The fluctuation relations, which characterize irreversible processes in Nature, are among the most important results in non-equilibrium physics. In short, these relations say that it is exponentially unlikely for us to observe a time-reversed process and, thus, establish the thermodynamic arrow of time pointing from low to high entropy. On the other hand, fundamental physical theories are invariant under time-reversal symmetry. Although in Newtonian and quantum physics the emergence of irreversible processes, as well as fluctuation relations, is relatively well understood, many problems arise when relativity enters the game. In this talk we explore the question of how the general relativistic effects enter into the fluctuation relations. We conclude that a positive entropy production emerges as a consequence of spacetime curvature.

Neuromorphic computing mimics the brain's architecture to create energy-efficient devices. Reconfigurable synapses are crucial for neuromorphic computing, which can be achieved through memory-resistive (memristive) switching. Graphene-based memristors have shown nonvolatile multi-bit resistive switching with desirable endurance. Through first-principles calculations, we study the structural and electronic properties of graphene in contact with an ultra-thin alumina overlayer and demonstrate how one can use charge doping to exert direct control over its interfacial covalency, reversibly switching between states of conductivity and resistivity in the graphene layer. We further show that this proposed mechanism can be stabilized through the p-type doping of graphene, e.g., by naturally occurring defects, the passivation of dangling bonds, or defect engineering.

Relevant reference: 10.1103/PhysRevResearch.5.043147

In this talk I will overview some of our recent works involving (i) the nonlocality of local Andreev conductances as a probe for topological Majorana wires in novel three-terminal superconducting 1D setups, asymmetrically coupled to normal leads [1], (ii) phase driving hole spin qubits in double quantum dots under simultaneous transverse (Rabi) and longitudinal (phase) drives [2], which enables tunable additional side bands and (some) immunity against noise, and (iii) beating-free magnetoresistitivity in 2D electron gases with strong (unmatched) spin-orbit and Zeeman interactions, in which a new condition for the vanishing of beatings is derived [3,4].  
 
*On leave from the University of São Paulo (IFSC).
[1] Dourado, Penteado, and Egues, arXiv:2303.01867.
[2] Bosco, Geyer, Camenzind, Eggli, Fuhrer, Warburton, Zumbühl, Egues, Kuhlmann, and Loss, arXiv:2303.03350, Phys. Rev. Lett., in press (Editors' suggestions).
[3] Candido, Erlingsson, Gramizadeh, Costa, Weigele, Zumbühl, and Egues, arXiv:2304.14327.
[4] Gramizadeh, Candido, Manolescu, Egues, and Erlingsson, arXiv:2306.02503.

Recently, twist bilayer van der Waals heterostructures have been the subject of substantial theoretical and experimental works due to many fascinating electrical, optical, and magnetic properties, such as unconventional superconductivity, ferroelectricity, and correlated insulator behavior for rotation angle between layers of order θ ∼ 1◦. [1, 2, 3] Moreover, quantum dots (QDs) in bilayer graphene (BLG) are a promising quantum information platform because of their long spin decoherence times, high sample quality, and tunability, whereas quantum rings (QRs) are the most natural systems to investigate quantum interference phenomenon in transport properties, Aharonov–Bohm oscillations and persistent currents. Within the context of moiré superlattice and quantum confinement systems, [4] in this talk, we present a systematic study of the energy levels of twisted BLG QDs and QRs, both in the absence and presence of an external perpendicular magnetic field. Results are obtained within the tight-binding model, with
interlayer hopping parameters defined by the Slater-Koster form, which takes into account the distance between the lattice points, which is fundamental for obtaining the Hamiltonian for inter-layer twisted systems. The confinement structures are modeled by a circular dotlike and ringlike-shape site-dependent staggered potential, which prevents edge effects. Due to a non-zero interlayer twist angle, the energy spectra exhibit features resulting from the interplay between characteristics of the AA and AB stacking orders that compose the moiré pattern of such twisted bilayer. Our findings show that, in the absence of a magnetic field, the energy levels of the QR scale with its width W according to a power law W^{−α}, whose exponent 1 ⪅ α ⪅ 2 depends on the twist angle. Moreover, assuming the so-called magic angle (θ = 1.08◦) for the interlayer twist, the lowest energy state oscillates as a function of the average radius of the ring, as a consequence of the different distributions of AA and AB stacking regions for each value of radius. In the presence of a perpendicular magnetic field, two sets of energy levels, which approach the Landau levels of infinite AA-staked and AB-staked BLG sheets, are observed, from which a variety of crossings between energy states emerges. Interestingly, these sets of energy states exhibit periodic (Aharonov-Bohm) oscillations as a function of the magnetic field, even for a QD, which reveals information about the moiré pattern of AA and AB stacked regions covered by the ring area.

