7 research outputs found
Graphene-Nanodiamond Heterostructures and their application to High Current Devices
Graphene on hydrogen terminated monolayer nanodiamond heterostructures provides a new way to improve carrier transport characteristics of the graphene, offering up to 60% improvement when compared with similar graphene on SiO2/Si substrates. These heterostructures offers excellent current-carrying abilities whilst offering the prospect of a fast, low cost and easy methodology for device applications. The use of ND monolayers is also a compatible technology for the support of large area graphene films. The nature of the C-H bonds between graphene and H-terminated NDs strongly influences the electronic character of the heterostructure, creating effective charge redistribution within the system. Field effect transistors (FETs) have been fabricated based on this novel herterostructure to demonstrate device characteristics and the potential of this approach
Frustration-induced anomalous transport and strong photon decay in waveguide QED
We study the propagation of photons in a one-dimensional environment
consisting of two non-interacting species of photons frustratingly coupled to a
single spin-1/2. The ultrastrong frustrated coupling leads to an extreme mixing
of the light and matter degrees of freedom, resulting in the disintegration of
the spin and a breakdown of the "dressed-spin", or polaron, description. Using
a combination of numerical and analytical methods, we show that the elastic
response becomes increasingly weak at the effective spin frequency, showing
instead an increasingly strong and broadband response at higher energies. We
also show that the photons can decay into multiple photons of smaller energies.
The total probability of these inelastic processes can be as large as the total
elastic scattering rate, or half of the total scattering rate, which is as
large as it can be. The frustrated spin induces strong anisotropic
photon-photon interactions that are dominated by inter-species interactions.
Our results are relevant to state-of-the-art circuit and cavity quantum
electrodynamics experiments.Comment: 5+13 pages, 3 + 6 figures. v2: changed title and presentatio
Accurate methods for the analysis of strong-drive effects in parametric gates
The ability to perform fast, high-fidelity entangling gates is an important
requirement for a viable quantum processor. In practice, achieving fast gates
often comes with the penalty of strong-drive effects that are not captured by
the rotating-wave approximation. These effects can be analyzed in simulations
of the gate protocol, but those are computationally costly and often hide the
physics at play. Here, we show how to efficiently extract gate parameters by
directly solving a Floquet eigenproblem using exact numerics and a perturbative
analytical approach. As an example application of this toolkit, we study the
space of parametric gates generated between two fixed-frequency transmon qubits
connected by a parametrically driven coupler. Our analytical treatment, based
on time-dependent Schrieffer-Wolff perturbation theory, yields closed-form
expressions for gate frequencies and spurious interactions, and is valid for
strong drives. From these calculations, we identify optimal regimes of
operation for different types of gates including SWAP, controlled-Z, and
CNOT. These analytical results are supplemented by numerical Floquet
computations from which we directly extract drive-dependent gate parameters.
This approach has a considerable computational advantage over full simulations
of time evolutions. More generally, our combined analytical and numerical
strategy allows us to characterize two-qubit gates involving parametrically
driven interactions, and can be applied to gate optimization and cross-talk
mitigation such as the cancellation of unwanted ZZ interactions in multi-qubit
architectures.Comment: 20 pages, 9 figures, 62 reference
Quantum Conductance in Silicon Oxide Resistive Memory Devices
International audienceResistive switching offers a promising route to universal electronic memory, potentially replacing current technologies that are approaching their fundamental limits. In many cases switching originates from the reversible formation and dissolution of nanometre-scale conductive filaments, which constrain the motion of electrons, leading to the quantisation of device conductance into multiples of the fundamental unit of conductance, G 0. Such quantum effects appear when the constriction diameter approaches the Fermi wavelength of the electron in the medium – typically several nanometres. Here we find that the conductance of silicon-rich silica (SiO x) resistive switches is quantised in half-integer multiples of G 0. In contrast to other resistive switching systems this quantisation is intrinsic to SiO x , and is not due to drift of metallic ions. Half-integer quantisation is explained in terms of the filament structure and formation mechanism, which allows us to distinguish between systems that exhibit integer and half-integer quantisation. D evices in which the electrical resistance can stably and repeatedly be switched between vastly different values hold great potential as future non-volatile memory elements. So-called Resistive Random Access Memories (RRAMs) exploiting such switching behaviour are actively being developed as alternatives to current technologies such as Flash, which are approaching their limits of scalability and power dissipation 1. We recently demonstrated switching in silicon oxide 2,
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New material platform for superconducting transmon qubits with coherence times exceeding 0.3 milliseconds
The superconducting transmon qubit is a leading platform for quantum computing and quantum science. Building large, useful quantum systems based on transmon qubits will require significant improvements in qubit relaxation and coherence times, which are orders of magnitude shorter than limits imposed by bulk properties of the constituent materials. This indicates that relaxation likely originates from uncontrolled surfaces, interfaces, and contaminants. Previous efforts to improve qubit lifetimes have focused primarily on designs that minimize contributions from surfaces. However, significant improvements in the lifetime of two-dimensional transmon qubits have remained elusive for several years. Here, we fabricate two-dimensional transmon qubits that have both lifetimes and coherence times with dynamical decoupling exceeding 0.3 milliseconds by replacing niobium with tantalum in the device. We have observed increased lifetimes for seventeen devices, indicating that these material improvements are robust, paving the way for higher gate fidelities in multi-qubit processors