Tag: qubits

Magnesium Protects Tantalum

Magnesium Protects Tantalum

UPTON, NY—Scientists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory have discovered that adding a layer of magnesium improves the properties of tantalum, a superconducting material that shows great promise for building qubits, the basis of quantum computers. As described in a paper just published in the journal Advanced Materials, a thin layer of magnesium keeps tantalum from oxidizing, improves its purity, and raises the temperature at which it operates as a superconductor. All three may increase tantalum’s ability to hold onto quantum information in qubits.

This work builds on earlier studies in which a team from Brookhaven’s Center for Functional Nanomaterials (CFN), Brookhaven’s National Synchrotron Light Source II (NSLS-II), and Princeton University sought to understand the tantalizing characteristics of tantalum, and then worked with scientists in Brookhaven’s Condensed Matter Physics & Materials Science (CMPMS) Department and theorists at DOE’s Pacific Northwest National Laboratory (PNNL) to reveal details about how the material oxidizes.

Those studies showed why oxidation is an issue.

“When oxygen reacts with tantalum, it forms an amorphous insulating layer that saps tiny bits of energy from the current moving through the tantalum lattice. That energy loss disrupts quantum coherence—the material’s ability to hold onto quantum information in a coherent state,” explained CFN scientist Mingzhao Liu, a lead author on the earlier studies and the new work.

While the oxidation of tantalum is usually self-limiting—a key reason for its relatively long coherence time—the team wanted to explore strategies to further restrain oxidation to see if they could improve the material’s performance.

“The reason tantalum oxidizes is that you have to handle it in air and the oxygen in air will react with the surface,” Liu explained. “So, as chemists, can we do something to stop that process? One strategy is to find something to cover it up.”

All this work is being carried out as part of the Co-design Center for Quantum Advantage (C2QA), a Brookhaven-led national quantum information science research center. While ongoing studies explore different kinds of cover materials, the new paper describes a promising first approach: coating the tantalum with a thin layer of magnesium.

Read more on BNL website

Image: Chenyu Zhou, a research associate in the Center for Functional Nanomaterials (CFN) at Brookhaven National Laboratory and first author on the study, with Mingzhao Liu (CFN), Yimei Zhu (CMPMS), and Junsik Mun (CFN and CMPMSD), at the DynaCool Physical Property Measurement System (PPMS) in CFN. The team used this tool to make tantalum thin films with and without a protective magnesium layer so they could determine whether the magnesium coating would minimize tantalum oxidation.

Credit: Jessica Rotkiewicz/Brookhaven National Laboratory

Transition metal insulators: The origin of colour

Transition metal insulators: The origin of colour

In a theoretical study, researchers have explained the vibrant colours of two compounds whose electronic properties seemingly prohibit such colouring. The hues exhibited by the two insulators originate from transitions in the spins of the electrons, which modify the way the materials absorb and reflect light in such a way as to create the bright colours. The theoretical framework employed by the team promises new insights in fields such as optoelectronics or in the study of qubits, the quantum bits used in quantum computers. 

Although colour is a familiar phenomenon, it is sometimes challenging to explain how the hues of certain materials come about. This is the case with insulators that contain transition metals. In these compounds, the energy gap between the valence band, in which the electrons are tightly bound to the atoms, and the conduction band, in which the electrons can move freely, is larger than the highest energy of photons of visible light—meaning that these materials should not absorb visible light. As the colour of a compound is complementary to the wavelengths it absorbs, we should thus perceive these insulators as being transparent instead of coloured. 

A team of researchers including the head of the European XFEL Theory group, Alexander Lichtenstein, now used two complementary theoretical methods to study the origin of colour in two typical transition metal insulators: nickel(II) oxide (NiO)—a green compound used in the production of ceramics and nickel steel as well as in thin-film solar cells, nickel–iron batteries, and fuel cells—and manganese(II) fluoride (MnF2), a pink material employed in the manufacture of special kinds of glass and lasers.

Read more on XFEL website

Image: Visualization of the orbital character of low-laying excitons in NiO, corresponding to a local ‘Frenkel’ exciton at an energy of 1.6 eV and a weakly bound, bright ‘Wannier-Mottâ’ exciton at an energy of 3.6 eV

Connecting the dots between material properties and qubit performance

Connecting the dots between material properties and qubit performance

Engineers and materials scientists studying superconducting quantum information bits (qubits)—a leading quantum computing material platform based on the frictionless flow of paired electrons—have collected clues hinting at the microscopic sources of qubit information loss. This loss is one of the major obstacles in realizing quantum computers capable of stringing together millions of qubits to run demanding computations. Such large-scale, fault-tolerant systems could simulate complicated molecules for drug development, accelerate the discovery of new materials for clean energy, and perform other tasks that would be impossible or take an impractical amount of time (millions of years) for today’s most powerful supercomputers.

An understanding of the nature of atomic-scale defects that contribute to qubit information loss is still largely lacking. The team helped bridge this gap between material properties and qubit performance by using state-of-the-art characterization capabilities at the Center for Functional Nanomaterials (CFN) and National Synchrotron Light Source II (NSLS-II), both U.S. Department of Energy (DOE) Office of Science User Facilities at Brookhaven National Laboratory. Their results pinpointed structural and surface chemistry defects in superconducting niobium qubits that may be causing loss. 

Read more on the BNL website

Image: Scientists performed transmission electron microscopy and x-ray photoelectron spectroscopy (XPS) at Brookhaven Lab’s Center for Functional Nanomaterials and National Synchrotron Light Source II to characterize the properties of niobium thin films made into superconducting qubit devices at Princeton University. A transmission electron microscope image of one of these films is shown in the background; overlaid on this image are XPS spectra (colored lines representing the relative concentrations of niobium metal and various niobium oxides as a function of film depth) and an illustration of a qubit device. Through these and other microscopy and spectroscopy studies, the team identified atomic-scale structural and surface chemistry defects that may be causing loss of quantum information—a hurdle to enabling practical quantum computers.

Disorder brings out quantum physical talents

Disorder brings out quantum physical talents

Quantum effects are most noticeable at extremely low temperatures, which limits their usefulness for technical applications. Thin films of MnSb2Te4, however, show new talents due to a small excess of manganese. Apparently, the resulting disorder provides spectacular properties: The material proves to be a topological insulator and is ferromagnetic up to comparatively high temperatures of 50 Kelvin, measurements at BESSY II show.  This makes this class of material suitable for quantum bits, but also for spintronics in general or applications in high-precision metrology.

Quantum effects such as the anomalous quantum Hall effect enable sensors of highest sensitivity, are the basis for spintronic components in future information technologies and also for qubits in quantum computers of the future. However, as a rule, the quantum effects relevant for this only show up clearly enough to make use of them at very low temperatures near absolute zero and in special material systems.

Read more on the HZB website

Image: The Dirac cone is typical for topological insulators and is practically unchanged on all 6 images (ARPES measurements at BESSY II). The blue arrow additionally shows the valence electrons in the volume. The synchrotron light probes both and can thus distinguish the Dirac cone at the surface (electrically conducting) from the three-dimensional volume (insulating).

Credit: © HZB