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Surface acoustic waves interacting with superconducting qubits and two level fluctuators causing charge drift

PTB - Kolloquium

Two different topics will be discussed. In the first part we discuss a new type of mechanical quantum device, where propagating surface acoustic wave (SAW) phonons serve as carriers for quantum information. At the core of our device is a superconducting qubit, designed to couple to SAW waves in the underlying substrate through the piezoelectric effect. The SAW waves propagate freely on the surface of the substrate, and we use a remote electro-acoustic transducer to address the qubit acoustically. Three different experiments are presented: i) Exciting the qubit with an electromagnetic signal we can “listen” to the SAW phonons emitted by the qubit. ii) Reflecting a SAW wave off the qubit, we observe a nonlinear reflection with strong reflection at low power and low reflection at high power. ii) Exciting the qubit with both an electromagnetic signal and with a SAW signal, we can do two-tone spectroscopy on the qubit. In all of these experiments we find a good agreement between experiment and theory.

In the second part of the talk we discuss new experiments where two level fluctuators (TLFs) are studied with a single electron transistor (SET). By applying a large voltage step to the gate electrode of the SET, we bring the TLFs out of equilibrium, and we study how they relax back into equilibrium. This is seen as a slow charge drift persisting over many hours. Analyzing the charge drift, we can deduce the density of TLFs, and we can also conclude that the TLFs relax by quantum tunneling, rather than by thermal activation. 

M.V. Gustafsson et al., Science 346, 207 (2014)

A. Pourkabirian et al., Phys. Rev. Lett. 113, 256801 (2014)