Bulletin of the American Physical Society
APS March Meeting 2021
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session C31: Quantum Computing with Topological SuperconductorsFocus Live

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Sponsoring Units: DQI Chair: Gilad BenShach, IBM 
Monday, March 15, 2021 3:00PM  3:12PM Live 
C31.00001: Measurement and Control of Chiral Edge States in Graphene Curt Richter, Son Le, Joseph Hagmann, Ji Ung Lee By using a unique ringshaped device based on hBN sandwiched graphene with two electrostatic pn junctions (pnJs) we have experimentally probed the interactions between integer quantumHall edgestates along the physical edges of graphene and at electrostatic edges defined by the pnJs. A first pnJ is used to decouple the edgestate of the energetically lowest Landau level (LL) from the others while a second pnJ senses the amount of interaction between the lowest LL and the rest. These multiplepnJ measurements show that all edgestates interact strongly and equilibrate along the graphene physical edge while also confirming our previous results [1] that the lowest LL edgestate is decoupled from the other LL’s along the electostatic pnJ. By combining electrostatic and etched edges, chiral edgestates can be spatially separated at appropriate pnJs and then returned and mixed at physical boundaries. These results illustrate methods to manipulate chiral edgestates in graphene and are an advancement towards demonstrating the control of topologically protected chiral systems necessary for quantum logic operations. 
Monday, March 15, 2021 3:12PM  3:24PM Not Participating 
C31.00002: Contextuality and memory cost of simulation of Majorana fermions Susane Calegari, Juani BermejoVega, Michal Oszmaniec Contextuality has been reported to be a resource for quantum computation, analogous to nonlocality which is a known resource for quantum communication and cryptography. We show that the presence of contextuality places new lower bounds on the memory cost for classically simulating restricted classes of quantum computation. We apply this result to the simulation of a model of quantum computation based on the braiding of Majorana fermions, namely topological quantum computation (TQC) with Ising anyons, finding a saturable lower bound in loglinear in the number of physical modes for the memory cost. TQC model lies in the intersection between two computational models: the Clifford group and the fermionic linear optics (FLO), a framework analogous to bosonic linear optics. We extend our results and prove that the lower bound in the memory required in an approximate simulation of the FLO model is quadratic in the number of physical modes. 
Monday, March 15, 2021 3:24PM  3:36PM Live 
C31.00003: Nonlocal signatures of hybridization between quantum dot and Andreev bound state Andreas Pöschl, Alisa Danilenko, Deividas Sabonis, Kaur Kristjuhan, Tyler Lindemann, Sergei Gronin, Geoffrey C. Gardner, Candice Thomas, Michael Manfra, Charles M Marcus Andreev bound states (ABS) in semiconductorsuperconductor hybrid structures have attracted considerable attention in the past decade. 
Monday, March 15, 2021 3:36PM  3:48PM Live 
C31.00004: Adiabatic fusion and nucleation of Majorana bound states Alireza Habibi, Thomas L Schmidt A semiconductor nanowire with strong spinorbit coupling and induced superconductivity can host Majorana bound states. Using them in topological quantum computation usually requires braiding them as well as nucleating and fusing pairs of Majorana fermions. In all these processes, the time evolution should be adiabatic in order to prevent any qubit errors triggered by transitions above the bandgap. In this study, we investigate the maximum speed for remaining in the adiabatic limit during these manipulations. In particular, we derive the criteria for nucleation and fusion of Majorana bound states to remain adiabatic. To obtain our results, we use the densitymatrix renormalization group and exact diagonalization as numerical techniques and compare those to analytical results from perturbation theory. 
Monday, March 15, 2021 3:48PM  4:00PM Live 
C31.00005: Transport and spectroscopy of junction Andreev bound states in halfshell nanowire transmon devices Alisa Danilenko, Deividas Sabonis, Georg W Winkler, Oscar Erlandsson, Anders Kringhøj, Bernard Van Heck, Peter Krogstrup, Charles M Marcus Semiconductor/superconductor hybrid nanowirebased devices in a cQED implementation are a promising platform for the detection of properties related to topological superconductivity. Here we study a gatecontrolled Josephson junction, formed by selectively removing Al in a lithographically defined segment of a hybrid InAs/Al halfshell nanowire. The junction is incorporated in a transmon device geometry which enables both lowfrequency transport studies and coherent time domain qubit control via cQED techniques. Variation of voltages on nearby electrostatic gates allows for drastic changes in the qubit frequency response as function of parallel magnetic field. The effects observed experimentally are in agreement with a theoretical model that suggests fluxmodulated energies of Andreev bound states within the junction. 
Monday, March 15, 2021 4:00PM  4:36PM Live 
C31.00006: Majorana qubits Invited Speaker: Sankar Das Sarma I will provide a status report on the progress toward topological quantum computing using nonAbelian Majorana qubits in the semiconductorsuperconductor hybrid platform. I will cover theory, experiments, materials development, and device fabrication. Much has happened, but a lot still needs to be done. I will propose a roadmap for the future. 
