Contact

email: tel: +4961511620325 fax: +4961511620402 address: Institut für Angewandte Physik
Technische Universität Darmstadt
Hochschulstrasse 4a
D64289 Darmstadt, Germany
Education & Experience
 • PhD in theoretical physics, Carnegie Mellon University, Pittsburgh (PA), USA
 • postdoctoral work: University of Basel (Switzerland), Harvard University (USA)
 • teaching experience: Theoretical SolidState Physics Course, University of Basel
 • scientificconsulting experience: REA, DGCONNECT (Euro. Commission), RGC Hong Kong
 • current position: senior research scientist, Theoretical Quantum Physics Group, TU Darmstadt
IN THE PRESS: coverage of recent work on Wtype entanglement at the main website of TU Darmstadt
coverage in the research magazine hoch3  forschen of TU Darmstadt
Research interests
 • Quantumstate engineering in AMO systems (e.g., ensembles of cold neutral
atoms in Rydberg states) and superconducting circuits (circuitQED systems).  • Liealgebraic and optimal quantum control, with applications to arrays of qubits
(quantumstate and operator control, controllability, quantumgate optimization).  • Analog quantum simulation of static and dynamic features of manybody systems
with Rydberg atoms, trapped atomic/molecular ions, and superconducting circuits.  • Strongcoupling regimes of particle(spin)boson systems and smallpolaron models
(groundstate, spectral, and entanglement properties; nonequilibrium dynamics).  • Digital quantum simulation of systems/models with coupled fermionic and bosonic
degrees of freedom and its applications (e.g., in coherent excitation transport).  • Interplay of dissipation, driving, and coherence in quantum manybody systems,
with implications for quantum control of open and periodicallydriven systems.  • Quantum annealing and its application in solving realworld optimization problems;
physical implementations of coherent quantum annealers of nonstoquastic type.
Selected talks
From analog simulations of smallpolaron physics to W–state engineering with
Theory Colloquium, Technical University of Darmstadt, Germany, December 9, 2019
a superconducting qubitresonator systemLiealgebraic aspects of quantum control in interacting spin1/2 (qubit) chains
Topological Insulators and Mathematical Science conference, Center for Mathematical Sciences
and Applications (CMSA), Harvard University, Cambridge (MA), USA, September 16, 2014Sharp transitions for small polarons: from theory to transmonbased realization
Condensedmatter seminar, Yale University, New Haven (CT), USA, April 3, 2014
PQI seminar,
University of Pittsburgh, Pittsburgh (PA), USA, September 25, 2014
Quantum simulation of smallpolaron formation with trapped ions
Control of Complex Quantum Systems workshop, The Kavli Institute for Theoretical Physics,
UC Santa Barbara, USA, March 26, 2013
Popularlevel summary of recent work:
Unifying framework for creating
largescale Wtype entanglement
with superconducting and Rydbergdressed qubits
Publications
[show/hide abstracts]1. 
Singleatom transport in optical conveyor belts: Enhanced shortcutstoadiabaticity approach S. H. Hauck, G. Alber, and V. M. Stojanović
Fast and nearly lossless
atomic transport, enabled by moving the confining trap,
is a prerequisite for many quantumtechnology applications. While theoretical
studies of this problem have heretofore focussed almost exclusively on simplified scenarios
(onedimensional systems, purely harmonic confining potentials, etc.), we investigate it
here in the experimentally relevant setting of a moving optical lattice (optical
conveyor belt). We model singleatom transport in this system by taking fully
into account its threedimensional, anharmonic confining potential. We do so using the
established method of shortcuts to adiabaticity (STA), i.e. an inverseengineering
approach based on LewisRiesenfeld invariants, as well as its recently
proposed modification known as enhanced STA (eSTA). By combining wellcontrolled
analytical techniques and the numerical propagation of a timedependent Schr\"{o}dinger
equation, we evaluate atomtransport fidelities within both approaches. Being obtained
for realistic choices of system parameters, our results are relevant for future experiments
with optical conveyor belts. Moreover, they reveal that in the system at hand the eSTA
method outperforms its STA counterpart for all but the lowest opticallattice depths.

