805 research outputs found

### Polaronic signatures and spectral properties of graphene antidot lattices

We explore the consequences of electron-phonon (e-ph) 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 nearest-neighbor electronic hopping integrals due to
lattice distortions (Peierls-type e-ph coupling). We compute the full momentum
dependence of the e-ph vertex functions for a number of representative antidot
lattices. Based on the latter, we discuss the origins of the previously found
large conduction-band quasiparticle spectral weight due to e-ph coupling. In
addition, we study the nonzero-momentum quasiparticle properties with the aid
of the self-consistent Born approximation, yielding results that can be
compared with future angle-resolved photoemission spectroscopy measurements.
Our principal finding is a significant e-ph mass enhancement, an indication of
polaronic behavior. This can be ascribed to the peculiar momentum dependence of
the e-ph interaction in these narrow-band systems, which favors small phonon
momentum scattering. We also discuss implications of our study for recently
fabricated large-period graphene antidot lattices.Comment: published versio

### Bare-excitation ground state of a spinless-fermion -- boson model and W-state engineering in an array of superconducting qubits and resonators

This work unravels an interesting property of a one-dimensional lattice model
that describes a single itinerant spinless fermion (excitation) coupled to
zero-dimensional (dispersionless) bosons through two different
nonlocal-coupling mechanisms. Namely, below a critical value of the effective
excitation-boson coupling strength the exact ground state of this model is the
zero-quasimomentum 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.Comment: final, published versio

### Extracting spectral properties of small Holstein polarons from a transmon-based analog quantum simulator

The Holstein model, which describes purely local coupling of an itinerant
excitation (electron, hole, exciton) with zero-dimensional (dispersionless)
phonons, represents the paradigm for short-range excitation-phonon
interactions. It is demonstrated here how spectral properties of small Holstein
polarons -- heavily phonon-dressed quasiparticles, formed in the
strong-coupling regime of the Holstein model -- can be extracted from an analog
quantum simulator of this model. This simulator, which is meant to operate in
the dispersive regime of circuit quantum electrodynamics, has the form of an
array of capacitively coupled superconducting transmon qubits and microwave
resonators, the latter being subject to a weak external driving. The magnitude
of $XY$-type coupling between adjacent qubits in this system can be tuned
through an external flux threading the SQUID loops between those qubits; this
translates into an {\em in-situ} flux-tunable hopping amplitude of a fictitious
itinerant spinless-fermion excitation, allowing one to access all the relevant
physical regimes of the Holstein model. By employing the kernel-polynomial
method, based on expanding dynamical response functions in Chebyshev
polynomials of the first kind and their recurrence relation, the relevant
single-particle momentum-frequency resolved spectral function of this system is
computed here for a broad range of parameter values. To complement the
evaluation of the spectral function, it is also explained how -- by making use
of the many-body version of the Ramsey interference protocol -- this
dynamical-response function can be measured in the envisioned analog simulator.Comment: 17 pages, 7 figure

### Dicke-state preparation through global transverse control of Ising-coupled qubits

We consider the problem of engineering the two-excitation Dicke state
$|D^{3}_{2}\rangle$ in a three-qubit system with all-to-all Ising-type
qubit-qubit interaction, which is also subject to global transverse
(Zeeman-type) control fields. The theoretical underpinning for our envisioned
state-preparation scheme, in which $|000\rangle$ is adopted as the initial
state of the system, is provided by a Lie-algebraic result that guarantees
state-to-state controllability of this system for an arbitrary choice of
initial- and final states that are invariant with respect to permutations of
qubits. This scheme is envisaged in the form of a pulse sequence that involves
three instantaneous control pulses, which are equivalent to global qubit
rotations, and two Ising-interaction pulses of finite durations between
consecutive control pulses. The design of this pulse sequence -- whose total
duration is $T\approx 0.95\:\hbar/J$, where $J$ is the Ising-coupling strength
-- leans heavily on the concept of the symmetric sector, a four-dimensional,
permutationally-invariant subspace of the three-qubit Hilbert space. We
demonstrate the feasibility of the proposed state-preparation scheme by
carrying out a detailed numerical analysis of its robustness to systematic
errors, i.e. deviations from the optimal values of the eight parameters that
characterize the underlying pulse sequence. Finally, we discuss how our
proposed scheme can be generalized for engineering Dicke states in systems with
$N \ge 4$ qubits. For the sake of illustration, we describe the preparation of
the two-excitation Dicke state $|D^{4}_{2}\rangle$ in a four-qubit system.Comment: extended version, accepted for publication in Phys. Rev.

### Digital quantum simulation of scalar Yukawa coupling: Dynamics following an interaction quench on IBM Q

Motivated by the dearth of studies pertaining to the digital quantum
simulation of coupled fermion-boson systems and the revitalized interest in
simulating models from medium- and high-energy physics, we investigate the
nonequilibrium dynamics following a Yukawa-interaction quench on IBM Q. After
adopting -- due to current quantum-hardware limitations -- a single-site
(zero-dimensional) version of the scalar Yukawa-coupling model as our point of
departure, we design low-depth quantum circuits that emulate its dynamics with
up to three bosons. In particular, using advanced circuit-optimization
techniques, in the one-boson case we demonstrate circuit compression, i.e.
design a shallow (constant-depth) circuit that contains only two CNOT gates,
regardless of the total simulation time. In the three-boson case -- where such
a compression is not possible -- we design a circuit in which one Trotter step
entails 8 CNOTs, this number being far below the maximal CNOT-cost of a generic
three-qubit gate. Using an analogy with the travelling salesman problem, we
also provide a CNOT-cost estimate for quantum circuits emulating the system
dynamics for higher boson-number truncations. Finally, based on the proposed
circuits for one- and three-boson cases, we quantify the system dynamics for
several different initial states by evaluating the expected fermion- and boson
numbers at an arbitrary time after the quench. We validate our results by
finding their good agreement with the exact ones obtained through classical
benchmarking.Comment: 19 pages, 21 figure

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