Programmed Assembly of Quantum-Dot Arrays on DNA Templates: Hardware for Quantum Computing? J. C. Wells 1 , K.A. Stevenson 2 , G. Muralidharan 2 , T. G. Thundat 2 , L. Maya 3 , J. Barhen 1 Center for Engineering Science Advanced Research 1 Computer Science and Mathematics Division 2 Life Sciences Division 3 Chemical and Analytical Sciences Division Oak Ridge National Laboratory, Oak Ridge, TN, 37831 wellsjc@ornl.gov stevensonka@bio.ornl.gov , murali@bio.ornl.gov ,, thundattg@ornl.gov , mayal@ornl.gov , barhenj@ornl.gov Abstract: This paper reports progress in the fabrication and characterization of an array of 1nm- scale colloidal particles (i.e., quantum-dot array) that can be operated to execute nontrivial and innovative computations, possibly including quantum logic. We discuss the actual fabrication of 2-nm metal clusters as an example of possible quantum dot implementation. Innovative and unconventional paradigms underlie the different stages of this work. For example, regular array geometry is achieved by directing appropriately derivatized metal clusters to preselected locations along a stretched strand of an engineered DNA sequence. © 1999 Optical Society of America OCIS Codes: (160.2100) Electro-optical materials; (160.4330) Nonlinear optical materials 1. Introduction The proposals for the physical implementations of quantum computation span virtually every branch of quantum physics (e.g., see [1]). Many of these proposals have been motivated by recent advances in nanoscale science and engineering, including, in particular, quantum dots and quantum-dot arrays (e.g., see [2-4]). An approach to quantum computing that has not received broad attention is that of the quantum implementation of the cellular automaton [1]. As Lloyd has indicated in his original work [5], arrays of weakly coupled quantum systems can be made to compute by subjecting them to a sequence of electromagnetic pulses of well-defined frequency and length, and programming such computers is accomplished by selecting the proper sequence of pulses. Local control of the qubits is not required. New theoretical work by Benjamin shows how relatively simple local rules would permit the implementation of some quantum computations [6]. This work points toward the fabrication of an array of quantum-confined electron systems that can be addressed globally in a spectroscopic fashion. In this context, we report progress [7] in the development and characterization regular arrays of nanometer- sized colloidal clusters (e.g., Au, Ag, and Pt). Regular array geometry is achieved by directing appropriately derivatized metal clusters to preselected locations along a stretched strand of an engineered DNA sequence. We are interested in exploring these interesting nanoscale systems for possible implementations of quantum logic. 2. Programmed assembly of nanoparticles The ability to assemble nanoparticles in a precise and controlled way is key to the fabrication of a variety of nanodevices. Networks of nanometer–sized metal or semiconductor islands, or quantum dots, may exhibit a variety of quantum phenomena, with applications in optical devices [8], nanometer-sized sensors [9], advanced computer architectures [10,11], ultra dense memories [12], and quantum-information science and technology [1]. The challenge is that fabrication of nanoparticle arrays in a time and cost effective manner remains a formidable task. Particle-based and e-beam lithography lack the required resolution. Scanning probe microscopy can be used for making molecular devices, but it is slow and impractical for mass production. A variety of other techniques have been demonstrated, including self-assembled monolayers [13], block copolymer template lithography [14] or electro deposition [15], and controlled deposition by cleaved edge overgrowth [16]. All of these techniques have limitations on the size of the particle and/or the pattern of the resulting array. Interest in the concept of self-assembled nanostructures led to the idea of using DNA as a scaffold or template for the programmed assembly of nanoscale arrays (see review by Storhoff and Mirkin [17]). Beginning in the 1980s Seeman et al. experimented with combining DNA fragments to produce geometrical shapes, including cubes [18], triangles [19], two-dimensional arrays [20,21,22] and various forms of DNA knots [23,24]. Using DNA as a structural molecule has many advantages. It can be easily synthesized in lengths up to 40 nm and double-stranded DNA can be joined end to end to produce longer linear molecules or more complex shapes. It can be modified with functional groups at predetermined sites to allow for the attachment of other molecules in a specific manner. DNA has been used previously in the programmed assembly of particles. Mirkin et al. [25,26,27] and Alivisatos et al. [29,30] have successfully attached oligonucleotide-derivatized nanoparticles to DNA using hybridization techniques. Alivisatos also bound gold particles to both single-stranded and double-stranded DNA modified with