Session Q29: Focus Session: Topologically Protected Qubits II

Q29.00010 : Detecting a Majorana-Fermion Zero Mode Using a Quantum Dot

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Authors: Dong E. Liu, Harold U. Baranger

We propose an setup for detecting a Majorana zero mode consisting of a spinless quantum dot coupled to the end of a p-wave superconducting nanowire [1]. The conductance through the dot is monitored by adding two external leads. We find that the Majorana bound state at the end of the wire strongly influences the conductance through the quantum dot: driving the wire through the topological phase transition causes a sharp jump in the conductance by a factor of 1/2. In the topological phase, the zero temperature peak value of the dot conductance (i.e. on resonance and symmetric coupling) is e$^2$/2h. In contrast, if the wire is in its trivial phase, the peak is e$^2$/h, or if a regular fermionic zero mode occurs, the conductance is 0. We also consider coupling the dot to both ends of the wire (two MBS), with a magnetic flux f through the loop. The conductance as a function of phase shows peaks at f/f0 = (2n+1)*pi which can be used to tune Flensberg's qubit system [PRL (2011)] to the energy degeneracy point. \\[4pt] [1] D. E. Liu and H. U. Baranger, PRB in press (2011); arXiv/1107.4338.

Session B29: Focus Session: Quantum Optics with Superconducting Circuits: Many-Body Physics, Phase Transitions, & Bistability

B29.00001 : Cavity-Free Photon Blockade Induced by Many-Body Bound States

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Authors: Huaixiu Zheng, Daniel Gauthier, Harold Baranger

We show theoretically that a variety of strong quantum nonlinear phenomena occur in a completely open one-dimensional waveguide coupled to an N-type four-level system. This system could be realized, for example, in experiments using superconducting circuits. We focus on photon blockade, photon-induced tunneling, bunching or anti-bunching, and the creation of single-photon states, all in the absence of a cavity. Many-body bound states appear due to the strong photon-photon correlation mediated by the four-level system. These bound states cause photon blockade, generating a sub-Poissonian single-photon source [1]. Such a source is crucial for quantum cryptography and distributed quantum networking; our work thus supports the notion that open quantum systems can play a critical role in the manipulation of individual, mobile quanta, a key goal of quantum communication. [1] H. Zheng, D. J. Gauthier, and H. U. Baranger, Phys. Rev. Lett. in press (2011), arXiv:1107.0309.

Session W17: Optics of Semiconductor Nanowires

Abstract: W17.00014 : Quantum Zigzag Phase Transition in Quantum Wires

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Authors: Abhijit C. Mehta, Cyrus J. Umrigar, Harold U. Baranger

We use Quantum Monte Carlo (QMC) techniques to study the quantum phase transition of interacting electrons in quantum wires to a quasi-one dimensional zigzag phase. The phase diagram of particles with Coulomb interaction that undergo a linear to zigzag transition is relevant to electrons in quantum wires [Meyer et al, PRL 2007] and ions in linear traps [Simshoni et al., PRL 2011]. Interacting electrons confined to a wire by a transverse harmonic potential form a Wigner crystal at low densities; as density increases, symmetry about the axis of the wire is broken and the electrons undergo a transition to a quasi-one-dimensional zigzag phase. Using QMC, we characterize this phase transition by measuring the power spectrum and addition energies.

Session Q5: Nonequilibrium and Gauge/Gravity Duality

Abstract: Q5.00009 : Dissipation-Induced Quantum Phase Transition in a Resonant Level

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Authors: Henok, Ivan, Dong, Huaixiu, Yuri, Alex Smirnov, Harold, Gleb

We measure conductance through a resonant level coupled to a dissipative environment, which suppresses tunneling rate at low energies. Our sample consists of a single-walled carbon nanotube quantum dot contacted by resistive metal leads that serve as the dissipative environment. We study the shape of the resonant conductance peak, with the expectation that its width and height, both dependent on the tunneling rate, will be suppressed at low temperatures. However, we observe distinct regimes, including a case where the resonant tunneling conductance reaches the unitary limit, despite the presence of dissipation. We discuss the implication of these findings for a dissipation-induced quantum phase transition and extract the scaling exponents.

Session T17: Focus Session: Electron, Ion, and Exciton Transport in Nanostructures - Quantum Transport II

Abstract: T17.00013 : Dependence of Conductance Resonanoces and Modulations on Channel Length in Asymmetric Quantum Point Contacts (QPCs)

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Authors: Hao Zhang, Phillip Wu, Albert Chang

Transport features below $2e^2/h$ show resonance peaks in highly asymmetric QPCs. As we increace the channel length, the number of peaks observable also increases. We characterize the number of peaks and/or oscillations as a function of channel length, when the QPC is tuned to or below the first quantum channel. The the number of peaks/oscillations appears to increase on average as the channel length increases. In addition, we find preliminary evidence that there is a correspondence between the resonance peaks and the zero-bias anomaly(ZBA) in the differential conductance. These behaviors are consistent with an interpretation based on the formation of quasi-bound-states within the QPC channel in the single-mode limit.

Session H18: Focus Session: Electron, Ion, and Exciton Transport in Nanostructures - Nanowires and Superlattices

Abstract: H18.00010 : Stochastic current switching in semiconductor superlattices: observation of non-exponential kinetics

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Authors: Yu. Bomze, H.T. Grahn, R. Hey, S.W. Teitsworth

We report the experimental measurement of first-passage-time distributions associated with noise-induced current switching in doped, weakly-coupled GaAs/AlAs superlattices, in a regime of nonlinear electronic transport where the static current-voltage ($I - V$) curves exhibit multiple branches and bistability. For applied voltages near the end of each branch, internal shot noise induces switching of measured current to the next branch with a stochastically varying switching time. Switching time distributions are constructed by carrying out up to $10^5$ measurements under identical initial conditions. We have implemented a novel, high bandwidth technique that permits measurement of switching times over very large dynamic range of approximately $10^9$, with measured times ranging from $4$ ns to $10$ s. For relatively small times ($<$ 10$\mu$s), the switching time distributions show exponential tails, as expected for activated escape from an initial metastable state. However, at larger times ($>$ 10 $\mu$s), the distributions exhibit approximate power law tails that extend over several decades of time, with additional fine structure. A rate equation model indicates the possible role of multiple, nearly degenerate metastable states in producing the long tail behavior.