Decoherence and dephasing in coupled Josephson-junction qubits

Michele Governale, Milena Grifoni, Gerd Schön

Christopher Nugroho, Vladimir Orlyanchik, Dale J. Van Harlingen

Yasunobu Nakamura, Fumiki Yoshihara, K. Harrabi +1 more

Norihito Kosugi, Shigemasa Matsuo, Toshiyuki Fujii +1 more

Current fluctuations in a Josephson phase qubit are considered to be a source of decoherence, especially for pure dephasing. One possible way of evading such decoherence is to employ an optimal operation point as used in flux and charge qubits, where the qubit is insensitive to the bias fluctuations. However, there is no optimal point in a phase qubit since qubit energy splitting becomes monotonically smaller with increasing the bias current. Here we propose a phase qubit with an optimal point by introducing qubit energy splitting that depends nonmonotonically on the current bias realized in capacitively coupled Josephson junctions. The effect of junction asymmetry on the optimal point is also investigated.

J. Q. You, Xuedong Hu, Sahel Ashhab +1 more

A flux qubit can have a relatively long decoherence time at the degeneracy point, but away from this point the decoherence time is greatly reduced by dephasing. This limits the practical applications of flux qubits. Here we propose a qubit design modified from the commonly used flux qubit by introducing an additional capacitor shunted in parallel to the smaller Josephson junction (JJ) in the loop. Our results show that the effects of noise can be considerably suppressed, particularly away from the degeneracy point, by both reducing the coupling energy of the JJ and increasing the shunt capacitance. This shunt capacitance provides a novel way to improve the qubit.

R. McDermott