Screening of an electron spin by an epitaxial superconducting island in a semiconductor nanowire

Project Information

SpinScreen

Grant agreement ID: 832645

DOI

10.3030/832645

Project closed

EC signature date 18 March 2019

Start date 1 September 2019

End date 31 August 2021

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 207 312,00

EU contribution

€ 207 312,00

Coordinated by

KOBENHAVNS UNIVERSITET
Denmark

Periodic Reporting for period 1 - SpinScreen (Screening of an electron spin by an epitaxial superconducting island in a semiconductor nanowire)

Reporting period: 2019-09-01 to 2021-08-31

Summary of the context and overall objectives of the project

Quantum computers can potentially tackle some problems faster than conventional computers but suffer from operational errors in their fundamental unit of processing, the qubit. A new type of qubit immune to error is needed, and it may be based on hybrid semiconductor-superconductor materials. This sort of hybrid can be miniaturized into a tiny island, the heart of the future error-free qubit. A key idea is to use a quantum dot to read the state of this qubit.

Both quantum dot and island can be conceived as small boxes of electrons. While the quantum dot is a box for single electrons, the island is a box for pairs of electrons, so-called Cooper pairs. When put together, the two boxes spill their electrons to each other, which provides a way of detecting the qubit state on the island.

A superconductor, however, interacts with a quantum dot in an additional way. The superconductor can donate an unpaired electron to bond with a single electron in the quantum dot if the spins of the two electrons point in opposite directions. The total spin is zero, so it is said to be screened. Until now, the way this interaction proceeded (if it proceeded at all) when the superconductor is miniaturized was unknown.

In SpinScreen, experiments were performed at -273 degrees Celsius on hybrid semiconductor-superconductor materials shaped into these boxes of electrons, with the main objective of elucidating their interactions.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Through these experiments, SpinScreen found out that the tiny size of the hybrid island favours electron bonding. The bonding depends on how strongly the electrons repel each other in the island. If the repulsion is strong, an individual electron can exist in the island besides the Cooper pairs, while if it is weak, only Cooper pairs can. Bonding occurs when the electron repulsion is sufficiently strong.

Next, SpinScreen investigated devices in which one quantum dot is coupled to two small hybrid islands. In this case, the electron in the quantum dot finds itself in an astounding dilemma. With which electron to bond, that of island #1 or that of island #2? The experiments in SpinScreen show that the electron resides in a state of quantum superposition where it bonds at the same time with the electrons in island #1 and island #2. The total spin is however not zero, in contrast to the single island case. This is an over-screened quantum state.

The experiments in SpinScreen went further, though. With the help of theoreticians for the interpretation of the data, it was found out that a new type of quantum states had been discovered. It turns out that in a superconducting island there are two forbidden energy gaps. The first one is the superconducting gap, formed by the pairing of electrons into Cooper pairs. The second one is a Coulombic gap, due to the electric (also called Coulomb) repulsion between electrons when the superconductor is made tiny as in an island. The result of this second gap is that the quantum states are no longer electron-hole symmetric, in contrast to purely superconducting quantum states. Electron-hole symmetry means that removing an electron from a superconductor costs the same energy as adding it. Superconducting Coulombic quantum states (this is the name created for these states in the project) emerge when the quantum dot is filled with even numbers of electrons. This was an unsuspected situation, with no parallel in large superconductors, and exceeded the original objective of SpinScreen.

These results were disseminated to the specialized physics community through one manuscript which has been submitted for peer-review. An additional manuscript is in preparation with the results on the two-island devices. The results were also presented in 7 international scientific conferences, 2 international workshops and 1 invited seminar at the University of Cincinnati.

The following outreach activities took place: 1) Project communications through SpinScreen’s twitter account (https://twitter.com/SpinScreen). 2) The Fellow narrated his experience as a MSCA Fellow to potential Latin American applicants at an Euraxess-LAC webinar (https://www.youtube.com/watch?v=-Rii_Oep03s&t=1115s). 3) The Fellow produced a short film showing the activities at the host laboratory, the Center for Quantum Devices (https://video.ku.dk/video/69812637/labtour-at-center-for-quantum). 4) The Fellow was panellist at the International Conference of Physics Students 2021 to answer questions from the students about the activities at the Center for Quantum Devices.

An objective of the MSCA Individual Fellowships is to train the Fellow. To aid the academic career of the Fellow, the Fellow took courses of Introduction of University Pedagogy and PhD student supervision. To advance his teaching skills, the Fellow also taught courses to MSc students on electronic quantum transport at the Niels Bohr Institute. In addition, the Fellow co-supervised four MSc students and one PhD student in their scientific projects. The projects have resulted in 2 publications, 1 manuscript submitted for peer-review, and 2 more manuscripts in preparation. These publications are/will be co-authored by the Fellow. At the end of the fellowship, the Fellow was promoted by the Niels Bohr Institute to the rank of Assistant Professor.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

SpinScreen pushes the state of the art in several ways. The effect of a quantum dot on a hybrid island is now understood to be multifaceted. The quantum dot will in general tend to break Cooper pairs and promote the presence of a single unpaired electron in the hybrid island. Moreover, SpinScreen has shown that the quantum dot produces new quantum states in the hybrid island, which did not exist in previously used large superconductors. Finally, SpinScreen has provided with direct proof of an over-screened quantum state. Before SpinScreen, such state had only been indirectly observed in quantum dots coupled to metallic islands. In SpinScreen, the energy gap of the hybrid island isolates this quantum state, a unique advantage of the superconductivity in the island.

This knowledge could find applicability in quantum technologies. The spin-zero quantum states may be used in the future as qubits themselves, as their lack of spin makes them immune to magnetic noise. Moreover, the over-screened state could be used to entangle two error-free qubits together, a requisite for qubit operations in a future quantum computer.

SpinScreen thus brings one step further the understanding of hybrid islands coupled to quantum dots, and it brings one step closer the promise of error-free quantum computation. Quantum computation can accelerate progress in fields currently capped by the limitations of conventional computation, such as drug discovery, chemistry, fertilizers, protein modelling, and others, which is where the final utility for society in this type of fundamental research lies on.

Device used to explore the interactions between a quantum dot and a hybrid island.

Sketch of the interactions in a quantum dot coupled to a hybrid superconducting island.