In circuit-based quantum computing the available gate set typically consists of single-qubit gates acting on each individual qubit and at least one entangling gate between pairs of qubits. In certain ...physical architectures, however, some qubits may be “hidden” and lacking direct addressability through dedicated control and readout lines, for instance, because of limited on-chip routing capabilities, or because the number of control lines becomes a limiting factor for many-qubit systems. In this case, no single-qubit operations can be applied to the hidden qubits and their state cannot be measured directly. Instead, they may be controlled and read out only via single-qubit operations on connected “control” qubits and a suitable set of two-qubit gates. We first discuss the impact of such restricted control capabilities on the performance of specific qubit coupling networks. We then experimentally demonstrate full control and measurement capabilities in a superconducting two-qubit device with local single-qubit control and iswap and controlled-phase two-qubit interactions enabled by a tunable coupler. We further introduce an iterative tune-up process required to completely characterize the gate set used for quantum process tomography and evaluate the resulting gate fidelities.
The possibility to utilize different types of two-qubit gates on a single quantum computing platform adds flexibility in the decomposition of quantum algorithms. A larger hardware-native gate set may ...decrease the number of required gates, provided that all gates are realized with high fidelity. Here, we benchmark both controlled-Z (CZ) and exchange-type (iSWAP) gates using a parametrically driven tunable coupler that mediates the interaction between two superconducting qubits. Using randomized benchmarking protocols we estimate an error per gate of 0.9±0.03 and 1.3±0.4% for the CZ and the iSWAP gate, respectively. We argue that spurious ZZ-type couplings are the dominant error source for the iSWAP gate, and that phase stability of all microwave drives is of utmost importance. Such differences in the achievable fidelities for different two-qubit gates have to be taken into account when mapping quantum algorithms to real hardware.
We report on light emission from biased metallic single-wall carbon nanotube (SWNT), multiwall carbon nanotube (MWNT) and few-layer graphene (FLG) devices. SWNT devices were assembled from tubes with ...different diameters in the range 0.7−1.5 nm. They emit light in the visible spectrum with peaks at 1.4 and 1.8 eV. Similar peaks are observed for MWNT and FLG devices. We propose that this light emission is due to phonon-assisted radiative decay from populated π* band states at the M point to the Fermi level at the K point. Since for most carbon nanotubes as well as for graphene the energy of unoccupied states at the M point is close to 1.6 eV, the observation of two emission peaks at ∼1.6 ± ∼0.2 eV could indicate radiative decay under emission or absorption of optical phonons, respectively.
Abstract
One of the main limitations in state-of-the art solid-state quantum processors is qubit decoherence and relaxation due to noise from adsorbates on surfaces, impurities at interfaces, and ...material defects. For the field to advance towards full fault-tolerant quantum computing, a better understanding of these microscopic noise sources is therefore needed. Here, we use an ultra-high vacuum package to study the impact of vacuum loading, UV-light exposure, and ion irradiation treatments on relaxation and coherence times, as well as slow parameter fluctuations of flux tunable superconducting transmon qubits. The treatments studied do not significantly impact the relaxation rate Γ
1
and the echo decay rate
$${{{\Gamma }}}_{2,{{{\rm{SS}}}^{{{\rm{e}}}$$
Γ
2
,
SS
e
at the sweet spot, except for Ne ion bombardment which reduces Γ
1
. In contrast, flux noise parameters are improved by removing magnetic adsorbates from the chip surfaces with UV-light and NH
3
treatments. Additionally, we demonstrate that SF
6
ion bombardment can be used to adjust qubit frequencies in situ and post-fabrication without affecting qubit relaxation and coherence times at the sweet spot.
Fragmentation threatens biodiversity globally. Manilkara maxima (Sapotaceae) is listed by the IUCN as threatened and is an economically and ecologically important tree species endemic to the Atlantic ...forest of southern Bahia, Brazil, a biodiversity hotspot. The objectives of this study were to examine the effect of fragment size on density and genetic diversity of this threatened species. We surveyed and sampled 222 individuals across 2 large forest sites and 1 site comprising 8 small fragments. We focused on 5 microsatellite loci that provided comparable genetic information (average 21 alleles locus super(-1)) as 2 congeners in other studies. Fragment size accounted for 71 and 56% of density variation in adult and sapling trees, respectively, but did not account for genetic variation. Rather, density accounted for 80% of allelic diversity and 70% of allelic richness in both life stages. The lack of relationship between genetic diversity and fragment size was driven in part by a co-variation in density and genetic diversity in 25 ha fragments. These small forests can have high densities of M. maxima, and in turn, can have as much genetic diversity and conservation value as larger fragments. However, the larger fragments are of unique conservation value because they hold the greatest number of reproductively mature individuals, the ones necessary for the recruitment of new individuals. Our results suggest that relatively high levels of gene flow are contributing to the high genetic diversity of saplings found in some of the small fragments.
Habitat fragmentation may lead to spatial genetic structuring of plant populations, but the magnitude of this effect differs among species. In this study, the effects of fragmentation on spatial ...genetic structure of Manilkara maxima, an ecologically important tree species endemic to the Atlantic forest of southern Bahia, Brazil, were examined. To address this issue, 222 individuals were sampled across two large-forest sites (200 and 400 ha) and one site comprising eight small fragments (5, 10 and 25 ha) roughly 35 y old. Five microsatellite loci were used. In general, M. maxima exhibited limited genetic structuring within and across the sites examined. At the landscape scale, genetic structure was not shaped by isolation by distance. Within individual sites, weak to moderate genetic structure was observed for both adults and saplings, which represented pre- and post-fragmentation, respectively. Sapling genetic structure did not clearly vary among the study sites, suggesting fragmentation has not affected spatial genetic structure. High levels of migration were observed, which would help maintain genetic connectivity. Taken together, these results suggest M. maxima, a long-lived tree that naturally occurs in low densities, may have limited spatial genetic structuring both within and across forest patches.
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