Feb 2016

Feb 1-Feb 5 Bo Liu, Feb 8-Feb 12 Max, Feb 15-Feb 19 Haiyuan Zou, Feb 22-Feb 26 Ahmet Kel

Feb 18
arXiv:1602.05201 [pdf, ps, other]
Fast control of semiconductor qubits beyond the rotating wave approximation
Yang Song, J. P. Kestner, Xin Wang, S. Das Sarma
Comments: 13 pages, 9 figures
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Physics (quant-ph)

We present a theoretical study of single-qubit operations by oscillatory fields on various semiconductor platforms. We explicitly show how to perform faster gate operations by going beyond the universally-used rotating wave approximation (RWA) regime, while using only two sinusoidal pulses. No complicated pulse shaping or optimal control sequences are required. We first show for specific published experiments how much error is currently incurred by implementing pulses designed using standard RWA. We then show that an even modest increase in gate speed would cause problems in using RWA for gate design in the singlet-triplet (ST) and resonant-exchange (RX) qubits. We discuss the extent to which analytically keeping higher orders in the perturbation theory would address the problem. More strikingly, we give a new prescription for gating with strong coupling far beyond the RWA regime. We perform numerical calculations for the phases and the durations of two consecutive pulses to realize the key Hadamard and $\frac{\pi}{8}$ gates with coupling strengths up to several times the qubit splitting. Working in this manifestly non-RWA regime, the gate operation speeds up by two to three orders of magnitude.

Feb 17
arXiv:1602.05138 [pdf, ps, other]
Quantum anomalies in superconducting Weyl metals
Rui Wang, Lei Hao, Baigeng Wang, C. S. Ting
Subjects: Superconductivity (cond-mat.supr-con)

We theoretically study the quantum anomalies in the superconducting Weyl metals based on the topological field theory. It is demonstrated that the Fermi arc and the surface Andreev bound state, characteristic of the superconducting Weyl metals, are the manifestations of two underlying phenomenon, namely the chiral anomaly and the parity-like anomaly, respectively. The first anomaly is inherited from the Berry curvature around the original Weyl points, while the second is the result of the superconductivity. We show that, all the fascinating topological behavior of the superconducting Weyl metals, either intranode FFLO or the internode BCS pairing state, can be satisfactorily described and predicted by our topological field theory.

Feb 16
arXiv:1602.04555 [pdf, ps, other]
π-flux Dirac bosons and Bogoliubov edge states of Bose-Einstein condensed atoms
Zhi-Fang Xu, Li You, Andreas Hemmerich, W. Vincent Liu
Comments: 6+5 pages, 3+8 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

Topological edge states and Dirac fermions are fascinating phenomena known for fermionic systems. Whether similar phenomena can occur for interacting bosons is an open question in fundamental science. The recent observation of $^{87}$Rb chiral $p_x\pm ip_y$ condensates in a checkerboard optical lattice further raises the question whether a bosonic superfluid can be topological. Here we show that Dirac bosons experiencing a $\pi$ Berry flux emerge in the elementary excitation spectra of this experimentally realized Bose system. Tuning a population bias between the observed $p_x+ip_y$ and $p_x-ip_y$ condensate components opens an energy gap in the bulk, which results in Chern invariant and topologically protected Bogoliubov edge modes. Different from previous topological scenarios that employ the concept of engineered spin-orbit coupling or the equivalent in single-particle Bloch bands, spontaneous chiral orbital order driven by many-body interaction is a crucial ingredient in our finding.

Feb 15
arXiv:1602.03892 [pdf, other]
Measuring second Chern number from non-adiabatic effects
Michael Kolodrubetz
Subjects: Quantum Gases (cond-mat.quant-gas); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Physics (quant-ph)

The geometry and topology of quantum systems have deep connections to quantum dynamics. In this paper, I show how to measure the non-Abelian Berry curvature and its related topological invariant, the second Chern number, using dynamical techniques. The second Chern number is the defining topological characteristic of the four-dimensional generalization of the quantum Hall effect and has relevance in systems from three-dimensional topological insulators to Yang-Mills field theory. I illustrate its measurement using the simple example of a spin-3/2 particle in an electric quadrupole field. I show how one can dynamically measure diagonal components of the Berry curvature in an over-complete basis of the degenerate ground state space and use this to extract the full non-Abelian Berry curvature. I also show that one can accomplish the same ideas by stochastically averaging over random initial states in the degenerate ground state manifold. Finally I show how this system can be manufactured and the topological invariant measured in a variety of realistic systems, from superconducting qubits to trapped ions and cold atoms.

 Feb 5
1. arXiv:1602.01723 [pdf, other]
Cavity-induced chiral states of fermionic quantum gases
Ameneh Sheikhan, Ferdinand Brennecke, Corinna Kollath
We investigate ultra-cold fermions placed into an optical cavity and subjected to optical lattices which confine the atoms to ladder structures. A transverse running-wave laser beam induces together with the dynamical cavity field a two-photon Raman-assisted tunneling process with spatially dependent phase imprint along the rungs of the ladders. We identify the steady states which can occur by the feedback mechanism between the cavity field and the atoms. We find the spontaneous emergence of a finite cavity field amplitude which leads to an artificial magnetic field felt by the fermionic atoms. These form a chiral insulating or chiral liquid state carrying a chiral current. We explore the rich state diagram as a function of the power of the transverse laser beam, the atomic filling, and the phase imprint during the cavity-induced tunneling. Both a sudden onset or a slow exponential activation with the transverse laser power of the self-organized chiral states can occur.


