Symposium LN01 : Materials Science to Empower Quantum Information Technologies (NEW!)

The past decade has seen tremendous success in predicting and experimentally discovering distinct classes of topological insulators (TIs) and semimetals. We review the field and we propose an electronic band theory that highlights the link between topology and local chemical bonding, and combines this with the conventional band theory of electrons. Topological Quantum Chemistry is a description of the universal global properties of all possible band structures and materials, comprised of a graph theoretical description of momentum space and a dual group theoretical description in real space. We classify the possible band structures for all 230 crystal symmetry groups that arise from local atomic orbitals, and show which are topologically nontrivial. We show how our topological band theory sheds new light on known TIs, and demonstrate the power of our method to predict a plethora of new TIs.

Majorana zero modes are fermion-like excitations that were originally proposed in particle physics by Ettore Majorana and are characterized as being their own anti-particle. In condensed matter systems Majorana zero modes occur as fractionalized excitations with topologically protected degeneracy associated with such excitations. For over a decade the only candidate systems for observing Majorana zero modes were the non-Abelian fractional quantum Hall state and chiral p-wave superconductors. In this talk, I will discuss theoretical understanding of the results from the experimental search for Majorana zero modes in spin-orbit coupled semiconductor-based topological superconducting systems. Following this I will discuss future possibilities in terms of modifications to the experiments to help confirm the presence of Majorana zero modes through more dramatic signatures.

Engineering coherent systems is a central goal of quantum science and quantum information processing. Point defects in diamond known as color centers are a promising physical platform. As atom-like systems, they can exhibit excellent spin coherence and can be manipulated with light. As solid-state defects, they can be produced at high densities and incorporated into scalable devices. Diamond is a uniquely excellent host: it has a large band gap, can be synthesized with sub-ppb impurity concentrations, and can be isotopically purified to eliminate magnetic noise from nuclear spins. Specifically, the nitrogen vacancy (NV) center has been used to demonstrate basic building blocks of quantum networks and quantum computers, and has been demonstrated to be a highly sensitive, non-invasive magnetic probe capable of resolving the magnetic field of a single electron spin with nanometer spatial resolution. However, realizing the full potential of these systems requires the ability to both understand and manipulate diamond as a material. I will present two recent results that demonstrate how carefully tailoring the diamond host can open new opportunities in quantum science.
First, currently-known color centers either exhibit long spin coherence times or efficient, coherent optical transitions, but not both. We have developed new methods to control the diamond Fermi level in order to stabilize a new color center, the neutral charge state of the silicon vacancy (SiV) center. This center exhibits both the excellent optical properties of the negatively charged SiV center and the long spin coherence times of the NV center, making it a promising candidate for applications as a single atom quantum memory for long distance quantum communication.
Second, color centers placed close to the diamond surface can have strong interactions with molecules and materials external to the diamond, which makes them promising for nanoscale sensing and imaging. However, uncontrolled surface termination and contamination can degrade the color center properties and give rise to noise that obscures the signal of interest. I will describe our recent efforts to stabilize shallow NV centers within 5 nm of the surface using new surface processing and termination techniques. Specifically, we are able to demonstrate reversible and reproducible control over the top layer of atoms. These highly coherent, shallow NV centers will provide a platform for sensing and imaging down to the scale of single atoms.
References
[1] B. C. Rose, D. Huang, Z. Zhang, A.M. Tyryshkin, S. Sangtawesin, S. Srinivasan, L. Loudin, M. L. Markham, A. M. Edmonds, D. J. Twitchen, S. A. Lyon, and N. P. de Leon, “Observation of an environmentally insensitive solid state spin defect in diamond,” arXiv:1706.01555 [cond-mat.mtrl-sci] (2017)
[2] B. C. Rose, G. Thiering, A.M. Tyryshkin, A. M. Edmonds, M. L. Markham, A. Gali, S. A. Lyon, and N. P. de Leon, “Strongly Anisotropic Spin Relaxation in the Neutral Silicon Vacancy Center in Diamond,” arXiv:1710.03196 [quant-ph] (2017)

One of the principal challenges facing all implementations of quantum computing is protection of qubits from the debilitating effects of decoherence. Through unwanted interactions with the host environment of the qubit information stored in local degrees of freedom (e.g. spin in a semiconductor quantum dot) is lost. Topological quantum computing seeks to mitigate or wholly overcome decoherence by utilizing non-local quantum degrees of freedom to represent a qubit state. These non-local topological degrees of freedom do not couple to local perturbations and are consequently less susceptible to decoherence. Devising and constructing suitable physical platforms for topological qubits poses challenge for materials science. Topological degrees of freedom known as Majorana zero modes (MZMs) are predicted to form in hybrid systems consisting of low dimensional semiconductors with strong spin-orbit coupling that have been proximitized by an s-wave superconductor and placed in an appropriate magnetic field to open a topological gap. In this way the hybrid system mimics a topological p-wave spin less superconductor. A nanowire constructed from this system should support MZM at its ends. In this talk I will describe our efforts to realize MZM in hybrid systems consisting of a two-dimensional electron gas (2DEG) in strong spin-orbit coupled InAs or InSb that is proximitized with an ordinary s-wave superconductor, typically aluminum. Structures are grown in a multi-chamber molecular beam epitaxy system. We present experimental evidence for the formation of MZMs in nanowires defined through top-down lithography in the Al-InAs 2DEG system. Important materials challenges including epitaxial control of the superconductor/semiconductor interface, control of charge transfer across the interface by bandgap engineering, control of the induced superconducting gap, and reduction of disorder in a near-surface 2DEG will be discussed. New results and the impact of material parameters on device performance will be emphasized.