2. 
Scalable Wtype entanglement resource in neutralatom arrays with Rydbergdressed resonant dipoledipole interaction V. M. Stojanović
While the Rydbergblockade regime provides the
natural setting for creating Wtype entanglement with cold neutral atoms, it is demonstrated here that a scalable entanglement
resource of this type can even be obtained under completely different physical circumstances. To be more precise, a special
instance of twisted W states  namely, πtwisted ones  can be engineered in onedimensional arrays of cold neutral
atoms with Rydbergdressed resonant dipoledipole interaction. In particular, it is shown here that this is possible even
when a (dressed) Rydberg excitation is coupled to the motional degrees of freedom of atoms in their respective, nearlyharmonic
opticaldipole microtraps, which are quantized into dispersionless (zerodimensional) bosons. For a specially chosen (``sweetspot'')
detuning of the offresonant dressing lasers from the relevant internal atomic transitions, the desired πtwisted W state
of Rydbergdressed qubits is the ground state of the effective excitation  boson Hamiltonian of the system in a broad window
of parameters. Being also separated from the other eigenstates by a gap equal to the singleboson energy, this state can be
prepared using a Rabitype driving protocol. The corresponding preparation times are independent of the system size and several
orders of magnitude shorter than the effective lifetimes of the relevant atomic states.

3. 
Conversion from W to GreenbergerHorneZeilinger states in the Rydbergblockade regime of neutralatom systems: Dynamicalsymmetrybased approach T. Haase, G. Alber, and V. M. Stojanović
We investigate the possibilities for
a deterministic conversion between two important types of
maximally entangled multiqubit states, namely, W and GreenbergerHorneZeilinger (GHZ) states,
in the Rydbergblockade regime of a neutralatom system where each atom is subject to four external
laser pulses. Such interconversions between W states and their GHZ counterparts have quite recently
been addressed using the method of shortcuts to adiabaticity, more precisely techniques based on
LewisRiesenfeld invariants [R.H. Zheng {\em et al.}, Phys. Rev. A 101, 012345 (2020)].
Motivated in part by this recent work, we revisit the W to GHZ stateconversion problem using a
fundamentally different approach, which is based on the dynamical symmetries of the system and a
Liealgebraic parametrization of its permissible evolutions. In contrast to the previously used
invariantbased approach, which leads to a stateconversion protocol characterized by strongly
timedependent Rabi frequencies of external lasers, ours can also yield one with timeindependent
Rabi frequencies. This feature makes our protocol more easily applicable experimentally, with the
added advantage that it allows the desired state conversion to be carried out in a significantly
shorter time with the same total laser pulse energy used.

4. 
BareExcitation Ground State of a SpinlessFermion—Boson Model and WState Engineering in an Array of Superconducting Qubits and Resonators V. M. Stojanović
This Letter unravels an interesting property
of a onedimensional lattice model that describes a single
itinerant spinless fermion (excitation) coupled to zerodimensional (dispersionless) bosons through two
different nonlocal coupling mechanisms. Namely, below a critical value of the effective excitationboson
coupling strength, the exact ground state of this model is the zeroquasimomentum Bloch state of a bare
(i.e., completely undressed) excitation. It is demonstrated here how this last property of the lattice model
under consideration can be exploited for a fast, deterministic preparation of multipartite W states in a
readily realizable system of inductively coupled superconducting qubits and microwave resonators.

5. 
Entanglementspectrum characterization of groundstate nonanalyticities in coupled excitationphonon models V. M. Stojanović
The polaron concept captures physical
situations involving an itinerant quantum particle (excitation) that
interacts strongly with bosonic degrees of freedom and becomes heavily bosondressed. While the Gerlach
Loewen theorem rules out the occurrence of nonanalyticities of groundstaterelated quantities for a broad class
of polaron models, examples were found in recent years of sharp transitions pertaining to strongly momentum
dependent interactions of an excitation with dispersionless (zerodimensional) phonons. On the example of a
lattice model with Peierlstype excitationphonon interaction, such levelcrossingtype smallpolaron transitions
are analyzed here through the prism of the entanglement spectrum of the excitationphonon system. By
evaluating this spectrum in a numerically exact fashion it is demonstrated that the behavior of the entanglement
entropy in the vicinity of the critical excitationphonon coupling strength chiefly originates from one specific
entanglementspectrum eigenvalue, namely the smallest one. While the discrete translational symmetry of the
system implies that those eigenvalues can be labeled by the bareexcitation quasimomentum quantum numbers,
here it is shown numerically that they are predominantly associated with the quasimomenta 0 and pi, including
cases where a transition between the two takes place deeply in the strongcoupling regime.

