Symmetric minimally entangled typical thermal states
Phys. Rev. B
, Volume 92(115105)
June
2015
Abstract: We extend White's minimally entangled typically thermal states approach (METTS) to allow Abelian and nonAblian symmetries to be exploited when computing finitetemperature response functions in onedimensional (1D) quantum systems. Our approach, called SYMETTS, starts from a METTS sample of states that are not symmetry eigenstates, and generates from each a symmetry eigenstate. These symmetry states are then used to calculate dynamic response functions. SYMETTS is ideally suited to determine the lowtemperature spectra of 1D quantum systems with high resolution. We employ this method to study a generalized diamond chain model for the natural mineral azurite Cu3(CO3)2(OH)2, which features a plateau at 13 in the magnetization curve at low temperatures. Our calculations provide new insight into the effects of temperature on magnetization and excitation spectra in the plateau phase, which can be fully understood in terms of the microscopic model. 
Crystallization in Ising quantum magnets
Science
, Volume 347(6229), page: 14551458
March
2015
Abstract: Dominating finiterange interactions in manybody systems can lead to intriguing selfordered phases of matter. For quantum magnets, Ising models with powerlaw interactions are among the most elementary systems that support such phases. These models can be implemented by laser coupling ensembles of ultracold atoms to Rydberg states. Here, we report on the experimental preparation of crystalline ground states of such spin systems. We observe a magnetization staircase as a function of the system size and show directly the emergence of crystalline states with vanishing susceptibility. Our results demonstrate the precise control of Rydberg manybody systems and may enable future studies of phase transitions and quantum correlations in interacting quantum magnets. 
Quantum State Engineering with Circuit Electromechanical ThreeBody Interactions
Phys. Rev. Lett.
, Volume 114, page: 173602
2015
Abstract: We propose a hybrid system with quantum mechanical threebody interactions between photons, phonons, and qubit excitations. These interactions take place in a circuit quantum electrodynamical architecture with a superconducting microwave resonator coupled to a transmon qubit whose shunt capacitance is free to mechanically oscillate. We show that this system design features a threemode polaritonmechanical mode and a nonlinear transmonmechanical mode interaction in the strong coupling regime. Together with the strong resonatortransmon interaction, these properties provide intriguing opportunities for manipulations of this hybrid quantum system. We show, in particular, the feasibility of cooling the mechanical motion down to its ground state and preparing various nonclassical states including mechanical Fock and cat states and hybrid tripartite entangled states. 
Quantum dynamics of propagating photons with strong interactions: a generalized inputoutput formalism
2015
Abstract: There has been rapid development of systems that yield strong interactions between freely propagating photons in one dimension via controlled coupling to quantum emitters. This raises interesting possibilities such as quantum information processing with photons or quantum manybody states of light, but treating such systems generally remains a difficult task theoretically. Here, we describe a novel technique in which the dynamics and correlations of a few photons can be exactly calculated, based upon knowledge of the initial photonic state and the solution of the reduced effective dynamics of the quantum emitters alone. We show that this generalized "inputoutput" formalism allows for a straightforward numerical implementation regardless of system details, such as emitter positions, external driving, and level structure. As a specific example, we apply our technique to show how atomic systems with infiniterange interactions and under conditions of electromagnetically induced transparency enable the selective transmission of correlated multiphoton states. 
Undecidability of the Spectral Gap (short version)
2015
Abstract: The spectral gap  the difference in energy between the ground state and the first excited state  is one of the most important properties of a quantum manybody system. Quantum phase transitions occur when the spectral gap vanishes and the system becomes critical. Much of physics is concerned with understanding the phase diagrams of quantum systems, and some of the most challenging and longstanding open problems in theoretical physics concern the spectral gap, such as the Haldane conjecture that the Heisenberg chain is gapped for integer spin, proving existence of a gapped topological spin liquid phase, or the YangMills gap conjecture (one of the Millennium Prize problems). These problems are all particular cases of the general spectral gap problem: Given a quantum manybody Hamiltonian, is the system it describes gapped or gapless? Here we show that this problem is undecidable, in the same sense as the Halting Problem was proven to be undecidable by Turing. A consequence of this is that the spectral gap of certain quantum manybody Hamiltonians is not determined by the axioms of mathematics, much as Goedels incompleteness theorem implies that certain theorems are mathematically unprovable. We extend these results to prove undecidability of other low temperature properties, such as correlation functions. The proof hinges on simple quantum manybody models that exhibit highly unusual physics in the thermodynamic limit. 
