We report on long-term measurements of a highly coherent, non-tunable superconducting transmon qubit, revealing low-frequency burst noise in coherence times and qubit transition frequency. We achieve this through a simultaneous measurement of the qubit's relaxation and dephasing rate as well as its resonance frequency. The analysis of correlations between these parameters yields information about the microscopic origin of the intrinsic decoherence mechanisms in Josephson qubits. Our results are consistent with a small number of microscopic two-level systems located at the edges of the superconducting lm, which is further con rmed by a spectral noise analysis. arXiv:1901.05352v2 [cond-mat.supr-con]
We report on the investigation of a superconducting anharmonic multi-level circuit that is coupled to a harmonic readout resonator. We observe multi-photon transitions via virtual energy levels of our system up to the fifth excited state. The back-action of these higher-order excitations on our readout device is analyzed quantitatively and demonstrated to be in accordance with theoretical expectation. By applying a strong microwave drive we achieve multi-photon dressing within our anharmonic circuit which is dynamically coupled by a weak probe tone. The emerging higher-order Rabi sidebands and associated Autler-Townes splittings involving up to five levels of the investigated anharmonic circuit are observed. Experimental results are in good agreement with master equation simulations.
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 directẐ coupling between neighboring qubits, being a pending quest in the field of quantum simulation.Quantum bits based on superconducting circuits are leading candidates for constituting the basic building block of a prospective quantum computer. A common element of all superconducting qubits is the Josephson junction. The nonlinearity of Josephson junctions generates an anharmonic energy spectrum in which the two lowest energy states can be used as the computational basis 1,2 . Over the last decade there has been a two order of magnitude increase in coherence times of superconducting qubits. This tremendous improvement allowed for demonstration of several major milestones in the pursuit of scalable quantum computing, such as the control and entanglement of multiple qubits 3,4 . Further increases in coherence times will eventually allow for building a fault tolerant quantum computer with a reasonable overhead in terms of error correction, as well as implementing novel quantum simulation schemes by accessing wider experimental parameter ranges 5 . While superconducting qubits embedded in a 3D cavity 6 have shown coherence times in excess of 100 µs 7 , this approach may impose some constraint on the scalability of quantum circuits. Since the Josephson junction itself does not limit qubit coherence 6 , comparably long lifetimes can also be achieved in a planar geometry by careful circuit engineering.In this paper, we present the design and characterization of a superconducting quantum circuit comprising a concentric transmon qubit 8 , schematically depicted in Fig. 1(a). The two capacitor pads forming the transmon's a) Electronic mail: jochen.braumueller@kit.edu. large shunt capacitance are implemented by a central disk island and a concentrically surrounding ring. The two islands are interconnected by two Josephson junctions forming a gradiometric SQUID. A 50 Ω impedance matched on-chip flux bias line located next to the qubit allows for fast flux tuning of the qubit frequency due to the imposed asymmetry. This guarantees high experimental flexibility and enables full tomographic control. The gradiometric flux loop design reduces the sensitivity to external uniform magnetic fields and thus to external flux ...
We present an argon ion beam milling process to remove the native oxide layer forming on aluminum thin films due to their exposure to atmosphere in between lithographic steps. Our cleaning process is readily integrable with conventional fabrication of Josephson junction quantum circuits. From measurements of the internal quality factors of superconducting microwave resonators with and without contacts, we place an upper bound on the residual resistance of an ion beam milled contact of 50 mΩ • µm 2 at a frequency of 4.5 GHz. Resonators for which only 6% of the total foot-print was exposed to the ion beam milling, in areas of low electric and high magnetic field, showed quality factors above 10 6 in the single photon regime, and no degradation compared to single layer samples. We believe these results will enable the development of increasingly complex superconducting circuits for quantum information processing.
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