44^th IFF Spring School

 

Overview

Two-dimensional fabric of integrated-circuit devices for fault tolerant quantum computing: superconducting qubits (green), high-Q resonators (gray), and I/O ports (yellow).

Quantum Information Science (QIS) is a cross-disciplinary subject that has arisen in the last twenty years.  It concerns itself with the consequences of our most complete description of the physical world (that is, quantum mechanics) for the reliable, secure, private, and rapid processing of information, both in communication and computation.  While its invention is often ascribed to the famous theoretical physicist Richard Feynman in the 1980s, his contributions were only one of many that initiated the field around that time.  While he perceived that new types of physical devices, in which the quantum laws of superposition and entanglement function at the logical level, could give new power in the simulation of quantum physics, it was others (Bennett, Brassard) who showed that the uncertainty principle led to fundamentally more secure ways of communicating secret messages.  It was yet others (Deutsch, Vazirani) who understood that quantum theory defined a new kind of Turing machine, and that not only quantum physics simulations, but potentially many other computational problems, are sped up in this machine.  And it was yet another (Shor) who found a simple, very fast algorithm for prime factorization.

 

Quantum Information is a very diverse subject pursued today in many different directions, by many hundreds of researchers internationally:  In theoretical physics, it has enlivened and sharpened the understanding of efficient representations of entangled many-particle wavefunctions, and has provided the prospect of applications for new concepts such as anyons and majorana fermions.  Information theorists has benefitted from having a rigorous extension of the basis of their field, in which classical information theory is subsumed into a greatly broader subject.  Theoretical computer scientists continue the search for new quantum algorithms, and have used quantum concepts to prove new results about the classification of computational complexity.  Coding theorists have had the new and subtle problem of quantum error correction to analyze and conquer.

 

Finite-element grid for detailed modeling of electromagnetics of three-dimensional cavity region (blue) containing a superconducting qubit (dark blue), externally connected to a feed line (dark red). After S. E. Nigg et al., " Black-box superconducting circuit quantization", http://arxiv.org/abs/1204.0587

Most strikingly, the program of experimental physics has been influenced in many directions by QIS.  State-of-the-art optics experiments transmit quantum states over long distances and perform precision manipulations in single quanta (atomic ions, impurity spins, quantum dots) in the quest to have working quantum cyyptography and quantum computing.  Quantum Hall systems, and topological insulators, are being assiduously examined for new elementary excitations for use as qubits.  In the course of ten years, superconducting devices have improved by over four orders of magnitude in their quantum coherence, a metric made precise by the ideas of quantum computing.  In the current stage, engineering arts such as amplifier design and analog signal processing are looking to take the stage.

Much of this points to the question that has been asked repeatedly in the last fifteen years, "when will the quantum computer be built".  While the ansswer remains "we don't know", it is possible that the scientific developments of the last couple of years will, in retrospect, have brought us to the point that a realistic path to the development of this technology is now clear.