High-Fidelity Measurements of Josephson Phase Qubits

Erik A. Lucero, M. Ansmann, Radoslaw C. Bialczak +7 more

A. Sadeghi, Hesam Zandi, Sina Ataollah Khorasani

We present a new configuration concept in which two similar Josephson junctions are coupled through a capacitor placed in parallel to a dc-superconducting quantum interference device (SQUID) to improve the characteristics of phase qubits. In real coupled quantum systems, because of mutual effects such as crosstalk, entangled quantum states cannot be independently measured. The proposed two-qubit system is demonstrated to have a negligible crosstalk, obtained from the application of a single measurement pulse and an appropriate external flux to one of the junctions and the dc-SQUID, respectively. Surprisingly, the theoretically predicted fidelity for a single-qubit design increases to 99.99% for typical Josephson junction parameters. The high-fidelity measurement is reached even with pulse times less than 1 ns, while the highest fidelities reported so far are obtained by wide temporal pulses through single-shot current pulse measurements. This is accomplished by proper coupling capacitance, which determines the coupling time, the measurement time, and their proportion. Furthermore, the ratio of decoherence time for this structure with respect to the one belonging to a typical qubit is found to be more than 1.42.

Tobias Wirth, Jürgen Lisenfeld, Alexander V. Lukashenko +2 more

Matthias Steffen, M. Ansmann, R. McDermott +7 more

We introduce a new design concept for superconducting phase quantum bits (qubits) in which we explicitly separate the capacitive element from the Josephson tunnel junction for improved qubit performance. The number of two-level systems that couple to the qubit is thereby reduced by an order of magnitude and the measurement fidelity improves to 90%. This improved design enables the first demonstration of quantum state tomography with superconducting qubits using single-shot measurements.

Baleegh Abdo, Nicholas T. Bronn, Oblesh Jinka +8 more

Abstract Nonreciprocal microwave devices play critical roles in high-fidelity, quantum-nondemolition (QND) measurement schemes. They impose unidirectional routing of readout signals and protect the quantum systems from unwanted noise originated by the output chain. However, cryogenic circulators and isolators are disadvantageous in scalable superconducting architectures because they use magnetic materials and strong magnetic fields. Here, we realize an active isolator formed by coupling two nondegenerate Josephson mixers in an interferometric scheme and driving them with phase-shifted, same-frequency pumps. By incorporating our Josephson-based isolator into a superconducting qubit setup, we demonstrate fast, high-fidelity, QND measurements of the qubit while providing 20 dB of protection within a bandwidth of 10 MHz against amplified noise reflected off the Josephson amplifier in the output chain. A moderate reduction of 35% is observed in T2E when the Josephson-based isolator is turned on. Such a moderate degradation can be mitigated by minimizing heat dissipation in the pump lines.

Erik A. Lucero, M. Ansmann, Radoslaw C. Bialczak +8 more