Alexander Golubov, head of MIPT's Laboratory of Quantum Topological Phenomena in Superconducting Systems, said: "With the operational function that we have proposed in these memory cells, there will be no need for time-consuming magnetisation and demagnetisation 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."
Golubov and his colleagues have proposed creating basic memory cells based on quantum effects in ‘sandwiches’ – or Josephson junctions - comprised of a dielectric (or other insulating material) between two layers of superconducting material. The electrons are able to tunnel from one layer of a superconductor to another through the insulating layer.
Josephson junctions with ferromagnets used as the middle layer are currently said to be 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 magnetisation 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.
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 magnetisation or magnetisation reversal of a ferromagnet.
Golubov added:"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."
