Per Delsing, Chalmers University of Technology Sweden
We present 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. This type of coupling can be very strong, and in our case exceeds the coupling to any external electromagnetic mode. 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. The low speed of sound also allows us to observe the emission of the qubit in the time domain, which gives clear proof that the dominant coupling is acoustic.
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.
iii) 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 comparison to photons, SAW phonons have several striking features. Their speed of propagation is around 105 times lower, and the wavelength at a given frequency correspondingly shorter, indeed very similar to the wavelength of visible light. The slow speed means that qubits can be tuned much faster than SAWs traverse inter-qubit distances on a chip, which can enable new dynamic schemes for trapping and processing quanta. SAW phonons furthermore give access to a regime where the size of the qubit substantially exceeds the wavelength of the quanta it interacts with, opposite to the point-like interaction in photonic systems.