The quantum Rabi model describes the fundamental mechanism of light-matter interaction. It consists of a two-level atom or qubit coupled to a quantized harmonic mode via a transversal interaction. In the weak coupling regime, it reduces to the well-known Jaynes–Cummings model by applying a rotating wave approximation. The rotating wave approximation breaks down in the ultra-strong coupling regime, where the effective coupling strength g is comparable to the energy ω of the bosonic mode, and remarkable features in the system dynamics are revealed. Here we demonstrate an analog quantum simulation of an effective quantum Rabi model in the ultra-strong coupling regime, achieving a relative coupling ratio of g/ω ~ 0.6. The quantum hardware of the simulator is a superconducting circuit embedded in a cQED setup. We observe fast and periodic quantum state collapses and revivals of the initial qubit state, being the most distinct signature of the synthesized model.
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 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 ...
Analyzing weak microwave signals in the GHz regime is a challenging task if the signal level is very low and the photon energy widely undefined. A superconducting qubit can detect signals in the low photon regime, but due to its discrete level structure, it is only sensitive to photons of certain energies. With a multi-level quantum system (qudit) in contrast, the unknown signal frequency and amplitude can be deduced from the higher level AC Stark shift. The measurement accuracy is given by the signal amplitude, its detuning from the discrete qudit energy level structure and the anharmonicity. We demonstrate an energy sensitivity in the order of 10 −3 with a measurement range of more than 1 GHz. Here, using a transmon qubit, we experimentally observe shifts in the transition frequencies involving up to three excited levels. These shifts are in good agreement with an analytic circuit model and master equation simulations. For large detunings, we find the shifts to scale linearly with the power of the applied microwave drive. Exploiting the effect, we demonstrated a power meter which makes it possible to characterize the microwave transmission from source to sample.
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