Of superconducting qubits it is possible to create two giant artificial atom and placed them in a common waveguide. Such atoms are protected from radiative decay and can communicate with each other. The quantum coupling constant is strong enough that allows you to confuse the atoms, or in other words, to realize dvuhmestnoe logical operation. It was done scientists from MIT, what is reported in the paper on arXiv.org

Quantum optics exploring how quantum objects interact with classical or quantum electromagnetic waves — in other words, with light. Quantum optics — a rather well-established field of physics and has about 60 years of research. Therefore, of great interest to physicists is the emergence and development of a completely new concept, which has previously not been studied experimentally. We are talking about the so-called giant atoms. They are able to interact with remote from each other points of the electromagnetic waves, oscillations which greatly differ in phase.

The atoms of any chemical element, which usually works quantum optics have quite certain radius. As a rule, it is equal to 10-10 meters. The wavelength of light, the frequency of which corresponds to one of the atomic transitions (or easier — resonant light) usually fall in the range of 10-6-10-7 meters. Being 1000 times smaller than the wavelength, the atom can be considered as a point object, and the physical processes of interaction of atom with radiation field are determined by the value of the field in one point having the coordinates of the atom (and maybe changing his position). This assumption is known as the dipole approximation, and until recently it almost did not know of exceptions, including remaining valid for Rydberg atoms.

The artificial atom called any quantum system, e.g. quantum dot, or superconducting qubit, if it is associated with radiation modes in a waveguide or resonator. Quantum-optical phenomena in such systems is extremely unusual because the coupling constant between the qubit and the field g is much larger than for the case of “natural” atoms. Superconducting qubits helped controlled to create N-photon States and even generate an arbitrary quantum light state in the microwave resonator, that has still not been able to do in other systems.

The situation with the dipole approximation drastically changes if we are not talking about “natural” and the artificial atoms. For the first time this became relevant after the successful implementations of superconducting qubits is related to the electromagnetic wave and a surface acoustic wave (saw) on the surface of a piezoelectric is presented. Earlier we wrote about hown physics under the direction of Oleg Astafiev first resonant tied superconducting qubit and the saw resonator in the quantum regime. The phase saw velocity by 5-6 orders of magnitude smaller than the speed of light. Therefore, for a typical qubit with frequencies of 5-10 GHz and the size of the order of hundreds of microns it turns out that the length of the saw-frequency qubits is equal to 10-7 m, and the dipole approximation is grossly violated. Moreover, we are dealing with the reverse situation: the size of the atom is orders of magnitude greater than the resonant length of the surfactant. Therefore, it is appropriate to call such an atom of a giant.