6. 
Quantum dynamics of the smallpolaron formation in a superconducting analog simulator
V. M. Stojanović and I. Salom
We propose a scheme for investigating
the nonequilibrium aspects of smallpolaron physics using an array
of superconducting qubits and microwave resonators. This system, which can be realized with transmon or
gatemon qubits, serves as an analog simulator for a lattice model describing a nonlocal coupling of a quantum
particle (excitation) to dispersionless phonons. We study its dynamics following an excitationphonon (qubit
resonator) interaction quench using a numerically exact approach based on a Chebyshevmoment expansion of
the timeevolution operator of the system. We thereby glean heretofore unavailable insights into the process of
the smallpolaron formation resulting from strongly momentumdependent excitationphonon interactions, most
prominently about its inherent dynamical timescale. To further characterize this complex process, we evaluate
the excitationphonon entanglement entropy and show that initially prepared bareexcitation Bloch states here
dynamically evolve into smallpolaron states that are close to being maximally entangled. Finally, by computing
the dynamical variances of the phonon position and momentum quadratures, we demonstrate a pronounced
nonGaussian character of the latter states, with a strong antisqueezing in both quadratures.

7.  Feasibility of singleshot realizations of
conditional threequbit gates in exchangecoupled qubit arrays with local control V. M. Stojanović
We investigate the feasibility of
singleshot Toffoli and Fredkingate realizations in qubit arrays with
Heisenbergtype exchange interactions between adjacent qubits. As follows from the Liealgebraic criteria of
controllability, such an array is rendered completely controllable—equivalent to allowing universal quantum
computation—by a Zeemanlike control field with two orthogonal components acting on a single “actuator”
qubit. Adopting this localcontrol setting, we start our analysis with piecewiseconstant control fields and
determine the global maxima of the relevant figure of merit (targetgate fidelity) by combining the multistart
based clustering algorithm and quasiNewton type local optimization. We subsequently introduce important
practical considerations, such as finite frequency bandwidth of realistic fields and their leakage away from
the actuator. We find the shortest times required for highfidelity Toffoli and Fredkingate realizations and
provide comparisons to their respective twoqubit counterparts—controlled NOT and exponential SWAP . In
particular, the Toffoligate time compares much more favorably to that of controlled NOT than in the standard
decompositionbased approach. This study indicates that the use of the singleshot approach can alleviate the
burden on controlgenerating hardware in future experimental realizations of multiqubit gates.

8. 
Nanofriction in cavity quantum electrodynamics
T. Fogarty, C. Cormick, H. Landa, V. M. Stojanović, E. Demler, and G. Morigi
The dynamics of cold trapped ions in a highfinesse resonator results from the interplay between the longrange Coulomb repulsion and the cavityinduced interactions. The latter are due to multiple scatterings of laser photons inside the cavity and become relevant when the laser pump is sufficiently strong to overcome photon decay. We study the stationary states of ions coupled with a mode of a standingwave cavity as a function of the cavity and laser parameters, when the typical length scales of the two selforganizing processes, Coulomb crystallization and photonmediated interactions, are incommensurate. The dynamics are frustrated and in specific limiting cases can be cast in terms of the FrenkelKontorova model, which reproduces features of friction in one dimension. We numerically recover the sliding and pinned phases. For strong cavity nonlinearities, they are in general separated by bistable regions where superlubric and stickslip dynamics coexist. The cavity, moreover, acts as a thermal reservoir and can cool the chain vibrations to temperatures controlled by the cavity parameters and by the ions phase. These features are imprinted in the radiation emitted by the cavity, which is readily measurable in stateofart setups of cavity quantum electrodynamics.

9. 
Neutral edge modes in a superconductor  topologicalinsulator hybrid structure in a perpendicular magnetic field R. P. Tiwari, U. Zülicke, C. Bruder, and V. M. Stojanović
We study the lowenergy edge states of a superconductor  3D topologicalinsulator hybrid structure (NS junction) in the presence of a perpendicular magnetic field. The hybridization of electronlike and holelike Landau levels due to Andreev reflection gives rise to chiral edge states within each Landau level. We show that by changing the chemical potential of the superconductor, this junction can be placed in a regime where the sign of the effective charge of the edge state within the zeroth Landau level changes more than once resulting in neutral edge modes with a finite value of the guidingcenter coordinate. We find that the appearance of these neutral edge modes is related to the level repulsion between the zeroth and the first Landau levels in the spectra. We also find that these neutral edge modes come in pairs, one in the zeroth Landau level and its corresponding pair in the first.

