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arXiv:1304.2322 · Physical Review Letters

Coherent Josephson Qubit Suitable for Scalable Quantum Integrated Circuits

R. Barends, J. Kelly, A. Megrant, D. Sank, Evan R. Jeffrey, Y. Chen, Yi Yin, B. Chiaro, J. Mutus, C. Neill, P. J. J. O’Malley, P. Roushan, J. Wenner, T. White, Andrew N. Cleland, John M. Martinis

Published Aug 22, 2013• 776 citations
arXivHTMLPDF

Abstract

We demonstrate a planar, tunable superconducting qubit with energy relaxation times up to 44 μs. This is achieved by using a geometry designed to both minimize radiative loss and reduce coupling to materials-related defects. At these levels of coherence, we find a fine structure in the qubit energy lifetime as a function of frequency, indicating the presence of a sparse population of incoherent, weakly coupled two-level defects. We elucidate this defect physics by experimentally varying the geometry and by a model analysis. Our "Xmon" qubit combines facile fabrication, straightforward connectivity, fast control, and long coherence, opening a viable route to constructing a chip-based quantum computer.

DOI
10.1103/physrevlett.111.080502

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We demonstrate a planar, tunable superconducting qubit with energy relaxation times up to 44 μs. This is achieved by using a geometry designed to both minimize radiative loss and reduce coupling to materials-related defects. At these levels of coherence, we find a fine structure in the qubit energy lifetime as a function of frequency, indicating the presence of a sparse population of incoherent, weakly coupled two-level defects. We elucidate this defect physics by experimentally varying the geometry and by a model analysis. Our "Xmon" qubit combines facile fabrication, straightforward connectivity, fast control, and long coherence, opening a viable route to constructing a chip-based quantum computer.
Scalable superconducting qubit architectures demand qubits with long coherence times—a critical prerequisite for developing practical quantum computers. To achieve a compact circuit footprint, we have designed a superconducting qubit featuring a meander-structured coplanar waveguide capacitor, termed the “Smon.” Numerical simulations of the electric field distribution demonstrate that the Smon exhibits a significantly lower energy participation ratio (EPR) than both Xmon qubits and conventional rectangular planar transmon qubits. For an in situ comparison between Smons and conventional rectangular planar transmons, we propose a two-dimensional (2D) hybrid qubit array test structure. The capacitor electrodes of the devices are fabricated using tantalum. Measurements of the Smon’s coherence properties yield an energy relaxation time ( T 1 ) ranging from 30 to 359 µs and a dephasing time ( T 2 ) between 18 and 20 µs. These results confirm that superconducting quantum circuits with enhanced integration density and long coherence times are experimentally feasible, thereby offering a viable pathway toward high-performance large-scale superconducting quantum computers.
We present a planar qubit design based on a superconducting circuit that we call concentric transmon. While employing a straightforward fabrication process using Al evaporation and lift-off lithography, we observe qubit lifetimes and coherence times in the order of 10 μs. We systematically characterize loss channels such as incoherent dielectric loss, Purcell decay and radiative losses. The implementation of a gradiometric SQUID loop allows for a fast tuning of the qubit transition frequency and therefore for full tomographic control of the quantum circuit. Due to the large loop size, the presented qubit architecture features a strongly increased magnetic dipole moment as compared to conventional transmon designs. This renders the concentric transmon a promising candidate to establish a site-selective passive direct Ẑ coupling between neighboring qubits, being a pending quest in the field of quantum simulation.
We present a planar qubit design based on a superconducting circuit that we call concentric transmon. While employing a straightforward fabrication process using Al evaporation and lift-off lithography, we observe qubit lifetimes and coherence times in the order of 10us. We systematically characterize loss channels such as incoherent dielectric loss, Purcell decay and radiative losses. The implementation of a gradiometric SQUID loop allows for a fast tuning of the qubit transition frequency and therefore for full tomographic control of the quantum circuit. Due to the large loop size, the presented qubit architecture features a strongly increased magnetic dipole moment as compared to conventional transmon designs. This renders the concentric transmon a promising candidate to establish a site-selective passive direct Z coupling between neighboring qubits, being a pending quest in the field of quantum simulation.
This paper presents the design, simulation, and fabrication process of a superconducting single qubit utilizing Josephson junctions in both planar and flip-chip architectures. The Xmon qubit, characterized by its cross-shaped coplanar waveguide structure and high scalability, is employed to demonstrate the enhancement in coherence, connectivity, and control speed. The Xmon qubit was modeled and validated using Python, Ansys HFSS, and Sonnet software. The fabrication process, including the transmission line and Josephson junction, is also introduced.
Gate-tunable transmons (gatemons) employing semiconductor Josephson junctions have recently emerged as building blocks for hybrid quantum circuits. In this study, we present a gatemon fabricated in planar Germanium. We induce superconductivity in a two-dimensional hole gas by evaporating aluminum atop a thin spacer, which separates the superconductor from the Ge quantum well. The Josephson junction is then integrated into an Xmon circuit and capacitively coupled to a transmission line resonator. We showcase the qubit tunability in a broad frequency range with resonator and two-tone spectroscopy. Time-domain characterizations reveal energy relaxation and coherence times up to 75 ns. Our results, combined with the recent advances in the spin qubit field, pave the way towards novel hybrid and protected qubits in a group IV, CMOS-compatible material.
The remarkable advances of quantum computation technology with superconducting qubits based on circuit quantum electrodynamics (QED) architecture have been achieved by improving control, protection and measurement of the quantum states at the same time. At the heart of all these quantum operations, the significant enhancement of the qubit coherence time during the last decades was the key. Even after all these advances, the coherence and relaxation time of superconducting qubits still requires further improvements toward fault-tolerant quantum computation. Here, we report our observation of a strongly enhanced lifetime of an in-situ tunable superconducting transmon qubit on a silicon substrate that is embedded in a three-dimensional copper cavity. We measured a lifetime of the qubit of up to 84 µs, which is the best reported value of an in-situ tunable transmon on a silicon substrate. In our experiment, the in-situ frequency tunability over a broad range enabled the Purcell factor to be controlled continuously by detuning the qubit frequency against the resonator frequency in the strong dispersive regime. The silicon substrate has its own importance because the substrate should be fully compatible with the conventional semiconductor processes so that scalability and multi-chip module capability are guaranteed. In order to control the Purcell factor with another parameter, we displaced the qubit position in the cavity and observed a longer relaxation time with a smaller coupling coefficient. We believe that this systematic study and the control technique of the Purcell effect in the circuit QED design, together with minimizing the microwave photon loss through an improvement of the fabrication, will contribute to the realization of a practical large-scale quantum computer based on superconducting qubit technology.