Materials with no net magnetization are generally not magneto-optically active. While this is individually true for a collinear antiferromagnet (AFM) and a topological insulator (TI), it is shown here that the magneto-optic Kerr effect (MOKE) emerges when the TI and AFM films are proximity coupled. Because of the lack of macroscopic magnetization, the AFM only couples to the spin of one of the TI's surfaces breaking time-reversal and inversion symmetry -- which leads to a tiny $\mu$deg MOKE signal. This small MOKE can be easily enhanced by 5 orders of magnitude, via cavity resonance, by optimizing the AFM and TI film thicknesses on the substrate. For slightly off-resonant structures, a 6 deg Kerr rotation can be electrically switched on by varying the Fermi energy. This requires less than 20 meV, which is encouraging for low power spintronics and magneto-optic devices. We further show that this simple structure is easily resilient to 5% material growth error.

Received 21 April 2020DOI:https://doi.org/10.1103/PhysRevB.101.209902©2020 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasSkyrmionsTopological insulatorsPhysical SystemsInterfacesCondensed Matter, Materials & Applied Physics

Adiabatic gate operations required to remain within the qubit subspace in an anharmonic oscillator can be slow when compared to qubit decoherence times. However, significant gate speedups are possible using methods such as derivative-removal-by-adiabatic-gate (DRAG) (Motzoi et al. in Phys Rev Lett 103:110501, 2009), which creates spectral-holes near unwanted transitions. We analyze the effect of DRAG on the transmon qubit in some detail for cosine and truncated Gaussian pulses. An accurate tight-binding multi-level transmon model is presented here along with a multi-level Lindblad model and time-evolution methods to remove phase oscillations. It is shown that in addition to DRAG, the simultaneous optimization of the pulse truncation, detuning and the pulse norm significantly reduces leakage errors. For sharply truncated Gaussian pulses, DRAG leads to faster gates that are also stable against pulse jitter. However, for slow rising pulse envelopes, DRAG is not effective. This is explained using spectral analysis. Overall this can lead to much faster reverse-engineered qubit gates soon.

It is shown here that 1/f(alpha) flux noise in conventional low-T-c SQUIDs is a result of low temperature superparamagnetic phase transitions in small clusters of strongly correlated color center defects. The spins in each cluster interact via long-range ferromagnetic interactions. Due to its small size, the cluster behaves like a random-telegraphic macrospin when transitioning to the superparamagnetic phase. This results in 1/f(alpha) noise when ensemble averaged over a random distribution of clusters. This model is self-consistent and explains all related experimental results which includes alpha similar to 0.8 independent of system size. The experimental flux-inductance-noise spectrum is explained through three-point correlation calculations and time-reversal symmetry-breaking arguments. Also, unlike the flux noise, it is shown why the second-spectrum inductance noise is inherently temperature dependent due to the fluctuation-dissipation theorem. A correlation-function calculation methodology using Ising-Glauber dynamics was key for obtaining these results.

In the Kerr rotation geometry, magneto-optic memory devices typically suffer from low figure-of-merit (FOM) and long write times. We show that skyrmions formed at the interface of a thin-film multiferroic and a topological insulator can give rise to high FOM magneto-optic Kerr effects (MOKEs). Huge differential MOKE can arise in parts of the phase diagram. Resonancelike features in the MOKE spectra arising from the induced low energy TI band gap, the multiferroic-film thickness, and the high energy Drude-like behavior are resolved and explained. The Fermi level dependence of the MOKE signatures is distinct for the different magnetic textures. This has broad implications for magnetic texture characterization, electro-optic modulators and isolators, and high density magnetic optic memory.

Strong coupling between the electronic states of monolayer transition metal dichalcogenides (TMDC) such as MoS$_2$, MoSe$_2$, WS$_2$, or WSe$_2$, and a two-dimensional (2D) photonic cavity gives rise to several exotic effects. The Dirac type Hamiltonian for a 2D gapped semiconductor with large spin-orbit coupling facilitates pure Jaynes-Cummings type coupling in the presence of a single mode electric field. The presence of an additional circularly polarized beam of light gives rise to valley and spin dependent cavity-QED properties. The cavity causes the TMDC monolayer to act as an on-chip coherent light source and a spontaneous spin-oscillator. In addition, a TMDC monolayer in a cavity is a sensitive magnetic field sensor for an in-plane magnetic field.