Feb 4
1. arXiv:1602.01363 [pdf, other]
Geometrically induced complex tunnelings for ultracold atoms carrying orbital angular momentum
J. Polo, J. Mompart, V. Ahufinger
We investigate the dynamics of angular momentum states for a single ultracold atom trapped in two dimensional systems of sided coupled ring potentials. The symmetries of the system show that tunneling amplitudes between different ring states with variation of the winding number are complex. In particular, we demonstrate that in a triangular ring configuration the complex nature of the cross-couplings can be used to geometrically engineer spatial dark states to manipulate the transport of orbital angular momentum states via quantum interference.


Feb 3
1. arXiv:1602.01062 [pdf, other]
Geometry and non-adiabatic response in quantum and classical systems
Michael Kolodrubetz, Pankaj Mehta, Anatoli Polkovnikov
In these lecture notes, partly based on the course taught at the Karpacz Winter School in March 2014, we discuss the close connections between non-adiabatic response of a system with respect to macroscopic parameters and the geometry of quantum and classical states. We center our discussion around adiabatic gauge potentials, which are the generators of the unitary transformations of the basis states in quantum systems and generators of special canonical transformations in classical systems. In quantum systems, expectation values of these potentials in the eigenstates are the Berry connections and the covariance matrix of these gauge potentials is the geometric tensor, whose antisymmetric part defines the Berry curvature and whose symmetric part is the Fubini-Study metric tensor. In classical systems one simply replaces the eigenstate expectation value by an average over the micro-canonical shell. We express the non-adiabatic response of the physical observables of the system through these gauge potentials. We also demonstrate the close connection of the geometric tensor to the notions of Lorentz force and renormalized mass. We show how one can use this formalism to derive equations of motion for slow macroscopic parameters coupled to fast microscopic degrees of freedom to reproduce and even go beyond macroscopic Hamiltonian dynamics. Finally, we illustrate these ideas with a number of simple examples and highlight a few more complicated ones drawn from recent literature.



2. arXiv:1602.00979 [pdf, other]
Detecting the BCS pairing amplitude via a sudden lattice ramp in a honeycomb lattice
Marlon Nuske, L. Mathey, Eite Tiesinga
We determine the exact time evolution of an initial Bardeen-Cooper-Schrieffer (BCS) state of ultra-cold atoms in a hexagonal optical lattice. The dynamical evolution is triggered by ramping the lattice potential up, such that the interaction strength Uf is much larger than the hopping amplitude Jf. The quench initiates collective oscillations with frequency |Uf|/(2π) in the momentum occupation numbers and imprints an oscillating phase with the same frequency on the order parameter Δ. The latter is not reproduced by treating the time evolution in mean-field theory. The momentum density-density or noise correlation functions oscillate at frequency |Uf|/2π as well as its second harmonic. For a very deep lattice, with negligible tunneling energy, the oscillations of momentum occupation numbers are undamped. Non-zero tunneling after the quench leads to dephasing of the different momentum modes and a subsequent damping of the oscillations. This occurs even for a finite-temperature initial BCS state, but not for a non-interacting Fermi gas. We therefore propose to use this dephasing to detect a BCS state. Finally, we predict that the noise correlation functions in a honeycomb lattice will develop strong anti-correlations near the Dirac point.

Feb 2

1. arXiv:1602.00189 [pdf, ps, other]
Spin-orbit coupling induced Dirac monopoles with polar-core vortex in spinor Bose-Einstein condensates
Ji Li, Yan-Mei Yu, Wu-Ming Liu
Magnetic monopoles describe isolated north or south magnetic poles. The practical detection of the free magnetic monopoles, due to their extremely heavy masses, remains challenging so far. However, Bose-Einstein condensates offer an ideal observable platform for probing monopoles. In particular, the recent realization of spin-orbit (SO) coupling, owing to its high controllability and tunability, provides an unprecedented opportunity to explore exotic structures and phase transitions of monopoles unrealizable elsewhere. Here we report Dirac monopoles attached to the polar-core vortex induced by SO coupling in ferromagnetic Bose-Einstein condensates, which can be readily observed via time of flight measurements in experiment. The results show that, a cyclic phase transition from Dirac monopoles with polar-core vortex to that with Mermin-Ho vortex, is induced by SO coupling. Our studies may open a new direction to realize exotic topological defects and phase transitions in quantum systems.


Feb 1
1. arXiv:1601.07968 [pdf, other]
Resonant Pairing of Excitons in Semiconductor Heterostructures
Sergey V. Andreev
We suggest indirect excitons in 2D semiconductor heterostructures as a platform for realization of a bosonic analog of the Bardeen-Cooper-Schrieffer superconductor. The quantum phase transition to a biexcitonic gapped state can be controlled in situ by tuning the electric field applied to the structure in the growth direction. The proposed playground should allow one to go to strongly correlated and high-temperature regimes, unattainable with Feshbach resonant atomic gases.