Experimental Demonstration of a Robust and Scalable Flux Qubit (0909.4321v1)

Abstract: A novel rf-SQUID flux qubit that is robust against fabrication variations in Josephson junction critical currents and device inductance has been implemented. Measurements of the persistent current and of the tunneling energy between the two lowest lying states, both in the coherent and incoherent regime, are presented. These experimental results are shown to be in agreement with predictions of a quantum mechanical Hamiltonian whose parameters were independently calibrated, thus justifying the identification of this device as a flux qubit. In addition, measurements of the flux and critical current noise spectral densities are presented that indicate that these devices with Nb wiring are comparable to the best Al wiring rf-SQUIDs reported in the literature thusfar, with a $1/f$ flux noise spectral density at $1 $Hz of $1.3^{+0.7}_{-0.5} \mu\Phi_0/\sqrt{\text{Hz}}$. An explicit formula for converting the observed flux noise spectral density into a free induction decay time for a flux qubit biased to its optimal point and operated in the energy eigenbasis is presented.

Overview of the Experimental Demonstration of a Robust and Scalable Flux Qubit

This paper presents an experimental demonstration of a novel rf-SQUID flux qubit, which is specifically designed to be robust against fabrication variations in Josephson junction critical currents and device inductance while being scalable for large-scale quantum information processing. The pioneering design strategy employed in this research introduces the compound-compound Josephson junction (CCJJ) rf-SQUID, extending the rf-SQUID topology to incorporate additional complexity that is instrumental in addressing challenges related to architecture, circuit density, and fabrication variation.

Theoretical and Experimental Framework

The theoretical groundwork establishes the CCJJ rf-SQUID as a qubit whose parameters can be tuned in situ to compensate for fabrication deviations. The Hamiltonian governing the behavior of these devices is articulated through a sophisticated understanding of the quantum mechanical dynamics of superconducting loops, integrating multiple Josephson junctions. This device incorporates both a compound Josephson junction structure and an L-tuner to adjust inductance dynamically as coupling elements within a quantum processor are varied.

Experimentally, the paper provides comprehensive characterization data for the CCJJ structure, the L-tuner, and the capacitance of the rf-SQUID. Through meticulous design and calibration, parameters such as persistent current $I_q^p$ and tunneling energy $\Delta_q$ are measured against independently calibrated design values, affirming the quantum mechanical beneficial operations of the CCJJ rf-SQUID flux qubit.

Numerical and Empirical Results

The paper provides exhaustive empirical results demonstrating the robustness of the CCJJ rf-SQUID against variations in Josephson junctions and inductance. The calibrated CCJJ demonstrates controllable persistent currents and tunneling energies that align well with theoretical predictions, without requiring extensive recalibration for fluctuations in device parameters. The reported low frequency spectral density of flux noise, at $1.3^{+0.7}_{-0.5}\,\mu\Phi_0/\sqrt{\text{Hz}$ demonstrates that the rf-SQUID, despite its complexity and use of Nb wiring, achieves noise levels comparable to yet superior to typical Al wiring devices.

Implications and Future Prospects

The advancements introduced in this paper carry significant implications for both quantum optimization and gate model quantum computation processors. The enhancements in robustness and scalability suggest that CCJJ rf-SQUID flux qubits could play a pivotal role in the realization of large-scale quantum systems. Moreover, theoretical models predict promising outcomes for achievable coherence times in experimental setups, which are essential for practical application in quantum computing paradigms.

The research propels foundational work towards achieving desirable conditions in quantum processors, while further investigations may explore mapping these results across differing materials and scales of quantum devices. The methods and insights presented here integrate seamlessly into pathways advancing superconducting quantum technologies, offering a roadmap for future developments in flux qubit architectures. Advanced exploration into this novel qubit design could address current barriers to scalability and noise mitigation, potentially ushering in transformative shifts in quantum information processing capabilities.