Interlayer electron transport through a graphene/hexagonal boron-nitride (h-BN)/graphene heterostructure is strongly affected by the misorientation angle. of the h-BN with respect to the graphene layers with different physical mechanisms governing the transport in different regimes of angle, Fermi level, and bias. The different mechanisms and their resulting signatures in resistance and current are analyzed using two different models, a tight-binding, nonequilibrium Green function model and an effective continuum model, and the qualitative features resulting from the two different models compare well. In the large-angle regime (theta > 4 degrees), the change in the effective h-BN band gap seen by an electron at the K point of the graphene causes the resistance to monotonically increase with angle by several orders of magnitude, reaching a maximum at theta = 30 degrees. It does not affect the peak-to-valley current ratios in devices that exhibit negative differential resistance. In the small-angle regime (theta < 4 degrees), umklapp processes open up new conductance channels that manifest themselves as nonmonotonic features in a plot of resistance versus Fermi level that can serve as experimental signatures of this effect. For small angles and high bias, the umklapp processes give rise to two new current peaks on either side of the direct tunneling peak.

Interlayer misorientation in transition metal dichalcogenides alters their interlayer distance, total energy, electronic band structure, and vibrational modes, but its effect on the interlayer resistance is not known. This study analyzes the interlayer resistance of misoriented bilayer MoS2 as a function of the misorientation angle, and it shows that interlayer misorientation exponentially increases the electron resistivity while leaving the hole resistivity almost unchanged. The physics, determined by the wave functions at the high symmetry points, are generic among the popular semiconducting transition metal dichalcogenides (TMDs). The asymmetrical effect of misorientation on the electron and hole transport may be exploited in the design and optimization of vertical transport devices such as a bipolar transistor. Density functional theory provides the interlayer coupling elements used for the resistivity calculations.

We model repetitive quantum error correction (QEC) with the single-error-correcting five-qubit code on a network of individually-controlled qubits with always-on Ising couplings, using our previously designed universal set of quantum gates based on sequences of shaped decoupling pulses. In addition to serving as accurate quantum gates, the sequences also provide dynamical decoupling (DD) of low-frequency phase noise. The simulation involves integrating unitary dynamics of six qubits over the duration of tens of thousands of control pulses, using classical stochastic phase noise as a source of decoherence. The combined DD/QEC protocol dramatically improves the coherence, with the QEC alone responsible for more than an order of magnitude infidelity reduction.

Low frequency flicker noise has been argued to occur in spatially extended metastable systems near a critical point (Bak et al., 1987). An Ising-Glauber model based method is suggested here to systematically obtain temperature dependentnth-order correlation functions forNarbitrary interacting two-level systems (TLSs). This model is fully consistent with existing methods to calculate1/fnoise spectra from TLSs and complements them. However, with as such noa prioriassumptions on the typical log normal distribution of fluctuation rates, it is shown that1/fαnoise manifests in two different cases: first in the thermodynamic limit on a 2D lattice with long range antiferromagnetic interactions at low temperatures and second in the case of a statistical ensemble of finite-sized spin clusters representing disorder, but where each cluster is ordered due to ferromagnetic interactions.

Qubit-qubit interactions can significantly boost quantum coherence times for Bell states. The coherence-time enhancements are, however, not monotonic and there exists a phase where further increasing the interaction is unhelpful. A resonator in a suggested circuit QED-type implementation of the Tavis-Cummings (Dicke) model, is shown to shift this transition point depending on the number of loaded photons. This allows the resonator to amplify the coherence enhancements in certain regimes. The interactions also induce unusual collapse and revivaltype behavior for the entanglement dynamics. An exact open quantum system's formalism-(quasi-Hamiltonians for the Dicke model) shows how a Bell state singlet-triplet qubit in a resonator can be protected against 1/f noise from randomly fluctuating two-level systems. Simple circuit level details are given for flux qubits.

