Abstract – Energy-efficiency has become a critical parameter in the design of high-performance computing systems. Typically, compute elements consume the most energy in current systems, with memories and interconnect networks close behind. This paper proposes an energy-efficient, high-connectivity, petascale computing system for the year 2020 timeframe by addressing the energy requirements of these three components.We start with projections based on purely evolutionary computer system design trends, then include the impact of breakthroughs in processor design, memory packaging and optical interconnect technologies. Based on these projections, we motivate the development for a 1-pJ/b optical intrasystem interconnect technology that significantly increases system interconnect bandwidth and relieves the distance-based energy dependence of electrical alternatives.We show that improvements in compute, memory, and IO, when simultaneously applied, become a vision for many-chip photonically-interconnected modules that could lead to an order of magnitude improvement in energy efficiency in the 2020 timeframe. The vision hinges on a high-density, energy-efficient optical link that can connect electronic compute and memory elements across short chip-to-chip distances while also capable of kilometer or longer spans across data centers. We discuss the power budget to enable such a link and review experimental progress toward creating an ultra-dense, hybrid-integrated low-power silicon photonic link that will enable this vision.