Weak localisation magnetoresistance and valley symmetry in graphene.

Due to the chiral nature of electrons in a monolayer of graphite (graphene) one can expect weak antilocalisation and a positive weak-field magnetoresistance in it. However, trigonal warping (which breaks p to −p symmetry of the Fermi line in each valley) suppresses antilocalisation, while inter-valley scattering due to atomically sharp scatterers in a realistic graphene sheet or by edges in a narrow wire tends to restore conventional negative magnetoresistance. We show this by evaluating the dependence of the magnetoresistance of graphene on relaxation rates associated with various possible ways of breaking a ’hidden’ valley symmetry of the system

Phenomenological noise model for superconducting qubits: two-state fluctuators and 1=f noise

We present a general phenomenological model for superconducting qubits subject to noise produced by two-state fluctuators whose couplings to the qubit are all roughly the same. In flux qubit experiments where the working point can be varied, it is possible to extract both the form of the noise spectrum and the number of fluctuators. We find that the noise has a broad spectrum consistent with 1=f noise and that the number of fluctuators with slow switching rates is surprisingly small: less than 100. If the fluctuators are interpreted as unpaired surface spins, then the size of their magnetic moments is surprisingly large.Comment: 7 pages, 2 figures, 1 tabl

Quantum transport thermometry for electrons in graphene

We propose a method of measuring the electron temperature $T_e$ in mesoscopic conductors and demonstrate experimentally its applicability to micron-size graphene devices in the linear-response regime ($T_e\approx T$, the bath temperature). The method can be {especially useful} in case of overheating, $T_e>T$. It is based on analysis of the correlation function of mesoscopic conductance fluctuations. Although the fluctuation amplitude strongly depends on the details of electron scattering in graphene, we show that $T_e$ extracted from the correlation function is insensitive to these details.Comment: 4 pages, 4 figures; final version, as publishe

Effective quantum volume, fidelity and computational cost of noisy quantum processing experiments

Today's experimental noisy quantum processors can compete with and surpass all known algorithms on state-of-the-art supercomputers for the computational benchmark task of Random Circuit Sampling [1-5]. Additionally, a circuit-based quantum simulation of quantum information scrambling [6], which measures a local observable, has already outperformed standard full wave function simulation algorithms, e.g., exact Schrodinger evolution and Matrix Product States (MPS). However, this experiment has not yet surpassed tensor network contraction for computing the value of the observable. Based on those studies, we provide a unified framework that utilizes the underlying effective circuit volume to explain the tradeoff between the experimentally achievable signal-to-noise ratio for a specific observable, and the corresponding computational cost. We apply this framework to recent quantum processor experiments of Random Circuit Sampling [5], quantum information scrambling [6], and a Floquet circuit unitary [7]. This allows us to reproduce the results of Ref. [7] in less than one second per data point using one GPU.Comment: 14 pages, 13 figure

Influence of trigonal warping on interference effects in bilayer graphene

Bilayer graphene (two coupled graphitic monolayers arranged according to Bernal stacking) is a two-dimensional gapless semiconductor with a peculiar electronic spectrum different from the Dirac spectrum in the monolayer material. In particular, the electronic Fermi line in each of its valleys has a strong p -> -p asymmetry due to trigonal warping, which suppresses the weak localization effect. We show that weak localization in bilayer graphene may be present only in devices with pronounced intervalley scattering, and we evaluate the corresponding magnetoresistance