Speaker
Description
We experimentally investigated the prospect for using superconducting transmon qubits as cryogenic quantum sensors of ionizing radiation. We interrogated a six-qubit chip with resonators inductively coupled on both sides of a central transmission line in a hanger geometry. Measurements were performed without and with a source of ionizing radiation applied, external to the cryostat at room temperature, consisting of the isotope Cobalt 60 gamma emissions at 1.17 MeV and 1.33 MeV. Radiative flux was controlled by varying the height of the source relative to the bottom of the cryostat. Without applied external radiation, we measured T1 relaxation times of 200 to 350 μs and Ramsey T2R dephasing times of 100 to 150 μs for the six qubits. Measurements performed without and with applied external radiation include power dependance of readout, state discrimination in the IQ plane, T1 traces, Ramsey traces, and single point in a T1 or Ramsey trace measurements with variable durations of applied radiation.
We detail how, when exposed to a strong flux of gamma rays, many aspects of the qubit measurement break down. As a specific example, we describe voltage displacement in the IQ plane of the qubit state readout signal under the effects of radiation. This is done by discriminating the ground and excited state measurements using an unsupervised learning algorithm, fitting the measurements to a standard resonator form [Probst], and noting the frequency change in the ground-state cluster. With radiation we observe significant voltage displacement of the resonator frequency associated with the qubit ground state but no discernable voltage displacement of the resonator frequency associated with the qubit excited state. This experimental observation is consistent with our measurement for which the interrogating tone is set to the resonator frequency associated with the qubit ground state. The analysis indicates a decrease in frequency of the readout resonator consistent with radiation induced quasi-particles in the film increasing film inductance. Applying these results, we describe prospective designs for superconducting transmon qubits as cryogenic detectors of ionizing radiation.
This work was supported by the LDRD program at SNL, a multimission laboratory managed and operated by NTESS under DOE NNSA contract DE-NA0003525. This work was performed in part at CINT, a U.S. DOE BES user facility. The six-qubit chip was fabricated and provided by the SQUILL Foundry at MIT Lincoln Laboratory, with funding from the LPS Qubit Collaboratory.
[Probst] Probst, S., Song, F. B., Bushev, P. A., Ustinov, A. V. & Weides, M. Efficient and robust analysis of complex scattering data under noise in microwave resonators. Review of Scientific Instruments 86, 024706 (2015).