[1] Y. Cao, V. Fatemi, S. Fang, K. Watanabe, T. Taniguchi, E. Kaxiras, and P. Jarillo-Herrero,
Nature 556(7699), 43 (2018).
[2] M. Yankowitz, S. Chen, H. Polshyn, Y. Zhang, K. Watanabe, T. Taniguchi, D. Graf, A. F.
Young, and C. R. Dean, Science 363(6431), 1059 (2019).
[3] Y. Cao, V. Fatemi, A. Demir, S. Fang, S. L. Tomarken, J. Y. Luo, J. D. Sanchez-Yamagishi,
K. Watanabe, T. Taniguchi, E. Kaxiras, et al, Nature 556(7699), 80 (2018).
[4] M. Mirzakhani, F. M. Peeters, and M. Zarenia. Physical Review B 101(7), 075413 (2020).

Superconducting qubits operating at microwave frequencies have shown great promise for quantum computation; however, they do not have an innate interface with optical photons in the telecommunication band. A quantum transducer is necessary for converting quantum information between microwave and optical frequencies. This presentation focus on quantum transduction using rare-earth ions in solid state systems, with a detailed discussion on the ion energy levels and transitions. We propose using magnetic materials with rare-earth dopants, harnessing the strong coupling between rare-earth spin transitions and magnons. We analyze this situation using a formalism similar to Ref. [PRL 113, 203601 (2014)]. We find that hosting rareearth elements within a magnet dramatically speeds up the transduction rate by more than two orders of magnitude, which gives several key benefits: potentially higher efficiency as it is less affected by device internal losses, higher fidelity operations with the superconducting qubits, and reduced device constraints.

Recent advances in fabrication and manipulation of two-dimensional (2D) materials have enabled the control over stacking order and rotation between layers of transition metal dichalcogenides into van der Waals heterobilayers. In such twisted bilayers, the moiré pattern created by the lattice mismatch between atoms of the two layers play an important role: it produces an effective potential profile along the plane for excitons, with potential minima emerging in regions with specific local stacking registries. The superlattice of moiré potentials results in a new band structure for the bilayer, and the quasi-particle related to this band is called "moiré exciton".
In principle, the most precise way to calculate such moiré exciton band structure would be to construct a unit cell of the moiré pattern, with all its atoms, and perform ab initio calculations of the corresponding electronic structure. However, the huge number of atoms in such unit cells, especially for very small twist angles, makes first principle calculations computationally expensive and prohibitive. In this talk, I suggest an alternative way to calculate moiré exciton band structures in a continuum model approximation, which allows us to investigate these quasi-particles in heterobilayers with arbitrarily small twist angles. Our results show that moiré excitons in MoS2/WSe2 exhibit a Dirac-like band structure, where the mass of the associated Dirac fermion can be controlled  by an external electric field.
Two consequences of this unique band structure will be discussed: (i) the trembling motion of the exciton wave function, due to the effect known as zitterbewegung - a solely quantum effect that has been predicted by E. Schrodinger in the 30's, but for which an experimental proof has been elusive; and (ii) the possibility of observing topologically protected moiré excitons in a grain boundary between different regions of the van der Waals stack.

Recently, spin centers in solids (e.g., Nitrogen-Vacancy (NV) center) have attracted significant attention due to their applications to quantum information science, e.g., spin qubit and quantum sensor [1-4]. However, to be able to create entanglement between NV centers, one requires having NVs coupled to each other. Unfortunately, the bare interaction between two NV centers is week for separations > 20 nm. This creates a key challenge once NV centers cannot be optically resolvable at these distances. Therefore, providing alternative schemes to couple NV centers over long distances became crucial to enable their use in quantum computation.

