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. We performed an experiment on a superconducting flux qubit, which is the 'classical' example of a macroscopic object that can be made to behave as a quantum particle. It is characterized by two states with opposite macroscopic currents in a loop. We were able to control the tunnel barrier between these states over a very wide range. We tuned qubit energy levels below the barrier and the same time e effectively cool the sample to near zero temperature. This allowed us to study the qubit behavior when we go from the range of low barriers and strong quantum tunneling to the regime where quantum tunneling gradually disappears as the barrier is increased. In particular, we manage to observe the natural quantum oscillations manifested in the tunneling of the long-living macroscopic magnetic moments. At very high barriers we see the how these oscillations fade away as the barrier is increased.
