124 research outputs found
Self-consistent Keldysh approach to quenches in weakly interacting Bose-Hubbard model
We present a non-equilibrium Green's functional approach to study the
dynamics following a quench in weakly interacting Bose Hubbard model (BHM). The
technique is based on the self-consistent solution of a set of equations which
represents a particular case of the most general set of Hedin's equations for
the interacting single-particle Green's function. We use the ladder
approximation as a skeleton diagram for the two-particle scattering amplitude
useful, through the self-energy in the Dyson equation, for finding the
interacting single-particle Green's function. This scheme is then implemented
numerically by a parallelized code. We exploit this approach to study the
correlation propagation after a quench in the interaction parameter, for one
(1D) and two (2D) dimensions. In particular, we show how our approach is able
to recover the crossover from ballistic to diffusive regime by increasing the
boson-boson interaction. Finally we also discuss the role of a thermal initial
state on the dynamics both for 1D and 2D Bose Hubbard models, finding that
surprisingly at high temperature a ballistic evolution is restored.Comment: 13 figure
Exact spectral function of a Tonks-Girardeau gas in a lattice
The single-particle spectral function of a strongly correlated system is an
essential ingredient to describe its dynamics and transport properties. We
develop a general method to calculate the exact spectral function of a strongly
interacting one-dimensional Bose gas in the Tonks-Girardeau regime, valid for
any type of confining potential, and apply it to bosons on a lattice to obtain
the full spectral function, at all energy and momentum scales. We find that it
displays three main singularity lines. The first two can be identified as the
analogs of Lieb-I and Lieb-II modes of a uniform fluid; the third one, instead,
is specifically due to the presence of the lattice. We show that the spectral
function displays a power-law behaviour close to the Lieb-I and Lieb-II
singularities, as predicted by the non-linear Luttinger liquid description, and
obtain the exact exponents. In particular, the Lieb-II mode shows a divergence
in the spectral function, differently from what happens in the dynamical
structure factor, thus providing a route to probe it in experiments with
ultracold atoms.Comment: 10 pages, 3 figure
Vortex entanglement in Bose-Einstein condensates coupled to Laguerre-Gauss beams
We study the establishment of vortex entanglement in remote and weakly
interacting Bose Einstein condensates. We consider a two-mode photonic resource
entangled in its orbital angular momentum (OAM) degree of freedom and, by
exploiting the process of light-to-BEC OAM transfer, demonstrate that such
entanglement can be efficiently passed to the matter-like systems. Our proposal
thus represents a building block for novel low-dissipation and long-memory
communication channels based on OAM. We discuss issues of practical
realizability, stressing the feasibility of our scheme and present an operative
technique for the indirect inference of the set vortex entanglement.Comment: 10 pages, 7 figures, RevTex
Enhancing qubit readout with Bayesian Learning
We introduce an efficient and accurate readout measurement scheme for single
and multi-qubit states. Our method uses Bayesian inference to build an
assignment probability distribution for each qubit state based on a reference
characterization of the detector response functions. This allows us to account
for system imperfections and thermal noise within the assignment of the
computational basis. We benchmark our protocol on a quantum device with five
superconducting qubits, testing initial state preparation for single and
two-qubits states and an application of the Bernstein-Vazirani algorithm
executed on five qubits. Our method shows a substantial reduction of the
readout error and promises advantages for near-term and future quantum devices.Comment: 7 pages, 4 figure
Decoherence in a fermion environment: Non-Markovianity and Orthogonality Catastrophe
We analyze the non-Markovian character of the dynamics of an open two-level
atom interacting with a gas of ultra-cold fermions. In particular, we discuss
the connection between the phenomena of orthogonality catastrophe and Fermi
edge singularity occurring in such a kind of environment and the memory-keeping
effects which are displayed in the time evolution of the open system
Orthogonality catastrophe as a consequence of qubit embedding in an ultra-cold Fermi gas
We investigate the behaviour of a single qubit coupled to a low-dimensional,
ultra-cold Fermi gas. The scattering between the system and the fermions leads
to the loss of any coherence in the initial state of the qubit and we show that
the exact dynamics of this process is strongly influenced by the effect of the
orthogonality catastrophe within the gas. We highlight the relationship between
the Loschmidt echo and the retarded Green's function - typically used to
formulate the dynamical theory of the catastrophe - and demonstrate that the
effect can be triggered and characterized via local operations on the qubit. We
demonstrate how the expected broadening of the spectral function can be
observed using Ramsey interferometry on the qubit.Comment: 4 and a bit pages, 3 figures. Updated versio
Quantum Otto cycle with inner friction: finite-time and disorder effects
The concept of inner friction, by which a quantum heat engine is unable to
follow adiabatically its strokes and thus dissipates useful energy, is
illustrated in an exact physical model where the working substance consists of
an ensemble of misaligned spins interacting with a magnetic field and
performing the Otto cycle. The effect of this static disorder under a
finite-time cycle gives a new perspective of the concept of inner friction
under realistic settings. We investigate the efficiency and power of this
engine and relate its performance to the amount of friction from misalignment
and to the temperature difference between heat baths. Finally we propose an
alternative experimental implementation of the cycle where the spin is encoded
in the degree of polarization of photons.Comment: Published version in the Focus Issue on "Quantum Thermodynamics
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