10. 
Transmonbased simulator of nonlocal electronphonon coupling: A platform for observing sharp smallpolaron transitions
V. M. Stojanović, M. Vanević, E. Demler, and L. Tian
We propose an analog superconducting quantum simulator for a onedimensional model featuring momentumdependent (nonlocal) electronphonon couplings of SuSchriefferHeeger and "breathingmode" types. Because its corresponding coupling vertex function depends on both the electron and phonon quasimomenta, this model does not belong to the realm of validity of the GerlachLoewen theorem that rules out any nonanalyticities in singleparticle properties. The superconducting circuit behind the proposed simulator entails an array of transmon qubits and microwave resonators. By applying microwave driving fields to the qubits, a smallpolaron Bloch state with an arbitrary quasimomentum can be prepared in this system within times several orders of magnitude shorter than the typical qubit decoherence times. We demonstrate thatby varying the externally tunable parametersone can readily reach the critical coupling strength required for observing the sharp transition from a nondegenerate (singleparticle) ground state corresponding to zero quasimomentum (Kgs=0) to a twofolddegenerate smallpolaron ground state at nonzero quasimomenta Kgs and Kgs. Through exact numerical diagonalization of our effective Hamiltonian, we show how this nonanalyticity is reflected in the relevant singleparticle properties (groundstate energy, quasiparticle residue, average number of phonons). We also show that the proposed setup provides an ideal testbed for studying the nonequilibrium dynamics of smallpolaron formation in the presence of strongly momentumdependent electronphonon interactions.

11. 
Analog superconducting quantum simulator for Holstein polarons
F. Mei, V. M. Stojanović, I. Siddiqi, and L. Tian
We propose an analog quantum simulator for the Holstein molecularcrystal model based on a superconducting circuit QED system in the dispersive regime. By varying the driving field on the superconducting resonators, one can access both the adiabatic and antiadiabatic regimes of this model, and the strong electronphonon coupling required for smallpolaron formation can be readily reached. We show that a smallpolaron state of arbitrary quasimomentum can be generated by applying a microwave pulse to the resonators. We also show that significant squeezing in the resonator modes can be achieved in the polaroncrossover regime through a measurementbased scheme.

12. 
Strategy for implementing stabilizerbased codes on solidstate qubits
T. Tanamoto, V. M. Stojanović, C. Bruder, and D. Becker
We present a method for implementing stabilizerbased codes with encoding schemes of the operator quantum error correction paradigm, e.g., the "standard" fivequbit and CSS codes, on solidstate qubits with Ising or XYtype interactions. Using pulse sequences, we show how to induce the effective dynamics of the stabilizer Hamiltonian, the sum of an appropriate set of stabilizer operators for a given code. Within this approach, the encoded states (ground states of the stabilizer Hamiltonian) can be prepared without measurements and preserved against both the time evolution governed by the original qubit Hamiltonian, and energynonconserving errors caused by the environment.

13. 
Quantum simulation of smallpolaron formation with trapped ions
V. M. Stojanović, T. Shi, C. Bruder, and J. I. Cirac
We propose an analog quantum simulation of smallpolaron physics using a onedimensional system of trapped ions acted upon by offresonant standing waves. This system, envisioned as an array of microtraps, in the singleexcitation case allows the realization of the antiadiabatic regime of the Holstein model. We show that the strong excitationphonon coupling regime, characterized by the formation of small polarons, can be reached using realistic values of the relevant system parameters. Finally, we propose measurements of the quasiparticle residue and the average number of phonons in the ground state, experimental probes validating the polaronic character of the phonondressed excitation.

