Research

Nanomechanical resonators (NAMR) are of interest because of their combination of high natural frequencies and large quality factors together with a wide range of potential applications. To fully utilize the properties of NAMRs or to observe mesoscopic quantum phenomena, it is typically necessary to cool the NAMR to the mechanical ground state. Current cooling schemes typically require the so-called strong confinement or resolved regime of cooling, which is difficult to achieve experimentally.
In contrast, we have discussed NAMR ground state cooling in the weak-confinement or unresolved sideband case. The NAMR is embedded in the loop of a flux qubit. The qubit is modelled as a three-level quantum system in Λ configuration, and time-dependent magnetic fluxes (TDMF) are applied to the qubit in such a way that detrimental carrier excitations without change in the motional quantum number are suppressed by quantum interference. This is achieved by inducing electromagnetically induced transparency in the qubit, and thereby tailoring the cooling laser absorption spectrum in such a way that it has a minimum at the carrier excitation frequency. Our system allows to control the Lamb-Dicke parameters over a wide range via the applied magnetic field or the working point. As the interference-based scheme allows to apply strong cooling fields, fast and efficient ground state cooling can be achieved.

Superconducting qubits, and in particular flux qubits, are promising candidates for a number of applications in quantum information science. A precise and efficient preparation of such qubits is a key enabling technique for further research. Most current work is based on special area pulses, which makes the preparation sensitive to unertainties in the driving fields.
To overcome this limitation, we discussed robust coherent control schemes for the creation of states of interest in a pair of interacting flux qubits. Our main results are based on Stark chirped Raman adiabatic passage, which is a coherent control scheme well-known in atomic quantum optics. This scheme is ideally suited for flux qubits, because their transition frequencies can easily be dynamically adjusted around their optimum working point. In particular, we demonstrated the preparation of arbitrary superposition of different Bell states composed of the collective ground and excited states, robust against imperfections in the driving fields.