Quantum two-level systems interacting with the surroundings are ubiquitous in nature. The interaction suppresses quantum coherence and forces the system towards a steady state. Suchdissipative processes are captured by the paradigmatic spin-boson model, describing a two-state particle, the „spin“, interacting with an environment formed by harmonic oscillators. A fundamental question to date is to what extent intense coherent driving impacts a strongly dissipative system. Here we investigate experimentally and theoretically a superconducting qubit strongly coupled to an electromagnetic environment and subjected to a coherent drive. This setup realizes the driven Ohmic spin-boson model. We show that the drive reinforces environmental suppression of quantum coherence, and that a coherent-to-incoherent transition can be achieved by tuning the drive amplitude. An out-of-equilibrium detailed balance relation is demonstrated. These results advance fundamental understanding of open quantum systems and bear potential for applications in quantum technologies.
The study of the interaction of light and matter has led to many fundamental discoveries as well as numerous important technologies. Over the last decades, great strides have been madein increasing the strength of this interaction at the single-photon level, leading to a continual exploration of new physics and applications. In recent years, a major achievement has been the demonstration of the so-called strong coupling regime, a key advancement enabling great progress in quantum information science. In this work, we demonstrate light-matter interaction over an order of magnitude stronger than previously reported, reaching a new regime of ultrastrong coupling (USC). We achieve this using a superconducting artificial atom tunably coupled to the electromagnetic continuum of a one-dimensional waveguide. For the largest values of the coupling, the spontaneous emission rate of the atom is comparable to its transition frequency. In this USC regime, the conventional quantum description of the atom and light as distinct entities breaks down, and a new description in terms of hybrid states is required. Our results open the door to a wealth of new physics and applications. Beyond light-matter interaction itself, the tunability of our system makes it promising as a tool to study a number of important physical systems such as the well-known spin-boson and Kondo models.
We report experiments on superconducting flux qubits in a circuit quantum electrodynamics (cQED) setup. Two qubits, independently biased and controlled, are coupled to a coplanar waveguideresonator. Dispersive qubit state readout reaches a maximum contrast of 72%. We find intrinsic energy relaxation times at the symmetry point of 7μs and 20μs and levels of flux noise of 2.6μΦ0/Hz‾‾‾√ and 2.7μΦ0/Hz‾‾‾√ at 1 Hz for the two qubits. We discuss the origin of decoherence in the measured devices. These results demonstrate the potential of cQED as a platform for fundamental investigations of decoherence and quantum dynamics of flux qubits.
We present experiments on the dynamics of a two-state parametric fluctuator in a superconducting flux qubit. In spectroscopic measurements, the fluctuator manifests itself as a doubletline. When the qubit is excited in resonance with one of the two doublet lines, the correlation of readout results exhibits an exponential time decay which provides a measure of the fluctuator transition rate. The rate increases with temperature in the interval 40 to 158 mK. Based on the magnitude of the transition rate and the doublet line splitting we conclude that the fluctuation is induced by quasiparticle tunneling. These results demonstrate the importance of considering quasiparticles as a source of decoherence in flux qubits.