14. 
Superfluid drag of twospecies BoseEinstein condensates in optical lattices
P. P. Hofer, C. Bruder, and V. M. Stojanović
We study twospecies BoseEinstein condensates in quasi twodimensional optical lattices of varying geometry and potential depth. Based on the numerically exact Bloch and Wannier functions obtained using the planewave expansion method, we quantify the drag (entrainment coupling) between the condensate components. This drag originates from the (short range) interspecies interaction and increases with the kinetic energy. As a result of the interplay between interaction and kinetic energy effects, the superfluiddrag coefficient shows a nonmonotonic dependence on the lattice depth. To make contact with future experiments, we quantitatively investigate the drag for mass ratios corresponding to relevant atomic species.

15. 
Preserving universal resources for oneway quantum computing
T. Tanamoto, D. Becker, V. M. Stojanović, and C. Bruder
The common spin Hamiltonians such as the Ising, XY, or Heisenberg model do not have ground states that are the graph states needed in measurementbased quantum computation. Various highlyentangled manybody states have been suggested as a universal resource for this type of computation, however, it is not easy to preserve these states in solidstate systems due to their short coherence times. Here we propose a scheme for generating a Hamiltonian that has a cluster state as ground state. Our approach employs a series of pulse sequences inspired by established NMR techniques and holds promise for applications in many areas of quantum information processing.

16. 
Electronphonon coupling in crystalline organic semiconductors: Microscopic evidence for nonpolaronic charge carriers
N. Vukmirović, C. Bruder, and V. M. Stojanović
We consider electron(hole)phonon coupling in crystalline organic semiconductors, using naphthalene for our case study. Employing a firstprinciples approach, we compute the changes in the selfconsistent KohnSham potential corresponding to different phonon modes and go on to obtain the carrierphonon coupling matrix elements (vertex functions). We then evaluate perturbatively the quasiparticle spectral residues for electrons at the bottom of the lowestunoccupied (LUMO) and holes at the top of the highestoccupied (HOMO) band, respectively obtaining $Z_{\textrm{e}}\approx 0.74$ and $Z_{\textrm{h}}\approx 0.78$. Along with the widely accepted notion that the carrierphonon coupling strengths in polyacenes decrease with increasing molecular size, our results provide a strong microscopic evidence for the previously conjectured nonpolaronic nature of bandlike carriers in these systems.

17. 
Quantumcontrol approach to realizing a Toffoli gate in circuit QED
V. M. Stojanović, A. Fedorov, A. Wallraff, and C. Bruder
We study the realization of a Toffoli gate with superconducting qubits in a circuitQED setup using quantumcontrol methods. Starting with optimized piecewiseconstant control fields acting on all qubits and typical strengths of XYtype coupling between the qubits, we demonstrate that the optimal gate fidelities are affected only slightly by a "lowpass" filtering of these fields with the typical cutoff frequencies of microwave driving. Restricting ourselves to the range of controlfield amplitudes for which the leakage to the noncomputational states of a physical qubit is heavily suppressed, we theoretically predict that in the absence of decoherence and leakage, within 75 ns a Toffoli gate can be realized with intrinsic fidelities higher than 90%, while fidelities above 99% can be reached in about 140 ns.

18. 
Controlling qubit arrays with anisotropic XXZ Heisenberg interaction by acting on a single qubit
R. Heule, C. Bruder, D. Burgarth, and V. M. Stojanović
We investigate anisotropic XXZ Heisenberg spin1/2
chains with control fields acting on one of the end spins, with the aim of exploring local quantum control in arrays of interacting
qubits. In this work, which uses a recent Liealgebraic result on the local controllability of spin chains with ``alwayson''
interactions, we determine piecewiseconstant control pulses corresponding to optimal fidelities for
quantum gates such as spinflip (NOT), controlledNOT (CNOT), and squarerootofSWAP.
We find the minimal times for realizing different gates depending on the anisotropy parameter Delta of the
model, showing that the shortest among these gate times are achieved for particular values of Delta larger
than unity. To study the influence of possible imperfections in anticipated experimental realizations of
qubit arrays, we analyze the robustness of the obtained results for the gate fidelities to random variations
in the controlfield amplitudes and finite rise time of the pulses. Finally, we discuss the implications of
our study for superconducting chargequbit arrays.