Monday, March 15, 2021 4:36PM  4:48PM Live 
C31.00007: Chargetransfer based operations revealing nonabelian statistics of Majorana bound states Ruben Seoane Souto, Karsten Flensberg, Martin Leijnse By now, there has been a lot of evidence of the existence of Majorana bound states (MBSs) at the ends of topological superconductors (TSs) [1]. However, a definitive proof of their topological origin will rely on the demonstration of their nonabelian statistics, emerging after the exchange of two or more MBSs in real space. Alternatively, MBSs can be exchanged in parameter space using chargetransfer operations using a quantum dot (QD) coupled to two TSs [2]. In this presentation I will analyze the efficiency of these operations [3]. Using a full counting statistics analysis, we set bounds to the optimal operation time scales. The lower bound depends on the phase difference, due to a partial decoupling of the different parity sectors. The upper bound is determined by poisoning processes due to the tunneling of quasiparticles to the MBSs. Using realistic parameters, we find that operations can be performed with a fidelity close to unity in the μs to ms timescales, demonstrating the absence of dephasing and accumulated dynamical phases. 
Monday, March 15, 2021 4:48PM  5:00PM Live 
C31.00008: Dynamics of the MajoranaTransmon qubit with timedependent voltage bias. Elena Lupo, Eytan Grosfeld, Eran Ginossar Hybrid topologicalsuperconducting systems engineered by combining superconducting circuits with 1D Majorana Bound States (MBS) have been proposed as possible platforms for quantum computation. In these systems the topological states are formed by inducing pwave superconductivity in nanowires in contact with the superconducting leads. Among these proposals there is the recent idea of encoding the quantum information in a basis of fermion parity states originating from an additional hybridization between MBSs of different nanowires placed across a Josephson Junction [1]. Even if the coupling between the two Majorana fermions sacrifices their full topological protection, the system gains a highly anharmonic spectrum and it has been proposed that it can be used for engineering a coherent qubit [2]. Here we focus on the theoretical study of this system's dynamics in the case of timedependent voltage bias. Within this scenario, we investigate possible protocols for implementing single qubit gates in the system, together with an analysis of the effect of the charge noise on them. 
Monday, March 15, 2021 5:00PM  5:12PM Live 
C31.00009: Paritytocharge conversion for readout of topological Majorana qubits Gábor Széchenyi, András Pályi

Monday, March 15, 2021 5:12PM  5:24PM 
C31.00010: Timons: superconducting gatemon qubits based on proximitized topological insulators Tobias W Schmitt, Malcolm Connolly, Michael Schleenvoigt, Chenlu Liu, Declan Burke, Oscar Kennedy, Tobias Lindstrom, Sebastian de Graaf, Abdur Rehman Jalil, Benjamin Bennemann, Stefan Trellenkamp, Florian Lentz, Elmar Neumann, Erwin Berenschot, Niels Tas, Kristof Moors, Gregor Mussler, Karl Petersson, Detlev Grützmacher, Peter Schüffelgen Superconducting transmon qubits are frontrunners in the race to build a scalable quantum computer. Gatemons are a transmon variant with the metaloxide Josephson junction replaced by a voltagecontrolled semiconductor, eliminating crosstalk and heating from fluxbias currents, and enabling new topologicallyprotected modes of operation. Gatemons with proximitized IIIVs are difficult to scale [1], or have short relaxation times due to losses in the host substrate [2]. Both require substantial magnetic fields to tune to the topological regime. Here we introduce a new gatemon platform based on VVI semiconductor (Bi_{x}Sb_{1x})_{2}Te_{3} 3D topological insulators. We use selective area growth and nanostencil lithography on silicon for scalable fabrication of lowloss TIgatemon (timon) circuits, and explore the prospect of using magnetic dopants to induce topological protection at zero field. Initial results suggest the timon platform is reliable and robust enough for nextgeneration gatemons [3]. 
Monday, March 15, 2021 5:24PM  5:36PM Live 
C31.00011: Calculating Tunneling Matrix Elements to Majorana Zero Modes Using the BarrierIntegrated MahauxWeidenmüller Formula David van Driel, Andrey Antipov Hybrid superconductorsemiconductor nanowires are considered a prime candidate for hosting Majorana zero modes. For their creation, readout and manipulation, one relies on tunable tunnel gates. More specifically, these gates govern the coupling between a nanowire and another system, including quantum dots, leads or another nanowire. Constructing a faulttolerant topological quantum computer relies on having finetuned tunnel couplings between all the subsystems. These couplings are probed in transport, by controlling the conductance of an island using tunnel gates. We employ the MahauxWeidenmüller formula to calculate the scattering matrix for an island connected to leads. Splitting off the tunnel junctions, one retrieves the tunneling matrix elements between the leads and system. Hence, we utilize this BarrierIntegrated MahauxWeidenmüller formula to estimate tunnel couplings to Majorana zero modes in a range of geometries, including realistic devices. 
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