As the field of superconducting quantum computing advances, the requirements for fabrication and materials has significantly clarified. In particular, this involves designing low temperature, low loss circuits with good RF properties. Specific categories that need to be addressed are intrinsic capacitive losses due to parasitic capacitive losses, unwanted modes in ground planes and RF losses in normal metal components. In this talk we will touch on each of these three areas by describing developments in superconducting films with optimized interfaces and novel junctions to avoid two level systems in amorphous materials, fabrication of self-aligned ground plane cross-overs, and new types of superconductors that can be integrated into the support circuitry of the superconducting devices.

Quantum materials exhibit unique properties to study the most fundamental aspects of quantum mechanics. Exploiting these phenomena for future quantum technology will require a materials platform that can combine them with reproducibility, uniformity, and scalability. Silicon germanium heterostructures offer the capability to combine the unparalleled industrial semiconductor technology with key elements of the quantum toolbox, including long quantum coherence, strong spin-orbit coupling, and superconductivity. We present our latest results on gate-defined quantum dots and superconductivity in Ge/SiGe quantum wells [1] and give a perspective on the wide possibilities that this materials system offers in the domain of quantum information.
Because of the abundance of their nuclear-spin-free isotopes, Si and Ge can host spin qubits with long quantum coherence and excellent qubit control, enabling the recent implementation of two-qubit quantum algorithms and strong electron phonon coupling in Si/SiGe devices. While these results are promising, quantum hardware beyond the few qubit regime will need to overcome additional challenges, such as gate connection bottlenecks. Here, an excellent reproducibility and uniformity are key and can enable qubit arrays with shared control [2] using the leverage of today’s semiconductor manufacturing.
Holes in Ge/SiGe heterostructures can provide the necessary high mobility and come with strong spin-orbit coupling for fast qubit operation. As an added bonus, Ge readily forms Ohmic contacts with metal leads which are diffused into the heterostructure. This not only significantly simplifies the fabrication and tuning processes for quantum dot devices, but also enables proximity-induced superconductivity in Ge quantum wells. We present the confinement of heavy holes with mobilities exceeding 500,000 cm2/Vs in quantum dots and find a remarkable uniformity of the gate control. Furthermore, gate-controlled superconductivity is demonstrated in these heterostructures. These first results confirm the great potential of Germanium as a leading material for quantum computing as well as a versatile platform for semiconductor – superconductor hybrids and the exploration of emergent physics.
[1] N.W. Hendrickx, D.P. Franke, A. Sammak, M. Kouwenhoven, D. Sabbagh, L. Yeoh, R. Li, M.L.V. Tagliaferri, M. Virgilio, G. Capellini, G. Scappucci, and M. Veldhorst, preprint at arxiv:1801.08869 (2018)
[2] R. Li, L. Petit, D.P. Franke, J.P. Dehollain, J. Helsen, M. Steudtner, N.K. Thomas, Z.R. Yoscovits, K.J. Singh, S. Wehner, L.M.K. Vandersypen, J.S. Clarke, and M. Veldhorst, preprint at arxiv:1711.03807 (2017)

Organic-inorganic perovskites have already exhibited an enormous potential for optoelectric devices, such as photovoltaics, light-emitting diodes and photodetectors, which can realize low-cost and large area devices with high performance. Especially, perovskites reveal effective light absorption and high charge carrier mobility which lead to exploiting sensitive and fast photodetectors for visual and chemical/biological image sensing. In previous researches, inorganic quantum dots (QDs) was applied to perovskite solar cell for effective energy alignment which improves charge transport.[1] Herein, we demonstrate enhanced performance of solution-processed photodetectors composed of CH₃NH₃PbI₃ perovskite films with CH₃NH₃PbX₃(X=I, Br) QDs as a charge transport layer. Meanwhile, highly bright and stable CH₃NH₃PbX₃ QDs are difficult to synthesize through currentligand-assisted reprecipitation(LARP) technique because of the organic solvent in the precursor and the difficulty of redispersion. We developa simple and highly reproducible synthesis method ofCH₃NH₃PbX₃QDs by modified LARP technique. The advanced synthesis technique has process of precipitation of QDs by anti-solvent and surfactants are already dissolved in poor solvent instead of precursor.Introducing the QD transport layer controlled optical properties ontoperovskite films results in more improved photo response in the devices caused by enhanced near-field luminescence and plasmonic resonance. The QDs can also promote the transport of photon-generated carriers from the active layer to electrode. Combining the advantages of facile solution processability and high charge carrier mobility, introduction of the QDs based on novel synthesis is expected to replace commercial silicon with perovskite photodetection system.
[1] L. Hu, W. Wang, H. Liu, J. Peng, H. Cao, G. Shao, Z. Xia, W. Ma and J. Tang, J. Mater. Chem. A, 2015, 3, 515