Variational matrix product operators for the steady state of dissipative quantum systems
2015
Abstract: We present a new variational method, based on the matrix product operator (MPO) ansatz, for finding the steady state of dissipative quantum chains governed by master equations of the Lindblad form. Instead of requiring an accurate representation of the system evolution until the stationary state is attained, the algorithm directly targets the final state, thus allowing for a faster convergence when the steady state is a MPO with small bond dimension. Our numerical simulations for several dissipative spin models over a wide range of parameters illustrate the performance of the method and show that indeed the stationary state is often well described by a MPO of very moderate dimensions. 
Optical control of internal electric fields in bandgap graded InGaN nanowires
Nano Lett.
, Volume 15(1), page: 332–338
2015
Abstract: InGaN nanowires are suitable building blocks for many future optoelectronic devices. We show that a linear grading of the indium content along the nanowire axis from GaN to InN introduces an internal electric field evoking a photocurrent. Consistent with quantitative band structure simulations we observe a sign change in the measured photocurrent as a function of photon flux. This negative differential photocurrent opens the path to a new type of nanowirebased photodetector. We demonstrate that the photocurrent response of the nanowires is as fast as 1.5 ps. 
Ultrafast photocurrents and THz generation in single InAsnanowires
2015
Abstract: To clarify the ultrafast temporal interplay of the different photocurrent mechanisms occurring in single InAsnanowirebased circuits, an onchip photocurrent pumpprobe spectroscopy based on coplanar striplines was utilized. The data are interpreted in terms of a photothermoelectric current and the transport of photogenerated holes to the electrodes as the dominating ultrafast photocurrent contributions. Moreover, it is shown that THz radiation is generated in the optically excited InAsnanowires, which is interpreted in terms of a dominating photoDember effect. The results are relevant for nanowirebased optoelectronic and photovoltaic applications as well as for the design of nanowirebased THz sources. 
Towards onchip generation, routing and detection of nonclassical light
Proc. SPIE 9373
2015
Abstract: We fabricate an integrated photonic circuit with emitter, waveguide and detector on one chip, based on a hybrid superconductorsemiconductor system. We detect photoluminescence from selfassembled InGaAs quantum dots onchip using NbN superconducting nanowire single photon detectors. Using the fast temporal response of these detectors we perform timeresolved studies of nonresonantly excited quantum dots. By introducing a temporal ?ltering to the signal, we are able to resonantly excite the quantum dot and detect its resonance uorescence onchip with the integrated superconducting single photon detector. 
Quantum GrossPitaevskii Equation
2015
Abstract: We introduce a noncommutative generalization of the GrossPitaevskii equation for onedimensional quantum field theories. This generalization is obtained by applying the DiracFrenkel timedependent variational principle to the variational manifold of continuous matrix product states. This allows for a full quantum description of the many body system including entanglement and correlations and thus extends significantly beyond the usual meanfield description of the GrossPitaevskii equation, which is known to fail for onedimensional systems. 
Thermodynamics of the BoseHubbard model in a Bogoliubov+U theory
2015
Abstract: We derive the Bogoliubov+U formalism to study the thermodynamical properties of the BoseHubbard model. The framework can be viewed as the zerofrequency limit of bosonic dynamical meanfield theory (BDMFT), but equally well as an extension of the meanfield decoupling approximation in which pair creation and annihilation of depleted particles is taken into account. The selfenergy on the impurity site is treated variationally, minimizing the grand potential. The theory containing just 3 parameters that are determined selfconsistently reproduces the T=0 phase diagrams of the 3d and 2d BoseHubbard model with an accuracy of 1 % or better. The superfluid to normal transition at finite temperature is also reproduced well and only slightly less accurately than in BDMFT. 
Ultrafast helicity control of surface currents in topological insulators with nearunity fidelity
Nature Communications
, Volume 6, page: 6617
2015
Abstract: In recent years, a class of solid state materials, called threedimensional topological insulators, has emerged. In the bulk, a topological insulator behaves like an ordinary insulator with a band gap. At the surface, conducting gapless states exist showing remarkable properties such as helical Dirac dispersion and suppression of backscattering of spinpolarized charge carriers. The characterization and control of the surface states via transport experiments is often hindered by residual bulk contributions yet at cryogenic temperatures. Here, we show that surface currents in Bi2Se3 can be controlled by circularly polarized light on a picosecond time scale with a fidelity near unity even at room temperature. We reveal the temporal separation of such ultrafast helicitydependent surface currents from photoinduced thermoelectric and drift currents in the bulk. Our results uncover the functionality of ultrafast optoelectronic devices based on surface currents in topological insulators. DOI: 10.1038/ncomms7617

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