Abstract
Recent years have shown steady progress towards molecular electronics1, 2, in which molecules form basic components such as switches3-5, diodes6 and electronic mixers7. Often, a scanning tunnelling microscope is used to address an individual molecule, although this arrangement does not provide long-term stability. Therefore, metal-molecule-metal links using break-junction devices8-10 have also been explored; however, it is difficult to establish unambiguously that a single molecule forms the contact11. Here we show that a single hydrogen molecule can form a stable bridge between platinum electrodes. In contrast to results for organic molecules, the bridge has a nearly perfect conductance of one quantum unit, carried by a single channel. The hydrogen bridge represents a simple test system in which to understand fundamental transport properties of single-molecule devices.