In single file, please!
At the ultimate limit of miniaturization electrons are transported one at a time
The world of nanoscience is synonymous for the quest for quantum limits. Such limits, however, manifest themselves at many different levels. An electrical current flowing through a wire is like water running from a faucet – there are simply so many electrons that they behave like a homogeneous medium. But the current flow can be constrained like sand trickling down in an hour glass. In a scanning tunneling microscope, this is pushed to the atomic limit, where an atomically sharp tip hovers a few Ångströms over a sample surface, such that the electrons have to "tunnel" through a vacuum barrier (tunnel junction) to get from the tip to the sample. The tunneling effect is a purely quantum mechanical phenomenon that describes the ability to pass through a barrier without actually having enough energy to overcome it – very much in analogy to digging a tunnel through a mountain. Cooling down close to absolute zero temperature reveals the charge quantization limit, but only indirectly. Even then the current flows like cars on a multilane highway, so it is impossible to know how many and on which lane they pass through the barrier. The next limit is to allow only one electron at a time.
To stay with the analogies, researchers in Stuttgart, Ulm, and Madrid have now turned the tunnel junction into a revolving door. Only one electron at a time is allowed to pass through the revolving door. It will remain in this door for a while and the exit on the other side. To realize such a junction, it comes down to a balance between supply and demand. Electrons occupy energy levels and typically there are many of them. "We were able to create a single energy level inside a small energy gap both on the tip and on the sample.”, says Haonan Huang, who is working on the mK-STM, which is an STM operating at 15 mK (–273.135°C). A single energy level inside an energy gap can be created by an atomic scale magnetic impurity coupled to a superconductor. The challenge here is to not only produce this single energy level on the sample, but also on the tip.
Due to these single energy levels, the electrons who tunnel through the junction have to do this one at a time. Despite this march of the electrons in single file, the charge transport is actually quite efficient leading to reduced dissipation. "This unique system, which is reduced to its elementary constituents, allows us to focus on the most fundamental aspects of tunneling. This is an unprecedented opportunity.", says Christian Ast, who leads the mK-STM group. Indeed, projecting this concept onto integrated circuitry could lead, for example, to more energy efficient electronics.
The success of this experiment is not just due to the low temperatures and the skilled preparation of the tunnel junction, but also to the extreme stability of the environment, in which the mK-STM is located. The home of the mK-STM is the Precision Laboratory of the Max Planck Institute for Solid State Research in Stuttgart featuring acoustically and electromagnetically isolated boxes with vibrationally decoupled concrete slabs. “The Precision Lab continues to serve us well in our quest to explore new quantum limits.", says Klaus Kern, who is a director at this institute.
Despite the elaborate experimental setup, the intricate sample preparation along with the extreme environmental conditions, the underlying theoretical explanation becomes surprisingly simple. "What is remarkable about this minimized configuration, is its nearly complete isolation from the environment, which is imposed by symmetry and allows us to describe it as a simple two-level system", says Joachim Ankerhold from Ulm University, whose group contributed the theoretical foundation. Indeed, the near isolation makes this system also interesting for using it as a qubit, the elementary building block of a quantum computer, or interacting with exotic matter, such as Majorana bound states.
What this international collaboration of scientists has, thus, achieved is stripping the tunneling process of all unnecessary features and synthesizing a tunneling current between single energy levels, thereby reducing the process to its bare essentials. They have exposed the fundamental mechanism providing a path for a new bottom-up approach with a new set of building blocks. "I am quite excited to see how reaching a quantum limit does not set an end to a long journey, but marks the beginning of a new one.", says Christian Ast, “The closer we look, the bigger the picture.”