# Quantum Computing: Theoretical *versus* Practical Possibility

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## Abstract

An intense effort is being made today to build a quantum computer. Instead of presenting what has been achieved, I invoke analogies from the history of science in an attempt to glimpse what the future might hold. Quantum computing is possible in principle—there are no known laws of Nature that prevent it—yet scaling up the few qubits demonstrated so far has proven to be exceedingly difficult. While this could be regarded merely as a technological or practical impediment, I argue that this difficulty might be a symptom of new laws of physics waiting to be discovered. I distinguish between “strong” and “weak” emergentist positions. The former assumes that a critical value of a parameter exists (one that is most likely related to the complexity of the states involved) at which the quantum-mechanical description breaks down, in other words, that quantum mechanics will turn out to be an incomplete description of reality. The latter assumes that quantum mechanics will remain as a universally valid theory, but that the classical resources required to build a real quantum computer scale up with the number of qubits, which hints that a limiting principle is at work.

## Keywords

Quantum computing quantum information entanglement strong emergence weak emergence philosophy of physics history of physics## Notes

### Acknowledgments

My research for this paper began under a John Templeton Fellowship, which allowed me to spend the summer of 2009 at the Institute of Quantum Optics and Quantum Information of the University of Vienna. I especially thank my hosts, Professor Anton Zeilinger and Professor Markus Aspelmeyer, who made this visit possible, and for many enlightening discussions with them and other scientists in the Institute. I am solely responsible, of course, for the (probably controversial) views I express. I also thank the Academy of Finland for financial support through Academy Research Fellowship 00857 and Projects 129896, 118122, 135135, and 141559. Finally, I thank an anonymous referee for helpful comments, and Roger H. Stuewer for his editorial work on my paper.

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