Capsule Reviews

Quantum Programming Languages: An Introductory Overview. R. RÜDIGER During the first quarter of the 20th century, much research which was later to form the foundations of computer science has been laid down. Quantum theory, being the theory of physical processes at an atomic and subatomic scale, was one of the subjects that was also studied then. However, the influence of quantum concepts in the actual development and application of computer science remained less visible than any other concepts. This was to change in the past two decades were a number of applications which used quantum concepts were developed. For instance, quantum cryptography became a commercial product and quantum systems were shown to speed some computational tasks. This led to the question of whether conventional programming models and languages can be sufficient for programming quantum systems. This paper reviews the research that has been carried out in the field of quantum programming languages (QPLs). First, the paper gives a summary of the terminology and notation of quantum theory. Then, the paper discusses briefly the interpretation of quantum theory and in particular the ‘superposition principle’ which states that ‘at any given time, a quantum system can be in more than one state’ and the consequences of the linearity of quantum mechanics. The author concludes that it is an open question whether a proper refinement of quantum theory exists but that nonetheless, attempts which reformulate quantum theory are very interesting. Thereafter, the author moves to the design issues of QPLs listing a number of desiderata targeted within the general goals of designing QPLS and some literature which relates to these desiderata. Possible directions towards designing QPLs are then named; these include high-level structures, quantum lambda-calculus and functional programming. Section 5 reviews some of the approaches in the literature that deals with the goals and the design of QPLs. In particular: