Measurements of Two-Cell Flux Qubits at Millikelvin Temperatures

Abstract

Abstract : We report on the study of Josephson junction two-cell flux qubits and phase qubits. We have made progress with fabrication of two-cell flux qubits based on small Al-AlOx-Al Josephson tunnel junctions. We succeeded in preparing junctions as small as 0.09 x 0.24 sq micrometres. We describe newly developed sample design which implements a silent two-cell qubit containing a fluxon trapping loop as a phase shifter. This design was submitted for fabrication at MIT Lincoln Lab foundry. We characterized several chips fabricated according to this design at MIT Lincoln Laboratory and observed a switching of the qubit from one classical state to the other. A new readout scheme which uses fast current pulses is under development. The states of superconducting quantum bits, as the Josephson flux and phase qubits, are distinguished by magnetic flux generated by a persistent current in a superconducting loop. This flux is read out by measuring the switching current of an inductively coupled SQUID detector. As the readout circuit contributes decoherence, its coupling to the qubit should be made as weak as possible. In the second part of this report, we demonstrate a new data evaluation method which maximizes the measurement contrast of the two qubit states, thus allowing to significantly decrease the coupling. The method is tested experimentally by measuring high-fidelity Rabi oscillations in phase qubits operated beyond the single-shot resolution limit. This technique is promising for multi-qubit circuits since it allows to read out the states of several qubits by a single weakly coupled SQUID. Our current work on Josephson phase qubits is focused on the study of temperature effects on decoherence. Using samples fabricated from different materials and origins, we measured Rabi and Ramsey oscillations in a wide range of temperatures. Unexpectedly, the decoherence times extracted from the data show only a very weak dependence on temperature.