SmaLL-ScaLE qUanTUm cOmPUTInG

I l l u s t r a t I o n b y C o h e r e n t I m a g e s SmaLL-ScaLE qUanTUm cOmPUTInG devices built on a variety of underlying physical implementations exist in the laboratory, where they have been evolving for over a decade, and have demonstrated the fundamental characteristics necessary for building systems. The challenge lies in extending these systems to be large enough, fast enough, and accurate enough to solve problems that are intractable for classical systems, such as the factoring of large numbers and the exact simulation of other quantum mechanical systems. The architecture of such a computer will be key to its performance. Structurally, when built, a “quantum computer” will in fact be a hybrid device, with quantum computing units serving as coprocessors to classical systems. The program, much control circuitry, and substantial preand postprocessing functions will reside on the classical side of the system. The organization of the quantum system itself, the algorithmic workloads for which it is designed, its speed and capabilities in meeting those goals, its interfaces to the classical control logic, and the design of the classical control systems are all the responsibility of quantum computer architects. In this article, we review the progress that has been made in developing architectures for full-scale quantum computers. We highlight the process of integrating the basic elements that have already been developed, and introduce the challenges that remain in delivering on the promise of quantum computing. The most famous development to date in quantum algorithms is Shor’s algorithm for factoring large numbers in polynomial time. While the vernacular press often talks of factoring large numbers “in seconds” using a quantum computer, in reality it is not even possible to discuss the prospective performance of a system without knowing the physical and logical clock speed, the topology of the interconnect among the elements, the number of logical quantum bits (qubits) available in the system, and the details of the algorithmic implementation—in short, without specifying the architecture. Figure 1 illustrates the impact that architecture can have on the bottom-line viability of a quantum computer; here, A Blueprint for Building a Quantum Computer