the recursive undecidability of the halting problem [8]. The halting problem is the problem of whether or not an arbitrary algorithm terminates or produces a particular output and terminates. Assume that the halting problem is decidable. Turing [8] proved that this assumption yields a contradiction. To construct the contradiction, consider
We study the observation of quantum Turing machines by allowing interactions between a quantum machine and its environment during the computation, whereas a quantum Turing machine --original model introduced by Deutsch-- remains isolated. We show that the introduction of observations leads to a weakening of the well formedness conditions of quantum Turing machines such that any (reversible or not) classical Turing machine is a special instance of a quantum machine. Moreover, observation of quantum Turing machines provides a formal solution to the halting process problem: the impossibility to know whether a given quantum Turing machine has actually reached its halting state. It also provides a more realistic abstract architecture of a quantum computer while most of the physical proposals of quantum computers are based on an hybrid classical-quantum architecture. However, we show that a natural formalisation of an observable quantum Turing machines leads to an over-powerful model solving undecidable problems. As a consequence, we introduce a restricted version of observable quantum Turing machines and we show that, under this restriction, any observable quantum Turing machine can be efficiently simulated by a quantum Turing machine. Finally, we discuss the potential application of the observable quantum Turing machine in the quest of a universal quantum Turing machine while recent papers have pointed out that a classical control is a key feature of a universal quantum machine.
FINDING: Undecidable problems extend from computation to quantum field theory and many-body physics, revealing fundamental limits on what can be known or computed about physical systems. | MATH: Halting problem: no Turing machine can decide if an arbitrary program halts (Turing 1936). Gödel incompleteness: any consistent formal system containing arithmetic has true but unprovable statements. Reducibility: problem A reduces to B if solving B solves A (A ≤ B). In QFT, spectral gap problem is undecidable (Cubitt, Perez-Garcia, Wolf 2015): no algorithm can determine if a quantum many-body system has a nonzero energy gap. | CONNECTION: Undecidability in physics often arises from encoding Turing machines into Hamiltonian lattice systems — this maps computational logic onto geometric lattice structures (e.g., 2D spin lattices with local interactions). The spectral gap problem's undecidability proof uses aperiodic tilings and translation-invariant Hamiltonians, linking to crystallographic symm Author: Andrew Stewart Caldin, Independent Researcher, UK. Part of the E8 Intelligence Research series. Platform: e8intelligence.com