Superconducting quantum-dot turnstile singles out electrons

The superconductor-quantum-dot-superconductor turnstile
Single-dot island: the superconductor–quantum-dot–superconductor turnstile

A metallic quantum dot sandwiched between two superconductors – which functions as an electronic turnstile, only permitting one electron through at a time – has been developed by researchers in France, Russia and Finland. By driving an AC voltage through the device, the researchers can control the tunnelling of electrons into and out of the dot. While such turnstiles have been made before, this is the first where electrons at only one quantum energy level are allowed to pass through. This, say the researchers, makes the device ideal for quantum-metrology applications.

The ability to control the flow of current down to the level of a single electron has been a recent and major achievement in condensed-matter physics. Indeed, producing and controlling single electrons within a circuit on a chip has many applications, from nanoelectronics to electron optics and even quantum technologies. In most quantum current sources, electrons are sent in single file along a conductor by using the repulsive Coulomb force between electrons. Previous single-electron turnstile devices have been based on superconducting single-electron transistor (SINIS) turnstiles, where a metallic region or an “island” is connected to two superconducting leads via insulating tunnel junctions. These devices take advantage of the fact that the energy gap in the density of states of superconductors is sharply defined.

One at a time

The tunnelling itself is controlled via a periodically varying voltage that lets electrons first tunnel into the island and then onwards, much like a revolving door. In principle, the electrons are meant to pass though in single file – thanks to the Coulomb force, only one electron can sit on the island at any time. However, in practice, extra electrons can squeeze along. SINIS turnstiles, therefore, are often prone to errors, especially due to the leakage of thermally activated electrons.

But the new turnstile – developed by Clemens Winkelmann and David van Zanten from the University of Grenoble, France, together with colleagues in Russia and Finland – uses a quantum dot as the island. The dot helps to block the undesirable electrons, making it a much more exact single-electron source. While the superconductor energy gap is sharp and therefore limits the tunnelling of any extra electrons, the metal island has a range of energy levels, which unwittingly allows extra electrons to slip in.

Gold bridges

By getting rid of the metal island altogether and replacing it with a quantum dot that is much smaller and has discrete energy levels, Winkelmann and colleagues were able to develop a “superconductor–quantum-dot–superconductor” (SQS) turnstile that is a much more accurate gatekeeper. The dot’s quantum energy states are widely spaced, meaning that electrons only sit at the ground state. To fabricate the SQS junctions, the team created nanometre-sized fractures in superconducting constrictions using a technique known as “electromigration”. Randomly dispersed gold nanoparticles of about 5 nm diameter bridge the fractures, acting as the quantum-dot junctions.

The researchers found that the SQS turnstile produces a stream of single electrons, all at the same fixed energy, with less than 1% error that would arise from any trespassing electrons. As the electrons that flow through the SQS do so at a single energy, the device is especially well-suited for quantum-metrology applications, such as defining the ampere. The team is now looking into the possibility of building a spin-polarized turnstile that operates via Zeeman splitting in a magnetic field.

The research is published in Physical Review Letters.

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