Advances in quantum devices have brought scalable quantum computation closer to reality. We focus on the system-level issues of how quantum devices can be brought together to form a scalable architecture. In particular, we examine promising silicon-based proposals. We discover that communication of quantum data is a critical resource in such proposals. We find that traditional techniques using quantum SWAP gates are exponentially expensive as distances increase and propose quantum teleportation as a means to communicate data over longer distances on a chip. Furthermore, we find that realistic quantum error-correction circuits use a recursive structure that benefits from using teleportation for long-distance communication. We identify a set of important architectural building blocks necessary for constructing scalable communication and computation. Finally, we explore an actual layout scheme for recursive error correction, and demonstrate the exponential growth in communication costs with levels of recursion, and that teleportation limits those costs. Index Terms-Quantum architecture, quantum computers, silicon-based quantum computing. I. INTRODUCTION M ANY important problems seem to require exponential resources on a classical computer. Quantum computers can solve some of these problems with polynomial resources, which has led a great number of researchers to explore quantum information processing technologies [1]-[7]. Early-stage quantum computers have involved a small number Manuscript

As quantum computing moves closer to reality the need for basic architectural studies becomes more pressing. Quantum wires, which transport quantum data, will be a fundamental component in all anticipated silicon quantum architectures. Since they cannot consist of a stream of electrons, as in the classical case, quantum wires must fundamentally be designed differently. In this paper, we present two quantum wire designs: a swap wire, based on swapping of adjacent qubits, and a teleportation wire, based on the quantum teleportation primitive. We characterize the latency and bandwidth of these two alternatives in a device-independent way. Furthermore, unlike classical wires, quantum wires need control signals in order to operate. We explore the complexity of the control mechanisms and the fundamental tension between the scale of quantum effects and the scale of the classical logic needed to control them. This "pitch-matching" problem imposes constraints on minimum wire lengths and wire intersections, leading us to use a SIMD approach for the control mechanisms. We ultimately show that qubit decoherence imposes a basic limit on the maximum communication distance of the swapping wire, while relatively large overhead imposes a basic limit on the minimum communication distance of the teleportation wire.

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Quantum computation has become an intriguing technology with which to attack difficult problems and to enhance system security. Quantum algorithms, however, have been analyzed under idealized assumptions without important physical constraints in mind. In this paper, we analyze two key constraints: the short spatial distance of quantum interactions and the short temporal life of quantum data.In particular, quantum computations must make use of extremely robust error correction techniques to extend the life of quantum data. We present optimized spatial layouts of quantum error correction circuits for quantum bits embedded in silicon. We analyze the complexity of error correction under the constraint that interaction between these bits is near neighbor and data must be propagated via swap operations from one part of the circuit to another.We discover two interesting results from our quantum layouts. First, the recursive nature of quantum error correction circuits requires a additional communication technique more powerful than near-neighbor swaps -too much error accumulates if we attempt to swap over long distances. We show that quantum teleportation can be used to implement recursive structures. We also show that the reliability of the quantum swap operation is the limiting factor in solid-state quantum computation.

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