Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Abstract: Because of their long coherence time and compatibility with industrial foundry processes, electron spin qubits are a promising platform for scalable quantum processors. A full-fledged quantum computer will need quantum error correction, which requires high-fidelity quantum gates. Analyzing and mitigating the gate errors are useful to improve the gate fidelity. Here, we demonstrate a simple yet reliable calibration procedure for a high-fidelity controlled-rotation gate in an exchange-always-on Silicon quantum processor allowing operation above the fault-tolerance threshold of quantum error correction. We find that the fidelity of our uncalibrated controlled-rotation gate is limited by coherent errors in the form of controlled-phases and present a method to measure and correct these phase errors. We then verify the improvement in our gate fidelities by randomized benchmark and gate-set tomography protocols. Finally, we use our phase correction protocol to implement a virtual, high-fidelity controlled-phase gate.

Abstract: In this paper, we investigated the annealing experiments of poly(mercaptopropylmethylsiloxane, PMMS) confined within two types of porous templates (anodic aluminium oxide, AAO, and silica) characterized by different pore diameter, d= 8-120nm, using different thermal protocols (varying significantly in cooling/heating rate) by means of Broadband Dielectric Spectroscopy (BDS) supported by the complementary Differential Scanning Calorimetry (DSC) and temperature-dependent contact angle, {\theta}, measurements. It was found that relaxation times obtained from routine temperature-dependent dielectric investigations deviate from the bulk behavior when approaching the glass transition temperature. Importantly, this confinement induced effect can be easily removed by the annealing experiments performed at some specific range of temperatures. The analysis of the dielectric data collected during isothermal experiments of confined samples that was beforehand cooled with different rates revealed that (i) constant rates of annealing gets longer with cooling and weakly depend on the rate of cooling, and (ii) activation energy of the equilibration process, E_a, varies with the reduction of the pore diameter and material the porous template is made of. In fact, there is significant reduction in E_a from ~62 to ~23 kJ/mol obtained for the annealing process carried out in AAO (d= 10 nm) and silica (d= 8 nm) membranes, respectively. Such significant change in E_a can be explained taking into account temperature-dependence of {\theta} of PMMS indicating a notable change in wettability between both surfaces upon cooling. As a consequence, one can expect that the mass exchange between interfacial and core molecules as well as adsorption-desorption processes occurring at the interface at lower temperatures must be affected.

Abstract: Studies of the formation of Landau levels based on the Schr\"odinger equation for electrons constrained to curved surfaces have a long history. These include as prime examples surfaces with constant positive and negative curvature, the sphere [Phys. Rev. Lett. 51, 605 (1983)] and the pseudosphere [Annals of Physics 173, 185 (1987)]. Now, topological insulators, hosting Dirac-type surface states, provide a unique platform to experimentally examine such quantum Hall physics in curved space. Hence, extending previous work we consider solutions of the Dirac equation for the pseudosphere for both, the case of an overall perpendicular magnetic field and a homogeneous coaxial, thereby locally varying, magnetic field. For both magnetic-field configurations, we provide analytical solutions for spectra and eigenstates. For the experimentally relevant case of a coaxial magnetic field we find that the Landau levels split and show a peculiar scaling $\propto B^{1/4}$, thereby characteristically differing from the usual linear $B$ and $B^{1/2}$ dependence of the planar Schr\"odinger and Dirac case, respectively. We compare our analytical findings to numerical results that we also extend to the case of the Minding surface.

Abstract: Valley degrees of freedom in transition-metal dichalcogenides influence thoroughly electron-phonon coupling and its nonequilibrium dynamics. We conducted a first-principles study of the quantum kinetics of chiral phonons following valley-selective carrier excitation with circularly-polarized light. Our numerical investigations treat the ultrafast dynamics of electrons and phonons on equal footing within a parameter-free ab-initio framework. We report the emergence of valley-polarized phonon populations in monolayer MoS$_2$ that can be selectively excited at either the K or K' valleys depending on the light helicity. The resulting vibrational state is characterized by a distinctive chirality, which lifts time-reversal symmetry of the lattice on transient timescales. We show that chiral valley phonons can further lead to fingerprints of vibrational dichroism detectable by ultrafast diffuse scattering and persisting beyond 10 ps. The valley polarization of nonequilibrium phonon populations could be exploited as information carrier, thereby extending the paradigm of valleytronics to the domain of vibrational excitations.

