Tuned Transition from Quantum to Classical for Macroscopic Quantum States

The boundary between the classical and quantum worlds has been intensely studied. It remains fascinating to explore how far the quantum concept can reach with use of specially fabricated elements. Here we employ a tunable flux qubit with basis states having persistent currents of 1???A carried by a million pairs of electrons. By tuning the tunnel barrier between these states we see a crossover from quantum to classical. Released from nonequilibrium, the system exhibits spontaneous coherent oscillations. For high barriers the lifetime of the states increases dramatically while the tunneling period approaches the phase coherence time and the oscillations fade away.QN/Quantum NanoscienceApplied Science

Strong Coupling of a Quantum Oscillator to a Flux Qubit at Its Symmetry Point

A flux qubit biased at its symmetry point shows a minimum in the energy splitting (the gap), providing protection against flux noise. We have fabricated a qubit of which the gap can be tuned fast and have coupled this qubit strongly to an LC oscillator. We show full spectroscopy of the qubit-oscillator system and generate vacuum Rabi oscillations. When the gap is made equal to the oscillator frequency ?osc we find the largest vacuum Rabi splitting of ?0.1?osc. Here being at resonance coincides with the optimal coherence of the symmetry point.Kavli Institute of NanoscienceApplied Science