19. 
Local quantum control of Heisenberg spin chains
R. Heule, C. Bruder, D. Burgarth, and V. M. Stojanović
Motivated by some recent results of quantum control theory, we discuss the feasibility of local operator control in arrays of interacting qubits modeled as isotropic Heisenberg spin chains. Acting on one of the end spins, we aim at finding piecewiseconstant control pulses that lead to optimal fidelities for a chosen set of quantum gates. We analyze the robustness of the obtained results for the gate fidelities to random errors in the control fields, finding that with faster switching between piecewiseconstant controls the system is less susceptible to these errors. The observed behavior falls into a generic class of physical phenomena that are related to a competition between resonance and relaxationtype behavior, exemplified by motional narrowing in NMR experiments. Finally, we discuss how the obtained optimal gate fidelities are altered when the corresponding rapidlyvarying piecewiseconstant control fields are smoothened through spectral filtering.

20. 
Polaronic signatures and spectral properties of graphene antidot lattices
V. M. Stojanović, N. Vukmirović, and C. Bruder
We explore the consequences of electronphonon (eph) coupling in graphene antidot lattices (graphene nanomeshes), i.e., triangular superlattices of circular holes (antidots) in a graphene sheet. They display a direct band gap whose magnitude can be controlled via the antidot size and density. The relevant coupling mechanism in these semiconducting counterparts of graphene is the modulation of the nearestneighbor electronic hopping integrals due to lattice distortions (Peierlstype eph coupling). We compute the full momentum dependence of the eph vertex functions for a number of representative antidot lattices. Based on the latter, we discuss the origins of the previously found large conductionband quasiparticle spectral weight due to eph coupling. In addition, we study the nonzeromomentum quasiparticle properties with the aid of the selfconsistent Born approximation, yielding results that can be compared with future angleresolved photoemission spectroscopy measurements. Our principal finding is a significant eph mass enhancement, an indication of polaronic behavior. This can be ascribed to the peculiar momentum dependence of the eph interaction in these narrowband systems, which favors small phonon momentum scattering. We also discuss implications of our study for recently fabricated largeperiod graphene antidot lattices.

21. 
Electronphonon coupling in graphene antidot lattices: An indication of polaronic behavior
N. Vukmirović, V. M. Stojanović, and M. Vanević
We study graphene antidot lattices  superlattices of perforations (antidots) in a graphene sheet  using a model that accounts for the phononmodulation of the pielectron hopping integrals. We calculate the phonon spectra of selected antidot lattices using two different semiempirical interatomic potentials. Based on the adopted model and the obtained phonon modes, we quantify the nature of chargecarriers in the system by computing the quasiparticle spectral weight due to the electronphonon interaction for an excess electron in the conduction band. We show that the phononinduced renormalization is much stronger than in graphene, with the effective electron masses exhibiting an interesting nonmonotonic dependence on the superlattice period for a given antidot diameter. Our study provides an indication of polaronic behavior and points to the necessity of taking into account the inelastic degrees of freedom in future studies of electronic transport in graphene antidot lattices.

22. 
Character of electronic states in graphene antidot lattices: Flat bands and spatial localization
M. Vanević, V. M. Stojanović, and M. Kindermann
Graphene antidot lattices have recently been proposed as a new breed of graphenebased superlattice structures. We study electronic properties of triangular antidot lattices, with emphasis on the occurrence of dispersionless (flat) bands and the ensuing electron localization. Apart from strictly flat bands at zero energy (Fermi level), whose existence is closely related to the bipartite lattice structure, we also find quasiflat bands at low energies. We predict the realspace electron density profiles due to these localized states for a number of representative antidot lattices. We point out that the studied lowenergy localized states compete with states induced by the superlatticescale defects in this system, which have been proposed as hosts for electronspin qubits. Furthermore, we suggest that local moments formed in these midgap zeroenergy states may be at the origin of a surprising saturation of the electron dephasing length observed in recent weak localization measurements in graphene antidot lattices.

23. 
Quantumentanglement aspects of polaron systems
V. M. Stojanović and M. Vanević
We describe quantum entanglement inherent to the polaron ground states of coupled electronphonon (or, more generally, particlephonon) systems based on a model comprising both local (Holsteintype) and nonlocal (Peierlstype) couplings. We study this model using a variational method supplemented by the exact numerical diagonalization on a system of finite size. By way of subsequent numerical diagonalization of the reduced density matrix, we determine the particlephonon entanglement as given by the von Neumann and linear entropies. Our results are strongly indicative of the intimate relationship between the particle localization/delocalization and the particlephonon entanglement. In particular, we find a compelling evidence for the existence of a nonanalyticity in the entanglement entropies with respect to the Peierlscoupling strength. The occurrence of such nonanalyticity  not accompanied by an actual quantum phase transition  reinforces analogous conclusion drawn in several recent studies of entanglement in the realm of quantumdissipative systems. In addition, we demonstrate that the entanglement entropies saturate inside the selftrapped region where the smallpolaron states are nearly maximally mixed.

