Novel Computing Architecture on Arrays of Josephson Persistent Current Bits Jie Han, Pieter Jonker Pattern Recognition Group, Department of Applied Physics, Delft University of Technology Lorentzweg 1, 2628 CJ, Delft, The Netherlands, {jie, pieter}@ph.tn.tudelft.nl ABSTRACT A superconducting qubit (or quantum bit), which consists of a micrometer-sized loop with three Josephson junctions, has two persistent currents of opposite direction as its two states. The states of the qubit can be brought into quantum coherence to perform quantum computing. Classical bits can also be obtained from these superconducting loops, making it possible to base a classical computer architecture. We study a novel computing structure based on these Josephson Persistent Current (PC) Bits, starting from elementary logic gates to a Random Access Memory (RAM). The investigation shows that the Josephson PC Bit technology would not surpass semiconductor technology in term of the device density, while it is a promising candidate for unltra-fast memory, which can be integrated with other technologies. The classical computer might also serve as pre and post processor for the quantum computing performed in the heart of the array. The Josephson PC circuits, therefore, seems a good vehicle for the study of the quantum computer paradigm. Keywords: computing architecture, Josephson, circuit simulation, classical computing, quantum computing 1 INTRODUCTION Silicon-based CMOS circuitry has dominated microelectronics for decades. Besides the continuous miniaturization of the elements used to construct computers, ultra-dense and fast systems based on other technologies are also envisioned and proposed. The quantum computer is probably the ultimate objective of this endeavor. Various physical systems were proposed for quantum computing. Among those mesoscopic superconducting circuits of ultra-small Josephson junctions, which can be produced by modern lithography, appear promising for integration in electronic circuits and large- scale applications. Recently, the quantum superposition of two macroscopic persistent-current states on superconducting Josephson circuits was detected and measured [1]. Thus, the proposed qubit (or quantum bit), which consists of a micrometer-sized loop with three Josephson junctions, is possible to be brought into quantum coherence to perform quantum computing. Classical bits can also be obtained from these superconducting loops by increasing its critical current, making it possible to base a classical computing architecture on these Josephson Persistent-Current (PC) bits. In this paper we study a novel computing structure based on the Josephson (PC) Bits, starting from elementary logic gates to a Random Access Memory (RAM) design. The proposed classical parallel computer could also be a significant part in the realization of quantum computers, for the same device can be used for both classical and quantum computing [3]. Although the classical computing part could as well be done with conventional CMOS technology, the study on the Josephson PC circuits might bring us an insight that cannot easily be achieved with other spin based devices for quantum computers, because of the problems involved in combining them with conventional technology. In the second section we present a close look at the Josephson PC circuit and focus on the properties we are interested in. In the following sections we discuss the design of a classical computing structure, from elementary logic gates to a Random Access Memory (RAM). Finally, some conclusions are drawn. 2 THE JOSEPHSON PC QUBIT/BIT A Josephson PC qubit in principle consists of a loop with three Josephson junctions in series that encloses a magnetic flux Φ driven by an external magnet (Fig.1). In particular when the enclosed magnetic flux is close to half a superconducting flux quantum 0 Φ (= e h 2 / , where h is Planck’s constant), the loop may have multiple stable persistent current states, and this system behaves as a particle in a double-well potential, where the classical states in each well correspond to persistent currents of opposite sign. The two classical states are coupled via quantum tunneling through the barrier between the wells, and the loop is a macroscopic quantum two level system. This system has two stable states 0 and 1 with opposite circulating persistent currents [1][2]. The qubit is operated by resonant microwave modulation of the enclosed magnetic flux by a superconducting control line. The superconducting control line is on top of the qubit, separated by a thin insulator. Measurement can be made with superconducting magnetometers [superconducting quantum interference devices (SQUIDs)] [2]. When the quantum tunneling does not take place, the system behaves classically so that on the loop there is a