Adiabatic or slowly varying gate operations are typically required in order to remain within the qubit subspace in an anharmonic oscillator. However significant speed ups are possible by using the two quadrature derivative-removal-by-adiabatic-gate(DRAG) technique, where a second time derivative pulse component burns a spectral hole near an unwanted transition. It is shown here, that simultaneous optimization of the detuning and the pulse norm in addition, further reduces leakage errors and significantly improve gate fidelities. However, with this optimization accounting for the AC Stark shift, there is a low spectral weight pulse envelope regime, where DRAG is almost not needed and where the two state error fidelities are stable against pulse jitter. Explicit time evolution calculations are carried out in the lab frame for truncated multi-level Transmon qubit models obtained from a tight-binding model.

We consider the possibility of observing continuous third-harmonic generation using a strongly driven, single-band one-dimensional metal. In the absence of scattering, the quantum efficiency of frequency tripling for such a system can be as high as 93%. Combining the Floquet quasi-energy spectrum with the Keldysh Green's function technique, we derive a semiclassical master equation for a one-dimensional band of strongly and rapidly driven electrons in the presence of weak scattering by phonons. The power absorbed from the driving field is continuously dissipated by phonon modes, leading to a quasi-equilibrium in the electron distribution. We use the Kronig-Penney model with varying effective mass to establish the growth parameters of an InAs/InP nanowire near optimal for third harmonic generation at terahertz frequency range.

Clusters of interacting two-level-systems, likely due to Farbe+(F(+)) centers at the metal-insulator interface, are shown to self-consistently lead to 1/f(α) magnetization noise [with α(T)≲1] in SQUIDs. Model calculations, based on a new method of obtaining correlation functions, explains various puzzling experimental features. It is shown why the inductance noise is inherently temperature dependent while the flux noise is not, despite the same underlying microscopics. Magnetic ordering in these systems, established by three-point correlation functions, explains the observed flux-inductance-noise cross correlations. Since long-range ferromagnetic interactions are shown to lead to a more weakly temperature dependent flux noise when compared to short-range interactions, the time reversal symmetry of the clusters is also not likely broken by the same mechanism which mediates surface ferromagnetism in nanoparticles and thin films of the same insulator materials.

We show that resonant coupling and entanglement between a mechanical resonator and Majorana bound states can be achieved via spin currents in a 1D quantum wire with strong spin-orbit interactions. The bound states induced by vibrating and stationary magnets can hybridize, thus resulting in spin-current induced 4π-periodic torques, as a function of the relative field angle, acting on the resonator. We study the feasibility of detecting and manipulating Majorana bound states with the use of magnetic resonance force microscopy techniques.

We propose the manipulation of Majorana edge states via hybridization and spin currents in a nanowire spin transistor. The spin transistor is based on a heterostructure nanowire comprising of semiconductors with large and small g-factors that form the topological and non-topological regions respectively. The hybridization of bound edge states results in spin currents and 4π-periodic torques, as a function of the relative magnetic field angle – an effect which is dual to the fractional Josephson effect. We establish relation between torques and spin-currents in the non-topological region where the magnetic field is almost zero and spin is conserved along the spin–orbit field direction. The angular momentum transfer could be detected by sensitive magnetic resonance force microscopy techniques.

We describe how a universal set of dynamically corrected quantum gates can be implemented using sequences of shaped decoupling pulses on any qubit network forming a sparse bipartite graph with always-on Ising interactions. These interactions are constantly decoupled except when they are needed for two-qubit gates. We analytically study the error operators associated with the constructed gates up to third order in the Magnus expansion, analyze these errors numerically in the unitary time evolution of small qubit clusters, and give a bound on high-order errors for qubits on a large square lattice. We prove that with a large enough toric code the present gate set can be used to implement fault-tolerant quantum memory.