Topological quantum computer | EPFL Graph Search

A topological quantum computer is a theoretical quantum computer proposed by Russian-American physicist Alexei Kitaev in 1997. It employs quasiparticles in two-dimensional systems, called anyons, whose world lines pass around one another to form braids in a three-dimensional spacetime (i.e., one temporal plus two spatial dimensions). These braids form the logic gates that make up the computer. The advantage of a quantum computer based on quantum braids over using trapped quantum particles is that the former is much more stable. Small, cumulative perturbations can cause quantum states to decohere and introduce errors in the computation, but such small perturbations do not change the braids' topological properties. This is like the effort required to cut a string and reattach the ends to form a different braid, as opposed to a ball (representing an ordinary quantum particle in four-dimensional spacetime) bumping into a wall. While the elements of a topological quantum computer origin

Microwave-to-Optical Transduction with Gallium Phosphide Electro-Optomechanical Devices

Simon Benjamin Klaus Hönl

Quantum computing is one of the great scientific challenges of the 21st century. Small-scalesystems today promise to surpass classical computers in the coming years and to enable thesolution of classically intractable computational tasks in the fields of quantum chemistry,optimization, cryptography and more.In contrast to classical computers, quantum computers based on superconducting quantumbits (qubits) can to date not be linked over long distance in a network to improve their computingcapacity, since devices, which preserve the quantumstate when it is transferred from onemachine to another, are not available. Several approaches are being pursued to realize such acomponent, one of themost promising to date makes use of an intermediary, micromechanicalelement that enables quantum coherent conversion between the information presentin the quantum computer and an optical fiber, without compromising the quantum natureof the information, via optomechanical interaction. This approach could allow fiber-opticquantum networks between separate quantum computers based on superconducting qubitsin the future.In this work a platformfor such a microwave-to-optic link was developed based on the piezoelectricmaterial gallium phosphide. This III-V semiconductor offers not only a piezoelectriccoupling between the electric field of a microwave circuit and a mechanicalmode, but also awide optical bandgap E_g = 2.26eV which reduces nonlinear optical absorption in the deviceand a large refractive index n(1550nm) = 3.01 which allows strong optical confinement atnear-infrared wavelengths.Importantly and in contrast to other approaches with gallium phosphide, an epitaxiallygrown, single crystal thin film of the material is integrated directly on a silicon wafer withpre-structured niobium electrodes by direct wafer-bonding. This opens up the possibility ofintegrating the device design presented here directly with superconducting qubits fabricatedwith this material system.A microwave-to-optical transducer design was simulated and fabricated in the galliumphosphideon-silicon platform. The device was found to exhibit large vacuum optomechanical couplingrates g0/2 pi ~ 290kHz and a high intrinsic optical quality factor Q >10^5 while at the same timepermitting electromechanical coupling to a microwave electrode. Coherent microwave-toopticaltransductionwas shown at room temperature for this device and the electromechanicalcoupling rate could be extracted from a model derived by input-output theory.The electromechanical coupling between the electro-optomechanical device and a superconductingqubit was estimated to be g/2 pi = O(200kHz) which indicates that strong couplingbetween the here presented device and a superconducting transmon qubit is achievable.In addition, superconducting microwave cavities with high quality factor at single photonenergy Q ~ 5x10^5 were fabricated and measured to verify that fabrication process of themicrowave-to-optical transducer is compatible with high-quality superconducting microwavecircuits.

EPFL2021

Canted antiferromagnetic phases in the candidate layered Weyl material EuMnSb2

Xiao Wang

EuMnSb2 is a candidate topological material which can be tuned towards a Weyl semimetal, but there are differing reports for its antiferromagnetic (AFM) phases. The coupling of bands dominated by pure Sb layers hosting topological fermions to Mn and Eu magnetic states provides a potential path to tune the topological properties. Here we present single-crystal neutron diffraction, magnetization, and heat-capacity data as well as polycrystalline 151Eu M??ssbauer data which show that three AFM phases exist as a function of temperature, and we present a detailed analysis of the magnetic structure in each phase. The Mn magnetic sublattice orders into a C-type AFM structure below TNMn = 323(1) K with the ordered Mn magnetic moment ??Mn lying perpendicular to the layers. AFM ordering of the Eu sublattice occurs below TNEu1 = 23(1) K with the ordered Eu magnetic moment ??Eu canted away from the layer normal and ??Mn retaining its higher temperature order. ??Eu is ferromagnetically aligned within each Eu layer but exhibits a complicated AFM layer stacking. Both of these higher-temperature phases are described by magnetic space group (MSG) Pn'm'a' with the chemical and magnetic unit cells having the same dimensions. Cooling below TNEu2= 9(1) K reveals a third AFM phase where ??Mn remains unchanged but ??Eu develops an additional substantial in-plane canting. This phase a' . We also find some evidence of short-range magnetic correlations associated with the Eu between 12 K ??? T ??? 30 K. Using the determined magnetic structures, we postulate the signs of nearest-neighbor intralayer and interlayer exchange constants and the magnetic anisotropy within a general Heisenberg model. We then discuss implications of the various AFM states in EuMnSb2 and their potential for tuning topological properties.

AMER PHYSICAL SOC2022

Experimental QND measurements of complementarity on two-qubit states with IonQ and IBM Q quantum computers

Christophe Marcel Georges Galland, Nicolas Schwaller, Valeria Vento

We report the experimental nondemolition measurement of coherence, predictability and concurrence on a system of two qubits. The quantum circuits proposed by De Melo et al. (Phys Rev Lett 98(25):250501, 2007) are implemented on IBM Q (superconducting circuit) and IonQ (trapped ion) quantum computers. Three criteria are used to compare the performance of the different machines on this task: measurement accuracy, nondemolition of the observable, and quantum state preparation. We find that the IonQ quantum computer provides constant state fidelity through the nondemolition process, outperforming IBM Q systems on which the fidelity consequently drops after the measurement. Our study compares the current performance of these two technologies at different stages of the nondemolition measurement of bipartite complementarity.

SPRINGER2022

Microwave-to-Optical Transduction with Gallium Phosphide Electro-Optomechanical Devices

Simon Benjamin Klaus Hönl

EPFL2021