Quantum information technologies is the name given to theories and applications of the principles of quantum mechanics as applied to computation and information processing. The former uses the properties of quantum mechanical objects to implement algorithms that theoretically can perform better than those that run on current “classical” digital computers. The most famous example to date is perhaps Shor’s algorithm for factoring integers, which if implemented for very large integers could render current encryption systems obsolete. The latter refers to quantum techniques applied to secure and unbreakable encryption, communication that reveals the presence of eavesdroppers, and so on. The book discussed here presents 28 papers that delve into various areas of research in these areas. Due to the length (624 pages) and technical depth of this material, in this review I will cover major themes rather than attempt to summarize each topic.

Although quantum computing is possible in theory, with certain algorithms developed and analyzed as having the potential noted above, in practice the technical difficulty of manipulating quantum-level objects has prevented development of more than very simple prototypes of computing and information-handling devices. For example, a quantum device has run Shor’s algorithm to successfully factor 15 as 3 × 5, with “high probability.”

The government of Japan sponsored a project that ran from 2010 to 2014 to investigate quantum information technologies. The project covered the following five areas that are presented here as sections of the book:

Quantum communication concerns the transmission of information using quantum processes, the encryption of data using keys generated using quantum processes, and creating extended distance transmissions using quantum repeaters, which can be thought of as range extenders.

Metrology and sensing investigates technologies for measuring quantum events. Of particular importance is the development of increasingly accurate measurements at smaller and smaller dimensions by exploiting the behavior of quantum objects. For example, illuminating a sample under a microscope using entangled photons yields a more accurate microscopic image than using single, non-entangled photons.

Coherent computing attacks combinatorial optimization problems, such as the max-cut problem, which seeks to divide the set of vertices of a graph into two subsets that meet specific criteria. Several devices are presented that utilize lasers pulsed into a network of crystals or a fiber ring. As the laser input is adjusted, the network builds into a steady state that represents a solution to the problem.

Quantum simulation seeks to employ a highly controllable quantum entity to simulate the behavior of a different quantum entity or to explore and confirm properties suggested by a theoretical model such as the Bose–Hubbard model of bosons interacting in a lattice.

Quantum computing involves creating quantum equivalents of the bits of a classical computer. Qubits, or quantum bits, on measurement return a 1 or a 0, the values of classical bits, but during computations qubits exist in a superposition of these two states. It is this characteristic that theoretically permits algorithms to execute faster using qubits than if run on a classical computer. The problem in handling qubits is their fragility: qubits tend to quickly change their state, a phenomenon called decoherence. Research described here implements qubits as “artificial atoms,” solid state devices that have the required quantum behaviors but that can be controlled and kept stable long enough to produce computations. For example, the qubit can be stored as the spin of an electron contained in a quantum dot. The electron’s spin can be controlled by laser pulses. Despite encouraging results, it is clear that quantum computing is in a very early stage. It remains unclear if or when devices with a sufficiently large number of qubits to perform complex computations can be built.

In each area, theory is developed and presented mathematically along with descriptions of hardware devices created to test the theory and experimental results. For anyone looking for information on the recent state of the art in the areas discussed above, this is a good reference. The writing is clear and precise for the most part, although technically deep and mathematically complex. Extensive references are provided with each paper.