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Register a new userMerged-Element Transmons: Design and Qubit Performance
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Added by
Harry Jonathon Mamin
Publication date
2021
fields
Physics
and research's language is
English
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We have demonstrated a novel type of superconducting transmon qubit in which a Josephson junction has been engineered to act as its own parallel shunt capacitor. This merged-element transmon (MET) potentially offers a smaller footprint and simpler fabrication than conventional transmons. Because it concentrates the electromagnetic energy inside the junction, it reduces relative electric field participation from other interfaces. By combining micrometer-scale Al/AlOx/Al junctions with long oxidations and novel processing, we have produced functional devices with $E_{J}$/$E_{C}$ in the low transmon regime ($E_{J}$/$E_{C}$ $lesssim$30). Cryogenic I-V measurements show sharp dI/dV structure with low sub-gap conduction. Qubit spectroscopy of tunab
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A merged-element transmon (MET) device, based on Si fins, is proposed and the steps to form such a FinMET are demonstrated. This new application of fin technology capitalizes on the anisotropic etch of Si(111) relative to Si(110) to define atomically flat, high aspect ratio Si tunnel barriers with epitaxial superconductor contacts on the parallel side-wall surfaces. This process circumvents the challenges associated with the growth of low-loss insulating barriers on lattice matched superconductors. By implementing low-loss, intrinsic float-zone Si as the barrier material rather than commonly used, lossy Al2O3, the FinMET is expected to overcome problems with standard transmons by (1) reducing dielectric losses; (2) minimizing the formation of two-level system spectral features; (3) exhibiting greater control over barrier thickness and qubit frequency spread, especially when combined with commercial fin fabrication and atomic-layer digital etching; (4) reducing the footprint by four orders of magnitude; and (5) allowing scalable fabrication. Here, fabrication of Si fins on Si(110) substrates with shadow-deposited Al electrodes is demonstrated. The formation of FinMET devices is expected to allow tunnel junction patterning with optical lithography. This facilitates uniform fabrication on Si wafers based on existing infrastructure for fin-based devices while simultaneously avoiding lossy amorphous dielectrics for tunnel barriers.
We provide a characterization and analysis of the effects of dissipation on oscillator assisted (qubus) quantum gates. The effects can be understood and minimized by looking at the dynamics of the signal coherence and its entanglement with the continuous variable probe. Adding loss in between successive interactions we obtain the effective quantum operations, providing a novel approach to loss analysis in such hybrid settings. We find that in the presence of moderate dissipation the gate can operate with a high fidelity. We also show how a simple iteration scheme leads to independent single qubit dephasing, while retaining the conditional phase operation regardless of the amount of loss incurred by the probe.
Rigidity of an ordered phase in condensed matter results in collective excitation modes spatially extending in macroscopic dimensions. Magnon is a quantum of an elementary excitation in the ordered spin system, such as ferromagnet. Being low dissipative, dynamics of magnons in ferromagnetic insulators has been extensively studied and widely applied for decades in the contexts of ferromagnetic resonance, and more recently of Bose-Einstein condensation as well as spintronics. Moreover, towards hybrid systems for quantum memories and transducers, coupling of magnons and microwave photons in a resonator have been investigated. However, quantum-state manipulation at the single-magnon level has remained elusive because of the lack of anharmonic element in the system. Here we demonstrate coherent coupling between a magnon excitation in a millimetre-sized ferromagnetic sphere and a superconducting qubit, where the interaction is mediated by the virtual photon excitation in a microwave cavity. We obtain the coupling strength far exceeding the damping rates, thus bringing the hybrid system into the strong coupling regime. Furthermore, we find a tunable magnon-qubit coupling scheme utilising a parametric drive with a microwave. Our approach provides a versatile tool for quantum control and measurement of the magnon excitations and thus opens a new discipline of quantum magnonics.
Quantum computation requires the precise control of the evolution of a quantum system, typically through application of discrete quantum logic gates on a set of qubits. Here, we use the cross-resonance interaction to implement a gate between two superconducting transmon qubits with a direct static dispersive coupling. We demonstrate a practical calibration procedure for the optimization of the gate, combining continuous and repeated-gate Hamiltonian tomography with step-wise reduction of dominant two-qubit coherent errors through mapping to microwave control parameters. We show experimentally that this procedure can enable a $hat{ZX}_{-pi/2}$ gate with a fidelity $F=97.0(7)%$, measured with interleaved randomized benchmarking. We show this in a architecture with out-of-plane control and readout that is readily extensible to larger scale quantum circuits.
The current-mirror circuit [A. Kitaev, arXiv:cond-mat/0609441 (2006)] exhibits a robust ground-state degeneracy and wave functions with disjoint support for appropriate circuit parameters. In this protected regime, Cooper-pair excitons form the relevant low-energy excitations. Based on a full circuit analysis of the current-mirror device, we introduce an effective model that systematically captures the relevant low-energy degrees of freedom, and is amenable to diagonalization using Density Matrix Renormalization Group (DMRG) methods. We find excellent agreement between DMRG and exact diagonalization, and can push DMRG simulations to much larger circuit sizes than feasible for exact diagonalization. We discuss the spectral properties of the current-mirror circuit, and predict coherence times exceeding 1 ms in parameter regimes believed to be within reach of experiments.
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