Quantum bit from high-temperature superconductor
Jülich, 13. February 2020 – Researchers propose material for powerful and compact Quantum Computers.
In the process of pursuing a different goal, scientists from Jülich, Münster, and Moscow found something that could one day pave the way for Quantum Computers to move out of specialized laboratories and into wider use. The key is a material for qubits that does not need to be cooled to near absolute zero.
Quantum Computers are expected to be able to solve certain calculations much faster than the world's fastest supercomputers. This could be useful for a wide variety of problems - from optimized traffic control and the design of more efficient materials, to research into active ingredients for new medicines. However, Quantum Computers currently only exist as prototypes in a laboratory, or for special applications.
Various technological strategies are being simultaneously pursued to realize more powerful Quantum Computers. Qubits, the bits of Quantum Computers, can for instance consist of trapped ions or superconducting circuits. In both cases, elaborate cooling systems are needed to bring the qubits to temperatures of about -273 °C, which is slightly more than 0 Kelvin. They are as expensive as a single-family house and require more space than a large refrigerator.
Researchers from Jülich, Münster, and Moscow have now found that it may be possible to make superconducting qubits, not only from the usual low-temperature superconductors, but also from high-temperature superconductors. This would allow far cheaper cooling technology, which a small suitcase could contain. It should also be possible to accommodate a larger number of such qubits on a chip than before, and the achievable computing speed should increase by orders of magnitude. The latter is due, among other things, to the longer lifetime of the excited state of at least 20 milliseconds at 5 Kelvin.
Initially, the researchers - led by Prof. Rafal Dunin-Borkowski, director at the Jülich institutes Ernst Ruska-Centrum and Peter Grünberg Institute, and Jun.-Prof. Dr. Carsten Schuck from the University of Münster - were researching components for single-photon detectors, for which the lower cooling should be sufficient. Such detectors are needed, for example, for encrypting data using quantum cryptography. Schuck's research group has extensive experience in the development of single-photon detectors based on low-temperature superconductors.
The basis of the new detector was to be nanowire made of yttrium barium copper oxide (YBCO), a material that is already superconducting below a comparatively warm -181.15 °C. Jülich has many years of experience in producing high-quality thin films from this high-temperature superconductor and possesses unique equipment and methods for this purpose - some of which were developed in-house. The researchers at Jülich cut the required wires into shape from the thin films using a focused ion beam.
"We experimented by letting photons hit nanowires of different widths, and measured the resistance that this creates in the superconductor," reports physicist Dr. Matvey Lyatti, who initially conducted research on the project in Münster and later in Jülich. The detection of the photons is based on this principle. "But the results at widths below 100 nanometers did not meet our expectations."
As it turned out, quantum effects come to light at 12-13 kelvin: the superconducting nanowire only assumes selected energy states. These could be used to encode information. For conventional quantum bits, this requires temperatures several hundred times lower, which are much more costly to achieve.
"Our results were so surprising that we could hardly believe it ourselves," recalls Jülich physicist Dr. Irina Gundareva. But the measurements ultimately convinced even the initially skeptical reviewers of the now published results in Nature Communications.
The researchers will continue their work on YBCO nanowires and plan to develop nanowire-based superconducting quantum circuits in the coming years. The ultimate goal is to make a desktop Quantum Computer as compact as possible. They are also still pursuing their goal of novel superconducting single-photon detectors that can be cooled by compact cryocoolers. This is because the YBCO nanowires studied also proved suitable for this purpose and showed significant advantages over existing technology in terms of the required cooling temperature as well as temporal signal resolution.
M. Lyatti et al. (2019): Energy-level quantization and single-photon control of phase slips in YBa2Cu3O7–x nanowires. Nature Communications (07 February 2020); DOI: 10.1038/s41467-020-14548-x
Dr. Matvey Lyatti
Peter Grünberg Institute - Semiconductor Nanoelectronics (PGI-9)
Tel: 02461 61-6482
Dr. Irina Gundareva
Ernst Ruska-Center for Microscopy and Spectroscopy with Electrons (ER-C) - Physics of Nanoscale Systems (ER-C-1)/ Peter Grünberg Institute - Microstructure Research (PGI-5)
Tel: 02461 61-6642
Angela Wenzik, Scientific journalist
Tel: 02461 61-6048