Scanning tunneling microscope topograph of nitride patches (blue) on a copper substrate (green) that serve as nano-workbenches for the assembly of atomic-scale prototype structures from magnetic atoms (green bumps).
Single spins in solid-state systems are often considered prime candidates for the storage of quantum information, and their interaction with the environment the main limiting factor for the realization of such schemes. The lifetime of an excited spin state is a sensitive measure of this interaction, but extending the spatial resolution of spin relaxation measurements to the atomic scale has been a challenge. We show how a scanning tunneling microscope can measure electron spin relaxation times of individual atoms adsorbed on a surface using an all-electronic pump-probe measurement scheme. The spin relaxation times of individual Fe-Cu dimers were found to vary between 50 and 250 nanoseconds. Our method can in principle be generalized to monitor the temporal evolution of other dynamical systems.
Scientists at IBM’s Almaden Research Center in San Jose are using the scanning tunneling microscope (STM) like a high-speed camera to record the behavior of individual atoms at a speed about one million times faster than previously possible.
This breakthrough could be used to study areas such as:
Quantum computing. Quantum computers are a radically different type of computer – not bound to the binary nature of traditional computers – with the potential to perform advanced computations that are not possible today. With today’s breakthrough, scientists will have a powerful new way to explore the feasibility of a novel approach to quantum computing through atomic spins on surfaces.
Information storage technologies. As technology approaches the atomic scale, scientists have been exploring the limits of magnetic storage. This breakthrough allows scientists to “see” an atom’s electronic and magnetic properties and explore whether or not information can be reliably stored on a single atom
Since the magnetic spin of an atom changes too fast to measure directly using previously available Scanning Tunneling Microscope techniques, time-dependent behavior is recorded stroboscopically, in a manner similar to the techniques first used in creating motion pictures, or like in time lapse photography today.
Using a “pump-probe” measurement technique, a fast voltage pulse (the pump pulse) excites the atom and a subsequent weaker voltage pulse (the probe pulse) then measures the orientation of the atom’s magnetism at a certain time after excitation. In essence, the time delay between the pump and the probe sets the frame time of each measurement. This delay is then varied step-by-step and the average magnetic motion is recorded in small time increments. For each time increment, the scientists repeat the alternating voltage pulses about 100,000 times, which takes less than one second.
In the experiment, iron atoms were deposited onto an insulating layer only one atom thick and supported on a copper crystal. This surface was selected to allow the atoms to be probed electrically while retaining their magnetism. The iron atoms were then positioned with atomic precision next to non-magnetic copper atoms in order to control the interaction of the iron with the local environment of nearby atoms.
The resulting structures were then measured in the presence of different magnetic fields to reveal that the speed at which they change their magnetic orientation depends sensitively on the magnetic field. This showed that the atoms relax by means of quantum mechanical tunneling of the atom’s magnetic moment, an intriguing process by which the atom’s magnetism can reverse its direction without passing through intermediate orientations. This knowledge may allow scientists to engineer the magnetic lifetime of the atoms to make them longer (to retain their magnetic state) or shorter (to switch to a new magnetic state) as needed to create future spintronic devices.
“This breakthrough allows us – for the first time – to understand how long information can be stored in an individual atom. Beyond this, the technique has great potential because it is applicable to many types of physics happening on the nanoscale,” said Sebastian Loth, IBM Research. “IBM’s continued investment in exploratory and fundamental science allows us to explore the great potential of nanotechnology for the future of the IT industry.”