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.

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

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.

Most of 2D superconductors are of type II, i.e., they are penetrated by quantized vortices when exposed to out-of-plane magnetic fields. In a presence of a supercurrent, a Lorentz-like force acts on the vortices, leading to drift and dissipation. The current-induced vortex motion is impeded by pinning at defects. Usually, the pinning strength decreases upon any type of pair-breaking interaction perturbs a system.

In the talk I will discuss surprising experimental evidences showing an unexpected enhancement of pinning in synthetic Rashba 2D superconductors when applying an in-plane magnetic field. When rotating the in-plane component of the field with respect to the driving current, the vortex inductance turns out to be highly anisotropic. We explain this phenomenon as a direct manifestation of Lifshitz invariant that is allowed in the Ginzburg-Landau free energy when space-inversion and time-reversal symmetries are broken. As demonstrated in our experiment [1], elliptic squeezing of vortices---an inherent property of the non-centrosymmetric superconducting condensate---provides an access to fundamentally new property of Rashba superconductors, and offers an entirely novel approach to vortex manipulation.

Another interesting feature of the non-centrosymmetric superconductors in the applied magnetic field is the supercurrent diode effect---the critical current in one direction exceeds its counterpart in the opposite one---what stems from the Cooper pairs with finite centre of mass momentum. In the pioneering experiment [2] we demonstrated the emergence of the supercurrent diode effect in the Josephson junctions based on synthetic Rashba superconductors made of Al-InAs quantum wells. In the talk, I will discuss novel experimental method---measurements of the Josephson inductance---and the semiquantitative microscopic model capturing all the essential features as observed in experiment. 

[1]        L. Fuchs, D. Kochan, C. Baumgartner, S. Reinhardt, S. Gronin, G. Gardner, T. Lindemann,

M. Manfra, C. Strunk, N. Paradiso; Physical Review X 12 (4), 041020 (2022).

[2]       C. Baumgartner, L. Fuchs, A. Costa, S. Reinhardt, S. Gronin, G. Gardner, T. Lindemann,

M. Manfra, P. Faria Junior, D. Kochan, J. Fabian, N. Paradiso, C. Strunk; Nature Nanotechnology 17 (1), 39 (2022).

Sum of powers of principal minors (SPPM) of matrices appears in the calculation of many quantities in physics and applied mathematics. We show that the calculation of the Renyi entropy of fermionic systems, partition function of the Hubbard model and certain problems in machine learning are related to the SPPM problem. Although there is a simple formula to calculate the sum of principal minors of arbitrary matrices, it has been already shown that calculation of other powers is a hard problem (probably no polynomial time algorithm exists).
In this talk I will first discuss how the Renyi (Shannon) entropy in quantum chains detects different phases and determines the universality classes. Then I will show how calculating this quantity boils down to a SPPM problem in certain quantum systems such as Ising chain and free fermions. Finally in the main part of the talk I will write a Grassmann representation for a generic matrix SPPM problem. This field theory-like representation shows interesting symmetries such as U^n(1),  symmetric group,  axial U(1), chiral and particle-hole symmetry. Using this representation one can make a mean-field approximation and find an excellent estimate for the Renyi entropy which can detect the phase transition. We show that some of the mentioned symmetries are broken in, for example, the ferromagnetic phase of the Ising chain.

Advances in parallel computation and algorithm implementation are leading to accurate quantum mechanical descriptions of materials systems with hundreds of atoms and thousands of electrons.  Electronic structure methods based on the density functional theory with periodic boundary conditions become the workhorse in the simulations of defects in semiconductors, interfaces, and surfaces, and noststructures.  Developments in hybrid functionals have enabled accurate description of defect levels in semiconductors and insulators, defect-related absorption and emission energies, surface and interface electronic structures.  In this presentation, it will be discussed how these advanced computational methods are employed in the discovery of novel quantum materials and their exotic properties and help guide experimental efforts in the materials characterization. Specific topics to be covered include emerging topological phases in rare-earth pnictides, embedded nanoparticles in semiconductors, defects in 2D materials, and novel interface phenomena that are promising for the next generation of electronics and spintronics.

Implementation of quantum information using single photons, using heralded or entangled source, have versatility to be manipulated on free space, fiber optics or photonic chips. Our project proposes the uses of photonic chips to miniaturize experimental implementations using quantum optics. The chip design can be done using a similar design of free space experimental setup or can be used designs only possible in wave-guide. The photonic chip is based on silicon nitride wave-guide, allowing the generation and manipulation of correlated photons in the visible and infrared light. This band of the electromagnetic spectrum allows the integration of different quantum computation platforms, such as: ion traps and alkaline atoms. Moreover, using semiconductor material, it is possible to make integrated single photon detectors. This project is our first step for all in one device quantum computation based on photon chips.

Materials informatics is a new field of research that uses tools from data science to design, discover and understand new materials. In Brazil, CNPq has recently approved an INCT on this subject, focusing on several different areas of condensed matter physics, materials chemistry and materials science. In this talk I will discuss the recent advances in this area and present two works focusing on the magnetic properties and also on spin splittings in 2D compounds.