Sergey V. Remizov

A qubit system coupled to a resonator may be treated as a quantum metamaterial. Quantum nature of such a system results in large variety of non-trivial effects. In particular, we study a parametrically driven system of qubits coupled to a single-mode resonator and show that the energy dissipation in the resonator can enhance population of the excited state of the qubits and generate the entanglement between qubits. The energy dissipation in the qubit subsystem can enhance the number of photons generated from vacuum. We demonstrate that these effects survive in the steady state as well. The optimal parameters for these two cases of dissipation-induced quantum effects amplification are shown to be results of fine balance of different processes in this hybrid system.

It has been shown that superconducting qubit systems, having high tunability, can be used as a platform for the experimental study of various effects of nonstationary quantum electrodynamics in a cavity. In particular, the dynamic Lamb effect can be implemented owing to a nonadiabatic change in the effective coupling between the subsystem of qubits and a cavity. This effect is manifested in the excitation of a qubit (atom) at the change in the Lamb shift of its levels. It is remarkable that the effect of energy dissipation in such parametrically excited systems can be very nontrivial: dissipation in one of the subsystems of the hybrid system can enhance quantum effects in the other subsystem. This refers to various phenomena such as parametric qubit excitation, generation of photons from vacuum, and creation and confinement of finite entanglement of qubits.

The influence of individual impurities of Fe on the electronic properties of topological insulator Bi₂Se₃ is studied by Scanning Tunneling Microscopy. The microscope tip is used in order to remotely charge/discharge Fe impurities. The charging process is shown to depend on the impurity location in the crystallographic unit cell, on the presence of other Fe impurities in the close vicinity, and on the overall doping level of the crystal. We present a qualitative explanation of the observed phenomena in terms of tip-induced local band bending. Our observations evidence that the specific impurity neighborhood and the position of the Fermi energy with respect to both the Dirac point and bulk bands have to be taken into account when considering the electron scattering on the disorder in topological insulators.

We consider a dissipative evolution of a parametrically driven qubit-cavity system under the periodic modulation of coupling energy between two subsystems, which leads to the amplification of counter-rotating processes. We reveal a very rich dynamical behavior of this hybrid system. In particular, we find that the energy dissipation in one of the subsystems can enhance quantum effects in another subsystem. For instance, optimal cavity decay assists the stabilization of entanglement and quantum correlations between qubits even in the steady state and the compensation of finite qubit relaxation. On the contrary, energy dissipation in qubit subsystems results in enhanced photon production from vacuum for strong modulation but destroys both quantum concurrence and quantum mutual information between qubits. Our results provide deeper insights to nonstationary cavity quantum electrodynamics in the context of quantum information processing and might be of importance for dissipative quantum state engineering.

We study quantum effects of strong driving field applied to dissipative hybrid qubit-cavity system which are relevant for a realization of quantum gates in superconducting quantum metamaterials. We demonstrate that effects of strong and non-stationary drivings have significantly quantum nature and can not be treated by means of mean-field approximation. This is shown from a comparison of steady state solution of the standard Maxwell–Bloch equations and numerical solution of Lindblad equation on a density matrix. We show that mean-field approach provides very good agreement with the density matrix solution at not very strong drivings f < f* but at f > f* a growing value of quantum correlations between fluctuations in qubit and photon sectors changes a behavior of the system. We show that in regime of non-adiabatic switching on of the driving such a quantum correlations influence a dynamics of qubit and photons even at weak f.