An international group of scientists from Russia, Great Britain and Germany demonstrated an alternative qubit design that can be used to build a quantum computer. The main element of this design are superconductor nano-wires. Already in the first experiments, the new superconducting qubit proved to be no worse than the traditional qubits built on the Josephson junctions.
Scheme and drawing of a new qubit Collaborations of scientists from the Russian Quantum Center and NITU MISiS (Russia), the University of London and the National Physical Laboratory in Teddington (United Kingdom), the University of Karlsruhe and the Institute of Photonic Technologies (Germany), and the Moscow Institute of Physics and Technology and Skoltech (Russia) managed to create a fundamentally new qubit , based not on the Josephson junction, which is a discontinuity in a superconductor, but on a continuous superconducting nano-wire. The work of researchers
published in Nature Physics.
Scientists predict great achievements for a quantum computer. The computational principle laid down in its basis, even now, allows one to solve super-complex problems. Although the universal quantum computer itself has not yet been created, researchers can already simulate chemical compounds and materials using qubits. Therefore, many scientific groups are working to improve the elements of a quantum computer. The work on studying and improving the basic computing cell of a quantum computer - qubit - is especially intensive.
There are several approaches to creating qubits. For example, created qubits operating in the optical range. However, they are difficult to scale, in contrast to qubits on superconductors operating in the radio band and based on the so-called Josephson junctions. Each such transition is a rupture of a superconductor, and, more precisely, a dielectric layer through which electrons tunnel.
The new qubit is based on the effect of quantum phase slippage - controlled periodic destruction and restoration of superconductivity in an ultrathin (about 4 nm thick) nano-wire, which in the normal state has a rather large resistance. For the first time, this theoretically predicted effect was observed experimentally by the head of this work, Oleg Astafyev, he now runs the laboratory of the “Artificial Quantum Systems” MIPT in Russia and is a professor at the University of London and the National Physical Laboratory in Teddington in the UK. His pioneering work was published in the journal Nature in 2012.
Professor Alexey UstinovAs one of the authors of the new work, Alexei Ustinov, who in Russia manages the RCC group and manages the Superconducting Metamaterials laboratory of NUST MISIS, and in Germany is a professor at the Karlsruhe Institute of Technology, has now managed to create a new type of superconducting devices, much like SQUIDs (SQUID, Superconducting Quantum Interference Device (“superconducting quantum interferometer”, a supersensitive magnetometer based on Josephson junctions). Only instead of the magnetic field, the interference in the new device is caused by an electric field that changes the electric charge on the island between the two nano-wires. These wires play the role of Josephson junctions in the device, and they do not require the creation of discontinuities and can be made of a single layer of superconductor. As Alexey Ustinov noted, in this work we were able to show that this system can work as a charge interferometer. “If the wire is divided into two sections, a thickening is made in the center, then changing the charge on this thickening can, in fact, periodically modulate the process of quantum tunneling of magnetic quanta through the wire, which is observed in this work.” This is a key point, proving that the effect is manageable and coherent, and that it can be used to create a new generation of qubits.
SQUID technologies have already found their application in a number of medical scanning devices, such as magnetocardiographs and magnetoencephalographs, in instruments that detect nuclear magnetic resonance, as well as in geophysical and paleogeological methods of rock exploration. Therefore, it is possible that their dual charge SQUIDs can cause serious changes not only in the world of quantum computers.
According to Professor Ustinov, scientists face many more fundamental tasks related to the study of the work of the new qubit. However, it is already clear that we are talking about qubits with not less (and, maybe, more) functionality, but much simpler to make. “Now the main intrigue is whether it is possible to build on this principle the entire set of elements of superconducting electronics. - noted Professor Ustinov. - The device we received, in principle, is an electrometer and measures the charge induced on the island of a superconductor, with an error of a thousand times smaller than the charge of an electron. We can control it with the highest accuracy, since this charge is not quantized, but induced. ”
“Now we are studying qubits on the principle of phase slippage in my group in Karlsruhe, and the coherence times that we get on them turn out to be surprisingly high. - says Professor Ustinov. - So far they are not much more than in ordinary qubits, but we just started working, and there is a chance that they will be big. For example, there is another important topic of defects in qubits - we recently received a grant from Google on it - these defects arise in the dielectric, in the tunnel barrier of the Josephson junction. Defects are excited due to the fact that in this zone large electric fields, in fact, all the voltage drops on a scale of 2 nm. If we imagine that the same fall occurs in a uniform wire, and it is not known where, in uniform “blurring” throughout the superconductor, then the fields that will arise here are much smaller. This means that the defects that are in the material of the qubit, here, most likely, will not manifest. And this means that we will be able to get qubits with a higher coherence time, which will help to cope with one of the main problems of qubits — not too much time of their quantum “life”. ”