Abstract: We present a theory for band-tuned metal-insulator transitions based on the Kubo formalism. Such a transition exhibits scaling of the resistivity curves, in the regime where $T\tau >1$ or $\mu \tau>1$, where $\tau$ is the scattering time and $\mu$ the chemical potential. At the critical value of the chemical potential, the resistivity diverges as a power law, $R_c \sim 1/T$. Consequently, on the metallic side there is a regime with negative $dR/dT$, which is often misinterpreted as insulating. We show that scaling and this `fake insulator' regime is observed in a wide range of experimental systems. In particular, we show that Mooij correlations in high-temperature metals with negative $dR/dT$ can be quantitatively understood with our scaling theory in the presence of $T$-linear scattering.

Abstract: Moir\'e materials have recently been established experimentally as a highly-tunable condensed matter platform, facilitating the controlled manipulation of band structures and interactions. In several of these moir\'e materials, Dirac cones are present in the low-energy regime near the Fermi level. Thus, fermionic excitations emerging in these materials close to the Dirac cones have a linear dispersion relation near the Fermi surface as massless relativistic Dirac fermions. Here, we study low-energy fermionic excitations of moir\'e Dirac materials in the presence of a mass gap that may be generated by symmetry breaking. Introducing a dynamical Fermi velocity and/or time-dependent mass gap for the Dirac quasiparticles, we exhibit the emergence of an analog of cosmological fermion pair production in terms of observables such as the expected occupation number or two-point correlation functions. We find that it is necessary and sufficient for quasiparticle production that only the ratio between the mass gap and the Fermi velocity is time-dependent. In this way, we establish that moir\'e Dirac materials can serve as analog models for cosmological spacetime geometries, in particular, for Friedmann-Lema\^itre-Robertson-Walker expanding cosmologies. We briefly discuss possibilities for experimental realization.

Abstract: We propose a theory of anisotropic band flattening in moir\'e systems at the $\Gamma$ valley. We find that in twisted anisotropic two-dimensional crystals with a rectangular unit cell of $C_{2z}$ or mirror symmetries, a larger effective mass anisotropy $\eta=m_y/m_x$ has a stronger tendency to be further enhanced compared to that of monolayer, which leads to correlated physics in one dimension effectively. We predict twisted bilayer black phosphorus (tBBP) has giant anisotropic flattened moir\'e bands ($\eta\sim10^4$) from ab initio calculations and continuum model, where the low energy physics is described by the weakly coupled array of one-dimensional wires. We further calculate the phase diagram based on sliding Luttinger liquid by including the screened Coulomb interactions in tBBP, and find a large parameter space may host the non-Fermi liquid phase. We thus establish tBBP as a promising and experimentally accessible platform for exploring correlated physics in low dimensions.

Abstract: We investigate nonequilibrium effects in the transport of interacting electrons in quantum conductors, proposing the nonlocal thermoelectric response as a direct indicator of the presence of interactions, nonthermal states and the effect of correlations. This is done by assuming a quantum Hall setup where two channels (connected to reservoirs at different temperatures) co-propagate for a finite distance, such that a thermoelectrical response is only expected if the electron-electron interaction mediates heat exchange between the channels. This way, the nonlocal Seebeck response measures the interaction strength. Considering zero-range interactions, we solve the charge and energy currents and noises of a non-equilibrium integrable interacting system, determining the universal interaction-dependent length scale of energy equilibration. Further, a setup with two controllable quantum point contacts allows thermoelectricity to monitor the interacting system thermalisation as well as the fundamental role of cross-correlations in the heat exchange at intermediate length scales.