24. 
Incommensurate superfluidity of bosons in a doublewell optical lattice
V. M. Stojanović, C. Wu, W. V. Liu, and S. Das Sarma
We study bosons in the first excited Bloch band of a doublewell optical lattice, recently realized at NIST. By calculating the relevant parameters from a realistic nonseparable lattice potential, we find that in the most favorable cases, the boson lifetime in the first excited band can be several orders of magnitude longer than the typical nearestneighbor tunneling time scales, in contrast with that of a simple singlewell lattice. In addition, for sufficiently small lattice depths, the excited band has minima at nonzero momenta incommensurate with the lattice period, which opens a possibility to realize an exotic superfluid state that spontaneously breaks the timereversal, rotational, and translational symmetries. We discuss possible experimental signatures of this novel state.

25. 
Unconventional interaction between vortices in a polarized Fermi gas
V. M. Stojanović, W. V. Liu, and Y. B. Kim
Recently, a homogeneous superfluid state with a single gapless Fermi surface was predicted to be the ground state of an ultracold Fermi gas with spin population imbalance in the regime of molecular BoseEinstein condensation. We study vortices in this novel state using a symmetrybased effective field theory, which captures the lowenergy physics of gapless fermions and superfluid phase fluctuations. This theory is applicable to all spinimbalanced ultracold Fermi gases in the superfluid regime, regardless of whether the original fermionpairing interaction is weak or strong. We find a remarkable, unconventional form of the interaction between vortices. The presence of gapless fermions gives rise to a spatially oscillating potential, akin to the RKKY indirectexchange interaction in nonmagnetic metals. We compare the parameters of the effective theory to the experimentally measurable quantities and further discuss the conditions for the verification of the predicted new feature. Our study opens up an interesting question as to the nature of the vortex lattice resulting from the competition between the usual repulsive logarithmic (2D Coulomb) and predominantly attractive fermioninduced interactions.

26. 
Nonlocal electronphonon coupling: Consequences for the nature of polaron states
V. M. Stojanović, P. A. Bobbert, and M. A. J. Michels
We develop a variational approach to an extended Holstein model, comprising both local and nonlocal electronphonon coupling. The approach is based on the minimization of a Bogoliubov bound to the Helmholtz free energy. The ambivalent character of nonlocal coupling, which both promotes and hinders transport, is clearly observed. Furthermore, a salient feature of our results is that the local and nonlocal couplings can compensate each other, leading to a reduction of polaronic effects and a quasifree character of the excitation. Our findings have implications for organic crystals of Πconjugated molecules, where this electronphonon coupling mechanism plays an important role.

27. 
Theory of polaron bandwidth narrowing in organic molecular crystals
K. Hannewald, V. M. Stojanović, P. A. Bobbert, J. M. T. Schellekens, G. Kresse, and J. Hafner
We present a theoretical description of polaron bandwidth narrowing in organic molecular crystals. Based on a solution of a HolsteinPeierls model for tightly bound electrons interacting with phonons, an explicit expression for the temperature dependence of the electronic bandwidths is found. This formula generalizes the result of Holstein polaron theory by treating local and nonlocal electronphonon coupling on equal footing. The usefulness of the method is demonstrated by model studies for oligoacene crystals from which microscopic insight into the relevance of the different coupling mechanisms is obtained.

28. 
A note on temperaturedependent band narrowing in oligoacene crystals
K. Hannewald, V. M. Stojanović, and P. A. Bobbert
We present a theoretical description of polaron band narrowing in oligoacene crystals due to electronlattice interaction. The analysis is based on a model which takes both local and nonlocal contributions to the electronphonon coupling into account. Different approximation schemes are discussed and compared. The theory is supplemented by quantitative abinitio calculations of the temperature dependence of polaron bandwidths in oligoacene crystals which show the important role of inplane nonlocal electronphonon coupling.