Here, we first propose hybrid quantum systems that couple and entangle spin centers over micron length scales through the quantized spin-wave excitations (magnons) of a magnetic material [5,6]. These magnons serve as a quantum bus that transfers the information between different NVqubits. We predict strong long-distance (µm) NV-NV coupling via magnon modes with cooperativities exceeding unity in ferromagnetic bar, waveguide and cylindrical structures [5,6]. Moreover, we explore and compare on-resonant transduction and off-resonant virtual-magnon exchange protocols, and discuss their suitability for generating or manipulating entangled states under realistic experimental conditions [6]. Due to the absence of magnon occupation decay of the off-resonant protocol, our results show this protocol is robust at temperatures up to T≈150mK [6]. Conversely, at lower temperatures the on-resonant protocol shows a faster gate operation, and can even outperform the off-resonance protocol for small magnon damping parameters [6]. Secondly, we experimentally determine the NV-NV coupling mediated by magnons for a diamond slab on top of a YIG bar [7]. This is obtained through the magnon-induced self-energy of the NV center, obtained by combining room-temperature longitudinal relaxometry and an analysis using the fluctuationdissipation and Kramers-Kronig relations [7]. We show our results are quantitatively consistent with our theoretical model [5,6] where NV centers are coupled to magnons by the dipole interaction.

[1] DR Candido, ME Flatté, PRX quantum 2 (4), 040310 (2021)

[2] DR Candido, ME Flatté, arXiv:2303.13370

[3] DR Candido, ME Flatté, arXiv:2112.15581

[4] U Zvi, DR Candido, A Weiss, AR Jones, L Chen, I Golovina, X Yu, S Wang, DV Talapin, ME Flatté, AP Esser-Kahn, PC Maurer, arXiv:2305.03075

[5] DR Candido, GD Fuchs, E. Johnston-Halperin, and ME Flatte, Materials for Quantum Technology 1, 011001 (2021).

[6] M Fukami, DR Candido, DD Awschalom, and ME Flatte, PRX Quantum 2, 040314 (2021).

[7] M Fukami, JC Marcks, DR Candido, LR Weiss, B Soloway, SE Sullivan, N Delegan, FJ Heremans, ME Flatté and DD Awschalom, arXiv:2308.11710

Neste seminário apresentarei detalhes sobre a estrutura e organização do Programa de Pós-Graduação em Física da Universidade Federal de Goiás (conceito CAPES 6). 

Hole spin qubits in silicon and germanium quantum dots are promising platforms for large-scale quantum computers because of their large intrinsic spin-orbit interaction, which permits efficient and ultrafast all-electric qubit control without additional components. I will present schemes to engineer this interaction in different architectures, e.g. in the squeezed Ge quantum dots proposed in [1], aiming to optimize quantum information processing. A large spin-orbit interaction mediates a strong coupling between hole spins and microwave photons. Hole spin-photon coupling is not only strong but is also electrically tunable and can be engineered to be longitudinal [2], where the microwave field couples to the phase of the spin. This type of coupling enables exact protocols for fast and high-fidelity two-qubit gates that could even work at high temperatures.
On the other hand, the spin-orbit interaction also couples the spin to charge noise, causing the qubit to decohere. To overcome this issue, I will discuss qubit designs that enable sweet spots where charge noise can be completely removed [3]. These sweet spots appear in hole spin qubits encoded in silicon fin field-effect transistors, devices commonly used in the modern semiconductor industry. In these qubits, the noise caused by hyperfine interactions with nuclear spins -another leading source of decoherence in spin qubits- is also strongly suppressed, greatly enhancing their coherence, and reducing the need for expensive isotopically purified materials [4]. Moreover, the large spin-orbit interaction in hole quantum dots enables phenomena that are out of reach in competing architectures. For example, in these systems the exchange interactions between nearby spins can be highly anisotropic, even at zero magnetic fields, opening the way to novel protocols to enhance the speed and fidelity of two-qubit gates in future quantum processors.

[1] Bosco et al (2021) PRB 104
[2] Bosco et al (2022) PRL 129
[3] Bosco Hetenyi Loss (2021) PRX Quantum 2
[4] Bosco and Loss (2021) PRL 127

Nowadays, one of the major scientific challenges is related to the development of chemical-free technologies that do not pose risks to human health and the environment. Thus, substituting petroleum-derived chemicals with renewable resources and using green methods for material synthesis have become central topics for a sustainable future. In this context, nanocellulose extracted from biomass is an excellent starting nano-block. It is abundant in nature, renewable, low in toxicity, and offers numerous practical applications, including existing technologies and the potential for developing innovative ones. During my presentation, I will demonstrate how it is possible to transform Brazil's most abundant agricultural waste, sugarcane bagasse, into renewable materials.