Low temperature measurement of SiGe properties for superconducting quantum circuits*

Superconducting circuits is a promising technology for building a scalable quantum computer. One of the shortcomings of this technology is that there is at the moment no technology available to transport quantum information in and out of the cryogenic environment that the circuit operates in. In order to transfer quantum information from the chip out to a room temperature environment the signal has to be converted from single microwave photons to something that will not be awash by the thermal noise at 300K. Lately there has been several proposals for how to enable such conversion using microwave to optical transducer. One approach is to exploit the electro-optical effect in strained SiGe. SiGe is a mature technology for on chip optics but is not well explored for superconducting circuits. We have fabricated quantum circuits in the form of transmon style quantum bits (qubits) on a substrate containing SiGe with a thin buffer layer of epitaxial Si. We find that that the introduction of SiGe in the substrate stack does not degrade the coherence properties of the transmon qubit. From the coherence measurements we are able to extract bounds for the loss tangent of SiGe at millikelvin temperatures.