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MOSCOW, March 15. /TASS /. A group of scientists from MIPT (Moscow Institute of Physics and Technology) and Moscow State University has developed a fundamentally new type of memory cell based on superconductors — this type of memory will be able to work hundreds of times faster than the types of memory devices commonly used today, MIPT said in a press-release.
"With the operational function that we have proposed in these memory cells, there will be no need for time-consuming magnetization and demagnetization processes. This means that read and write operations will take only a few hundred picoseconds, depending on the materials and the geometry of the particular system, while conventional methods take hundreds or thousands of times longer than this," said the corresponding author of the study, Alexander Golubov, the Head of MIPT’s Laboratory of Quantum Topological Phenomena in Superconducting Systems.
Golubov and his colleagues have proposed creating basic memory cells based on quantum effects in "sandwiches" of a superconductor — dielectric (or other insulating material) — superconductor. The electrons in these "sandwiches" (they are called "Josephson junctions") are able to tunnel from one layer of a superconductor to another, passing through the dielectric like balls passing through a perforated wall.
Today, Josephson junctions are used both in quantum devices and conventional devices. For example, superconducting qubits are used to build the D-wave quantum system. Josephson junctions with ferromagnets used as the middle of the "sandwich" are currently of greatest practical interest. In memory elements that are based on ferromagnets the information is encoded in the direction of the magnetic field vector in the ferromagnet. However, there are two fundamental flaws with this process: firstly, the low density of the "packaging" of the memory elements - additional chains need to be added to provide extra charge for the cells when reading or writing data, and secondly the magnetization vector cannot be changed quickly, which limits the writing speed.
The group of physicists from MIPT and MSU proposed encoding the data in Josephson cells in the value of the superconducting current. By studying the superconductor-normal metal/ferromagnet-superconductor-insulator-superconductor junctions, the scientists discovered that in certain longitudinal and transverse dimensions the layers of the system may have two energy minima, meaning they are in one of two different states. These two minima can be used to record data - zeros and ones.
Diagram of the junction. S – superconductor, I – insulating tunnel barrier, F – ferromagnet, N – normal metal, shaded area – potential barrier arising in the superconducting zoneMIPT’s Laboratory of Quantum Topological Phenomena in Superconducting Systems
In order to switch the system from "zero" to "one" and back again, the scientists have suggested using injection currents flowing through one of the layers of the superconductor. They propose to read the status using the current that flows through the whole structure. These operations can be performed hundreds of times faster than measuring the magnetization or magnetization reversal of a ferromagnet.
"In addition, our method requires only one ferromagnetic layer, which means that it can be adapted to so-called single flux quantum logic circuits, and this means that there will be no need to create an entirely new architecture for a processor. A computer based on single flux quantum logic can have a clock speed of hundreds of gigahertz, and its power consumption will be dozens of times lower," said Golubov.
The study was supported by the Russian Science Foundation Project No. 15-12-300, and Mega-Grant No. 14Y26.31.0007 of the Government of the Russian Federation for state supported research conducted under the supervision of leading scientists.
Article published in the journal Applied Physics Letters.
Superconducting currents when reading various states of the memory cell. The greater current the larger arrowMIPT’s Laboratory of Quantum Topological Phenomena in Superconducting Systems