Native Approach to Controlled-Z Gates in Inductively Coupled Fluxonium Qubits.

The fluxonium qubits have emerged as a promising platform for gate-based quantum information processing. However, their extraordinary protection against charge fluctuations comes at a cost: when coupled capacitively, the qubit-qubit interactions are restricted to XX interactions. Consequently, effective ZZ or XZ interactions are only constructed either by temporarily populating higher-energy states, or by exploiting perturbative effects under microwave driving. Instead, we propose and demonstrate an inductive coupling scheme, which offers a wide selection of native qubit-qubit interactions for fluxonium. In particular, we leverage a built-in, flux-controlled ZZ interaction to perform qubit entanglement. To combat the increased flux-noise-induced dephasing away from the flux-insensitive position, we use a continuous version of the dynamical decoupling scheme to perform noise filtering. Combining these, we demonstrate a 20 ns controlled-z gate with a mean fidelity of 99.53%. More than confirming the efficacy of our gate scheme, this high-fidelity result also reveals a promising but rarely explored parameter space uniquely suitable for gate operations between fluxonium qubits.

Fluxonium: An Alternative Qubit Platform for High-Fidelity Operations.

Superconducting qubits provide a promising path toward building large-scale quantum computers. The simple and robust transmon qubit has been the leading platform, achieving multiple milestones. However, fault-tolerant quantum computing calls for qubit operations at error rates significantly lower than those exhibited in the state of the art. Consequently, alternative superconducting qubits with better error protection have attracted increasing interest. Among them, fluxonium is a particularly promising candidate, featuring large anharmonicity and long coherence times. Here, we engineer a fluxonium-based quantum processor that integrates high qubit coherence, fast frequency tunability, and individual-qubit addressability for reset, readout, and gates. With simple and fast gate schemes, we achieve an average single-qubit gate fidelity of 99.97% and a two-qubit gate fidelity of up to 99.72%. This performance is comparable to the highest values reported in the literature of superconducting circuits. Thus our work, within the realm of superconducting qubits, reveals an alternative qubit platform that is competitive with the transmon system.

Dirac-cone induced gating enhancement in single-molecule field-effect transistors.

Using graphene as electrodes provides an opportunity for fabricating stable single-molecule field-effect transistors (FETs) operating at room temperature. However, the role of the unique graphene band structure in charge transport of single-molecule devices is still not clear. Here we report the Dirac-cone induced electrostatic gating effects in single-molecule FETs with graphene electrodes and a solid-state local bottom gate. With the highest occupied molecular orbital (HOMO) as the dominating conduction channel and the graphene leads remaining intrinsic at zero gate voltage, electrostatic gating on the HOMO and the density of states of graphene at the negative gate polarity reinforces each other, resulting in an enhanced conductance modulation. In contrast, gating effects on the HOMO and the graphene states at the positive gate polarity are opposite. Depending on the gating efficiencies, the conductance can decrease, increase or remain almost unchanged when a more positive gate voltage is applied. Our observations can be well understood by a modified single-level model taking into account the linear dispersion of graphene near the Dirac point. Single-molecule FETs with Dirac-cone enhanced gating have shown high performances, with the modulation of a wide range of current over one order of magnitude. Our studies highlight the advantages of using graphene as an electrode material for molecular devices and pave the way for single-molecule FETs toward circuitry applications.

Efficient Fabrication of Stable Graphene-Molecule-Graphene Single-Molecule Junctions at Room Temperature.

We present a robust approach to fabricate stable single-molecule junctions at room temperature using single-layer graphene as nanoelectrodes. Molecular scale nano-gaps in graphene were generated using an optimized fast-speed feedback-controlled electroburning process. This process shortened the time for creating a single nano-gap to be less than one minute while keeping a yield higher than 97 %. To precisely control the gap position and minimize the effects of edge defects and the quantum confinement, extra-narrow grooves were pre-patterned in the graphene structures with oxygen plasma etching. Molecular junctions were formed by bridging the nano-gaps with amino-functionalized hexaphenyl molecules by taking advantage of chemical reactions between the amino groups at the two ends of the molecules and the carboxyl groups at the edges of graphene electrodes. Electronic transport measurements and transition voltage spectroscopy analysis verified the formation of single-molecule devices. First-principles quantum transport calculations show that the highest occupied molecular orbital of hexaphenyl is closer to the Fermi level of the graphene electrodes and thus the devices exhibit a hole-type transport characteristics. Some of these molecular devices remained stable up to four weeks, highlighting the potential of graphene nano-electrodes in the fabrication of stable single-molecule devices at room temperature.

Origin and mechanism analysis of asymmetric current fluctuations in single-molecule junctions.

The measurements of molecular electronic devices usually suffer from serious noise. Although noise hampers the operation of electric circuits in most cases, current fluctuations in single-molecule junctions are essentially related to their intrinsic quantum effects in the process of electron transport. Noise analysis can reveal and understand these processes from the behavior of current fluctuations. Here, in this study we observe and analyze the faint asymmetric current distribution in single-molecule junctions, in which the asymmetric intensity is highly related to the applied biases. The exploration of high-order moments within bias and temperature dependent measurements, in combination with model Hamiltonian calculations, statistically prove that the asymmetric current distribution originates from the inelastic electron tunneling process. Such results demonstrate a potential noise analysis method based on the fine structures of the current distribution rather than the noise power, which has obvious advantages in the investigation of the inelastic electron tunneling process in single-molecule junctions.

Stereoelectronic Effect-Induced Conductance Switching in Aromatic Chain Single-Molecule Junctions.

Biphenyl, as the elementary unit of organic functional materials, has been widely used in electronic and optoelectronic devices. However, over decades little has been fundamentally understood regarding how the intramolecular conformation of biphenyl dynamically affects its transport properties at the single-molecule level. Here, we establish the stereoelectronic effect of biphenyl on its electrical conductance based on the platform of graphene-molecule single-molecule junctions, where a specifically designed hexaphenyl aromatic chain molecule is covalently sandwiched between nanogapped graphene point contacts to create stable single-molecule junctions. Both theoretical and temperature-dependent experimental results consistently demonstrate that phenyl twisting in the aromatic chain molecule produces different microstates with different degrees of conjugation, thus leading to stochastic switching between high- and low-conductance states. These investigations offer new molecular design insights into building functional single-molecule electrical devices.