Lasing and cooling with Josephson qubits

speaker: Alexander Shnirman (University of Karlsruhe)

abstract: Lasing and cooling with Josephson qubits Superconducting qubits coupled to electric or nanomechanical resonators display effects previously studied in quantum electrodynamics (QED) and extensions thereof. Here we study a driven qubit coupled to a low-frequency tank circuit. When the qubit is driven to perform Rabi oscillations, with Rabi frequency in resonance with the oscillator, the latter can be driven far from equilibrium. Blue detuned driving leads to a population inversion in the qubit and strong enhancement of the population of oscillator modes, reaching for sufficiently high quality circuits lasing behavior (``single-atom laser"). For red detuning the qubit cools the oscillator. This behavior persists at the symmetry point where the qubit-oscillator coupling is quadratic and decoherence effects are minimized. Here the system realizes a ``single-atom-two-photon laser". In another parameter regime we demonstrate Sisyphus cooling as well as its exact opposite, Sisyphus amplification. The combination of red-detuned driving of the coupled system with spontaneous emission processes, creates a situation where in each oscillation cycle of the LC circuit energy is transferred from the LC circuit to the qubit, similar to Sisyphus who always had to roll a stone uphill. For blue-detuning the same mechanism creates excitations in the LC circuit with a tendency towards lasing and the characteristic line-width narrowing. In this regime ``lucky Sisyphus'' always rolls the stone downhill. Parallel to the experimental demonstration we analyze the system theoretically and find quantitative agreement, which allows us to extract the system parameters.

timetable:
Fri 5 Dec, 14:30 - 15:00, Sala Stemmi
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