Split transmons have been widely used in making quantum processors for building scalable and fault-tolerant superconducting quantum computers. With tunable external flux threading the superconducting quantum interference devices (SQUIDs) of qubits, split transmons have a degree of freedom for tuning frequencies but introduce flux noise as an important dephasing factor at the same time. Here, an improved version of a transmon is proposed and is called a ``triple-Josephson-junction transmon'' (tri-JJ transmon). A tri-JJ transmon is obtained by shunting another Josephson junction to the SQUID of a split transmon, which can produce much more different local minima and maxima of qubit frequencies tuned by external flux as ``sweet spots'' than those of split transmons. The first-order effect of the flux noise at the sweet spots can be eliminated and the corresponding pure-dephasing times are estimated as about $200\phantom{\rule{4pt}{0ex}}\textmu{}\mathrm{s}$ by considering only critical current noise. Tri-JJ transmons are compatible with the process and the protocol of control and measurement of split transmons and can be competitive candidates for constructing a high-performance superconducting quantum processor.Split transmons have been widely used in making quantum processors for building scalable and fault-tolerant superconducting quantum computers. With tunable external flux threading the superconducting quantum interference devices (SQUIDs) of qubits, split transmons have a degree of freedom for tuning frequencies but introduce flux noise as an important dephasing factor at the same time. Here, an improved version of a transmon is proposed and is called a ``triple-Josephson-junction transmon'' (tri-JJ transmon). A tri-JJ transmon is obtained by shunting another Josephson junction to the SQUID of a split transmon, which can produce much more different local minima and maxima of qubit frequencies tuned by external flux as ``sweet spots'' than those of split transmons. The first-order effect of the flux noise at the sweet spots can be eliminated and the corresponding pure-dephasing times are estimated as about $200\phantom{\rule{4pt}{0ex}}\textmu{}\mathrm{s}$ by considering only critical current noise. Tri-JJ transmons are compatible with the process and the protocol of control and measurement of split transmons and can be competitive candidates for constructing a high-performance superconducting quantum processor.

Physical Review ADec 13, 20240 citations

Investigation of 1/f flux noise in SQUIDs and Superconducting Qubits

Antonio Puglielli, S. Sendelbach, Taylor Klaus +1 more

Bulletin of the American Physical SocietyMar 19, 20130 citations

Flux qubit in a dc SQUID with the 4 π period Josephson effect

Wenchao Huang, Qifeng Liang, Dao‐Xin Yao +1 more

We propose a superconducting flux qubit in a dc SQUID structure, formed by a conventional insulator Josephson junction and a topological nanowire Josephson junction with Majorana bound states. The zero-energy Majorana bound states transport 4 π period Josephson currents in the nanowire junction. The interplay between this 4 π period Josephson effect and the convectional 2 π period Josephson effect in the insulator junction induces a double-well potential energy landscape in the SQUID. As a result, the two lowest-energy levels of the SQUID are isolated from the other levels. These two levels show counterpropagating supercurrents, thus can be used as a flux qubit. We reveal that this flux qubit has the merits of stability to external noises, tolerance to the deviation of system parameters, and scalability to large numbers. Furthermore, we demonstrate how to couple this flux qubit with the Majorana qubit by tuning the junction parameters, and how to use this coupling to manipulate the Majorana qubit.We propose a superconducting flux qubit in a dc SQUID structure, formed by a conventional insulator Josephson junction and a topological nanowire Josephson junction with Majorana bound states. The zero-energy Majorana bound states transport 4 π period Josephson currents in the nanowire junction. The interplay between this 4 π period Josephson effect and the convectional 2 π period Josephson effect in the insulator junction induces a double-well potential energy landscape in the SQUID. As a result, the two lowest-energy levels of the SQUID are isolated from the other levels. These two levels show counterpropagating supercurrents, thus can be used as a flux qubit. We reveal that this flux qubit has the merits of stability to external noises, tolerance to the deviation of system parameters, and scalability to large numbers. Furthermore, we demonstrate how to couple this flux qubit with the Majorana qubit by tuning the junction parameters, and how to use this coupling to manipulate the Majorana qubit.

Europhysics Letters (EPL)May 1, 20153 citations

1/f Flux Noise in Josephson Phase Qubits

R. McDermott, Radek Bialczak, M. Ansmann +8 more

Bulletin of the American Physical SocietyMar 11, 20088 citations

Proposal for a flux qubit in a dc SQUID with the $4π$ period Josephson effectProposal for a flux qubit in a dc SQUID with the $4π$ period Josephson effect

Wenchao Huang, Qifeng Liang, Dao‐Xin Yao +1 more

Constructing qubits which are suitable for quantum computation remains a notable challenge. Here, we propose a superconducting flux qubit in a dc SQUID structure, formed by a conventional insulator Josephson junction and a topological nanowire Josephson junction with Majorana bound states. The zero energy Majorana bound states transport $4π$ period Josephson currents in the nanowire junction. The interplay between this $4π$ period Josephson effect and the convectional $2π$ period Josephson effect in the insulator junction induces a double-well potential energy landscape in the SQUID. As a result, the two lowest energy levels of the SQUID are isolated from other levels. These two levels show contradicting circulating supercurrents, thus can be used as a flux qubit. We reveal that this flux qubit has the merits of stability to external noises, tolerance to the deviation of system parameters, and scalability to large numbers. Furthermore, we demonstrate how to couple this flux qubit with the Majorana qubit by tuning the junction parameters, and how to use this coupling to manipulate the Majorana qubit.Constructing qubits which are suitable for quantum computation remains a notable challenge. Here, we propose a superconducting flux qubit in a dc SQUID structure, formed by a conventional insulator Josephson junction and a topological nanowire Josephson junction with Majorana bound states. The zero energy Majorana bound states transport $4π$ period Josephson currents in the nanowire junction. The interplay between this $4π$ period Josephson effect and the convectional $2π$ period Josephson effect in the insulator junction induces a double-well potential energy landscape in the SQUID. As a result, the two lowest energy levels of the SQUID are isolated from other levels. These two levels show contradicting circulating supercurrents, thus can be used as a flux qubit. We reveal that this flux qubit has the merits of stability to external noises, tolerance to the deviation of system parameters, and scalability to large numbers. Furthermore, we demonstrate how to couple this flux qubit with the Majorana qubit by tuning the junction parameters, and how to use this coupling to manipulate the Majorana qubit.

arXiv:1501.03315Jan 14, 20150 citations

Low-frequency Flux Noise in SQUIDs and Superconducting Qubits

S. Sendelbach, David J. Hover, A. Kittel +2 more

Bulletin of the American Physical SocietyMar 11, 20080 citations

Proposal for a flux qubit in a dc SQUID with the

4π

period Josephson effect

4π

period Josephson currents in the nanowire junction. The interplay between this

4π

period Josephson effect and the convectional

2π

period Josephson effect in the insulator junction induces a double-well potential energy landscape in the SQUID. As a result, the two lowest energy levels of the SQUID are isolated from other levels. These two levels show contradicting circulating supercurrents, thus can be used as a flux qubit. We reveal that this flux qubit has the merits of stability to external noises, tolerance to the deviation of system parameters, and scalability to large numbers. Furthermore, we demonstrate how to couple this flux qubit with the Majorana qubit by tuning the junction parameters, and how to use this coupling to manipulate the Majorana qubit.