Andreas Kurcz

Hybrid quantum magnetism in circuit QED: from spin-photon waves to many-body spectroscopy.

We introduce a model of quantum magnetism induced by the nonperturbative exchange of microwave photons between distant superconducting qubits. By interconnecting qubits and cavities, we obtain a spin-boson lattice model that exhibits a quantum phase transition where both qubits and cavities spontaneously polarize. We present a many-body ansatz that captures this phenomenon all the way, from a the perturbative dispersive regime where photons can be traced out, to the nonperturbative ultrastrong coupling regime where photons must be treated on the same footing as qubits. Our ansatz also reproduces the low-energy excitations, which are described by hybridized spin-photon quasiparticles, and can be probed spectroscopically from transmission experiments in circuit QED, as shown by simulating a possible experiment by matrix-product-state methods.

Energy concentration in composite quantum systems

The spontaneous emission of photons from optical cavities and from trapped atoms has been studied extensively in the framework of quantum optics. Theoretical predictions based on the rotating wave approximation (RWA) are, in general, in very good agreement with experimental findings. However, current experiments aim at combining better and better cavities with large numbers of tightly confined atoms. Here we predict an energy concentrating mechanism in the behavior of such a composite quantum system which cannot be described by the RWA. Its result is the continuous leakage of photons through the cavity mirrors, even in the absence of external driving. We conclude with a discussion of the predicted phenomenon in the context of thermodynamics.

Rate-equation approach to cavity-mediated laser cooling

The cooling rate for cavity-mediated laser cooling scales as the Lamb-Dicke parameter

Rotating wave approximation and entropy

\ensuremath{\eta}

Sonoluminescence and quantum optical heating

squared. A proper analysis of the cooling process hence needs to take terms up to

Concentrating Energy by Measurement

{\ensuremath{\eta}}^{2}

Laser cooling of a trapped particle with increased Rabi frequencies

in the system dynamics into account. In this paper, we present such an analysis for a standard scenario of cavity-mediated laser cooling with

Comparing cavity and ordinary laser cooling within the Lamb–Dicke regime

\ensuremath{\eta}\ensuremath{\ll}1

Driven spin-boson Luttinger liquids

. Our results confirm that there are many similarities between ordinary and cavity-mediated laser cooling. However, for a weakly confined particle inside a strongly coupled cavity, which is the most interesting case for the cooling of molecules, numerical results indicate that more detailed calculations are needed to model the cooling process accurately.

The Interspersed Spin Boson Lattice Model

This Letter studies composite quantum systems, like atom-cavity systems and coupled optical resonators, in the absence of external driving by resorting to methods from quantum field theory. Going beyond the rotating wave approximation, it is shown that the usually neglected counter-rotating part of the Hamiltonian relates to the entropy operator and generates an irreversible time evolution. The vacuum state of the system is shown to evolve into a generalized coherent state exhibiting entanglement of the modes in which the counter-rotating terms are expressed. Possible consequences at observational level in quantum optics experiments are currently under study.