Russian and Japanese scientists have created a superconducting structure with the properties of a single atom
Scientists of the Physics Institute named after PN Lebedev RAS (FIAN), in collaboration with Japanese colleagues, managed to create a superconducting chip that reproduces the “work” of a single atom, and even demonstrated with its help a number of quantum effects, including lasing.
The current prototype of the device, developed at the research laboratory of the NEC corporation, where FIAN employees work under contract, is a metal film structure on an ordinary silicon chip. It was made by the method of electronic lithography – by sputtering aluminum films at various angles through a germanium mask formed by reactive ion etching. The object is rather complex, consisting of various elements: waveguide lines supplying and removing microwave radiation, a resonator, and an island superconducting structure, which contains nanometer tunnel junctions.
It is this island that can be called an artificial atom. The analogy with the atom is that discrete energy levels arise for a charge placed here, a Cooper pair, or a single electron. In this case, the states that are capable of occupying charges can be pre-constructed by the researcher. “Unlike ordinary ones, artificial atoms with predetermined properties can be made on a chip,” says one of the authors of the work, Yuri Pashkin. – The distance between energy levels can be set in a wide range – for example, voltage or magnetic field. Due to their large dimensions compared to ordinary atoms, artificial ones interact much more strongly with electromagnetic radiation. These properties make it possible to study quantum optics on a chip in the limiting case when the optical medium is reduced to a single atom. ”
By changing the parameters of an artificial atom and its connection with external elements, scientists can form “their” energy levels, different from those created by nature in the elements listed in the periodic table. Having created such an atom, it can be used as an element of a technical device – for example, a quantum generator. To do this, it is necessary to ensure the so-called inverse population, that is, to make sure that the population of the upper energy level exceeds the population of the level lying below.
Such an experiment has already been carried out. “The active optical medium in the form of a single atom is coherently connected to the microwave cavity,” explains Oleg Astafiev, NEC employee. – Inverse population in an artificial atom was created by passing a direct current (current pumping). When the rate of generation of photons by an atom exceeded the rate of their decay in the cavity, the accumulation and emission of photons took place in it, which was then amplified and detected. The strong bond between the atom and the resonator here, in contrast to conventional lasers and masers, leads to a thresholdless lasing regime. ”
The radiation of conventional laser devices is formed in a wide spectral range, including optical. In artificial atoms, the radiation frequency is much lower than the frequency of visible light (in general, it depends on the size of the energy gap of the superconductor). Thus, the first created device, where an aluminum film with a superconducting gap of about 0.2 MeV serves as a superconductor, operates at a frequency of about 0.01 THz. Generators of such frequencies can find application in computers based on superconducting elements (including quantum ones), since they are easily combined with them. If a film made of a high-temperature superconductor is used, the radiation frequency can be increased by 1–2 orders of magnitude, and then it will fall into the terahertz range.
This range is an intermediate region that is difficult to access both for classical methods of radiation generation and for quantum-optical methods (lasers). However, a whole class of extremely important applied problems is associated with its development. Spectroscopy in the THz range can be used in a wide variety of fields – from the detection of explosives, narcotic substances and toxicants to medical express diagnostics of the air exhaled by a person. “The terahertz range is poorly mastered,” says Vladimir Pudalov, head of the department of high-temperature superconductivity and nanostructures at the Lebedev Physical Institute. – And now a way has been found how to approach him with all grace. Fifteen years ago, physicists learned to create artificial atoms and watch how a single electron, two electrons, three, a multielectron system lives in them. But that was a game, now it is a working device. This is an exceptional success. ”
Prepared on the basis of the materials of the Physics Institute. P.N. Lebedev RAS.