A possible sighting of Majorana states Nearly 80 years ago, the Italian physicist Ettore Majorana proposed the existence of an unusual type of particle that is its own antiparticle, the so-called Majorana fermion. The search for a free Majorana fermion has so far been unsuccessful, but bound Majorana-like collective excitations may exist in certain exotic superconductors. Nadj-Perge et al. created such a topological superconductor by depositing iron atoms onto the surface of superconducting lead, forming atomic chains (see the Perspective by Lee). They then used a scanning tunneling microscope to observe enhanced conductance at the ends of these chains at zero energy, where theory predicts Majorana states should appear. Science, this issue p. 602; see also p. 547 Scanning tunneling microscopy is used to observe signatures of Majorana states at the ends of iron atom chains. [Also see Perspective by Lee] Majorana fermions are predicted to localize at the edge of a topological superconductor, a state of matter that can form when a ferromagnetic system is placed in proximity to a conventional superconductor with strong spin-orbit interaction. With the goal of realizing a one-dimensional topological superconductor, we have fabricated ferromagnetic iron (Fe) atomic chains on the surface of superconducting lead (Pb). Using high-resolution spectroscopic imaging techniques, we show that the onset of superconductivity, which gaps the electronic density of states in the bulk of the Fe chains, is accompanied by the appearance of zero-energy end-states. This spatially resolved signature provides strong evidence, corroborated by other observations, for the formation of a topological phase and edge-bound Majorana fermions in our atomic chains.

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Observation of Majorana fermions in ferromagnetic atomic chains on a superconductor

We describe experimental signatures of Majorana fermion edge states, which form at the interface between a superconductor and the surface of a topological insulator. If a lead couples to the Majorana fermions through electron tunneling, the Majorana fermions induce resonant Andreev reflections from the lead to the grounded superconductor. The linear tunneling conductance is 0 (2e(2)/h) if there is an even (odd) number of vortices in the superconductor. Similar resonance occurs for tunneling into the zero mode in the vortex core. We also study the current and noise of a two-lead device.

https://dspace.mit.edu/openaccess-disseminate/1721.1/52593

Majorana Fermion Induced Resonant Andreev Reflection

Quantum mechanics can help to solve complex problems in physics and chemistry, provided they can be programmed in a physical device. In adiabatic quantum computing, a system is slowly evolved from the ground state of a simple initial Hamiltonian to a final Hamiltonian that encodes a computational problem. The appeal of this approach lies in the combination of simplicity and generality; in principle, any problem can be encoded. In practice, applications are restricted by limited connectivity, available interactions and noise. A complementary approach is digital quantum computing, which enables the construction of arbitrary interactions and is compatible with error correction, but uses quantum circuit algorithms that are problem-specific. Here we combine the advantages of both approaches by implementing digitized adiabatic quantum computing in a superconducting system. We tomographically probe the system during the digitized evolution and explore the scaling of errors with system size. We then let the full system find the solution to random instances of the one-dimensional Ising problem as well as problem Hamiltonians that involve more complex interactions. This digital quantum simulation of the adiabatic algorithm consists of up to nine qubits and up to 1,000 quantum logic gates. The demonstration of digitized adiabatic quantum computing in the solid state opens a path to synthesizing long-range correlations and solving complex computational problems. When combined with fault-tolerance, our approach becomes a general-purpose algorithm that is scalable.

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Digitized adiabatic quantum computing with a superconducting circuit, part I: Theory

The principle that perturbation in quantum statistics should be accompanied by application of an appropriate magnetic field has been successful in giving a simple understanding of major qualitative features of the fractional quantized Hall states and related anyon superconducting states. In these applications, the starting point is one or more filled Landau levels. Here we consider the question of perturbation around free fermions. We argue that very near this point the statistical interactions are weak and their effects calculable; nevertheless they have the important qualitative consequence that a p-wave BCS pairing instability is triggered. The result is a new line of incompressible states in the (inverse) filling-fraction-statistics plane. This line extrapolates to a state obeying Fermi statistics at filling fractio 12, which is a candidate to describe electron states. A variety of techniques is then employed to elucidate the properties of this state and the unusual quasiparticles it supports. We believe the state is in the same universality class as one Halperin proposed based on grouping electrons into pairs of tightly bound bosonic molecules, which form a correlated state of the Laughlin type. We report the results of extensive numerical work which establishes firmly the existence of an incompressible state with the properties we predict, including the very unusual quasiparticles, for simple model potentials. We also investigate the situation for realistic potentials, and conclude that a paired Hall state of the type investigated here is a good candidate to describe real 2d electron gases, especially for thick samples and higher Landau levels, quite possibly including the state at filling fraction 52 that has already been observed.

Paired Hall States

The control of quantum systems is of fundamental scientific interest and promises powerful applications and technologies. Impressive progress has been achieved in isolating quantum systems from the environment and coherently controlling their dynamics, as demonstrated by the creation and manipulation of entanglement in various physical systems. However, for open quantum systems, engineering the dynamics of many particles by a controlled coupling to an environment remains largely unexplored. Here we realize an experimental toolbox for simulating an open quantum system with up to five quantum bits (qubits). Using a quantum computing architecture with trapped ions, we combine multi-qubit gates with optical pumping to implement coherent operations and dissipative processes. We illustrate our ability to engineer the open-system dynamics through the dissipative preparation of entangled states, the simulation of coherent many-body spin interactions, and the quantum non-demolition measurement of multi-qubit observables. By adding controlled dissipation to coherent operations, this work offers novel prospects for open-system quantum simulation and computation.

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An Open-System Quantum Simulator with Trapped Ions

This review article summarizes the requirement of low temperature conditions in existing experimental approaches to quantum computation and quantum simulation.

Low-temperature environments for quantum computation and quantum simulation

https://iopscience.iop.org/article/10.1088/1674-1056/abd762/pdf

Zhenhai Hu

Papers

Low-temperature environments for quantum computation and quantum simulation

Low-temperature environments for quantum computation and quantum simulation